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

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 alpha-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 alpha-Cu and zeta-Cu17Ge3 phases with dendritic structure. By annealing at 700 degrees C for various times and followed quenching processing, the alloys successfully transformed into the supersaturated alpha-Cu single-phase structure. To confirm the color difference between the supersaturated single alpha-phase alloys with a high Ge content and the as-cast single alpha-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 alpha-phase alloys and as-cast single alpha-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 alpha-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.