Preparation of hard magnetic materials based on nitrogenated rare-earth iron alloys

Nd Fe11Ti, Nd Fe10.5 Mo1.5 and Nd Fe10.75 Mo1.25 alloys were synthesized by reduction-diffusion calciothermic process (RDC) from neodymium chloride (NdCl3), iron, titanium, molybdenum and reduction agent (metallic calcium). The effect of process variables, like temperature, time, excess amount of NdCl3, heating rate, and composition variation of the Nd Fe12-xMox (1 ≥ x ≥ 2). Mother alloys in which 1:12 phase is major were nitrogenated by gas-solid reaction with N2 and by chemical reaction with sodium zide (Na N3). In addition, the influence of reducing particle size of the powdered mother alloys in the nitrogenation step with Na N3 were studied. As prepared and interstitially modified Nd Fe11 Ti, Nd Fe10.5 Mo1.5 and Nd Fe10.75 Mo1.25 alloys with nitrogen , were characterized by X-ray diffraction, Moessbauer spectroscopy, thermomagnetic, SEM and EDS. Nitrogenation by gas-solid reaction with N2 is found to be not promising, since resulted Curie temperatures (Tc) were lower than literature values. However, nitrogenation by chemical reaction with Na N3 was efficient with higher or same Tc than previous reported results. The average increases on Tc and volumetric expansion were 200 deg C and 4%, respectively. Milling of the mother alloys before nitrogenation at 330 deg C is preferred because reaction kinetics is enhanced. Nevertheless, at 450 deg C, a competition between the interstitially modified compound formation (alloy + N) and alloy dissociation has occurred, resulting in a Fe-α phase increase. (author)