Uwe Paul

Nickel aluminides: a step toward industrial application

Abstract Intermetallics based on nickel aluminide are subjects of an ongoing development of new high-temperature materials for application as heat shields for combustion chambers and as first-row vanes in industrial gas turbines. Due to the complex geometry of these parts and the complicated machinability of NiAl intermetallics, these components have to be manufactured by near-net-shape casting. This casting technique required an adaptation of the investment casting process to the special properties of NiAl alloys. For this purpose, a special ceramic mold system was developed to prevent casting defects and excessive mold–metal reactions. Suitable casting parameters were established for the production of grain-refined, crack-free heat shields. Accompanying numerical simulations were performed to optimize the investment casting process for the NiAl-alloy helped to save development time and to reduce the number of casting trials noticeably. These modeling results for various casting conditions (mold and pouring temperature, cooling rate) were compared with outcomes of casting trials in aspects of porosity, mold filling and temperature-field development during the casting process.

Inhomogeneities in single-crystal components

Abstract The temperature distribution during single-crystal solidification of the Ni-based superalloys SRR99 and CMSX-6 by the Bridgman process was measured at various withdrawal rates in dummy turbine blades. Asymmetric heat flux and cross-section transients influence the geometrical formation of the liquidus isotherm and consequently of dendritic growth. The probability of the formation of structural inhomogeneities such as microporosity, zebras, mosaic structures, etc. is largely dependent on the curvature of the solidification front, which in turn depends on the geometrical arrangement of the ceramic cluster and on the withdrawal rate. The proneness to recrystallization during γ′ solutioning at cross-section transients due to casting-induced deformation and misaligned dendrites was also examined. Recrystallization remains confined to highly deformed areas as long as microscopic inhomogeneities acting as obstacles are present during heat treatment. High cycle fatigue tests showed that defects such as porosity and local recrystallization reduce the fatigue strength to about half the fatigue strength of the defect-free materials.