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Atomistic Simulations of Screw Dislocations in Titanium

Abstract

This dissertation contains a variety of atomistic calculations concerning screw dislocations in pure or nearly-pure titanium alloys. Many aspects of these dislocations are considered. First, the core structures of -type screw dislocations in pure titanium at zero temperature and no applied stress are calculated. The parameters required to converge calculations of the cores structures are identified. The energy differences between core structures were found to be small, which suggests that at practical temperatures there will be occupation of more than one core structure configuration. Next, the effects of applied stresses with zero resolved shear stress on these dislocations are considered. The results suggest that non-Schmid stresses alter the critical resolved shear stress by inducing changes in the equilibrium core structure. This corroborates earlier conjecture that the non-Schmid slip behavior of screw dislocations in α-titanium alloys is due to non-planar core structure. Chapter 5 describes a new parameter based on elasticity theory for dislocation core identification and classification. This parameter is applied to the study of dislocation core structures in an empirical potential, examining the dependence of the core orientation on temperature and applied non-Schmid stresses. It is found that increasing the temperature leads to a larger variation in the core orientations. It is also found that applied stress on the [11̄00] axis normal to the dislocation line direction leads to a transition from the prismatic core structure to the pyramidal core structure. This transition appears to shift to lower pressure at higher temperature, suggesting that entropy of the core structures is important. It is demonstrated that temperature/non-Schmid stress conditions that permit easier access to the prismatic core orientation lead to higher strain rates for the same applied shear stress. In Chapter 6 I introduce a novel interstitial shuffling mechanism to explain how dilute concentrations of interstitial impurities can promote increased planarity of dislocation slip. This invites design of titanium alloys with increased tolerance to variations in interstitial impurity content. Finally, in Chapter 7 nucleation of the β to α phase transition in pure titanium is examined through a combination of elasticity theory and molecular dynamics simulation. Screw dislocations in the β phase act as heterogeneous nucleation sites and increase the α phase growth rate, but also restrict the orientation of the α nuclei to certain directions along which the strain field of the dislocation aligns with the strain required to complete the Burgers transformation path.

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