The Role of Cu in Sintered Nd-Fe-B Magnets: ab initio Study

Trace addition of Cu is an effective method to improve the coercivity of sintered NdFeB magnets via improving the microstructure. The efficiency of Cu doping depends on the distribution of Cu in the multi-phase microstructure of the NdFeB magnet. To understand and control the Cu redistribution, the site preference of Fe substitution by Cu in Nd₂Fe₁₄B (2:14:1) and their substitution energies have been calculated by a first-principles density functional method. The total energy calculations show that all the substitution energies of Fe by Cu in 2:14:1 are positive, indicating Cu tends to avoid entering 2:14:1 phase. In particular, the substitution energy of Cu at the 16k₁ site (Fe) has a value of 55 meV/Cu per unit cell, implying the substitution of Fe (16k₁ site) by Cu in 2:14:1 could occur at high temperature (above 650 K). It is expected that a very small amount of Cu (1.5 at.% or so) will dissolve in 2:14:1 during induction-melting sintering process (above 1600 K) while depleting from the 2:14:1 grains to the grain boundary region during the post-sinter annealing process. The redistribution of Cu in Nd-rich phase will lower its melting point and promote the homogeneous distribution of Nd-rich phase along the grain boundary of 2:14:1 phase, enhancing the coecivity in sintered NdFeB.