Uncovering deformation twins in intermetallic AuSn and Au₅Sn

Intermetallic compounds AuSn with a double hexagonal close-packed (dhcp) structure and Au5Sn with a trigonal structure constituted the eutectic Au-20Sn alloy. By using high-resolution scanning transmission electron microscopy, first-principles density functional theory calculations, and topological models, we characterized three deformation twins, {1012} twin in AuSn and {1012} and {1011} twins in Au5Sn and quantified their elementary parameters, K1, 711, K2 and 712. {1012} twin in AuSn has six equivalent variants corresponding the dhcp structure. The elementary twinning disconnection (TD) is (b3,3h0) with the shear vector b3 = 0.062 (1011) and the step height of three atomic layers. The trigonal structure of Au5Sn exhibits a three-fold rotation symmetry around the [0001] axis, {1012} and {1011} twins in Au5Sn have two sets of three equivalent variants. These twins could be either conventional twins or pseudo twins. For conventional {1012} twins with low energy twin boundary, the elementary TD is (b3, 3h0) with b3 = 0.059 (1011) and the step height of three atomic layers. For two sets of {1011} twins, the elementary TD is (b6, 6h0) with shear vector b6 = 0.086 (1012) and the step height of six atomic layers. Pseudo twins could be created through the gliding of (b2, 2h0) TDs. This work provides the precise knowledge and comprehensive understanding of deformation twins in intermetallic compounds with complex structures.