Towards the development of dual phase constitutive relations for Ti-6Al-4V based on the mechanical threshold stress model

Abstract

In this work single-crystal MTS models for the alpha- and beta-phases of the Ti-6Al-4V alloy were developed at the slip system level for use within the Viscoplastic Self Consistent 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. To properly model the constitutive behavior, modifications were made to the Mechanical Threshold Stress model to 1) extend applicability to high-strain rate behavior where thermal dislocation activation transitions to dislocation drag mechanism and 2) more accurately model the initial work-hardening at the onset of plastic deformation. The results were found to match reported experimental data to within 5%. Simulations were also carried out to model the surface texture evolution of the Ti-6Al-4V alpha- and beta-phases undergoing high-speed machining processes. X-ray diffraction and electron backscatter diffraction measurements were made on as-received Ti-6Al-4V samples to provide input texture data as an initial state for simulations while measurements made on post-machined samples were used to validate simulated texture results. Because experimental measurements provide texture information at the surface as well as below the surface, novel methods were devised to subtract out the sub-surface texture information and better represent the experimental texture at the surface. High-speed machining simulation texture results showed good qualitative agreement with this method.