Grain size gradient nickel alloys II: Fatigue properties

Grain size gradient nickel alloys were tested in a rotating-bending fatigue machine and found to display better fatigue behavior than non-gradient, homogeneous nickel specimens having the same average grain size. This demonstrates that the overall fatigue properties are improved through the concept of a grain size gradient. The following reasons are suggested for the improved fatigue behavior. For the case of a gradient alloy with fine grains at the exterior and coarse grains in the center, the combined effect of a fine-grained surface and the presence of Al2O3 particles (grain growth inhibitor) increases fatigue crack initiation resistance, while the coarse-grained central region increases fatigue crack propagation resistance. For an alloy having the reversed grain size gradient, the coarse-grained surface increases crack propagation resistance near the threshold region, and the central, Al2O3 - added region contributes to fatigue resistance by enhancing yield strength. In both cases, when the applied stress is small, the fatigue crack propagates intergranularly at the beginning of the fracture process, but changes to a transgranular mode with striations as the stress intensity factor ΔK increases. At higher stresses the fatigue crack growth occurred only in a transgranular mode with striations. Grain size was found to exert a stronger influence upon the overall fatigue behavior than do the dispersed Al2O3 particles, probably because of the two opposing effects of oxide particles on crack front deflection and cavity formation. The portion of intergranularly fractured surface area decreases with an increase in the applied stress, and fatigue life increases with an increase in the portion of intergranularly fractured surface area. © 1989