On the primary creep of CMSX-4 superalloy single crystals

The effect of orientation on the primary-creep mechanism of the Ni-based single-crystal superalloy CMSX-4 is examined. Four specimens with orientations within 20 deg of the 001] axis are deformed at 750 °C and 750 MPa and show a decreasing amount of primary creep as the tensile axis approaches the 001]–011] symmetry boundary. Of these, specimen N lies within 1 deg of 013] and shows a negligible primary creep and a very low secondary creep rate. For this specimen, direct observation in the transmission electron microscope (TEM) and analysis of the shape change show that 〈112〉{111}, stacking-fault shear, is absent. Analysis of the dislocations present in the gamma precipitates and at the γ/γ′ interface shows that the only dislocations present have the Burgers vectors a/2101] and a/210-1], which cannot combine to nucleate the dislocations necessary for stacking-fault shear. Thus, it is argued that the reason for the low degree of primary creep for orientations close to the 001]–011] symmetry boundary is not the interaction between two equally stressed 〈112〉{111} systems, but the lack of either. As the orientation moves away from the 001]–011] boundary, the range of dislocation Burgers vectors increases and stacking-fault shear is nucleated; the amount of primary creep increases as a consequence.