Thermodynamic Explanation for the Large Difference in Improving Shape Memory Effect of Fe–Mn Alloys by Co and Si Addition

Abstract

The authors investigate the shape memory effect (SME) and microstructure evolution of the Fe-25Mn, Fe-25Mn-6Co, and Fe-25Mn-6Si alloys before and after deformation at room temperature (RT) through ex situ optical observation. After deformation, the stress-induced reverse transformation of thermal e martensite, an unrecovered deformation mode, can be easily observed in the Fe-25Mn and Fe-25Mn-6Co alloys, but it can be hardly observed in the Fe-25Mn-6Si alloy. The lower values of chemical driving force for e ! g transformation at RT lead to the occurrence of the stress-induced reverse transformation of thermal e martensite and the poorer SME in the Fe-Mn and Fe-Mn-Co alloys than in the Fe-Mn-Si alloy.

Key Factors Achieving Large Recovery Strains in Polycrystalline Fe–Mn–Si‐Based Shape Memory Alloys: A Review

“…Recently, based on thermodynamic calculated results, Chen et al revealed that Si can remarkably increase the chemical driving force of FCC → HCP martensitic transformation at RT, but Co cannot do that, as shown in Figure c . The reason is that Si exhibits a stronger capability of lowering the T N temperature than Co. Consequently, Si addition can remarkably reduce magnetic free energy difference (Figure b).…”

“…The reason is that Si exhibits a stronger capability of lowering the T N temperature than Co. Consequently, Si addition can remarkably reduce magnetic free energy difference (Figure b). As the result of the lower chemical driving force of FCC → HCP martensitic transformation at RT, an unrecovered deformation mode, that is, the stress‐induced reverse martensitic transformation of thermally‐induced HCP martensite, took place in Fe–25Mn and Fe–25Mn–6Co alloys, when deformed at RT . By contrast, it was difficult for the occurrence of stress‐induced reverse martensitic transformation of thermally‐induced HCP martensite in a Fe–25Mn–6Si alloy .…”

“…As the result of the lower chemical driving force of FCC → HCP martensitic transformation at RT, an unrecovered deformation mode, that is, the stress‐induced reverse martensitic transformation of thermally‐induced HCP martensite, took place in Fe–25Mn and Fe–25Mn–6Co alloys, when deformed at RT . By contrast, it was difficult for the occurrence of stress‐induced reverse martensitic transformation of thermally‐induced HCP martensite in a Fe–25Mn–6Si alloy . Accordingly, the SME must be better in the Fe–25Mn–6Si alloy than in the Fe–25Mn and Fe–25Mn–6Co alloys.…”

“…Accordingly, the SME must be better in the Fe–25Mn–6Si alloy than in the Fe–25Mn and Fe–25Mn–6Co alloys. Furthermore, they proposed that the chemical driving force of FCC → HCP martensitic transformation should be as large as possible for obtaining a good SME in Fe – Mn – Si‐based SMAs …”

“…The in the Fe–Mn‐based alloys can be calculated by the following equation: . (Reproduced with permission from ref . Copyright 2016, John Wiley and Sons)…”