The work reveals an important fundamental scientific problem regarding the influence of thermally initiated nanocrystallization processes in amorphous metal alloys based on aluminum and iron on their atomic structure, interphase morphology, elemental composition, and physicochemical properties.
In particular, it was found that the initial samples of AMA systems Al‒RE‒Ni/Fe (RE Y, Gd and Dy) and Fe‒Nb‒B‒ RE (RE Y, Gd, Tb and Dy) have a special cluster structure and are formed, mainly, from atoms of basic elements, forming homoatomic pairs Al‒Al and Fe‒Fe. RE and Ni/Fe atoms (in the case of Al-AMA) also participate in the formation of short-range order. The interatomic distances indicate a donor–acceptor electron interaction in Al‒TM pairs, and a typical metallic bond is observed between Al and RE atoms. In the case of iron-based AMA, only the interaction of RE ‒B can be asserted. At the same time, a decrease in the Fe‒B distance, compared to the sum of the atomic radii of the elements, was recorded only in Fe82Nb2B14Dy2 AMA, which may be related to the peculiarities of the cluster structure in the presence of dysprosium itself. The existence of three main types of atomic pairs in the first coordination sphere, namely Al‒Al, Al‒ RE, and Al‒Ni in the starting Al-AMA, the formation of Al nanocrystals leads to the delamination of the amorphous matrix into two phases enriched in RE or Ni, respectively. A similar process is not observed in the case of Fe-AMA, the reason for which is the presence of a large number of Nb‒Nb atomic pairs, which, concentrating on intercluster boundaries, form diffusion layers during annealing. The analysis of the kinetics of isothermal crystallization showed that the growth of grains (nanocrystals) has a diffusion-controlled nature, and therefore the niobium layers limit the growth and prevent the aggregation of nanoparticles.
Magnetic isotherms of RE-doped (Y, Gd, Tb, Dy) AMAs of the Fe–Nb–B system at a temperature of 2 K in a magnetic field with an induction value of up to 7 T describe the ferro/ferrimagnetic behavior of samples with saturation below 1 T. AMAs are divided into two groups depending on the magnitude of the magnetic reversal effect (MI effect). Alloys of the first group (Y- and Gd-doped AMAs) are characterized by relatively low values of the degree of magnetic irreversibility - about 5% at low temperatures, while in the case of the second group (alloys doped with Dy and Tb) - more than 50% at 2 K.
If the Fe82Nb2B14Y2 and Fe82Nb2B14Gd2 alloys are characterized by a high value of μ (530 and 750, respectively), then in the AMAs of the second group, the RE additions cause local magnetic anisotropy, which is responsible for the decrease in magnetic susceptibility by almost five times.
Electrochemical studies have shown that the thickness and density (porosity) of the protective oxide-hydroxide layers that form on the surface of the AMA samples are two important factors that determine the corrosion resistance of the investigated alloys of the Al‒RE‒Ni/Fe and Fe‒Nb‒B‒RE systems under the action of electrochemical loads. Thus, an increase in the number of polarization cycles of the studied AMA electrodes leads to a decrease in oxidation currents in NaCl aqueous solutions and, conversely, to their growth in an alkaline (KOH) environment as a result of thickening/densification and destruction of protective layers, respectively. At the same time, processes of electrochemical release of gases and reversible red/ox transformations of iron compounds also play an important role.
AMA electrodes of the Al‒RE‒Ni/Fe and Fe‒Nb‒B‒RE systems possess electrocatalytic properties with respect to HER from aqueous alkaline solutions, while the efficiency of the process increases significantly with the use of annealed samples. The electrocatalytic activity of the investigated AMA samples is caused by an increase in the surface concentration of Ni (for Al-AMA) and RE (for Fe-AMA) as a result of electrochemical oxidation-reduction of the AMA surface and, accordingly, annealing. It was found that the most promising electrode materials for the electrochemical release of hydrogen from alkaline solutions are the annealed samples of AMA Al87Ni8Y4Dy1 and Fe84Nb2B14Gd2.
