【作者】 康慧君；

【导师】 郭景杰； 李新中；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 博士

【摘要】 本论文选择Al-6wt%Mn（-2.5wt.%Be）合金为研究对象。通过定向凝固技术研究初生相Al₆Mn化合物的生长形貌及其演变过程，揭示不同凝固条件对Al₆Mn化合物生长行为的影响规律。另外，通过将第三组元Be加入Al-Mn合金中，研究少量Be的加入对其凝固特性及准晶I相形成的影响，探讨Be加入后合金中化合物相和准晶相的形貌特征与生长机制。在此基础上，研究了不同凝固条件下定向凝固Al-Mn-（Be）合金的力学性能，分析了两种合金的断裂行为和强化机制，探索制备具有化合物相或准晶相弥散分布在基体中凝固组织的高强原位复合材料的工艺。这为实现化合物相形貌的主动控制，发展先进化合物或准晶颗粒原位增强金属基复合材料提供必要的理论基础和实践指导。在较低的生长速度（V=1m/s）下，Al₆Mn相主要以拉长的，且具有尖锐棱角的小平面形式析出。结合定向凝固试样的EBSD和晶体结构分析表明，Al₆Mn晶体在定向凝固条件下沿晶体学方向[001]择优生长，而且相对于其它低指数面，（011）和（101）面更密堆。所以，在低生长速度下，Al₆Mn相趋向于形成由四个{011}晶面和四个{101}晶面组成的八面体形貌。通过对实际三维形貌的分析，结合晶体结构决定的晶面关系和外部生长环境，建立了Al₆Mn相近平衡生长过程模型。本论文基于Al₆Mn相正交结构的晶格参数，结合实际的形貌，量化了Al₆Mn （001），（011）和（101）面之间相对生长速度与形貌的关系。随着生长速度的增加，Al₆Mn相的形貌由实心的截顶多面体或棱柱转变为空心的棱柱再到槽状形貌，最后直至发达的枝晶（V=1000m/s），实现了Al₆Mn化合物相的生长方式由小平面生长转变为非小平面生长。生长速度的增加，引起了由体积扩散决定的角和边加速生长，形成空心的棱柱和槽状形貌。同时，还导致了固/液界面过冷度的增加，促进了Al₆Mn相的连续生长，形成枝晶状的Al₆Mn相。本文建立了冷却速率增大后小平面向非小平面生长转变的熔体团簇模型，很好地解释了Al₆Mn相生长方式的转变。利用Helmholtz公式揭示了（001）和（101）晶面表面能与复杂正方晶系Al₆Mn相临界形核形貌的关系，发现Al₆Mn相的生长是由二维晶核和螺型位错层生长机制共同控制的，两种机制均在密堆的（101）和（011）面上进行。提出了Al₆Mn相的不断形核和生长交替进行的沟槽重复形核生长模型。Be的加入使二元相图向高Mn区移动，引入三种金属间化合物相，λ-Al₄Mn、H1相和Be4AlMn相，明显细化了铸态和定向凝固的组织。同时，Be的加入有效地促进了准晶I相的形成，表现为减小了I相形成所需的Mn含量和冷却速度。在定向凝固过程中，其临界冷却速度形成范围被确定为1.5².5K/s。在定向凝固Al-6wt%Mn-2.5wt.%Be合金中，随着生长速度的增加，初生析出相和共晶基体中的化合物相发生了竞争生长。初生相先由λ-Al₄Mn转变为H1相，最后转变为准晶I相。基体也由（-Al+λ-Al₄Mn）转变为（-Al+H1）相进一步转变为（-Al+I相）共晶。同时，伴随其三维形貌的变化：六棱柱λ-Al₄Mn→六棱柱H1相→六瓣花状H1相→枝晶H1相→小平面I相→花瓣I相→枝状I相。基于伪I13簇并结合H1相的高分辨图像，建立了H1相的结构模型，通过其结构相似性，很好地解释了上述的转变过程。λ-Al₄Mn、H1相和Be4AlMn相的三维形貌与其晶体结构密切相关，其生长方式均为在密排面上的二维晶核层生长。随着生长速度的增加，初生准晶I相的形貌和生长方式均发生了转变，即由小平面的五角十二面体簇形貌转变为花瓣枝晶形貌，再到非小平面的发达枝晶形貌。由此，建立了五角十二面体簇形貌在二次、三次和五次方向的二十面体对称模型，并提出了准晶I相的生长过程模型，发现准晶I相是通过二维晶核层生长机制在五次面上进行生长的。利用定向凝固方法获得了金属间化合物和准晶I相强化复合材料。对其力学性能研究发现，两种成分Al-6wt%Mn和Al-6wt%Mn-2.5wt.%Be合金的室温力学性能随生长速度的增加而增加，而延伸率先减小后增加，这主要是由强化相的性质决定。Al-6wt.%Mn和Al-6wt.%Mn-2.5wt.%Be合金在生长速度为1000μm/s时，抗拉强度最高同时具有较高的延伸率，分别为188和244MPa及16.66%和12.01%。两种合金的断裂方式都随生长速度的增加由脆性转变为韧性。Be的加入明显改善了合金尤其是高生长速度合金的性能。这主要归因于Al-6wt.%Mn-2.5wt.%Be合金中除了具有颗粒强化和基体强化外，还包括准晶I相的界面强化。准晶I相的强化作用使高生长速度下Al-6wt.%Mn-2.5wt.%Be合金也表现出很好的高温力学性能，在200℃呈现出与室温接近的强度和35%的大延伸率。更多还原

【Abstract】 In this study, Al-6wt.%Mn（-2.5wt.%Be） alloys have been selected forinvestigation. The growth morphology and evolution process of primary Al₆Mnphase during the directional solidification have been investigated, thereby revealingthe effect of solidification condition on the growth behavior of Al₆Mn phase.Meanwhile, effects of minor Be addition on the solidification property andformation of icosahedral quasicrystal have been investigated by adding the thirdelement Be into the hypereutectic Al-Mn alloy. The morphological feature andgrowth mechanism of the intermetallic compound and icosahedral quasicrystal afterBe addition have been discussed. Based upon those work, the mechanical propertiesof directionally solidified Al-Mn-（Be） alloy with different solidification parametershave been investigated. In addition, the fracture behavior and strengtheningmechanisms of two alloys have been analyzed. A technology for preparing a high-strength in-situ composite with