Study of CuAu I by transmission electron microscopy

Abstract

The thin foil transmission electron microscope technique has been used to study the ordering of CuAu I. At low annealing temperatures ordered plate-like CuAu I nuclei are formed which are coherent with the disordered matrix and parallel to the (101) planes. At higher annealing temperatures short range strain accommodations in the form of microtwinning occur at first and later long range accommodations in the form of coarse twin lamellae. The latter result from the accommodations to strains set up by the growth of ordered lattice domains whose nucleation centers had different c-axis orientations. The orientation of the twin planes is (101) and the twins appear to be nucleated by other twins or on free surfaces and not by dislocations. The ordered domains become disordered at the twin interfaces but remain intact within the lamellae. The increase in mechanical strength upon ordering is related to the coherency strains between ordered nuclei and disordered matrix and the decline of the mechanical strength is due to the elimination of coherency strains by twinning.