Identifier

etd-07092004-113612

Degree

Master of Science (MS)

Department

Mechanical Engineering

Document Type

Thesis

Abstract

Grain growth is the process that takes place during annealing of polycrystalline materials; its major feature is a systematic increase in grain size. Two different types of grain growth can be distinguished: the normal and abnormal grain growth. During normal grain growth, the microstructure exhibits a uniform increase in grain size with time and the grain size distribution follows the log-normal distribution with the grain sizes ranging from 0 - 2.2 times the average grain size (R). On the contrary, when the abnormal grain growth is the dominant mechanism, there are certain grains (abnormal grains) in the microstructure that grow much faster than the majority of the grains and in the end consume the fine-grained matrix around them. There has been a lot of work done in the field of abnormal grain growth, but the actual mechanism of abnormal grain formation and development from a uniform grain size distribution is not fully understood. In this study, various aspects of abnormal grain growth are investigated using a mesoscopic simulation approach. Our investigation focuses on two possible venues that are, in general, considered as main sources promoting abnormal grain growth. The role of both the geometrical inhomogeneities (size effect) and grain boundary (GB) anisotropic properties are investigated. Simulations are done on various microstructures in which there are certain fraction of preexistent large grains (size greater than 2.2), as well as on microstructures in which a given fraction of grains has different GB properties (mobility and energy). Our simulation results indicate that the presence of some large grains in the initial microstructure does not promote the abnormal grain growth. However, when certain grains in the microstructure have grain boundary energies below a certain threshold or mobilities above a certain threshold value relative to the rest of the grains, the microstructure may evolve by abnormal grain growth.

Date

2004

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Dorel Moldovan

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