Electrochemical grain refining of a metal

Abstract

A method for surface and subsurface grain refining of a bulk hydrogen-absorbing metal includes the steps of cathodically charging the bulk hydrogen-absorbing metal with an electric current in the presence of a source of hydrogen to hydride the hydrogen-absorbing metal, and, changing polarity of the electric current to dehydride the hydrogen-absorbing metal. The method results in improvement to hardness and/or wear resistance of the metal, particularly titanium alloys such as Ti-6Al-4V. Metals treated with this method are particularly useful for medical implants and vehicle parts in which improved hardness and/or wear resistance is required.

Description

FIELD OF THE INVENTION

The present invention is directed to electrochemical grain refining of a metal, particularly a titanium alloy.

BACKGROUND OF THE INVENTION

Medical (e.g. orthopedic) implants are used to replace or augment existing biological structures, for example bones, in the bodies of humans and other animals. Materials used to construct such implants are in direct contact with the body through an interface between the material's surface and the body's bones, tissues and/or extracellular fluids. Therefore, the tribiological (friction, wear, durability) properties of the implant are important considerations, and the surface nano/micro structure of the material used to construct the implant plays a very important role in determining the tribiological properties of the implant. For example, wear of the implant's surface can lead to loosening of the implant and to releasing of debris into the body, therefore increasing wear resistance and/or surface hardness of implant materials is important.

There has been extensive work on surface modification of materials to understand and enhance surface performance of implants. One approach is to modify the surface topography by creating a rough, or porous surface on the implant to increase the surface area available for bone/implant apposition. A natural consequence of these surface modifications, however, is an increase in metal ion release due to increased surface contact with corrosive media in the body. A further complication is an increase in wear debris due to increased surface friction, which will also result in increased metal ion release rates and loosening of the implant.

Another approach is to provide the implant's surface with a bioactive coating, for example bioactive glass. Bioactive glass is intended to allow stable mechanical fixation of the implant to bone. However, bioactive glass has a non-compatible thermal expansion coefficient relative to the metal, for example Ti-based alloys, used in the implant. Non-compatible thermal expansion coefficients leads to cracking of the glass due to thermal stresses that occur during the heating/cooling cycle used to apply glass films to the metal. Additionally, bioactive glass rapidly dissolves in body fluids when implanted.

Other surface coatings have been tried with the aim of improving the interface bond between the implant and bone. These include hydroxyapatite (HA) coatings produced by plasma spray or ion implantation. While these methods were effective in improving short-term bone-implant bonding without any fibrous tissue formation, long-term performance of such coatings is severely lacking due to adhesion problems of the coating on the metal of the implant and due to poor control of dissolution rate of the coatings in bodily fluids.

Yet another approach to surface modification of implants is to coat the implant's surface with hard materials focussing on increasing wear resistance. For example, titanium nitride may be coated on to an implant surface by chemical vapor deposition (CVD) and physical vapor deposition (PVD). Although these methods provide the implant with excellent wear resistance, the deposited layers suffer from lack of adherence as the interaction between the coating and the substrate is not a requisite for coating growth in either CVD or PVD.

Low energy nitrogen ion bombardment plasma nitriding is one of the most recent methods for improving wear and corrosion behavior of metallic alloys. In plasma nitriding, a Ti-based substrate is directly involved in the formation of the coating, which results in excellent adhesion of the coating to the substrate. However, the inherently high cost of plasma nitriding reduces cost-effectiveness of this method.

Hydrogenation/dehydrogenation techniques are very effective in forming mesoscopic crystalline (˜1 μm), submicrocrystalline (SMC, <1 am) and even nanocrystalline (NC, <0.1 μm) structures. Techniques such as thermal hydrogen processing (THP) and a combination of electrochemical hydrogenation and thermal dehydrogenation have been reported. In these approaches, hydrogen is added to a metal alloy by controlled diffusion from a hydrogen environment, which can be either gaseous or electrolytic hydrogen environments. After processing, hydrogen is then removed by a controlled vacuum anneal (a thermal process).

