Study of deformation in single crystal superalloys has traditionally focussed on creep. Yield has had less attention as this is generally not a critical property for turbine blade applications, but is nevertheless important, particularly so where stress concentrations result in LCF fatigue challenges.

We present observations on yield of the commercial single crystal superalloy, CMSX-4*, at yield as a function of temperature and strain rate. These show a range of dislocation mechanisms spanning the regimes of yield and creep dependent on the deformation temperature and strain rate. Although the initial yield point is very consistent over a wide range of strain rates, the subsequent behaviour differs greatly. At high strain rates, typical of commercial tensile tests, a plateau followed by an abrupt but limited period of work-hardening is observed. As the strain rate is lowered the material displays a raid drop in flow stress with the strain before this happens being particularly sensitive to strain rate. The paper will characterise the deformation mechanism at various stages during flow and as a function of strain rate. Observations of the mechanism demonstrate that the reduction in flow stress is associated with a change in deformation mechanism from shear by coupled a/2<110> dislocations separated by Anti-Phase Boundary (APB) to super-partial dislocations separated by stacking faults. This latter mechanism is similar to deformation seen during creep at the same temperatures and the strain rates are close to rapid creep rates observed during primary creep. We look closely at the nature of the stacking faults and associated partials showing that they are in many respects the same as those seen during creep. We discuss the role these and other mechanisms play in determining the strength and work-hardening behaviour of the alloy.