Adjacent phase-governed stability of retained austenite and hetero-deformation-induced hardening in TRIP-assisted steels

The phase constituent is pivotal in modulating the transformation-induced plasticity (TRIP) effect and hetero-deformation-induced (HDI) hardening, which govern the work-hardening behavior of multi-phase TRIP-assisted steels. This study investigated retained austenite (RA) mechanical stability and the distinct work-hardening responses of two TRIP-assisted steels with contrasting matrices: a ferrite-based matrix (TRIP-F) and a bainite-based matrix (TRIP-B), both featuring comparable volume fractions and characteristics of RA. The results reveal that about 44.6% RA grains in the TRIP-F sample were surrounded by soft ferrite, which possessed significantly lower mechanical stability than the TRIP-B sample. It is attributed to the diminished hydrostatic constraint provided by the ferrite matrix, leading to the premature onset of martensitic transformation. Crucially, the pronounced strain partitioning between the soft ferrite matrix and the hard strain-induced martensite in the TRIP-F sample triggers substantial HDI hardening. The HDI hardening emerges as the dominant mechanism in the later deformation stage, sustaining the high work-hardening rate even after the exhaustion of the TRIP effect. In contrast, the minimal hardness gradient between the bainite matrix and RA in the TRIP-B sample restricts the development of the HDI stress. Thus, the work hardening behavior of the TRIP-B sample was primarily governed by the TRIP effect. This work provides new insights into tailoring phase constituents to optimize the hardening mechanisms for enhanced mechanical properties of advanced steels.