High temperature shape memory alloys

Shape memory alloys (SMAs) with high transformation temperatures can enable simplifications and improvements in operating efficiency of many mechanical components designed to operate at temperatures above 100 degrees C, potentially impacting the automotive, aerospace, manufacturing and energy exploration industries. A wide range of these SMAs exists and can be categorised in three groups based on their martensitic transformation temperatures: group I, transformation temperatures in the range of 100-400 degrees C; group II, in the range of 400-700 degrees C; and group III, above 700 degrees C. In addition to the high transformation temperatures, potential high temperature shape memory alloys (HTSMAs) must also exhibit acceptable recoverable transformation strain levels, long term stability, resistance to plastic deformation and creep, and adequate environmental resistance. These criteria become increasingly more difficult to satisfy as their operating temperatures increase, due to greater involvement of thermally activated mechanisms in their thermomechanical responses. Moreover, poor workability, due to the ordered intermetallic structure of many HTSMA systems, and high material costs pose additional problems for the commercialisation of HTSMAs. In spite of these challenges, progress has been made through compositional control, alloying, and the application of various thermomechanical processing techniques to the point that several likely applications have been demonstrated in alloys such as Ti-Ni-Pd and Ti-Ni-Pt. In the present work, a comprehensive review of potential HTSMA systems are presented in terms of physical and thermomechanical properties, processing techniques, challenges and applications.