Development of Ni-Ti based shape memory alloys for actuation and control

This paper compares shape memory alloys (SMA) with other “smart” materials such as piezoelectric, magnetostrictive and electrorheological materials, and discusses the ongoing collaborative research between Auburn University and Johnson Controls, Inc. Discussion of the attributes and present challenges in the use of SMA in engineering components is given and emphasis is placed on martensite transformation characteristics of NiTi and NiTiHf shape memory alloys. The use of SMAs in actuator applications depends on the phase transformation temperatures of the SMAs and their stability as an actuating member. The objective of this study is to investigate the influence of thermal-mechanical processing on the martensitic transformation temperature of NiTi and NiTiHf shape memory alloys. Since the martensitic transformation is responsible for the shape memory effect, results from this study are used to design alloys and heat treatment procedures that will yield the desired switching properties. A Ti-rich binary NiTi shape memory alloy and two ternary SMAs having the compositions Ni₄₉Ti_51-xHf_x with 1 at.% and 3 at.% Hf are investigated. The influence of Hf content, cold work (CW) and heat treatment (HT) temperature on the martensitic transformation in these three SMAs has been thermally analyzed using differential scanning calorimetry (DSC). Results show that, at low HT, the martensitic transformation temperature decreases with increasing CW for all the SMAs studied. At high HT, the martensite transformation temperature is almost insensitive to CW