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Abstract:
The effect of replacement of molybdenum with small amount of tungsten on the stability of cobalt based super-alloys of Co-Ni-Mo-Al-Ta-Ti class has been presented. A small addition of tungsten (W) in Co-30Ni-(5-x)Mo-10Al-2Ta-2Ti-2W alloys stabilizes the cuboidal morphology of precipitates and increases the gamma' volume fraction. A 2 at% addition of W causes an increase of 60 degrees C in solvus temperature of the base superalloy to reach a value of 1130 degrees C with a slight increase of mass density to 8.79 g/cc. Beside partitioning into gamma', W also shifts the partitioning preference of Mo from the gamma' phase in 0W alloy to that of equal partitioning in both gamma and gamma' phases in 2W alloy. An interfacial confinement of Mo atoms could be observed at the gamma/gamma' interfaces that reduces interface energy leading to enhanced microstructural stability. The experimentally determined temporal evolution of average precipitate size in the 2W alloy at the temperatures of 800, 900 and 950 degrees C suggests a matrix diffusion limited coarsening kinetics. The estimated coarsening rate constant at 900 degrees C follows a quasi-steady state model and is comparable to those observed for W and Re containing Co-based superalloys. The activation energy for gamma' precipitate coarsening is estimated to be 258 +/- 6 kJ/mole, which is comparable to the Mo diffusion in the gamma-Co matrix suggesting Mo diffusion still controls the precipitate coarsening in the 2W alloy.
