Magnesium (MG) Alloy and Method of Producing Same

Abstract

Embodiments of a magnesium (Mg) alloy and method for producing the same are disclosed. One such embodiment, among others, is a method for producing a magnesium (Mg) alloy, comprising the steps of: (a) producing a Mg powder aggregate by mixing Mg powder and at least one strengthening agent, the strengthening agent selected from: a carbon, a metal, and a combination thereof; (b) agglomerating the aggregate; and (c) sintering the agglomerated aggregate to produce the Mg alloy. Preferably, although not necessarily, steps (a) and (b) are performed using a ball mill. Moreover, the strengthening agent may be, for example but not limited to, carbon nanotubes, copper, tin, titanium, or silicon carbide. The resulting Mg alloy comprises nano-scale crystalline and/or micro-scale crystalline lattice structures and a yield strength that is at least as high as steel, exhibiting a yield strength that is about 320 MPa to 500 MPa.

Description

CLAIM OF PRIORITY

This application claims priority to and the benefit of provisional application No. 61/668,132, filed on Jul. 5, 2012, which is incorporated herein by reference in its entirety.

BACKGROUND

Traditional cast and wrought magnesium (Mg) alloys have been very limited in their usage in connection with automotive structural applications and other structural applications due to poor performance at cold or even warm forming conditions, caused by low ductility and low fracture toughness. Although Mg is much lighter than steel (Fe3C) and aluminum (Al) as one of the prominent candidates for lightweight material design, the strength level of Mg is still lower than steel and aluminum and therefore undesirable for use in connection with automotive and other structural applications.

SUMMARY OF THE INVENTION

Embodiments of a new magnesium (Mg) alloy and method for producing the same are disclosed herein.

One such embodiment, among others, comprises a method for producing a magnesium (Mg) alloy, comprising the steps of: (a) producing a Mg powder aggregate by mixing Mg powder and at least one strengthening agent, the strengthening agent selected from the group consisting of: carbon, metal, and a combination thereof; (b) agglomerating the Mg powder aggregate; and (c) sintering the agglomerated aggregate to produce the Mg alloy. Preferably, although not necessarily, steps (a) and (b) are performed using a ball mill. Moreover, the strengthening agent may be, for example but not limited to, carbon nanotubes, copper, tin, titanium, or silicon carbide. The resulting Mg alloy comprises nano-scale crystalline lattice structures, micro-scale crystalline lattice structures, or both, and exhibits a yield strength that is at least as high as steel. The strength of the resulting Mg alloy can be altered and designed with the amount of the strengthening agent mixed. The yield strength is about 320 MPa to 500 MPa.

Another embodiment, among others, comprises a method for producing a Mg alloy, comprising the steps of: (a) mixing and agglomerating in a ball mill a Mg powder aggregate having a Mg powder and at least one strengthening agent, the strengthening agent selected from the group consisting of: carbon, metal, and a combination thereof; and (b) sintering the agglomerated aggregate to produce the Mg alloy. Moreover, the strengthening agent may be, for example but not limited to, carbon nanotubes, copper, or titanium. The resulting Mg alloy comprises nano-scale crystalline lattice structures, micro-scale crystalline lattice structures, or both, and exhibits a yield strength that is at least as high as steel.

Other embodiments, methods, apparatus, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following detailed description. It is intended that all such additional embodiments, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims.

DETAILED DESCRIPTION

It is desirable to improve both ductility and strength levels of Mg alloys by innovative manufacturing methods in which the grain size is significantly reduced, the grain boundary bounding resistance (GB energy) improved, and the dislocation/twinning behavior favorable to deformation. The present disclosure includes an innovative manufacturing method to produce highly ductile and high strength Mg alloys.

The method involves synthesizing Mg metal in its powder form, mixed with one or more strengthening agents by high-energy deformation processes. The strengthening agents can be (a) carbon-based, such as but not limited to, carbon (C) nanotubes or (b) metallic-based, such as but not limited to, copper (Cu) or titanium (Ti). In some embodiments, the metals have a hexagonal close-packed crystal structure. Elemental powders of a purity of about 99.9% or better and a particle size of about −120 mesh are used for producing samples.

Next, the aggregate of the Mg powder and the strengthening agent are agglomerated using any suitable apparatus. In the preferred embodiment, the Mg powder and the strengthening agent are mixed in and agglomerated in a conventional ball mill. Preferably, although not necessarily, the added agents are ductile.

The ball milling is conducted in a standard lab-scale shaker mill with hardened steel balls and vial. To avoid oxidation, the powders are sealed in a container under a noble-gas environment (i.e., nitrogen (N₂) or other inert gases), opened after a period of up to about 24 hours of ball milling, and subsequently put in a case for subsequent cold pressing and elevated temperature sintering. The vial temperature during ball milling process is kept constant by gas cooling. All operations concerning the Mg samples were carried out without exposure to air. The optical microscopy of the powder in an early stage of milling showed deformation by twinning and re-twinning within the grains, developing sub-grain boundaries, which eventually defined nanometre-sized grains. The grain size reduction examined using X-Ray Diffraction (XRD) revealed a rapid decrease and then saturation of the grain size at the nanometer level.

After a number of hours of high-energy ball milling, the aggregate is processed by high temperature sintering (heating) at particular temperature-time histories to further enhance ductility and fracture toughness.

Optionally, after ball milling the Mg mixture and prior to sintering, the mixture can be processed in a compaction device to shape it.

After sintering, the samples were subjected to X-ray and Transmission Electron Microscope (TEM) analysis in order to quantify the microstructural state. In addition, multi-scale modeling simulations were performed to understand the mechanisms of mixing Mg powders with ductile and strengthening agents.

