A detailed study was made of the changes in orientation of monocrystalline superalloys which were crept to fracture in tension using stresses of 200 to 850MPa at 750 to 1050C. Specimens of SRR99 with nominal axial orientations of [001], [111] and [011], and specimens of CMSX-4 with [111] and [011] orientations, were considered. Electron back-scattering diffraction analysis of [011] and [111] crystals which had been tested at stresses above 450MPa revealed highly deformed regions (about 1μm in size) that were rotated by 60º about <111>. Transmission electron microscopy confirmed that this orientational relationship arose from twinning. Such features were not observed in the case of [001] specimens.

Twin Formation during Creep in Single Crystals of Nickel-Based Superalloys. M.G.Ardakani, M.McLean, B.A.Shollock: Acta Materialia, 1999, 47[9], 2593-602