Superelastic and Shape-Memory Fe-Based Wire and Direct Wire Casting Thereof

Abstract

Herein is provided a ferrous shape memory alloy (SMA) wire and processes for production of ferrous shape memory alloy wire that do not require crystallographic texturing processes to achieve superior superelastic and SMA wire properties. The shape memory alloy wire includes an elongated wire body with a longitudinal-axis length of iron alloy material and has a cross-sectional wire diameter that is less than about 1 millimeter. The iron alloy material has an oligocrystalline crystallographic morphology along the longitudinal-axis length. The iron alloy material has a ′-fcc crystallographic matrix and a volume fraction of ′-LH crystallographic precipitates in the ′-fee crystallographic matrix.

We claim: 
     
         1 . A shape memory alloy wire comprising:
 an elongated wire body including a longitudinal-axis length of iron alloy material and having a cross-sectional wire diameter less than about 1 millimeter;   said iron alloy material having an oligocrystalline crystallographic morphology along said longitudinal-axis length; and   said iron alloy material having a  -fcc crystallographic matrix and including a volume fraction of γ′-L1 2  crystallographic precipitates in the γ-fcc crystallographic matrix.   
     
     
         2 . The shape memory alloy wire of  claim 1  wherein said oligocrystalline crystallographic morphology comprises a population of crystalline grains having an average grain size that is greater than half of said cross-sectional wire diameter. 
     
     
         3 . The shape memory alloy wire of  claim 1  wherein said longitudinal-axis length of iron alloy material has a surface area that is greater than a total area of polycrystalline grain boundaries of the oligocrystalline crystallographic morphology of the iron alloy material. 
     
     
         4 . The shape memory alloy wire of  claim 1  wherein said oligocrystalline crystallographic morphology comprises a population of crystalline grains having substantially random grain orientation. 
     
     
         5 . The shape memory alloy wire of  claim 1  wherein said oligocrystalline crystallographic morphology comprises a population of crystalline grains having a crystallographic texture with a multiples of random distribution value of no more than 15. 
     
     
         6 . The shape memory alloy wire of  claim 1  wherein said oligocrystalline crystallographic morphology comprises a population of crystalline grains having a crystallographic texture with a multiples of random distribution value of no more than 10. 
     
     
         7 . The shape memory alloy wire of  claim 1  wherein said iron alloy material exhibits an α′ martensite phase, and wherein said iron alloy material comprises a composition consisting of between about 20 at. % and about 35 at. % nickel (Ni), no more than about 20 at. % cobalt (Co), between about 7 at. % and about 20 at. % aluminum (Al), no more than about 12 at. % total of an element selected from tantalum (Ta), niobium (Nb), titanium (Ti), tungsten (W), molybdenum (Mo), vanadium (V), silicon (Si), chromium (Cr), copper (Cu), antimony (Sb), tin (Sn), zirconium (Zr), hafnium (Hf), thallium (Tl), germanium (Ge), gallium (Ga), and indium (In), and no more than about 2 at. % total of an element selected from boron (B), carbon (C), and nitrogen (N), with iron (Fe) as a remaining balance of said iron alloy composition. 
     
     
         8 . The shape memory alloy wire of  claim 1  wherein said iron alloy material comprises a composition consisting of Fe-(27-35 at. %)Ni-(0-20 at. %)Co-(5-17 at. %)Al-(0-10 at. %)(Ta, Nb, Ti, Mo, V, W)-(0-1 at. %)B. 
     
     
         9 . The shape memory alloy wire of  claim 1  wherein said iron alloy material comprises a composition consisting of Fe-(27-30 at. %)Ni-(12-18 at. %)Co-(9-13 at. %)Al-(1-3 at. %)(Ta, Nb, Ti)-(0-0.1 at. %)B. 
     
     
         10 . The shape memory alloy wire of  claim 1  wherein said iron alloy material comprises a composition consisting of Fe-28Ni-17Co-11.5Al-2.5Ti-0.05B (atomic %). 
     
     
         11 . The shape memory alloy wire of  claim 1  wherein said volume fraction of γ′-L1 2  crystallographic precipitates in the γ-fcc crystallographic matrix is at least about 10%. 
     
