Study Of Fracture Behavior In Aluminum Alloys Based On Dislocation Slip

The damage mechanics of metallic materials is a hot topic in material science and engineering area. The fracture process of metallic materials is affected by void growth and coalescence directly, most of the researchers supposed that metal is isotropic, but they ignored the factors of crystallographic orientation, inter- and intra-granular inhomogeneous deformation. In fact, all the above factors determine the process of void growth and coalescence, so it is necessary to investigate the influence of crystallographic orientation and grain scale heterogeneous plastic deformation on the fractural behavior of metals.Integrating with crystal plasticity mechanics, the growth and coalescence behaviors of voids in aluminum single crystal and on the grain boundary were studied by microscopic and macroscopic analysis, through theoretic analysis, numerical calculation and experimental verification, one kind of heat treatment for improving the fracture toughness of high strength aluminum alloys was suggested. The main creative points are as follows:(i) To calculate the change of void volume during deformation, the void is regarded as one kind of material with very small Young's modulus and Poisson's ratio. Comparing with integral method and prescribed displacement method, it avoids compiling a long program, and it can solve the problem in prescribed displacement method, which can't consider the influence of crystallographic orientation on the growth rate of void. Besides it can calculate the effective stress and strain of the unit cell.(ii) A compression experiment of a cylindrical void in aluminum single crystal was designed. The rule of the change of crystallographic orientation around the voids and the change of void shape was determined.(iii) The influence of crystallographic orientation on void growth and coalescence in FCC single crystal was simulated with 3D crystal plasticity finite element theory, and the reasons for the change of void shape, void growth direction, void coalescence and crack formation were revealed.(iv) Using 3D crystal plasticity finite element theory and linking the crystallographic orientation directly with the rate of void growth and the fracture behavior of material, an effective method was offered to analyze the influence of the original crystallographic orientation in the material on its fracture properties. (v) The effect of misorientation on the void growth and coalescence was first investigated quantitatively by using the multi-point constraints method and prescribed displacement method. Based on the dislocation slip theory, a two-grain model was established to analyze the influence of high angle grain boundary on the void growth and coalescence, which firstly gives the reason why recrystallization structure is easy to fail by intergranular fracture.(vi) The effects of solution treatment on the fracture toughness of 7A55 and 7050 aluminum alloys were investigated by experiments, and the relation of soluble particles and recrystallization with the fracture toughness was established. The temperature incremental solution heat treatment was suggested to promote the solutionization and reduce the recrystallization effectively, and the strength and fracture toughness of the alloys were improved.