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

摘要

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 theB2 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 Ni2Al3structure (same space group) formed by the ordering of vacancies on the nickel sublattice in aluminum‐ri