常规凝固条件下Mg-Zn-Y三元稳定准晶的研究

对常规凝固条件下Mg-Zn-Y系中稳定二十面体准晶相Mg3Zn6Y1的形成进行了研究,分析了按照严格化学计量比配制的大体积单相准晶的组织形态、凝固过程中的相变点、以及准晶在稀溶液中自由生长时的形貌变化.实验发现,按照严格化学计量比配制的准晶合金组织中准晶相占有较大的体积分数,在自由生长的情况下该准晶形貌为花瓣状,此外还有部分拉长的六边形及五边形组织.该准晶是在624.95℃通过包晶反应形成的.     

含准晶相Al－Cu－Fe合金的球磨非晶化研究

应用球磨方法使含准晶相的Ａｌ６５Ｃｕ２０Ｆｅ１５合金转变为非晶相。利用Ｘ射线结构分析和扫描电镜研究了经不同时间球磨的合金粉末的相结构及形貌。结果表明，Ａｌ－Ｃｕ－Ｆｅ二十面体准晶相比合金中的少量晶体相更易于非晶化，且在非晶化过程中没有中间相生成。应用高温差热分析仪（ＤＴＡ）和Ｘ射线衍射技术（ＸＲＤ），分别研究了该非晶粉末的晶化特性。     

强磁场下Cu-50%Ag合金定向凝固过程中的组织及固液界面形貌演变

目的 研究强磁场下Cu-50%(质量分数)Ag合金定向凝固过程中的组织演变、固液界面形貌变化及溶质迁移行为,分析强磁场对金属凝固过程的作用机制,为强磁场下的金属材料制备提供理论借鉴和指导。方法 在不同的凝固速率与磁场条件下进行定向凝固和淬火实验,对合金的定向凝固组织、糊状区与固液界面形貌以及溶质分布行为进行考察。结果 强磁场破坏了凝固组织的定向生长,使凝固组织转变为枝晶与等轴晶共存的形貌;强磁场诱发了熔体对流,减少了糊状区中溶质的含量;强磁场改变了固液界面处的溶质分布和固液界面形貌,破坏了固液界面的稳定性。结论 强磁场通过洛伦兹力和热电磁力的共同作用,诱发了糊状区内液相的纵向环流,改变了固液界面及糊状区中的组织形貌与元素分布。     

高温合金定向凝固过程中枝晶生长与溶质对流数值模拟

目的 针对高温合金叶片在定向凝固过程中容易出现雀斑缺陷,从而导致叶片报废的问题,对定向凝固枝晶生长与溶质对流进行模拟研究,以揭示雀斑缺陷的形成规律。方法 针对CM247LC合金定向凝固过程,采用相场模型模拟凝固过程枝晶生长,采用格子Boltzmann模型模拟溶质浓度差引起的自然对流。采用基于双重网格的GPU并行算法对相场-格子Boltzmann模型进行数值求解。研究在不同晶体取向角度与取向差条件下的枝晶形貌、对流速度及溶质羽流的演变规律。结果 当晶体取向角度不同时,在枝晶生长过程中,液相区域的平均对流速度均表现为周期性变化。当晶体取向角度较大时,随着晶体取向角度的变大,一次枝晶臂间距变大。当枝晶间存在晶体取向差时,溶质羽流倾向于在发散型晶界附近发起;随着晶体取向差的增大,溶质羽流发起时间提前。溶质羽流的形成阻碍了枝晶尖端及附近枝晶侧臂的生长。结论 晶体取向角度对溶质羽流形成的影响较小,较大的晶体取向差对溶质羽流的形成有促进作用。     

冷却速率对Mg68Zn28Y4自生复合材料I相形成的影响

采用两种不同冷却方式的普通铸造技术在Mg68Zn28Y4合金中制备了镁合金稳定态二十面体准晶相,分析了冷却速率对准晶相形成、分布、数量和晶粒尺寸的影响.通过扫描电镜、能谱分析仪和透射电子显微分析技术,确定了合金的组织、相成分及准晶相结构.结果表明:Mg68Zn28Y4三元合金在室温冷却过程中,准晶相直接从液相形核、长大;在铸铁模型腔内冷却时,铸件断面的温度梯度小,温度场较为平坦,沿铸型纵横截面准晶相分布均匀、晶粒尺寸约为10～15μm;在铸铁板表面冷却时,铸件断面的温度梯度较大,导致冷却速率显著影响准晶相的分布、数量及形貌.准晶相的晶粒尺寸为10～60μm不等,分布由均匀弥散到不均匀散布再到均匀.     

