Evolution of equiaxed alpha phase during heat treatment in a near alpha titanium alloy

The evolution of equiaxed primary α phase (α_p) during post-deformation heat treatment of near-α titanium alloy Ti-5.6Al-3.8Sn-3.2Zr-1.0Ta-0.5Mo-0.4Nb-0.35Si was studied. This alloy was isothermally compressed at 1019 °C and subsequently heat treatment at 1024 °C for times ranging from 0.5 to 24 h. The recrystallization behavior and boundary splitting within equiaxed α_p were analyzed by crystallographic orientation and microstructure observations. The results showed that the formation of internal (sub)boundaries within the α_p was by the strong recovery. In most cases, it is difficult to obtain a refinement of big equiaxed α_p by boundary splitting and α_p grains were still contiguous with “peanut” shape at relatively short heat treatment time (≤24 h, 1024 °C). The groove in the “peanut” structure was indicative of boundary of α/α. Only in a few cases, when an isolated β grain can form within the interior α_p along the (sub)boundary, refinement of α_p grain may be achieved due to the obvious reduction of boundary splitting distance. These observations were rationalized on the basis of the classical Mullins grooving analysis.