Date of Award

2015-01-01

Degree Name

Master of Science

Department

Physics

Advisor(s)

Ramon Ravelo

Abstract

Most metals and alloys undergo plastic deformation at shear stresses and/or pressures in the Kbars regime. On the other hand, typical pressure-induced structural transitions, take place at much higher pressures, in the tens of GPa range. Surprisingly, there has not been to date, a systematic study on the role plastic deformation plays on the dynamics of defect-mediated phase transformations. The main purpose of this work was to investigate via atomistic simulations, the contribution of pre-existing defect densities on the kinetics of structural transformations under dynamic loading. We have carried out molecular dynamics simulations - including large-scale non-equilibrium molecular dynamics (NEMD), of defect-mediated phase transformations under shock and quasi-isentropic compression (QIC). An analytical embedded atom method (EAM) description is used to model a fcc-bcc phase transition (PT) boundary chosen to occur below or above the elastic-plastic threshold in

order to model systems undergoing a PT with and without plasticity. As expected, for conditions in which plastic deformation precedes the phase transformation, the defect-mediated PT proceeds at faster rates than the corresponding defect-free ones. The bcc fraction growth rate can be correlated with a sharp decrease in the dislocation densities originally present in the parent phase.

Language

en

Provenance

Received from ProQuest

File Size

121 pages

File Format

application/pdf

Rights Holder

Punam Ghimire

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