To exploit the full potential of advanced high-strength steels (AHSS), a more in-depth understanding of the complex micromechanical interaction of thin-film retained austenite (RA) and lath martensite is indispensable. Inspired by the medium-Mn steel microstructure, a three-dimensional micromechanical modeling approach is therefore proposed in the present work, embedding the thin RA films explicitly into the hierarchical lath martensite structure. This enables systematic studies of the effect of RA film thickness and volume fraction on the local stresses and strains as well as their partitioning within the microstructure. The investigations reveal that with shrinking RA volume fraction, both stress and especially strain heterogeneity in the thin-film RA intensifies. In the martensite blocks, stress and strain heterogeneity also intensifies, although stresses are generally more heterogeneously, and strains much more homogeneously, distributed than in RA. The results underline the key role of RA with thin-film morphology for further optimizing AHSS microstructures.

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.

The stability of retained austenite (RA) is a critical factor for enhancing the mechanical performance of advanced high-strength steels. This study experimentally investigated the influence of microstructural factors on RA stability, isolating them from the influence of chemical composition. This is done through a comparative analysis of two-phase (RA/martensite) and one-phase (austenite) microstructures with nearly identical austenite compositions in medium-Mn steels. This approach enabled a focused examination of microstructural factors influencing austenite stability without the influence of composition. The experimental results were further correlated with a thermodynamic Ms model to determine the significance of different microstructural factors.

A major part of this thesis is dedicated to understanding microstructural factors and their influence on austenite stability. A range of characterization techniques were employed. (1) Scanning electron microscopy (SEM) coupled with electron backscatter diffraction (EBSD) was used to characterize the apparent microstructure. (2) Dilatometry and X-ray diffraction (XRD) were used to assess austenite stability during cooling, and (3) In-situ high-energy X-ray diffraction tensile test was used to assess austenite stability during deformation. The results showed distinct microstructure effects on the thermal and mechanical stabilities of RA, signifying the importance of the microstructural effects on γ/RA stability. 

In addition, another part of this thesis explored the kinetics of lath martensite formation in Fe and Fe alloys based on ultra-rapid cooling experiments. These experiments provided rare isothermal information on the transformation, indicating that substitutional alloying elements such as Cr, Ni, and Ru have small effects on the rate of lath martensite formation, in contrast to the behavior observed for interstitial C. A mathematical model based on the Arrhenius equation was developed to predict the rate and temperature dependence of lath martensite formation in Fe-C alloys.

Stabiliteten hos restaustenit (RA) är en kritisk faktor för att förbättra den mekaniska prestandan hos avancerade höghållfasta stål. Denna studie undersökte experimentellt påverkan av mikrostrukturella faktorer på RA-stabiliteten, och isolerade dem från effekterna av kemisk sammansättning. Detta görs genom en jämförande analys av tvåfasiga (RA/martensit) och enfasiga (austenit) mikrostrukturer med nästan identiska austenitsammansättningar i medel-Mn stål. Detta tillvägagångssätt möjliggjorde en fokuserad undersökning av mikrostrukturella faktorer som påverkar austenitstabiliteten utan påverkan av sammansättning. De experimentella resultaten korrelerades ytterligare med en termodynamisk Ms-modell för att bestämma betydelsen av olika mikrostrukturella faktorer.

En stor del av denna avhandling är tillägnad förståelsen av mikrostrukturella faktorer och deras inverkan på austenitstabilitet. En rad tekniker användes. (1) Svepelektronmikroskopi (SEM) kopplad med elektronbackscatter-diffraktion (EBSD) användes för att karakterisera mikrostrukturen. (2) Dilatometri och röntgendiffraktion (XRD) användes för att bedöma austenitstabilitet under kylning, och (3) In-situ högenergiröntgendiffraktionsdragtest användes för att bedöma austenitstabilitet under deformation. Resultaten visade distinkta mikrostruktureffekter på den termiska och mekaniska stabiliteten hos RA, vilket indikerar vikten av de mikrostrukturella effekterna på γ/RA-stabiliteten.

Dessutom har kinetiken för lattmartensitbildning i Fe och Fe-legeringar baserat på ultrasnabba kylningsexperiment undersökts. Dessa experiment ger sällsynt isotermisk information om martensitbildning, och tyder på att substitutionella legeringselement så som Cr, Ni och Ru har små effekter på bildnings hastigheten av lattmartensit, i motsats till det komplexa beteende som observerats för interstitiellt C. En matematisk modell baserad på Arrhenius-ekvationen utvecklades för att förutsäga hastigheten och temperaturberoendet av bildining av lattmartensit i Fe-C-legeringar.