Discrete Dislocation Dynamics (DD) models provide a framework to advance the understanding of plasticity. However, existing DD models currently do not account for multiphysical effects. Multiphysical phenomena are often present during plastic deformation. Two particular examples are the electromechanical behavior of plastically deformed piezoelectric materials and the thermomechanical behavior of metals under high strain rate plastic deformation. Thus, I present two new DD models, that take these behaviors into account. The basic carriers of plastic deformation are dislocations, which are crystallographic defects. Therefore, in the two new DD models, dislocations are directly modeled as crystallographic line defects in an elastic continuum. These models are based on the Extended Finite Element Method (XFEM), which is a versatile tool used to analyze discontinuities, singularities, localized deformations, and complex geometries. The XFEM captures the slip from edge dislocations by way of Heaviside step enrichment function.

In the 1950s the direct observation of dislocations became possible, stimulat ing the interest of many research workers in the dynamics of dislocations. This led to major contributions to the understanding of the plasticity of various crys talline materials. During this time the study of metals and alloys of fcc and hcp structures developed remarkably. In particular, the discovery of the so-called in ertial effect caused by the electron and phonon frictional forces greatly influenced the quantitative understanding of the strength of these metallic materials. Statis tical studies of dislocations moving through random arrays of point obstacles played an important role in the above advances. These topics are described in Chaps. 2-4. Metals and alloys with bcc structure have large Peierls forces compared to those with fcc structure. The reasons for the delay in studying substances with bcc structure were mostly difficulties connected with the purification techniques and with microscopic studies of the dislocation core. In the 1970s, these difficulties were largely overcome by developments in experimental techniques and computer physics. Studies of dislocations in ionic and covalent bonding materials with large Peierls forces provided infonnation about the core structures of dislocations and their electronic interactions with charged particles. These are the main subjects in Chaps. 5-7.

Along with numerous illustrative examples, this text provides an overview of the dynamic behavior of dislocations and its relation to plastic deformation. It introduces the general properties of dislocations and treats the dislocation dynamics in some detail.

There has been a trend of miniaturization in recent technological advances, particularly through the development of microelectromechanical systems (MEMS). To cope with the demand for increasing performance from ever smaller components, alternatives to traditional scaling techniques is required, for example, by exploiting scale-dependent material properties. The investigation of material behaviour through computer simulations is an attractive alternative to experimental techniques which are limited by scale and cost. Metallic crystalline solids are commonly the material of choice for MEMS components. The majority of a metal's capacity for deformation is irreversible, otherwise known as plasticity. The dislocation -- a defect in the crystal structure at the atomic level -- acts as the microscopic carrier of plasticity. The Discrete Dislocation Dynamics (DD) family of numerical models serves as a bridge between an atomistic and a continuum description of plasticity at the mesoscale. In continuum models, plasticity is captured through the homogenization of localized effects induced by dislocation activity. With DD models, the activity of discrete dislocations is instead explicitly simulated. Conventional DD models are purely mechanical and are based on a quasi-static formulation. For the purpose of high strain-rate loading scenarios, they fail to capture the localized thermal effects which emerge, as well as the inertial effects which are particularly relevant. As such, the fully Dynamic and coupled Thermo-Mechanical Dislocation Dynamics model (DTM-DD) was developed in this thesis to address the limitations of existing DD models in the context of high strain-rate plasticity. Inertia was included via an elastodynamic description of material behaviour and the consideration of dislocation mass; and thermal influences, through thermo-mechanical coupling and the temperature dependence of dislocation parameters. Using the DTM-DD, the high strain-rate plastic behaviour of metals was investigated. The interaction and interference of elastic waves was observed; and the implications and convergence of dynamic dislocation motion was determined. The framework of extension load testing was presented to investigate the influence and strain-rate sensitivity of system and dislocation parameters to inertial and thermal effects. The selection of the thermal boundary condition was identified to significantly influence the simulated material response. The nature of temperature dependence, as investigated through parameter studies of dislocation drag and nucleation strength, was shown to be a competition between influences causing material softening and hardening. The DTM-DD was extended to investigate the effect of loading rate on the nano-indentation of a thin film sample. Loading rate-dependent propagation of dislocation nucleation and slip as a plastic front was observed. Ultimately, the investigations using the DTM-DD demonstrate that the interplay between inertial and thermal effects are highly complex in a fully dynamic and thermo-coupled system.

This publication is based upon lectures given during a well-received course on physical metallurgy and originally intended for students specializing in fields related to metallic materials. But, as the author points out, metallic materials are the most widely investigated group of materials and their study therefore gives a good basis for understanding how other materials can be made to reveal interrelationships between their structures and properties; especially with regard to those properties associated with strain. Similar types of rule can then be applied to other materials, in spite of their apparent differences.