Crack initiation and propagation in a single crystal Ni-base superalloy with respect to TCP phase formation during low cycle fatigue

Abstract

Miniature specimens made of a single crystalline (SX) Ni-base superalloy with the nominal chemical composition of CMSX-4® have been tested in low cycle fatigue (LCF) at 950°C. The specimens were 20 mm long dog bone shaped and cut from SX slabs with their length axis parallel to different defined crystallographic orientations. The SX was solidified with dendritic microstructure showing interdendritic solidification pores aligned parallel to the growth direction, which was also the <100> direction of the cubic SX. For achieving high creep resistance a precipitation hardening treatment was performed, which generated a typical γ-/γ’-microstructure with cubic ’-precipitates separated by channels. Creep resistance was further improved via solid solution strengthening of the channels by enrichment of the alloying elements Co, Cr, Re, and W. Specimens having different orientations with respect to the crystallographic system and the growth direction were tested in LCF until fracture under load control with triangular load pattern and a load ratio of about 0.62 to 0.65 in tension. Fractography revealed that cracks initiated at interdendritic pores, either in the volume or at the surface. On length sections two crack propagation paths were identified (1) perpendicular to the load direction within the γ-channels (see Figure 1) or (2) along {111}-slip planes (see Figure 2). In both cases intermetallic phases containing refractory alloying elements occurred, decorating the crack paths and slip planes, respectively. Such intermetallic phases, presumably topologically closed packed (TCP) phases, were further only observed close to the surface within a thin seam of 20 µm. Since in the specimen’s remaining volume no such phases appeared, it is assumed that their localized formation is linked to local stress-/strain-concentrations at pores, crack tips, and shear bands. While TCP formation is associated with depletion of solid solution hardening elements of the γ-phase, having negative influence on creep properties if occurring dispersed in the volume, its influence on fatigue properties appears to be ambiguous. It seems that localized formation of TCP-phases may reduce the crack propagation velocity either of cracks along γ-channels by causing crack tip unloading and crack path deviations or of cracks along slip planes by inhibiting further shear deformation. The effects of the observed microstructural mechanisms will be discussed with respect to LCF life.