The influence of gaseous environments on fatigue crack growth in a nickel-copper alloy

Cyclic crack propagation rates for a 65 pct nickel-33 pct copper alloy in low pressure, 0.013 MPa (100 torr), environments of hydrogen, oxygen, and nitrogen gas were compared to a reference crack propagation rate in a 1.3µPa vacuum. Crack propagation rates were determined over a range of temperatures for vacuum and hydrogen gas at a constant cyclic stress intensity. Crack propagation in the gaseous environment results in an increased crack propagation rate compared to growth rates in vacuum and a unique fracture mor-phology for each environment. Parallel investigations using transmission electron microscopy showed a unique dislocation structure adjacent to the fracture surface corre-sponding to each fracture morphology and environment. Fracture modes were transgran-ular in vacuum and nitrogen gas, transgranular with crystallographically-oriented features in oxygen gas, and intergranular over a range of temperature in hydrogen. A mechanism is suggested to explain gaseous environmental effects based on dislocation-gas atom inter-action.

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