Effect of solidification rate and composition on microstructure and crystallization in Al-Ni-La metallic glasses

Abstract

Al87La7Ni6-xAgx (x = 0,1) alloys, prepared by copper mould casting and melt-spinning at different wheel speeds, were studied by X-ray diffraction, scanning electron microscopy, differential scanning calorimetry and microhardness measurements. The Cu-mould cast ingot shows a hypereutectic microstructure. At the highest cooling rate (wheel speed 40 m/ s) the ribbon is completely amorphous, while at lower wheel speeds mixed structures comprising amorphous and crystalline phases or only crystalline phases were observed. Both the ribbons (with and without Ag containing alloys) show two stage crystallization process on annealing. Crystallization kinetics was analysed by Kissinger and Johnson-Mehl-Avrami approaches. The low values of activation energy reflect the instability of these amorphous alloys. Microhardness of all the ribbons was examined at different temperatures and was correlated with the corresponding evolution of phases.