Abstract
The fatigue fracture mechanism of a nickel-based single crystal (NBSC) superalloy with recrystallized grains was studied at 550 °C by in situ observation with a scanning electron microscope (SEM) for the first time. Multiple crack initiations associated with recrystallized grain boundaries and carbides were observed. By analysis of the slip traces and crack propagation planes, the operated slip systems were identified to be octahedral for both single crystal substrate and recrystallized grains. Distinct crystallographic fractures dominated, accompanied by recrystallized grain boundary associated crack initiations. This is different from the widely reported solely intergranular cracking at high temperature. Fatigue crack growth rate curves showed evident fluctuation, due to the interaction of fatigue cracks with local microstructures and the crack coalescence mechanism. Both the recrystallized grains and the competition between different slip systems were responsible for the deceleration and acceleration of fatigue microstructurally small crack behavior.
Highlights
Nickel-based single crystal (NBSC) superalloys have been widely used as high temperature materials in aircraft turbines as well as in land-based gas turbines [1], due to its excellent mechanical properties under high temperature
The material used in this study was a nickel-based single crystal superalloy, which was developed for fabricating high-performance gas-turbine blades [17]
G, the primary crack showed an evident increase in crack opening displacement and out of plane displacement. It is associated with the fact: (i) the primary crack is long enough to provide a higher ∆K and a larger COD; (ii) the nickel-based single crystal (NBSC) substrate has different activated slip orientations, causing a deceleration of crack propagation in the substrate, corresponding to “g” in the which leads to remarkable increase in the primary crack length
Summary
Nickel-based single crystal (NBSC) superalloys have been widely used as high temperature materials in aircraft turbines as well as in land-based gas turbines [1], due to its excellent mechanical properties under high temperature. Xie G et al [6] studied the characteristics of recrystallized grain boundaries in the creep crack initiation behavior of a directionally solidified Ni-base superalloy. The recent studies on directionally solidified DZ4 superalloy indicated that the influence of recrystallization was closely related to the microstructure of recrystallized grains It may decrease the fatigue life of DZ4 [10,12] or even increase the fatigue life by a dense recrystallized layer composed of refined grains [13]. Zhao Y et al [11] reported that the effect of recrystallized grains led to an evident decrease in fatigue life in directionally solidified superalloy DZ40M They suggested the effects of the cracking of recrystallized grain boundaries and twin boundaries on fatigue life. The underlying mechanism for the variation of crack path and small crack growth rates was discussed
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