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

As an important parameter in Fe-Mn-Si based shape memory alloys, the stacking fault probability of gamma-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 gamma --> epsilon transformation, M-s, the critical stress required for inducing epsilon-martensite, sigma(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 sigma M and the reciprocal of P-sf, as sigma(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 sigma(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 gamma-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 sigma M and increase n, so that the shape memory effect is improved. The possible mechanism is discussed. (C) 1998 Acta Metallurgica Inc.