Effect of thermal activation and vibrational dynamics of dislocations on thermodynamic dislocation theory

自青铜时代以来，人类使用加工金属已有5000多年的历史。塑性变形金属具有一些有益的特性，如高强度和不可逆性，可作为工具和武器使用。位错作为线缺陷，是晶体塑性变形的主要载体，位错的运动、相互作用和湮灭对材料的力学行为有显著影响。尽管金属的经验加工技术已经有了很长的历史，位错也已经被观察到70年了，但我们仍然缺乏位错子结构发展及其与硬化响应关系的基本理论。最近，位错微观结构的熵被引入到材料建模中，这被认为弥补了这种“知识的缺乏”。2010年，Langer， Bouchbinder和Lookman提出了涉及位错熵的热力学位错理论。利用这一理论和一组很少的物理参数，模拟了铜、铝和钢在几十年的应变速率和从室温到熔点的一半的平面应变压缩中的大量应力-应变曲线，得到了与实验结果定量一致的结果，证明了该理论的实用性。除了在宏观工程上的应用外，还尝试通过引入过量位错来探索微观组织及其相关的力学响应，并解释了基于过量位错运动物理机制的运动硬化、包辛格效应和尺寸效应。热力学位错理论仍有发展空间来解释材料结构的层次，例如涉及位错与其他材料缺陷的相互作用，例如空位，晶界，沉淀。由于理论中只使用了位错-位错相互作用的控制特征脱剥落机制，因此它可能对单相金属有效，而对其他金属则不充分。在实际应用中，对于多相强化合金的析出物，由于位错系统的振动而引起的热激活屏障旁路起着重要的作用。因此，我项目的主要目标是发展由析出相引起的硬化的热力学位错理论的扩展。两个目标成为主要的兴趣，(i)采用热振动的影响的位错线的理论和（ii）参与位错旁路机制。

Processed metals have been used by human beings for more than 5000 years since the bronze age. Plastically deformed metals have some beneficial properties, such as high strength and irreversibility, as tools and weapons to be used. Dislocations, as line defects, are the primary carriers of plastic deformation in crystals and dislocation‘s motion, interaction and annihilation influence remarkably the mechanical behaviour of materials. Despite the long history of empirical processing technology in metals and 70 years since dislocations were observed, we still lack fundamental theories of dislocation substructure development and its relation to hardening responses. Recently, the entropy of dislocations microstructures is involved into material modelling, which is believed to compensate for this “lack of knowledge”. In 2010 Langer, Bouchbinder, and Lookman have proposed thermodynamic dislocation theory involving entropy of dislocations. Using this theory with a set of few physics-based parameters, the numerous stress-strain curves in plane strain compression for copper, aluminum and steel over several decades of strain rate and from room temperature to one half of melting point have been simulated and obtained quantitative agreements with experimental results, which attested the usefulness of the theory. In addition to the applications in macroscopic engineering, one attempt to explore microstructures and associated mechanical responses, has been made through introducing excess dislocations, and the kinematic hardening, Bauschinger effect, and size effect based on the physical mechanism of movement of excess dislocations have been explained. Thermodynamic dislocation theory still has spaces to develop to account for hierarchy of material structures, such as to involve the interactions of dislocations to other material defects, e.g. vacancies, grain boundaries, precipitates. Since only depinning mechanism, a controlling feature of dislocation-dislocation interaction, is used in the theory, it may be valid in single phase metals but insufficient in others. For precipitate of multiphase strengthened alloys in practice, thermal activated bypass of barriers owing to the vibration of dislocation system plays important role. Thus, the main target of my project is to develop an extension of thermodynamic dislocation theory for hardening induced by precipitates. Two objectives become of prime interest, (i) adoption of the effect of thermal vibration of dislocation lines into the theory and (ii) involvement of dislocation bypass mechanism.