The stability of retained austenite is a key factor in the design of advanced high-strength steels that exhibit excellent mechanical performance, including high strength and high ductility/toughness. However, the contribution of certain microstructural factors, such as the morphology and size of the austenite, and the surrounding matrix, to this stability is still not fully understood, partly due to the inherent difficulties in separating these factors in multiphase microstructures. Therefore, this study uniquely compared the stabilities of retained austenite in two-phase microstructures with bulk austenitic microstructures of the same composition, across four medium-Mn steels upon quenching. By fixing the austenite chemical composition, we could exclude the influence of composition and examine the influence of other factors, such as morphology, size, and the surrounding matrix, on the stability of austenite. Our experimental results showed that retained austenite in the two-phase microstructures was more stable than the bulk austenitic microstructures of the same composition, regardless of morphology and size. Analysis using thermodynamic calculations revealed that neither the steel composition nor the size alone could explain the high stability of the retained austenite in the two-phase microstructures. Instead, we propose that microstructural factors, including size, morphology, and matrix, have a significant influence on the metastable austenite in two-phase microstructures. While these factors have been studied previously, our study introduces a novel perspective by excluding the influence of the austenite composition, thus contributing to a more comprehensive understanding of retained austenite stability. These findings may guide the design of advanced steels and highlight the importance of considering the contribution of these microstructural factors in tailoring the stability of metastable austenite.

Given the critical role that metastable retained austenite (RA) plays in advanced high-strength steel (AHSS), there is significant interest in obtaining a comprehensive understanding of its stability, to achieve excellent mechanical properties. Despite considerable attention and numerous studies, the significance of individual contributions of various microstructural factors (size, crystallographic orientation, surrounding phases, etc.) on the stability of RA remain unclear, partly due to the difficulty of isolating the direct effects of these factors. In this study, we examined the influence of microstructural factors while minimizing the effect of chemical composition on the mechanical stability of RA. We accomplished this by comparing the austenite (γ) stability in two distinct microstructures: a two-phase RA/martensite microstructure and a one-phase γ microstructure, both with nearly identical γ compositions. We employed in situ high-energy X-ray diffraction during uniaxial tensile testing conducted at both room temperature and 100 °C, facilitating the continuous monitoring of microstructural changes during the deformation process. By establishing a direct correlation between the macroscopic tensile load, phase load partitioning, and the γ/RA transformation, we aimed to understand the significance of the microstructural factors on the mechanical stability of the RA. The results indicate that very fine RA size and the surrounding hard martensitic matrix (aside from contributing to load partitioning) contribute less significantly to RA stability during deformation than expected. The findings of this study emphasize the critical and distinct influence of microstructure on γ/RA stability.

Isothermal information is rarely available for the formation of martensite in Fe or Fe alloys due to a very high rate of transformation compared to the rate of heat conduction. Such information has now been extracted for lath martensite in some sets of Fe alloys from available information on ultra-rapid quenching but only at a single temperature for each alloy, related to its two MS temperatures. The temperature dependence could, thus, be studied only on binary sets of alloys. Those results have been applied to mathematical models based on the Arrhenius equation and illustrated with Arrhenius plots. For three sets of binary Fe alloys, a large group of rates came close to the rate of an almost pure and carbon-free Fe-C alloy. It illustrated that Cr, Ni, and Ru in low contents have relatively small effects on the rate of formation of lath martensite in Fe. It also demonstrated that the present measurements have considerable reproducibility. In contrast, a set of Fe-C alloys did not give a straight line in the Arrhenius plot. Using a new mathematical model based on the concept of the Arrhenius equation to express the effect of carbon, it was possible to predict the rate of formation of lath martensite for Fe-C alloys with fixed C content and their temperature dependencies which are not available experimentally due to the very high rate of formation.