Microstructurally small fatigue crack propagation and retardation at grain boundary in a polycrystalline Ni-base superalloy

Abstract

Fatigue crack propagation in a polycrystalline Ni-base superalloy was experimentally investigated. Special focus was put on the effect of grains crystal orientation and grain boundary on fatigue crack propagation, employing bi-crystal, coarse grain, fine grain CT specimens extracted from a directional solidified superalloy, MGA-1400. A series of experiments revealed that trans-granular crack propagation was affected by the primary and secondary crystal orientation of grains, and fundamental mechanism of Stage I cracking along slip planes could be interpreted by the experimental and analytical findings in the single crystal material. It was also found that the grain boundary in the bi-crystal specimen caused the retardation of crack propagation depending on crack propagation modes just after the grain boundary. In case the crack propagated in Stage I manner after the grain boundary, crack retardation became more pronounced due to the Stage I + II cracking.