Distinction between thermal and stress-induced martensitic transformations and inhomogeneity in internal stress

Abstract

A concept of inhomogeneous internal stress called shape change stress (SCS) was introduced in martensitic transformations (MTs). The SCS was considered to be required because martensite plates formed on cooling must deform the surrounding untransformed parent. The concept enabled successful elucidation of the reason for the progress of thermally induced MTs (TIMTs) with changing temperature and for that of stress-induced MTs (SIMTs) under constant stress on the basis of the Gibbs phase role. Decreasing SCS with decreasing specimen size (mass) explained the downward shifts of experimental equilibrium temperatures T*0 = (Ms + Af)/2 with decreasing specimen mass m so far observed in the TIMTs of some shape memory alloys. Furthermore, the concept that no SCS is generated either in SIMTs or at the ultimate end of m = 0 in TIMTs rationalized the following observation on a monocrystalline Cu-13.4Al-4.2Ni (mass%) alloy. That is, the variations of Ms and Mf with m converged at the extrapolation to m = 0, and those of As and Af also did; Ms# = Mf# and As# = Af# (the superscript # indicates the transformation temperatures at the ultimate end of m = 0). The former coincided with TL (0) where the dependence of transformation stress σL on temperature was extrapolated to stress zero, and the latter coincided with TU (0) where a similar extrapolation in the case of reverse transformation stress σU was performed.