NiFeGa ferromagnetic shape memory alloys and magnetic properties of phase change thermal effects

Title: NiFeGa ferromagnetic shape memory alloys and magnetic properties of phase change thermal effects of Author: Yu Huajun Degree awarded by: University of Electronic Science and Technology Keywords: ferromagnetic shape memory alloy;; Ni-Fe-Ga;; martensite;; magnetic properties;; magnetocaloric effect Abstract:
Ferromagnetic shape memory alloys in recent years, studies have found a set of magnetic shape memory and magnetic field induced strain in one of the new smart materials, sensors and actuators are expected to become key components such as the material of choice. The Ni-Fe-Ga ferromagnetic shape memory alloys have magnetic anisotropy can be large and thermal processing performance, etc., the rich physical contents include: first, the alloy is Magnetic lifter ferromagnetic intermetallic compounds, it's hot parent phase with L2_1/B2 cubic structure, while the martensite phase modulation is usually a complex layer structure. Second, it is the thermoelastic martensitic transformation, the martensitic transformation characteristics and chemical composition of alloys by heat treatment and other effects. Third, the alloy with high saturation magnetization and large magnetic anisotropy constant. Therefore, the project has a very broad application prospects, and research scholars from various countries has gradually become one of the key material.
In this paper, arc melting and suction casting method of preparation of the Ni-Fe-Ga (-X) series alloys, a systematic study of Ni-Fe-Ga (-X) alloy microstructure, microstructure, martensitic change behavior and magnetic properties to study the chemical composition of the fourth alloying elements and heat treatment processes such as on the Ni-Fe-Ga alloy of impact. Improved preparation of Ni-Fe-Ga (-X) alloy method, attempted to improve the thermal effect of the magnetic alloy channels for Ni-Fe-Ga alloy lay the foundation for engineering applications.
The results show that the chemical composition of Ni-Fe-Ga alloy at room temperature and phase structure of very significant. Room temperature when Ni_ (73-x) Fe_xGa_ (27) series of alloys of Fe content from 18at% to 20at%, the alloy from the http://www.999magnet.com/products/131-magnetic-lifter martensitic phase transition into a single body with Austin. X-ray diffraction and TEM selected area electron diffraction revealed, Ni55Fe_ (18) Ga_ (27) alloy at room temperature monoclinic martensite phase modulation of the 14M structure. It also found that Ge, Al, Co and Cu alloying elements such as the Fourth of Ni-Fe-Ga alloy has an important impact on organizational structure. A certain amount of Ge element substitution Ni_ (56.5) Fe_ (17) Ga_ (26.5) in the alloy part of the group element, to make alloy martensitic at room temperature into Austin from the bulk phase; Co Add so Ni_ (56.5) Fe_ (17) Ga_ (26.5-z) Co_z alloy microstructure at room temperature from the martensite phase and γ phase formed, with the Co content increases, γ-phase increase in the number size increases; Ni_ (56.5) Fe_ (17) Ga_ (26.5-x) Al_x and Ni_ (56.5-z) Fe_ (17) Ga_ (26.5) Cu_z not only by the martensitic alloy or austenitic form, and there is a variety of phase structures.
Ni-Fe-Ga martensitic alloy chemical composition and behavior of alloying elements is very sensitive to the fourth. In Ni_ (73-x) Fe_xGa_ (27) series alloys, the martensitic transformation temperature M_s the Ni_ (55) Fe_ (18) Ga_ (27) alloy decreased to 31.7 ℃ Ni_ (52.5) ​​Fe_ (20.5) Ga_ (27) alloy -118.3 ℃; in Ni_ (73.5-y) Fe_yGa_ (26.5) series alloys, M_s the Ni_ (56.5) Fe_ (17) Ga_ (26.5) alloy decreased to 108.9 ℃ Ni_ (54.5) Fe_ (19) Ga_ (26.5) alloy of 28.1 ℃. Fixed Fe / Ga ratio is constant, Ni-Fe-Ga alloys with martensitic transformation temperature increases linearly with Ni content increased, specifically, in Ni_ (56.5 + z) Fe_ (17) Ga_ (26.5) series alloys, M_s 54.5at% from the Ni content increased when the Ni content of 39.6 ℃ to 58.5at% when 139.9 ℃. Ge elements found in both the alternative alloys of Ni or Fe, or Ga, are significantly lower temperature martensitic alloys, Ge substitution Ni martensitic phase transition temperature decreased the fastest, Ge, followed by alternative Fe, Ge Ga substitution slowest: The Co substitution Ni-Fe-Ga alloys Ga atoms significantly improved the alloy's martensitic transformation temperature. Preliminary view should be from the valence electron concentration, the size factor and a combination of factors such as degree of order study the role of martensitic transformation temperature.
Heat treatment process is also affected Ni-Fe-Ga alloy martensitic transformation behavior is one important factor. The results show that the martensitic transformation temperature decreased significantly compared with unannealed, and then rises with increasing annealing temperature; and annealing time, the alloy of the reverse martensitic transformation temperature gradually increased, Ma bulk transition temperature first increases, until the annealing time 5 h, then the extension of the annealing time decreased. The order degree of parent phase alloy, crystal defects and internal stress such as heat treatment process is affecting the temperature of the martensitic alloy may cause.
In the strong magnetic field magnetization, Ni-Fe-Ga (-X) alloy exhibits a common feature, namely the high-temperature austenitic alloy was easily magnetized, easily saturated; low temperature martensite phase shows hard magnetization, saturation Slow features. In Ni_ (56.5-x) Fe_ (17) Ga_ (26.5) Ge_x (x = 0,0.5,1.0,2.0) alloy, alloy and the saturation magnetization of the magnetic crystal anisotropy constant, along with the increase of Ge content decreases; Curie temperature increases with the Ge atom concentration decrease. By isothermal magnetization curves of the Ni-Fe-Ga (-X) alloy magnetocaloric effect. The results showed that, Ni_ (55) Fe_ (18) Ga_ (27) alloy is not very high maximum magnetic entropy change in the 50 kOe magnetic field, the maximum value of only -2.0J/kgK, difficult as magnetic refrigeration technology magnetic refrigerant. Despite the addition of Al and Ge alloy increases the maximum magnetic entropy change, but still can not achieve the requirements of magnetic refrigeration technology. However, through the study found that changing the alloy composition, magnetic atoms alloying is to improve the magnetocaloric effect may be an effective way.
Room temperature compression test results show that, Ge elements added to improve the NiFeGa alloy compressive strength, improve the elongation of the alloy. (Ni_ (56.5) Fe_ (17) Ga_ (26.5)) _ (100-x) Ge_x compressive strength and elongation of alloys with Ge content increases. Degree Year: 2009