Effect of Prior Deformation of Austenite on the γ→ε Martensitic Transformation in Fe–Mn Alloys

The influence of prior deformation of austenite (γ) on the formation of hcp (ε) martensite in Fe–Mn binary alloys during subsequent cooling was investigated. The formation of bcc (α′) lath martensite in deformed austenite was also examined for comparison. In both of the martensitic transformations to α′ in the Fe-9%Mn alloy and to ε in the Fe-16%Mn and Fe-24%Mn alloys, the transformation start temperature (Ms) monotonously decreases with increasing the reduction of austenite at 773 K. It is demonstrated that the decrease in Ms temperature for the γ→ε transformation is much larger than that for the γ→α′ transformation. The Ms temperature of the Fe-24%Mn alloy is lowered over 80 K by 40% reduction and the structure becomes fully austenitic at room temperature, whereas the decrease in Ms temperature of the Fe–9Mn alloy is 16 K. The decrease in Ms temperature by the prior deformation of austenite is quantitatively accounted for by the strengthening of austenite and the extra energy required for the transformation dislocations to advance through forest dislocations in austenite. The marked stabilization of austenite against the γ→ε martensitic transformation is mainly associated with the smaller increase in chemical driving force for the γ→ε transformation with decreasing temperature than that for the γ→α′ transformation.