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

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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.