Mechanism of low-temperature ordering in equiatomic CuAu alloy : III. Electron-microscopic studies of the second stage of ordering

Electron microscopy was used to study the structural features of the A → L1 0 phase transformation in the CuAu alloy at the second ordering stage characterized by a sharp increase in the axial ratio c/a of the ordered phase. This increase in the c/a ratio was shown to be caused by a polytypic shear transformation resulting in the formation of the Z phase. The Al + L1 0 regions that underwent shear shifts due to the formation of stacking faults and their periodic arrangement represent a new perfect phase, in which the ordered arrangement of copper and gold atoms differs from that characteristic of the L1 0 phase.