Method and System for Fabricating Bulk Metallic Glass Sheets

Abstract

This invention describes a method and hardware of how to deform metallic glasses under low force and stabilized conditions to fabricate thin and large area metallic glass sheets. It is based on a combination of thermoplastic rolling and stretching and typically combined with a pre-heating method. The predominant mode of deformation is dependent on the BMG conditions such as thickness, viscosity, and crystallization time.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Application No. 61/919,158, filed on Dec. 20, 2013 and U.S. Provisional Application No. 62/025,558, filed on Jul. 17, 2014, the subject matter of each of which is herein incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under W911NF-11-1-0380, awarded by the United States Army Research Office. The U.S. Government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus for deforming metallic glasses to fabricate metallic glass sheets, strips and ribbons.

BACKGROUND OF THE INVENTION

Bulk metallic glasses (BMGs), which are also known as bulk solidifying amorphous alloy compositions, are a class of amorphous metallic alloy materials that are regarded as prospective materials for a vast range of applications because of their superior properties such as high yield strength, large elastic strain limit, and high corrosion resistance.

A unique property of BMG is that they have a super-cooled liquid region (SCLR), ΔTsc, which is a relative measure of the stability of the viscous liquid regime. The SCLR is defined by the temperature difference between the onset of crystallization, Tx, and the glass transition temperature, Tg of the particular BMG alloy. These values can be conveniently determined by using standard calorimetric techniques such as DSC (Differential Scanning Calorimetry) measurements at 20° C./min.

Generally, a larger ΔTsc is associated with a lower critical cooling rate, though a significant amount of scatter exists at ΔTsc values of more than 40° C. Bulk-solidifying amorphous alloys with a ΔTsc of more than 40° C., and preferably more than 60° C., and still more preferably a ΔTsc of 70° C. and more are very desirable because of the relative ease of forming. In the supercooled liquid region the bulk solidifying alloy behaves like a high viscous fluid. The viscosity for bulk solidifying alloys with a wide supercooled liquid region decreases from 10¹² Pa·s at the glass transition temperature to 10⁷ Pa·s and in some cases to 10⁵ Pa·s. Heating the bulk solidifying alloy beyond the crystallization temperature leads to crystallization and immediate loss of the superior properties of the alloy and it can no longer be formed.

Superplastic forming (SPF) of an amorphous metal alloy involves heating it into the SCLR and forming it under an applied pressure. The method is similar to the processing of thermoplastics, where the formability, which is inversely proportional to the viscosity, increases with increasing temperature. In contrast to thermoplastics however, the highly viscous amorphous metal alloy is metastable and eventually crystallizes.

Crystallization of the amorphous metal alloy must be avoided for several reasons. First, it degrades the mechanical properties of the amorphous metal alloy. From a processing standpoint, crystallization limits the processing time for hot-forming operation because the flow in crystalline materials is order of magnitude higher than in the liquid amorphous metal alloy. Crystallization kinetics for various amorphous metal alloys allows processing times between minutes and hours in the described viscosity range. This makes the superplastic forming method a finely tunable process that can be performed at convenient time scales, enabling the net-shaping of complicated geometries.

The ability of an amorphous metal alloy to be thermoplastically formed is described by its formability a parameter which is directly related to the interplay between the temperature dependent viscosity and time for crystallization. Crystallization has to be avoided during TPF of an amorphous metal alloy since it degrades the amorphous metal alloy's properties and retards its formability. Therefore, the total time elapsed during TPF of the amorphous metal alloy must be shorter than the time to crystallization.

Sheets are one of the most important shapes of metals either as a final product or as feedstock for further processing. In particular, for metallic glasses, sheets are highly desirable since they are thin in one dimension and, in such geometries, bulk metallic glasses (BMGs) often show bending ductility. For example, it has been shown that medium range BMGs with a thickness of ˜1 mm exhibit bending ductility.

However, fabrication of BMG sheets, especially large BMG sheets, has been challenging and the prior art has not demonstrated sheets exceeding about 10 cm×10 cm in size.

Conventional casting is not suited for the fabrication of BMG sheets that are fabricated through a rolling process, because the process requires contradictive requirements—fast cooling on the one hand to avoid crystallization and slow cooling on the other hand to fill the entire mold cavity. Twin-roll casting must be conducted in high vacuum or protective atmosphere, and has challenging control issues. Cold rolling of BMGs is very limited, because at room temperature BMGs plastically deform highly localized through the formation of shear bands, where the vast majority of strain is localized, as oppose to homogenous deformation required for rolling. As a consequence, the deformation achieved during cold rolling is highly non-homogeneous and, for feedstock materials exceeding about 1 mm in thickness, leads to direct fracture.

Deformation at temperatures within the supercooled liquid region of the particular BMG (T_g<T<T_x) has also been explored for shaping and processing. At temperatures within the supercooled liquid region for practical strain rates, BMGs deform homogenously at low stresses, ˜ 1/100 of the room temperature yield strength, and the access to such low flow stresses has been explored for fabricating thin BMG discs.

Attempts have also been made to roll the BMG within the supercooled liquid region. Most successful has been an approach where heating is carried out through heating plates that are passed with the feedstock through the rollers. Here however, the BMG is in permanent contact with the heating plates which sandwich and thereby heat the BMG. As such, the main benefit of rolling, the reduction of the contact area between mold (roll) and material, is sacrificed, and deformation is rather limited. The largest and thinnest pieces achievable by this method are approximately 7 cm×5 cm, with a thickness of about 0.4 mm. Since the feedstock is in permanent contact with the mold (heating plates), it has not possible to achieve large thin sheets with this technique.

For a thermoplastic forming process, where the BMG is in permanent contact with the mold, the increase in radius is proportional to timê^1/8 for constant pressure. Therefore, either an impractical long time (by far exceeding the crystallization time, which defines the maximum processing time) is necessary to achieve a large radius or an impractically high pressure is needed. Thus, techniques where the entire BMG is in contact with the mold are not suitable for fabricating large and thin BMG sheets.

U.S. Pat. No. 8,485,245 to Prest et al., the subject matter of which is herein incorporated by reference in its entirety, describes a method of pouring molten BMG alloys onto a molten metal bath having a higher density and a lower temperature than the BMG melt in a float chamber to spread and solidify the BMG melt and form a BMG sheets. This process mainly relies on solidification of the BMG melt.

However, it is difficult to control thickness since the resultant equilibrium sheet thickness is defined by the gravity and surface tension of the BMG melt and the molten bath. In addition, an inert gas environment and/or vacuum is required in the molten alloy chamber and the float chamber.

U.S. Pat. Pub. No. 2013/0025746 to Hofmann et al., the subject matter of which is herein incorporated by reference in its entirety, describes a method of fabricating BMG sheets through twin roll casting of BMG melts in an inert environment. The method involves injecting molten BMG melts into cold rolls to solidify metallic glass melts into sheets. As an additional option, the exit BMG sheets can be further thermoplastically formed into thinner sheets by a subsequent set of rolls.

In this process, the BMG melt is processed first through cold rolls for solidification casting and then through a subsequent set of hot rolls for hot rolling. This twin roll casting method solidifies the BMG melts into sheets and must be done in vacuum or in an inert environment. In addition, the thickness of BMG sheets though twin roll casting is not desirable, as it is often non-uniform, with a typical thickness of less than about 200 μm. Thermoplastic rolling by the subsequent set of rolls after twin roll casting also requires a very high rolling stress to further deform the thin sheets and achieve thinner sheets.

