Investigation Of Preparation Of Fe₃Al-Based Alloys By Mechanical Alloying And Low-Temperature Sintering

Fe₃Al alloys have been potentially developed for use in thermal protection systems, engines and aircraft turbines due to their good properties, such as high specific strength, high specific modulus, high corrosion resistance property and high thermal strength. However, their application is currently limited by the low ductility and poor workability at room temperature. And it has been reported that (micro-) alloying and grain refining can be better ways to improve mechanical properties. Therefore, a new technology, which consists of mechanical alloying and conventional pressure-less sintering in argon atmosphere, has been put forward to fabricate Fe₃Al alloys in order to receive ameliorated properties.Firstly, Using X-ray diffraction (XRD), scanning electron microscopy (SEM) and laser particle analyzer, an orthogonal test was carried out to optimize the technological parameters of novel high energy ball milling on Fe₇₂-Al₂₈ alloy powders. The results show that is of benefit to fining the grain size and Particle Sizes of Fe-Al alloy powders that the ratio of ball to powder is 60:1, the amount of stearic acid (PLA) is 1wt. %, and the milling speed is 225rpm.Based on optimum techniques for the high energy ball milling, the phases and morphologies of mechanical alloying powders in the different time (5h, 10h 15h) were analyzed by XRD and SEM respectively. The result shows that the nanocrystalline supersaturated solid solution Fe (Al) was formed during milling the Fe₇₂-Al₂₈ powder. It was found that with increased milling time, the morphologies of powder gradually became flaky. In the whole process, no Fe-Al intermetallic compound and amorphous phase were found to come into being. According to Miedema theoretical model, for the Fe₇₂-Al₂₈, the intermetallic compound was most stable. The Various kinds of defects induced by milling process leaded to the transitions of non-stabilization. At the same time the Grain boundary, surface and defection formed by MA process greatly enhances the diffusion of Al by lowing the activation energy of its diffusion in Fe lattices.Secondly, using CASPET program which is the first-principles pseudopotential plane-wave methods based on the density functional theory, the grain-boundary (GB) doping effects of impurity or solute atoms in Fe₃Al (B2) intermetallic compound were theoretically forecasted. The results show that the GB segregation of Ti and Nb atoms decreases the intergranular fracture energy, and enhances the IGB tendency of Fe₃Al (B2) alloys. Contrarily, the GB can be strengthened by GB segregation of Cr, Mn and Mo atoms and consequently the IGB is restricted. Thereinto the effect of Cr on the strength and toughness of Fe₃Al alloy is best. After selected Cr as alloying element, structural evolution and final products of Fe₇₀-Al₂₅-Cr₅ mixed-powders processing by mechanical alloying have been studied at the first time. The result showed that the addition of Cr didn't change the formation of Fe (Al, Cr) solid solution. The evolution of powder morphologies is similar to that in the Fe₇₂-Al₂₈. But the particle size of final product increased significantly compared with that of former. This is related to the fact that the addition of Cr element improved the ductility of powder.Finally, the microstructure and mechanical properties of Fe₃Al-based alloys by mechanical and sintering in argon atmosphere were studied. It was shown that the structural phase of sintered sample is mainly partially ordered Fe₃Al B2 structure. An island structure of the microstructure is observed which is related to the uneven distribution characteristics of Al elements in powders during MA. The mechanical properties of sintered simple were significantly improved comparing with casting ones. Obvious quasi-cleavage fracture was observed on the base of analysis of fractographies, which shows that the sample occur definite model distortion. With the compactness of 95% and 94% (Cr), compressive yielding strength and compressive strain of 832Mpa, 962Mpa (Cr) and 29.90%, 31.36% (Cr), It shows that the plasticity of Fe₃Al-based alloy could be improved by adding Cr element.