Intra-regional strain partitioning within initial austenite regions in medium Mn steels

Metastable austenite (γ) plays a critical role in determining the mechanical response of advanced steels. However, despite the wide recognition of interphase strain partitioning between γ and the constituent phases, the evolution of strain heterogeneity within the initial γ regions during deformation remains poorly understood. Here, using in situ scanning electron microscope tensile tests combined with microscopic digital image correlation, we systematically investigate the grain-scale spatiotemporal evolution of microstrain in intercritically annealed medium Mn steels with and without 0.1 wt.% V, with particular focus on the initial γ regions where martensite (α′) forms during deformation via strain-induced martensitic transformation (SIMT). We reveal a dual strain partitioning behavior, in which intra-regional strain partitioning within the γ/α′ regions is more pronounced than interphase strain partitioning between the ferrite (α) and γ/α′ regions. We further show that the initial γ grains exhibit crystallographic orientation-dependent deformation responses, with high-Schmid-factor grains favoring stronger intragranular-dominated plasticity, whereas low-Schmid-factor grains deform mainly through intergranular accommodation, thereby establishing an early local strain contrast. This strain contrast is then amplified by SIMT and redistributed by interface-mediated strain transfer, leading to pronounced intra-regional strain partitioning. V microalloying further intensifies this strain partitioning by promoting SIMT and increasing phase mechanical contrast. These findings demonstrate that, beyond the conventionally emphasized interphase strain partitioning, intra-regional strain partitioning within the initial γ regions is a key mesoscale deformation feature in medium Mn steels, extending the current understanding of deformation heterogeneity in γ-containing advanced steels.