The origin of coarse macrograin during thermo-mechanical processing in a high temperature titanium alloy

Coarse macrograins have been observed in aerospace components for a high temperature near-α titanium alloy Ti60. The present work was inspired to understand the origin of coarse macrograin and examine how strain affects the macro/microstructure. Thus, thermomechanical processing was conducted in α + β phase field to simulate industrial production. This alloy with a bimodal microstructure was isothermally pre-deformed at 1019 °C and a constant strain rate of 0.1 s⁻¹ to height reductions of 15, 35, 55 and 75%. Heat treatment was conducted following deformation at 1024 °C for 30 min. The results showed that coarse macrograins originated from inadequate recrystallization of prior β grains of initial billet. Deformation in the α + β phase field does not form many new orientation β grains but slightly scatters β grain orientations around the orientations of big prior β grains in small strain samples. Each of different coarse macrograins corresponds to a family of prior β grains with similar crystallographic orientations with about 20° of spread, which is outlined by secondary α (α_s) plates having nearly parallel basal planes with similar crystallographic orientations. The size of macrograin decreases with increasing strain, which is consistent with the extent of recrystallization of prior β grain. These understandings can be applied to optimize processing routes for the control of final macro/microstructure and properties in near-α titanium alloys.