Twin relationship in between the variant-pair of η precipitates in the Al-Zn-Mg-Cu aluminium alloy

For Al-Zn-Mg-Cu aluminium alloys, 15 types of η precipitates would possess the symmetrically variants distributed on the closed planes of the Al matrix, parallel to the (0001)η, (21̅1̅0)η, or (101̅0)η interfaces of precipitates. The η2 precipitates, possessing the crystallographic orientation of (111̅1̅)al // (0001)η2 and [110]al // [101̅0]η2, would exhibit four equivalent variants, i.e., η2(1) to η2(4), on the (11̅1̅)al habit planes. In the present work, along the zone axis of [110]al // [101̅0]η, the edge-on configurations showing the twin-like atomic arrays would occur as the growth/coalescence of two η2 variants grown on (111̅1̅)al and (11̅1)al planes, respectively. The twin relationship can be revealed in term of the crystallographic relationship at 70.5° with respect to their habit planes. Alternatively, on the (11̅1̅0)al image, it also indicates the nearly-twinning configuration in between the variant-pair of η2.