- A Mg—Li—Ca alloy having between 3.0 wt. % and 7.0 wt. % Li as described above, and between 0.10 wt. % and 1.0 wt. % Ca as described above. In one exemplary embodiment, the Mg—Li—Ca alloy is 93.75 wt. % Mg—6 wt. % Li—0.25 wt. % Ca or 93 wt. % Mg—6 wt. % Li—1 wt. % Ca.
- A Mg—Li—Ca-RE alloy having between 3.0 wt. % and 7.0 wt. % Li as described above, between 0.10 wt. % and 1.0 wt. % Ca as described above, and between 0.25 wt. % and 7.0 wt. % RE as described above.
- A Mg—Li—Ca—Al alloy having between 3.0 wt. % and 7.0 wt. % Li as described above, and between 1.0 wt. % and 6.0 wt. % combined Al and Ca, including 0.10 wt. % to 1.0 wt. % Ca and 0.9 wt. % to 5 wt. % Al as described above.
- A Mg—Li—Ca—Al-RE alloy having between 3.0 wt. % and 7.0 wt. % Li as described above, between 1.0 wt. % and 6.0 wt. % combined Al and Ca including 0.10 wt. % to 1.0 wt. % Ca and 0.9 wt. % to 5 wt. % Al as described above, and between 0.25 wt. % and 1.0 wt. % RE as described above.

Wire Constructs Including Mg—Li—Ca—(Al)-(RE)

An alloy in accordance with the present disclosure may first be formed in bulk, such by casting an ingot, continuous casting, or thixomolding of the desired material.

This bulk material is then formed into a suitable pre-form material (e.g., a rod, plate or hollow tube) by hot-working the bulk material into the desired pre-form size and shape. For example, the ingot may be melted using an arc-melting, cold crucible technique in order to cast rod stock. The rod stock may then be subjected to one or more iterations of warm or hot working, such as forging or hot extrusion, in order to effect a large area reduction (e.g., 8:1) resulting in an intermediate rod stock. For purposes of the present disclosure, warm or hot working is accomplished by heating the material to an elevated temperature above room temperature and performing desired shaping and forming operations while the material is maintained at the elevated temperature. Full annealing may optionally be performed after hot working to achieve equiaxed microstructure.

This intermediate rod stock may then be subjected to conventional iterative cold working and annealing, as further described below, to create an initial coarse wire structure ready for final processing. Each iterative cold work process imparts cold work which is stored in the material microstructure, as further described herein, and this stored cold work is relieved by fully annealing the material between draws, thereby enabling the next iterative cold working process. In full annealing, the cold-worked material is heated to a temperature sufficient to substantially fully relieve the internal stresses stored in the material, thereby relieving the stored cold work and “resetting” cold work to zero.

In addition to a coarse wire material, other forms and shapes may be produced by similar repetitive cold-forming and annealing cycles. Constructs of potential use in a medical device include rods, wires, tubes, sheets or plate products.

For the present Mg—Li—Ca materials, full annealing is accomplished at a temperature about 350° C. for at least 60 minutes. Alternatively, a full anneal can be accomplished with a higher temperature, such as between 375° C. and 400° C., for a shorter time, such as between 60 seconds and 30 minutes. Of course, a relatively higher temperature annealing process can utilize a relatively shorter time to achieve a full anneal, while a relatively lower temperature will typically utilize a relatively longer time to achieve a full anneal. In addition, annealing parameters can be expected to vary for varying wire diameters, with smaller diameters shortening the time of anneal for a given temperature. Whether a full anneal has been accomplished can be verified in a number of ways as well known in the art, such as microstructural examinations using scanning electron microscopy (SEM), mechanical testing for ductility, strength, elasticity, etc., and other methods.

The resulting coarse wire material may then be finally processed into a final form, such as a fine wire suitable for integration into a stent or other medical device. Exemplary wire constructs are described in further detail below.

1. Monolithic Wires

FIG. 4aillustrates a cross-section ofmonolithic wire31, made entirely of a first material α having outer cross-sectional diameter D_W.Monolithic wire material31 is made of the present Mg—Li—Ca alloy, and may have finished diameter D_Wand an axial length as required or desired for a particular application. The production, characteristics and use ofwire31 is described in detail below.

