Amorphous soft magnetic alloys
Amorphous alloys are a new special class of precision alloys that differ from crystalline alloys in structure, manufacturing method, region of existence on the temperature-time diagram and properties.
In the 60s, experiments on the rapid cooling of metal melts, which were carried out in order to obtain a submicroscopic metal structure, found that in some cases the crystal lattice in the metal is completely absent, and the arrangement of atoms is characteristic of a structureless, amorphous body. The structure of amorphous alloys is similar to the structure of a frozen liquid and is characterized by the absence of long-range order in the arrangement of atoms. It turned out that the amorphous metal has completely different, not similar properties to the crystalline metal. It becomes several times stronger, its resistance to corrosion increases, its electromagnetic characteristics change, and even one of the most stable constants - the modulus of elasticity. Unlike alloys with a crystalline structure, the production technology of which has serious problems associated with the antagonism of the properties of the components at the crystallization stage, in amorphous alloys all the necessary components are perfectly combined and coexist. With ultrafast cooling, the alloy solidifies before the antagonist components have time to show their antagonism. This opens up the broadest possibilities of finding the optimal combinations of components to obtain specific properties. Amorphous alloys are called metallic glasses. Interest in them is growing rapidly.
First of all, researchers were interested in the ferromagnetic properties of alloys based on iron, nickel and cobalt. The soft magnetic properties of metal glasses generally turned out to be better than the properties of permalloy, moreover, these properties are more stable. The amorphous state of the alloys is achieved by selecting the chemical composition and using a special technology for cooling from the melt at a rate higher than the critical one determined for each composition. The absence of dislocations leads to the fact that metal glasses are superior in strength to the best alloyed steels. Their high hardness results in excellent wear resistance. Another major advantage of amorphous metal alloys is their extremely high corrosion resistance. In many highly corrosive environments (sea water, acids), metal glasses do not corrode at all. For example, the corrosion rate of an amorphous alloy containing iron, nickel and chromium in a hydrochloric acid solution is practically zero. Apparently, the main reason for such a high corrosion resistance of amorphous alloys is that, having no crystal lattice, they are devoid of characteristic crystal "defects" - dislocations and, most importantly, grain boundaries. The high packing density of atoms in the crystal near these "defects" decreases so sharply that along them "enemy agents" can easily penetrate into the metal. It is important that the defect-free structure of the amorphous alloy is imparted to that thin oxide film that forms on its surface at the initial stages of the corrosion process and further protects the metal from direct contact with the "aggressor". The specificity of the technologies allows the production of amorphous alloys in the form of ribbons less than 40 microns thick.
For the manufacture of amorphous alloys in the form of strips, a cooling method is usually used, in which a jet of liquid metal is directed at a certain speed onto the surface of a rapidly rotating cylinder made of a material with high thermal conductivity. A microwire with an amorphous structure is made by melting a metal enclosed in a glass tube with a conical bottom by high-frequency currents, drawing out and cooling a thin capillary filled with metal. When heated, amorphous alloys transform into a crystalline state. For the stable operation of products made of amorphous alloys, it is necessary that their temperature does not exceed the maximum operating temperature for each alloy (T work max). Currently, the most widespread are soft magnetic amorphous alloys, which combine high magnetic and mechanical properties. Soft magnetic amorphous alloys - ferromagnetic alloys with a narrow hysteresis loop. A feature of soft magnetic amorphous alloys in comparison with crystalline ones is a large (about 20%) content of nonmagnetic elements, such as boron, silicon, carbon, phosphorus, etc., necessary to preserve the amorphous structure. The presence of these elements reduces the maximum saturation induction in amorphous alloys in comparison with crystalline ones and increases the temperature coefficient of magnetic properties. The same elements increase the electrical resistance, increase the hardness and strength of amorphous alloys, as well as their corrosion resistance. Since the beginning of the eighties, amorphous materials have been widely used in radio and electrical products, which are used instead of permalloy, ferrites, electrical steels, and magnetodielectrics.
Nanocrystalline magnetic alloys
The second representative of a new class of metastable fast-cooled alloys and an active rival of amorphous alloys are nanocrystalline alloys. Their feature is an ultrafine crystalline structure. The size of crystals (nanoparticles) in these alloys ranges from 1 to 10 nm. Nanocrystalline and amorphous alloys are the closest relatives. Their "relationship" is based on two circumstances. The first is structural similarity. As is known, the structure of amorphous alloys has a short-range order, i.e., it consists of ordered microgroups of atoms, the sizes of which are close to the sizes of nanograins of nanocrystalline alloys. Secondly, it is the production technology. Currently, the most widespread method for obtaining a nanostructure is controlled crystallization from the initial amorphous state.
Thus, the amorphous alloy is the "parent" base of the nanocrystalline alloy.
The structure of a nanocrystalline alloy is a two-phase system, one of the phases of which are nanocrystals, and the other is a residual amorphous matrix. The properties of a nanoalloy depend on the composition, size, and amount of nanocrystals, as well as their relationship to the amorphous phase. Silicon and iron are the basis of economical raw materials. Having a high saturation induction (1.2 T), good temperature stability in a wide temperature range from -60 to +180 ° C, the new nanocrystalline material has excellent characteristics in the high-frequency region at the level of amorphous cobalt-based alloys. Moreover, the new alloy is much more economical.
Precise control of the parameters of annealing of toroids wound from a ribbon allows one to regulate the required material properties (for example, the shape of the hysteresis loop, the level of magnetic permeability, the squareness coefficient, and specific losses) over a wide range. At the same time, good quality at an affordable price is becoming an increasingly important indicator of the competitiveness of nanocrystalline material in comparison with ferrites and permalloy.
