In-situ high-energy X-ray diffraction study of the early-stage decomposition in 2 : 17-type Sm-Co-based permanent magnets

The complex interaction between precipitation and dislocations challenges the determination of the effective precipitate nucleation temperature (T_nd) of dislocation-bearing supersaturated solid solutions, in particular, for the 2:17-type Sm-Co-based permanent magnets that evolve gradual formation and dissociation of dislocations during cellular precipitation. In this study, the early-stage decomposition behavior of a solution-treated Sm₂₅Co_50.2Fe_16.2Cu_5.6Zr_3.0 (wt.%) alloy with highly-faulted 2:17H (hexagonal Sm₂Co₁₇) was investigated using in-situ high-energy synchrotron X-ray diffraction (HES-XRD) and ex-situ high-resolution transmission electron microscopy (HR-TEM). The nucleation and growth of 1:5H (hexagonal SmCo₅) precipitate during heating and isothermal tempering were observed in situ, clarifying a dispute in 2:17-type Sm-Co-based magnets. The effective nucleation temperature T_nd of 1:5H precipitates was determined to be ∼760 °C, which is much higher than the starting transformation temperature of 2:17H to 2:17R (rhombohedral Sm₂Co₁₇) phase governed by diffusion-controlled glides of Shockley partials, T_a∼240 °C. TEM studies revealed that a pre-aging treatment at 550 °C (far below T_nd, but above T_a) causes dissociation of partial dislocations whereas a pre-aging treatment at 750 °C (slightly below T_nd) promotes the nucleation of 1:5H precipitates. As a result, after whole-process heat-treating, the final magnet with pre-aging at 750 °C possesses better magnetic properties than the one with pre-aging at 550 °C and the one without pre-aging. These results reveal that upon heating thermally-activated motion of dislocations occurs prior to sufficient atomic clustering into precipitate nuclei in 2:17-type Sm-Co-based magnets, providing direct guidance for designing proper material processing towards high-performance.