Small additions of up to 1 wt.-% magnesium improve the stress-rupture life of nickel-2½ vol.-% alumina to a level comparable with that of nickel–thoria (TD nickel) at 1150°C. The effect of magnesium on the dispersoid/matrix interfacial energy has been measured and the kinetics of coarsening of the alumina particles as a function of magnesium content has been studied. Alloys with the best stress-rupture-life properties contain particles that coarsen least rapidly. The reason for this is shown to be that magnesium reduces the diffusion coefficient for the slowest-moving atomic species in the matrix, and, when present in concentrations < 0.3 wt.-%, decreases the dispersoid/matrix interfacial energy.
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