Effect of Co Addition On the Microstructure, Martensitic Transformation and Shape Memory Behavior of Fe-Mn-Si Alloys

The shape memory effect observed in ternary Fe-Mn-Si and Fe-Mn-Co alloy systems is associated with austenite (γ) to ε martensite transformation. Although, a large numbers of research publications are available in literature on the shape memory behavior of these ternary alloys, there are none so far on the shape memory behavior of quaternary Fe-Mn-Si-Co alloys. In this work, the effect of Co addition on the microstructure, martensitic transformation and shape memory behavior of Fe-Mn-Si alloys has been studied. A series of Fe-30Mn-6Si-xCo (x = 0-9 wt.%) alloys with varying Co content was prepared by vacuum arc melting and characterized in terms of microstructure, mechanical behavior and shape memory property. The investigations revealed that the microstructure of Fe-Mn-Si-Co alloys remains single-phase austenite till 5% Co addition, beyond which it is a two-phase microstructure consisting of γ + (Fe,Co)₅Mn₃Si₂ intermetallic compound. It was observed that the γ-ε martensite transformation start temperature (M_s) decreases with Co addition, though the martensite to austenite start transformation temperature (A_s) is almost constant. Room temperature mechanical tests reveal that the yield strength of these alloys decreases with Co addition. The magnitude of shape recovery of Fe-Mn-Si-Co alloys is less than that of the Fe-30Mn-6Si alloy; however, Co addition appears to improve the shape recovery of these quaternary alloys.  This improvement in recovery is possibly related to the formation of higher amounts of stress induced ε martensite. This paper will try to rationalize these observations on the basis of microstructure, phase transformation, mechanical properties and the kinetics of martensite transformation predicted by the CALPHAD approach.