摘要
Fe_3Al合金因其高比强度，高比模量、中高温环境中强抗腐蚀能力及热强性等特点，而被公认为是航空和高温结构材料领域内具有重要应用价值的新材料。但由于室温脆性及其加工性能差，限制了其进一步的应用。已有研究表明，(微)合金化、晶粒细化是改善材料力学性能的有效途径之一。为此本文采用机械合金化(MA)和常压氩气气氛保护烧结相结合的方法来制备Fe_3Al合金，以求达到改善的力学性能。
本文首先利用XRD、激光粒度分析测试仪，借助正交实验手段，优化了在新型高能球磨机上制备Fe_(72)-Al_(28)合金粉末工艺。结果表明：采用球料比60：1、过程控制剂硬脂酸1wt．％，球磨机转速为225rpm时，有利于减小Fe_(72)-Al_(28)合金粉末的晶粒尺寸和粒度。利用XRD、SEM等多种测试手段，研究了Fe_(72)-Al_(28)混合粉末不同球磨时间(5h、10h、15h)产物的物相、形貌的变化。研究表明：MA终产物为纳米晶的过饱和固溶体Fe(Al)，Fe_(72)-Al_(28)粉末在形貌上逐渐变成片状，粒度逐渐变小。在合金化过程中没有非晶相及Fe-Al金属间化合物的生成。利用Miedema半经验理论对上述合金成分进行了热力学分析，结果表明：对于Fe_(72)-Al_(28)这一配比，热力学上Fe_3Al金属间化合物最为稳定。MA过程中产生的晶格畸变能、晶界能及无序能是使Fe_(72)-Al_(28)形成亚稳态Fe(Al)固溶体的根本原因，同时MA过程形成的晶界、表面及缺陷大大降低了扩散活化能从而实现Al在Fe中的低温扩散。
其次本文采用基于密度泛函理论的第一原理赝势平面波方法CASTEP程序，基于Rice-Wang热力学模型，对Fe_3Al金属间化合物中有关微量元素的晶界掺杂效应进行理论计算，探究Ti、Nb、Cr、Mn和Mo原子的晶界偏聚对Fe_3Al(B2)晶体沿晶断裂功的影响规律，为进一步选择出理想的合金元素提供理论参考。研究表明：Ti、Nb的晶界偏聚会显著弱化晶界，促进Fe_3Al的室温沿晶脆性；而Cr、Mn、Mo等元素的晶界偏聚会显著强化晶界，从而抑制室温沿晶脆性的发生，其中Cr的韧化能力最强。在选择Cr作为合金元素之后，本文研究了Fe_(70)-Al_(25)-Cr_5混合粉末的机械合金化过程，研究表明：合金元素的加入并没有改变过饱和固溶体Fe(Al，Cr)的生成。粉末形貌基本与Fe_(72)-Al_(28)演化过程相同，但终产物的粒度与前者相比明显增大，这与Cr的加入对粉末塑性的改善有关。
最后本文初步地研究了利用机械合金化和氩气气氛保护烧结制备的Fe_3Al基合金的显微组织及其力学性能。研究结果表明，烧结块体物相上以部分有序的B2结构Fe_3Al组织为主，显微形貌呈岛状结构，这种结构是由于MA时复合粉末外层局部富Al导致。烧结试样在力学性能上比铸态形态下有明显提高，试样压缩断口呈准解理断裂，呈现出塑性特征。其致密度为95％、94％(含Cr)，室温压缩屈服强度为832Mpa、962Mpa(含Cr)、压缩率29．90％、31．36％(含Cr)，说明Cr元素的加入对Fe_3Al基合金的塑性有明显改善。
Fe_3Al 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 Fe_3Al 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 Fe_(72)-Al_(28) 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 Fe_(72)-Al_(28) 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 Fe_(72)-Al_(28), 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 Fe_3Al (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 Fe_3Al (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 Fe_3Al alloy is best. After selected Cr as alloying element, structural evolution and final products of Fe_(70)-Al_(25)-Cr_5 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 Fe_(72)-Al_(28). 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 Fe_3Al-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 Fe_3Al 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 Fe_3Al-based alloy could be improved by adding Cr element.
引文
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