The role of grain size and shape in strengthening of dispersion hardened nickel alloys

Abstract

Thermomechanical processing was used to develop various microstructures in Ni, Ni-2ThO 2, Ni-20Cr, Ni-20Cr-2ThO 2, Ni-20Cr-10W and Ni-20Cr-10W-2ThO 2, and the influence of microstructure on room temperature and elevated temperature strength was investigated. The yield strength at 25°C increased with substructure refinement according to the Hall-Petch relation. It was found that substructure refinement was a much more potent means of strengthening at room temperature than was dispersion hardening. At elevated temperature (1093°C), the most important microstructural feature affecting strength of dispersion hardened nickel alloys was the grain aspect ratio, i.e. grain length, L, divided by grain width, l. The yield strength and creep strength increased linearly with increasing L l and the following relation was obeyed: σ = σ e + K( L l −1) , where σ e = strength at L l = 1 (equiaxed grains) and K = a constant, denned as the grain aspect ratio coefficient. The significance of this relation is discussed in terms of grain boundary sliding, which is a major mode of deformation at elevated temperatures.