U.S. Pat. Pub. No. 2014/0064043 to Tsuchiya et al., the subject matter of which is herein incorporated by reference in its entirety, describes a method of fabricating timepiece springs through single-roll casting of BMG melts into sheets followed by superplastic rolling of the BMG sheets to eliminate the casting induced pinholes on the surfaces of the BMG timepiece springs.

Similarly to Hofmann, the process proceeds from cold rolls to hot rolls and relies on solidification of BMG melts in an inert environment. This results in pinholes on surfaces of BMG sheets when the BMG solidifies in a state where air remains on the surface of the BMG or in the interior. The thickness is also not controllable (i.e., is non-uniform and typically below about 200 μm). The superplastic hot rolling step is only designed for smoothing the surface of the BMG sheets; it cannot deform the sheet due to the high hydrostatic stress state during hot rolling. Finally, no uni-axial thermoplastic stretching is applied.

U.S. Pat. No. 8,613,814 to Kaltenboeck et al and U.S. Pat. Pub. No. US 2014/0047888 to Johnson et al., the subject matter of each of which is herein incorporated by reference in its entirety, describe a method of rapidly heating and forging a bulk metallic glass using a rapid capacitor discharging technology within milliseconds. Although this method can soften BMGs very rapidly, the thermoplastic deforming or forging stress for this process is very high due to the high hydrostatic stress under forging and the high strain rate within the short time scale, thus limiting its use in fabricating large BMG sheets.

Thus, there remains a need in the art for a method that can deform metallic glasses under practical conditions (e.g., in air and under practical feasible pressures) in a controllable manner to produce bulk metallic glass sheets having arbitrary thicknesses and sizes. In addition, there remains a need in the art for a fabrication method that can produce large, thin bulk metallic glass sheets.

SUMMARY OF THE INVENTION

It is an object of the present invention to fabricate bulk metallic glass sheets.

It is another object of the present invention to fabricate bulk metallic glass sheets having a uniform thickness.

It is another object of the present invention to fabricate large bulk metallic glass sheets under favorable processing conditions.

It is still another object of the present invention to fabricate bulk metallic glass sheets having complex patterns.

It is still another object of the present invention to fabricate bulk metallic glass strip.

It is still another object of the present invention to join bulk metallic glass sheets or strips by hot rolling.

It is yet another object of the present invention to a system for fabricating bulk metallic glass sheets and strips.

To that end, in one embodiment the present invention relates generally to a method of fabricating a bulk metallic glass sheet, the method comprising the steps of:

• • a) preheating a bulk metallic glass feedstock to a temperature sufficient to soften the bulk metallic glass feedstock but that does not significantly contribute to a consumed crystallization time of the bulk metallic glass; and
  • b) thermoplastically rolling the pre-heated bulk metallic glass feedstock between a set of heated rollers maintained at a processing temperature of the bulk metallic glass;
  • wherein the bulk metallic glass feedstock is reduced in thickness to produce a bulk metallic glass sheet.

The present invention also relates generally to a system for fabricating a bulk metallic glass sheet, the system comprising:

a) a set of pre-heating plates, wherein the set of pre-heating plates are capable of sandwiching a bulk metallic glass feedstock therebetween to preheat the bulk metallic glass feedstock to a temperature sufficient to soften the bulk metallic glass feedstock but that does not significantly contribute to a consumed crystallization time of the bulk metallic glass;

b) a set of rotatable heated rollers maintained at a processing temperature of the bulk metallic glass, wherein as the set of heated rollers rotates, the heated rollers thermoplastically roll the bulk metallic glass feedstock received from the step of pre-heating plates therebetween to thin the bulk metallic glass feedstock into a bulk metallic glass sheet; and

c) a stretching mechanism capable of stretching the rolled bulk metallic glass sheet exiting the set of heated rollers under controlled velocity

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying figures, in which:

FIGS. 1A, 1B, 1C and 1D depict various methods of thinning metallic glasses in their supercooled liquid state.

FIG. 2 depicts two methods of BMG processing technologies.

FIGS. 3A and 3B demonstrate the relationship between formability and processing temperature.

FIGS. 4A and 4B depict the effect of preheating on the maximum deformation that can be achieved during hot rolling with negligible stretching force.

FIG. 5 depicts a temperature protocol of a single step (conventional) processing protocol along with the processing protocol of the present invention.

FIG. 6 depicts a Time-Temperature-Transformation diagram for Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ BMG.

FIG. 7 depicts maximum dimensions of rolled Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ BMG sheets.

FIG. 8 depicts a schematic of a rolling system combined with a stretching method.

FIG. 9 depicts a comparison of the rolling force and stretching force for the same speed (3 mm/s).

FIG. 10 depicts a comparison of the maximum shear stress and maximum hydrostatic pressure at thermoplastic rolling as a function of sheet thickness.

FIG. 11A depicts an as rolled Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ BMG sheet and FIG. 11 depicts the Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ BMG sheet after removing surface oxidation.

FIG. 12 depicts a rolling apparatus for BMG material in accordance with the present invention.

FIG. 13 depicts an exploded view of the key components of the rolling apparatus in accordance with the present invention.

FIG. 14 depicts an example of the preheating plate in accordance with the present invention.

FIG. 15 depicts as-rolled Pd₄₃Ni₁₀Cu₂₇P₂₀ samples.

FIG. 16 plots examples of making patterns in BMG sheets either in-plane or out-of-plane using a patterned roller FIG. 17 depicts examples of making patterns in BMG sheets using thermoplastic compressing or blow molding methods.

FIG. 18 depicts an example of an eyeglass frame patterned into a BMG sheet processed in accordance with the present invention.

FIG. 19 depicts rolling of a sandwich of a bulk metallic glass feedstock material to join entirely (a) or partially (b) and deform the sandwich of material.

Also, while not all elements may be labeled in each figure, all elements with the same reference number indicate similar or identical parts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates generally to a method and apparatus for deforming metallic glasses under low force and stabilized conditions to fabricate large area bulk metallic glass sheets as well as bulk metallic glass strips and ribbons. By “sheet” what is meant is a broad stretch or surface of bulk metallic glass. By “strip” or “ribbon” what is meant is a long narrow piece of bulk metallic glass. In both instances, the bulk metallic glass sheet or strip is fabricated to have a thickness of less than about 1 mm.

As described above, for prior art based on thermoplastic rolling, the rolls are typically cold (˜room temperature), which is much lower than the desired rolling temperature. Thereby, upon rolling, the BMG cools and its flow stress increases rapidly. A key aspect of the current invention is to establish the opposite—the BMG feedstock temperature increases when approaching the rolls and the amount by which it increases is optimized taking into consideration consumption of formability prior to contacting the rolls. This minimizes the time required to reach rolling temperature and optimizes viscosity during rolling to allow large deformations, enabling separation and laminar flow.

Surprisingly, it was found that obtaining large thin BMG sheets by using just hot rolling is difficult because of the hydrostatic pressure needed to reach a practical shear deformation (where η is the viscosity, U is the rolling speed, R and h are the radius of roller and sheet thickness). The hydrostatic pressure can be considered as losses because the shear deformation only depends on the gradient of pressure; the overwhelming majority of the clamping force only hydrostatically compresses the BMG without any permanent deformation. For typical dimensions and thickness reduction, the hydrostatic pressure consumes most of the forces exerted and exceeds pressures that can be generated by practically achievable clamping forces for hot-rolling mills. In short, thermoplastic rolling by itself does not allow for the fabrication of BMG sheets exceeding dimensions of 20 cm×20 cm with a thickness of less than about 0.05 cm, even for those very few metallic glasses having a minimum practically achievable viscosity lower than 10⁷ Pa·s.

