Thermally activated amorphous phase formation in cold-rolled multilayers of Al–Ni, Al–Ta, Al–Fe and Zr–Cu

Abstract

Intermixed structures of different binary systems (Al80Ni20, Al75Ta25, Al75Fe25 and Zr67Cu33) were prepared by repeated cold rolling with intermediate folding of elemental foils. The microstructural development during the rolling procedure and after annealing were measured by X-ray diffraction, scanning and transmission electron microscopy. While the rolling procedure yielded mostly nanometer sized crystalline (nanocrystalline) morphologies, amorphous regions were formed in all investigated samples after thermal exposure of the samples under argon atmosphere at temperatures between 560°C and 1400°C. The amorphous phases are formed in localized sample regions of up to 20 μm in size which had local oxygen levels between 10 and 40 at.%. While amorphous areas of an Al80Ni20 sample crystallized on heating in the beam of a transmission electron microscope with rejection of oxygen, thicker amorphous regions were unaffected by this thermal treatment. The thermal stability of the amorphous regions together with the observation of the in situ crystallization of thinner sample areas indicate that the oxygen content acts to stabilize the amorphous phase and prevents the growth of intermetallic crystals.