Microstructural changes in a single crystal Ni-base superalloy induced by plastic straining

Morphological changes in the γ/γ′ microstructure of a single crystal Ni-base superalloy, CMSX-4, induced by plastic pre-strains and a subsequent heat treatment were experimentally investigated. To study the effects of the straining temperature, uni-axial tensile plastic strains were introduced into the solid cylindrical specimens at room temperature, 700 °C and 900 °C. For the evaluation of the roles of strain distribution and residual stress field, compressive loading was applied to ring shaped specimens at the three temperatures. It was found from the experiments that the uni-axial plastic strain at 700 °C and 900 °C could induce directional coarsening after subsequent heat treatment at 1080 °C in the same manner as the rafting phenomena, while the plastic strain applied at room temperature caused a localized morphological change only near slip bands. It was also found by the experiments using the ring shaped specimens that the directional coarsening was governed by the sign and magnitude of residual stress rather than plastic strain. A series of experiments were systematically discussed in terms of the temperature dependencies of the plastic deformation and inhomogeneous relaxation of misfit strain at the γ/γ′ interface.