An investigation of the Ni₅Al₃ phase

The crystallography of the phases obtained by heat treating a Ni-34.7 at. % Al alloy with 0.9 at. % Hf and 0.04 at. % B was studied in detail using x-ray diffraction techniques. Particular emphasis was placed on characterizing the Ni₅Al₃ phase, which is obtained by quenching the B2 NiAl phase and aging the resultant martensite. The structure of the Ni₅Al₃ phase is confirmed to be orthorhombic with space group D¹⁹_2h (Cmmm) and lattice parameters of a₀ = 0.7475 nm, b₀ = 0.6727 nm, and c₀ = 0.3732 nm. The transformation from B2 NiAl to L1₀ martensite is shown to involve a Bain distortion of the c/a ratio from 0.707 to 0.853, and the further ordering transformation to Ni₅Al₃ involves a further increase of this c/a ratio to 0.900.