Phase-field modelling of martensitic transformation in low alloyed steel considering the effect of large elasto-plastic deformation

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Martensitic steel is characterized by its ultra-high strength, finding its application in automotive, aerospace, power and other industries. Despite its wide technical importance, theoretical understanding of martensitic transformation kinetics is limited till today. This is mainly due to the extremely rapid transformation kinetics, which makes experimental observation almost impossible, and due to the complex symmetry operations active at the atomic level. Only a set of classical examples, in particular in shape memory alloys with low transformation strains, can be considered as well understood today.The phase-field method is a promising tool for unravelling mechanisms of microstructure evolution in general. Coupled to the state of the art crystal plasticity models, it allows taking into account all relevant mechanisms of martensitic transformation in steels, and investigating their influence on the microstructure formation and mechanical properties. In this project, a three-dimensional phase-field study of martensitic transformation in low-carbon steel is proposed. The full transformation strains of up to 20% are considered for a set of 24 martensite symmetry variants with Kurdjumov-Sachs orientation relationship. We aim to revealing the mechanisms responsible for the formation of the complex, hierarchical lath-martensite microstructure. The focus lies on three major mechanisms, namely the effect of autocatalytic martensite nucleation on the resulting microstructure, the effect of plastic relaxation on the martensite start and finish temperature and its effect on mechanical properties after complete transformation. This study will provide new insight into the kinetics of martensitic transformation and explain the formation of lath martensite microstructure by allowing to individually consider the influence of various material and process parameters, ranging from cooling rate through the elasticity parameters dependence on the alloys composition to the effect of plasticity in austenite and martensite.