Effect of stacking fault probability on γ– ε martensitic transformation and shape memory effect in Fe–Mn–Si based alloys

Abstract

As an important parameter in Fe–Mn–Si based shape memory alloys, the stacking fault probability of γ-phase, P sf, has been determined by means of X-ray diffraction profile analysis and connected with some macro-behavior such as the starting temperature of the thermo-induced γ→ ε transformation, M s, the critical stress required for inducing ε-martensite, σ M, the strain-hardening exponent, n, etc. It is revealed that a linear relationship between M s and the reciprocal of P sf is established as M s=372−0.113/ P sf, and also between σ M and the reciprocal of P sf, as σ M=29+0.212/ P sf. It is shown that the strain-hardening exponent n changes monotonically with P sf. Adding Cr or N, and increasing the content of Mn in a Fe–30Mn–6Si alloy will decrease P sf, resulting in the reducing of M s and n, and the increasing of σ M. The existence of quenched-in vacancies will promote the enhancement of P sf, that leads to the increase in M s. The variation of γ-grain size has no obvious effect on P sf, so that it does not affect the M s temperature. The thermo-mechanical cycling will increase P sf, in turn will lower σ M and increase n, so that the shape memory effect is improved. The possible mechanism is discussed.