THP processes have been shown to improve yield strength, elongation, hardness, hot-workability and ductility of the metal. It is thought that these improvements are due to modification of grain microstructure of the metal. A typical THP process includes subjecting a titanium alloy to a hydrogenation treatment, an elevated temperature β solution treatment, a moderate temperature eutectoid decomposition and finally an elevated temperature vacuum dehydrogenation treatment. It is believed that after hydrogenation, γ titanium hydride precipitates in a phase of the hydrogenated specimen. During the eutectoid decomposition treatment a mixture consisting of α+γ hydride nucleates from β_H matrix. During the dehydrogenation treatment, the γ titanium hydrides gradually lose their hydrogen content resulting in recrystallization of fine α/β phases and refined microstructure.

A combination of electrochemical hydrogenation and thermal dehydrogenation has been reported by Wu et al. in U.S. Pat. No. 5,178,694 issued Jan. 12, 1993 and in Metall. Trans. A, 24A, 1181-1185 (1993). An electrochemical technique was utilized to dissolve hydrogen into a titanium alloy instead of a thermal technique. A thermal technique was used to dehydride the alloy. The results showed that a fine and equiaxed grain was formed. The processed specimens showed an improvement in surface hardness, which was attributed to associated recrystallization as in THP processes.

Both THP and Electrochemical/THP processes use hydrogen as a temporary alloying element to refine surface and subsurface grains at the metal surface. However, both THP and Electrochemical/THP processes require high temperatures (above 800° C.). Additionally, wear resistance and surface hardness of the treated material could be improved.

While there has been considerable advance in the surface modification of metals, the prior art methods all suffer from one or more disadvantages including high capital investment, high energy consumption, furnace chamber size limitations, complicated operations and controls, and coating separation from substrates. There remains a need in the art for a method of surface modification that ameliorates one or a combination of the disadvantages while providing a material with excellent tribiological properties, for example wear resistance and surface hardness.

SUMMARY OF THE INVENTION

There is provided a method for surface and subsurface grain refining of a bulk hydrogen-absorbing metal comprising: cathodically charging the bulk hydrogen-absorbing metal with an electric current in the presence of a source of hydrogen to thereby hydride the hydrogen-absorbing metal; and, changing polarity of the electric current to thereby dehydride the hydrogen-absorbing metal.

There is also provided a method for surface and subsurface grain refining of a bulk titanium alloy comprising: cathodically charging the bulk titanium alloy with an electric current in an aqueous inorganic acid or an aqueous inorganic base to thereby hydride the titanium alloy; and, changing polarity of the electric current to thereby dehydride the titanium alloy.

Metals useful in the present invention have hydrogen-absorbing capacity, although high sensitivity to hydrogen is not required. Preferably, the hydrogen-absorbing capacity is in a range of from about 50 mAh/g to about 999 mAh/g, more preferably from about 150 mAh/g to about 999 mAh/g. The metal is provided in bulk form, preferably in the form that will be used commercially. Preferably the metal is biologically compatible, although for non-biological purposes there is no requirement that the metal be biologically compatible.

Some examples of hydrogen-absorbing metals are titanium-based metals, lanthanum-based metals, magnesium-based metals, cerium-based metals, zirconium-based metals and cadmium-based metals. Cadmium-based metals are not particularly preferred as cadmium is toxic and is therefore of limited value in biological applications. Titanium alloys and lanthanum alloys (e.g. Misch metals) are particularly preferred. Titanium alloys are more preferred. Titanium alloys comprise titanium (Ti) and significant quantities of one or more alloying elements, for example aluminum (Al) and vanadium (V). Titanium alloys may contain small amounts of impurities or incidental elements, for example, carbon (C), nitrogen (N), iron (Fe), yttrium (Y), oxygen (O) and/or hydrogen (H), although such impurities or incidental elements can sometimes affect the overall properties of the alloy. A particularly preferred titanium alloy is Ti-6Al-4V.

