The Phase Diagram Of Ti-6al-4v At High-Pressures And High-Temperatures

We report results from a series of diamond-anvil-cell synchrotron x-ray diffraction and large-volume-press experiments, and calculations, to investigate the phase diagram of commercial polycrystalline high-strength Ti-6Al-4V alloy in pressure-temperature space. Up to similar to 30 GPa and 886 K, Ti-6Al-4V is found to be stable in the hexagonal-close-packed, or alpha phase. The effect of temperature on the volume expansion and compressibility of alpha-Ti-6Al-4V is modest. The martensitic alpha -> omega (hexagonal) transition occurs at similar to 30 GPa, with both phases coexisting until at similar to 38-40 GPa the transition to the omega phase is completed. Between 300 K and 844 K the alpha -> omega transition appears to be independent of temperature. omega-Ti-6Al-4V is stable to similar to 91 GPa and 844 K, the highest combined pressure and temperature reached in these experiments. Pressure-volume-temperature equations-of-state for the alpha and omega phases of Ti-6Al-4V are generated and found to be similar to pure Ti. A pronounced hysteresis is observed in the omega-Ti-6Al-4V on decompression, with the hexagonal structure reverting back to the alpha phase at pressures below similar to 9 GPa at room temperature, and at a higher pressure at elevated temperatures. Based on our data, we estimate the Ti-6Al-4V alpha-beta-omega triple point to occur at similar to 900 K and 30 GPa, in good agreement with our calculations.