Factors Influencing the Return to Equilibrium of Quenched Beta Ti Alloys and of Massively Transformed TiAl-Based Alloys

The return to equilibrium of quenched beta Ti alloys and of massively transformed TiAl alloys are discussed in this article. In the case of Ti-15-3 (Ti-15V-3Sn-3Al-3Cr), it is shown that the addition of carbon leads to very significant refinement of the alpha, which precipitates during the aging of quenched samples and to the virtual elimination of grain boundary alpha. Correspondingly, there is no enrichment of oxygen at grain boundaries during aging of the C-containing alloy but significant enrichment in the C-free samples. The precipitation of alpha 2 in annealed massively transformed cast samples of TiAl alloys is shown to lead to very significant refinement of the as-cast structure and thus to a significant improvement in properties. The cast samples were either hot isostatically pressed (“hipped”) or annealed to precipitate the alpha, and it has been shown that the pressures used in “hipping” strongly influence the microstructure and properties. This unexpected result is shown to be due to the fact that the molar volume of alpha 2 is larger than that of gamma, and that cooling within the hot isostatic press or after annealing is rapid enough to retain the different proportions of alpha formed at the high temperature.