Flow Curve of Superalloy 718 under Hot Forming in a Region of γ” Precipitation

Abstract

This study aims to formulate a constitutive equation to accurately determine the flow stress of superalloy 718 for effective production of gas turbine disks. A hot-compression test with superalloy 718 at temperatures ranging from 900 to 1000°C, a reduction rate of 67%, and strain rates of 0.1, 1, and 10 s⁻¹, respectively, was conducted to analyze the flow stress in the dynamic precipitation region. An accurate flow stress curve was obtained for each strain rate and initial temperature. The flow curves obtained at a deformation temperature of 900°C and strain rates of 0.1 and 1 s⁻¹, represent a combination of work-hardening and dynamic recovery. Dynamic recrystallization (DRX) behavior was observed under other deformation conditions. At a deformation temperature of 950°C and each strain rate, the strain at the onset of DRX (ε_c) decreases, and DRX tends to occur rapidly. In addition, the steady-state stress at a strain rate of 1 s⁻¹ was greater than that at a higher strain rate of 10 s⁻¹. The lowest steady-state stress among all the experimental conditions was observed at a strain rate of 10 s⁻¹. This may be attributed to the role of nucleation sites, precipitation hardening caused by dynamically precipitated γ” phases at approximately 950°C and a strain rate of 1 s⁻¹, and dynamic softening effects due to significant heat generated by deformation at a strain rate of 10 s⁻¹. A new constitutive equation for the generalized flow curve of superalloy 718 was obtained by considering these metallurgical phenomena.