Uncovering deformation twins in intermetallic AuSn and Au5Sn

Intermetallic compounds AuSn with a double hexagonal close-packed (dhcp) structure and Au ₅ Sn 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, { 10 1 ¯ 2 } twin in AuSn and { 10 1 ¯ 2 ¯ } and { 10 1 ¯ 1 } twins in Au ₅Sn and quantified their elementary parameters, K ₁, η ₁, K ₂ and η ₂. { 10 1 ¯ 2 } twin in AuSn has six equivalent variants corresponding the dhcp structure. The elementary twinning disconnection (TD) is ( b ₃,3h ₀) with the shear vector b ₃ = 0.062 〈 10 1 ¯ 1 ¯ 〉 and the step height of three atomic layers. The trigonal structure of Au ₅Sn exhibits a three-fold rotation symmetry around the [ 0001 ] axis, { 10 1 ¯ 2 ¯ } and { 10 1 ¯ 1 } twins in Au ₅Sn have two sets of three equivalent variants. These twins could be either conventional twins or pseudo twins. For conventional { 10 1 ¯ 2 } twins with low energy twin boundary, the elementary TD is ( b ₃, 3h ₀) with b ₃ = 0.059 〈 10 1 ¯ 1 〉 and the step height of three atomic layers. For two sets of { 10 1 ¯ 1 } twins, the elementary TD is ( b ₆, 6h ₀) with shear vector b ₆ = 0.086 〈 1 ¯ 012 〉 and the step height of six atomic layers. Pseudo twins could be created through the gliding of ( b ₂, 2h ₀) TDs. This work provides the precise knowledge and comprehensive understanding of deformation twins in intermetallic compounds with complex structures.