The inventors of the present invention have found that stretching deformation of BMGs is an effective use of deformation force where essentially all the exerted forces are used for deformation. However, when using only stretching, instabilities (necking), especially at high stretching speed, are observed. As a consequence, the thickness variation is unacceptably high and worsens for multi-pass processing as illustrated in FIG. 1A.

However, when one or more of hot-rolling and stretching under a controlled velocity (not force) down a negative temperature gradient are combined, the forces to deform the bulk metallic glass can be drastically reduced, and large, thin sheets can be achieved. Furthermore, the combination results in high stability, with very little thickness variations of the fabricated sheets.

Rapid cooling is known to affect the mechanical properties of BMGs. However it has never been reported that deformation during solidification can also affect mechanical properties. The inventors of the present invention discovered that with the method described herein, the sample deforms until it solidifies. In addition, a higher bending ductility is measured for samples that are stretched during solidification compared to a sample that has not been stretched.

The present invention relates generally to a method and system for fabricating metallic glass sheets with controllable dimensions. Metallic glasses are deformed under low force and stabilized conditions to fabricate thin and large area metallic glass sheets. The present invention has the ability to produce metallic glass sheets that are both large (e.g., at least 20 cm×40 cm, depending on the particular BMG) and thin (i.e., thicknesses of less than about 1 mm, preferably less than about 0.5 mm, most preferably less than about 0.1 mm, and even thinner, depending on the particular BMG).

The present invention is based on a thermoplastic rolling technique which is optionally, but preferably, combined with stretching deformation. It is based on a combination of one or more of thermoplastic rolling and stretching combined with a pre-heating method. The predominant mode of deformation is dependent on the BMG conditions such as thickness, viscosity, and crystallization time. The method described herein avoids crystallization during fabrication of the sheets, particularly with large and thin sheets made from BMGs having low formability as further described below. Through preheating, the available processing time that is consumed is reduced, which allows for the use of thick feedstock material. Deformation during hot-rolling is used to reduce thicknesses below we rely on only very limited deformation; minimum requirements are the smoothing of perturbations in the thickness.

Thus, in one embodiment, the present invention relates generally to a method of fabricating a bulk metallic glass sheet, the method comprising the steps of:

• • a) preheating a bulk metallic glass feedstock to a temperature sufficient to soften the bulk metallic glass feedstock but that does not significantly contribute to a consumed crystallization time of the bulk metallic glass; and
  • b) thermoplastically rolling the pre-heated bulk metallic glass feedstock between a set of heated rollers maintained at a processing temperature of the bulk metallic glass;
  • wherein the bulk metallic glass feedstock is reduced in thickness to produce a bulk metallic glass sheet.

In the first processing step, the metallic glass is heated to a temperature of 0.8T_g<T_preheat<1.4T_g (T_g: glass transition temperature during heating with 20 K/min in ° C.) to pre-heat the feedstock BMG for the subsequent rolling step. This pre-heating step reduces heating time required for rolling and softens the BMG, which allows effective heat transfer between roller and BMG feedstock, and thereby rapid heating to the roller temperatures of 0.7T_x<T_process<1.3T_x (T_x: crystallization temperature during heating with 20 K/min in ° C.). This pre-heating step becomes less effective (or can even be skipped) if the sheets are already thin, e.g., upon several passes when the thickness is below 1 mm. Subsequently, the BMG feedstock is hot-rolled at temperatures of 0.7T_x<T_process<1.3T_x.

Hot-rolling is used for thinning the feedstock but becomes very ineffective when

$\frac{R}{h}·\frac{\Delta h}{h}>>1$

(essentially for thin sheets), where R is the radius of rollers, Δh and h are the thickness reduction and film thickness, respectively, because hydrostatic pressure increases very fast in the lubrication approximation regions. Typically, the contribution of rolling force to plastic deformation in these regions is less than 10%. However, another benefit from hot-rolling is that any perturbations (initial thickness variation) which cause “necking” instability can be reduced, which helps to stabilize the subsequent stretching process.

Upon exiting of the bulk metallic glass sheet from the set of heated rollers, the bulk metallic glass sheet is optionally, but preferably, exposed to a stretching force. Surprisingly, this stretching force is typically much smaller than the clamping force required for hot-rolling of metallic glasses to achieve the same deformation rate. The stretching tbrce is applied over a temperature gradient and the bulk metallic glass sheet moves with respect to the temperature gradient. This stabilizes the process and prevents instabilities from developing during the associated thinning.

One requirement for stabilizing the stretching is that stretching occurs under velocity controlled conditions, not under force controlled conditions. Thus, all of the stretching force is used for tension deformation, while to achieve similar deformation rate during hot-rolling, a very large hydrostatic pressure must be exerted. The forces that are exerted to build the hydrostatic pressure do not thin the feedstock and can be considered as losses. Thus, the stretching force is dramatically smaller than the clamping force exerted during hot-rolling, particularly during the later stages where the BMG feedstock is thin.

As a result, thin and large BMG sheets can be achieved with the combined rolling and stretching process described herein. Thus, a more effective use of deformation forces and a much broader range of BMG alloys (i.e., even those with a (practical achievable) minimum viscosity of up to 10¹⁰ Pa·s) can be formed into sheets which would not be possible through thermoplastic based rolling alone.

Surprisingly, it was found that the deformation also affects mechanical properties of the BMG through generation of free volume or equivalent by raising the ratio of bulk modulus over shear modulus. The solidification during stretching results in a more ductile BMG than if the BMG is simply hot-rolled. Therefore, the method described herein can create BMGs with increased ductility, even though during typical processing conditions they would be brittle. For example, stretch rolling was found to increase the bending ductility from less than 2% to over 3% of a 0.8 mm ribbon of Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ bulk metallic glass alloy.

Such a method realization minimizes formability consumption prior to the feedstock touching the rolls, and minimize required deformation stresses through a velocity controlled stretching in a temperature gradient that is stabilized and for thicker feedstock assisted by a hot-rolling processing step.

FIGS. 1A, 1B, 1C and 1D depict various methods of thinning metallic glasses in their supercooled liquid state and in which stretching is achieved through velocity control (rather than force control). FIG. 1A (Case 1) illustrates constant stretching at a uniform temperature, resulting in overall necking of the feedstock and a non-uniform thickness. FIG. 1B (Case 2) illustrates a temperature gradient field using only stretching. As seen in FIG. 1B, the typical existence of surface defects on BMG feedstocks act as “perturbations” during stretching without hot rolling down a temperature gradient. These perturbations can continue to grow during stretching. resulting in local necking behavior. FIG. 1C (Case 3) illustrates that by hot rolling without stretching down a temperature gradient, very limited strain can be created to deform the BMG feedstocks due to a very high hydrostatic pressure in hot rolling. Finally, FIG. 1D (Case 4) illustrates that by combining hot rolling and stretching, perturbations can be highly eliminated and a much higher strain with a steady state thermoplastic deformation can be achieved.

FIG. 2 depicts two methods of BMG processing technologies. Route 1 is a liquid casting process that relies on fast quenching of liquid melts to form BMGs, while route 2 relies on thermoplastic forming of BMGs within the supercooled liquid state.

