High Temperature Combined Fatigue and Creep Behavior of Polycrystalline and Single Crystal Nickel Base Superalloys

An overview is presented on the high temperature combined creep and fatigue behaviour of polycrystalline and single crystal nickel base superalloys. Particular reference is made to extensive experimental studies carried out on the polycrystalline alloy MAR M002 alloy and its single crystal equivalent SRR99. hi general, the high temperature mechanical properties of the single crystal alloy are superior to the conventional polycrystalline alloy. This is mainly attributed to the lack of grain boundaries in the single crystal alloy. This lack of grain boundaries also leads to material response which is dependent on crystal orientation. The mechanical response and fatigue lives were examined when the alloys were subjected to total strain controlled limits, at constant temperatures between 750 °C and 1050 °C. Constant strain dwells were introduced at peak tensile and compressive strains. Comparisons between the high temperature mechanical response of polycrystalline and single crystal alloys can be made by introducing an orientation function based on the anisotropic elastic response of the single crystal. Mean stresses generated during combined fatigue-creep loading played a major role in determining the fatigue life of the alloys. For the single crystal alloys fatigue lives must be correlated with orientation modified total strain ranges for mean stress effects to be quantified Comparisons with other single crystal alloys are also made. Finally, a significant improvement in the high temperature fatigue-creep lives of single crystal over polycrystalline alloys is evident.