Hardness Alnico alloys

Hard magnetic materials these materials cannot be magnetized nor demagnetized easily (fig. 11.5.5b). Examples of such materials are Alnico alloys containing iron, alloys of rare earths and Fe-Cr-Co alloys. 

Alnico alloys are brittle and hard, they can only be machined by grinding, spark erosion, and electrochemical milling, and they resist atmospheric corrosion well up to 500 °C (Fiepke, 1990). The mechanical behavior, in particular creep, of Alnico-type, Fe-Ni- Al alloys has been studied in detail, and both Fe-rich alloys with precip-... 

Alnico /al-ma-koh/ (Trademark) Any of a group of very hard brittle alloys used to make powerful permanent magnets. They contain nickel, aluminum, cobalt, and copper in various proportions. Iron, titanium, and niobium can also be present. They have a high remanence and coercive force. 

Table 33.9 compares some of the important properties of hard ferrites with hard metal alloy magnets such as alnico and high-energy (BH > 80kJ/m ) rare earth magnets such SmCos. 

Cobalt ores are often found in association with copper(II) sulfide. Cobalt is a silver-gray metal and is used mainly for alloying with iron. Alnico steel, an alloy of iron, nickel, cobalt, and aluminum, is used to make permanent magnets such as those in loudspeakers. Cobalt steels are hard enough to be used as surgical steels, drill bits, and lathe tools. The color of cobalt glass is due to a blue pigment that forms when cobalt(II) oxide is heated with silica and alumina. 

In alloy steels, iron is mixed with carbon and varying amounts of other elements, mainly metals. Added metals produce desired properties such as hardness and corrosion resistance (Cr), resistance to wear (Mn), toughness (Ni), heat resistance (W and Mo), and springiness (V). Stainless steel is a well-known, corrosion-resistant alloy steel. It contains ten to 30 percent chromium and sometimes nickel and/or silicon. Because of its outstanding magnetic properties, Alnico steel is used to make permanent magnets. Alnico magnets are used in voltmeters and ammeters to rotate the cod of wire connected to the pointer. 

Common examples of magnetically hard materials are high carbon steels, precipitation hardened alloys e.g., Alnico), and sintered or bonded fine-particle alloys e.g., ferrites, rare earth alloys). The earliest examples of rare earth magnets are SmCos and Sm2Coi7, with recent developments focused on the incorporation of Fe rather than other costly transition metals. Whereas iron-rare earth alloys such as R2Fei7 have... 

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