Sintering of Ferromagnetic Materials at Lower Temperatures in Hydrogen I. Sm2Co17 Alloys

Abstract

Scanning electron microscopy and X-ray energy-dispersive spectroscopy were employed to study the sintering of powders from the induction-melted industrial ferromagnetic Sm₂(Co,Fe,Zr,Cu)₁₇ alloy by the hydrogenation, disproportionation (HD), desorption, recombination (DR) (HDDR) route. The HD stage proceeded at 700°C and DR at 950°C. The experimental results showed that sintering of the powders occurred at the HD stage to produce a mechanically integral highly porous material. The porosity of the sintered materials was found to decrease as the compaction pressure and powder particle refinement increased. The powder compaction pressure was estimated to range from 2 to 5 t/cm². The decrease in sintering temperature was attributed to the higher diffusion rate of the alloy components resulting from the decrease in particle size, hydrogen-initiated phase transformations, and the hydrogen solid solution present in the alloy. Phase transformations occurred when the pressure changed at high temperatures. If the hydrogen pressure was high, the intermetallic was not thermodynamically stable and disintegrated (disproportionated) into several phases. If the hydrogen pressure was low (vacuum), the rare earth metal hydride was thermodynamically unstable and disintegrated, while the rare earth metal interacted with other phases to form the starting intermetallic. These phenomena are due to chemical reactions within a solid body, proceeding through the diffusion of components. The new sintering method for ferromagnetic materials has process advantages over existing methods: it does not require holding at the highest heating temperatures or usage of complex dies or complex equipment and results in the production of anisotropic nanostructured materials. Ways to improve the properties of sintered materials at low temperatures (in particular, increasing the homogeneity of their microstructure and decreasing the porosity) are proposed, such as optimization of sintering parameters and homogenization of the powders by particle size.