Retained austenite stability in third-generation advanced high-strength steels: Thermodynamic, mechanical, and kinetic frameworks for forming and crash performance

Third-generation advanced high-strength steels (3G-AHSS) exploit retained austenite (RA) to deliver exceptional strength–ductility via the TRIP effect, yet performance depends on when RA transforms during forming and crash. This review consolidates recent progress in quantifying RA stability through thermodynamic, mechanical, and kinetic descriptors. Key experimental tools, including in-situ diffraction, dilatometry, and calorimetry, are compared to clarify what each stability metric truly measures in practice today. We discuss composition-driven control of martensite-start temperature ( M s ) and show how processing-mediated partitioning and chemical heterogeneity make “effective M s ” path dependent. Mechanical stability is framed using M d concepts and stress-assisted transformation, emphasizing sensitivity to stress state, morphology (film versus blocky RA), and model-based prediction including data-driven approaches. Kinetic stability is treated as a time–temperature problem, covering thermal exposure, decomposition windows, and microstructural barriers that can either preserve RA or render it TRIP-inactive. Finally, we synthesize strain-rate effects relevant to crash loading, where adiabatic heating and rapid kinetics can shift transformation pathways and energy absorption. The paper closes with practical guidelines for alloy/process design and for characterization protocols that link laboratory metrics to component-level loading histories.