Dependence of the high-temperature low-cycle fatigue behaviour of the monocrystalline nickel-base superalloys CMSX-4 and CMSX-6 on the γ/γ′-morphology

Strain-controlled high-temperature (950 and 1050°C) low-cycle fatigue (LCF) experiments were conducted on monocrystalline specimens of the γ′-hardened nickel-base superalloys CMSX-4 and CMSX-6. The goal was to investigate the effect of different γ/γ′-morphologies on the LCF behaviour. For this purpose, the LCF tests were performed on specimens with three different initial γ/γ′-morphologies, namely cuboidal γ′-particles, γ/γ′-rafts perpendicular and γ/γ′-rafts parallel to the stress axis, respectively. The raft structures had been introduced by a small high-temperature creep pre-deformation (plastic strain ≤0.4%) in tension or compression. The LCF tests showed that the fatigue lives were reduced for specimens with γ/γ′-rafts perpendicular to the stress axis and extended for specimens with γ/γ′-rafts parallel to the stress axis. Specimens with cuboidal γ/γ′-particles exhibited intermediate fatigue lives. Fractographic investigations revealed the reasons for this behaviour. The fatigue cracks usually followed along the γ-channels or the γ/γ′-interfaces and avoided cutting the γ′-phase. Thus, fatigue cracking was facilitated when the γ/γ′-rafts were perpendicular to the stress axis and obstructed, when the γ/γ′-rafts lay parallel to the stress axis.