Variant selection and its effect on texture inTi-6Al-4V

  • Gideon Obasi

Student thesis: Phd

Abstract

Titanium alloys are strong candidates for the aerospace industry and biomaterial applications because of their low density, high strength-to-weight ratio and very high strength even at temperatures up to 600°C. Like many other engineering alloys, titanium alloys are prone to strong preferred crystallographic orientation development during thermomechanical processing. Part of the titanium processing route is to heat treat the material above the β transus for the purpose of homogenization and associated phase transformation. This heat treatment dramatically affected the microstructure and texture evolution. Theoretically, such heat treatment should result in a nearly random texture if all variants during alpha→ β →alpha phase transformation are active. In reality, significant textures are observed after such a heat treatment process.The present project aims at developing a detailed understanding of the root cause for this relatively strong texture by means of EBSD and in-situ neutron diffractionstudies. The effect of β grain growth on variant selection during β to alpha phase transformation has been investigated by using two variants of Ti‐6Al‐4V with and without 0.4 wt% yttrium addition. The aim of adding yttrium was to control β grain growth above the β transus by pinning grain boundaries with yttria. Both materials were first thermomechanically processed to generate a similar starting microstructures and crystallographic textures. Subsequently, both materialswere solution heat treated above the β transus followed by slow cooling to promote growth of the alpha lath structure from grain boundary alpha. Detailed EBSD and insitu neutron diffraction analysis were carried out to study microstructure and texture evolution. The variant selection calculation suggests that more variant selection occurred in convectional material with a large grain size compared to material with yttrium addition. Insitu measurements showed that β texture strengthened significantly above the β transus with increasing β grain size. There was no significant variant selection during alpha→ β transformation;variant selection noticeably increased during β→alpha transformation with increasing β grain size. Additional interrupted cooling experiments followed by EBSD analysis showed early nucleation of alpha variants with a "butterfly morphology" from β grain boundaries that have a pair of β grain with a common pole. These observations suggest reduced nucleation energies for alpha formation in such circumstances allowing extensive growth of these alpha variants into unoccupied β grains making it a dominant variant. The influence of rolling temperatures (i.e. at 800 ºC and 950 ºC) to producedifferent starting texture, on texture evolution and variant selection during alpha→ β→alpha transformation was also investigated. Laboratory X-ray, EBSD and in-situ neutrondiffraction texture analyses were carried out. Even though the transformation textureis stronger at 800 ºC, the degree of variant selection is stronger in materials rolled at950 ºC compared to material rolled at 800 ºC. Here, the enhanced variant selectionfor the material rolled at 950 ºC was related to the different β texture. It is suggestedthat the combination of a particular β texture components promote variant nucleationthat can increase the likelihood of having β grain pairs with a common pole.
Date of Award1 Aug 2012
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorMichael Preuss (Supervisor)

Keywords

  • Ti-6Al-4V, Grain growth, Texture evolution, Variant selection and Texture memory

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