Microstructure and internal oxidation property of ball-milled Ag-Zn alloy powder

Influence of ball-milling time on the microstructure and internal oxidation properties of Ag-Zn alloy powder was investigated by XRD and SEM. The results show that the grain size decreased rapidly while the microstrain increased sharply at the initial stage of ball milling. After ball-milling for 25 h, the change became gentle. After 100 h, the grain size and microstrain were 20 nm and 0.55%, respectively. The milling process may be divided into four stages, i.e. flattening, breaking & refining, balancing of breakage and cold welding, and welded conglomerations. The internal oxidation velocity of Ag-Zn alloy powder increased with the milling time prolonging. The oxidation ratio of Ag-Zn powder milled for 100 h reached the maximum in 0.5 h, which increased by 25% compared with that of the milling-free powders. During internal oxidation, ZnO were grown as spicule and flake with big sizes on the surface of the milling-free powders, while on the surface of milled powders ZnO was shaped mainly as spicule with small sizes. Nodules of virtually pure silver atoms formed on the surface of powder during internal oxidation, which is attributed to the compressive stress caused by the expansion of ZnO in the silver-matrix during internal oxidation.