The effect of boron addition and aluminum stoichiometry on mechanical properties and solidification structure in $Ni_3Al$ were studied. Specimens with varying boron and aluminum concentrations were prepared by arc melting and rapid solidification of melt spinning.
Room temperature ductility and fracture behavior depended greatly on deviation from aluminum stoichiometry. When tensile tested as melt spun state, boron improved both ductility and yield strength dramatically in hypostoichiometry (24% Al) and stoichiometry (25% Al) $Ni_3Al$. But in hyperstoichiometry (26% Al) $Ni_3Al$, boron did increase yield strength, but it did not improve the ductility of the alloy. However, annealing at 800℃ or 1000℃ for 2 hours after melt spinning for the B-doped hyperstoichiometry $Ni_3Al$ resulted in about 10% tensile elongation. And fracture mode also changed from pure intergranular fracture mode to mixed mode of intergranular and transgranular fracture occurring simultaneously.
It was found that yield strength of $Ni_3Al$ increased with increased boron concentration regardless of both aluminum stoichiometry and annealing temperature. However, the degree of strengthening was more pronounced in hyperstoichiometry $Ni_3Al$.
It is proposed that the difference in the degree of strengthening was due to phase stability or APB energy differences rather than simple solid solution hardening.
The solidification structure of $Ni_3Al$ also changed markedly with varying aluminum and boron concentrations. In hypostoichiometry $Ni_3Al$, grain contained dendrites of similar shape. But in hyperstoichiometry $Ni_3Al$, grain contained irregularly shaped dendrites considered as β phase which did not transform to γ' phase.
In hypostoichiometry $Ni_3Al$, grain boundary second phase precipitated as boron concentration exceeded 0.75%. But in stoichiometry and hyperstoichiometry $Ni_3Al$, second phase cannot be observed up to 1% boron concentration.
