Production of Functional Nanocrystalline Materials in Hydrogen

Abstract

On the example of KS25 and KS37 samarium-cobalt-base commercial alloys and LaNi 4.5 Al 0.5 alloy, we show the possibility, in principle, of obtaining functional materials in the nanocrystalline state with the help of a planetary mill in hydrogen medium. Milling with a rotational speed of 600 rpm during 24 h leads to the disproportionation of KS25 and KS37 alloys into samarium hydride and iron‐cobalt (cobalt) and of LaNi4.5 Al0.5 into Ni3 Al and amorphous products. After vacuum annealing up to 1181 K, the main phases of samarium-cobalt materials are recombined. The crystallite sizes after annealing are 58 ‐ 72 and 70 nm for KS25 and KS37, respectively. We established that LaNi 4.5 Al 0.5 alloy is not recombined in vacuum, and the nanocrystalline state in it can be reached by milling up to 30 min. The crystallite sizes constitute 45 ‐ 78 nm. Among nanocrystalline materials, the investigations of which have proceeded rapidly for the last two decades, functional, in particular, magnetic, materials occupy an important place. Changing the character of interaction in ferromagnetic materials after their passage from microcrystalline to nanocrystalline state, one can substantially improve the properties of magnets [1 ‐ 3]. In particular, in microcrystalline (where the crystallite sizes are from several micrometers and higher) sintered magnets of the system rare-earth metal‐transition metal (boron), magnetic domains interact magnetostatically. In a single-phase magnetic material with crystallite sizes from several to tens of nanometers, single-domain crystallites enter into magnetic exchange interaction across grain boundaries [4 ‐ 6]. As a result, the residual magnetization of the material grows. If a nanocrystalline material consists of soft-magnetic and hard-magnetic phases, then, due to exchange magnetic interaction, the residual magnetization and coercive force grow, which increases the magnetic energy [7 ‐ 10]. At present, the magnetic energy ((BH) max ) of the most powerful magnets based on neodymium-iron-boron alloys exceeds ∼ 400 kJ / m 3 [11], whereas, according to theoretical predictions, the magnetic energy of permanent magnets in the nanocrystalline state is higher than ∼ 715 kJ / m 3 [12]. Such high parameters will be characteristic of anisotropic sintered magnets in nanocrystalline state. This will be a new qualitative leap in the development of equipment functioning on permanent magnets, which will be characterized by miniaturization of the corresponding devices and new, in principle, directions of materials science and instrument making. Among the most widespread methods of obtaining magnetic materials in the nanocrystalline state, we should mention crystallization of vapors of components of the used alloy, pouring out of the molten alloy onto a copper water-cooled rotating drum, and synthesis in various mechanical mills [3]. In all these cases, amorphous, in the first stage, materials pass to the nanocrystalline state by means of annealing. The synthesis of magnetic materials in the nanocrystalline state by the mechanical method is usually carried out in an inert atmosphere [13]. In [14 ‐ 16], on the example of compounds of the samarium‐cobalt system, a possibility is shown to apply