Modelling the orientation and direction dependence of the critical resolved shear stress of nickel-base superalloy single crystals

The orientation and direction dependence of the critical resolved shear stress was determined experimentally for the chromium-rich superalloy SC 16 at 650, 750 and 850°C and a constant strain rate of 10⁻³ s⁻¹. The results are used to establish an extended Schmid law for octahedral slip in the temperature and orientation range in which cross-slip pinning of dislocation pairs in the γ′ phase takes place. Normal Schmid behaviour was assumed for orientations near [111], for which cube slip was activated on a macroscopic level. Differences between some commercial superalloys are worked out and can be attributed to morphology and volume fraction of the γ′ phase. The orientation dependence and asymmetry effects increase in the order NIMONIC 105, SC 16, René N4. The orientation range where macroscopic cube slip can be expected increases in the same order. A close inspection of the parameters which are responsible for non-Schmid behaviour suggests that, in addition to cross-slip pinning, a matrix effect must be operating as well, partly counteracting the behaviour expected for mono-phase γ′ crystals.