Abstract: In the present study, a single parameter governing the substructure and the strengthening for martensitic transformation was tentatively explored by detailing the microstructure and the strengthening of a Fe-15 wt.%Cr binary alloy subjected to thermal cycle under high pressure (cooled at 10 °C s–1 from 1050 °C under hydrostatic pressure of 1.0–4.0 GPa). Experimental results show that high pressure makes martensitic transformation occur in a Fe-15Cr alloy that traditionally has no high-temperature austenite under atmospheric pressure. The phase transformation begins with the pairing of twinned variants, and the strengthening is solely dependent upon the density of dislocations and variants. The austenite strength at the transformation temperature governs the substructure and the induced strengthening by influencing: (1) The critical size below which twinned variants are solely allowed; (2) the orientation spreading of the pioneer twinned variants toward Bain pairs; (3) the variant thickness and in turn the strengthening extent. The present study sheds light on tuning the substructure and hardening during martensitic transformation via the austenite strength, showing potential scientific and technological importance.

Key words: Austenite strength, Substructure and strengthening, Martensitic transformation, Twinned variants, High pressure