Starting with a coarse, initial wire construct as described above, the present Mg—Li—Ca material may be formed into a final wire construct by final drawing and/or annealing to produce a monolithic wire ready to be used in vivo. For purposes of the present disclosure, cold-working methods effect material deformation at or near room temperature, e.g. 20-30° C. The total cold work imparted tomonolithic wire31 during a drawing step can be characterized by the following formula (I):

\begin{matrix} cw = 1 - {(\frac{D_{2}}{D_{1}})}^{2} \times 100 % & (I) \end{matrix}

wherein “cw” is cold work defined by reduction of the original material area, “D₂” is the outer cross-sectional diameter of the wire after the draw or draws, and “D₁” is the outer cross-sectional diameter of the wire prior to the same draw or

True strain is an alternative expression of total imparted cold work. True strain is calculated according to the following formula (II):

\begin{matrix} ts = \ln ({(\frac{D_{1}}{D_{2}})}^{2}) & (II) \end{matrix}

wherein “ts” is cold work expressed as true strain, “ln” is the natural logarithm operator, and D₁and D₂are the diameter prior to cold work conditioning and after cold work conditioning respectively. Further discussion of exemplary cold work conditioning processes are presented in U.S. Patent Application Publication No. 2010/0075168, entitled FATIGUE DAMAGE RESISTANT WIRE AND METHOD OF PRODUCTION THEREOF, filed Sep. 18, 2009, and assigned to the present assignee, the disclosure of which is hereby expressly incorporated by reference herein in its entirety.

Referring toFIG. 3A, the cold work step may be performed by the illustrated drawing process.Wire31 is drawn through a lubricateddie36 having an output diameter D₂, which is less than initial diameter D₁ofwire31 prior to the drawing step. The outer diameter ofwire31 is accordingly reduced from pre-drawing diameter D₁to drawn diameter D₂, imparting cold work cw and true strain is as set forth in equations (I) and (II) above. draws. If the drawing step is the final step of imparting cold work to create a finished wire, diameter D₂at the output of die36 equals diameter D_Wof monolithic wire31 (FIG. 4a).

Although wire drawing is the illustrated method of introducing cold work cw into the material ofwire31, it is contemplated that cold work may be imparted by a number of other processes within the scope of the present disclosure. For example, net cold work may be accumulated inwire31 by cold-swaging, rolling the wire (e.g., into a flat ribbon or into other shapes), extrusion, bending, flowforming, or pilgering. Cold work may also be imparted by any combination of techniques including the techniques described here, for example, cold-swaging followed by drawing through a lubricated die finished by cold rolling into a ribbon or sheet form or other shaped wire forms. In one exemplary embodiment, the cold work step by which the diameter ofwire31 is reduced from D₁to D₂is performed in a single draw and, in another embodiment, the cold work step by which the diameter ofwire31 is reduced from D₁to D₂is performed in multiple draws (e.g., through lubricated dies having successively smaller output diameters) which are performed sequentially without any annealing step therebetween.

Thermal stress relieving, otherwise known in the art as annealing, at a nominal temperature not exceeding the melting point of either the first or second materials, may be used to improve the ductility ofwire31 after cold work application. The softening point of the present Mg—Li—Ca materials, and therefore their behavior during an annealing process of a particular time and temperature, can be controlled by introducing a particular amount of cold work into the wire material. As noted above, deformation energy is stored in the cold-worked structure as accumulated cold work, and this energy serves to reduce the amount of thermal energy required for stress relief of the wire material.

For certain exemplary Mg—Li—Ca materials, the above-described cold work processing facilitates annealing of the composite structure at temperatures in the range of 60 to 80% of the melting point of the material, e.g., between about 200° C. and 400° C. The time of annealing may be between 1 second and 60 minutes, with lower temperatures resulting in longer annealing times and higher temperatures resulting in shorter annealing time, as described in detail above. Moreover, the particular annealing time and temperature will also depend on the specific wire material and wire size, as also noted above. The annealing parameters may be chosen on a case-by-case basis to provide the desired ductility and strength.

Post-cold-work annealing may be used for applications in which a wire with annealed mechanical properties (i.e. low relative tensile strength and high ductility) may be desired. In applications where higher tensile strength is desired and a lower relative ductility is acceptable, annealing after the cold work processing step(s) may be omitted.