Electrochemical processes are typically conducted in an electrochemical cell having a working electrode and a counter electrode, and possibly a reference electrode, suspended in an electrolyte. The general construction of electrochemical cells is well known to one skilled in the art. The working electrode is made of the hydrogen-absorbing metal to be treated. The counter electrode may comprise any suitably conductive material, for example platinum (Pt) or graphite. If a reference electrode is required or desired, standard electrodes, for example saturated calomel and Hg/HgO electrodes, may be used.

The electrolyte may comprise any suitable ionic species in liquid or gaseous form. Preferably, the electrodes are suspended in aqueous solutions of ionic species. Suitable ionic species may be generated from salts (e.g. alkali metal halides such as sodium chloride or potassium chloride), acids and/or bases. Preferably, the ionic species are generated from acid or bases.

The hydrogen-absorbing metal is cathodically charged in the presence of a source of hydrogen to hydride the metal. The source of hydrogen may be any hydrogen-containing species that will provide hydrogen atoms to be absorbed by the metal under cathodic charging conditions. The source of hydrogen is preferably, an aqueous acid or base since aqueous acids and based provide for both the hydrogen source and the electrolyte. Aqueous acids are more preferred.

Of the acids, inorganic acids are preferred. Inorganic acids include, for example, hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, perchloric acid, among others. Aqueous solutions of the acid should contain a high enough concentration of hydrogen ion (hydronium ion) to permit hydrogenation of the metal during cathodic charging. Preferably, the concentration of hydrogen ion in solution is in a range from about 0.1 to 10 M, more preferably from about 2 to 6 M.

Of the bases, inorganic bases are preferred. Inorganic bases include, for example, alkali metal hydroxides (e.g. sodium hydroxide, potassium hydroxide) and alkaline earth hydroxides (e.g. magnesium hydroxide and calcium hydroxide). Aqueous solutions of the base should contain a high enough concentration of hydroxide ion to permit hydrogenation of the metal during cathodic charging. Preferably, the concentration of hydroxide ion in solution is in a range from about 0.05 to 6 M, more preferably from about 1 to 6 M.

To dehydride the metal, the polarity of the electrochemical cell may be changed. Typically, direct current (DC) has been used in the art to perform hydrogenation. If direct current (DC) is used to hydride the metal, a cycle or time course may be set up whereby the DC cathodically charges the hydrogen-absorbing metal for a first period of time, and then the polarity of the DC is reversed to anodically charge the metal for a second period of time. Two or more charging cycles may be conducted whereby the polarity of the DC is periodically reversed to alternate between cathodic and anodic charging of the metal. A simpler way of providing alternation between cathodic and anodic charging (i.e. alternation between hydrogenation and dehydrogenation) is to use alternating current (AC), whereby the polarity is automatically changed on a regular periodic basis. Charging programs for hydriding and dehydriding the metal depend on the type of current used. The process is generally faster using AC as the process is kinetically sluggish using DC. The use of AC is preferred over DC.

For DC, hydrogenation is preferably performed at a current density in a range of from about 0.01 to about 20 mA/cm² for a period of time in a range of from about 1 to about 200 hours. Preferably, the current density remains constant during hydrogenation. Dehydrogenation is preferably performed at a current density in a range of from about 0.01 to about 1 mA/cm² for a period of time in a range of from about 1 to about 400 hours. Preferably, the current density remains constant during dehydrogenation. Preferably, hydrogenation is performed continuously until maximum hydrogen absorption is achieved, and then dehydrogenation is performed continuously until complete or near complete dehydrogenation is achieved. One cycle comprises one period of hydrogenation and one period of dehydrogenation. The process may comprise more than one cycle and the number of cycles depends on the desired depth of grain refinement.

