A Study of the Influence of Thermo-Mechanical Processing on VIM-VAR Melted NiTi Shape Memory Alloys

NiTi shape memory alloy wires are used in the manufacture of numerous medical devices, many of which are employed in fatigue critical applications. Besides melting, thermo-mechanical processing plays a significant role in influencing their fatigue life. This study aims at understanding the properties of wires drawn by two different thermo-mechanical processes - (i) marforming (0.250'' to 0.021'') and (ii) ausforming (0.250'' to 0.085'') followed by marforming (0.085'' to 0.021''). The final wire, 0.021'' in diameter, is straight annealed and subjected to thermal and mechanical characterization studies. The fatigue characteristics of the wires are studied by rotating bending fatigue testing.  The material studied in this work is melted by vacuum induction melting followed by vacuum arc re-melting (VIM-VAR). The nominal composition of the alloy is Ni_50.8Ti_49.2.