Al-Cu-Cr非稳定准晶及其类似相的断裂韧性

用雾化法制得的Al65 Cu20 Cr15准晶粉末与纯铝热压合成复合材料,在热压和热处理状态下复合材料中的颗粒为尺寸在35-160μm范围的Al-Cu-Cr二十面体准晶和十次准晶及其类似相.采用维氏压痕实验技术测试估算了Al-Cu-Cr准晶颗粒的断裂韧性.结果表明,热压状态下的Al-Cu-Cr二十面体准晶材料的断裂韧性约为0.82MPa·m1/2,经退火处理后得到的Al-Cu-Cr十次准晶及其类似相材料的断裂韧性约为1.07MPa·m1/2,与Al-Cu-Cr二十面体准晶材料相比有所提高.在退火和淬火两种状态下Al-Cu-Cr十次准晶及其类似相材料的断裂韧性无明显变化.     

含冷晶相Al—Cu—Fe合金的球磨非晶化研究

应用政治协商会议磨方法使含冷晶相的Ａｌ６５Ｃｕ２０Ｆｅ１３合金转变为非晶相，利用Ｘ射线结构分析和扫描电研究了经不同的时间球磨的合金粉末的结构及形貌。结果表明，Ａｌ－Ｃｕ－Ｆｅ二十面体准晶相比合金中少的少量本相更易于非晶化，且在非晶化过程中没有中间相生成。应用高温差热分析仪（ＤＴＡ）和Ｘ射线射衍射技术的（ＸＲＤ），分别研究了该非晶粉末的晶化特性。     

镁合金中的准晶及其相变

二十面体准晶(IQC)相强化的Mg-Zn-Y/RE合金具有优良的室温和高温力学性能,且可通过热变形处理进一步提高其力学性能。但此类Mg合金在铸造和热处理时往往会形成有害的晶体相,从而弱化IQC相的强化作用。准确了解此类Mg合金中的相变类型、序列和温度,尤其是IQC相的变化,对通过优化处理工艺提高材料性能非常重要。为深入了解Mg合金中准晶相变的规律和机制,选用成分为Mg_(95)Zn_(4.3)Y_(0.7)的合金作为研究对象,采用原位加热透射电子显微术实时研究了IQC相到面心立方W相Zn_3Mg_3Y_2和六方H相Zn_3Mg Y的相变,及W/Mg和H/Mg界面上生长IQC的固态相变。IQC→W和IQC→H的相变温度分别为720 K和727 K。金属间化合物准晶一般由液态形成,但原位加热观察发现升温至573 K和593 K时,IQC相颗粒就在六方H相和立方W相的表面上固态外延形核生长。借助像差校正扫描透射电子显微术研究了准晶和晶体间界面的原子结构,从原子尺度解释了金属间化合物准晶在固态条件下的起源。固态准晶相变的研究为认识准晶起源开辟了新天地。     

Ti-6Al-4V三元合金焊接熔池凝固组织模拟

将二元合金元胞自动机模型与热力学相平衡求解软件相结合,提出了一种三元合金元胞自动机模型.对模型的稳定性进行了验证,模拟了Ti-6Al-4V三元合金焊接熔池凝固过程中枝晶的生长形貌、溶质浓度的分布以及扰动振幅对枝晶生长的影响,并进行了金相实验验证.结果表明,模型稳定性良好,能够实现三元合金焊接熔池凝固过程的数值模拟;熔池中枝晶择优生长显著,且存在晶界偏析现象;Al元素与V元素在液相中的浓度分布规律大致相同;随着扰动振幅的增大,熔池中的枝晶数量逐渐增加,枝晶臂间距减小,竞争生长进一步加强;模拟结果与实验结果基本吻合.     

大块深过冷Al-Mn-(Si,B)合金准晶相的初生凝固

用“玻璃覆盖、熔盐净化及热循环”复合方法实现了大块Al－Mn－（Si;B）熔体的深过冷,获得了100K的过冷度,并自发生成准晶相．研究了准晶相的形核及生长形貌特点,结果表明,准晶相为初生凝固相．初生准晶相的形成归因于推晶相的固液界面能低于竞争晶体相.     