For AC, hydrogenation and dehydrogenation recur periodically as the polarity changes on a regular basis. Therefore, in using AC, multiple hydrogenation/dehydrogenation cycles occur automatically. The total charging time when using AC is preferably in a range of from about 1 hour to about 50 hours, more preferably from about 20 hours to about 50 hours. During the total charging time, hydrogenation is preferably performed at a pulse current density (I₁) in a range of from about 0.01 to about 100 mA/cm² for a time period (t₁) of from about 2 to about 120 seconds. Dehydrogenation is preferably performed at a pulse current density (I₂) in a range of from about 0.01 to about 100 mA/cm² for a time period (t₂) of from about 2 to about 120 seconds. The current densities change with change in potential, therefore, there is no fixed value for the current density during hydriding and dehydriding. Current density changes in relation to the amount of hydrogen absorbed by the metal. As the metal absorbs hydrogen, the current density drops, and as the metal loses absorbed hydrogen the current density increases. Finally, the total time and the time periods for hydrogenation and dehydrogenation depend in part on the magnitude of the current density.

Pulse potentials in AC depend on the electrolyte. In acid solutions, the pulse potential for hydrogenation (E₁) is preferably about −1.3 to −0.5 V. The pulse potential for dehydrogenation (E₂) is preferably about −0.5 to −0.1 V. In base solutions, the pulse potential for hydrogenation (E₁) is preferably about −1.9 to −1.4 V. The pulse potential for dehydrogenation (E₂) is preferably about −1.1 to −0.5 V.

The present method may be conducted at any suitable temperature. For aqueous electrolyte systems, the temperature is preferably in a range of from about 0° C. to about 100° C. For simplicity, the method is preferably conducted at ambient temperature. Lower temperatures slow the rate of hydrogenation and dehydrogenation, whereas higher temperatures drive dissolved hydrogen out of solution more rapidly. Optimal temperature may be readily determined by one skilled in the art by simple experimentation in light of the particular metal being treated.

Once method parameters have been determined for a particular treatment application, charging of the metal may occur for the total time without having to change any of the parameters. Thus, the electrochemical method of the present invention is very simple to operate and control. The total charging time typically depends on how thick of a treated layer is desired, which depends on the particular commercial application. Longer charging time results in a thicker hardened layer, i.e. a larger depth of refinement.

Other processing of the metal may be performed before and/or after the electrochemical grain refining process of the present invention. For example, the metal may be mill annealed, shaped, polished, degreased, cleaned, etched, etc. In situations where it is undesirable to grain refine all surfaces of the metal, certain areas may be sealed to prevent treatment at those areas.

The present electrochemical method of grain refinement does not require thermal treatment of the metal during either hydrogenation or dehydrogenation, since hydrogenation and dehydrogenation are both conducted electrochemically. Surprisingly, such an electrochemical method leads to unexpectedly large improvements in surface hardness and wear resistance of the metal. Furthermore, such an electrochemical process is useful for metals having low hydrogen sensitivity and for a wider variety of hydrogen-absorbing metals than processes with a thermal component. Other advantages of the present method in comparison to methods involving thermal processes include: lower capital investment; lower energy consumption; excellent scalability with lower scale-up cost; and, simpler operation and controls. In addition, since the present method does not require surface coating, there is no problem of a surface layer coating separating from a bulk material.

Another surprising benefit of the present invention is that the dehydriding step is sufficiently kinetically facile that there is no need for the metal to contain any components to lower the kinetic barrier to dehydriding. It is known in the art of rechargeable metal hydride batteries that dehydriding hydrogen-absorbing metals, especially titanium alloys, is often kinetically sluggish. For this reason, additives (e.g. nickel in the form of TiNi or Ti₂Ni) are often added to titanium alloys to lower the kinetic barrier to dehydrogenation in order for dehydriding to occur sufficiently quickly. In the present process, such additives are not required, although they could be present if desired. Therefore, the metal may be nickel-free.

A further surprising benefit of the present invention is that even with metals that are not highly hydrogen sensitive, it is possible to conduct the dehydriding step at a low rate, referred to as “trickling”, towards the end of the dehydriding to ensure complete dehydrogenation.

Metals treated by the present electrochemical method are useful in any commercial application in which surface hardness and/or wear resistance is desirable. For example, medical implants (e.g. pace makers, orthopedic implants, etc.) and vehicle parts (e.g. automotive, aircraft, aerospace, etc.) are two preferred commercial applications. Surface and subsurface treatment of medical implants, particularly orthopedic implants, is a particularly preferred use of the present method.

