Mechanism of boron-induced strengthening of grain boundaries in Ni₃Al

Abstract

Poor cohesive strength of grain boundaries in strongly ordered Ni₃Al is due to porous structure of grain boundaries with crack-like microcavities which open up under small tensile stresses. Interstitial B atoms in Ni-rich Ni₃Al reduce the strength of directional bonding between Ni and Al atoms in the interior of the grains. When two such grains with reduced Ni—Al bonding meet each other, the atoms near the grain boundary can relax more easily and the size of the microcavities is reduced. Furthermore, interstitial B atoms increase ductility in the interior of grains so that dislocations can be easily generated to shield the tips of cracks initiated from the cavities, providing increased resistance to intergranular fracture. Evidence for the above mechanism of B-induced strengthening of grain boundaries in Ni₃Al is presented.