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

Abstarct 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, A f , increases with increase in pre-strain amount, though the reverse transformation start temperature, A S , 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 Fe 5 Ni 3 Si 2 -type intermetallic π -phase within these plates.