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

自青铜时代以来，加工金属已被人类使用了5000多年。塑性变形金属具有一些有益的性质，如高强度和不可逆性，作为可使用的工具和武器。位错作为线缺陷是晶体塑性变形的主要载体，位错的运动、相互作用和湮没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和观察到位错70年的历史，但我们仍然缺乏关于位错亚结构发展及其与硬化响应的关系的基础理论。最近，位错微结构的熵被引入到材料建模中，这被认为可以弥补这种“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。利用该理论和一组较少的物理参数，模拟了铜、铝和钢在几十年的应变率下以及从室温到熔点一半的平面应变压缩过程中的大量应力-应变曲线，并与实验结果进行了定量比较，证明了该理论的有效性。除了在宏观工程中的应用外，还尝试通过引入过量位错来探索微观结构和相关的力学响应，并解释了基于过剩位错运动的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的空间，以解释材料结构的层次，如涉及位错与其他材料缺陷的相互作用，如空位、晶界、析出物。由于理论中只使用了位错-位错相互作用的控制特征-脱钉机制，因此它在单相金属中可能是有效的，而在其他相金属中是不充分的。对于实际中多相强化合金的析出，位错系振动引起的势垒热激活旁路起着重要作用。因此，我的项目的主要目标是发展热力学位错理论的扩展，用于析出物诱导的硬化。两个目标变得非常重要，(I)将位错线的热振动效应引入理论和(Ii)位错旁路机制的参与。

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.