Further features of the invention will be described or will become apparent in the course of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:

FIG. 1A is a schematic representation of an electrochemical processing program for potential pulse;

FIG. 1B is a schematic representation of an electrochemical processing program for direct current;

FIG. 2 is an optical micrograph showing optical morphology of a cross-section of untreated Ti-6Al-4V;

FIG. 3 is an optical micrograph showing optical morphology of a cross-section of Ti-6Al-4V treated with a process of the present invention;

FIG. 4A is a photograph of indentations on the surface of an untreated Ti-6Al-4V specimen after hardness measurements using a 10 g load;

FIG. 4B is a photograph of indentations on the surface of an untreated Ti-6Al-4V specimen after hardness measurements using a 50 g load;

FIG. 5A is a photograph of indentations on the surface of a Ti-6Al-4V specimen treated with a process of the present invention after hardness measurements using a 10 g load; and,

FIG. 5B is a photograph of indentations on the surface of a Ti-6Al-4V specimen treated with a process of the present invention after hardness measurements using a 50 g load.

FIG. 6 is a graph comparing surface hardness (HV) of untreated and treated Ti-6Al-4V specimens under 10 g, 50 g, 100 g, 200 g, 300 g and 400 g loads.

DESCRIPTION OF PREFERRED EMBODIMENTS

In a process of the present invention, either DC or AC (pulse current) may be used for the hydrogenation and dehydrogenation steps in order to refine grain sizes of a metal specimen. FIG. 1A graphically represents an electrochemical processing program for potential pulse (AC). FIG. 1B graphically represents an electrochemical processing program for a constant current (DC) program.

Electrochemical cells used to conduct the inventive processes in the following Examples comprised counter electrodes and reference electrodes. The counter electrodes were platinum (Pt), and the reference electrodes were saturated calomel electrode for acidic solutions and Hg/HgO electrode for basic solutions.

In all Examples of the invention below, the hydrogen-absorbing metal is a titanium alloy, namely, Ti-6Al-4V. In all of the Examples, Ti-6Al-4V ELI sheets with a thickness if 1.83 mm manufactured by RMI Titanium company were mill annealed at 787° C. (1450° F.) for 15 minutes and then air cooled. Specimens were then cut the desired shape and size for each Example. The elemental composition of Ti-6Al-4V is provided in Table 1.

TABLE 1
Composition of Ti—6Al—4V (wt %)
C	N	Fe	Al	V	Y	O	Ti	H
0.02	0.008	0.213	6.16	3.92	<50 ppm	0.12	Balance	32 ppm

EXAMPLES

Example 1

Optical Microscopy

Two Ti-6Al-4V specimens were cut to a size of 10 mm×10 mm×1.83 mm and sealed with epoxy resin on one side of each specimen. The specimens were abraded with emery papers of up to 600 grits, polished with 0.05 μm alumina, degreased with acetone and then ultrasonically cleaned with deionized water. One of the two specimens was electrochemically treated by a process of the present invention in an electrochemical cell and the other was left untreated. The treated specimen was exposed to a 6 M KOH solution at 22° C. and treated with a potential pulse for 5 hours. The pulse parameters were E₁=−1.93 V, E₂=−1.13 V, t₁=10 s, t₂=10 s. The 6 M KOH solution was prepared with analytically pure KOH and deionized water.

The untreated and treated specimens were etched with Kroll's reagent and observed with an optical microscope. FIG. 2 is an optical micrograph showing optical morphology of a cross-section of the untreated specimen. In FIG. 2, the light area is the metal and the dark area below the light area is not part of the metal. The surface of the metal is at the interface between the light and dark areas. FIG. 3 is an optical micrograph showing optical morphology of a cross-section of the treated specimen. The gray area labeled as L in FIG. 3 is a layer in which grain refinement has occurred due to the electrochemical treatment of the metal. It is evident from the micrographs that extensive grain refinement has occurred in the treated specimen in comparison to the untreated specimen. It can be seen from the scale in the upper left corner of FIG. 3, that the refined layer is about 10 μm thick, i.e. the depth of the hardened layer is about 10 μm. The clear demonstration of grain refining effect of the present method indicates that further grain refining of the grain structure of hydrogen-absorbing metals to nano grain or even amorphous structure is achievable using the present method.

