The effect of creep exposure on microstructure stability and tensile properties of single crystal nickel based superalloy CMSX-4

The effect of creep exposure on microstructure stability and tensile properties of single crystal nickel-based superalloy CMSX-4 was studied. The single crystal samples with [001] crystallographic orientation were prepared by directional solidification in a Bridgman type of furnace. The creep exposure at 950 degrees C, applied stresses ranging from 60 to 230 MPa for various time up to 2000 h leads to the degradation of initial cuboidal gamma/gamma' microstructure. Coarsening of cuboidal gamma'(Ni-3(Al, Ti)) precipitates, formation of transient microstructure consisting of coarsened gamma' precipitates and rafts and finally development of well rafted gamma/gamma' microstructure are observed in the crept specimens. The width of the gamma' rafts decreases and their length increases with increasing applied stress and creep time. The width of the gamma (Ni-based solid solution) channels increases with increasing applied stress and creep time. The creep rafting decreases the offset yield stress and ultimate tensile strength in the temperature from range 20 to 950 degrees C when compared to those of the heat treated specimens with cuboidal gamma/gamma' microstructure. On the other hand, creep rafting increases significantly room and high temperature ductility of the studied alloy. The creep fracture surfaces exhibit palisade morphology formed by propagation of cracks along (001) crystallographic planes in directions perpendicular and parallel to loading direction.