Since the discovery of magnetic field induced strain (MFIS), Ni-Mn-Ga Heusler alloys have attracted considerable attention. These materials have a variety of response functions, capable of changing shape and size under the influence of external magnetic fields. So far, the MFIS of 10M, 14M and NM martensitic structure single crystal alloys is 7.1%, 11.2% and 12%, respectively. Despite the considerable strain, the application of MGIS is practically limited to single crystal materials. Due to the high cost of monocrystalline manufacturing, polycrystalline Ni-Mn-Ga alloys have also been extensively studied. The main limitation of obtaining large MFIS in polycrystalline alloys is the grain boundary constraint, which hinders the movement of the twin grain boundaries and also causes fractures along the grains. To reduce its negative effects and enhance MFIS, scholars have proposed coarse grain size and strong texture materials. However, coarse grains and strongly woven components can easily lead to strain/stress incompatibilities between adjacent grains. One of the existing ways to overcome the problem of incompatibility is to introduce a certain degree of porosity. There is currently no effective solution to the incompatibility problem of dense polycrystalline Ni-Mn-Ga materials.
Researchers from the Polish Academy of Sciences conducted a comprehensive study on the microstructure and crystallography characteristics of Ni50.2Mn28.3Ga21.5 fast quenching tape after heat treatment. The paper was published in Acta Materialia under the title "Evolution of microstructure and crystallographic texture of Ni-Mn-Ga melt-spun ribbons exhibiting 1.15% magnetic field-induced strain."
Thesis Link:
https://doi.org/10.1016/j.actamat.2021.117237
Under the atmosphere of argon, a block alloy with a nominal composition of Ni50.2Mn28.3Ga21.5 was prepared by induction melting, and after induction melting, it was sprayed on a copper wheel rotating at a linear velocity of 25m/s by induction melting, and the thin strip obtained was heat treated under different conditions, including: 573K×1h; 873K×1h; 1073K×1h; 1173K×1h; 1173K×72h, air cooling.
The study found that when the annealing temperature reached 1173 K, as the temperature increased, the temperature of Ms and As had a tendency to change to higher temperatures, which was consistent with single crystal materials of the same composition, but after 72 h, their temperatures decreased again. The magnetization intensity of all thin bands has two mutations. After cooling from 400 K, the paramagnetic transition of L21 austenite greatly increases the magnetization intensity. Curie temperature (Tc) below 1173K increases with increased annealing temperature, but is independent of annealing time. This effect can be attributed to magnetic exchange, which is greatly influenced by the Mn-Mn distance and is directly related to the degree of atomic order.
Figure 1 Relationship between the degree of order of Ni50.2Mn28.3Ga21.5 strip and the influence of heat treatment
Figure 2 BSE/SEM image of Ni50.2Mn28.3Ga21.5 with thin cross-section
Figure 3 EBSD diagram of ni-mn-Ga band after 1173K ×1h and 1173K × 72h annealing
Figure 4 In the direction of the magnetic field perpendicular to the strip surface, the magnetization properties of thin bands of 1173K×1h and 573K × 72h
Fig. 5 SEM plot after annealing at 1173 k/72 h (a) initial state and (b) applied magnetic field
In this paper, the microstructure and crystal texture of Ni50.2Mn28.3Ga21.5 fast quenching thin band under different heat treatment conditions are studied. The results show that as the annealing temperature increases, both the martensitic phase transition and the magnetic phase transition move in the direction of high temperature, which is related to the increase in atomic order, and not to the grain growth. As the annealing temperature increases, the volume fraction of 10M martensitic increases, reaching a complete phase transition. In addition, the heat treatment ensures a uniform radial structure with a strongly woven thin strip of <100 > fibers, resulting in an MMIS. 10M Ni50.2Mn28.3Ga21.5 Fast Quenching Thin Strip after 1173K ×72h heat treatment, MFIS up to 1.15%. The MFIS value of polycrystalline materials is relatively large, which reduces the incompatibility between adjacent grains and facilitates strain regulation during deformation. This study effectively improves the incompatibility problem of dense polycrystalline Ni-Mn-Ga materials. (Text: Breaking the Wind)
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