Kinetical analysis of the heat treatment procedure in SmCo 5 and other rare-earth transition-metal sintered magnets

Abstract

In the processing of all types of commercial sintered rare-earth transition-metal magnets (SmCo 5, Sm(CoCuFeZr) z , NdFeB) a post-sintering heat treatment is included, which is responsible for large increase of the coercive field. During this post-sintering heat treatment, there are phase transformations with diffusion of the alloying elements, moving the system towards the thermodynamic equilibrium. Due to the larger size of the rare-earth atoms, the diffusion of the rare-earth atoms in the lattice of rare-earth transition-metal phases like SmCo 5, Sm 2(Co, Fe) 17 or Nd 2Fe 14B should be very slow, implying that the diffusion of the rare-earth atoms should be controlling the overall kinetics of the process. From the previous assumption, a parameter named “diffusion length of rare-earth atoms” is introduced as a tool to study the kinetics of the heat treatment in rare-earth magnets. Detailed microstructural characterization of SmCo 5 and NdFeB magnets did not indicate significant microstructural changes between sintering and heat treatment temperatures and it was suggested that the increase of coercivity can be related to decrease of the content of lattice defects. The sintering temperature is high, close to melting temperature, and in this condition there are large amount of defects in the lattice, possibly rare-earth solute atoms. Phase diagram analysis has suggested that a possible process for the coercivity increase can be the elimination of excess rare-earth atoms, i.e. solute atoms from a supersatured matrix. The “diffusion length of rare-earth atoms” estimated from diffusion kinetics is compatible with the diffusion length determined from microstructure. For the case of SmCo 5, it was found that the time of heat treatment necessary is around 20 times lower if an isothermal treatment at 850 °C is substituted by a slow cooling from sintering temperature 1150 to 850 °C. These results give support for the thesis that the coercivity increase is related to elimination of lattice defects. Although this example is for SmCo 5 magnets, this analysis can be used for the optimization of the heat treatment procedure in other cases, and it is also discussed how this method could be applied for Sm(CoCuFeZr) z and NdFeB sintered magnets.