High Temperature Dwell Fatigue Crack Growth in Cold-Worked and Direct-Aged 718PlusTM

Abstract

The high temperature dwell fatigue crack growth behaviours of 30% cold-drawn and direct-aged ATI 718PlusTM were studied. This was compared to an annealed and direct aged variant of the same material. This was in order to rationalise the effects of cold work. Tests were conducted at 650 °C using different dwell times, in air and in relative vacuum, at R = 0.1. It was found that the cold-worked and direct-aged condition produced slower dwell fatigue crack growth rates in all tests. This has been linked to the larger elongated grain morphology and the dislocation structure retained after cold working. Increased crack tortuosity and roughness-induced crack closure are considered to enhance the cycle-dependent crack growth properties. Superior resistance to environmentally assisted intergranular failure is related to greater crack tip stress relaxation and more homogeneous grain boundary deformation.