Alnico is a family ofironalloys which, in addition to iron, are composed primarily ofaluminium (Al),nickel (Ni), andcobalt (Co), hence the acronym[1]al-ni-co. They also includecopper, and sometimestitanium. Alnico alloys areferromagnetic, and are used to makepermanent magnets. Before the development ofrare-earth magnets in the 1970s, they were the strongest permanent magnet type. Other trade names for alloys in this family are:Alni, Alcomax, Hycomax, Columax, andTiconal.[2]
The composition of alnico alloys is typically 8–12% Al, 15–26% Ni, 5–24% Co, up to 6% Cu, up to 1% Ti, and the rest is Fe. The development of alnico began in 1931, when T. Mishima in Japan discovered that an alloy of iron, nickel, and aluminum had acoercivity of 400 oersteds (32 kA/m), double that of the best magnet steels of the time.[3]
Alnico alloys can be magnetised to produce strongmagnetic fields and have a high coercivity (resistance to demagnetization), thus making strong permanent magnets. Of the more commonly available magnets, onlyrare-earth magnets such asneodymium andsamarium-cobalt are stronger. Alnico magnets produce magnetic field strength at their poles as high as 1500 gauss (0.15 tesla), or about 3000 times the strength ofEarth's magnetic field. Some alnico brands areisotropic and can be efficiently magnetized in any direction. Other types, such as alnico 5 and alnico 8, areanisotropic, each having a preferred direction ofmagnetization, or orientation. Anisotropic alloys generally have greater magnetic capacity in a preferred orientation than isotropic types. Alnico'sremanence (Br) may exceed 12,000 G (1.2 T), its coercivity (Hc) can be up to 1000 oersteds (80 kA/m), itsmaximum energy product ((BH)max) can be up to 5.5 MG·Oe (44 T·kA/m). Therefore, alnico can produce a strong magnetic flux in closed magnetic circuits, but has relatively small resistance against demagnetization. The field strength at the poles of any permanent magnet depends very much on the shape and is usually well below the remanence strength of the material.
Alnico alloys have some of the highestCurie temperatures of any magnetic material, around 800 °C (1,470 °F), although the maximal working temperature is typically limited to around 538 °C (1,000 °F).[4] They are the only magnets that have useful magnetism even when heatedred-hot.[5] This property, as well as itsbrittleness and high melting point, results from the strong tendency toward order due tointermetallic bonding between aluminum and other constituents. They are also one of the most stable magnets if handled properly. Alnico magnets are electrically conductive, unlike ceramic magnets.[citation needed] Alnico 3 has a melting temperature of 1200 - 1450 °C.[6]
| MMPA class | IEC code ref. | Composition by weight (Fe comprises remainder) | Magnetic properties | Physical properties | Thermal properties | |||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Max. energy product, (BH)max | Residual induction, Br | Coercive force, Hc | Intrinsic coercive force, Hci | Density | Tensile strength | Transverse modulus of rupture | HRC | Thermal expansion coefficient (10−6 per °C) | Electrical resistivity, at 20 °C (μΩ·cm) | Reversible temp. coefficient, (% per °C) | Curie temp. | Max. service temp. | ||||||||||||||||||