     
         12 . The shape memory alloy wire of  claim 1  wherein said volume fraction of γ′-L1 2  crystallographic precipitates in the γ-fcc crystallographic matrix is at least about 30%. 
     
     
         13 . The shape memory alloy wire of  claim 1  wherein said longitudinal-axis length of iron alloy material is at least about 1 meter. 
     
     
         14 . The shape memory alloy wire of  claim 1  wherein said cross-sectional wire diameter is less than about 300 microns. 
     
     
         15 . The shape memory alloy wire of  claim 1  wherein said 2 oligocrystalline crystallographic morphology comprises a population of crystalline grains having a crystallographic texture with a multiples of random distribution value of no more than 15, and wherein said volume fraction of γ′-L1 2  crystallographic precipitates in the γ-fcc crystallographic matrix is at least about 25%, whereby said length of iron alloy exhibits at least one of a tensile strength of at least about 1 GPa and a superelastic strain of at least about 5%. 
     
     
         16 . A shape memory alloy wire comprising:
 an elongated wire body including a longitudinal-axis length of iron alloy material of at least about 1 meter and having a cross-sectional wire diameter less than about 1 millimeter;   said iron alloy material consisting of Fe-(27-35 at. %)Ni-(0-20 at. %)Co-(5-17 at. %)Al-(0-10 at. %)(Ta, Nb, Ti, Mo, V, W)-(0-1 at. %)B;   said iron alloy material having an oligocrystalline crystallographic morphology along said longitudinal-axis length; and   said iron alloy material having a γ-fcc crystallographic matrix and including a volume fraction of at least about 10% of γ′-L1 2  crystallographic precipitates in the γ-fcc crystallographic matrix.   
     
     
         17 . A method for producing a ferrous shape memory alloy wire comprising:
 mixing a shape memory alloy composition consisting of between about 20 at. % and about 35 at. % nickel (Ni), no more than about 20 at. % cobalt (Co), between about 7 at. % and about 20 at. % aluminum (Al), no more than about 12 at. % total of an element selected from tantalum (Ta), niobium (Nb), titanium (Ti), tungsten (W), molybdenum (Mo), vanadium (V), silicon (Si), chromium (Cr), copper (Cu), antimony (Sb), tin (Sn), zirconium (Zr), hafnium (Hf), thallium (Tl), germanium (Ge), gallium (Ga), and indium (In), and no more than about 2 at. % total of an element selected from boron (B), carbon (C), and nitrogen (N), with iron (Fe) as a remaining balance of said iron alloy composition, to obtain a resulting shape memory alloy mixture;   heating the shape memory alloy mixture to form a melted shape memory alloy;   casting the melted shape memory alloy mixture to form a polycrystalline ferrous shape memory alloy wire having a length of at least about 1 meter and a diameter of no more than about 300 microns;   solutionizing the polycrystalline ferrous shape memory alloy wire for a duration that transforms the polycrystalline ferrous shape memory alloy wire to an oligocrystalline ferrous shape memory alloy wire and that produces a γ-fcc phase matrix in the oligocrystalline ferrous shape memory alloy wire; and   ageing the solutionized oligocrystalline ferrous shape memory alloy wire to produce in the oligocrystalline ferrous shape memory alloy wire a volume fraction of at least about 10% of γ′-L1 2  precipitates;   wherein each of said casting, solutionizing, and ageing steps, and any processing of said solutionized and aged oligocrystalline ferrous shape memory alloy wire subsequent to said ageing step, is conducted without solid state deformation of said ferrous shape memory alloy wire that increases crystallographic texture of the oligocrystalline ferrous shape memory alloy wire.   
     
     
         18 . The method of  claim 17  wherein casting the melted shape memory alloy mixture comprises melt spinning. 
     
     
         19 . The method of  claim 17  further comprising applying a training load to the aged and solutionized oligocrystalline ferrous shape memory alloy wire to obtain superelasticity of at least about 5% in the solutionized ferrous shape memory alloy wire. 
     
     
         20 . The method of  claim 17  wherein mixing a shape memory alloy composition comprises mixing a shape memory alloy consisting of Fe-(27-35 at. %)Ni-(0-20 at. %)Co-(5-17 at. %)Al-(0-10 at. %)(Ta, Nb, Ti, Mo, V, W)-(0-1 at. %)B.