Influence of Heat Treatment Parameters on Properties of NdRFeMCoB Sintered Magnets

Influence of multi-step heat treatments on magnetic properties and structure of three magnet types NdRFeMCoB: high-energy (Bt < 1, 3 T) (A); high-coercive (iHc < 2000 kA/m) (B) and thermally stable (αb> |0, 02| %/K in the temperature interval 293–373K) (C) was investigated. Significant affect of condition of Nd2Fe14B phase on high-coercive condition development is shown. High values of iHc in magnets of NdRFeMCoB type can be provided by creating of elasto-stressed condition of Nd2Fe14B phase and minimal stresses values near boundary phases during heat treatments. The highest results on iHc are achieved after multi-step heat treatments. The temperature of the last step of heat treatment for optimal chemical compositions of the alloys (in dependence on the ratio of such elements as Al, Cu, Ga, Pr, Co) is in the interval of 725–905K (preferably, 775–825K). The value of iHc of magnet (B) Nd13,2Tb1,8FebaiCo5Mo3,8Al0,9B9(at. %) after heat treatment {1175–1075K (4–12 ks) - hardening + 625–725K (3,6–36 ks) + 810K (3, 6 ks) - hardening} was 2600 kA/m. It was possible to attain the following properties for the magnets (C) (Nd0.22Pr0.44Tb0.20Ho0.10Gd0.04)15(Fe0.73Co0.27)baiAl0.99Cu0.1Ga0.1B8.5(at. %) after optimal thermal treatment (1175K (3,6–7,2 ks) with slow (12–16 ks) cooling to 675K and subsequent endurance at T=775K for 3,6 ks - hardening}: Br= 1,0 T, iHc = 1560 kA/m, BHmax= 192 kJ/m3, αb> |0, 015| %/K in the temperature interval 293–373K.