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

Strain-controlled high-temperature (950 and 1050 degrees C) low-cycle fatig ue (LCF) experiments were conducted on monocrystalline specimens of the gam ma'-hardened nickel-base superalloys CMSX-4 and CMSX-6. The goal was to inv estigate the effect of different gamma/gamma'-morphologies on the LCF behav iour. For this purpose, the LCF tests were performed on specimens with thre e different initial gamma/gamma'-morphologies, namely cuboidal gamma'-parti cles, gamma/gamma'-rafts perpendicular and gamma/gamma'-rafts parallel to t he stress axis, respectively. The raft structures had been introduced by a small high-temperature creep pre-deformation (plastic strain less than or e qual to 0.4%) in tension or compression. The LCF tests showed that the fati gue lives were reduced for specimens with gamma/gamma'-rafts perpendicular to the stress axis and extended for specimens with gamma/gamma'-rafts paral lel to the stress axis. Specimens with cuboidal gamma/gamma'-particles exhi bited intermediate fatigue lives. Fractographic investigations revealed the reasons for this behaviour. The fatigue cracks usually followed along the gamma-channels or the gamma/gamma'-interfaces and avoided cutting the gamma '-phase. Thus, fatigue cracking was facilitated when the gamma/gamma'-rafts were perpendicular to the stress axis and obstructed, when the gamma/gamma '-rafts lay parallel to the stress axis. (C) 1999 Elsevier Science S.A. All rights reserved.