| The Fe-Ni alloys, as a soft magnetic material with high permeability, low coercivity, excellent processing performance and high stability, are widely applied in read-write magnetic storage devices, electromagnetic shielding, and electrical machines. However, the Fe-Ni alloys generally show the low hardness and strength, complex preparation process and high molding temperature, which limit its applications in such as high-performance wear-resistant magnetic heads. This paper aims to prepare a high hardness and strength Fe-Ni-P ternary magnetic alloy at a lower temperature by liquid phase sintering (LPS) and provides the experimental and theoretic bases for developing new iron nickel alloys and expanding the applications of iron nickel alloys. At the same time, the influence of process parameters and P on the structure, composition and properties of sintered samples, as well as the sintering mechanism will be studied.On the basis of others’research, the coated Fe-23.69 wt.%Ni-2.67 wt.%P composite powder has been prepared by electroless nickel plating on the surface of spherical carbonyl iron powder of 3-5μm in this paper. After the pretreatment and micro-pressure molding, the composite powder was subjected to vacuum sintering at 950 ℃,975 ℃,1000 ℃, and 1025℃ for 0.5 h and at 950 ℃ for extended holding time of 1 h and 4 h respectively. The microstructure, phase constitution, chemical component, density, magnetic properties, hardness and compressive properties of different sintered samples were systematically investigated. Meanwhile the formation energy and density of states of phases were calculated and analyzed by using the first principle method.The results show that, the Fe-Ni-P composite powder is mainly composed of a-Fe and amorphous Ni-P coating of 0.5-1 μm. During the sintering process of Fe-Ni-P composite powder, a persistent liquid sintering can be available. It is found that the microstructure of sintered samples is related to sintering temperature and Fe/Ni atomic ratio in solid-liquid diffusion layer. The higher Fe/Ni atomic ratio of the diffusion layer at lower temperature (950 ℃ and 975 ℃) can extend the solid solution phase, causing the divorced eutectic transformation. When sintered at higher temperature (1000 ℃ and 1025 ℃), the regular eutectic transformation dominates the whole process due to the lower Fe/Ni atomic ratio in the diffusion layer. All the sintered Fe-Ni-P alloys are composed of α-Fe(Ni,P), γ-Fe(Ni,P) and (Fe,Ni)3P. α-Fe(Ni,P) and γ-Fe(Ni,P) solid solutions are metastable phase with the weaker bonding ability and mainly distributed in grains. While the (Fe,Ni)3P phase, as an intermetallic compound with the thermodynamic and structural stability as well as stronger bonding ability, has the higher hardness and is mainly distributed in the grain boundary. As the increase of sintering temperatures or holding times (950 ℃), for all the sintered samples, the grain size grows up obviously, the density decreases after an initial increase, the hardness increases greatly, and the compressive properties monotonously decreases. The magnetic properties seem much less sensitive to the sintering temperature, but can be improved effectively with the holding time (950 ℃) increasing. The study found that the Fe-Ni-P alloy sintered at 950 ℃ for 1 h has the best comprehensive performance with the high hardness (349 HBW), compressive strength (2088.44 MPa), and saturation magnetization (124.00 emu/g) in this paper. Adding P in the Fe-Ni alloys can realize the liquid phase sintering, lower the sintering temperature, increase the structural stability, improve the hardness and strength of alloys substantially, however, shows less influence on the magnetic properties and increasing the brittleness of materials. |
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