Microstructure Evolution of the Ti-46Al-8Nb-2.5V Alloy during Hot Compression and Subsequent Annealing at 900 °C.

TiAl alloys are high-temperature structural materials with excellent comprehensive properties, and their ideal service temperature range is about 700-950 °C. High-Nb containing the Ti-46Al-8Nb-2.5V alloy was subjected to hot compression and subsequent annealing at 900 °C. During hot compression, work-hardening and strain-softening occurred. The peak stresses during compression are positively correlated with the compressive strain rates and negatively correlated with the compression temperatures. The α2 phase exhibited a typical (0001)α2 basal plane texture after hot compression, while the ß0 and γ phases did not show a typical strong texture. Subsequent annealing at 900 °C of the hot-compressed samples resulted in significant phase transformations, specifically the α2 → γ and ß0 → γ phase transformations. After 30 min of annealing, the volume fraction of the α2 phase decreased from 39.0% to 4.6%. The microstructure characteristics and phase fraction after 60 min of annealing were similar to those after 30 min. According to the calculation of Miller indexes and texture evolution during annealing, the α2 → γ phase transformation did not follow the Blackburn orientation relationship. Multiple crystal-oriented α2 phases with nanoscale widths (20~100 nm) precipitate within the γ phase during the annealing process, which means the occurrence of γ → α2 phase transformation. Still, the γ → α2 phase transformation follows the Blackburn orientation relationship.

Low Temperature Phase Transformations in Copper-Quenched Ti-44.5Al-8Nb-2.5V Alloy.

In this study, an easily controlled transformation similar to the ß + α → ß + α + γ and the analysis of metastable phases in a ß solidifying Ti-44.5Al-8Nb-2.5V alloy were investigated. Therefore, a liquid alloy copper-quenching followed by annealing at an application temperature (850 °C) has been carried out. Following quenching, a microstructure composed of several supersaturated phases-the basket-weave ß0 (ßbv) phase, the plate-like α2 (αp) phase and the stripe-like γ (γs) phase-was obtained. In the annealing processes, phase transformations in the prior ßbv and αp phases domain corresponded nicely to the ß + α → ß + α + γ transformation during solidification. Also, in the annealed γs phase, the kinetics of the phase transformations involving the metastable L12 phase was firstly detected by transmission electron microscopy (TEM). The L12 phase had a lattice structure similar to the γ phase, whereas the composition of the phase was similar to the α2 phase. The formation of the γ pre-twin phase with an anti-phase boundary (APB) was detected in the γs phase of the matrix. The orientation relationships between the γs and precipitated: γ (γp) phase are <101]γs//<114]γp, (10 1 ¯ )γs//( 1 ¯ 10)γp and (0 1 ¯ 0)γs//(22 1 ¯ )γp.