Damage accumulation during creep deformation of a single crystal superalloy at 1150 °C

Microstructural degradation in the CMSX-4 single crystal superalloy during creep deformation at 1150 °C and 100 MPa is studied. Tensile deformation in the 〈0 0 1〉 direction is considered due to its technological importance. Under these conditions, rafting of the γ′ structure is completed quickly and within the first 10 h. It is demonstrated that the creep rupture event is highly localised, the instability being associated with a critical and well-defined strain being reached with failure occurring within a further few tens of hours. It is shown that the high strain rates and shear stresses associated with the rupture process are sufficient to cause realignment of the rafted γ′ structure with respect to the γ matrix. Creep cavitation damage near to the rupture surface is prevalent, at microporosity inherited from the casting process but more significantly, at topologically close-packed (TCP) phases and associated pores and voids formed in their vicinity which have formed via vacancy condensation. Hot isostatic pressing (HIPing) prior to creep testing reduces the incidence of casting microporosity, but the creep rupture life is not improved significantly. It is suggested that it is the formation of TCP phases which limits creep rupture life.