The long-period antiphase-boundary-modulated structures in Cu3+xAl1−x

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Abstract
Observations by means of high-resolution electron microscopy (HREM) and electron diffraction (ED) of long-time annealed Cu3+xAl1−x martensite and α-Cu-Al precipitates reveal a large but limited number of commensurate uniform L12-based long-period antiphase-boundary-(LPAPB-)modulated structures (M=7/5, 4/3, 13/10, 9/7, 14/11 and 5/4). The satellite configurations in ED and the very regular antiphase boundary (APB) sequences in HREM support Fujiwara's model of a (commensurate) square-wave modulating function. The observation of the coexistence of different simple commensurate structures excludes the stability of more complex interpolating phases and suggests that the period of the LPAPB structures varies in a discontinuous way with external parameters such as the annealing temperature and the composition (via the electron-to-atom ratio). Also defective non-equilibrium antiphase boundary (APB) configurations, such as node-like junctions of minority domains, are analysed by means of HREM.