Diffusional transformation in Ti6Al4V alloy during isothermal compression
(1. Council for Scientific and Industrial Research (CSIR), Pretoria 0001, South Africa;
2. Department of Materials Science and Metallurgical Engineering, University of Pretoria, Pretoria 0001, South Africa)
2. Department of Materials Science and Metallurgical Engineering, University of Pretoria, Pretoria 0001, South Africa)
Abstract: Thermodynamic calculation of the two-phase Ti alloy was completed using CompuTherm Pandat？ and Ti data base, followed by isothermal compression of Ti6Al4V (Grade 5), with an initial colony lamellar structure that was performed in the (α+β) and β-phase field. Microstructural evolution and phase transformation were investigated using X-ray diffraction, scanning and transmission electron microscopy. The presence of the Ti3Al or α2 (hcp), the phase stability and transition temperatures were predicted by the Gibbs free energy-temperature and phase fraction-temperature diagrams. The isothermal compression in the (α+β)-phase field is characterized by reorientation and localized kinking of α/β lamellae, and cracking at α/β interphase regions. While in the α→β-phase transformation area, deformation in β-phase and at α/β interphase boundaries, extensive transformation of α into β-phase, martensitic transformation and spheroidization of α-laths mainly characterize this isothermal compression. A complete transformation of α into β single phase occurs in the β-phase field. Ti3Al or α2 (hcp), β (bcc) and α (hcp)-phase, and additional hcp α'''' and orthorhombic α'''''''' phases in a deformed Ti6Al4V are revealed. The flow stress level, the dynamic recovery and dynamic globularization are affected by deformation temperature.
Key words: Ti6Al4V; phase transformation; spheroidization; thermodynamic calculation