After production, thefinished wire31 may then be braided into the shape of a stent such as that ofFIG. 1A, knitted into the shape of a stent such as that ofFIG. 1B, or otherwise formed into a medical device such as a vascular or gastric stent, aneurysm clotting device, or blood filter, for example. For the foregoing applications,wire31 will typically be drawn to a final finish diameter between 20 μm and 250 μm.

2. Composite Wires

FIG. 2 illustratescomposite wire30, includingshell32 surroundingcore34.

Bimetalliccomposite wire30 has a circular cross section and extends along a longitudinal axis and includes outer shell, sheath, ortube32 made of a first biodegradable material and a core34 made of a second biodegradable material.Outer shell32 may be formed as a uniform and continuous surface or jacket, such thatwire30 may be coiled, braided, or stranded as desired.Core34 completely fills the bore formed throughouter shell32 such thatcomposite wire30 forms a solid wire construct, but with two different materials.

In an exemplary embodiment, at least one of the two biodegradable materials used forcomposite wire30 is formed from the present Mg—Li—Ca alloy material. It is contemplated thatouter shell32 andcore34 may be formed from the same material or different materials, and that either shell32 orcore34 may be formed from any of the present Mg—Li—Ca materials as required or desired for a particular application.

The other of the two biodegradable materials may be any of a number of biodegradable materials in accordance with the present disclosure. In one embodiment, one of theshell32 orcore34 may be iron-based, such as pure metallic iron (Fe), an anti-ferromagnetic iron-manganese alloy (Fe—Mn) such as Fe-30Mn or Fe-35Mn, or another iron-based alloy (Fe alloy). In another embodiment, one of theshell32 orcore34 may be magnesium-based, such as pure magnesium (Mg) or a magnesium-based alloy (Mg alloy) such as ZM21 (Mg-2Zn-1Mn), AE21 (Mg-2Al-1RE), AE42 (Mg-4Al-2RE), WE43 (Mg-4Y-0.6Zr-3.4RE, as in yttrium, zirconium, RE). In another embodiment, one of theshell32 orcore34 may be a zinc-based.

For purposes of the present disclosure, bimetalcomposite wire30 can be expressed as a first material forshell32 and a second material forcore34, where the second material is specified as comprising a specified balance percentage of the total wire cross-sectional area. “DFT” is interposed between the two materials to indicate that the material is “drawn filled tubing,” i.e.,composite wire30 as shown inFIG. 2. For example, onecomposite wire30 made in accordance with the present disclosure may be defined as Fe-DFT-25% MgLi, which is 75% iron and 25% of the present Mg—Li—Ca materials.

FIGS. 4band 4cillustrate variable relative proportions ofcore34 andshell32 ofcomposite wire30.FIG. 4ashows amonolithic wire material31 made entirely of a first material α having outer cross-sectional diameter D_W. In an exemplary embodiment,monolithic wire material31 may be a magnesium-lithium alloy such as the alloys described in Example 2 below.FIG. 4bshows awire30, such as a wire for a stent, in which shell32 is made of a first material α occupying 75% of the total cross-sectional area ofwire30, whilecore34 is formed from a second material β occupying the balance (25%) of the cross-sectional area ofwire30.FIG. 4cshows awire30, such as a wire for a stent, in which shell32 is made of a first material α occupying 43% of the total cross-sectional area ofwire30, whilecore34 is formed from a second material β occupying the balance (57%) of the cross-sectional area ofwire30. For Example 1, test materials in accordance with the present disclosure and benchmark alloys including 316L stainless steel, MP35N® and NiTi were procured as with outer diameters D_Wof 125 μm.Stent40, made fromwire30 and/or31, has outside diameter D_S, which may be about 7 mm.Stent40 may be a tubular mesh stent scaffold manufactured from

wires

30,31, or a combination of

wires

30 and31. Exemplary such stents are available from biomedical materials supplier, Fort Wayne Metals (Fort Wayne, Ind., USA). An exemplary strut thickness (i.e., wire diameter D_W) of 127 μm and expanded tubular diameter D_Sof 7 mm, as perFIG. 3(d), are selected as dimensions similar to current self-expanding stent designs which are used in peripheral vessel scaffolding.

