Study of the hydrogen decrepitation process of Nd-Fe-B alloys with different Nd content and the addition of Nb and Ga as doping metals

The objective of this work is to analyze the effect of Nd concentration and additions of Nb and Ga on the hydrogen decrepitation of gas-atomized powders after grain growth annealing at 1150 °C for 5 h. Ga and Nb additions reduced the grain size upon atomization. Thermodynamic calculations have demonstrated that, in general, the amount of liquid at 1150 °C increases with Nd concentration and with the addition of Nb or Nb-Ga. Grain growth during annealing is accelerated when the Nd content is raised or with the addition of Ga, which can be explained by the larger amount of liquid in the alloy. On the other hand, Nb forms precipitates that delay grain growth, especially if the amount of liquid is low. Comparing the grain size distribution of the annealed samples with the particle size distribution of hydrogen decrepitated powders allowed identifying the dominant fracture mechanism. Some transgranular cracking occurred in all compositions, increasing the fraction of fine irregular particles. When the Nd concentration is raised, the particle size is reduced. In general, the dominant mechanism of fracture is intergranular crack propagation. However, the transgranular fracture was the dominant mechanism in Ga-containing alloys. Particle shape was not sensitive to the compositional changes evaluated in this work.