Mechanism of 〈1 1 2〉/3 slip initiation and anisotropy of γ′ phase in CMSX-4 during creep at 750 °C and 750 MPa

Creep deformation defects from tensile creep tests carried out at 750 °C under 750 MPa in the approximate orientations [1̄ 3 6], [0 0 1], [0 1 1] and [1 1 1] for CMSX-4 single crystals have been identified carefully, using transmission electron microscope combined with computer simulated microimages. A series of results have been obtained. (i) It was confirmed that 〈1 1 2〉/3 slip in γ′ phase mainly involved the reaction and dissociation of two 〈1 1 0〉/2 dislocations which had different Burgers vectors and met at the γ/γ′ interfaces where the product 〈1 1 2〉/6 partials were trapped. (ii) At 750 °C and 750 MPa, the selectivity among {1 1 1}〈1 1 2〉 slip systems was sensitive to Schmid factors. The reason for the anisotropy was that the resolved shear stress of highly stressed {1 1 1}〈1 1 2〉 slip systems in the [1̄ 3 6], [0 0 1] and [0 1 1] orientations were just around the critical resolved shear stress of 〈1 1 2〉/3 faulting in γ′ phase which was estimated to be ∼300 MPa. Hence, a slight decrease of a RSS could switch off the operation of a {1 1 1}〈1 1 2〉 slip system, or vice versa. (iii) The activation of {1 1 1}〈1 1 0〉 slip systems was a necessary but not sufficient factor of initiating 〈1 1 2〉/3 faulting in the γ′ particles. The operation of 〈1 1 2〉/3 slip and density of superlattice stacking faults were controlled by the RSSs acting on those {1 1 1}〈1 1 2〉 slip systems.