Prior art techniques for fabricating BMG sheets which are based on liquid state processing, such as twin roll casting, single roll casting, injection molding or pouring molten melts into bath, are all conducted by the processing method of route 1. As discussed above, there are several main disadvantages of route 1, such as very high temperature (above liquid temperature), rapid cooling to avoid crystallization (narrow processing window), the need for high vacuum or inert atmosphere, very limited control.

In contrast, there are many advantages for fabricating BMG sheets based on the processing method of route 2, including much lower temperature and a larger processing window. In addition, the route 2 processing technique may also be feasible to conduct in air.

The ability to thermoplastically process BMGs is quantified in the temperature dependent formability F=l_crst/η. As shown in FIGS. 3A and 3B, formability increases with increasing processing temperature. This behavior is surprising and appears to be ubiquitous among BMGs. A wide range of BMGs have been studied, some of which are summarized in Pitt, E. B., G. Kumar, and J. Schroers, Temperature dependence of the thermoplastic formability in bulk metallic glasses. Journal of Applied Physics, (2011) 110 (4).

This behavior has dictated the processing protocol described herein for the practical fabrication of large, high quality, and thin sheets. As a consequence, in order to maximize the deformation during thermoplastically deforming a metallic glass, a high temperature is chosen, as shown in FIG. 3A, which depicts that the formability of a metallic glass is a function of crystallization time and viscosity which are both strong functions of temperature. Surprisingly, for all the considered BMGs, the formability increases with increasing processing temperature. However, with increasing temperature, the crystallization time decreases. Thus, in order to roll large sheets an effort has to be made to access high temperatures while at the same time avoiding crystallization. Experimentally realizing this highly formable state can be very challenging.

To solve this problem, the present invention uses a processing protocol that optimizes the pre-rolling condition. Formability is low, yet the BMG feedstock softens sufficiently to enable rapid heating to rolling temperature (i.e., processing temperature within a specific temperature range) and also optionally, but preferably, uses stretch deformation as the major deformation process. Stretch deformation requires much lower forces than deformation during hot-rolling, and thus can deform at a lower temperature and conserve formability.

The pre-heating of the BMG feedstock is an effective first processing step of the method described herein which leads to overall larger possible deformation that is obtainable by the processes of the prior art. The effect of pre-heating on the maximum deformation that can be achieved during hot-rolling with negligible stretch rolling forces (lateral sheet size) is schematically shown in FIGS. 4A and 4B.

FIG. 4A depicts the maximum deformation that can be achieved during hot rolling without the use of any preheating. As shown in FIG. 4A, without preheating the BMG feedstock only slowly heats to the desired rolling temperature, which causes slow deformation of the BMG feed stock, while consuming formability.

In contrast, as shown in FIG. 4B, when the BMG feedstock is preheated from T_pre-heat to T_roll, there is a faster deformation of the BMG, which conserves formability, and makes it possible to fabricate a large, thin BMG sheet. The pre-heating step sufficiently softens the BMG so that the rollers form a close contact with the BMG, thus heating the BMG more rapidly.

For the BMGs with highest formability, one can practically realize a rolling operation ˜f=10⁻⁴ Pa⁻¹. This corresponds approximately to 1 minute processing time at a viscosity of 10⁶ Pa·s. In a typical thermoplastic processing protocol, the feedstock is heated to the processing temperature. This process consumes approximately 1 minute. However, only a fraction contributes to the consumption of formability, as illustrated in FIG. 5. This is the temperature region close to the set temperature, T_set±10%. The reason that only a small fraction must be considered is because the crystallization time decreases rapidly with increasing temperature, and the heating rate decreases when approaching the set temperature. The heating rate is proportional to the difference of feedstock temperature and set temperature. In contrast, in the present invention, instead of heating to the ideal rolling temperature directly the BMG feedstock is pre-heated, generally to a lower temperature first. Here the crystallization time is longer, and thus a significantly smaller fraction of formability is consumed as illustrated in FIGS. 3A and 3B.

As seen in FIG. 5, due to the 2-step process utilized in the present invention, consumption of formability during heating can be drastically reduced. In the current process, heating rate decreases when approaching T_pre-heat (with negligible formability consumption), and increases in the temperature region where significant formability is consumed due to a different heating mechanism from T_pre-heat to T_roll.

The present invention takes advantage of the exponential dependence of the crystallization time with temperature. The feedstock BMG is heated to a temperature where the crystallization time is very long, (i.e., at T_g, the crystallization time is on the order of one day or more). However, from a thermal point of view, this temperature is close to the processing temperature. Thus, the temperature of the pre-heated feedstock only has to be increased by a few tens of degrees through the set of heated rollers to achieve the processing temperature and rapid and precise heating can be achieved by feeding the pre-heated feedstock BMG through the set of heated rollers. Subsequently, cooling can be achieved through natural convection of the processing environment or can be enhanced by forcing gas or liquid on the exiting sheet. Thus, the use of the preheating step results in only a small overall deformation and can also cause damage to the rollers. Thus, the bulk metallic glass is preheated to a temperature where it can remain for a long period of time compared to its available time at processing temperature, which, in one embodiment is at least 5 times longer, preferably at least 10 times longer. In other words, the bulk metallic glass feedstock is preheated to a temperature sufficient to soften the bulk metallic glass feedstock but that does not significantly contribute to a consumed crystallization time of the bulk metallic glass. At the same time, this pre-heating temperature is close to the rolling temperature from a thermal aspect, and the BMG feedstock has softened sufficiently that it can be thermoplastically deformed readily.

FIG. 6 illustrates a Time-Temperature-Transformation diagram for a Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ BMG. As seen from FIG. 6, crystallization time rapidly decreases with increasing temperature.

FIG. 7 illustrates the maximum rolled Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ BMG sheets at different temperatures. The initial feedstocks are all 1.7 mm thick, 14 mm diameter discs. The number of passes prior to crystallization is indicated. As seen in FIG. 7, using Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ BMG, it determined that 440° C. was the best processing temperature from a rolling point of view to yield the highest thickness reduction possible and thinnest samples possible.

Typically, rolling is performed in several passes, which may range from between 3 and 15 passes. At 440° C., the Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ BMG exhibits a processing window of 5˜6 minutes before crystallization. During a one-step heating process, ˜25 seconds at 440° C. are consumed for each pass among the total 5˜6 minutes. The actual rolling (contact of feedstock with roller) depends on, for example, the sheet length, roller radius, rolling speed and, for typical BMG rolling, the actual rolling takes about 10 seconds. Therefore, rolling through 10 passes consumes approximately 6 minutes of processing time, meaning that the BMG sample would have already started to crystallize. The maximum is thus only about 9 passes before the BMG sample crystallizes. If a non-ideal low processing temperature is chosen, such as 420° C., 8 minutes of processing time is are available, and thus more than 10 passes can be carried out before the sample crystallizes. However, the achieved deformation is low due to the significantly increased viscosity at this lower temperature. Other temperatures are also shown in FIG. 7.

In the present invention, the pre-heating temperature is optimized to a temperature that is high but that is low enough that it does not significantly contribute to the consumed crystallization time. This temperature is close to the processing/roll temperature so that the temperature can be rapidly raised to the processing temperature.

The reasons for the rapid heating ability in this processing step are as follows:

• • (1) The temperature is already close to the processing temperature, typically >T_roll−about 20% to about 35%, more preferably about 30%; and
  • (2) The softness of the pre-heated state of the BMG feedstock results in intimate contact of the entire contact line, and thus rapid heat transfer can be achieved.

