Structural Changes Associated with the Pseudoelastic Response of Fe-Based Shape Memory Alloys

Abstract

The pseudoelastic responses of two types of iron base shape memory alloys (SMAs) were introduced and discussed. The former was based on Fe-Mn-Si system, obtained by classical (CM) and by powder metallurgy (PM) manufacturing. The latter was based on Fe-Ni-Co system being processed by a non conventional technology comprising melt spinning and heat treatment. In the case of FeMnSibased SMAs, CM specimens obviously experienced larger ductility and a more pronounced pseudoelastic response while PM specimens were stiffer and underwent larger work-hardening. On the other hand, melt spun FeNiCo-based SMAs revealed an outstanding superelasticity in the case of micro-indentation tests. By means of scanning electron microscopy (SEM) observations, a martensitic morphology was identified in FeMnSi-based SMAs while FeNiCo-based SMAs revealed an austenitic structure. The presence of both α’ and ε martensites was confirmed in FeMnSibased SMAs by means of X-ray diffraction (XRD). In fully austenitic melt-spun and aged FeNiCo-based SMAs, no martensite was indentified on XRD patterns. These results sustain the conclusion that FeMnSi-based SMAs, that contain pre-existing martensite, experienced a pseudoelastic behavior caused by crystallographic reorientation of martensite plate variants while austenitic FeNiCo-based SMAs experienced a reversible stress-induced martensitic transformation, at room temperature.