Effect of temperature range on extent of microstructural degradation during thermal cycling of unidirectionally solidified eutectic composite

Abstract

A refractory carbide strengthened nickel-base alloy was directionally solidified at 5 mm h^—1 to produce a regular well-aligned eutectic alloy consisting of niobium carbide fibres in a nickel–chromium matrix. The fibrous microstructure which is extremely stable under isothermal annealing conditions suffers severe degradation during thermal cycling. The upper cycling temperature and the temperature range of the cycling treatment were found to strongly affect the extent of fibre degradation, which increased as both upper cycling temperature and temperature range increased. Fibre degradation occurred by nucleation and growth of nodules at the corners of some fibres and by the dissolution of other fibres. Possible mechanisms for such microstructural breakdown are discussed.