Characterization of Hot Deformation of near Alpha Titanium Alloy Prepared by TiH₂-Based Powder Metallurgy

TiH₂-basd powder metallurgy (PM) is one of the effective ways to prepared high temperature titanium alloy. To study the thermomechanical behavior of near-α titanium alloy and proper design of hot forming, isothermal compression test of TiH₂-based PM near-α type Ti-5.05Al-3.69Zr-1.96Sn-0.32Mo-0.29Si (Ti-1100) alloy was performed at temperatures of 1123-1323 K, strain rates of 0.01-1 s^-1, and maximum deformation degree of 60%. The hot deformation characteristics of alloy were analyzed by strain hardening exponent (n), strain rate sensitivity (m), and processing map, along with microstructure observation. The flow stress revealed that the difference in softening/hardening behavior at temperature of 1273-1323 K and the strain rate of 1 s^-1 compared to the lower deformation temperature and strain rate. The strain hardening exponents at temperatures of 1123 K are all negative under all strain rates, and the most severe flow softening with minimum value of n was observed at 1123 K and 1 s^-1. The strain rate sensitives showed that the peak region with m value greater than 0.5 generally appeared in the high temperature range of 1273-1323 K, while strain rate sensitivity at low temperature behaved differently with strain rates. The processing map developed for strain of 0.6 exhibited high power dissipation efficiency at high temperatures of 1273-1323 K and a low strain rate of 0.01 s^-1, due to microstructure evolution of β phase. The decrease of strain rate at 1323 K resulted in the formation of globularization of α lamellae. The instability domain of flow behavior was identified in the temperature range of 1123-1173 K and at the strain rate higher than 0.01 s^-1 reflecting the localized plastic flow and adiabatic shear banding, and inhomogenous microstructure. The variation of power dissipation energy (η) slope with strain demonstrated that the power dissipation mechanism during hot deformation has been changed from temperature-dependent to microstructure-dependent with the increase of temperature for the alloy deformed at 0.1 s^-1. Eventually, the optimum processing range to deform the material is at 1273-1323 K and a strain rate range of 0.01-0.165 s^-1 (lnε˙ = -4.6--1.8).