Crystallization and precipitation structures of quasicrystalline phase in rapidly solidified Al-Mn-X ternary alloys

Tranmission electron microscopic observations were carried out to reveal the solidification and precipitation structures of rapidly solidified Al-Mn-X alloys (X=Si or Zr), with particular focus on the formation of the icosahedral quasicrystals. On increasing the manganese content, the distribution of the icosahedral quasicrystals in the rapidly solidified alloys changed from the type of cell-boundary segregation to that of one component in the eutectic and the primarily dendritic phase. The precipitation of the icosahedral quasicrystals from the supersaturated solid solutions of the present alloys was restricted in the surface layers and/or largeangle grain boundaries. The orientation relationships between the icosahedral quasicrystals and the aluminium matrix were such that three mutually perpendicular two-fold axes of the icosahedron lie along the [100], , and [011] axes of the fcc crystallographic directions. The 15-fold symmetry diffraction pattern frequently obtained from (111) and/or (233) matrix planes could be explained by considering axial rotation between the five-fold and three-fold symmetry axes of the icosahedron and cubic lattices. It was suggested that the hierarchy of the free energies of the icosahedral phase and the other crystalline phases was very close.     

A study of orthorhombic phases in aluminium-transition metal alloys

In recent decades some complex crystalline phases, as also many rational approximants to quasicrystalline phases with rather large unit cells, have been reported with orthorhombic symmetry in aluminium-transition metal (Al-TM) alloys. Furthermore, quite a few quasicrystalline phases, icosahedral as well as decagonal, forming in Al-TM alloys on normal or rapid solidification have been interpreted during the last decade as multiply-twinned orthorhombic crystals growing as superstructures of an orthorhombic cell that forms through welding in three perpendicular directions in the liquid state of 13-atom icosahedral clusters. Following exemplification of this new approach to quasicrystals based on the analysis of the Debye-Scherrer diffraction data from the comparatively defect-free Al-Cu-Fe icosahedral phase, the three types of orthorhombic phases in aluminium-rich Al-TM alloys, numbering 36 in all, have been examined as icosahedral cluster compounds nucleating from icosahedral atomic clusters present in the molten alloys. A detailed analysis of their lattice parameters supports the postulate that all such phases can be viewed as complex and, occasionally, as very large superstructures of a small basic orthorhombic cell. Dedicated to the dearly cherished memory of Professor Sir William Hume-Rothery F.R.S. (1899–1968) in his birth centenary year.     

Icosahedral clusters,icosaheral order and stability of quasicrystals—a view of metallurgy

We review the stability of various icosahedral quasicrystals (iQc) from a metallurgical viewpoint. The stability of stable iQcs is well interpreted in terms of Hume-Rothery rules, i.e. atomic size factor and valence electron concentration, e/a. For metastable iQcs, we discuss the role of phason disorder introduced by rapid solidification, in structural stability and its interplay with chemical order and composition.     

Quasicrystals and crystalline phases in Al65Cu20Fe10Cr5 alloy

An alloy of compositon Al₆₅Cu₂₀Fe₁₀Cr₅ forms a mixture of icosahedral and decagonal quasicrystals upon rapid solidification from the liquid state. These quasicrystals are highly faulted as revealed by electron microscopy studies. After heat treatment, the alloy transforms into a new orthorhombic modification. This crystal is studied by X-ray and transmission electron microscopy and emphasis is put on its close resemblance to the decagonal phase.     

Mechanical spectroscopy of hydrogen-absorbing quasicrystals

The H-induced damping peak in icosahedral quasicrystals, generally attributed to a Snoek-type reorientation mechanism, was studied in comparison between Zr_69.5Cu₁₂Ni₁₁Al_7.5 and Ti₅₃Zr₂₇Ni₂₀ using the vibrating-reed technique. Whereas the peak in Zr_69.5Cu₁₂Ni₁₁Al_7.5 is practically the same as in the amorphous phase (in accordance with icosahedral short-range order in metallic glasses), significant differences in shape and position of the peak are found between the two quasicrystalline alloys. These might possibly be used as a probe to distinguish between icosahedral structure types. Another interesting result is a higher intrinsic damping of quasicrystalline Zr_69.5Cu₁₂Ni₁₁Al_7.5, compared to the amorphous phase, between 450 and 650 K.     