Example 2

Hardness

Ti-6Al-4V specimens were cut to a size of 10 mm×10 mm×1.83 mm and sealed with epoxy resin on one side of each specimen. The specimens were abraded with emery papers of up to 600 grits, polished with 0.05 μm alumina, degreased with acetone and then ultrasonically cleaned with deionized water. Some of the specimens were electrochemically treated by a process of the present invention in an electrochemical cell and the others were left untreated. The treated specimens were exposed to a 1 M H₂SO₄ solution at 80° C. and treated with a potential pulse. The 1 M H₂SO₄ solution was prepared with analytically pure H₂SO₄ and deionized water. The pulse parameters for the treated specimens were based on a trickling design and were E₁=−0.9 V, E₂=−0.5 V, t₁=10 s, t₂=10 s for 1 hour, and then E₁=−0.5 V, E₂=−0.1 V, t₁=10 s, t₂=10 s for 2 hours.

To measure hardness of the specimens, a Vickers microhardness tester was used with 10 g, 50 g, 100 g, 200 g and 300 g loads for 20 seconds. Due to roughness of the electrochemically treated specimens, the treated specimens were slightly polished with 0.05 μm alumina before the hardness was measured. FIGS. 4A and 4B show photographs of indentations on the surface of the untreated specimens after hardness measurements using a 10 g load (FIG. 4A), and a 50 g load (FIG. 4B). FIGS. 5A and 5B shows photographs of indentations on the surface of treated specimens after hardness measurements using a 10 g load (FIG. 5A), and a 50 g load (FIG. 5B). It is evident from the photographs that treated specimens are harder than untreated specimens as the indentations are not as deep on the treated specimens.

Table 2 lists surface hardness values (HV) of the untreated specimens (Untreated 1-5), the electrochemically treated specimens (Treated 1-5), untreated comparative specimens (Untreated A-B) and treated comparative specimens (Comp A-B). The comparative specimens are Ti-6Al-4V specimens treated with electrochemical/thermal methods as described in U.S. Pat. No. 5,178,694 (Comp A) and in Metall. Trans. A, 24A, 1181-1185 (1993) (Comp B). The hardness values for Untreated A and Comp A at 400 g are taken from U.S. Pat. No. 5,178,694. The hardness values for Untreated B and Comp B at 100 g are taken from the Metall. Trans. A paper, with Comp B being the average of the sixteen hardness values listed in Table III of the Metall. Trans. A paper. Table 2 also provides the numerical difference (A) between the treated and untreated specimens.

TABLE 2
Surface Hardness (HV)
	10 g	50 g	100 g	200 g	300 g	400 g
Specimen	load	load	load	load	load	load
Untreated 1	315	—	—	—	—	—
Treated 1	358	—	—	—	—	—
Δ1	43	—	—	—	—	—
Untreated 2	—	292	—	—	—	—
Treated 2	—	363	—	—	—	—
Δ2	—	71	—	—	—	—
Untreated 3	—	—	295	—	—	—
Treated 3	—	—	359	—	—	—
Δ3	—	—	64	—	—	—
Untreated 4	—	—	—	307	—	—
Treated 4	—	—	—	379	—	—
Δ4	—	—	—	72	—	—
Untreated 5	—	—	—	—	298	—
Treated 5	—	—	—	—	365	—
Δ5	—	—	—	—	67	—
Untreated A	—	—	—	—	—	325
Comp A	—	—	—	—	—	340
ΔA	—	—	—	—	—	15
Untreated B	—	—	325	—	—	—
Comp B	—	—	349	—	—	—
ΔB	—	—	24	—	—	—

