Hard-magnetic precision alloys

Hard-magnetic alloys (magnetically hard materials) are magnetized till saturation and are overmagnetized in relatively strong magnetic fields with magnetic force of thousands and ten of thousands of A/m. Overmagnetization is change of the direction of the sample magnetization intensity to the opposite one affected by an external magnetic field. They have a high coercitive force, remanent magnetic induction and magnetic energy on the demagnetization section ("hysteresis loop “back”). Magnico, alni, vicalloy, some ferrites and compounds of rare-earth elements with cobalts are used as hard-magnetic materials.

The considered materials are based on Fe-Ni-Al, Fe-Co-Ni-Al, Fe-Cr-Co, Fe-Co-V chemical systems, Fe-Cr-Ni, alloyed steels (Cr, Co, Mo, W) - alloys for permanent magnets; Fe-Co-V, Fe-Co-Ni-V, Fe-Co-Cr-V, Fe-Cr-W, Fe-Co-W-Mo - alloys for hysteresis motors; Fe-Co-Cr, Fe-Ni, Fe-Co-Ni - alloys for memory elements. Fe - iron, Ni - nickel, Al - aluminum, Co - cobalt, Cr - chromium, V - vanadium, W - tungsten, Mo - molybdenum.

By the main application hard-magnetic alloys can be divided into three groups.

Table 1

Group	Grade	General specification	Intended use
For permanent magnets	52Co11V (52CoVW) 52Co12V (52CoVNb) 52Co13V (52CoVA)	Coercitive force by induction is HcB=15÷40 kA/m, magnetic energy (BH)max=12÷28 T·kA/m	Small magnets with thickness or diameter 0.2-3 mm
	ECr3 EW6(E7W6) ECr5Co5 ECr9Co15Mo2 (ECr9Co15Mo)	Coercitive force by induction within HcB=5÷12 kA/m, magnetic energy - (BH)max=1÷2 T·kA/m	Cheap magnets
For hysteresis motors	52Co10V (52CoV10) 52Co11V (52CoVW) 52Co12V (52CoVNb) 52Co13V (52CoVA) 35CoV10Ni	Field intensity under maximum permeability is Hμmax=12÷33 kA/m, hysteresis loop convexity factor - γμmax=0,5÷0,6	Laminated rotors of medium- and high-power machines
	35CoCr4V 35CoCr6V 35CoCr8V	Hμmax=3.2÷8.6 kA/m, γμmax=0.55÷0.68. Increased nominal resistance ρ≈0.8 µΩ·m. Low sensitivity to drawing temperature (if compared with 52CoV alloys)	Laminated rotors of low-power motor working under frequencies over 1000 Hz
For memory elements	35CoCr12 35CoCr15 37CoCr12	The values measured in the field are as follows: maximum magnetic field intensity for this magnetization cycle: Hmax=16 kA/m; coercitive force Hc=2.0÷5.6; maximum induction for this magnetization cycle Bmax=1.6÷1.9 T; squareness ratio Bτ/Bmax>0.85	Ferrite elements with external memory

The properties of one or another material depend on its chemical composition, production method and some other factors. So the properties of hard-magnetic precision alloys should be considered within groups described in Classification section. They are given in “General specification” column, Table 1.

The main grades of precision hard-magnetic alloys are given in Classification section, Table 1. The chemical composition and other requirements are regulated by GOST 10994-74 and GOST 17809-72 standards.

Hard-magnetic materials have three main industrial applications: production of permanent magnets; production of active parts of rotors used in hysteresis motors; creation of memory elements of control, automation and communication systems, magnetic recording media.

Permanent magnets are used for creating the set magnetic field intensity or set magnetic fluxes in a certain working space.

In rotors of hysteresis motors hard-magnetic alloys are used for forming the rotor torque and work in an alternative magnetic field with intensity from 1.6 to 32 kA/m depending on the motor configuration and intended use.

Alloys for memory elements of control, automation and communication systems are used in form of semi-permanent or alternative magnets subject to many magnetization cycles during operation (~10⁹-10¹⁰). As for elements with external memory (ferrite) semi-hard alloys only control quick-response small relays (magnetically-operated switches) with fixed contacts made of 52A-VI high-permeability alloy. In elements with internal memory - magnetically-operated switches with memory or memory magnetically operated sealed contacts - the fixed contacts are made of a semi-hard magnetic alloy that controls contact closure and break.

Hard-magnetic materials are produced in form of flat (strips, sheets) and round (wire, rods) products. A certain application determines the blank use.