Effect of hydrogen pressure on disproportionation kinetics of Nd-Fe-B alloy

In the anisotropic HDDR treatment of the Nd-Fe-B alloy, the critical step determining the texture in the final product is the disproportionation. In the present study, the effect of hydrogen pressure on the disproportionation kinetics of the Nd12.5Fe80.6B6.4Ga0.3Nb0.2 alloy was examined by isothermal thermopiezic analysis (TPA). It was found that the overall reaction rate of disproportionation of the hydrogenated Nd12.5Fe80.6B6.4Ga0.3Nb0.2 alloy was influenced strongly by the hydrogen pressure. With the hydrogen pressure of 0.1 and 0.06 MPa, the overall reaction rate of disproportionation was controlled by simple linear relationship. However, the overall reaction rate of disproportionation with the low hydrogen pressure of 0.02 MPa was expressed by parabolic rate equation. The activation energy for the disproportionation of Nd12.5Fe80.6B6.4Ga0.3Nb0.2 alloy with hydrogen pressure of 0.02 MPa was significantly higher than those with hydrogen pressure of 0.1 and 0.06 MPa. The activation energy for the disproportionation with hydrogen pressure of 0.1, 0.06 and 0.02 MPa was calculated to be approximately 272, 262, and 321 kJ/mol H, respectively. It was suggested that the disproportionation of the alloy with initial hydrogen pressure of 0.02 MPa might take place via different mechanisms from that with initial hydrogen pressure of 0.1 and 0.06 MPa.