Anisotropic creep in CMSX-4 in orientations distant from 〈001〉

Abstract

Constant load creep tests have been performed on the single crystal nickel base superalloy CMSX-4 in crystallographically unstable orientations distant from the 〈001〉 pole of the standard stereographic triangle at 1023 K (750°C) and varying stress levels. Orientations distant from the 〈001〉–〈111〉 boundary had high creep rates and relatively low rupture lives, while the orientations closer to it had relatively low initial creep rates and longer rupture lives. Electron back scatter diffraction patterns, cross section shape change measurements on creep tested specimens, and optical microscopy indicate that the operating slip system is of the type {111}〈112〉 in orientations far from the 〈001〉–〈111〉 boundary. The results are explained based on the resolved shear stress acting on the operating slip system taking into consideration the rotations and changes in cross section. A theory based on the ease of generation of superlattice intrinsic stacking faults is proposed to account for the observed creep rates in various orientations. A finite element model based on the slip system approach is used to demonstrate the validity of the experimental results.