For example, if the BMG is preheated at 390° C. rolled and is then rolled at 440° C., crystallization occurs after 35 minutes, which is much longer than the crystallization time scale of 5˜6 min at 440° C., without a preheating step. Thus, consumption of formability is drastically reduced during heating to the pre-heating temperature and can be consumed during the rolling step. As such, by using the process described herein, the BMG feedstock can be rolled up to ˜16 passes before crystallization, as compared with ˜10 passes for one-step processing.

As described herein, only by thermoplastic rolling was it found that the forces required to reach a desired thickness reduction were dramatically high, in particular for sheet like dimensions (i.e., where the dimensions out-of-plane are far smaller than the dimensions in-plane). However, if a stretching step is added as illustrated in FIG. 8, then the thickness of sheet after exiting from the first set of rollers can be further reduced because of the stretching force, F.

The stretching force can be applied by various methods, including, for example, by means of (1) a second pair of “cold” rollers (i.e., maintained at a lower temperature than the processing temperature of the heated rollers) which rotate at a higher velocity than the first set of heated rollers to create a stretching force; or (2) by a velocity controlled pulling mechanism. Other methods of applying a stretching force to the bulk metallic glass as it exits the first set of heated rollers would also be known to those skilled in the art. What is critical for stretching is both controlled velocity and a negative temperature gradient.

The stretching force is calculated according to σ=F/(h*W), wherein W is the width of the sheet. The stress in the stretched part is proportional to 1/h. This means that as the thickness of rolled sheet decreases, the driving stress will increase until the sheet breaks. However, it was surprisingly found that variations in feedstock thickness can be reduced such that even some thickness perturbation prior to stretching can be tolerated without the BMG growing in an instable manner.

The requirements for stabilizing the combination of compression and stretch rolling are as follows:

• • (1) A negative temperature gradient when the BMG sheet exits the first set of heated rollers. Thereby, with increasing distance from the first set of heated rollers, the viscosity increases, and thus the deformation resistance increases;
  • (2) Stretching is realized by precisely controlling the displacement. In other words, stretching is velocity controlled, as opposed to force controlled. Typically viscosity is at least substantially constant; and
  • (3) The variation of thickness that occur during stretching can be flattened by subsequent rolling passes.

FIG. 8 illustrates schematics of a rolling system combined with a stretching method. where a second set of “cold” rollers is used to control stretching of the rolled sheet to make it thinner.

The deformation mechanisms for thermoplastic rolling and stretching are different. For thermoplastic rolling, thickness reduction is from shear deformation (similar to squeezing flowing), while for stretching, thickness reduction is from tensile deformation.

To quantify the preferable deformation modes for different BMG process conditions, such as thickness, viscosity, and crystallization time, the thermoplastic rolling and stretching forces can be calculated.

In the following calculation, the problem is simplified to a 2D problem, i.e., it was assumed that the thickness of sheet is far smaller than its other dimensions, and for BMGs deformed in the supercooled liquid region, the BMGs were considered as incompressible Newtonian fluids. The Reynolds number

$\left(\mathrm{Re}=\frac{\rho U_0L}{\eta }\right)$

in practice is far smaller than 1, and thus the body forces such as gravity and inertia terms can be neglected.

Based thereon, the pressure at rolling can be determined using a no-slip boundary condition as shown in Equation (1):

$\begin{array}{cc}p\left(x\right)=\frac{3\eta U\sqrt{h_mR}}{2h_m^2}\left(\left(4-\frac{3h_1}{h_m}\right)\left(\arctan \left(\frac{x}{\sqrt{h_mR}}\right)-\arctan \left(\sqrt{\frac{h_1}{h_m}-1}\right)\right)+\frac{x}{\sqrt{h_mR}}\frac{\frac{x^2}{h_mR}\left(4-\frac{3h_1}{h_m}\right)+4-\frac{5h_1}{h_m}}{{\left(1-\frac{x^2}{h_mR}\right)}^2}+\frac{\left(\sqrt{\frac{h_1}{h_m}-1}\right)\left(\frac{3h_1}{R_m}-2\right)}{\frac{h_1}{h_m}}\right)& \left(1\right)\end{array}$

wherein η is the viscosity, U is the rolling speed, x is along the rolling direction, h_m, R, h₁ are the gap between rollers, radius of rollers and sheet thickness at exit, respectively.

The rolling force can then be calculated according to Equation (2):

F₁=w∫_−lp^x1p(x)dx  (2)

The stretching force is estimated according to Equation (3):

$\begin{array}{cc}{F}_s=2w\eta h{\stackrel{.}{\varepsilon }}_{\mathrm{xx}}\approx 2w\eta h\frac{\Delta U}{L_0}& \left(3\right)\end{array}$

wherein w is the width of sheet, ΔU is the speed difference between the two ends of sheet and L₀ is the length within which the thickness reduction takes place.

To compare the rolling and stretching forces, a thickness reduction of Δh/h₀=40% is used as an example and it is assumed that the roller gap is half of the initial sheet thickness, w=10 cm, η=10⁶ Pa·s, U=3 mm/s, ΔU=U and L₀=1 cm, which is reasonable for practical control. e.g. high temperature needs to be cooled down to stabilize the deformation. In one embodiment, the deformation forces of thermoplastic rolling and stretching for the same deformation achieved during the deformation time differ by at least 10 times.

The results are shown in FIG. 9, which compares the rolling force and stretching force for the same speed (3 mm/s), where thickness reduction Δh/h₀=40%, roller gap is half of the initial sheet thickness, sheet width=10 cm and viscosity=10⁶ Pa·s. Very surprisingly, the forces required to reach the same speed and thickness reduction rate during stretching are dramatically lower than during rolling. Thus, to achieve a sheet of 0.5 mm thickness, the rolling force is approximately a thousand times higher than the stretching force.

Based on Equation (3), it is obvious that all of the stretching force is used for plastic deformation, while for thermoplastic rolling, shear stress rather than hydrostatic pressure contributes to plastic deformation. The maximum shear stress and maximum hydrostatic pressure were compared for the same rolling parameters as in FIG. 9 as function of thickness as shown in FIG. 10. As illustrated in FIG. 10, the maximum hydrostatic force increases dramatically fast as the sheet thickness reduced. In addition the maximum hydrostatic force is always far larger than the maximum shear stress, which means that making thin sheets by thermoplastic rolling becomes less and less efficiency as the sheet thins. 15.

One of the major benefits of the present invention is that it allows for the fabrication of sheets in an ambient atmosphere. In contrast, most of the alternative methods previously proposed for fabricating sheet-like BMG parts, including methods such as casting, twin-roll casting and alloy melt forming, require vacuum or an inert gas or reducing environment, which are impractical in a manufacturing environment. Obviously. most BMGs oxidize during the suggested temperatures and processing conditions. However, it was surprising found that the oxidation is superficial and limited to the surface of the sheet. The oxide layer only stays on the top (thin) depth of the surface (1-2 μm). FIG. 11A illustrates an as-rolled Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ BMG sheet with a thickness ˜450 μm (T_roll=440° C., T_pre-heat=420° C. for 13 passes). FIG. 11B illustrates this same BMG sheet after removing 1-2 microns surface.

