【作者】 张学敏；

【导师】 曾卫东；

【作者基本信息】 西北工业大学 ， 材料加工工程， 2007， 硕士

【摘要】 金属材料的韧性断裂是塑性加工过程中常见的失效形式和影响热加工性的重要因素，历来都是先进塑性加工领域的研究热点。随着有限元模拟技术和损伤力学的不断发展，如何建立合适的热变形开裂准则，预测和避免缺陷的产生已成为缺陷仿真预测迫切需要解决的难题。本文以热变形极易开裂的Ti40阻燃合金为研究对象，以各种室温下适用的开裂准则为基础，引入Zener-Hollomon因子，对Ti40合金的变形机理及开裂行为进行了系统的研究。主要研究内容和结果如下： 研究了Ti40合金高温变形过程中变形温度和应变速率对流动应力的影响规律，揭示了流动软化和不连续屈服现象的影响因素和机理，发现不连续屈服现象与大量可动位错从晶界突然增殖有关。 揭示了Ti40合金的高温变形机理。发现变形温度低于950℃，以动态回复为主；高于950℃，发生动态再结晶。动态再结晶的形貌随应变速率的变化而变化：应变速率较高时(＞1s^-1)，动态再结晶晶粒呈项链状沿原始B晶界分布，沿晶界析出的Ti₅Si₃颗粒是再结晶晶粒的核心：应变速率较低时(＜0.1s^-1)，发生了锯齿状的连续再结晶，亚晶形核是其形核的主要机制。 研究了Ti40合金的开裂机理。发现低温、高应变速率下，变形以45°剪切开裂为主；温度较高时，以平行于压缩轴方向的纵裂和豆腐渣式开裂为主。V₂O₅挥发导致接近表面的晶界产生空洞，是合金热变形开裂的诱因。 揭示了Ti40阻燃合金热变形开裂的临界变形量与变形温度和应变速率的关系。结果表明：变形温度越高，应变速率越低，材料的临界变形量越大。发现变形温度和应变速率的综合作用可用单变量Zener-Hollomon因子来表示，且开裂的临界变形量与lnZ呈线性关系，从而大大减少试验次数。 基于DEFORM3D有限元平台，建立了Ti40合金等温热压缩过程的有限元分析模型，并对6种典型的室温韧性开裂准则进行了分析比较。发现基于空洞长大聚合的Oyane模型可适用于Ti40阻燃合金高温变形。发现Oyane准则的临界开裂C_f值与LnZ值也符合线性关系，从而建立了基于Zener-Hollomon因子的Ti40合金热变形开裂准则，并获得了验证。更多还原

【Abstract】 In the field of metal forming, the study of workability or formability of material and defects during metal forming is the focus of researchers. Workability is generally determined by the occurrence of ductile fracture and therefore limitations are set by the appearance of surface or internal cracks within regions that are highly strained due to extensive material flow. The advent of numerical methods to handle large deformation plasticity has made it possible to analyze the occurrence of cracks during the metalworking process. In order to predict the occurrence of defects, it is necessary to establish a reliable fracture criterion in hot forming. Ti40 burn resistant titanium alloy, which was easily to crack during the ingot cogging procedure, was used to systematically research its deformation mechanism and fracture behavior based on various fracture criteria. Zener-Hollomon parameter, which considered the influence of both temperature and strain rate, was introduced to establish a new criterion in hot forming. The research method and main conclusions of this paper as follows:The flow stress-strain curves were used to study the flow softening and discontinuous yielding phenomenon. Flow softening of Ti40 alloy arises as a result of structural instabilities such as adiabatic heating, generation of a softer texture during deformation and flow localization. The discontinuous yielding phenomenon has been related to the rapid generation of mobile dislocations from grain boundary sources, leading to hot deformation proceeding from the grain boundary region inward.The deformation mechanism at high temperature was investigated. It was found that dynamic recovery appears at temperatures less than 950℃ and dynamic recrystallization zone appears at temperatures higher than 950℃. The appearances of dynamic recrystallization varied with the strain rates: ’necklace’ recrystallization which was associated with Ti₅Si₃ particles precipitating along beta grain boundaries occurred at higher strain rates (>1s^-1), and a typical continuous recrystallization with serrated grain boundaries which was attributed to substructure occurred at lower strain rates (<0. 1s^-1).The fracture behaviors and mechanism of Ti40 alloy were disclosed. It was found that shear cracking along 45° orientation with respect to the compression axis更多还原