Micromechanical Simulation of Initiation and Propagation of Cleavage Cracks in the Ductile-brittle Transition Region

随着温度的降低，典型的工程材料，如大多数钢，变得越来越脆。其原因是微观失效模式由微孔洞的延性长大转变为穿晶解理。体心立方金属由延性机制向脆性机制转变而导致的宏观断裂韧性下降是工程应用中的一个严重问题，因为在制造或使用过程中形成的裂纹状缺陷会显著降低构件的强度和寿命。在韧脆转变区，单晶和多晶的断裂韧度测量值相差几个数量级。据作者所知，目前还没有任何模型可以解释这种差异。本研究项目的目的是为理解这一现象作出重大贡献。为此，应建立一个细观力学有限元模型，以颗粒(偏析、夹杂物)和颗粒的形式离散地解析铁素体钢裂纹尖端的微观组织。在模型扩展的不同阶段，应研究和量化相应的子机制的影响，如颗粒脱粘、颗粒断裂、晶界和颗粒位错堆积以及相邻颗粒解理面的不相容。在此基础上，优化显微组织，提高断裂韧性。

With decreasing temperature typical engineering materials like the most steels become more and more brittle. The reason is the transition of the microscopic failure mode from ductile growth of micro-voids to transgranular cleavage of grains. The drop of the macroscopic fracture toughness caused by the transition from the ductile to the brittle mechanism is a severe problem in engineering applications of body-centered cubic metals since crack-like defects formed during manufacturing or in operation may reduce strength and life-time of a component significantly. In the ductile-brittle transition region the measured fracture toughness values of single crystals and poly-crystals differ by several orders. To the knowledge of the authors no model can explain this discrepancy yet. It is the aim of the present research project to contribute significantly to the understanding of this phenomenon. For this purpose a micromechanical FEM model shall be developed which resolves the microstructure of a ferritic steel at the crack tip discretely in form of particles (segregations, inclusions) and grains. Within different stages of extension of the model the effects of the relevant submechanisms like debonding of particles, fracture of particles, dislocation pile-up at grain boundaries and particles and incompatibility of cleavage planes of neighboring grains shall be investigated and quantified. On this basis the microstructure shall be optimized to increase the fracture toughness.