Chaotic and uniform regimes in incommensurate antiphase boundary modulated Cu3Pd alloys (18-21 at.%Pd)

High-resolution electron microscopy, electron diffraction and optical diffraction have been used to analyse in detail the antiphase boundaries (APBs) constituting the long-period structure (LPS) in low-Pd Cu-Pd alloys. The evolution of APB density, periodicity and sharpness was studied as a function of temperature in view of the correlation of the observations with statistical models such as the ANNNI-model and the phason-soliton descriptions. The measurement of defect widths from high-resolution images is critically assessed, and a way of quantifying the degree of wavyness and periodicity of the APBs is proposed. At high temperature, close to T c , the APB can be characterized as wavy and with a substantial width, and thus to have a mean position which is not directly related to the basic structure (incommensurate M-values). At low temperature, near the L1 2 - LPS transition, the APBs are sharp and pinned to the lattice. The APB width increases with increasisng annealing temperature. These results are shown to be in good agreement with statistical models.