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

马氏体钢具有超高强度的特点，在汽车、航空航天、电力等行业得到了广泛的应用。尽管马氏体相变动力学具有广泛的技术重要性，但到目前为止，人们对马氏体相变动力学的理论了解还很有限。这主要是由于极快的相变动力学，这使得实验观察几乎是不可能的，而且由于复杂的对称操作在原子水平上活跃。目前只有一组经典的例子，特别是低相变应变的形状记忆合金，才能被人们所理解。相场法是揭示微观组织演化机制的一种很有前途的工具。与最先进的晶体塑性模型相结合，它可以考虑钢中马氏体相变的所有相关机制，并研究它们对组织形成和力学性能的影响。本课题对低碳钢的马氏体相变进行了三维相场研究。对于一组具有Kurdjuov-Sachs取向关系的24个马氏体对称变种，考虑了高达20%的全相变应变。我们的目标是揭示复杂的、层次化的板条马氏体微结构的形成机制。重点研究了自催化马氏体形核对显微组织的影响、塑性松弛对马氏体初始态和终态温度的影响以及完全相变后对力学性能的影响三个主要机制。通过单独考虑不同材料和工艺参数的影响，从冷却速度到弹性参数对合金成分的依赖，再到奥氏体塑性和马氏体塑性的影响，这项研究将为马氏体相变动力学提供新的见解，并解释板条马氏体组织的形成。

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.