Porosity Clusters and Recrystallization in Single-Crystal Components

Abstract

The influence of cross-section transients on dendritic single-crystal growth in the presence of asymmetric heat flux was investigated for CMSX-6 and SRR99, two nickel-base superalloys. The probability of the formation of porosity clusters is largely dependent on the curvature of the solidification front, which in turn depends on the geometrical arrangement of the cluster and on the withdrawal rate. The proneness to recrystallization during J-solutioning at the cross-section transients due to casting-induced deformations and misaligned dendritic growth was also examined. Recrystallization remains confined to the highly deformed areas as long as microstructural inhomogeneities, e.g. eutectics, are present during the heat treatment. HCF-testing established that these kinds of defects (porosity clusters as well as recrystallized grains) reduce the fatigue strength to about one half of that of defect-free material.