| Al | Ni | Co | Cu | Ti | (MGOe) | (kJ/m3) | (gauss) | (mT) | (Oe) | (kA/m) | (Oe) | (kA/m) | (lb/in3) | (g/cm3) | (psi) | (MPa) | (psi) | (MPa) | Near Br | Near max. energy prod. | Near Hc | (°C) | (°F) | (°C) | (°F) | |||||
| Isotropic cast AlNiCo | ||||||||||||||||||||||||||||||
| Alnico 1 | R1-0-1 | 12 | 21 | 5 | 3 | - | 1.4 | 11.1 | 7200 | 720 | 470 | 37 | 480 | 38 | 0.249 | 6.9 | 4000 | 28 | 14000 | 97 | 45 | 12.6 | 75 | |||||||
| Alnico 2 | R1-0-4 | 10 | 19 | 13 | 3 | - | 1.7 | 13.5 | 7500 | 750 | 560 | 45 | 580 | 46 | 0.256 | 7.1 | 3000 | 21 | 7000 | 48 | 45 | 12.4 | 65 | -0.03 | -0.02 | -0.02 | 810 | 1490 | 450 | 840 |
| Alnico 3 | R1-0-2 | 12 | 25 | - | 3 | - | 1.35 | 10.7 | 7000 | 700 | 480 | 38 | 500 | 40 | 0.249 | 6.9 | 12000 | 83 | 23000 | 158 | 45 | 13.0 | 60 | |||||||
| Anisotropic cast AlNiCo | ||||||||||||||||||||||||||||||
| Alnico 5 | R1-1-1 | 8 | 14 | 24 | 3 | - | 5.5 | 43.8 | 12800 | 1280 | 640 | 51 | 640 | 51 | 0.264 | 7.3 | 5400 | 37 | 10500 | 72 | 50 | 11.4 | 47 | -0.02 | -0.015 | +0.01 | 860 | 1580 | 525 | 975 |
| Alnico 5DG | R1-1-2 | 8 | 14 | 24 | 3 | - | 6.5 | 57.7 | 13300 | 1330 | 670 | 53 | 670 | 53 | 0.264 | 7.3 | 5200 | 36 | 9000 | 62 | 50 | 11.4 | 47 | |||||||
| Alnico 5-7 | R1-1-3 | 8 | 14 | 24 | 3 | - | 7.5 | 59.7 | 13500 | 1350 | 740 | 59 | 740 | 59 | 0.264 | 7.3 | 5000 | 34 | 8000 | 55 | 50 | 11.4 | 47 | |||||||
| Alnico 6 | R1-1-4 | 8 | 16 | 24 | 3 | 1 | 3.9 | 31.0 | 10500 | 1050 | 780 | 62 | 800 | 64 | 0.265 | 7.3 | 23000 | 158 | 45000 | 310 | 50 | 11.4 | 50 | -0.02 | -0.015 | +0.03 | 860 | 1580 | 525 | 975 |
| Alnico 8 | R1-1-5 | 7 | 15 | 35 | 4 | 5 | 5.3 | 42.2 | 8200 | 820 | 1650 | 131 | 1860 | 148 | 0.262 | 7.3 | 10000 | 59 | 30000 | 207 | 55 | 11.0 | 53 | -0.025 | -0.01 | +0.01 | 860 | 1580 | 550 | 1020 |
| Alnico 8HC | R1-1-7 | 8 | 14 | 38 | 3 | 8 | 5.0 | 39.8 | 7200 | 720 | 1900 | 151 | 2170 | 173 | 0.262 | 7.3 | 10000 | 59 | 30000 | 207 | 55 | 11.0 | 54 | -0.025 | -0.01 | +0.01 | 860 | 1580 | 550 | 1020 |
| Alnico 9 | R1-1-6 | 7 | 15 | 35 | 4 | 5 | 9.0 | 71.6 | 10600 | 1060 | 1500 | 119 | 1500 | 119 | 0.262 | 7.3 | 7000 | 48 | 8000 | 55 | 55 | 11.0 | 53 | -0.025 | -0.01 | +0.01 | 860 | 1580 | 550 | 1020 |
| Isotropic sintered AlNiCo | ||||||||||||||||||||||||||||||
| Alnico 2 | R1-0-4 | 10 | 19 | 13 | 3 | - | 1.5 | 11.9 | 7100 | 710 | 550 | 44 | 570 | 45 | 0.246 | 6.8 | 65000 | 448 | 70000 | 483 | 45 | 123.4 | 68 | |||||||
| Anisotropic sintered AlNiCo | ||||||||||||||||||||||||||||||
| Alnico 5 | R1-1-10 | 8 | 14 | 24 | 3 | - | 3.9 | 31.0 | 10900 | 1090 | 620 | 49 | 630 | 50 | 0.250 | 6.9 | 50000 | 345 | 55000 | 379 | 45 | 11.3 | 50 | |||||||
| Alnico 6 | R1-1-11 | 8 | 15 | 24 | 3 | 1 | 2.9 | 23.1 | 9400 | 940 | 790 | 63 | 820 | 65 | 0.250 | 6.9 | 55000 | 379 | 100000 | 689 | 45 | 11.4 | 54 | |||||||
| Alnico 8 | R1-1-12 | 7 | 15 | 35 | 4 | 5 | 4.0 | 31.8 | 7400 | 740 | 1500 | 119 | 1690 | 134 | 0.252 | 7.0 | 50000 | 345 | 55000 | 379 | 45 | 11.0 | 54 | |||||||
| Alnico 8HC | R1-1-13 | 7 | 14 | 38 | 3 | 8 | 4.5 | 35.8 | 6700 | 670 | 1800 | 143 | 2020 | 161 | 0.252 | 7.0 | 55000 | 379 | 45 | 11.0 | 54 | |||||||||
