Modulated structures in AuNi alloys

Abstract

A modulated structure has been shown to develop on {100} planes in certain gold-nickel alloys on aging at temperatures in the vicinity of 100–200°C. The critical composition is at 45 at.% Ni and the maximum temperature at which these structures occurs is 220°C. The boundary of the modulated structure region is nearly symmetric, with extreme compositions at approximately 20 and 60 at.% Ni. This structure has been observed by means of electron diffraction and is characterized by the formation of satellite reflections on 〈100〉 axes around each fundamental peak. The modulated structures exhibited a composition amplitude of approximately 5 at. % and characteristic wavelengths which varied from 6.5 Å at 54 at.% Ni to 13 Å at 22 at.% Ni. An activation energy of 13,500 cal. per mol. was calculated for the rate limiting step in the low temperature region, corresponding to the energy of migration for vacancies in this composition range. The satellite reflections disappeared on aging just above the critical temperature ; on reaging at a lower temperature, the satellites did not reappear. This is taken to indicate that the presence of a large concentration of quenched-in excess vacancies is required for the development of the modulated structure. The formation of the modulated structure may be associated with the postulated spinodal decomposition in this system, although the theories of spinodal decomposition do not yet adequately account for the role of vacancies.