Progress in the Preparation of Rare Earth Magnetic Refrigeration Materials by Thermal Deformation
Magnetic refrigeration technology based on the magnetocaloric effect of materials has intrinsically reversible high-efficiency thermodynamic properties and environmental advantages, and is expected to become one of the most potential alternatives to gas compression refrigeration. my country’s La-Fe-Si alloy with independent intellectual property rights is internationally recognized as one of the magnetic refrigeration materials with great application potential. However, the magnetocaloric functional phase of this material is formed by the peritectic solidification reaction, and it is difficult to directly obtain the magnetocaloric phase by traditional casting methods. Usually, the as-cast alloy needs to be homogenized and heat treated at a high temperature of about 1273 K for several weeks, which has a long preparation cycle and high energy consumption, which seriously hinders the development of magnetic refrigeration technology. Previous studies have mostly used methods such as increasing heat treatment temperature, accelerating solidification rate, composition optimization or powder metallurgy to accelerate the formation of the magnetocaloric phase, but at the expense of sacrificing the magnetocaloric properties and mechanical properties of the material or introducing other complex processes. Therefore, one of the key technologies to break through the preparation of La-Fe-Si magnetic refrigeration materials is to find a simple and effective new way to regulate the formation process of the magnetocaloric phase, so as to efficiently obtain a magnetic working medium with excellent comprehensive properties.
Recently, the Magnetic Phase Change Materials team of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences systematically carried out research on 923 K hot forging LaFe13.92Si1.4 alloys with deformations of 31.3% to 84.4%, and found that the formation rate of the magnetocaloric phase in the deformed samples was significantly higher than that of the undeformed sample, and the formation rate increases with the increase of the deformation amount. The microstructure of the deformed samples was characterized by backscattered electron diffraction. It was observed that the α-Fe and La-rich phases in the hot forging samples were distributed in layers, and the continuous dynamic recrystallization occurred in the deformation process. density grain boundaries (Fig. 1a). The singular layered structure of the vertical and horizontal grain boundary structure increases the nucleation sites of the magnetocaloric phase, and the high density of grain boundaries is beneficial to promote element diffusion. After heat treatment at 1323 K for 1 h, the 84.4% hot forged sample can obtain a magnetocaloric phase as high as 82.2 vol.%, and the magnetic entropy becomes 14.6 J/kg K under a 2 T magnetic field. In contrast, the undeformed sample contained only 24.1 vol.% magnetocaloric phase under the same conditions, and the magnetic entropy changed to 1.3 J/kg K (Fig. 1b). The large deformation sample was heat-treated for 12 h and then subjected to hydrogen absorption treatment, and a sheet-like refrigerant with a Curie temperature of 309 K and remained intact 20 × 10 × 1 mm3 was obtained, and the magnetic entropy change was as high as 19.4 J/kg K (Fig. 1c). ), and preliminarily realized the preparation of high comprehensive performance magnetic refrigerant. This research on the phase-forming behavior and magnetocaloric effect of La-Fe-Si alloys based on thermoplastic deformation has an important guiding role in the microstructure design and processing of rare-earth magnetic refrigeration materials. Scientific questions such as associations with phasing behavior provide a powerful tool. In addition, the near-net forming characteristics of hot forging deformation are also very suitable for the batch preparation of La-Fe-Si alloys. Relevant results were published in Acta Materialia 221 (2021) 117334 (https://doi.org/10.1016/j.actamat.2021.117334), a well-known journal in the field of metal materials.
This work was supported by the National Natural Science Foundation of China and the Ningbo Municipal “Science and Technology Innovation 2025” major special project.
Fig. 1 (a) Microstructure of 84.4% hot forging deformed sample; (b) Magnetic entropy change of different samples after heat treatment at 2 T; (c) Sample and magnetic entropy of 84.4% deformed sample after heat treatment for 12 h and hydrogen absorption Change