The effects of temperature and pressure on the hydrogen decrepitation and hydrogen desorption of Sm2TM17 sintered magnets

This study investigated the impact of hydrogen pressure and processing temperature on the hydrogen decrepitation of Sm 2 TM 17 sintered magnets (where TM = Co, Fe, Cu and Zr). Sm and Co are ‘critical elements’, hence there is interest in recycling these materials from end-of-life applications such as surface mounted permanent magnet motors and actuators. Hydrogen Decrepitation (HD) is a potential recycling technology, but the effect of different processing parameters has yet to be fully explored. Three commercial grades of Sm 2 TM 17 sintered magnets with different compositions and microstructures were subjected to HD treatments over 72 h, at hydrogen pressures of 2 and 18 bar and temperatures between 25 and 300 °C. The resulting powders were characterised to assess their particle size and morphology, hydrogen content and degassing behaviour. Thermal activation temperatures of 100–150 °C allowed for significant decrepitation to be observed across all Sm 2 TM 17 compositions at both 2 and 18 bar pressures. Degassing analysis showed that all decrepitated powders examined released hydrogen in a single step, with peak hydrogen desorption occurring between 200 and 260 °C. At HD temperatures of 300 °C the reaction ceased as degassing took precedence over hydrogen absorption. Particle size analysis indicated that adjusting HD processing parameters altered powder size greatly, e.g. increasing temperature increased particle size whereas increasing pressure decreased particle size. The sample with the smallest cell size, greatest Cu and Zr and lowest Fe content absorbed the least amount of hydrogen. It also generated the coarsest powder and degassed at the lowest temperatures. This has been linked to this sample having less of the main interstitial hydride forming 2:17 rhombohedral phase found in the magnet nanostructure. • Thermal activation of ≥50 °C facilitates HD at a low pressure of 2 bar. • HD powder size can be controlled by altering pressure and temperature. • A greater proportion of 2:17 R phase may increase the extent of HD reaction. • Increased temperature shifts cracking from transgranular to intergranular fracture. • Competing hydriding and desorption occur during HD at temperatures >150 °C.