L10 nanotwinned gold-rich Au–Cu–Pt

Abstract: Ternary Au–Cu–Pt alloys, of unequal gold and copper atomic contents and containing 2.5wt.% Pt, are ordered isothermally at 250 or 400°C after annealing at 650°C and quenching in water. The alloys are characterized by transmission electron microscopy, X-ray diffraction, hardness and tensile testing. With 75 and 76.5wt.% Au, the microstructural evolution depends on the ordering temperature. At 400°C, a classical nanotwinned “polytwin” structure with, locally, two L10 crystal orientation variants is formed; this evolves into a two-phase, L10 +A1, checkerboard-like microstructure after 106 s. At 250°C, ordering develops a stable (up to 105 s) structure locally containing all three L10 crystal variants arranged in a network of {110} twins roughly 30nm wide. At both ordering temperatures the 78wt.% Au alloy develops a similar three-variant nanotwinned structure that also remains stable up to 105 s. The 75 and 76.5wt.% Au alloys display a peak in hardness after roughly 120 s at both 400°C and 250°C; with 78wt.% Au, peak hardness is not reached at 105 s. With all three alloys, superior hardness and tensile strength, coupled with lower ductility, are obtained with the three-variant nanotwinned structure formed at 250°C compared with the more classical polytwin structure that develops at 400°C. [Copyright &y& Elsevier]