BORON IN POLYCRYSTALLINE NI3AL - MECHANISM OF ENHANCEMENT OF DUCTILITY AND REDUCTION OF ENVIRONMENTAL EMBRITTLEMENT

A model is proposed to explain on a unified basis the role of boron in enhancing ductility and reducing environmental embrittlement in polycrystalline Ni3Al. The grain boundaries in strongly ordered Ni3Al have a porous structure with crack-like microcavities, which can open up under small stresses. Furthermore, atomic hydrogen, generated by the reaction of environmental moisture with Al, can diffuse to the tips of the microcavities under stress and can cause embrittlement. It is argued that strong bonding between interstitial B atoms and Ni atoms in B-doped Ni3Al reduces the strength of directional bonding between Ni and Al atoms in the interior of the grains. When two such grains with weakened Ni-Al bonding meet each other, the atoms near the grain boundary can relax easily and close up the microcavities. As a result, the ductility is enhanced in B-doped Ni3Al. The environmental embrittlement is also reduced, because a small amount of environmental hydrogen cannot nucleate microcracks. Numerous experimental observations have been explained with the proposed mechanism.