In one embodiment, the present Mg—Li—Ca materials may be particularly useful ascore34 ofcomposite wire30 in order to facilitate cold-work processing and its associated control over the overall mechanical properties ofwire30. As noted above, magnesium and its alloys typically comprise a hexagonal-close-packed (HCP) crystal structure which possesses low ductility at room temperature due to intrinsically limited slip systems, primarily confined to the basal plane. Addition of Li to the base Mg material, however, has been found to increase ductility and, therefore, cold workability. At the same time, where iron or iron-alloy materials are employed incomposite wire30, it is desirable to conduct wire processing by cold-working methods in order to maximize the mechanical properties of the iron or iron-alloy. Where such iron or iron-alloy is used forshell32 and the present Mg—Li—Ca material is used forcore34, the iron or iron-alloy serves as a sheath to confine the present Mg—Li—Ca material, thereby inducing a compressive stress during cold work processing (e.g., wire drawing as described below). The ductility of the present Mg—Li—Ca alloy enables such processing techniques and therefore promotes maximization of mechanical properties by obviating any need for unwanted intermediate stress-relief (e.g., by annealing or high-temperature processing as described herein) that might otherwise be necessary to formcomposite wire30 with a magnesium alloy.

Further discussion of bimetal composite wires made from biodegradable constituent materials are further described in U.S. Patent Application Publication No. 2011/0319978, filed Jun. 24, 2011 and entitled BIODEGRADABLE COMPOSITE WIRE FOR MEDICAL DEVICES, the entire disclosure of which is hereby expressly incorporated herein by reference.

The selection of materials forshell32 andcore34 will inherently determine the absolute and relative biodegradation rates of these materials, and may be chosen by one of ordinary skill in the art in accordance with such considerations. For example,shell32 may be formed of a relatively slower-biodegrading material andcore34 may be formed of a relatively faster-biodegrading material. In this arrangement, overall degradation will occur at a slow pace until the relatively fast-degradingcore34 begins to be exposed. At this stage, e.g. in the case of awire construct30 having an iron or iron alloyouter shell32 and a magnesium or magnesium alloy core, an electrochemical potential will drive the more rapid degradation of thecore34. In some designs, this intermediate degradation point may leave behind a thin iron or iron alloy outer shell which will possess reduced flexibility more similar to the vascular wall, thereby permitting more natural vessel movement and reactivity. Further, the remaining hollowouter shell32 of iron or iron-alloy will present additional surface area to fluid contact in vivo, thereby causing the material to degrade more quickly than a comparable monolithic iron or iron alloy wire.

In other embodiments, this arrangement may be reversed, whereinshell32 may be formed of a relatively faster-biodegrading material andcore34 may be formed of a relatively slower-biodegrading material. In this arrangement, the degradation process is expected to consumeouter shell34 and leave an intermediate and mostlycontinuous core34. Similar to the embodiment described above, this relatively thin core element will provide improved flexibility, an increased rate of bioabsorption, and a concomitantly improved vessel healing response with a reduced risk of thrombosis, particle embolization, and restenosis compared to a monolithic bioabsorbable wire.

Forcomposite wires30 incorporated into stents (e.g., as shown inFIGS. 1A and 1B) or other in vivo structures, the first biodegradable material (i.e., outer shell32) may be chosen to degrade in vivo at a slower rate than the second biodegradable material (i.e., core34), such that overall structural integrity and strength are substantially maintained for a period of time after initial implantation while the slower-degradingouter shell32 bioabsorbs or bioresorbs. After theouter shell32 erodes enough to expose the core34 to biodegradation by interaction with substances in the in vivo environment, a relatively rapid biodegradation occurs as noted above. This construction modality is particularly useful where approximately equal amounts of the first and second biodegradable materials are used, but any relative proportions of the present Mg—Li—Ca materials and a second material may be used as noted above.

Moreover, stents made from wire produced in accordance with the present disclosure provide well-designed control over the mechanics and pace of the overall degradation rate of the constituent wires (and therefore, also of the stent structure itself), thereby facilitating therapeutic optimization.

It is also contemplated that antiferromagnetic alloys of iron and manganese may be used in eithershell32 orcore34 ofwire30 for magnetic resonance imaging compatibility.

Optionally, shell32 of the wire may be partially or fully coated with a biodegradable polymer35 (FIG. 2) that may be drug-eluting to further inhibit neointimal proliferation and/or restenosis. Suitable biodegradable polymers include poly-L lactic acid (PLLA) and poly-L glycolic acid (PLGA), for example. The wire may be coated either before, or after being formed into a stent.

