טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentSheinkman Sagi
SubjectThe Collective Effect of a Dislocation Network on their
Climb Rate
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Dan Mordehai
Full Thesis textFull thesis text - English Version


Abstract

One of the great challenges in solid mechanics is to prevent the degradation in strength of components. In particular, material may deform at high temperatures even at low constant stresses, a deformation mode known as deformation creep. One of the microstructural mechanisms that governs deformation by creep is dislocation motion due to absorption or emission of vacancies, a mechanism called dislocation climb. Despite its importance, it is assumed by most climb models in literature that the dislocation network is homogeneous and dilute, which allows treating each dislocation as an isolated sink (or source) for vacancies. This assumption of isolated independent sinks is far from being true in many cases, such as in dislocation dipoles, dislocation pile ups, dislocation walls etc. While these models consider the mechanical interaction between dislocations they do not take into account the collective effect of the dislocation network on the diffusion field around them, which in turn determines how the climb rate of each dislocation is affected from the whole network. Therefore, the importance of the dislocation network on deformation creep is still far from being understood.

In this study we developed a climb model that accounts for the collective effect of the dislocation network, by solving the diffusion equation for vacancies in a region with a general dislocation distribution. The definition of the sink strength is extended, to account for the contributions of neighbouring dislocations to the climb rate. The model is then applied to dislocation dipoles and dislocation pile-ups, which are dense dislocation structures and it is found that the sink strength of dislocations in a pile-up is reduced since the vacancy field is distributed between the dislocations. In particular, this coordinated climb motion of neighbouring dislocations is important to the disassembly of dislocation pile-ups. Finally, we discuss how considering the interaction elastic energy between the dislocations and vacancies alters the results.