Distinctive role of plastic and creep strain in directional coarsening of a Ni-base single crystal superalloy

The distinctive role of plastic and creep strain in directional coarsening of a single crystal Ni-base superalloy, CMSX-4, was experimentally and analytically investigated. First, solid cylindrical samples were subjected to uniaxial tensile plastic strain at 900 °C followed by thermal aging at 1080 °C. Microstructural observation revealed that the extent of directional coarsening became more pronounced with increasing plastic strain until it reached 0.2%; no further coarsening was induced with the introduction of additional plastic strain. Second, several combinations of plastic and creep strain were applied to the samples. The magnitude of the compressive creep strain at 1080 °C was varied with the plastic strain fixed at 1.5% at 900 °C. The results indicated that the effect of 0.15% creep strain was completely balanced with that of the 1.5% plastic strain; hence, the effects of the plastic and creep strain on the morphological changes differed quantitatively. Finally, elastic-plastic and creep finite element analyses were conducted to rationalize the experimental results. The analytical investigations suggested that the directional coarsening was dependent on the inelastic strain gap between the γ and γ' phases, which was attributed to an inhomogeneous relaxation of the local misfit stress field. The difference in the roles of plastic and creep strain in the coarsening was observed to quantitatively correspond to the difference in the localized inelastic deformations.