Abstract
Grain boundaries are thought to be the primary demagnetization sites of precipitate-hardening 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets with a unique cellular nanostructure, leading to a poor squareness factor as well as a much lower than ideal energy product. In this work, we investigated the grain boundary microstructure evolution of a model magnet Sm25Co46.9Fe19.5Cu5.6Zr3.0 (wt. %) during the aging process. The transmission electron microscopy (TEM) investigations showed that the grain boundary region contains undecomposed 2:17H, partially ordered 2:17R, 1:5H nano-precipitates, and a Smn+1Co5n−1 (n = 2, 1:3R; n = 3, 2:7R; n = 4, 5:19R) phase mixture at the solution-treated state. After short-term aging, further decomposition of 2:17H occurs, characterized by the gradual ordering of 2:17R, the precipitation of the 1:5H phase, and the gradual growth of Smn+1Co5n−1 compounds. Due to the lack of a defect-aggregated cell boundary near the grain boundary, the 1:5H precipitates are constrained between the 2:17R and the Smn+1Co5n−1 nano-sheets. When further aging the magnet, the grain boundary 1:5H precipitates transform into Smn+1Co5n−1 compounds. As the Smn+1Co5n−1 compounds are magnetically softer than the 1:5H precipitates, the grain boundaries then act as the primary demagnetization sites. Our work adds important insights toward the understanding of the grain boundary effect of 2:17-type Sm-Co-Fe-Cu-Zr magnets.
Highlights
The alloys of 2:17-type Sm-Co-Fe-Cu-Zr are the strongest high-temperature permanent magnets and have been widely applied in advanced industries such as large-efficiency motors, magnetic bearings, and the momentum wheel of satellite communications because they can maintain a high maximum energy product ((BH)max) and high coercivity (Hcj) at elevated temperatures [1,2,3,4,5,6,7]
The magnets usually contain various types of microstructural deficiencies, which weaken the pinning force against domain walls (DWs) motions locally and lead to an inhomogeneous demagnetization process as well as a poor squareness factor. These microstructural deficiencies include: (i) the intersections between 1:5H precipitates and the co-existing 1:3R precipitates [14], (ii) the cell edges that usually contain stacking faults (SFs) or the 2:17R’ intermediate phase [20], (iii) the defect-aggregated cell boundaries (DACBs) free of the 1:5H phase [21], and (iv) the grain boundaries with sparser 1:5H cell boundary precipitates than the grain interiors and phase mixture of Zr-stabilized Smn+1Co5n−1 (n = 2, 1:3R; n = 3, 2:7R; n = 4, 5:19R) precipitates that are magnetically softer than the 1:5H precipitates [15,22,23,24,25,26,27]
The squareness factor was evaluated by Hk/Hcj, where Hk is the knee point field of the demagnetization curve at which 80% Jr is retained
Summary
The alloys of 2:17-type Sm-Co-Fe-Cu-Zr are the strongest high-temperature permanent magnets and have been widely applied in advanced industries such as large-efficiency motors, magnetic bearings, and the momentum wheel of satellite communications because they can maintain a high maximum energy product ((BH)max) and high coercivity (Hcj) at elevated temperatures [1,2,3,4,5,6,7]. The magnets usually contain various types of microstructural deficiencies, which weaken the pinning force against DW motions locally and lead to an inhomogeneous demagnetization process as well as a poor squareness factor These microstructural deficiencies include: (i) the intersections between 1:5H precipitates and the co-existing 1:3R precipitates [14], (ii) the cell edges that usually contain stacking faults (SFs) or the 2:17R’ intermediate phase [20], (iii) the defect-aggregated cell boundaries (DACBs) free of the 1:5H phase [21], and (iv) the grain boundaries with sparser 1:5H cell boundary precipitates (i.e., larger cells) than the grain interiors and phase mixture of Zr-stabilized Smn+1Co5n−1 (n = 2, 1:3R; n = 3, 2:7R; n = 4, 5:19R) precipitates that are magnetically softer than the 1:5H precipitates [15,22,23,24,25,26,27]. It is necessary to investigate how the grain boundary microstructure evolves during the aging process, i.e., how the concurrently happening recrystallization, precipitation of 1:5H precipitates, and growth of Smn+1Co5n−1 precipitates interact with each other
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