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Substructure strengthening in nickel alloys Clegg, Maurice Alexander

Abstract

Pure Nickel, 80% Ni-20% Cr, 98% Ni-2% ThO₂ and 78% Ni-20% Cr-2% ThO₂ have been studied to compare the mechanisms of dispersion strengthening in the latter two materials. All four materials were subjected to a wide range of thermo-mechanical treatments including cold rolling reductions up to 90% and annealing treatments which in the case of the materials containing ThO₂ were at temperatures up to 0.97Tm (1400°C). In each condition the materials were examined by an x-ray line profile technique to determine the distribution of non-uniform lattice strain, the coherent crystallite domain size and the twin and stacking fault probabilities. The x-ray data were also interpreted in terms of dislocation configurations. Supporting transmission electron microscopy was carried out on each material. Tensile tests were done at room temperature and at an elevated temperature on all materials to determine the strength properties and initial work hardening rate. It was concluded that the room temperature strength and the high temperature strength of all materials were determined by the presence of a fine domain or cell size and that the high temperature strength was also determined by the degree of polygonization of the fine substructure boundaries. In the case of the dispersion strengthened alloys the substructure boundaries were stabilized by the presence of ThO₂. It was observed that the Ni-Cr-ThO₂ material developed a different lattice strain distribution during cold rolling than did the Ni-ThO₂. This was characterized by a very small domain size and a high level of lattice strain averaged over short distances. It is proposed that this strain distribution favours recrystallization and grain growth and leads to the inferior structural stability in cold rolled Ni-Cr-Th0₂ at elevated temperatures compared to Ni-ThO₂.

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