【作者】 刘文辉；

【导师】 张新明；

【作者基本信息】 中南大学 ， 材料加工工程， 2008， 博士

【摘要】 金属材料损伤破坏机理一直是材料科学与工程领域研究的热门课题。孔洞长大和聚合直接影响金属材料的断裂过程,以往的研究大多把金属当成各向同性的材料,未考虑晶体取向、晶内及晶间的不均匀塑性变形等因素。而事实上,这些因素决定了孔洞长大和聚合的过程。因此,有必要从细观的角度来研究晶体取向以及晶粒尺度上的不均匀塑性变形等对金属材料断裂行为的影响。本文结合晶体塑性力学,采用宏观和细观研究相结合及计算模型、理论分析、实验验证相结合的方法,开展孔洞在铝单晶和晶界上长大与聚合行为的研究工作。通过研究,提出了一种改善高强铝合金断裂韧性的热处理方法。本文主要创新点如下:(1)提出了把孔洞视为一种弹性模量和泊松比都非常小的材料来计算孔洞体积的方法。与积分法、给定位移法相比,该方法既避免了积分法的大量编程运算,又解决了给定位移法不能考虑晶体取向对孔洞长大速率影响的难题,且在计算中能直接求解单胞的有效应力和有效应变。(2)通过设计平面应变条件下含圆柱形孔洞的铝单晶压缩实验,探明了孔洞周围的晶体取向变化以及孔洞本身的变形规律。(3)采用3D晶体有限元探求了FCC晶体中晶体取向对球形孔洞长大和聚合过程的影响,揭示了晶体取向影响孔洞形状、孔洞长大方向、孔洞加速聚合以及裂纹形成的原因。(4)采用3D晶体有限元模型把晶体取向与孔洞长大速率以及材料的断裂直接联系在一起,为定量分析初始取向或织构对材料断裂性能的影响提供了一种有效的分析方法。(5)首次采用给定位移和多点约束的方法定量地研究了取向差影响孔洞长大和聚合的物理本质;建立了分析大角度晶界对孔洞长大和聚合影响的双晶模型,首次从位错滑移理论的角度揭示了再结晶组织易发生沿晶断裂的本质。(6)在研究了固溶制度对7A55和7050铝合金断裂韧性影响规律的基础上,建立了可溶性粒子与再结晶组织对合金断裂韧性的关系,提出了一种使合金既充分固溶又有效抑制再结晶的慢速升温固溶的热处理方法,使合金在具有高强度的同时,提高了合金的断裂韧性。更多还原

【Abstract】 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.更多还原