The inventors have found however that required for the limitation of the oxide to the surface of the BMG sheet is a laminar flow. Therefore, the present invention is limited to processing conditions which result in laminar flow and avoids turbulences. Conditions for such flow can be defined through the Reynolds number, where a Reynolds number significantly smaller than 1 is required to result in laminar flow. For the rolling process, Reynolds number can be calculated as

$\mathrm{Re}=\frac{\rho \mathrm{Uh}}{\eta },$

where ρ, η and h are density, viscosity and thickness of feedstock respectively, and U is the rolling speed. The requirement of laminar flow for the present invention include the following conditions: a rolling thickness of 10 μm-20 mm; viscosity: 10³˜10¹⁰ Pa·s, density: 3˜20 g/cm³; rolling speed: 1-200 cm/min, respectively, all of which yields an Re≈10⁻¹³-10⁻³<<1.

BMGs that can be processed in accordance with the present invention can be formally divided into four groups, even though they fall on a continuum as shown in Table 1.

TABLE 1
Classes of BMGs
Class of BMGs	Example alloys	ηmin (Tx) [Pa · s]	F (Tx) [Pa − 1]	stretching
Excellent	Pt57.5Cu14.7Ni5.3P22.5	<105	>10−4	required only for
formability	Pd43Ni10Cu27P20			large sheets (e.g.,
	Zr35Ti30Cu7.5Be27.5
High formability	Zr44Ti11Cu10Ni10Be25	106~107	10−4~10−5	Required for
	Au49Ag5.5Pd2.3Cu26.9			medium and large
	Si16.3Mg65Cu25Y10			sheets (e.g.,
Medium	Fe48Cr15Mo14Er2C15B6	>107~108	<10−5~10−6	Required
formability	Ni59Zr16Ti13Si3Sn2Nb7
Low formability	Ni62Nb38	>109	<10−6	Required and only in
				limited geometries
				possible

The need for stretching depends in part on the particular class of BMG as well as the desired size of the finished BMG sheet.

BMGs with excellent formability exhibit a viscosity of 10⁶ Pa·s at T_x. Their formability is larger or equal to 10⁻⁴ Pa⁻¹. Pre-heating during the initial stages of rolling for thick feedstocks (>3 mm) is still required. Often, for this BMG class, the processing temperature can be chosen to be lower, and thus a larger processing time is available and crystallization can be more readily avoided. Stretching is required only for large sheets (i.e., having dimension greater than about 40 cm by about 20 cm).

BMGs with high formability exhibit a viscosity of 10⁶-10⁷ Pa·s or smaller at Tx. Their formability is between about 10⁻⁴-10⁻⁵. Pre-heating during the initial stages of rolling for thick feedstocks (>3 mm) is still required. Stretching is required for large and medium sheets (i.e., having dimension greater than about 20 cm by about 10 cm).

BMG alloys with medium formability require stretching for most geometries. With a clamping force of 30 kN, the final size that can be achieved with these alloys is noticeably different than for the high formability BMGs. Large sheets can only be achieved through stretching. BMG sheets having dimensions of about 20 cm by about 10 cm are possible using the techniques described herein.

BMG alloys low formability have a viscosity larger than 10⁹ Pa s at T_x (F<10⁻⁶ Pa⁻¹) have low formability and deform insignificantly during rolling. Thus, the vast majority of deformation must occur during stretching. Pre-shaped thin plates can be still thinned when including stretching. BMG sheets having dimensions of about 20 cm by about 3 cm are possible using the techniques described herein.

However, in all cases where the BMG feedstock has a thickness of >3 mm, it is necessary to preheat the feedstock material prior to rolling. For feedstock dimensions, e.g., after several passes where the feedstock is sheet-like with thickness <1 mm, heating through the rollers may become sufficient and pre-heating is no longer required.

Another key aspect of the present invention is precise temperature and processing time control. Precise temperature control can be achieved by controlling the temperature of the heating elements. For example, a thermocouple feedback can be placed inside the set of heated rollers and used to control the temperature of heating cartridges within the set of heated rollers.

In the alternative, temperature control can also be accomplished using radiation heat from outside of the set of heated rollers. Rollers can also be heated by submersion in a heated liquid. Other temperature control means that allow for precise temperature control would also be known to those skilled in the art and are usable in the present invention.

As illustrated in FIGS. 12 and 13, in one embodiment the rollers are heated with resistance heaters which are controlled through PID-control, in which thermocouples, positioned close to the surface of the rollers, are used to measure the temperature. It is highly desirable that temperature uniformity on the surface of the roll deviates less than 5 degrees Celsius throughout the whole roller.

As described above, the pre-heater is a key element of the present invention because it reduces formability consumption of the BMG. The requirements for the pre-heater are that the BMG feedstock is sufficiently softened so that the BMG does not damage the roller, conforms to the roller shape, and meanwhile minimizes formability consumption.

In one embodiment, as shown in FIG. 11, the preheater may comprise two heating plates (top and bottom) that heat the feedstock as the feedstock passes through the pre-heating step. Temperature control during pre-heating is critical, however not as critical as during rolling. The pre-heater optionally, but preferably, has a heating and control mechanism independent from that of the rollers. Techniques to heat and control the pre-heater can be similar to what has been suggested for the rollers.

The preheater shown in FIG. 11 allows for temperature control within ±5 degrees Celsius. In the present example, the two heating plates press against feedstock simply by means of gravity. However, depending on the particular class of BMG, as well as the particular properties of the BMG within the class as well as the geometry of the BMG feedstock, additional forces can be added. Such force can be adjusted to be higher or lower as is needed.

With increasing lateral sheet dimensions, the tendency of the BMG to stick to the rollers can be problematic. Sticking of the BMG to the rollers must be solved in order to maintain flatness, heat conductivity, and continuous formability during the entire rolling process. The tendency of the feedstock to stick to the rollers can be quantified by comparing the driving force to stick due to decreased system energy due to adhesion and the resistance force due to increased strain energy due to bending. For Newton fluid, the increased bending energy due to adhesion is shown in Equation (4):

$\begin{array}{cc}{U}_b=M·\theta =\frac{\lambda I\Omega }{R}\theta & \left(4\right)\end{array}$

wherein λ=3η is the tension viscosity of the sheet, Ω is the angle velocity of roller, and I=h₂³/12 is the cross-sectional moment of inertia.

On the other hand, the adhesion energy of the system is shown in Equation (5):

U_s=(γ_s-v−γ_s-l)·s=(γ_s-v−γ_y-l)Rθ=γ_l-v cos β·Rθ  (5)

wherein β is the contact angle between metallic glass and roller.

Thus, to prevent sticking during rolling, it is required that:

$\begin{array}{cc}\frac{\partial U_b}{\partial \theta }>\frac{\partial U_s}{\partial \theta }\mathrm{Or}& \left(6\right)\\ {\gamma }_{1-v}\cos \beta <\frac{\eta h_2^3}{4R^2}\Omega & \left(7\right)\end{array}$

This sticking tendency has not been observed for other metals and is believed to occur in BMGS due to the low accessible viscosity with BMGs or corresponding flow stresses, and as a consequence, the intimate contact between roller and BMG feedstock. Thus, in order to reduce sticking during hot rolling, the adhesion energy must be decreased.

