Effect of impurities (graphite) on the high-temperature creep-properties of NIAL

Previous high-temperature compression creep studies of NiAI have shown peculiar behaviour in the temperature range 700 to 900° C, which was perhaps due to precipitation
of impurities in the matrix. To isolate the "impurity" effect, high-purity NiAI samples with 0.15 at% and 0.20 at% carbon (graphite) additions have been creep tested
at four temperatures between 700 and 850° C. Addition of graphite has been shown to produce a significant reduction in the creep strength of the alloys. However, alloys with
higher graphite concentrations have shown better creep resistance than those with lower graphite concentrations. Transmission electron microscopy indicates the presence of
competing softening and hardening mechanisms in the alloys. Softening is due to the graphite particles acting as a dislocation source. Hardening results from a grain-boundary hardening mechanism due to the graphite particles segregating at grain boundaries and a dislocation—impurity (fine graphite) interaction, developing a Cottrell-like atmosphere.