Study of tensile behaviour of Fe base shape memory alloys during mechanical cycling

Abstract

The governing mechanism of Shape Memory Alloys (SMAs) relays on a reversible martensitic transformation between parent (austenite) and stress induced martensite phases. Martensite transformation is a first order phase transition which involves the presence of an invariant (habit) plane which serves as an interface between the two transforming phases. In the specific case of FeMnSi-base SMAs the formation and reversion of ϵ-hexagonal close 7packed (hcp) stress induced martensite to γ-face centred cubic (fcc) austenite proceeds by stacking fault migration. However, the reaction is not perfectly reversible, in such a way that an amount of retained stress induced martensite, that does not revert to austenite during unloading, is accumulated after each loading-unloading cycle and this evolution is directly related to that of permanent strain. The present paper investigates the relationship between macroscopic and microscopic effects of mechanical cycling. In this purpose, tensile loading-unloading tests were applied to specimens prepared from Fe-18Mn-3Si-7Cr-4Ni SMA (mass. %) by powder metallurgy routine. The tensile tests were applied by means of two experimental test protocols: (i) with constant maximum stress and (ii) with constant maximum strain. In each case, the effect of the number of applied cycles was investigated, while monitoring the evolution of total and permanent strains and of maximum stress and permanent strain, respectively. By means of dedicated software, the variation tendencies of the above mechanical parameters, with the number of cycles have been determined. Finally, the evolution of stress induced martensite has been discussed as a result of microstructural analysis.