Effect of Initial Microstructure on Hot Deformation Behavior of Tc25g Alloy: Comparison between Basket-Weave and Equiaxed Microstructure

The initial microstructure and its evolution during hot deformation are of critical importance with regard to the performance of high-temperature titanium alloys. This study is concerned with isothermal compression tests of the TC25G alloy, which has been subjected to pre-treatment to produce both basket-weave and equiaxed microstructures. The tests were conducted at deformation temperatures ranging from 1163 K to 1223 K of the α+β phase zone, with strain rates from 0.01 s-1 to 1.0 s-1, and height reductions of 50% and 70%. The findings reveal that the flow stress of the initial basket-weave microstructure is greater than that of the equiaxed microstructure. A constitutive equation was developed and validated through experiments, which demonstrated variations in the stress exponent n and deformation activation energy Q between the two pretreated microstructures as strain increased from 0.15 to 1.0. The predicted flow stress closely matched the experimental data, fulfilling engineering application requirements. Recrystallisation was observed to increase with elevated deformation temperatures and recrystallized grains demonstrated a rapid growth rate following the loss of primary α phase constraint in the basket-weave microstructure. Higher deformation rates were found to enhance recrystallized grain boundaries within the equiaxed microstructure, although the grain size remained unaltered. Transmission electron microscopy observations revealed that the lamellar α primary phase underwent spheroidization via grain boundary separation, with deformation in the basket-weave sample mainly due to lamellar α phase distortion and twinning. The secondary α phase present within the β phase acted as a reinforcement, impeding the movement of dislocations. In the equiaxed microstructure, deformation occurred in both the primary α and β phases, with the β phase exhibiting lower resistance to deformation due to recrystallization. These findings elucidate the influence of the initial microstructure on the high-temperature deformation behavior of the titanium alloy TC25G.