The influence of orientation on the stress rupture properties of nickel-base superalloy single crystals

The influence of orientation on the stress rapture properties of MAR-M247 single crystals was studied. Stress rupture tests were performed at 724 MPa and 774 °C where the effect of anisotropy is prominent. The mechanical behavior of the single crystals was rationalized on the basis of the Schmid factors for the operative slip systems and the lattice rotations which the crystals underwent during deformation. The stress rupture lives at 774 °C were found to be greatly influenced by the lattice rotations required to produce intersecting slip, because second-stage creep does not begin until after the onset of intersecting slip. Crystals which required large rotations to become oriented for intersecting slip exhibited a large primary creep strain, a large effective stress level at the onset of steady-state creep, and consequently, a short stress rupture life. Those crystals having orientations within about 25° of the 001] exhibited significantly longer lives when their orientations were closer to the 001]-011] boundary of the stereographic triangle than to the 001]-1l 1] boundary, because they required smaller rotations to produce intersecting slip and the onset of second-stage creep. Thus, the direction off the 001], as well as the number of degrees off the 001], has a major influence on the stress rapture lives of single crystals in this temperature regime. REBECCA A. MacKAY, formerly Graduate Assistant, Case Western Reserve University, Cleveland, OH RALPH D. MAIER, formerly Assistant Professor, Case Western Reserve University