finely dispersed intermetallic or icosahedralquasicrystal has been developed. This work provides a theoretical foundation andpractical guidance for the active control of the morphologies of intermetalliccompounds and the development of advanced in-situ intermetallic or quasicrystalparticles-reinforced metal matrix composite materials.During directional solidification, at a low growth rate （V=1m/s）, Al₆Mnprecipitates in a faceted growth with sharp edges and corners and exhibits elongatedmorphology. EBSD results and crystal structural analysis indicate that Al₆Mncrystal has a preferred growth direction along the crystallographic [001] directionduring directional solidification. Comparing with other low-index crystal plane,（011） and （101） is more closely packed. Therefore, at a low growth rate, Al₆Mntends to form an octahedron morphology enclosed by four {011} planes and four{101} planes. A nearly equilibrium growth model of Al₆Mn has been establishedbased on the analysis of actual3-D morphologies. Theoretically, the crystalmorphology is determined by the competition between various important crystalplanes. Based on the lattice parameter of orthorhombic Al₆Mn and combined withmorphological analysis, a relationship between morphologies and growth-rate ratiosof （001）,（011） and （101） planes different planes has been built.As the growth rate increases, the morphologies of Al₆Mn phase transit fromsolid truncated polyhedral or prism to hollow prism, and further to a groovemorphology and ultimately developed dendrites （V=1000m/s）, and also the growthpattern transits from faceted to non-faceted. Increasing growth rates lead to an accelerated angular growth mechanism mainly determined by volume diffusion, andhollow prism and groove morphologies form. Meanwhile, increasing growth rateslead to a large undercooling ahead of the S/L interface, which favors the continuousgrowth of Al₆Mn and formation of dendritic Al₆Mn. This paper proposes a melt-cluster model during the faceted-non-faceted transition when the cooling rateincreases. This model can be used to illustrate the transition of growth pattern ofAl₆Mn.A relation between the surface energy of （001） and （101） crystal planes and thecritical nucleus morphology in the complex orthorhombic Al₆Mn has been builtusing Helmholtz formula. The growth of Al₆Mn is controlled by the2-D nucleusand screw-dislocation layer growth mechanisms, which are conducted on theclosely-packed （101） and （011） planes. The frequent renucleation and growth modelhas also been proposed. The nucleation of a new crystal terminates the growth of aparent crystal, and sequent growth of the new crystal also may be terminated by anew nucleation event.The addition of Be results in the shift of binary phase diagram toward the Mn-rich side and the appearance of three intermetallic compounds, namely λ-Al₄Mn、H1and Be4AlMn, and significantly fined the microstructure of the as-cast anddirectionally solidified samples. Meanwhile, the addition of Be effectivelypromotes the formation of icosahedral quasicrystal, evidenced by the reduced Mnconcentration and cooling rate required for the formation of icosahedral phase. Inthe directionally solidified sample, the critical cooling rate determined is within1.5².5K/s.During directionally solidification of Al-6wt.