Oxidation Behaviour of Microstructurally Highly Metastable Ag-La Alloy.

A new silver-based alloy with 2 wt.% of lanthanum (La) was studied as a potential candidate for electric contact material. The alloy was prepared by rapid solidification, performed by the melt spinning technique. Microstructural examination of the rapidly solidified ribbons revealed very fine grains of α and intermetallic AgLa particles, which appear in the volume of the grains, as well as on the grain boundaries. Rapid solidification enabled high microstructural refinement and provided a suitable starting microstructure for the subsequent internal oxidation, resulting in fine submicron-sized LaO oxide nanoparticle formation throughout the volume of the silver matrix (α). The resulting nanostructured Ag-LaO microstructure was characterised by high-resolution FESEM and STEM, both equipped with EDX. High-temperature internal oxidation of the rapidly solidified ribbons essentially changed the microstructure. Mostly homogeneously dispersed nano-sized LaO were formed within the grains, as well as on the grain boundaries. Three mechanisms of internal oxidation were identified: (i) the oxidation of La from the solid solution; (ii) partial dissolution of finer AgLa particles before the internal oxidation front and oxidation of La from the solid solution; and (iii) direct oxidation of coarser AgLa intermetallic particles.