Grain Aspect Ratio Strengthening of Dispersion Hardened Metals.

Abstract : The effect of a prestrain dislocation substructure on the tensile creep of MgO single crystal specimens compared with annealed specimens was investigated. Specimens having a substructure consisting of subgrains were slightly stronger than those having a uniform dislocation distribution but the strongest condition were annealed specimens having few dislocations. Without strong pinning, the prestrain dislocation substructure merely provided a higher density of mobile dislocations. Substructural strengthening of an oxide dispersion hardened metal, TD-NiCr showed that a dislocation substructure added only a modest amount to the creep strength and the effect appeared to be sensitive to the level of prestrain used. The temperature and strain rate dependence of the grain aspect ratio effect on TD-Ni was investigated at 871 and 600 C over a range of strain rate. The strengthening effect of elongated grains persisted down to 600 C although strengthening due to grain refinement also contributed at 600 C. The strength increased with increasing grain aspect ratio (G.A.R.) linearly up to a ratio of 10 to 15, after which the strength increased only slowly with further increase in G.A.R. This limit of G.A.R. = 10 to 15 was independent of temperature and strain rate. An expression combining grain boundary sliding and deformation of the grains duplicated the form of the G.A.R. effect on creep strength. (Author)