Figure 1: Schematic of the thermal profile, microstructural evolution, and the β-transus (as a function of hydrogen content) during HSPT and subsequent heat treatments to produce Ti-6Al-4V with bi-modal and globularized microstructures.

Densification is achieved during β-phase sintering step, activated by the presence of hydrogen. Homogeneous precipitation of low-temperature phases, enabled by the presence of hydrogen, forms an ultrafine-grained (UFG) acicular microstructure of α + α₂ + β + (δ-TiH₂) during phase transformation. This is then transformed during dehydrogenation into an UFG structure of α + β. During heat treatment, β grains grow to accommodate the increased equilibrium phase fraction and the UFG α colonies coalesce to form globularized α_p, which is driven by grain boundary energy. If the alloy is cooled slowly, the β grains recede to the low temperature phase fraction, leaving a fully globularized microstructure. If the material is cooled quickly, the β grains transform into either lamellar or acicular α colonies or martensite, depending on cooling rate, leaving a bi-modal microstructure.