Amorphous and nanocrystalline soft magnetic alloys

Amorphous soft magnetic alloys

Amorphous alloys – a new special class of precision alloys, different from the crystalline alloys by its structure, method of production, range of existence on the temperature-time diagram and properties.

Down in the 60's the experiments on rapid cooling of molten metals carried out to obtain submicroscopic structure of the metal, discovered that in some cases the crystal lattice of the metal was absent, and the arrangement of atoms wastypical for structureless, amorphous solid.The structure of amorphous alloys is similar to that of a frozen liquid and is characterized by a lack of long-range ordering in the arrangement of atoms.

It was found that amorphous metal has quite different properties that are not similar to the crystalline metal. It becomes several times stronger, its corrosion resistance increases, its electromagnetic characteristics and even one of the most enduring constants – modulus of elasticity – are changing.In contrast to alloys with a crystalline structure, the technology of which has serious problems associated with antagonism of component properties during crystallization, all the necessary components are getting along well in amorphous alloys. During rapid cooling the alloy solidifies before the antagonist components show their antagonism. It opens up tremendous opportunities for searching optimal combinations of components to obtain specific properties. Amorphous alloys were called metallic glasses. Interest to them is increasing rapidly.

First of all the researchers are interested in the ferromagnetic properties of Fe, Ni and Co-based alloys.Soft magnetic properties of metallic glasses are mostly better than properties of permalloys, moreover, these properties are more stable. Amorphous state of alloys is achieved by selecting the chemical composition and the use of special cooling technology of molten metal at a rate above the critical value determined for each composition.The absence of dislocations leads to the fact that the metallic glass is even stronger than even the best alloyed steels. The high hardness results in their excellent durability. Another important advantage of amorphous metallic alloys is their extremely high corrosion resistance. Melallic glasses do not corrode in many highly corrosive environments (sea water, acids). For example, the corrosion rate of an amorphous alloy containing iron, nickel and chromium in hydrochloric acid solution is practically zero. Apparently, the main reason for high corrosion resistance of amorphous alloys is resulted by absence of crystal lattice,they also have no specific "defects" of crystals – dislocations and, most importantly, the boundaries between grains.High atoms density in a crystal near these "defects" is reduced so dramatically that"enemy agents" easily get inside the metal through them.

It is important that defect-free structure of the amorphous alloy is attached to the thin oxide film that forms on its surface on the initial stages of the corrosion process and subsequently protects the metal from direct contact with the "aggressor." The specificity of the technology allows to produce amorphous alloys in the form of ribbons with thickness less than 40 microns.

The common method of producing amorphous alloys in the the form of ribbons is cooling, when a jet of molten metal is sent to the surface of a rapidly rotating cylinder made ​​of a material with high thermal conductivity at a certain speed.

Microwire with an amorphous structure is produced by using high-frequency current to melt the metal encased in a glass tube with a conical bottom, pulling and cooling of a thin capillary tube filled with metal. Amorphous alloys pass into the crystalline state when heated.

For the stable operation of the product of amorphous alloys it is necessary that for each alloy their temperature does not exceed the maximum operating temperature (T op. max). Magnetic amorphous alloys, which combine high magnetic and mechanical properties the most widely used now. Magnetic amorphous alloys are ferromagnetic alloys with a flat hysteresis loop. The feature of the soft magnetic amorphous alloys in comparison with the crystaliine is high (about 20%) content of nonmagnetic elements such as boron, silicon, carbon, phosphorus, and so on which are necessary to maintain the amorphous structure.

The presence of these elements reduces the maximum saturation induction of amorphous alloys comparing to those of a crystalline alloys and increases the temperature coefficient of magnetic properties. These elements increase the electrical resistance, increase the hardness, strength and corrosion resistance of amorphous alloys. Since the early eighties amorphous materials have been widely used in radio and electrical products instead of permalloys, iron, electrical steel and ferrites.

 

Nanocrystalline magnetic alloys

The second in a new class of metastable rapidly cooled alloys and active opponents of amorphous alloys are nanocrystalline alloys. Their feature is the super fine crystalline structure.

The crystal size (nanoparticles) in these alloys ranges from 1 to 10 nm. Nanocrystalline and amorphous alloys are closest relatives. Their "kinship" is based on two circumstances. First is their structural similarity.

As we know, the structure of amorphous alloys has short-range order, i.e. it consists of ordered microgroups of atoms whose size is close to the size of nanograins in nanocrystalline alloys.  The second is the technology of production.

Currently, the most common method of obtaining the nanostructure is controlled crystallization from the initial amorphous state. Thus, the amorphous alloy is the "parent" basis for the nanocrystalline alloy. The structure of the nanocrystalline alloy is a two-phase system, where nanocrystals are the first phase, and the other is the residual amorphous matrix. Properties of nano alloy depend on the composition, size and number of nanocrystals, as well as their relations with the amorphous phase. Silicon and iron are the basis for economical raw materials. With high saturation induction (1.2 T), good thermal stability over a wide temperature range from -60 to 180ºC, a new nanocrystalline material has excellent characteristics at high frequenciescomparable with amorphous Co-based alloys.Therefore, the new alloy is much more economical. Precise control of parameters of the annealing of strip wound toroids allows to adjust the desired properties of the material in a wide range (for example, the shape of the hysteresis loop, the level of permeability, squareness ratio, the specific loss).

At the same time, good quality at an affordable price is becoming more significant indicator of competitiveness of the nanocrystalline material compared with ferrites and permalloy.

Advantages

Regardless of the application, the use of amorphous and nanocrystalline cores when designing inductive components typically provides the following advantages:

• Reduced weight
• Reduced losses on the coil due to reduction in number of turns
• Extended temperature range from -60 to 125ºC
• Increased stability of properties and durability
• High accuracy for measuring devices
• Increased efficiency of the devices

Comparison of characteristics of amorphous and nanocrystalline soft magnetic materials with those of the traditional ones:

Material _________ Properties	Electrical steel	Permalloy		Ferrite Mn-Zn	Amorphous		Nanocrystalline
		50 Ni	80 Ni		Co-based	Fe-based	Fe-based
Magnetic induction amplitude, Bm (T)	2,0	1,55	0,74	0,5	0,58	1,56	1,16
Coercitive force, Hc (Oe)	0,5	0,15	0,03	0,1	0,005	0,03	0,01
Initial permeability, µi	1500	6000	40000	3000	60000	5000	70 000
Max. permeability, µm	20000	60000	200000	6000	1000000	50000	600 000
Specific resistance, p (µOhm/cm)	50	30	60	1000000	120	130	130
Curie temperature, Тс (°C)	750	500	500	140	255	415	560
Crystallization temperature, Тx (°C)	-	-	-	-	530	550	515
Max. operating temterature, Т (°C)				100	90	150	180
Optimal frequency range, f (kHz)	0...1	0...10		10...	10...100000

PC "MSTATOR" produces a wide range of ribbons of AMAG amorphous and nanocrystalline alloys at width of 1 to 30 mm and a thickness of 15 to 25 microns for supply to customers and use in their own production of toroidal cores and complete electromagnetic components presented in the Products section.