In this paper the microstructural instability of a recently developed powder-processed alloy, RR1000, is investigated and the corresponding influence on the creep-fatigue crack growth behaviour scrutinised. Equilibrium phase calculations of the alloy composition predict the presence of the TCP (Topologically Close Packed) phase sigma at the intended operating temperature of the alloy. Subsequent expousures at elevated temperatures reveal intergranular sigma phase precipitation. The kinetics of singma phase formation has been determined experimentally by quantitative x-ray diffraction of exposed specimens. The effect of this grain boundary modification during prolonged exposure on the mechanical properties of the alloy was then investigated by testing specimens with two thermal histories: as-heat-treated (with a standard solution and ageing heat treatment developed for RR1000) and exposed to contain 1 wt
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