Optimization of thermal processing parameters of Ti555211 alloy using processing maps based on Murty criterion

Isothermal compression testing of Ti555211 titanium alloys was carried out at deformation temperatures from 750 to 950 °C in 50 °C intervals with a strain rate of 0.001–1.000 s⁻¹. The high-temperature deformation behavior of the Ti555211 alloy was characterized by analysis of stress–strain behavior, kinetics and processing maps. A constitutive equation was formulated to describe the flow stress as a function of deformation temperature and strain rate, and the calculated apparent activation energies are found to be 454.50 and 207.52 kJ·mol⁻¹ in the α + β-phase and β-phase regions, respectively. A processing map based on the Murty instability criterion was developed at a strain of 0.7. The maps exhibit two domains of peak efficiency from 750 to 950 °C. A ~60 % peak efficiency occurs at 800–850 °C/0.001–0.010 s⁻¹. The other peak efficiency of ~60 % occurs at ≥950 °C/0.001–0.010 s⁻¹, which can be considered to be the optimum condition for high-temperature working of this alloy. However, at strain rates of higher than 1.000 s⁻¹ and deformation temperatures of 750 and 950 °C, clear process flow lines and bands of flow localization occur in the high-temperature deformation process, which should be avoided in Ti555211 alloy hot processing. The mechanism in stability domain and instability domain was also discussed.