Abstract
The effect of thermal cycling on the microstructural stability of a unidirectionally solidified eutectic alloy has been studied by electron microscopy. Such alloys are candidate materials for use as turbine blades in advanced aeroengines and as such must have a high degree of microstructural stability. The initial alloy microstructure, which consisted of a uniform distribution of predominantly square-section niobium carbide rods in a nickel-chromium base alloy, suffered severe microstructural degradation on thermal cycling, the highly faceted carbide/matrix interface breaking down to give irregularly shaped carbide cross-sections. The driving force for this breakdown process is not reduction of surface energy as in conventional precipitate coarsening, but is related to fibre dissolution and reprecipitation processes. These are aided by the high dislocation density at the interface generated by the thermal expansion mismatch between the two phases and by the high stress gradients that exist there. Means of improving alloy performance by compositional modification are discussed.
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