| During the last decade, the emergence of high entropy alloys (HEAs) attracts so many attentions because of their unique phase structures and excellent properties. The design of high entropy alloy is a breakthrough of the traditional alloy design. The high entropy effect facilitates the alloys to form simple solid solutions rather than intermetallic compounds. The severe lattice distortion and sluggish diffusion effects lead the HEAs to exhibit high strength, high hardness, high softening resistance and so on. In addition, investigations show that, HEAs have potential applications in functional materials, such as the magnetic materials. For traditional soft magnetic alloys, such as Fe-Si, Fe-Co and amorphous alloys, and the magnetic field induced shape memory alloy, Ni2MnGa, the brittleness limits their industrial application and rises the production cost. It is so.important to develop magnetic materials with good ductility. The emergence of HEA brings hope to this, since by carefully designing the composition, the HEAs can have best combination of strength and ductility. Besides, the mostly investigated HEAs always contain ferromagnetic elements Co, Fe, and Ni, which implies this kind of alloys can show magnetic behaviors. But little effort has been put into this field. So, this work mainly focuses on designing the magnetic HEAs and studying the relationship between phase structures and properties.Firstly, based on the high entropy concept, a system of CoFeNi(AlSi)x alloys are designed and investigated by adding Si and Al into the CoFeNi alloy. The saturation magnetization (Ms) decreases with increasing the content of Al and Si, while the strength and hardness show the opposite trend. Excessive addition of Al and Si makes the alloys so brittle. Among all, the CoFeNi(AlSi)0.2 alloy shows the best combination of ductility and soft magnetic property. It exhibits high saturation magnetization (1.15 T), high electrical resistivity (69.5μΩ·cm), excellent plasticity, but a little higher coercivity (1400 A/m). For the as-cast alloy, it always contains defects, such as elemental segregation, holes, internal stress. In order to decrease the coercivity, different techniques, such as suction casting, heat treatment, and Bridgman solidification are used to uniform the microstructure, reduce the internal stress or form columnar grain with large grain size. All these methods can help to significantly lower the coercivity, and make the alloy be comparable to silicon steel which shows potential application in soft magnetic field.Then, in order to design high entropy alloys which may show magnetic-field-induced shape memory effect, Mn and Ga are chose to be added into the CoFeNi alloy. And according to the composition ratio of the typical magnetic-fie Id-induced shape memory alloy, Ni2MnGa, the contents of Co and Fe are adjusted to form the CoxFe1-xNiMnGa HEAs with the hope to improve the plasticity. However, results show that the high entropy of mixing can not inhibit the formation of ordered phases, but to some extent, it can decrease the ordering, which lead the alloy not showing the shape memory effect, since the martensite transformation disappears or be delayed to temperature lower than -170℃. The intrinsic brittleness has not been changed using this method, however, the fracture mechanism turns to be the combination of intergranular fracture and cleavage fracture. Replacing Ni by Co can increase the Curie temperature and saturation magnetization which can make the alloy to be candidate for high temperature soft magnetic material.Since the magnetic moment of Ni-Mn-Ga alloys is mainly from the Mn atom, Mn is firstly added to the ternary CoFeNi alloy, and then the mostly used elements in shape memory alloys, such as Al, Ga, and Sn are also added to form CoFeNiMnX (X=Al, Ga, Sn) HEAs. Investigation shows that, the addition of Al, Ga, and Sn favors the alloys to form ordered phases, which leads the alloys to show high strength and low ductility. However, all these elements addition can make the alloys to present higher saturation magnetization, especially the CoFeMnNiAl alloy, for which the Ms can be 147 Am2/kg, which can be comparable to CoFeNi alloy. The DFT simulation shows the anti-ferromagnetism Mn performed in CoFeMnNi alloy is prohibited by Al addition, which helps to turn the anti-ferromagnetism to ferromagnetism. However, the Ms is not proportional to the Al content. For CoFeNiMnAlx (x=0.25,0.5,0.75,1.0,1.25,1.5,1.75,2.0) alloys, the highest Ms can only be obtained at x=1.0. With further increasing the Al content, the Ms can decrease. Too much Al addition impairs the saturation magnetization. Besides, the Ms is closely related to the crystal structure. For the same alloy, the higher volume fraction of the BCC phase, the higher the Ms can reach. |
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