Formation of cellular/lamellar nanostructure in Sm2Co17-type binary and ternary Sm-Co-Zr magnets

• Zr concentration above 1.4 at.% promotes cellular/lamellar nanostructure formation in ternary SmCo 7.6 Zr 0.1 . • In absence of Cu, small concentration gradients in the cellular nanostructure cause near zero coercivity in SmCo 7.6 Zr 0.1 . • 2:17 phase twins are suggested to facilitate the formation of 1:5 cell boundary phase. 2:17 SmCo magnets with a quinary composition of Sm(Co,Cu,Fe,Zr) 7+δ 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 1.4 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.