Effect of Grain Size on Strength, Ductility and Fracture in Recrystallized Ni3Al Doped with Boron

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Abstract
Tensile tests at room temperature were performed on recrystallized Ni3Al doped with boron having grain sizes from 1.6 to 208 μm to determine the effect of grain size on the strength, ductility and fracture. Both yield and flow stresses depend on grain size in agreement with a Hall-Petch relation when the specimen size effect is taken into account. The increase in Hall-Petch slope with strain is explained in terms of the difference in work-hardening rate for different grain sizes. The increase of lattice friction stress with strain is found to be analogous to the single crystal properties. While the {111} cracking is predominant without necking for coarse-grained specimens, the slant-type fracture and cup-cone fracture preceded by a necking is mainly observed for intermediate and fine-grained specimens, respectively. Large elongation is developed when the intersection of slip bands becomes insufficient to produce the {111} cracking, and the void formation which results in the cup-cone fracture is retarded.