Formation of cellular/lamellar nanostructure in Sm$_2$Co$_{17}$-type binary and ternary Sm-Co-Zr magnets

Abstract

2:17 SmCo magnets with a quinary composition of Sm(Co,Cu,Fe,Zr)$_{7+{\delta}}$ are industrially relevant hard magnets used in high temperature and corrosive environments. Their complex cellular/lamellar nanostructure, consisting of ordered 2:17 phase cells, 1:5 phase cell boundaries and Z-phase (Zr-rich) lamellae, is essential for their high coercivity. However, the system's complexity makes it challenging to determine the contribution of each element or microstructural feature to coercivity. To disentangle the microstructure-property relationships, we simplified the system to binary and ternary SmCo$_{7.7-x}$Zr$_x$ (with $x = 0$ and 0.1) magnets and conducted detailed micro- to atomic-scale analyses. Only Zr-containing magnets formed a cellular/lamellar nanostructure akin to industrial magnets, in Zr-rich regions with at least 1 at.% Zr, but without achieving high coercivity due to low elemental gradients in absence of Cu across cell boundaries. Data from Zr-poor areas of SmCo$_{7.6}$Zr$_{0.1}$ suggests that 2:17 phase twin boundaries facilitate cellular nanostructure formation by providing inhomogeneities for heterogeneous nucleation.