On the contrasting effects of parent austenite grain size on athermal and deformation-induced martensitic transformation in an Fe-18Cr-12Ni (wt.%) alloy

The parent austenite grain size (PAGS) plays a critical role in governing the austenite to martensite transformation in steels, as it directly influences the stability of the austenite phase. Therefore, a thorough understanding of the PAGS effect is essential for the design of advanced steels with optimized performance. In this study, we investigate the influence of PAGS on both athermal and deformation-induced martensitic transformation (AMT and DIMT) in an Fe-18Cr-12Ni (wt.%) alloy. In situ and ex situ high-energy synchrotron X-ray diffraction (HEXRD) measurements were conducted during tensile loading and after cooling, respectively. These measurements were complemented by electron microscopy to elucidate the underlying mechanisms governing the PAGS effect on martensitic transformation. The results reveal PAGS exerts opposite effects on AMT and DIMT. Grain refinement increases the barrier for martensitic transformation onset, and in the case of AMT, this leads to a lower martensite start temperature (Ms) and suppressed transformation. Conversely, although grain refinement delays DIMT and raises the critical stress required for its initiation, the associated Hall-Petch strengthening enhances the flow stress, thereby promoting DIMT at higher strain levels. These findings offer valuable insights for the microstructural design of austenitic steels and contribute to the modeling of martensitic transformation behavior under varying mechanical and thermal conditions.