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Stacking faults, deformation-induced martensite and micromechanics of metastable austenite in steels studied by high-energy synchrotron X-ray diffraction

Time: Fri 2021-12-17 10.00

Location: Ångdomen, Osquars backe 31, Stockholm

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Language: English

Subject area: Materials Science and Engineering

Doctoral student: Benjamin Neding , Egenskaper

Opponent: Professor Dmytro Orlov, Lunds universitet

Supervisor: Professor Peter Hedström, Strukturer

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Austenitic stainless steels are known for their remarkable corrosion resistance and exhibit a very high ductility and toughness. They posses the face centered cubic crystal structure. Depending on the chemical composition of the austenite, the austenite may be metastable and during plastic deformation may undergo a deformation-induced phase transformation into α′-martensite - and that α′-martensite is partly responsible for the steel’s remarkable mechanical properties. In order to predict and control the occurrence and extent of the deformation-induced transformation into α′-martensite, it is crucial to have a profound knowledge of that transformation. Such knowledge is important to further improve austenitic stainless steels but also to contribute to the development of third generation advanced high strength steels that possess a multi-phase microstructure since the deformation behavior of the austenitic phase crucially influences their bulk deformation properties. Accordingly, in order to improve the current knowledge about the micromechanical behavior of steels and to predict the deformationbehavior of metastable austenite reliably, parameters affecting the austenite’s deformation behavior need to be described and quantified. This work contributes to such knowledge by investigating experimentally the effect of temperature, chemical composition, and grain orientationon the deformation behavior of austenite. The investigations were performed mainly with the aid of high-energy X-ray diffraction(HEXRD). HEXRD is a measurement method that allows to examine the materials’ response to plastic deformation in a bulk specimen non destructively. With HEXRD phases and their fraction evolution, latticestrains and stresses, and the stacking fault energy (γSF) can be followed in situ as the sample is subjected to load. Moreover, the high spatial resolution of HEXRD allows line profile analysis, i.e. to study the shape of diffraction peaks in order to quantify the formation of dislocations and stacking faults as well as their evolution during the course of deformation. Also, high-energy X-ray diffraction microscopy (HEDM) measurementwere conducted in order to resolve and follow the deformation behavior of individual grains embedded within the polycrystalline bulk during deformation. This is important to understand the effect of grain orientation, grain neighborhood, and grain morphology on the deformation behavior of individual grains and consequently the deformationbehavior of the bulk as a whole.

The temperature effect on γSF was studied both on powders of three austenitic model alloys with different alloy compositions using an in situ temperature HEXRD experiment and on a commercial 301LN bulk specimen with the aid of an in situ tensile loading experiment. It was found that the γSF increases significantly with increasing temperature. Moreover, the temperature induced increase of γSF significantly influences the predominantly active deformation mechanism. At low temperatures, large fractions of stacking faults, ε- and α′-martensite formed, which also reflects on the properties of the steel by a high work hardening rate. With increasing temperature, and consequently increasing γSF, the formation of stacking faults, ε- and α′-martensite becomes less predominant. As a result a significant decrease in work hardening with increasing temperature was observed. Moreover, it was found, that at elevated temperatures, the dissociation of dislocation into partial dislocation occurs at significantly higher strain. In addition to temperature, grain orientation was found to affect the deformation behavior of austenitic steels substantially. Grains deformed along [100] form predominantly stacking faults, whereas grains deformed along [111] mainly deform via dislocation glide. Grain orientation also played a key role in the formation of deformation-induced phases. Crystalline austenitic regions oriented with their {111} at 45° to external load were found to transform preferentially into ε-martensite before further transforming into α′-martensite, whereas crystalline austenitic regions oriented with their {111} at 0° and 90° to the load, transformed directly into α′-martensite, without transforming into ε-martensite first.The knowledge acquired by studying single phase austenitic steel was expanded to medium Mn steels (MMnS), possessing a multi-phase microstructure. It was found that the average bulk deformation behavior of medium Mn steels is crucially affected by the interdependencies between the micromechanical deformation behavior and the stability of the austenite, which can be controlled by tuning microstructure and austenite composition.The contribution of this work is to increase the knowledge of the deformation-induced martensitic phase transformations of metastable austenite, its dependence with γSF, temperature, and the correlation with parameters affecting the deformation behavior in the bulk which are not considered in the γSF.