Investigation Of Mechanical Alloying Process Of Fe₇₅Si₂₅ And The Stability Of The Product

Fe₃Si alloy is considered as a kind of attractive functional material, owing to its negative temperature coefficient of electrical resistivity, fine corrosion resistance and excellent soft magnetic properties. However, its low ductility and poor formability prevent industrial application. Mechanical alloying (MA) is expected to produce Fe₃Si alloy with high performance. The mechanical alloying process of Fe₇₅Si₂₅ powders and the stability of the product are studied in this paper.The phases, microstructures, morphologies, cross-sections of mechanical alloying powders were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and electron probe microstructure analysis (EPMA) respectively. The results of MA experiments show that MA leads to an expansion of the crystalline disordered solid solution phase. When Si content is 25at.%, the alloying product is nanostructured a-Fe(Si) solid solution. No amorphous phase form during the mechanical alloying process. Two kinds of Fe-Si powders are obtained in the process, one is a composite powder with a typical laminar structure, and the other is a powder with iron as the inner core. Because the diffusion distance of the central area of the latter is larger, the process of its mechanical alloying trails. The mechanical alloying almost finished after 30h milling. Based on the results of the experiments and the knowledge of the mechanism of the mechanical alloying of the ductile/brittle component, a model is established represented by Fe₇₅Si₂₅.When the product of mechanical alloying was heat treated at 400 ℃, the transition of α-Fe(Si)→Fe3Si was discovered. A heat treatment of the 30h-millined powder at 500℃ leaded to the complete transition from α-Fe(Si) to Fe₃Si. And the grain size of the Fe₃Si remains in the nanometer region. The changes of phase composition and structure during the mechanical milling of the Fe₃Si are studied. The results prove that the main phase changed into α-Fe (Si) solid solution after milling for 80h.Based on the Miedema's semi-empirical theory, the thermodynamic model forFe-Si was established. When the component is Fe7sSi25, the free energy of FeaSi is the lowest. Mechanical milling introduces high energy into the material being processed. This energy can be stored in Fe3Si as the distortion of crystal lattice, grain boundaries and atomic disorder, which make the free energy of FeaSi higher than in the a-Fe (Si) solid solution. When grain size reaches 15nm, the increment of energy is large enough for the transition of a-Fe (Si)—Fe3Si. The MA process greatly enhances the diffusion of Si by lowing the activation energy of its diffusion in Fe lattices. The diffusion coefficient is 1015 times larger than the simple one. The high-energy milling, in which a lot of grain boundaries and surfaces as well as the dynamic diffusion are formed, can further offer conditions for the diffusion of Fe-Si system at low temperature.The a-Fe (Si) solid solution exhibits excellent soft magnetic properties. The milling time and heat treatment exert important effect on the magnetic properties of the Fe-Si alloy powders.