To form wire30 (FIG. 2),core34 is inserted withinshell32 to form a wire construct, and an end of the wire construct is then tapered to facilitate placement of the end into a drawing die. The end protruding through the drawing die is then gripped and pulled through the die to reduce the diameter of the construct and bring the materials ofcore34 andshell32 into physical contact. After an initial draw, the inner diameter of the shell will close on the outer diameter of the core such that the inner diameter of the shell will equal the outer diameter of the core whereby, when viewed in section, the inner core completely fills the outer shell.

For example, as shown inFIG. 3B,wire30 is drawn through a lubricateddie36 having an output diameter D_2S, which is less than diameter D_1Sofwire30 prior to the drawing step. The outer diameter ofwire30 is accordingly reduced from pre-drawing diameter D_1Sto drawn diameter D_2S, imparting cold work cw as described in detail above with respect tomonolithic wire31. Drawing imparts cold work to the material of both shell32 andcore34, with concomitant reduction in the cross-sectional area of both materials. That is, each drawing step reduces the cross section ofwire30 proportionately, such that the ratio of the sectional area ofcore34 to the overall sectional area ofwire30 is nominally preserved as the overall sectional area ofwire30 is reduced. Referring toFIG. 3, the ratio of pre-drawing core outer diameter D_1Cto pre-drawings shell outer diameter D_1Sis the same as the corresponding ratio post-drawing. Stated another way, D_1C/D_1S=D_2C/D_2S. Further details regarding wire drawing of composite wires are discussed in U.S. Patent Application Serial No. 2009/0260852, filed Feb. 27, 2009, entitled “ALTERNATING CORE COMPOSITE WIRE,” the entire disclosure of which is incorporated by reference herein. In an exemplary embodiment, thefinished wire30 may be a fine wire, having a finished diameter D_2Sof between 20 μm and 1 mm. In another embodiment,wire30 may have a finished diameter D_2Sup to 2.5 mm.

The fully dense (i.e., solid cross-section)composite wire30 may be annealed after drawing, similar towire31 discussed above.

Wire Properties

1. Strength and Ductility

The yield strength ofwires30 and/or31, and thus its resilience, is influenced by the amount of strain-hardening deformation applied towires30 and/or31 to achieve the final diameter D_W, and by the thermal treatment applied after drawing the wire (if any). The ability to vary the strength and resilience of

wires

30,31 allows use of the wire in resilient designs, such as for self-expanding stents, or for plastic-behaving designs, such as for balloon-expanding stents. As described herein, the present Mg—Li—Ca alloys are highly ductile, and therefore tolerate large amounts of cold work before fracture. This ductility and cold workability enable flexible design parameters for finished Mg—Li—Ca alloy products, by facilitating selection from a wide range of strength and resilience properties depending on the amount of applied cold work.

Generally speaking, cold work processing of the present Mg—Li—Ca materials may be used to increase stress to fracture, with a corresponding decrease in overall strain to fracture for many of the present Mg—Li—Ca alloy materials. Varying levels of cold work may be applied in order to achieve varying levels of material strength and ductility.

Strength is positively correlated with cold work/true strain, as demonstrated below for various exemplary alloys. For purposes of the present disclosure, the positive correlation of strength and true strain can be assumed to be approximately linear between the low and high levels given below.

As further described below, the present Mg—Li—Ca materials have the ability to undergo large amounts of cold work without fracture. This large capacity for cold work enables a wide range of cold work strengthening options. After performing cold work, yield strengths (YS) are improved, and are in excess of 200 MPa. Ultimate tensile strengths (UTS) are at least about 270 MPa.

Mg—Li—Ca alloy material made in accordance with the present disclosure, including a relatively small amount of Ca (e.g., 0.25%) has sufficient ductility to allow cold work up to 98% without fracture. This low-Ca Mg—Li—Ca material with 98% cold work exhibits yield strength YS of 276 MPa and ultimate tensile strength UTS of 334 MPa. Thus, the addition of a relatively small amount of Ca to the binary Mg—Li material significantly increases ductility while strength as compared to the binary Mg—Li alloy.

