Lattice parameters and bonding characteristics of boron-doped hafnium containing trinickel aluminide alloys

The lattice parameters of boron-doped γ′ phase (Ni3Al) containing hafnium were measured at room temperature after quenching from 1130°C. The lattice parameter a in angstroms is given by the following best fit expression: a=6.9987−7.52XNi−3.785XAl+4.47XNi2+4.38XAl2+2.00XNiXAl. The composition dependence of the partial volumes of Ni, Al and Hf calculated from lattice parameters suggest that in alloys containing 75 at.% Ni, the boron atoms occupy the edge centered octahedral sites. Hafnium atoms tend to occupy the aluminium sites when the hafnium content is low while they start occupying nickel sites also when the hafnium content is increased. In nickel-rich alloys containing 77 at.% Ni, the boron atoms tend to occupy some of the body centered octahedral sites in addition to the edge centered octahedral sites. The lattice parameters of the binary stoichiometric γ′ phase measured as a function of quenching temperature show an unusual temperature dependence. The lattice parameter tends to increase with increase in quenching temperature up to about 1000°C and then tends to decrease slightly beyond 1000°C.