IMPURITY EFFECTS ON ATOMIC BONDING IN NI3AL

First-principles electronic structure calculations based on the full-p otential linear-muffin-tin-orbital method have been employed to study the contrasting effects of boron and hydrogen on the electronic struct ure of the L 1(2) ordered intermetallic Ni3Al. The total energy, the s ite- and l-projected densities of states, and the impurity-induced cha rge-density characteristics are calculated for various impurity config urations, to investigate the effects of local environment on the elect ronic structure. Total-energy calculations show that both boron and hy drogen impurities prefer to occupy octahedral interstitial sites that are entirely coordinated by six nickel atoms. Our results suggest that the underlying mechanism of the boron-induced strengthening in Ni3Al is the Ni-d and B-p hybridization between the nearest-neighbor nickel and boron sites. This results in an enhancement of the intraplanar met allic bonding between the nickel atoms, an enhancement of interstitial bonding charge, and reduction of the bonding-charge directionality ar ound the Ni atoms on the (001) NiAl planes. In contrast, hydrogen is f ound to enhance the bonding-charge directionality near some Ni atoms a nd to reduce the interstitial charge, suggesting that it promotes poor local cohesion. When both boron and hydrogen are present in Ni3Al, th e dominant changes in the electronic structure are induced by boron an d the charge distribution resembles that of Ni3Al+B. These results are broadly consistent with the notion of boron as a cohesion enhancer an d hydrogen as an embrittler.