High-Temperature Deformation Behavior of a Ti-6Al-7Nb Alloy in Dual-Phase (a plus β) and Single-Phase (β) Regions

The present study aimed to characterizing the microstructure evolution of a Ti-6Al-7Nb biomedical type titanium alloy during hot working through hot compression tests. The hot deformation cycles were conducted under the strain rate of 0.0025, 0.025, and 0.25 s(-1) in the temperature range of 850-1150 degrees C where both dual-phase (alpha + beta) and single-phase (beta) regions could be accessible. The flow stress behavior of the material for the entire deformation regime was interpreted via microstructural observations. The results indicated that in the single-phase beta region (1050-1150 degrees C), the dynamically recrystallized (DRX) grains were formed at the deformed and elongated beta grain boundaries as a necklace-like structure. The variations in the dynamically recrystallized grain size were determined to follow the Zener-Hollomon relationship where DRX grain size was decreased by reducing the temperature and increasing the strain rate. The alloy deformation characteristics in alpha + beta region were somewhat different. During deformation in the upper alpha + beta temperature range (e.g., 1000 degrees C), the beta phase would accommodate most of the deformation, while alpha regions remained undeformed. In the lower alpha + beta temperature range (e.g., 850-950 degrees C), the kinking/bending of alpha lamellae as well as the subsequent globularization of alpha layers were postulated to be responsible for the observed flow softening behavior.