Microstructures' effects on high temperature fatigue failure behavior of typical superalloys

This work examines the influence of microstructures on the fatigue failure behavior of the typical superalloys, GH864 and FGH96. A series of heat treatments were carried out to vary the grain size, the precipitation of carbide, and the γ′ phase. The fatigue failure behavior at 650 °C is demonstrated by crack growth tests, both allowing a 90 s dwell time and omitting any dwell time. Results from the as-received materials reveal four periods in the whole failure process: incubation, initiation, propagation, and instant fracture. The resistance of each period can be evaluated according to its proportion of the whole fatigue failure process. The alloy GH864, for example, shows an intersection of crack initiation and propagation when the solution temperature is around 1046 °C or the carbide continuous coefficient (fc) is close to 0.56. An optimum point should exist where the strengthening effects of grain size and carbide precipitation achieve perfect matching for a normal alloy. The smaller sizes of secondary γ′ (γII′) and tertiary γ′ (γIII′) phases do present better crack growth resistance in their fatigue tests conducted without dwell time. The superior crack growth resistance of butterfly-shaped γII′, as compared to octagonally shaped γII′, indicates the influence of morphology on crack growth behavior. Moreover, the comprehensive parameter σs/(1−ψ) offers a helpful evaluation tool for determining crack growth resistance; its application is confirmed by experiments. All of these results produce not only a method for evaluating the life of engineering metals, but also provide a standard for designing highly damage-tolerant superalloys.