The data in Table 2 is represented graphically in FIG. 6. It is evident from Table 2 and FIG. 6 that the increase in hardness (A) is greater in Ti-6Al-4V specimens treated by the electrochemical process of the present invention compared to Ti-6Al-4V specimens treated by processes of the prior art. In particular, the results at a load of 100 g show that the increase in hardness value (A) of a treated specimen over an untreated specimen for a specimen treated by a process of the present invention (Treated 3) is over 2.5 times greater than the increase in hardness value (A) of a specimen treated with an electrochemical hydrogenation step and a thermal dehydrogenation step (Comp B). The results above demonstrate that the present electrochemical hydrogenation/dehydrogenation method improves the surface hardness of Ti-6Al-4V. The results also demonstrate that the present electrochemical method can result in a greater improvement in surface hardness in comparison to a method that involves thermal dehydrogenation.

Example 3

Wear Resistance

Three Ti-6Al-4V specimens were cut to a size of 20 mm×20 mm×1.83 mm and sealed with epoxy resin on one side of each specimen. The specimens were abraded with emery papers of up to 600 grits, polished with 0.05 μm alumina, degreased with acetone and then ultrasonically cleaned with deionized water. Two of the three specimens were electrochemically treated by a process of the present invention in an electrochemical cell and the other was left untreated. The treated specimens were exposed to a 1 M H₂SO₄ solution at 80° C. and treated with a potential pulse. The 1 M H₂SO₄ solution was prepared with analytically pure H₂SO₄ and deionized water. The pulse parameters for both of the treated specimens were based on a trickling design and were E₁=−0.9 V, E₂=−0.5 V, t₁=10 s, t₂=10 s for 0.5 hour, and then E₁=−0.5 V, E₂=−0.1 V, t₁=10 s, t₂=10 s for 2.5 hours.

Wear resistance of the untreated and treated specimens was tested using a pin-on-disk apparatus based on ASTM G99-95a: Standard Test Method for Wear Testing with a Pin-on-Disk Apparatus. The untreated and treated specimens were used as the disk. The material of the pin was WC-6% Co and the diameter of the pin ball was ⅛ inch. The load applied to the disk was 100 g. The rotation speed of the disk was 60 rpm an the total rotation number was 2200. The track radius was 5 mm. The wear tracks on the specimens after pin-on-disk tests was analyzed using profilometry, and the wear loss was calculated based on the following formula assuming that there was no significant pin wear. $\mathrm{Disk}\mathrm{volume}\mathrm{loss},{\mathrm{mm}}^3=\frac{\pi \left(\mathrm{wear}\mathrm{track}\mathrm{radius},\mathrm{mm}\right){\left(\mathrm{track}\mathrm{width},\mathrm{mm}\right)}^3}{6\left(\mathrm{sphere}\mathrm{radius},\mathrm{mm}\right)}$

Table 3 lists wear loss data of the untreated and treated specimens.

TABLE 3
Wear Loss
Specimen  Wear Loss (mm3)
Untreated 0.089
Treated 1 0.071
Treated 2 0.068

Wear resistance increase for treated specimen 1 is (0.089−0.071)*100/0.089=20%. Wear resistance increase for treated specimen 2 is (0.089−0.068)*100/0.089=24%. The average wear resistance increase was about 22%. Thus, the results have demonstrated that the electrochemical method of the present invention can result in an initial increase in wear resistance of over 22% of Ti-6Al-4V in comparison to untreated specimens.

Other advantages which are inherent to the invention are obvious to one skilled in the art. The embodiments are described herein illustratively and are not meant to limit the scope of the invention as claimed. Variations of the foregoing embodiments will be evident to a person of ordinary skill and are intended by the inventor to be encompassed by the following claims.

Claims

1. A method for surface and subsurface grain refining of a bulk hydrogen-absorbing metal comprising:

(a) cathodically charging the bulk hydrogen-absorbing metal with an electric current in the presence of a source of hydrogen to thereby hydride the hydrogen-absorbing metal; and,

(b) changing polarity of the electric current to thereby dehydride the hydrogen-absorbing metal.

