Ordering of the phases CoAl and NiAl

Abstract

In a recent paper West (1964) reports nuclear magnetic resonance (N.M.R.) intensity measurements for certain intermetallic compounds, which indicate a high degree of order in all compounds studied, particularly CoAl and NiAl. The relatively large intensities at compositions appreciably different from the equiatomic value for these two phases are accounted for by assuming that a relatively large volume of the material remains ideally ordered and that excess atoms, or vacancies, are not distributed at random. In particular it is suggested that even in NiAl the vacancies are gathering in small groups and that these eventually allow the formation of the Ni₂Al₃ structure. It is the purpose of this note to enlarge a little on the information concerning the ordering which is obtained from x-ray powder photographs and to suggest that considerable caution is required when N.M.R. results are interpreted in terms of ordering effects which conflict with x-ray observations.