Stress/temperature phase diagrams as a tool for shape memory alloy selection and processing

Abstract
One of the difficulties of applying shape memory alloys to real applications is dealing with how applied stress affects the transformation temperatures and phase sequence. This paper presents stress-temperature phase transformation diagrams of the type described by Todoroki, Tamura and Suzuki [l]. The data employed to construct these diagrams relates to a previous study of binary NiTi alloys that have varied alloy compositions, prior cold work and heat treatment temperatures [2]. The diagrams presented in this paper graphically display how phase sequence as well as transformation temperatures vary with processing and alloying parameters. In addition, it is shown that the stress-temperature diagrams change after repeated thermal transformations against applied stress. Increasing M_s temperatures and decreasing A_s temperatures result in narrower thermal hysteresis and loss of the intermediate R-phase.