2. The method of claim 1, wherein the electric current is AC.

3. The method of claim 1, wherein the electric current is DC.

4. The method of claim 1, wherein the hydrogen-absorbing metal is nickel-free.

5. The method of claim 1, wherein the hydrogen-absorbing metal comprises a titanium alloy.

6. The method of claim 5, wherein the titanium alloy is Ti-6Al-4V.

7. The method of claim 1, wherein the source of hydrogen is an aqueous acid or base.

8. The method of claim 1, wherein the source of hydrogen is an aqueous inorganic acid or an aqueous inorganic base.

9. The method of claim 8, wherein the aqueous inorganic acid has a concentration in a range of from 0.1 M to 10 M, and the aqueous inorganic base has a concentration in a range of from 0.05 M to 6 M.

10. The method of claim 1, wherein the source of hydrogen is aqueous sulfuric acid or aqueous potassium hydroxide.

11. The method of claim 2, wherein

the AC during hydriding has a current density (I1) in a range of from 0.01 to 100 mA/cm2 and a pulse period (t1) in a range of from 2 to 120 seconds,

the AC during dehydriding has a current density (I2) in a range of from 0.01 to 100 mA/cm2 and a pulse period (t2) in a range of from about 2 to about 120 seconds, and

total time for hydriding/dehydriding is in a range of from 1 hour to 50 hours,

temperature is in a range of from 0° C. to 100° C.,

and wherein

(a) in an acidic environment, the AC has a pulse potential for hydriding (E1) of from −1.3 to −0.5 V, a pulse potential for dehydriding (E2) of from about −0.5 to −0.1 V, or,

(b) in a basic environment, the AC has a pulse potential for hydriding (E1) of from −1.9 to −1.4 V a pulse potential for dehydriding (E2) of from −1.1 to −0.5 V.

12. The method of claim 3, wherein the metal is hydrided at a current density in a range of from 0.01 to 20 mA/cm2 for a period of time of from 1 to 200 hours per cycle at a temperature in a range of from 0° C. to 100° C., and the metal is dehydrided at a current density in a range of from 0.01 to 1 mA/cm2 for a period of time in a range of from 1 to 400 hours per cycle at a temperature in a range of from 0° C. to 100° C.

13. The method of claim 1, wherein dehydriding is conducted initially at a first rate and then subsequently at a second rate, the second rate being lower than the first rate.

14. A method for surface and subsurface grain refining of a bulk titanium alloy comprising:

(a) cathodically charging the bulk titanium alloy with an electric current in an aqueous inorganic acid or an aqueous inorganic base to thereby hydride the titanium alloy; and,

(b) changing polarity of the electric current to thereby dehydride the titanium alloy.

15. The method of claim 14, wherein the titanium alloy is Ti-6Al-4V.

16. The method of claim 14, wherein the electric current is AC.

17. The method of claim 16, wherein the aqueous inorganic acid comprises sulfuric acid having a concentration in a range of from 0.1 M to 10 M, and the aqueous inorganic base comprises potassium or sodium hydroxide having a concentration in a range of from 0.05 M to 6 M.

18. The method of claim 17, wherein

the AC during hydriding has a current density (I1) in a range of from 0.01 to 100 mA/cm2 and a pulse period (t1) in a range of from 2 to 120 seconds,

the AC during dehydriding has a current density (I2) in a range of from 0.01 to 100 mA/cm2 and a pulse period (t2) in a range of from about 2 to about 120 seconds, and

total time for hydriding/dehydriding is in a range of from 1 hour to 50 hours,

temperature is in a range of from 0° C. to 100° C.,

and wherein

(a) in the aqueous inorganic acid, the AC has a pulse potential for hydriding (E1) of from −1.3 to −0.5 V, a pulse potential for dehydriding (E2) of from about −0.5 to −0.1 V; or,

(b) in the aqueous inorganic base, the AC has a pulse potential for hydriding (E1) of from −1.9 to −1.4 V a pulse potential for dehydriding (E2) of from −1.1 to −0.5 V.

19. The method of claim 18, wherein dehydriding is conducted initially at a first rate and then subsequently at a second rate, the second rate being lower than the first rate.