On the residual stress relaxation in Inconel 718 superalloys at high temperature by real-time neutron diffraction

Residual stresses in nickel-based superalloy engineering structure develop during various manufacturing stages, which can result in component distortion or reduction of performance reliability during high temperature service. Experimental quantification of residual stress built-up and its relaxation at high temperatures provides the time-dependent evolution and model validation for residual stress predictions. Via in situ time-of-flight neutron diffraction, a real-time study of residual stress relaxation is carried out on Inconel 718 (IN 718) blocks with different initial residual states after oil or water quenching and annealing processes. The residual stresses are largely relieved after 14 ∼ 20 h at 718 °C, and multi-stage kinetics of relaxation are observed. The staged feature is correlated to the competing stress relaxation mechanisms in the atomic scale that are clued from the evolution of neutron diffraction data. Although with large initial discrepancy in samples due to different thermal treatments, the final residual stresses tend to be within small variations after the relaxation.