Effect of Pre-straining on the Shape Recovery of Fe-Mn-Si-Cr-Ni Shape Memory Alloys

The effect of pre-straining on the shape recovery behavior of Fe-14Mn-6Si-9Cr-5Ni (wt pct) shape memory alloy (SMA) has been studied. The shape recovery associated with the reverse ε martensitic transformation, i.e., ε → γ, was characterized by dilatometry using specimens which were pre-strained to different extent (0 to 14 pct). Dilatometric studies revealed that in Fe-Mn-Si-Cr-Ni SMA, the shape recovery takes place in two stages: (i) in the first stage, the unpinned fraction of stress-induced ε martensite reverts back to parent phase γ in the temperature regime of 353 K to 653 K (80 °C to 380 °C) and (ii) in the second stage the remaining “pinned” ε martensite is unpinned by the decomposition of deformation-induced α′ martensite in the temperature range of 743 K to 893 K (470 °C to 620 °C). The amount of recovery in the first stage decreases with pre-strain, whereas it increases in the second stage. The ε → γ transformation finish temperature, Af, increases with increase in pre-strain amount, though the reverse transformation start temperature, AS, remains unaffected. Microstructural characterization revealed that the amount of deformation-induced α′ martensite depends on the mode of straining and the crystallographic texture of the starting material. The reversion of α′ martensite is seen to occur by the precipitation of Fe5Ni3Si2-type intermetallic π-phase within these plates.