The present invention proposes the following strategies to reduce the sticking tendency of the BMG feedstock to the roller:

• • (1) Reducing microscopic surface area—it was observed that sticking tendency reduces with a very high surface finish. Thus, in an attempt to reduce the sticking tendency, roughness was decreased. At first, the sticking worsened, but below a roughness that compares with a mirror finish, a decrease in sticking is observed;
  • (2) Surface chemistry—when choosing a roller material that is poorly wetted by the BMG. What this means is that the wetting angle, β in Equation 7 is larger than 90 degrees (Equation 7 always holds). For example, nitrides and other ceramic roller surfaces reduce sticking (increase the wetting angle) whereas a metallic roller surface promotes sticking.
  • (3) Related to surface chemistry is oxidation of BMG feedstock. The oxidized surface of the BMG feedstock also reduces sticking. For example, it was found that a Pt_57.5Cu_14.7Ni_5.3P_22.5 BMG with the lowest oxidation tendency sticks the most to the rollers, whereas a Pd₄₃Ni₁₀Cu₂₇P₂₀ BMG sticks significantly less and the least tendency to sticking occurs with a Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ BMG alloy that was tested, as illustrated in FIG. 15.
  • (4) Another way to prevent sticking of the BMG is to decrease the surface energy of BMG (γ_l-v in Equation (7)), by adding lubricants.

FIG. 15 depicts photographs of as-rolled Pt_57.5Cu_14.7Ni_5.3P_22.5 samples. As seen in FIG. 15, the Pd₄₃Ni₁Cu₂₇P₂₀ sample may stick to the top and bottom rollers simultaneously causing it to tear along the centerline.

The method described herein allows for the fabrication of ultrasmooth sheets of bulk metallic glasses with low oxidation rates.

Furthermore, once the BMG has been fabricated into a large, thin sheet, other processing steps may then be performed to form the BMG sheet into more complex design shapes.

For example, FIG. 16 plots examples of making patterns in BMG sheets either in-plane or out-of-plane using a patterned roller. As shown in FIG. 16, afler the BMG has been processed through the preheating step and rolled through the set of heated rollers followed by the stretching step as described in detail above, the BMG may be processed through a subsequent set of heated rollers having an in-plane or out-of-plane pattern disposed thereon to pattern the BMG sheet. The set of patterned rollers are maintained at the same or a different processing temperature as the first set of heated rollers to impose the pattern, either in-plane or out-of-plane, onto the BMG sheet.

These same patterns can also be fabricated by using thermoplastic compressing or blow molding methods as illustrated in FIG. 17. In a similar fashion, after the BMG has been processed through the preheating step and rolled through the set of heated rollers followed by the stretching step, the BMG may be subjected to a compression molding or a blow molding process as is generally known in the art. Examples of these techniques are described, for example, in U.S. Pat. No. 8,641,839 to Schroers et al. and in U.S. Pat. Pub. No. 2013/0306262 to Schroers et al., the subject matter of each of which is herein incorporated by reference in its entirety.

In addition, the molding step may also be performed to mold the bulk metallic glass sheet into a mold cavity. If desired, a shearing step may be performed to cut the bulk metallic glass sheet into outlines set by the mold cavity. In another preferred embodiment, a deformation step may be performed to corrugate the bulk metallic glass sheet into out of plane deformations set by the mold cavity.

By using these methods as well as other similar patterning methods for BMGs, even very complicated shapes such as eyeglass frames as illustrated in FIG. 18, can be fabricated using the methods described herein.

In another preferred embodiment of the present invention, the bulk metallic glass feedstock may comprise a plurality of bulk metallic glass pieces that are joined in step b). As shown in FIG. 19, a plurality of bulk metallic glass pieces forming a “sandwich” may be joined by rolling. This joining step can be complete, in which the plurality of bulk metallic glass pieces are completely joined together without any gaps. In the alternative, the joining locations may be controlled to prevent the pieces of bulk metallic glass from joining in certain locations such that only portions of the bulk metallic glass pieces are joined. In this method, in one embodiment, various materials such as salts and polymers can be interspersed within the bulk metallic glass pieces to prevent joining of the bulk metallic glass pieces in certain locations. Other methods that can prevent joining of materials in certain areas such that only portions of the bulk metallic glass pieces are joined may also be usable in the practice of the invention.

The present invention also relates generally to a system for forming bulk metallic glass sheets from bulk metallic glass feedstock materials, the system comprising:

a) a set of pre-heating plates, wherein the set of pre-heating plates are capable of sandwiching a bulk metallic glass feedstock therebetween to preheat the bulk metallic glass feedstock to a temperature sufficient to soften the bulk metallic glass feedstock but that does not significantly contribute to a consumed crystallization time of the bulk metallic glass;

b) a set of rotatable heated rollers maintained at a processing temperature of the bulk metallic glass, wherein as the set of heated rollers rotates, the heated rollers thermoplastically roll the bulk metallic glass feedstock received from the step of pre-heating plates therebetween to thin the bulk metallic glass feedstock into a bulk metallic glass sheet; and

c) a stretching mechanism capable of stretching the rolled bulk metallic glass sheet exiting the set of heated rollers under controlled velocity.

As discussed above, the stretching mechanism moves with respect to a negative temperature gradient, wherein as the bulk metallic glass is pulled or stretched from the set of heated rollers it cools to a temperature below the processing temperature of the BMG.

In one embodiment, the stretching mechanism comprises a velocity controlled pulling mechanism, wherein the velocity controlled pulling mechanism pulls the bulk metallic glass sheet as it exits from the set of heated rollers. This stretching mechanism preferably pulls the bulk metallic glass sheet at a faster rate than the bulk metallic glass proceeds through the set of heated rollers.

In another embodiment, the stretching mechanism comprises a set of set of rotatable cool rollers. In this embodiment, the set of rotatable cool rollers are maintained at a lower temperature than the set of heated rollers and receive the bulk metallic glass sheet therebetween as it exits from the set of heated rollers. The set of cool rollers also preferably rotate at a faster rate than the set of heated rollers.

As also discussed above, following the rolling and stretching steps, the system of the invention may also comprise a set of rotatable patterned rollers that impose a pattern onto the bulk metallic glass sheet after it exits the stretching mechanism. In the alternative, following the rolling and stretching steps, various molding processes may be performed to impose a pattern onto the bulk metallic glass sheet.

In one embodiment, the system of the invention may comprise multiple systems connected in series to continuously produce a thin bulk metallic glass sheet. For example, the system may comprise multiple stations of at least one additional set of heated rollers and at least one additional set of cool rollers, wherein the bulk metallic glass is further thinned. If necessary, a set of preheater plates may be positioned before the at least one additional set of heated rollers. However, the preheater plates may not be necessary between every station if the bulk metallic glass sheet remains at a sufficient temperature for rolling between the set of heated rollers. As discussed herein, the “cool” rollers are only cool relative to the processing temperature maintained by the set of heated rollers. If the bulk metallic glass sheet does not cool significantly between the next subsequent station, an additional preheater step may not be required. Thus the optimal configuration of the set of preheater plates, set of heated rollers and set of cool rollers within the system can be determined by one skilled in the art.

The set of heated rollers may optionally comprise a surface coating thereon that provides a smooth, non-stick surface. Other methods of preventing the bulk metallic glass from sticking to the heated rollers are discussed above. In addition, the set of heated rollers preferably comprise a hard metal that is sufficiently strong at the processing temperature of the bulk metallic glass.

The set of cool rollers may have a rough surface, so that the set of cool rollers grip the bulk metallic glass sheet as it exits the set of heated rollers.

