In-situ high-energy synchrotron X-ray diffraction experiments during uniaxial tensile loading are performed to investigate the effect of temperature (25, 45 and 70 degrees C) on the deformation behavior of a 301 metastable austenitic stainless steel. The micromechanical behavior of the steel at the three deformation temperatures is correlated with the stacking fault energy (gamma(SF)) experimentally determined through the same in-situ X-ray experiments. The applied measurements provide a unique possibility to directly interrogate the temperature dependent gamma(SF) in relation to the active bulk deformation mechanism in a metastable austenitic stainless steel. The determined gamma(SF) is 9.4 +/- 1.7 mJ m(-2) at 25 degrees C, 13.4 +/- 1.9 mJ m(-2) at 45 degrees C and 25.0 +/- 1.1 mJ m(-2) at 70 degrees C. This relatively minor change of gamma(SF) and temperature causes a significant change of the dominant deformation mechanism in the alloy. At room temperature (25 degrees C) significant amounts of stacking faults form at 0.05 true strain, with subsequent formation of large fractions of deformation-induced alpha' and epsilon-martensite, 0.4 and 0.05, at 0.4 true strain, respectively. With increasing temperature (45 degrees C) fewer stacking faults form at low strain and thereupon also smaller alpha' - and epsilon-martensite fractions form, 0.2 and 0.025, at 0.4 true strain, respectively. At the highest temperature (70 degrees C) plastic deformation primarily occurs by the generation and glide of perfect dislocations at low strain, while at higher strain these dislocations dissociate to form stacking faults. The alpha'-martensite fraction formed is significantly less at 70 degrees C reaching 0.1 at 0.4 strain, whilst epsilon-martensite is not found to form at any strain at this temperature. The temperature-dependent mechanical behavior of the alloy is consistent with the observed dominant deformation mechanisms; the strong work hardening from the TRIP effect at low temperature, and low gamma(SF), decreases significantly with increasing temperature, and gamma(SF).

The mechanical response, on a microscopic and macroscopic level, and the deformation-induced martensitic transformation (DIMT) were investigated in multi-phase medium Mn steels (MMnS) with 6, 8 and 9 wt% Mn using in situ high-energy synchrotron x-ray diffraction during tensile loading. Prior to the in-situ analysis, a similar heat treatment finishing with an intercritical annealing was imposed on all MMnS. The initial microstructure prior to tensile loading was investigated by electron backscatter diffraction analysis. The volume fraction of austenite (gamma) after the heat treatment decreases from 60.2% to 50.7%, and 23.6% with increasing Mn content from 6 to 8 and 9 wt% Mn, respectively. This is mainly due to the difference in the formation of athermal alpha '-martensite. Athermal epsilon-martensite also formed in the MMnS with 8 and 9 wt% Mn, whereas no athermal epsilon-martensite formed in the MMnS with 6 wt% Mn. The alloys have quite different deformation behavior due to the different microstructures, and the majority of the load is carried by the phase that forms a continuous network throughout the steel, which in turn influences the DIMT. These results reveal the importance of assessing both phase-specific strain/stress and the inherent mechanical stability of the austenite in order to predict the macroscopic mechanical properties of the steel. As an example, this is witnessed by the comparison of MMnS9 and MMnS8. Austenite in MMnS9 bears about half the load as compared to austenite in MMnS8 during early deformation due to a continuous network of athermal alpha '-martensite resulting in significant load partitioning from austenite to alpha '-martensite. Thus, the mechanical driving force for DIMT in MMnS9 is reduced and therefore causes lower DIMT kinetics in MMnS9 than in MMnS8, even though MMnS9 has lower inherent austenite stability.

A Cu precipitation-mediated austenitic transformation during ageing treatment of a 15 & ndash;5 PH stainless steel is revealed through atom probe tomography, in situ synchrotron X-ray diffraction and computational thermodynamics and kinetics. The austenitic transformation is proposed to occur through the pathway: Cu precipitation at the martensite/retained austenite interfaces or at martensite lath boundaries -> partitioning of austenite stabilizing elements towards interfaces of the Cu precipitates -> reverted austenite formation.

The formation of stacking faults and dislocations in individual austenite (fcc) grains embedded in a polycrystalline bulk Fe-18Cr-10.5Ni (wt.%) steel was investigated by non-destructive high-energy diffraction microscopy (HEDM) and line profile analysis. The broadening and position of intensity, diffracted from individual grains, were followed during in situ tensile loading up to 0.09 strain. Furthermore, the predominant deformation mechanism of the individual grains as a function of grain orientation was investigated, and the formation of stacking faults was quantified. Grains oriented with [100] along the tensile axis form dislocations at low strains, whilst at higher strains, the formation of stacking faults becomes the dominant deformation mechanism. In contrast, grains oriented with [111] along the tensile axis deform mainly through the formation and slip of dislocations at all strain states. However, the present study also reveals that grain orientation is not sufficient to predict the deformation characteristics of single grains in polycrystalline bulk materials. This is witnessed specifically within one grain oriented with [111] along the tensile axis that deforms through the generation of stacking faults. The reason for this behavior is due to other grain-specific parameters, such as size and local neighborhood.

The dependence of stacking fault energy (γSFE) on temperature in austenitic Fe–Cr–Ni alloy powders was investigated by in situ high energy synchrotron X-ray diffraction and ab initio calculations in the temperature range from −45 °C to 450 °C. The X-ray diffraction peak positions were used to determine the stacking fault probability and subsequently the temperature dependence of γSFE. The effect of temperature on the diffraction peak positions was found to be mainly reversible; however, recovery of dislocations occurred above about 200 °C, which also gave an irreversible contribution. Two different ab initio-based models were evaluated with respect to the experimental data. The different predictions of the models can be explained by their respective treatment of the magnetic moments for Cr and Ni, which is critical for the alloy compositions investigated. Ab initio calculations, taking longitudinal spin fluctuations (LSF) into consideration within the quasi-classical phenomenological model, predict a temperature dependence of γSFE in good agreement with the experimentally evaluated trend of increasing γSFE with increasing temperature: |ΔγSFE/ΔT|=0.05mJm−2/K. The temperature effect on γSFE is similar for all three investigated alloys: Fe–18Cr–15Ni, Fe–18Cr–17Ni, Fe–21Cr–16Ni (wt pct), while their room temperature γSFE are evaluated to be 22, 25, 20 mJ m−2, respectively.