ThMn12-Type Alloys for Permanent Magnets

Iron-rich compounds with the tetragonal ThMn12-type structure have the potential to meet current demands for rare-earth-lean permanent magnets with high energy density and operating temperatures of 150-200 degrees C. However, while it is normal for magnet technology to lag behind the development of underlying magnetic material, this gap has always been unusually large for ThMn12-type magnets. The gap has widened further in recent years, as excellent combinations of intrinsic magnetic properties have been obtained in compounds synthesized with a smaller amount of structure-stabilizing elements (e.g., SmF11V or Sm0.8Zr0.2Fe9.2CO2.3Ti0.5) or with no such elements (i.e., SmFe9.6Co2.4 thin films). The search for promising compounds continues-with increasing help coming from theoretical calculations. Unfortunately, progress in the development of magnets beyond polymer-bonded interstitially modified powders remains marginal. The introduction of lanthanum (La) was found to stabilize low-melting-temperature minority phases in Sm(Fe,Ti)(12) alloys, thus allowing for liquid-phase sintering for the first time. The high reactivity of La, however, has apparently undermined the development of coercivity (H-c). A controlled crystallization of the initially suppressed ThMn12-type phase makes bulk magnetic hardening possible, not only in Sm-Fe-V alloys (in which it has been known since the 1990s), but also is in La-added (Ce,Sm)(Fe,Ti)(12) alloys. The properties of the bulk-hardened alloys, however, remain unsatisfactory. Mechanochemically synthesized (Sm,Zr)(Fe,Si)(12) and (Sm,Zr)(Fe,Co,Ti)(12) powders may become suitable for sintering into powerful fully dense magnets, although not before a higher degree of anisotropy in both alloys and a higher H-c in the latter alloy have been developed. (C) 2020 THE AUTHORS. Published by Elsevier LTD on behalf of Chinese Academy of Engineering and Higher Education Press Limited Company.