UNUSUAL LATTICE DEFECTS IN NON-STOICHIOMETRIC NIAL

Planar defects, which frequently appear to be terminated by a dislocation at one end only, have been observed by transmission electron microscopy in aluminium-rich alloys of the ordered body-centred cubic phase NiAl. Analyses of their geometry and contrast show that the faults lie on cube planes with displacement and associated dislocation vectors normal to the fault plane. The faults are considered to be platelets of aluminium atoms sandwiched between layers of nickel vacancies. Fault contrast fades with distance from the terminating dislocation(s) as a result of the gradual relaxation of the compressive stresses associated with the configuration. This method of accommodating excess aluminium atoms is discussed with reference to previous density, x-ray and nuclear magnetic resonance results (Bradley and Taylor 1937, Cooper 1963, 1964, Eltis and Hall 1965, West 1964). © 1969 Taylor & Francis Group, LLC.