%Mn-2.5wt.%Be alloy, with theincrease of growth rates, competitive growth of intermetallic compounds betweenprimary and eutectic phases takes place. The primary phase transits from λ-Al₄Mnto the H1phase, and ultimately icosahedral quasicrystal. The matrix transits from（-Al+λ-Al₄Mn） to （-Al+H1）, and ultimately （-Al+Icosahedral） eutectic.Meanwhile, the3-D morphology concurrently transits: Hexagonal prism λ-Al₄Mn→Hexagonal prism H1→Six-petal H1phase→Dendritic H1phase→Facetedicosahedral phase→Flower-like icosahedral phase→Dendritic icosahedral phase.Based on the pseudo I13cluster model and high resolution transmission electronmicroscopy of H1phase, the structural model of H1phase has been proposed. Suchtransition is well demonstrated based on the similarity of the structure. The3-Dmorphology is closely related to the crystal structure of λ-Al₄Mn, Be4AlMn and H1phase. Their growth patterns are the2-D nucleation layer growth on the closely-packed planes. As the growth rate increases, the morphology of primary icosahedralquasicrystal transits from faceted pentagonal dodecahedral cluster to flower-likedendrites, and ultimately non-faceted dendrites. Based on the actual morphologiesof icosahedral quasicrystal, the icosahedral symmetric model of the pentagonaldodecahedral cluster has been established along2-fold,3-fold and5-fold direction.The growth model of icosahedral quasicrystal has also been proposed, andrevealing that the icosahedral quasicrystal is grown on the5-fold plane by2-Dnucleation-layer growth mechanism.The mechanical properties of directionally solidified Al-6wt.%Mn and Al-6wt.%Mn-2.5wt.%Be alloys have been investigated. It is indicated that room-temperature mechanical properties of both alloys increase with the increase ofgrowth rates. But the elongation reduces first and then increases as the growth ratesincrease, which can be attributed to the properties of strengthening phases. At agrowth rate of1000μm/s, both two alloys have a highest ultimate tensile strengthand a relative large elongation. The ultimate tensile strength and elongation are188and244MPa,16.66%and12.01%, respectively. The fracture mode of two alloystransits from brittle fracture to ductile fracture as the growth rates increase.Compared with binary phase, the addition of Be significantly enhances themechanical properties of the alloy, especially the alloy with a high growth rate.This is mainly due to the interfacial strengthening effect of icosahedral quasicrystalexpected for the particle strengthening and matrix strengthening effects in the Al-6wt.%Mn-2.5wt.%Be alloy. Due to the strengthening effect of icosahedralquasicrystal, the Al-6wt.%Mn-2.5wt.%Be alloy with a large cooling rate exhibitsexcellent mechanical properties under high temperature. At200℃，the alloy has astrength closed to that under room temperature and a large elongation of35%.更多还原