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Abstract

The following manuscript discusses the fundamentals of glassy metallic alloys comprising Ce and Gd rare earth (RE) elements. It is of interest to observe to which extent the RE-based alloys inherit the behaviour of the respective RE and how the properties change with the variation between Al and RE. The core of this study lies in the characterisation of structure and magnetic behaviour since the former detrimentally affects the latter.
Besides containing one RE, the composition of quaternary alloys is made of Al, Fe and Cu in order for the general stoichiometry to read (Al1-xREx)62Fe13Cu25. Alloy design is based on the variation of Al and RE contents, whereas x=RE/(RE+Al), in order to test the impact of RE on the structure, microstructure, thermal behaviour and magnetic properties. A range of glassy alloys is found throughout the composition landscape, but they differ in the ability to hinder crystallisation, because some specimens are fully glassy whereas others contain crystallites, concurrently with a glassy matrix.
A set of Ce-based glasses exhibits a completely glassy structure when ratio x corresponds to the region of deep eutectics in the Al-Ce phase diagram. Microstructural investigation confirms this with the lack of crystallites in transmission electron microscopy (TEM) micrographs and the presence of diffuse rings in the selected area electron diffraction (SAED). The lowest temperature of glass transition (Tg) in Ce-based glasses appears already at 350 K, which, in general, is one of the lowest values for metallic glasses. Afterwards, crystallisation (Tx) appears with a lag of 10-50 K. The magnetic behaviour of Ce-based alloys is paramagnetic (PM) at room temperature (RT), whereas their local structure, crystallite content and composition do not significantly influence the magnetisation, which falls in the range 0.3-0.5 emu/g. Low-temperature magnetism of Ce-based x=0.75 alloys displays a shift from PM to antiferromagnetic (AFM) state, with a spin glass behaviour.
The structure of Gd-based alloys manifests a more reduced vitrification ability than that of Ce-based counterparts, which is consistent with a 300 K higher liquidus line in Al-Gd diagram. TEM observation of microstructure at atomic scale shows crystallites within the glassy matrix. The temperatures Tg and Tx are higher compared to the Ce-based system, which coincides with the higher melting point of Gd. Magnetic behaviour of Gd-based glasses at RT changes from PM to ferromagnetic (FM) and magnetisation increases from 4 to 16 emu/g. Below the blocking temperature, PM is replaced by AFM state. The evidence of spin glass behaviour appears as well. Local atomic structure of the glasses imparts a hybridisation effect, which couples the adjacent magnetic moments of the constitutive elements.
In addition to the variation of x, the ratio y=Fe/(Fe+Cu) between the immiscible Fe and Cu was altered, resulting in strong glasses for Cu-rich compositions. The glasses’ magnetisation, however, is higher in the case of Fe-rich alloys.
The ribbons of Ce- and Gd-based metallic glasses were consolidated into cylinders in the range between Tg and Tx, via pulsed electric current sintering, while maintaining a glassy structure. The diffuse hump in XRD pattern of a composite is broadened due to overlapping of Ce- and Gd-based ribbons, while thermal analysis and magnetisation preferentially follow the behaviour of Gd-based glass.