Nanocrystalline Mixed Structure Prepared by Crystallization of Cu–Ag–La Amorphous Alloys

Abstract

New amorphous alloys in the Cu-Ag-La system were found to form in a wide composition range of 20 to 70 at% Ag and 3 to 10%La by melt spinning, despite that the Cu-Ag binary system is an insoluble type and no fcc single phase could be obtained in the binary system by melt spinning. The origin of the remarkable effect of La on the glass formation is probably due to the difficulty of the precipitation of a crystalline phase from the supercooled liquid containing La. The liquid consisting of the three constituent elements with significantly different atomic sizes (La>Ag>Cu) is expected to have a highly dense random packed structure which inhibits the precipitation. These ternary amorphous alloys have good bending ductility. The Vickers hardness is in the range of 310 to 330 and tends to increase with increasing La content. The amorphous alloys containing less than 6 at% La subjected to continuous heating crystallized through two stages of Am → Am + fccCu(Ag) + fccAg(Cu) → Cu + Ag + unknown compound. The Cu(Ag) and Ag(Cu) phases are in a metastable state enriched with Ag and Cu, respectively. Particles of these phases have an ellipsoidal morphology with a diameter of about 3 nm and a length of about 8 nm. This study emphasizes the synthesis of the nanoscale mixed structure consisting of two kinds of fcc phases by the crystallization of the Cu-Ag-La amorphous alloys containing the insoluble elements as main constituent components.