In-situ electrical resistivity measurements : study of magnetic and phase transitions and solid-HDDR processes in Nd-Fe-B-type alloys

Nd-Fe-B type alloys have been characterised by means of in situ electrical resistivity measurements. The potential of this technique for monitoring various phenomena relevant to the hydrogenation, disproportionation, desorption and recombination (HDDR) processing of Nd-Fe-B-type alloys is assessed, together with an evaluation of its capacity for delineating magnetic and phase transitions. The effects of external parameters, such as hydrogen pressure and processing temperature, and of intrinsic parameters, such as alloy composition and initial microstructure, on the kinetics of the solid-HDDR process have been investigated. It was found that the amount of neodymium-rich intergranular phase present in the material had a significant influence on the rates of disproportionation and recombination reactions. At 620 deg C, the recombination process takes place as a solid-solid reaction, and this has a marked effect on the reaction rate. It was also found that the disproportionation process is very sensitive to the H pressure and the dependence of the overall process on the processing temperature between 620-900 deg C has been determined.