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 characterized by means of in-situ electr ical resistivity measurements. The potential of this technique for mon itoring various phenomena relevant to the hydrogenation, disproportion ation, desorption and recombination (HDDR) processing of Nd-Fe-B-type alloys is assessed, together with an evaluation of its capacity for de lineating magnetic and phase transitions. The effects of external para meters, such as hydrogen pressure and processing temperature, and of i ntrinsic parameters, such as alloy com position and initial microstruc ture, on the kinetics of the solid-HDDR process have been investigated . It was found that the amount of neodymium-rich intergranular phase p resent in the material had a significant influence on the rates of dis proportionation and recombination reactions. At 620 degrees C, the rec ombination process takes place as a solid-solid reaction, and this has a marked effect on the reaction rate. It was also found that the disp roportionation process is very sensitive to the hydrogen pressure and the dependence of the overall process on the processing temperature be tween 620 and 900 degrees C has been determined.