FIGS. 12 and 13 depict drawings of the preheater plates and the set of heated rollers in accordance with the present invention. As set forth in FIGS. 12 and 13, preheater plates 2 and 4 are configured to receive the bulk metallic glass feedstock material. As discussed above, the preheater plates may sandwich the bulk metallic glass feedstock material using gravity alone or may optionally utilize an additional means of providing pressure to provide more intimate contact between the preheater plates and the bulk metallic glass feedstock material.

Servo motors 6 control the rotation of the set of heated rollers 8 and 10. A load and displacement sensor 12 may be used to provide feedback on the position of the rollers. A set of jack screws can be used to control the gap between the set of heated rollers 8 and 10 based on information received from the load and displacement sensor 12.

FIG. 13 depicts a drawing of an exploded view of the key components of the rollers 8 and 10. As seen in FIG. 13, cartridge heaters 16 are placed lengthwise along the rollers 8 and 10 along with thermocouples 18. The preheater plates 2 and 4 each also have cartridge heaters 20 and thermocouples going through them.

FIG. 14 depicts an example of the preheating plates 2 and 4 in accordance with the present invention. The measuring tape is in inches.

As described herein, the present invention allows for the fabrication of large, thin bulk metallic glass sheets having a uniform thickness. In addition, the present invention allows for the fabricating of bulk metallic glass strips having a uniform thickness. The present invention also allows for the further fabrication of complex patterns in the bulk metallic glass sheets. In addition, the present invention allows for the continuous fabrication of arbitrary complex shapes. Finally, the present invention also allows for the joining of bulk metallic glasses of the same type or of a different type by hot rolling. Other uses of the present invention to fabricate bulk metallic glass sheets having desired features, textures, thicknesses and configurations would also be known to those skilled in the art and are within the scope of the present invention.

It should also be understood that the following claims are intended to cover all of the generic and specific features of the invention described herein and all statements of the scope of the invention that as a matter of language might fall there between.

Claims

1. A method of fabricating a bulk metallic glass sheet, the method comprising the steps of:

a) preheating a bulk metallic glass feedstock to a temperature sufficient to soften the bulk metallic glass feedstock but that does not significantly contribute to a consumed crystallization time of the bulk metallic glass; and

b) thermoplastically rolling the pre-heated bulk metallic glass feedstock between a set of heated rollers maintained at a processing temperature of the bulk metallic glass; and

c) stretching the bulk metallic glass sheet after the bulk metallic glass sheet exits the set of heated rollers;

wherein the bulk metallic glass feedstock is reduced in thickness to produce a bulk metallic glass sheet; and

wherein processing conditions during steps a) to c) are such that a Reynolds number Re<10−3 of the bulk metallic glass is maintained.

2. (canceled)

3. The method according to claim 1, wherein the pre-heating temperature is between about 0.8 times the glass transition temperature, Tg, and 1.4 times the glass transition temperature, as measured in degrees Celsius, of the bulk metallic glass feedstock.

4. (canceled)

5. The method according to claim 1, wherein stretching occurs in a negative temperature gradient.

6. The method according to claim 1, wherein stretching occurs by a controlled velocity.

7. The method according to claim 1, wherein steps a) to c) are repeated to obtain a desired thickness of the bulk metallic glass sheet.

8. The method according to claim 1, wherein steps a) and b) are conducted in air.

9. (canceled)

10. (canceled)

11. (canceled)

12. The method according to claim 1, wherein the thermoplastic rolling step reduces perturbations in thickness that can grow in magnitude during the subsequent stretching step.

13. (canceled)

14. (canceled)

15. The method according to claim 1, wherein the bulk metallic glass feedstock comprises a plurality of bulk metallic glass pieces, wherein the plurality of bulk metallic glass pieces are joined in step b).

16. The method according to claim 15, wherein the joining locations are controlled, wherein only portions of the bulk metallic glass pieces are joined.

17. The method according to claim 1, wherein stretch rolling increases the bending ductility of the bulk metallic glass sheet.

18. The method according to claim 1, further comprising the step of imposing a pattern on the bulk metallic glass sheet after step c).

19. The method according to claim 18, wherein the pattern is imposed on the bulk metallic glass sheet by rolling the bulk metallic glass sheet through a set of patterned rollers, wherein the set of patterned rollers are maintained at a processing temperature of the bulk metallic glass sheet, wherein the rollers impose the pattern onto the bulk metallic glass sheet.

20. The method according to claim 18, wherein after step c) a molding step is performed to impose the pattern onto the bulk metallic glass sheet.

21. The method according to claim 18, wherein after step c) a molding step is performed to mold the bulk metallic glass sheet into a mold cavity.

22. The method according to claim 21, wherein a shearing step is performed to cut the bulk metallic glass sheet into outlines set by the mold cavity.

23. The method according to claim 21, wherein a deformation step is performed to corrugate the bulk metallic glass sheet into out of plane deformations set by the mold cavity.

24. The method according to claim 18, wherein the pattern has a length scale of less than 1 mm.

25. The method according to claim 24, wherein the pattern has a length scale of less than 0.5 mm.

26. The method according to claim 18, wherein the pattern comprises mold cavities.

27. (canceled)

28. (canceled)

29. (canceled)

30. (canceled)

31. (canceled)

32. A system for fabricating a bulk metallic glass sheet, the system comprising:

a) a set of pre-heating plates, wherein the set of pre-heating plates are capable of sandwiching a bulk metallic glass feedstock therebetween to preheat the bulk metallic glass feedstock to a temperature sufficient to soften the bulk metallic glass feedstock but that does not significantly contribute to a consumed crystallization time of the bulk metallic glass;

b) a set of rotatable heated rollers maintained at a processing temperature of the bulk metallic glass, wherein as the set of heated rollers rotates, the heated rollers thermoplastically roll the bulk metallic glass feedstock received from the step of pre-heating plates therebetween to thin the bulk metallic glass feedstock into a bulk metallic glass sheet, wherein the set of heated rollers comprise a hard metal that is sufficiently strong at the processing temperature of the bulk metallic glass; and

c) a stretching mechanism capable of stretching the rolled bulk metallic glass sheet exiting the set of heated rollers under controlled velocity.

33. The system according to claim 32, wherein the stretching mechanism moves along a negative temperature gradient.

34. The system according to claim 32, wherein the stretching mechanism is controlled by velocity as it exits from the set of heated rollers.

35. The system according to claim 34, wherein the stretching mechanism pulls the bulk metallic glass sheet at a faster rate than the bulk metallic glass proceeds through the set of heated rollers.

36. The system according to claim 34, wherein the stretching mechanism comprises a set of rotatable cool rollers, wherein said set of rotatable cool rollers are maintained at a lower temperature than the set of heated rollers and receive the bulk metallic glass sheet therebetween as it exits from the set of heated rollers.

37. The system according to claim 34, where in the stretching mechanism comprises a clamping device that receives the bulk metallic glass sheet therebetween as it exits from the set of heated rollers.

38. The system according to claim 36, wherein the set of cool rollers rotate at a faster rate than the set of heated rollers.

39. The system according to claim 32, further comprising a set of rotatable patterned rollers, wherein the patterned rollers impose a pattern onto the bulk metallic glass sheet after it exits the stretching mechanism.

40. The system according to claim 39, wherein the set of patterned rollers are maintained at a processing temperature of the bulk metallic glass.

41. (canceled)

42. (canceled)

43. (canceled)

44. The system according to claim 32 wherein the steps are performed in air.

45. (canceled)

46. (canceled)