부산대학교 도서관

This thesis develops an understanding of the microstructure and texture evolution, it identi- fies the corresponding micromechanisms which occur during deformation of two titanium alloys, Ti-6Al-4V and commercial purity titanium (CPTi). Particular emphasis is placed on the thermo- mechanical processing required to produce globular α-phase morphologies and the concomitant evolution in texture. In-situ synchrotron X-ray diffraction recrystallisation experiments were performed in order to trace parent to globular textures. During rolling, the α (0002) basal texture softens due to kinking of the colony α-phase laths. During recrystallisation the texture is observed to strengthen and slightly rotate as highly strained, misorientated areas of the laths dissolve and globularisation occurs. It is observed that, recrystallisation times strongly effect the transformed secondary α-phase texture. Short annealing times produce Bürgers related secondary α orientations but at longer annealing times the secondary α takes the orientation of the surrounding globularised primary α grains. In another body of work, neutron diffraction tensile tests were used in order to measure the change in lattice d-spacings of two plate product forms of Ti-6Al-4V. A two-phase elasto-plastic (EPSC) self-consistent model was further developed to rationalise the findings. The loading response of the two plates are distinct and are well reproduced by the model. It was found that microstrains were generally larger in the unidirectionally rolled material than the cross-rolled. Low critical resolved shear stresses for slip were required to reproduce the near-zero strains in the (0002). Finally to conclude the work, the micromechanics of textured Grade One Commercial Purity Titanium (CPTi) is fully examined using in-situ neutron diffraction experiments and post hoc EBSD. Both tension and compression twinning is observed when compressing the texture in the axial and hoop direction. Limited twining is observed when loading axially in tension. Analysis of the twin types revealed {10⁻12}< ⁻1011> tension and {11⁻22}<11⁻23>compression twins are dominant. Tests revealed a strong reorientation of the texture towards the loading direction in the axially compressed samples. Twins were observed to be larger when loaded in the axial direction than in the hoop direction in compression. Which has been interpreted in light of current dislocation theories. EPSC modelling can closely reproduce both the microstrains and the resultant texture. The critical resolved shear stresses found are consistent with macroscopic flow curves, lattice strains and textures.