Deformation and microstructure evolution above the B2 transus of Ti-22Al-25Nb (at%) orthorhombic alloy

Hot compression deformations above the B2 transus of Ti-22Al-25Nb (at%) orthorhombic alloy were conducted from 20 degrees C above the B2 transus to 100 degrees C above the transition point with 20 degrees C intervals. Four deformation strain rates including 10 s(-1), 1 s(-1), 0.1 s(-1) and 0.01 s(-1) were employed in order to study the effect of strain rates on the deformed microstructures. The constitutive relation establishment has been attempted using Zener-Hollomon parameter. The analyses indicate that exponential law equation is not applicable for Ti-22Al-25Nb alloy in present conditions while the power law equation and the hyperbolic sine law equation are both effective. However, the power law equation is more applicable than the exponential law equation since the linear correlation coefficient between lnZ and In sigma is 0.989 while the correlation coefficient between InZ and Insinh(alpha sigma) is 0.972. It is found that strain rates and deformation temperatures have remarkable effect on the deformed microstructures, Dynamic recovery (DRV) dominated the deformation mechanism at high strain rates (10 s(-1) and 1 s(-1)) while dynamic recrystallization (DRX) occurred in the microstructures deformed at low strain rate (0.1 s(-1) and 0.01 s(-1)). The DRX are mainly continuous DRX (CDRX) although some discontinuous DRX (DDRX) grains are observed at prior high angle grain boundaries and triple junctions of grain boundaries.