Study On Mechanical Alloying And Magnetic Properties Of Fe-Si System

Mechanical alloying (MA), which is a new technology of solid state reaction, has attracted much attention of materials researchers recently, because it can synthesize metastable materials such as amorphous phase, nanocrystallines, super-saturated solid solution, etc. In this paper, the mechanical alloying process and magnetic properties of Fe-Si system are studied.The results of mechanical alloying experiments show that when Si <=30at.%, the alloying product is nanostructured a-Fe(Si) solid solution, while the MA products of Fe5oSi5o and Fe3oSi7o are the compound FeSi and the mixture of a-Fe(Si)+FeSi+p-FeSi2, respectively. What's more, in the full composition range, there is no amorphous phase formed during the mechanical alloying process of Fe-Si system.Based on the Miedema's semi-empirical theory, the thermodynamic model for binary system is established to analyze the mechanical alloying of Fe-Si system. In the full composition range, the formation energy of amorphous phase is higher than that of solid solution, so Fe-Si system tends to form solid solution firstly. The coercive reaction of mechanical alloying is the essential reason that Fe25Si75 forms the metastable a-Fe(Si) super-saturated solid solution. Finally, Fe50Si50 and Fe30Si70 can both meet the thermodynamic conditions to form the intermetallic compound FeSi and the mixture of cc-Fe(Si)+FeSi+p-FeSi2, respectively. In conclusion, the results of the thermodynamic analysis on mechanical alloying of Fe-Si system agree well with the experimental results.The super fine Si powders formed during MA are dispersed in Fe particles to form the composite structure, which provides preconditions for the diffusion of Si in Fe lattices and the solid-state reaction between Fe and Si. The MA process greatly enhances the diffusion of Si by lowing the activation energy of its diffusion in Fe lattices and forming the nanocrystallines. 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 temperatures.The Fe-Si alloys prepared by MA have excellent soft magnetic properties. The relationship of their saturation magnetization and coercive force with Si concentration is the same as that of polycrystalline and single crystalline Fe-Si alloys. When the content of Si is 12at.%, Fe-Si alloys can get the best properties. In addition, the milling time and heat treatment also have important effect on the magnetic properties of the Fe-Si alloy powders.