The Study Of Microstructure And Properties Of Casting Of Fe-Al Intermetallic Compoud

In this paper, Fe₃Al intermetallic compouds was prepared in medium frequencyinduction melting furnace and solidified in the permanent mold. After heat treated（1000℃/15h homogenizing annealing+furnace cooling+600℃/1h tempering+oilquenching）, the microstructural, high temperature oxidation resistance, hot corrosionresistance and fracture mechanism were investigated by means of optical microscpe,SEM, EPMA, X-ray diffraction and mechanical property test. The results are shownthat:（1） Fe₃Al alloy as-cast is composed of fine and uniform crystal grains andmetallographic structure looks black, white and gray. After heat treatment the crystalgrains grew up and the dispersive phase separated out, but metallographic structurelooks black and white. The XRD analysis indicates that Fe₃Al alloy is composed ofFe₂AlCr, FeAl and Fe₃Al both after as-cast and heat treatment, while all XRD peaksshifted to the right and width of the diffraction peak decreased after heat treatment.EPMA analysis shows the dispersive phase is Fe₂AlCr. The microstructure of Fe₃Al alloy after heat treatment is known as the gray matrix and black strips of Cr-rich phaseby TEM analysis.（2） The oxidation kinetics of Fe₃Al alloy at700℃and850℃obeys parabolic law,but it is inconformity at1000℃. Fe₃Al alloy owns excellent high temperatureoxidation resistance of700¹000℃. The oxide scales at700℃of125h are consistedof FeAl2O4, Cr₂O₃and a little Al₂O₃, the oxide film formation. While at850℃and1000℃, the oxide scales are composed of a loose Al₂O₃top layer and internal oxidelayer consisted of FeAl₂O₄, Fe₂O₃and Cr₂O₃, but the Al₂O₃oxidation film has notcompletely covered the internal oxidation layer for125hour. There is no wrinklingand peeling off between oxide layer and matrix. Sheet or ridge-like Al₂O₃appearedduring the oxidation process of Fe₃Al alloy is formed by diffusion and selectiveoxidation. The holes can be observed in the oxidation process, and the equilibrium theholes may reach depends in a certain degree on the geometry of the alloy grain size.（3） Corrosion resistance of Fe₃Al alloy at700℃and800℃is significantly betterthan1Cr18Ni9Ti. The corrosion products of Fe₃Al alloy at700℃of20h are mainlysulfides （Cr₂S₃, CrS, Al₂S₃and FeCr₂S₄） with a small amount of oxide. A completeand protective film of Al₂O₃could not be formed, and S atom was easy to get throughthe oxide film layer and react with the alloy to generate sulfide. However, the S atoms could not further diffuse to the matrix. Corrosion products of Fe₃Al alloy at800℃of20h are mainly oxides（FeFe₂O4、FeCr₂O₄、Al₂O₃、Fe₂O₃and CrO）. Erosion mixed ofoxide and sulfide happened in this time. S atoms passed through the oxide layer andpermeated to the inward along the grain boundary. At last, porous corrosion productformed.（4） A weak point of the edge of the sample in the tensile test, when stretched to acertain load, a split source and the expansion of the crack initiation source toward thefavorable orientation of cleavage （the macrop performance of the river pattern） untilcompletely penetrate the sample, the occurrence of a typical brittle cleavage fracture.In the compression tests, the samples had a large plastic deformation. Specimenscracked at the notch root of at the same time in three-point bending test and four-pointbending test. Once the cracks appeared in the notch root, accumulated energy madethe material fracture. Damage of specimens originated in the plastic stage of thethree-point bending test, while damage originated in the brittle stage of the four-pointbending test.