Mechanical properties of conventionally cast,directionally solidified,and single-crystal superalloys

In this paper the authors compare the creep and low-cycle fatigue properties of conventional, directionally solidified, and single-crystal castings produced from nickel-base superalloys. A brief historical review describes the reasons for the evolution from wrought to cast product through directionally solidified to modern single-crystal (‘monocrystal’) castings. The influence of microstructural variations produced by the casting conditions, such as porosity and grain size, on creep and low-cycle fatigue properties are illustrated. The important aspects of postsolidification heat treatment, hot isostatic pressing, and the damaging effects of impurities are described for conventional castings. The results of controlling the microstructures produced by directional solidification especially by high temperature gradient solidification are demonstrated by comparing the creep properties of directionally solidified materials with those of the conventionally cast alloys in long-term tests. The creep and low-cycle fatigue properties depend on the stress direction relative to the crystallographic directions of the material for both directionally solidified and single-crystal castings. For single crystals, individual alloys show variable dependences of properties on the crystallographic directions. Directionally solidified materials show advantages in thin sections and are less sensitive to the effects of impurities compared to conventional castings.

MST/329