Koji Kakehi
Department of Mechanical Engineering, Graduate School of Engineering, Tokyo Metropolitan University, Hachioji 192-0397The yield strength of high strength nickel-base superalloys for gas turbine blades increases with increasing temperature up to a peak temperature (650~800°C). The influence of crystallographic orientation on the high-temperature mechanical properties of single-crystal nickel-base superalloys was investigated by a systematic study of the deformation mechanism at a temperature above the peak temperature. Tensile and creep tests were carried out at 900°C. Initial tensile orientations were selected over a wide range on the standard stereographic triangle. In the tensile test, Schmid's law, based on an assumption of {111}<101> slip system, broke down because of the cross slip of superlattice dislocation pairs from the unique slip plane. The effect of both cube slip in the γ' phase and by-passing mechanism is to give a suppression of the yield strength in [111] orientation. In the creep test, it was found that the orientations in which the operation of {111}<101> slip systems is unstable exhibited the planar slip fracture surfaces and low ductilities; on the contrary, the stable-slip orientations exhibited the large ductilities, and the specimen with [111] orientation exhibited the longest rupture life as a result of a low Schmid factor for the {111}<101> slip systems. Therefore, creep deformation occurs primarily by viscous slip of the relaxed a/2<101> dislocation pairs controlled by climb of the anti phase boundary. Furthermore, the operation of cube slip in the γ' phase, as well as octahedral slip which shears both γ'-γ phases, gave a good account of the orientation dependence of creep strength.
single crystals, nickel-base superalloy, crystallographic orientation, creep strength, elevated-temperature tensile strength, viscous slip, octahedral slip, cube slip