The deformation and texture evolution of Ti-10V-2Fe-3Al

2017-03-02T00:40:00Z (GMT) by Zhao, Juan
Beta titanium alloys form one of the most versatile classes of materials with respect to processing, microstructure, and mechanical properties. Owing to their body centered cubic structure, β titanium alloys offer an attractive alternative to the α+β and α alloys as they usually possess higher specific strengths, better heat treat ability, deep hardening properties, and lower processing temperatures. However, beta titanium alloys are very sensitive to deformation parameters and may produce quite different microstructures and further various mechanical properties. It is necessary to explore the effect of deformation parameters on the microstructure evolution, including testing temperature, strain rate and strain. Texture introduced by deformation may produce a strong anisotropy of mechanical and other properties, and can also lead to the localization of these properties. Therefore, in order to make use of the advantages of β titanium alloys, we must also understand the evolution of texture. The aim of this project is to reveal the deformation mechanisms and the evolution of texture during hot deformation in β titanium based on compression tests, optical microscopy (OM), scanning electron microscopy (SEM) and texture evolution by Electron backscattered diffraction (EBSD) and x-ray texture observation. The microstructure, texture and mechanical response of forged Ti-10V-2Fe-3Al plates were investigated. Forged plates of as-received Ti-10V-2Fe-3Al alloy features fine α phase in large β grains. Yield locus tests were performed which confirmed that β phase {001} <110> texture resulted in lower mechanical anisotropy in the forged plate. The relationship between α phase and β matrix texture was characterized by EBSD. The Burgers relationship is observed between these two phases in the as-received state, and this is preserved from the forging process and following 760 °C heat treatment. After further heat treatment at 820 °C, both β phase and α phase form recrystallization textures and the Burgers relationship is no longer obvious. Furthermore, through thickness texture inhomogeneity was studied in the as-received plate. A rotated cube texture is widely spread at the surface, and more intense at quarter thickness and center. At the surface there is a tendency to form the {111} <112> recrystallization texture while in the quarter thickness and center, the restoration mode is mainly recovery. Following solution treatment in single β phase field, deformation in α+β phase field was performed, which presented a considerable effect on α phase morphologies. The microstructure of α phase is basically an evolution of lamellar α breaking up into shorter lamellar and coarsening into globular morphologies. Dynamic recovery is the main restoration mechanism for β phase. The dominant orientations of beta phase include a strong rotated cube texture and weak <111> orientations parallel to compression direction. The orientation relationship between α phase and β matrix follows the Burgers relationship and deformation in α+β phase field does not destroy this orientation relationship. Deformation at above β transus temperature leads to strong <001> and weak <111> textures. With increasing temperature and decreasing strain rate, the <001> texture gets strengthened gradually and <111> orientations show weakening tendency. The volume fractions of dynamic recrystallization are lower than 10% under all the deformation conditions in the single β phase field, showing the main dynamic restoration mechanism is also dynamic recovery at above β transus temperature tested in this thesis. Moreover, deformation parameters show significant effects on microstructure evolution. At low strain rate, dislocation annihilation and dynamic recovery are very effective, resulting in significantly larger subgrain size and less pancaked original grains. Besides dynamic recovery, continuous dynamic recrystallization by progressive lattice rotation is also observed, which occurs at high strain rate and low temperature. Geometric dynamic recrystallization hardly occurs, as HAGB spacing is much larger than the subgrain size at the highest strain of 0.9. Discontinuous dynamic recrystallization is rarely observed in the sing β phase field in Ti-10V-2Fe-3Al.