Experimental and theoretical study of Ti-6Al-4V to 220 GPa

We report results from an experimental and theoretical study of the ternary alloy Ti-6Al-4V to 221 GPa. We observe a phase transition to the hexagonal $\omega$ phase at approximately 30 GPa, and then a further transition to the cubic $\beta$ phase starting at 94–99 GPa. We do not observe the orthorhombic $\gamma$ and $\delta$ phases reported previously in pure Ti. Computational studies show that this sequence is possible only if there is significant local atomic ordering during the compression process, yet insufficient atomic diffusion to reach the phase-separated thermodynamic equilibrium state.

Figure 1

Diffraction profiles collected from Ti64 on compression from 30.7 to 44.2 GPa in a neon pressure-transmitting medium. The arrow above the 33.4 GPa profile identifies the (110/101) reflection from the $\omega$ phase, which is first evident at 32.7 GPa. Diffraction peaks from the $\alpha$ and $\omega$ phases, and from the Cu pressure marker and the Ne pressure medium, are indexed.

Figure 2

Diffraction profiles collected from Ti64 on compression from 106.3 GPa to 127.1 GPa without a pressure transmitting medium, showing disappearance of $\omega$-phase peaks that mark the transition from the hexagonal $\omega$ phase to the cubic $\beta$ phase. Diffraction peaks from the $\omega$ phase and the Cu pressure marker are indexed in the 106.3 GPa profile and peaks from the $\beta$ phase are indexed in the 127.1 GPa profile.

Figure 3

Pressure dependence of the $c/a$ ratio for the hexagonal $\omega$ phase of Ti64, as obtained from the least (no PTM) and most (neon PTM) hydrostatic compression experiments. The calculated $c/a$ ratio, assuming local ordering of the Al and V atoms, is shown for comparison. The horizontal dashed line indicates the “ideal” $c/a$ ratio of $\sqrt{3/8}=0.612$, where the hexagonal lattice is pseudo cubic. The vertical arrow indicates the experimentally determined $\alpha \to \omega$ phase boundary.

Figure 4

Compressibility of Ti64 to 174 GPa at RT, as determined from sample with no PTM (solid symbols) and from the sample with a neon PTM (open symbols). The solid line shows the Vinet fit to the $\alpha$-phase data. The dashed lines shows the calculated data at 0 K, excluding thermal expansion. Peak overlap between the $\omega$ and $\beta$ phases means that the atomic volume of the latter cannot be determined below $\sim$120 GPa (see text for details).

Figure 5

Calculated enthalpy differences between $\omega$ and $\beta$ phases relative to that of $\alpha$ phase and as a function of increasing pressure. The transition pressures are taken from where the lower curves cross. The upper graph shows the predicted transition pressures if the alloying atoms are ordered randomly, whereas the lower graph shows the transition pressures if the locations of the alloying atoms are locally ordered so as to be distributed as far apart as possible.