Mg—Li—Ca alloy material including a larger amount of Ca (e.g., 1.0%) has sufficient ductility to allow cold work up to 88% without fracture. This higher-Ca Mg—Li—Ca material with 88% cold work exhibits yield strength YS of 240 MPa and ultimate tensile strength UTS of 271 MPa. Thus, the addition of a relatively larger amount of Ca to the binary Mg—Li material increases strength while somewhat decreasing ductility, as compared to the binary Mg—Li alloy.

Thus, the present Mg—Li—Ca alloys have the ability, in view of their cold workability, to be produced and modified without utilizing elevated temperatures for such production. Room-temperature or lower-temperature production of the finished wire product represents a significant efficiency, particularly for large-scale production, and therefore minimizes production cost.

2. Fatigue Endurance

Fatigue endurance of the present Mg—Li—Ca alloys can be enhanced by imparting cold work to the material, as described in detail above, and then performing a controlled anneal of the material such that a refined, substantially equiaxed grain structure is achieved.

This enhanced fatigue endurance, bioabsorbable wires and stents made in accordance with the present disclosure can initially withstand flexion of mobile vessels of the extremities, give sufficient time for vessel remodeling, and then biodegrade. Thus, the present wire is ideally suited for use in stents implanted in high-flexion areas (i.e., extremities) and other demanding applications.

3. Medical Device Applications

In view of the foregoing material properties for wires made of the present Mg—Li—Ca alloys, it can be seen that stents and wires made in accordance with the present disclosure offer the ability to optimize design to account for, e.g., anatomy, blood and cell compatibility, long term endothelial functionality, fracture resistance, and patient-specific rates of bioabsorption. Such design optimization can be provided by, for example, cold work conditioning, thermomechanical processing, and material selection, and wire size and/or geometry and discussed above

For example, wires and stents made in accordance with the present disclosure allow a surgeon to implant a naturally reactive stent over a predetermined term and to plan for the stent to completely biodegrade after the predetermined term. In this way, use of the present wires and wire constructs can reduce or eliminate late complications such as late-stent-thrombosis, relative vessel occlusion and lifelong anti-platelet therapy. When used in self-expanding, biocompatible, and biodegrading stent designs the present wire can further extend this treatment option to the challenging vasculature of the extremities.

Still another advantage of the present wire is the opportunity to offer controllable degradation rates of stents to allow patient-dependent time for vessel remodeling. As noted above, patient-specific stent degradation rates also offer long-term benefit by allowing unimpeded reintervention and natural long term vasoreactivity.

Example

The following non-limiting Example illustrates various features and characteristics of the present invention, which is not to be construed as limited thereto.

In this Example, exemplary monolithic Mg—Li—Ca alloy wires in accordance with the present disclosure were produced, tested and characterized, particularly with regard to material workability and mechanical strength.

1. Production of Mg—Li—Ca Alloy Materials

Four alloys were selected, each having 6 wt. % Li, selected additional alloying elements of Al, Ca and/or RE, and the balance Mg. The composition of each of these alloys is set forth below in Table 5. Ingots of each alloy were induction melted and extruded at 300° C. to a diameter of approximately 4.5 mm. The extruded ingots were then iteratively cold-drawn using standard methods, as described in detail above, with intervening annealing at 350° C.

The iterative draw-anneal cycles were repeated as necessary until a wire was produced at a diameter of 0.9 mm, at which point a final anneal was performed to create a stress-relieved base wire ready for processing in accordance with the present disclosure.

Starting with respective samples of the 0.9 mm diameter wire, cold work was imparted to the material by drawing in accordance with the procedure described above. The amount of cold work tolerated by each material is shown in Table 5.

Mechanical performance was then evaluated for each cold worked sample via a uniaxial tensile test on an Instron Model 5565 test machine available from Instron or Norwood, Mass., USA). More specifically, destructive uniaxial tension testing of the wire materials was used to quantify the ultimate strength, yield strength, axial stiffness and ductility of candidate materials, using methods described inStructure-Property Relationships in Conventional and Nanocrystalline NiTi Intermetallic Alloy Wire, Journal of Materials Engineering and Performance18, 582-587 (2009) by Jeremy E. Schaffer, the entire disclosure of which is hereby expressly incorporated herein by reference. These tests are run using servo-controlled Instron load frames in accordance with industry standards for the tension testing of metallic materials.

