Gaseous interactions with Sm–Fe and Sm–Fe–Ta intermetallic alloys

Abstract

Abstract SmFe based alloys, grain refined by the HDDR process and interstitially modified with nitrogen, are potential candidates for commercially produced high energy permanent magnets. In order to obtain the optimum properties a thorough understanding of the starting material and processing parameters is required. The microstructures of two cast alloys, Sm 13.8 Fe 82.2 Ta 4.0 and Sm 13.7 Fe 86.3, were carefully examined with an SEM equipped with EDX and the exact stoichiometries of the observed phases were determined. The SmFeTa material was found to contain significant amounts of TaFe 2 as well as the Sm 2 Fe 17 , SmFe 2 , SmFe 3 phases observed in the binary SmFe material but without the α-iron dendrites which are characteristic of the latter material. XRD was used to monitor the disproportionation of the material over a range of temperatures. Nanocrystalline powders for subsequent nitriding were prepared from the cast alloys via the HDDR process, with the recombination stage carried out under vacuum between 700 and 850°C. Subsequent nitriding of the powders resulted in a lattice expansion and an increase in coercivity with values of 360 kA/m being obtained for the SmFe and 680 kA/m for the SmFeTa materials, respectively. Multiple applications of the HDDR process to the same material prior to nitriding were found to have little effect on the magnetic properties of the SmFe alloy although significant improvements with values of 1040 kA/m and an interesting contraction of the unit cell volume of the Sm 2 (FeTa) 17 phase were observed for the SmFeTa material.