Pushing The Limit to Achieve NiTi SMA Actuating Members that are Dimensionally Stable and have High Transformation Temperatures

Abstract

AbstractIn binary NiTi shape memory alloys (SMA), the highest martensite (M) and austenite (A) transformation temperatures (TT) occur in the annealed condition. The highest TT also occur in near equiatomic NiTi alloys that have an excess of Ti. However, the NiTi alloy composition and condition that have the highest TT produce actuating elements that are generally short lived, have poor mechanical properties, and plastically deform (creep) under low stress levels. Cold working the SMA followed by a memory imparting stress relieving heat treatment (HT) produces actuating elements that are long lived, have good mechanical properties, and are resistant to creep under moderate stress levels. However, in obtaining these desirable properties through thermal-mechanical processing, the M and A TT are significantly decreased, which limits the upper ambient temperature in which the actuating element can operate.A dimensionally stable actuating member with high TT can be achieved by thermal cycling (under stress) a NiTi SMA wire that has received prior thermal-mechanical processing. Cycling under an applied axial stress can increase the M TT of a SMA wire. Data showing the influence of thermal cycling on the TT of axially stressed SMA wires, that were cold drawn followed by HT at different memory imparting temperatures, are presented and discussed. For a NiTi SMA wire (A finish TT = 111 °C in the annealed condition) having approximately 40% cold reduction in area, 400°C for 1 hour memory imparting HT, and 10 Ksi axial stress, the M start TT (Ms) and A finish TT (Af) increase from 26°C and 79°C respectively after 10 thermal transformation cycles to 62 °C and 83.5 °C respectively after 10,000 thermal transformation cycles.