Relationship between austenite dislocation density introduced during thermal cycling and M_s temperature in an Fe-17 Wt Pct Mn alloy

The reason why thermal cycling decreases the martensite start (M_s) temperature of an Fe-17 wt pct Mn alloy was quantitatively investigated, based on the nucleation model of ε martensite and a thermodynamic model for a martensitie transformation. The M_s temperature decreased by about 22 K after nine cycles between 303 and 573 K, due to the increase in shear-strata energy (ΔG_sh) required to advance the transformation dislocations through dislocation forests formed in austenite during thermal cycling. The ΔG_sh value increased from 19.3 to 28.8 MJ/m³ due to the increase in austenite dislocation density from 1.5 × 10¹² to 3.8 × 10¹³/m² with the number of thermal cycles (in this case, up to nine cycles). The austenite dislocation density increased rapidly for up to five thermal cycles and then increased gradually with further thermal cycles, showing a good agreement with the increase in austenite hardness with the number of thermal cycles.