Substructure strengthening in dispersion-strengthened nickel alloys

Pure nickel, 80 pct Ni-20 pct Cr, 98 pct Ni-2 pct ThO₂, and 78 pct Ni-20 pct Cr-2 pct ThO₂ were studied in a wide range of thermomechanical conditions to identify strengthening mechanisms in the dispersion-strengthened materials. An X-ray line profile technique was used to determine the distribution of lattice strain, the crystallite domain size and the incidence of twins and stacking faults. Transmission electron microscopy was carried out, and tensile tests were done at room temperature and at an elevated temperature. It was found that cold deformation of Ni?ThO₂ did not produce lattice strains as large as was the case with pure nickel and Ni?Cr. However, deformation of Ni?Cr?ThO₂ did generate high lattice strains, due it is thought to the influence of chromium on cross-slip. The materials containing high lattice strains recrystallized more readily on annealing or testing at high temperature. It was concluded that room temperature strength was related to domain size without regard to composition in the series investigated. Strengthening by particle-dislocation interaction was not thought to be applicable when the domain size was small compared to the interparticle spacing, or at elevated temperatures. High temperature strength was determined primarily by the presence of a polygonized dislocation substructure which was stabilized by the thoria dispersion.