| The demand for improved performance in jet engines has led to a steady increase in the operating temperatures for gas turbine disks and blades. Modern nickel-base superalloys have been developed to withstand the resulting intense combination of thermal and mechanical loading. While improvements in alloy chemistry and material processing have largely overcome strength limitations, fatigue crack propagation at high temperatures still restricts the useful life of hot components. Environmental embrittlement controls high-temperature fatigue crack growth in polycrystalline nickel-base superalloys. However, the physics of this process is complex, and the detailed thermal, chemical, and mechanical mechanisms of crack growth are still not fully understood. Different mechanisms may be critical, depending on the particular alloy system and the specific thermomechanical loading. In some cases, two or more mechanisms may interact to cause crack growth.; This study focuses on one mechanism that is believed to be important in the fatigue response of high-temperature superalloys: intergranular fracture driven by the infiltration and embrittlement of grain boundaries by oxygen. A novel space-time finite element model is developed to study this mechanism. The numerical model provides the basis for generic investigations of the kinetics of quasi-static crack growth along a brittle interface (such as an oxidized grain boundary) embedded in an elastic-viscoplastic material. Additional studies illuminate the complex interactions between stress-assisted grain-boundary diffusion, inelastic material response and grain-boundary cleavage. |
