Mechanically alloyed Sm(CoFe) permanent magnets

Abstract

The use of mechanical alloying as a processing method for the solid-state synthesis of new materials has received considerable attention in recent years. Of particular interest has been the synthesis of rare earth permanent magnet materials. Many of the rare-earth transition metal alloys studied to date exhibit similar structures and properties in the as-milled condition. In particular, the hard magnetic phase does not form during mechanical alloying. Instead, either a single amorphous phase is formed or a two-phase mixture is developed during milling which consists of crystallites of the transition metal, (i.e. Fe, Co), and an amorphous phase containing the rare earth element. Crystallization of the as-milled structure into a nanocrystalline hard magnetic phase (i.e. Nd{sub 2}Fe{sub 14}B or SmCo{sub 5}) normally requires a heat treatment at temperatures in the range of 600--700 C. With Sm{sub 2}Fe{sub 17}N{sub 2.7}, nitriding must be carried out after crystallization since Sm{sub 2}Fe{sub 17}N{sub 2.7} decomposes to samarium nitride and {alpha}-Fe at temperatures above 500 C. Mechanically alloyed and heat treated structures have been found to exhibit remarkably high values of coercivity, H{sub c}. For example, values of H{sub c} exceeding 75 kOe have been obtained in SmCo{sub 5}.