High pressure effect on the substructure and hardness of IF steel during martensitic transformation

Highlights: • High pressure raises the propensity for fine twinned variants during martensitic transformation, and (block) boundary strengthening and (forest) dislocation strengthening lead to substantial hardening from 80 to 780 HV independent of interstitial atoms. In the present investigation, the microstructure and hardening of an IF steel after one thermal cycle (heating to 1050°C and holding for 30 min followed by 10°C/s cooling to room temperature) under hydrostatic pressure of 1-5 GPa were studied. Experimental results show that typical lath martensite was induced, giving rise to significant hardening from 80 to 780 HV. The lath martensite shows hierarchical packet-block-lath structure and obeys the classical K-S orientation relationship between martensite and austenite. High pressure influences the propensity and size of martensitic variants. As the pressure increases from 1 to 5 GPa: 1) single-variant blocks gradually replace those with dual-variants of same Bain group; 2) twin-related variants become predominant; 3) variants decrease their thickness from micron- to nano-scale. The hardening mechanism was analyzed assuming a linear additivity of carbon-independent contributions from (block) boundary strengthening and (forest) dislocation strengthening. High pressure was proposed as an effective method to widely tune the martensitic transformation independent of alloy element, showing potential scientific and technological importance.