Order twinning of AuCu II in the AuCu-3at.%Ag pseudobinary alloy

Structural features of the AuCu II ordered phase associated with order twinning were studied by means of high-resolution electron microscopy in the AuCu-3at.%Ag pseudobinary alloy. Two types of order twinning were observed during ageing just below the critical temperature for ordering, i.e. macrotwinning and microtwinning. The macrotwinning held the (01̄1) twin relation, whereas microtwinning occurred on the {101} planes and made microtwin platelets along the [101] and [1̄01] directions. A slight difference in the orientation of the twin boundaries was distinguished between the two orientation variants of the microtwin platelets as a necessary consequence of the axial ratio c:a of the AuCu II ordered phase. A disordered f.c.c. phase which was predicted from the coherent phase diagram of AuCu-Ag system and the phase rule was found adjacent to a tip of the microtwin platelets when they were in contact with other variants of the microtwin platelets as an inevitable consequence of strain accommodation.