“Ʌ”-shaped trend of stress relaxation stability of Inconel718 superalloy with initial stress increasing

Stress relaxation tests were carried out to identify stress relaxation behavior of Inconel718 superalloy under different initial stresses. Experimental results indicate that: increasing initial stress from 260 MPa to 1160 MPa, stress relaxation stability of Inconel718 superalloy rises firstly and then falls. Above all, thermodynamic models of stress relaxation under different initial stresses are constructed, and the release of elastic strain energy density (▵W) and release ratio of elastic strain energy density (η) are identified to quantify thermodynamic characteristics of stress relaxation. The trends of ▵W and η demonstrate that plastic deformation decreases firstly and then increases with initial stress increasing. Besides, constitutive equation and relevant parameters are used to describe kinetic characteristics of stress relaxation behavior. Plastic strain rate increases continuously, but the tendency of stress exponent suggests that work hardening strengthens in the beginning and then weakens remarkably. Due to the coupling effect between the two factors, the accumulation of plastic deformation drops firstly and then rises significantly. Finally, based on microstructure evolution, it can be found that, at low initial stress, stress relaxation consists of two stages, and dislocation movement and grain boundary migration are dominant mechanisms of stress relaxation. While at high initial stress, stress relaxation can be divided into three stages. Twinning is the main mechanism in the first stage, leading to obvious plastic deformation. When it comes to the second stage, dislocation movement and grain boundary migration are dominant reasons. Eventually, stress relaxation achieves the third stage, referred to as the steady state.