The Sm-Fe- V based 1:12 permanent magnets

Abstract

Criticality of the rare earth (RE) metals has encouraged materials researchers to explore RE-lean/free alternatives to Nd-Fe-B and Sm-Co permanent magnets. In this context, the Sm-based compounds with ThMn12 (1:12) structure exhibiting intrinsic magnetic properties comparable to that of Nd2Fe14B compounds are considered as one of the alternatives. This thesis aims to develop Sm-based 1:12 magnets with magnetic properties comparable to Nd-Fe-B magnets without using critical raw metals such as Nd, Dy, Tb, Co and with reduced use of Sm. For this purpose, the studies are divided into three parts. The first part of the investigations focuses on the 1:12 phase stabilization and improvement of the intrinsic magnetic properties in the.SmFe12-xVx (x = 2, 1.5, 1.0, 0.5) systems. The second part deal with transferring the intrinsic properties into extrinsic properties, and the main task consisted in developing coercivity on stoichiometric and off-stoichiometric SmFe10V2 alloys. Two approaches were used to achieve this goal (i) through grain size refinement using powder metallurgy route and rapid solidification, and (ii) via bulk magnetic hardening by precipitation of the eutectic Sm-La phase. The third part is devoted to the consolidation of the mechanical milled powders into bulk magnets using hot compaction and hot deformation processes. During this process, the bulk materials developed the proper microstructure and thus the magnetic properties. A maximum coercivity of 1.06 T with M3T = 0.59 T, Mr = 0.42 T and (BH)max = 28 kJ m-3 was obtained in Sm12Fe73V15 isotropic hot compacted. The Curie temperature was measured to be 330°C and the temperature coefficients of remanent magnetization and coercivity were 0.14% C-1 and 0.39% C-1, respectively. Minor hysteresis loops indicate a coercivity mechanism similar to that of the nanocrystalline Nd-Fe-B magnets. Magnets with modified compositions Sm-Fe-(V,M) (M = Ti, Mo, Cu) were synthesized in order to investigate the effect on the magnetic properties of Sm12Fe73V15 when V was reduced or partially substituted by another transition metal. All the mechanically milled powders were successfully consolidated into fully-dense magnets. The most striking result was that magnets with compositions Sm12Fe76.5V11.5 and Sm12Fe73V7.5Mo7.5 developed a texture perpendicular to the deformation direction.