OXIDATION‐ASSISTED FATIGUE CRACK GROWTH BEHAVIOR IN ALLOY 718‐PART I. QUANTITATIVE MODELING

Abstract

Abstract This paper presents a new model to predict the high temperature, intergranular crack growth rate behavior in alloy 718 at 650°C. The model is based on the concept that the crack tip oxidation‐fatigue damage is a nonlinear process governed by the chemical‐mechanical interaction along the grain boundary fracture path at the crack tip. A concept of two‐stage oxidation mechanism was used here. This mechanism depends on the rate of formation of the chromia layer in relation to the build‐up of other oxide types at the crack tip. The saturation of the Cr 2 O 3 build‐up signifies the occurrence of the oxide passivation effect. The determination of the amount of Cr 2 O 3 depends on the amount of both oxygen diffused along the affected grain boundary and chromium transported via a mobile dislocation network. Here, both the grain boundary and effective dislocation pipe are treated as short‐circuit diffusion paths along which the diffusion process can be described using Whipple's solution. The model yields sufficient information to correlate the amount of Cr 2 O 3 with the reduction in the grain boundary ductility within the affected oxide zone. The grain boundary ductility is balanced by the effective strain at the crack tip resulting from the external loading. This balance defines the fracture criterion of the model and permits the calculation of the crack advance per cycle which is the ultimate goal of this work.