Effect of Wall Thickness on the Microstructure and Properties of a Fourth-Generation Single Crystal Superalloy Dd91

Abstract

The solidification process of a single crystal superalloy with different thicknesses was simulated by Pro-cast software. The effects of wall thickness on the as-cast microstructure, element segregation and microstructure were investigated using optical microscopy, scanning electron microscopy, and electron probe micro-analyzer techniques, and stress rupture properties of the alloy with different wall thickness were compared. The results indicate that the larger thickness leads to a slower solidification rate of the alloy and stronger dendritic segregation. Slower solidification rate and stronger dendritic segregation result in an increased of eutectic fraction, primary dendrite arm spacing, and porosities density. Stress-rupture test to specimens with various thicknesses (0.3mm, 0.5mm, 1.0mm, and 1.5mm) was conducted at 1100°C/150MPa. It was shown that the stress-rupture life of the alloy increase with larger thickness under the range of 0.3mm to 1.5mm. Samples with different thicknesses have different specific surface area, which can affect the internal morphology of γ' precipitates and the precipitation of carbides after heat treatment. The porosity and morphology of γ' precipitates varied with thickness, which lead to a different extent of reduction in the effective load-bearing cross-section of the castings and the formation of discontinuous Al2O3 oxide layers. The thin-wall effect is influenced by both the internal microstructure variations within the alloy and surface oxidation, which act as dual factors in this regard.

Keywords: Ni-Base superalloy, thickness, Microstructure, properties