As of 2018, Alnico magnets cost about 44 USD/kg (US$20/lb) or US$4.30/BHmax.[7]
Alnico magnets are traditionally classified using numbers assigned by the Magnetic Materials Producers Association (MMPA), for example, alnico 3 or alnico 5. These classifications indicate chemical composition and magnetic properties. (The classification numbers themselves do not directly relate to the magnet's properties; for instance, a higher number does not necessarily indicate a stronger magnet.)[8]
These classification numbers, while still in use, have been deprecated in favor of a new system by the MMPA, which designates alnico magnets based onmaximum energy product in megagauss-oersteds andintrinsic coercive force as kilo oersted, as well as an IEC classification system.[8]
Alnico magnets are produced bycasting orsintering processes.[9] Cast alnico is produced by conventional methods usingresin bonded sand molds, which can be intricate and detailed, thereby allowing for complex shapes to be produced.[10] The produced alnico magnet typically has a rough surface.[11] This process has higher initial tooling costs for mold creation.[12] Sintered alnico magnets are formed using powdered metal manufacturing methods. While sintering can also produce a range of shapes, it may not be as suitable for extremely intricate or detailed designs compared to casting.[10][13]
Most alnico produced is anisotropic, meaning that during manufacturing the grains are precipitated with their magnetic axes parallel. Anisotropic alnico magnets are oriented by heating above a critical temperature and cooling in the presence of a magnetic field. Both isotropic and anisotropic alnico require proper heat treatment to develop optimal magnetic properties. Without it, alnico's coercivity is about 10 Oe, comparable to technical iron, a soft magnetic material. After the heat treatment alnico becomes a composite material, named "precipitation material"—it consists of iron- and cobalt-rich[14] precipitates in a rich-NiAl matrix.
Alnico's anisotropy is oriented along the desired magnetic axis by applying an external magnetic field to it during the precipitate particle nucleation, which occurs when cooling from 900 °C (1,650 °F) to 800 °C (1,470 °F), near theCurie point. There are local anisotropies of different orientations without an external field due to spontaneous magnetization. The precipitate structure is a "barrier" against magnetization changes, as it prefers few magnetization states requiring much energy to get the material into any intermediate state. Also, a weak magnetic field shifts the magnetization of the matrix phase only and is reversible.
Alnico magnets are widely used in industrial and consumer applications where strong permanent magnets are needed. Examples areelectric motors, electricguitar pickups,microphones,sensors,loudspeakers,magnetron tubes, andcow magnets. In many applications they are being superseded byrare-earth magnets, whose stronger fields (Br) and larger energy products (B·Hmax) allow smaller-size magnets to be used for a given application.
The high-temperature resistance of alnico magnets leads to many uses that cannot be filled by less resistant magnets, such as inmagnetic stirring hotplates.