Two-dimensional long period structures in Cu-Pd. A study of the mechanism of the transition from a one-dimensional LPS to a two-dimensional LPS

The nucleation and formation mechanisms by which two-dimensional long period structures (2D-LPS) are generated from one-dimensional long period structures (1D-LPS) in Cu 3 Pd alloys are analysed. A series of alloys with very specific heat treatments was prepared to permit an analysis of different static steps in the transition from 1D-LPS to 2D-LPS. This analysis is based on studies by high-resolution electron microscopy and electron diffraction, which were essential to substantiate a crystallographic model for the transformation mechanism. The proposed mechanism ascribes the formation of the 2D-LPS to the propagation of parallel sets of hairpin-shaped configurations of non-conservative antiphase boundaries. Their progression in a direction perpendicular to the 1D-LPS boundaries gives rise to the 2D-LPS. Displacement vector analysis, and detailed observations of boundary geometries, support the proposed crystallographic model. The nucleation of the transformation proceeds in two ways, both related to the behaviour of non-conservative antiphase boundaries. Single non-conservative antiphase boundaries become unstable and acquire a meandering configuration in a direction perpendicular to the 1D-LPS boundaries. Segments of non-conservative antiphase boundaries, present at coherent intervariant boundaries, progress inwards with respect to one of the variants, producing the parallel hairpin configurations. Particular observations concerning spacing variations of the second set of antiphase boundaries near the intervariant boundaries can be explained unambiguously by the model.