Thomas Mack

Characterization of phases of aluminized nickel base superalloys

Aluminum rich coatings, built up by a diffusion zone and a NiAl-cover layer, can protect the surface of turbine blades against oxidation. Within the single crystalline substrate and the adjacent layer, phases in the range of several tens of nanometers up to a few micrometers develop during production and operation of the turbine blade, were characterized. Investigations with transmission electron microscopy, nanoindentation and local crystal orientation mapping with a scanning electron microscope have been carried out in order to determine composition, morphology and distribution of the different phases. The diffusion zone has in general a defined orientation relative to the superalloy substrate and is built up by at least three phases embedded in a softer matrix, with significant differences in nanohardness. Local internal stress states in the diffusion zone are estimated. The NiAl-cover layer is a coarse columnar grained, non-textured B2 ordered intermetallic NiAl-phase.

Influence of tungsten and rhenium concentration on creep properties of a second generation superalloy

Abstract Creep properties at three different temperatures (1123, 1253 and 1373 K) for five single crystal alloys with varying Re and W content are presented. Tungsten content has been varied between 3.0 and 3.6 wt.%, rhenium between 2.3 and 2.9 wt.%. Creep properties, such as minimum creep rate, time to reach 1, or 2% strain and time to failure have been evaluated. For the low temperature regime, two parts of rhenium can be replaced by one part of tungsten. In the intermediate range, the ratio cW/cRe≈1.3 has to be fulfilled with the constraint that cRe≥2.3 wt.%. At high temperature the sum of W plus Re concentration determines the time to failure.