Two Typical Microstructures of Ti–6.6Al–1.7Mo–2.3V–1.9Zr Alloy Fabricated by Vacuum Hot Pressing of Powders with the Spherical Shape of Particles

Two typical microstructures of Ti–6.6Al–1.7Mo–2.3V–1.9Zr (TA15) titanium alloy were successfully fabricated by vacuum hot pressing using TA15 metallic powders of two different sizes with the spherical shape of particles. The size of prior β grains was consistent with the size of the as-received TA15 alloy powder. The microstructure of TA15 alloys differed depending on the size of the initial powder, forming Widmanstätten patterns for the sample from coarse powder or equiaxed microstructure for fine powder. The microstructure evolution during the vacuum hot pressing included solid-state phase transition and powder compact. In the temperature-rise period, the solid-state phase transition occurred (α → β). The anterior β-grain only grew to the original powder interface, which means that it would not coarsen causing its size to exceed that of the original powder. The solid-state phase transition occurred (β → α) when the temperature decreased during the subsequent cooling process. The nuclei of grain boundaries α appeared at the grain boundary of the anterior β-grain. Then the nuclei of grain boundaries α grew together enclosing the anterior β-grain. The grain boundaries α belonged to a certain anterior β-grain and could provide nucleation sites for the α-colonies of the two adjacent anterior β-grains. Finally, the α colonies grew into the anterior β-grain forming the Widmanstätten structure. The two typical microstructures will likely affect the mechanical properties of the TA15 alloys. An improvement in tensile properties was evident in the TA15 alloys (equiaxed microstructure) fabricated from a fine powder compared to their predecessors, consisting of colonies α microstructure fabricated from the coarse powder. To be specific, the tensile strength increased from 849 to 898 MPa and the ductility growth was from 5.5 to 6.5%.