GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Wednesday, April 24, 2019

12:00 PM
in Love 295

will be held the

DISSERTATION DEFENSE

for

Jason Allen

"Towards the Development of Dual Phase Constitutive Relations for Ti-6Al-4V Based on the Mechanical Threshold Stress Model"

Committee Members:

Prof. Hamid Garmestani, Advisor, MSE

Prof. Steven Liang, ME

Prof. Naresh Thadhani, MSE

Prof. Preet Singh, MSE

Prof. David McDowell, ME

Abstract:

High-speed machining of Ti-6Al-4V parts often subjects the workpiece surface to increased temperatures, high strains and strain-rates and can lead to phase transformations, recrystallization, growth and grain size gradients. The description of the mechanical behavior of dynamic processes depends greatly on a constitutive model being able to account accurately for changes in the flow stress with variations in strain-rate and temperature that occur during processing. The Mechanical Threshold Stress (MTS) model is a physical-based internal state variable model that accounts for this behavior remarkably well. While much work has been invested in parameterizing the MTS model for various material systems, relatively little attention has been given to dual or multiphase systems. Additionally, most applications of the MTS model have been used to describe bulk material behavior without regard to the constitutive behavior at the slip system level. 

To properly model the constitutive behavior, modifications were made to the MTS model to 1) extend applicability to high-strain rate behavior where thermal dislocation activation transitions to dislocation drag mechanisms and 2) more accurately model the initial work-hardening at the onset of plastic deformation. Using these modifications, single-crystal MTS models for the alpha- and beta-phases of the Ti-6Al-4V system were then developed at the slip system level for use within the Viscoplastic Self Consistent (VPSC) crystal plasticity model to describe the stress-strain behavior of polycrystalline dual-phase Ti-6Al-4V undergoing high strain-rate compression over a wide range of temperatures. Simulations using these models were also carried out to describe the surface texture evolution of the Ti-6Al-4V alpha- and beta-phases undergoing high-speed machining processes.