Mise en évidence par diffraction électronique de la mise en ordre des atomes de nickel en excès par rapport à la stoechiométrie dans les alliages β'-NiAl riches en nickel: formation d'une surstructure Ni₂Al

In electron diffraction patterns of nickel-rich β′-NiAl alloys, many anomalies are observed. One of these is the appearance of diffuse intensity maxima between the reflexions of the B2 structure. This is explained by the short-range ordering of the excess nickel atoms on the simple cubic sublattice occupied only by aluminum atoms in the stoichiometric, perfectly ordered NiAl alloy. After annealing Ni–37.5 at.% Al and Ni–37.75 at.% Al for one week at 300–400°C, the diffuse intensity maxima transformed into sharp superstructure reflexions. These reflexions are explained by the formation of the four possible variants of an ordered hexagonal superstructure corresponding to the Ni₂Al composition (space group: Pm1, no. 164). This structure is closely related to the Ni₂Al₃ structure (same space group) formed by the ordering of vacancies on the nickel sublattice in aluminum-rich β′-NiAl alloys.