Method of pre-aging nitihf shape memory alloys and parts therefrom with uniform microstructures and superior properties

Abstract

A method to produce a high strength NiTiHf alloy, a NiTiHfZr alloy or a NiTiZr alloy are disclosed. The alloys comprise less than about 10 atomic percent of Hf, Hf+Zr, or Zr, respectively. The alloys, devices containing the alloys and methods of producing the devices are also disclosed.

1 . A method to produce a high-strength NiTi(Hf/Zr) alloy with homogeneous distribution of H-phase precipitates, with between about 2 and about 8% recoverable compression strain, comprising:
 processing a NiTi(Hf/Zr) alloy by at least one method of a casting method, an additive manufacturing method, a drawing method, a forging method, an extrusion method, a powder metallurgy method, or combinations thereof, wherein the composition of the NiTi(Hf/Zr) alloy comprises between 50 and 53 atomic percent of Ni, between about 1 and 10 atomic percent of Hf, Zr, or combinations thereof, less than about 5 atomic percent total % of incidental materials, and the balance of the composition being Ti;   annealing the NiTi(Hf/Zr) workpiece to produce an annealed NiTi workpiece; and   pre-aging the annealed NiTi workpiece at a temperature between about 100° C. and about 400° C. for a pre-aging time period between about 1 and 24 hours to produce the high-strength NiTi alloy.   
     
     
         2 . The method of  claim 1 , wherein the pre-aging temperature is about 300° C. 
     
     
         3 . The method of  claim 2 , wherein the pre-aging time period is about 12 hours. 
     
     
         4 . The method of  claim 1 , further comprising quenching the annealed NiTi workpiece at a temperature between about −100° C. and about 200° C. prior to pre-aging. 
     
     
         5 . The method of  claim 1 , wherein a compressive yield strength of the high-strength NiTi alloy is between about 2 GPa and about 4 GPa. 
     
     
         6 . The method of  claim 1 , further comprising a heat treatment at a heat treatment temperature of between about 400° C. and about 600° C. for between about 10 minutes and about 24 hours. 
     
     
         7 . The method of  claim 1 , wherein the annealing is solution annealing. 
     
     
         8 . The method of  claim 7 , wherein the solution annealing temperature is between about 700° C. and about 1150° C. 
     
     
         9 . The method of  claim 1 , wherein a primary precipitate is not formed following the preaging step. 
     
     
         10 . The method of  claim 1 , wherein the NiTi alloy is of the formula Ni50+x(Ti+Hf)49.9-1x wherein x is between 0.1 and 2.9. 
     
     
         11 . The method of  claim 10 , wherein Hf in the NiTiHf alloy is between about 0.1 to 10%. 
     
     
         12 . The method of  claim 1 , wherein the NiTi alloy is of the formula Ni50+x(Ti+Zr)49.9-1x wherein x is between 0.1 and 2.9. 
     
     
         13 . The method of  claim 10 , wherein Zr in the NiTi alloy is between about 0.1 to 10%. 
     
     
         14 . The method of  claim 10 , wherein the NiTi alloy further comprises Zr, wherein the Zr in the NiTi alloy is between about 0 and about 10 atomic percentage of a percentage of Hf in the NiTiHf alloy of a composition NiTiHfZr. 
     
     
         15 . A NiTi alloy, wherein a recoverable yield of the alloy is between about 2 and about 8%, and wherein a compressive yield strength of the alloy is greater than about 1.5 GPa. 
     
     
         16 . The NiTi alloy of  claim 15 , wherein a compressive yield strength of the high-strength NiTi alloy is between about 2 GPa and about 4 GPa. 
     
     
         17 . The NiTi alloy of  claim 15 , wherein a composition of the NiTi alloy is Ni 50.3-53. Ti 49.7-x Hf x  wherein x is between 6 and 9, 3 and 6, or 1 and 3. 
     
     
         18 . The NiTi alloy of  claim 15 , further comprising incidental materials of up to about 1 atomic percent of any individual transition metals, up to about 1 atomic % of a nonmetal, and combinations thereof, wherein a total maximum amount of the incidental materials is about 5 atomic percentage. 
     
     
         19 . A biomedical implant, comprising a NiTi alloy, wherein a composition of the NiTi alloy is Ni 50.3-53. Ti 49.7-x Hf x  wherein x is between 6 and 9, 3 and 6, or 1 and 3.