A 127 mm gage length and crosshead speed of 12.7 mm/min was used for the tensile testing.

TABLE 5

Exemplary monolithic Mg—Li wires

Alloy	Composition by wt. %	Cold Work (%)

1	94Mg—6Li	94
2	89.5Mg—6Li—4Al—0.5RE	75
3	93.75Mg—6Li—0.25Ca	98
4	93Mg—6Li—1Ca	88

2. Characterization of Mechanical Properties in Tension

a. Strength

Alloy #1 (an Mg—Li base binary alloy, Table 5) exhibited good ductility, achieving 94% cold work without fracture. Yield strength YS, shown inFIG. 5B, was measured at 243 MPa while ultimate tensile strength UTS was measured at 305 MPa.

Alloy #2 (Table 5), which was an Mg—Li alloy with Al and RE additions, reduced the attainable cold work to 75%. Referring toFIG. 5B, however, it can be seen thatalloy #2 demonstrated a dramatically improved strength as compared toalloy #1, as shown inFIG. 5B. More particularly,alloy #2 had a yield strength YS of 455 MPa and an ultimate tensile strength UTS of 495 MPa.

For alloy #3 (Table 5), the Mg—Li base binary alloy was used with the addition of 0.25% Ca. Formability was improved with respect toalloy #1, with cold work increasing to 98%. In addition,alloy #3 achieved a moderate gain in strength with a yield strength YS of 276 MPa and an ultimate tensile strength UTS of 334 MPa.

In alloy #4 (Table 5), a higher level of Ca was composited with the base Mg—Li binary alloy. The resulting wire material was able to withstand less cold work, at 88%. Yield strength YS and ultimate tensile strength UTS also both fell, to 240 MPa and 271 MPa respectively.

In all material samples tested in the present Example, addition of cold work was enabled by the 6 wt. % Li constituent and resulted in strengthening of the material. This effect was most pronounced inAlloy 2, reaching a UTS of 495 MPa as illustrated inFIG. 5B.

FIG. 5A is a plot of stress-strain data for individual wire samples ofalloy #2 prepared in accordance with the present Example, and cold worked to various levels as specified in the legend ofFIG. 5A.

As illustrated, increasing cold work levels were associated with increasing stress to fracture but generally decreasing overall strain to fracture. With no cold work, testing an as-annealed wire, engineering strain to rupture was measured at 10.6%, with an ultimate tensile strength UTS of 289 MPa. With 20% cold work, engineering strain to rupture dropped to 2.9% but ultimate tensile strength UTS increased to 367 MPa. At 50% cold work, engineering strain to rupture measured 4.1% while ultimate tensile strength UTS further increased to 415 MPa. At 60% cold work, engineering strain to rupture measured 3.0% while ultimate tensile strength UTS increased to 474 MPa. At 64% cold work, engineering strain to rupture fell again to 2.5% while ultimate tensile strength UTS rose again to 487 MPa. At 75% cold work, the highest tested in this Example, engineering strain to rupture reached a low point of 1.5% while ultimate tensile strength UTS reached a high of 495 MPa.

The present Mg—Li—Ca alloys tested in this example demonstrate the ability, in view of their cold workability, to be produced without utilizing elevated temperatures for such production. Room-temperature or lower-temperature production represents a significant efficiency, particularly for large-scale production, and therefore minimizes production cost. Thus, the high levels of cold work tolerated by the Mg—Li—Ca alloy wires of the present Example are amenable to an efficient, cost-effective production method.

Excellent strength is an added benefit of cold working for the present Mg—Li—Ca materials using including Li and Ca as described herein. The UTS of the present Mg—Li—Ca alloys was high in all cases, and inparticular alloy 2 demonstrated a significantly higher YS and UTS as compared to other Mg—Li materials of comparable ductility.

The addition of as little 6 wt. % Li has been shown to induce a cubic structure in the Mg—Li—Ca alloys tested in this Example, rather than the predominantly HCP crystal structure exhibited by other magnesium alloys. This cubic structure dramatically improves the ductility of the material, enabling high levels of cold work and obviating any need for hot-forming processes. Accordingly, Mg—Li—Ca alloys in accordance with the present example facilitate a decrease in processing costs and an increase in cold-work-induced strengthening.

While this invention has been described as having an exemplary design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.