Constitutive modeling and processing map for hot deformation of Ti–15Mo–3Al-2.7Nb-0.2Si

Abstract

Hot tensile tests of a metastable β alloy, Ti–15Mo–3Al-2.7Nb-0.2Si, were conducted in the wide temperature range of 750–950 °C with strain rates from 0.01 to 0.00005s−1. The flow behavior, constitutive modeling, strain rate sensitivity exponent (m), processing map and microstructural evolution were investigated. The results showed that the flow stress decreases with increasing temperature and decreasing strain rate. Discontinuous yielding was observed in the flow stress curves at high strain rates. The apparent activation energy under different discrete true strains was much higher than the self-diffusion activation energy of pure α or β-Ti. A constitutive model considering the strain effect is proposed with an average relative error of 8%. The m-value distribution and processing maps show that the stability region corresponds to a higher m value (>0.2) or higher power dissipation efficiency (η > 0.33), and the deformation mechanism relates to dynamic recrystallization (DRX), grain boundary sliding (GBS) and dynamic recovery (DRV). An instability region appeared at temperatures of 800–825 °C and high strain rates of 0.01–0.001s−1. The microstructural observations demonstrate that DRV with the formation of subgrains is the main deformation mechanism in the β field, accompanied by DRX and rotation of the β grains. When the testing temperatures were lower than Tβ, DRV was the main deformation mechanism of the deformed region. Static recovery was the primary mechanism for the grip regions.