Microstructure characteristics and optimization of 2:17-type Sm-Co sintered magnets with different iron content

Highlights

• The microstructure is strongly correlated to solid solution treatment condition.

• Cu precipitates in nano-scale are found in the as-solutionized magnet.

• A prolonged solid solution treatment duration can improve the magnetic properties.

• Reversible transformation of microstructure and magnetic properties is achieved.

Abstract

In this work, the dependence of microstructure and magnetic properties on the iron content for Sm(Co_balFe_xCu_0.073Zr_0.024)_7.6 (x = 0.226, 0.233 and 0.24) magnets has been systematically studied. The magnet with x of 0.226 (relatively low Fe content) shows a homogeneous microstructure and optimal magnetic properties after it has undergone a solid solution treatment for only 2 h at 1443 K. However, under the same solid solution treatment, a largely uneven composition distribution is observed in magnets with an increased Fe content (x = 0.24). As a result, the peak and gradient of Cu concentration in the cellular boundaries are lower, giving rise to poor magnetic properties. It is impressive that there Cu precipitates are found in the as-solutionized magnet though the cellular structure has not yet occurred. Our results show that a prolonged solid solution treatment (t_s) is necessary to improve the microstructure and magnetic properties of the magnets with higher Fe content. Moreover, the optimal t_s can be shortened by increasing the solid solution temperature (T_s). However, when the T_s is too high, the composition is not uniform but segregated, and the magnet shows poor magnetic properties as well. Furthermore, it is also found that the homogeneous microstructure and optimal magnetic properties are deteriorated when the magnets undergo a solution treatment at much higher T_s. However, it can be recovered if the magnet is further solution treated at the proper conditions. Our results suggest that the transformation of microstructure and magnetic properties can be controlled reversibly by adjusting the solution treated conditions.

Introduction

2:17 Sm-Co based sintered magnets find wide applications in many different fields including aerospace, military technology and electric cars et al., because of their high Curie temperature which leads to excellent magnetic properties at high temperatures [1], [2], [3], [4], [5]. As it is well known, the magnetic properties of the magnets are closely correlated to their microstructure [6], [7], [8]. It is well known that a solid-solution treatment processing at high temperatures is important for the Sm₂Co₁₇-type magnets to obtain the ideal microstructure needed [9], [10], [11]. Through this treatment, a disordered TbCu₇ type 1:7H phase is formed which evolves after a low temperature aging into a complex microstructure consisting of 2:17R cells with a rhombohedral Th₂Zn₁₇ structure surrounded by 1:5H cellular boundaries with a hexagonal CaCu₅ structure coexisting with a Zr-rich lamellar phase [12], [13]. The single 1:7H phase is conducive to the formation of uniform cell structure and enhances the magnetic properties of the magnets. The iron-rich 2:17R cells play a crucial role on the high magnetization, while the Cu-rich 1:5H cellular boundary phase is responsible for the domain wall pinning, and it is the key to achieve high intrinsic coercivity (H_cj) and a large knee-point (H_knee) (a magnetic field at which the corresponding magnetization is equal to 0.9 × B_r, where B_r is the remanence) [14], [15], [16], [17], [18], [19], [20]. Recently, Shang et al. reported that the micron-size microstructure and composition of the as-solution-treated magnets have a profound influence on the cellular structure and the Cu content distribution, which determines the magnetic properties of the magnets [21]. In our previous work, it was also found that a critical time of the solution treatment is prerequisite to form homogeneous microstructure in the as-solutionized magnets [22].

The chemical composition of the 2:17 magnets also affects the morphology and uniformity of their microstructure [23], [24], [25]. To improve the maximum energy product of the 2:17 magnets, more Fe is added (at the expense of Co) increasing the saturation magnetization of the Sm₂(Co, Fe)₁₇ matrix phase [26], [27]. To obtain a homogeneous microstructure and optimal magnetic properties in the higher Fe content, an appropriate solution process should be employed.

In this work, the microstructure and magnetic properties of Sm₂Co₁₇-type sintered magnets with different Fe content have been systematically studied and the solutionized process was optimized to improve both the H_cj and H_knee in the higher Fe content magnets. These results provide a clue to preparing high-performance Sm₂Co₁₇-type sintered magnets with optimized H_cj and H_knee.