Influence of Ta Addition on Austenite Stability and Strain-Induced Martensite Transformation in Sintered Fe-7Mn Alloy

This study examines the effect of Ta addition on austenite stability and strain-induced martensitic transformation behavior in Fe–7Mn alloys fabricated by powder metallurgy. Fe–7Mn–xTa alloys (x = 0, 1, and 1.5 wt.%) were produced via mechanical alloying followed by spark plasma sintering, achieving nearly full relative density for all compositions. With increasing Ta content, the initial retained austenite fraction significantly increased, reaching 80.55 vol.% in the Fe–7Mn–1.5Ta alloy. EBSD analysis revealed a grain coarsening tendency with Ta addition, indicating that the increase in retained austenite fraction could not be explained solely by grain refinement. Compression tests up to 20% strain showed strain-induced martensitic transformation in all alloys, with substantially more pronounced transformation observed in the Fe–7Mn–1.5Ta alloy. The Burke–Matsumura–Tsuchida model showed that the austenite stability parameter (k), where higher values indicate lower stability, increased from 3.89 to 10.62 with Ta addition. Ta thus exhibits a dual effect: promoting retained austenite after sintering while reducing its deformation stability. The hardening efficiency per unit martensite fraction decreased with Ta content, and a preliminary correlation between k and hardening efficiency suggests that austenite stability governs the mechanical response of Fe–Mn-based alloys.