Preparation And Characterization Of Fe₃Si Based Transition-metal Silicide Layer And Nanocomposite Powders

The bond type of Intermetallic compounds (IMC) is a mixture of metallic bond and covalent bond. The brittleness at room temperature and poor strength at high temperature are the key problems for the application of this material. Fe₃Si have excellent soft magnetic property, high temperature oxidation resistance and corrosion resistance, the property of which changes with its Si content. When the silicon content is 6.5 wt%, Fe-Si alloy has low iron loss and coercivity, high magnetic permeability, with it's magnetostriction close to zero. When the silicon content is 25.2 at%, Fe₃Si alloy can even resist the corrosion from boiling sulfuric acid. The ductility can be increased if Plastic phase is introduced into IMC, but the high-temperature strength and high temperature oxidation resistance will be damaged. In this paper Fe₃Si were made as the coating of metal to avoid these problems of toughened IMC. In addition, IMC have the R effect that the intensity changes from low to high and then high to low with the temperature rising. This effect can make the Intermetallic Compounds/Ceramic matrix composites (I/CMC) have the potential to become high-temperature structural materials. Fe₃Si/Al₂O₃ nanocomposite powders were synthesized by reactive mechanical alloying. The reaction product has clean interface, which avoids the problem of the introduction of oxides impurities during conventional ball milling. The results obtained in this dissertation will play a guidance role for solving the problem of brittleness at room temperature and poor high temperature strength.Fe₃Si type transition-metal silicide layer deposited on AISI 304 stainless steel were formed in low melting point molten salts at 800℃. The phase of the silicide layer was analyzed by X-ray diffraction. The micrographs and the composition of the cross section of the silicide layer were studied by scanning electron microscope attached with energy dispersive X-ray spectrometer(EDS)attachment. The siliconizing mechanism in molten salts was analyzed as well. It has been shown that the phase of the silicide layer is rich in Cr and Ni alloying elements and Si is evenly distributed. The defects nearby the interface of the silicide layer/matrix include Kirkendall voids and big layer defects. The defects present zonal distribution and the width is about 1/3 of the silicide layer. The layer without defects is very dense.As seen in the phase diagram, Fe₃Si has a wide Si content. Si content in the transition-metal silicide layer are 13.21%,19.12% and 22.03% (at%), which are obtained by changing the proportion of siliconizing agent and then adding SiO₂ as another source. The mechanism of aided siliconizing is studied as well.The tension behavior of the samples were tested on an universal tension apparatus, the influence of the silicide layer on the tension behavior of the AISI 304 stainless steel samples were studied as well. The results show that brittle fracture occurs in the cross section and the silicide layer fractures along the kirkendall voids band at the same time. The interface of the silicide layer/matrix is tight as before. The stress-strain curves in the elastic stage and strain-hardening stage of AISI 304 siliconized at 800℃are similar as that of AISI 304 stainless steel. The stress-strain curves in the initial elastic stage of AISI 304 siliconized at 900℃for 5h almost perpendicular to the abscissa. The load in the initial elastic stage is taken by the silicide layer. The material in the big holes outside kirkendall voids band was remained after the axial extension test. The EDS data show that the material in the holes is salt. This result confirmed the formation mechanism of the defect in the silicide layer.The cyclic oxidation behavior of the silicide layer and the AISI 304 stainless steel was studied at 800℃and 900℃. The oxidation kinetic curve of the Fe₃Si type silicide layer obeys a parabolic rule. The oxidation resistance of the silicide layer at 800℃is a little better than that of AISI 304 stainless steel. The oxidation resistance of the silicide layer at 900℃is much better than that at 800℃, and the oxidation film created at 900℃is more compact than 800℃. The AISI 304 stainless steel failed at 900℃. The thermal stress made a crack between the silicide layer and the matrix in the cyclic oxidation at 800℃. Diffusion of Si and Cr through the interface of the silicide layer/matrix enhanced the bonding strength of the interface at 900℃in the cyclic oxidation. The diffusion avoided the stress cracking along the interface. The composite oxide film consisting of SiO₂, Cr2O3, Cr3O4 and Fe₂O₃ accounts for the excellent high temperature oxidation properties.The solid phase reaction can be induced by mechanical force in the Fe₂O₃-Si-Al powders. Fe₃Si/Al₂O₃ composite powders were obtained by milling Fe₂O₃-Si-Al powders for 20 h. The main phase after milled for 1h is still raw material powder. The reaction between Si and Fe₂O₃ occurs, and as a result some SiO₂ was formed. After annealed at 900℃for 1h, the solid phase reaction occurred. Al₂O₃,Fe₃Si and FeSi were obtained, and the intermediate product SiO₂ remained. The solid phase reaction occurred in Fe₃O₄-Si-Al powders after milled for 1h.