Field heat treatment of Chromindur alloys

Low cobalt permanent magnets with high energy products (∼5MG-Oe) have been obtained in ternary Cr-Co-Fe (Chromindur) alloys via heat treatment in a magnetic field. The alloys investigated here were developed by S. Jin[1],[2]. Compositions studied were in the range of 28∼33%Cr, 10.5∼13.5%Co, bal Fe, with no other additive elements. After a solution heat treatment at 900 followed by quenching to room temperature, the alloys were cooled in a DC magnetic field of 1000 Oe from above the spinodal temperature (Ts C) to C at various controlled cooling rates. Optimum magnetic properties were obtained by using progressively slower rates as a given alloy was cooled. The particle size (wavelength) is related to the cooling rate at the high temperature end (670°C to 610°C). The compositional variation (amplitude) is optimized by heat treatment at the lower temperatures (610°C to 525°C). The alloy of 28%Cr-10.5%Co-Fe composition has a wide initial cooling rate window of 65°C±25°C/ hour. With magnetic field treatment this alloy developed values of Br = 12700, Hc= 570 and (BH)max∼ 5MG.Oe with the high squareness ratio (Br/Bs = 0.96). These values are comparable to those of Alnico 5.