The ductility and strength of the Mg alloy can be tailored and designed by the control of the volumetric fraction of ductile and strengthening agents added to the Mg powders and the associated processing parameters, such as the duration of ball milling, the sintering temperature, the duration of sintering, and the sintering pressure level. The strength of the resulting Mg alloy can be altered and designed with the amount of the strengthening agent mixed, and the yield strength will be in the range of about 320 MPa to 500 MPa.

There are many applications for the Mg alloy and the method for making same. One application of the method is that it has proven to be an effective processing technique for producing nanocrystalline and/or microcrystalline metal allloys. Nanocrystalline and/or microcrystalline metal alloys have shown improved chemical, physical and mechanical properties due to the ultra-fine grain structure and the high volume of grain boundaries. Mg is a hexagonal closed pack (HCP) metal with a melting point of about 923 degrees K, which is similar to that of aluminum. The major attraction of Mg is the low density, which gives its high specific strength. At room temperature, plastic deformation of Mg is limited to basal slip and twinning, and hence, the ductility of Mg is relatively low (about −14% compared to about −25% of aluminum in tension). However, the ductility of Mg can be improved by suppressing twin formation through grain refinement, and because finer grains reduced the grain boundary back stresses allowing easier accommodation of grain boundary sliding and rotation.

Another application of the enhanced Mg alloys produced by the method of the present disclosure is used for storage (housing) of hydrogen (H₂) due to the high hydrogen storage capacity, the low cost and weight of Mg powders. The hydrogen storage properties of MgH₂ are significantly enhanced by a proper engineering of the microstructure and surface. Ball milling, which is used for fabrication of nanocrystalline and/or microcrystalline Mg alloy, improves both the morphology of the powders and the surface activity for hydrogenation. Mg alloys can be produced in a nanocrystalline and/or microcrystalline form by mixing tin (Sn) and/or silicon carbide (SiC) powders with Mg powders, which gives remarkable improvement of absorption/desorption kinetics. The hydriding properties are further enhanced by catalysis through nano-particles of palladium (Pd) located on Mg surface. Nanocrystalline and/or microcrystalline Mg alloy with such a catalyst exhibits an outstanding hydrogenation performance: very fast kinetics, operation at lower temperatures than conventional Mg and no need for activation.

Other applications of the Mg alloy are, for example but not limited to, the manufacture of automotive parts, sporting goods (such as football helmets and football helmet facemasks, etc.), etc.

It should be further noted that mechanical milling of the above described method also improves the corrosion resistance of Mg in passive conditions (KOH solution) as well as in more active corrosion conditions (borate solution). A Mg oxide (MgO) enrichment in the milled powders, which seems to be essentially related to the powder oxidation during the milling, contributes to the improved corrosion resistance of milled Mg, since MgO is thermodynamically much less reactive than Mg in the presence of water. Additional investigations, such as TEM or AFM/STM characterizations, are useful to investigate the improved surface layers of milled powders.

It should be emphasized that the above-described embodiments of the present disclosure, particularly, any “preferred” embodiments, are merely possible non-limiting examples of implementations, merely set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) of the disclosure without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of the present disclosure.

Claims

1. A method for producing a magnesium (Mg) alloy, comprising the steps of:

producing a Mg powder aggregate by mixing Mg powder and at least one strengthening agent, the strengthening agent selected from the group consisting of: a carbon, a metal, and a combination thereof;

agglomerating the aggregate; and

sintering the agglomerated aggregate to produce the Mg alloy.

2. The method of claim 1, wherein the producing and agglomerating steps are performed in a ball mill.

3. The method of claim 1, wherein the agent is at least one selected from the group consisting of: carbon nanotubes, copper, tin, titanium, silicon carbide, and a combination thereof.

4. The method of claim 1, wherein the agent is a metal with a hexagonal close-packed crystal structure.

5. The method of claim 1, further comprising the step of compacting the agglomerated aggregate prior to performing the sintering step.

6. The method of claim 1, wherein during the agglomerating step, the aggregate is sealed in a container with an inert gas in order to prevent oxidation of the aggregate.

7. A magnesium (Mg) alloy produced by the method of claim 1.

8. A method for producing a magnesium (Mg) alloy, comprising the steps of:

mixing and agglomerating in a ball mill a Mg powder aggregate having a Mg powder and at least one strengthening agent, the strengthening agent selected from the group consisting of: a carbon, a metal, and a combination thereof; and

sintering the agglomerated aggregate to produce the Mg alloy.

9. The method of claim 8, further comprising the step of preventing oxidation of the aggregate during the agglomerating step by introducing an inert gas into the ball mill.

10. The method of claim 8, wherein the agent is at least one selected from the group consisting of: carbon nanotubes, copper, tin, titanium, silicon carbide, and a combination thereof.

11. A magnesium (Mg) alloy comprising a structure selected from the group consisting of: a nano-scale crystalline lattice structure, a micro-scale crystalline lattice structure, and a combination thereof; and exhibiting a yield strength that is about 320 MPa to 500 MPa.

12. The Mg alloy of claim 11, wherein the alloy comprises Mg and a strengthening agent, and wherein the strengthening agent is at least one selected from the group consisting of: a carbon, a metal, and a combination thereof.

13. The Mg alloy of claim 12, wherein the agent is a metal with a hexagonal close-packed crystal structure.

14. The Mg alloy of claim 11, wherein the alloy comprises Mg and a strengthening agent, and wherein the strengthening agent is at least one selected from the group consisting of: carbon nanotubes, copper, tin, titanium, silicon carbide, and a combination thereof.