Effect of Initial Microstructure and Strain Rate on the Hot Deformation Behavior of ATI425 Alloy in Two-Phase α/β Region

In this paper, the effect of initial microstructure on hot deformation behavior of ATI 425 alloy has been investigated by constitutive equations. The results revealed that the initial equiaxed microstructure has superior hot compression properties in comparison with martensite microstructure. In the case of equiaxed microstructure, the shear bands have not appeared via hot compression tests in the range of 700-900 °C and strain range of 0.001-1 s⁻¹, while for martensite microstructure, the shear bands have been promoted via low temperature and high strain rate. According to the constitutive equations, the content of thermal activation energy for studied initial microstructures was computed. The processing map indicated that by increasing the temperature and decreasing the strain rate, the coefficient of power dissipation was significantly improved. It is found that the dynamic globularization kinetic of ATI425 is directly sensitive to deformation parameters. True stress–strain curve exhibited a peak stress followed by noticeable flow softening; the values of peak stress and flow softening rate showed dependence on starting microstructure. Eventually, the strain hardening behavior of the alloy was investigated using Kocks–Mecking approach. For both initial microstructures, by increasing hot compression temperature from 700 to 900 °C, the rate of dislocation annihilation rate was considerably accelerated; consequently, the strain hardening rate (theta) was significantly decreased.