Expansion of chromatic color range and improvement of mechanical properties on Cu–Ge supersaturated solid solution alloys

Abstract

The Cu–Ge solid solution alloys have a limited color range and relatively low strength due to its limitation of Ge solubility for a solid solution α-Cu phase at room temperature. To extend chromatic color range and to improve mechanical properties of Cu–Ge solid solution alloys, the Cu–Ge alloys with relatively higher Ge content than solubility limit on equilibrium state were fabricated by induction casting. The as-cast Cu–Ge alloys with 7–10 at.%Ge exhibited dual-phase structure composed of α-Cu and ζ-Cu17Ge3 phases with dendritic structure. By annealing at 700 °C for various times and followed quenching processing, the alloys successfully transformed into the supersaturated α-Cu single-phase structure. To confirm the color difference between the supersaturated single α-phase alloys with a high Ge content and the as-cast single α-phase alloys with low Ge content, the reflectivity analysis, CIE L*a*b* color space system, and color difference indicator were used. As a result, it was confirmed that there was a distinct color difference between supersaturated single α-phase alloys and as-cast single α-phase alloys, and the reflectivity and color difference of the alloys significantly depended on Ge content. Moreover, the Vickers hardness of as-cast and solution treated single α-phase alloys highly improved with an increase of Ge content. The influence of Ge content on phase formation, chromaticity and mechanical properties of solid solution Cu–Ge alloys was systematically discussed.