Icosahedral clusters,icosaheral order and stability of quasicrystals—a view of metallurgy∗

Abstract

We review the stability of various icosahedral quasicrystals (iQc) from a metallurgical viewpoint. The stability of stable iQcs is well interpreted in terms of Hume-Rothery rules, i.e. atomic size factor and valence electron concentration, e/a. For metastable iQcs, we discuss the role of phason disorder introduced by rapid solidification, in structural stability and its interplay with chemical order and composition.     

Covalency, semiconductor-like and thermoelectric properties of Al-based quasicrystals: icosahedral cluster solids

A unified picture of icosahedral cluster solids obtained by comparing Al-based icosahedral quasicrystals to boron-based cluster solids, which consist of icosahedral clusters of the same group III element, B, is discussed. Metallic-covalent bonding conversion in icosahedral clusters of Al and B was demonstrated by molecular orbital calculations. According to the electron density distribution obtained using the maximum entropy method and the Rietveld method, the bonding conversion phenomenon occurs even in cluster solids such as some Al- and B-based icosahedral approximant phases (Al₁₂Re, α-AlMnSi, α-rhombohedral boron). The covalent bonds observed by the electron density analysis give rise to extremely low carrier density and small density of states at E_F in α-AlMnSi and perhaps in icosahedral quasicrystals. In β-rhombohedral boron, the electronic structure and properties depend on slight variation in atomic environment of the icosahedral multiple-shell structure of the crystals. As metallic atoms are doped into specific atomic sites of β-rhombohedral boron, the atomic structure, the electrical conductivity and its temperature dependence approach those of AlLiCu icosahedral quasicrystal and equal those of AlPdRe icosahedral quasicrystal. From the absolute density measurement, it was revealed that the concept of rigid sphere packing, i.e. metallic bonding, is not applicable to AlPdRe icosahedral quasicrystal. Various semiconductor-like behaviors of Al-based icosahedral quasicrystals were demonstrated by means of not only conventional conductivity and Hall coefficient measurements but also modulated photocurrent measurements. In addition, the thermoelectric properties of AlPdRe quasicrystal were estimated. The potential for application of the icosahedral quasicrystals as thermoelectric materials is also discussed.     

Al—Mn二十面体准晶相的形态观察

用扫描电镜和透射电镜以熔态单辊急冷的Ａｌ－Ｍｎ条带炽的二十面体准晶相的形态进行观察，发现在冷却速度高的与辊筒面接触的区域，准晶相形态呈“花”状，而在远离接触面的区域，则呈较规则的枝晶状，两种形态的准晶似乎都是在量亚晶粒的相互聚集由于先形成的准晶中心核处向外生长，由于成核密度的差异，枝晶状准晶的尺寸是“花”状准晶的数倍。     

Three-dimensional silicon inverse photonic quasicrystals for infrared wavelengths

Quasicrystals are a class of lattices characterized by a lack of translational symmetry. Nevertheless, the points of the lattice are deterministically arranged, obeying rotational symmetry. Thus, we expect properties that are different from both crystals and glasses. Indeed, naturally occurring electronic quasicrystals (for example, AlPdMn metal alloys) show peculiar electronic, vibrational and physico-chemical properties. Regarding artificial quasicrystals for electromagnetic waves, three-dimensional (3D) structures have recently been realized at GHz frequencies and 2D structures have been reported for the near-infrared region. Here, we report on the first fabrication and characterization of 3D quasicrystals for infrared frequencies. Using direct laser writing combined with a silicon inversion procedure, we achieve high-quality silicon inverse icosahedral structures. Both polymeric and silicon quasicrystals are characterized by means of electron microscopy and visible-light Laue diffraction. The diffraction patterns of structures with a local five-fold real-space symmetry axis reveal a ten-fold symmetry as required by theory for 3D structures.     

Covalency,semiconductor-like and thermoelectric properties of Al-based quasicrystals: icosahedral cluster solids

A unified picture of icosahedral cluster solids obtained by comparing Al-based icosahedral quasicrystals to boron-based cluster solids, which consist of icosahedral clusters of the same group III element, B, is discussed. Metallic-covalent bonding conversion in icosahedral clusters of Al and B was demonstrated by molecular orbital calculations. According to theelectron density distribution obtained using the maximum entropy method and the Rietveld method, the bonding conversion phenomenon occurs even in cluster solids such as some Al- and B-based icosahedral approximant phases (Al₁₂Re, α-AlMnSi, α-rhombohedral boron). The covalent bonds observed by the electron density analysis give rise to extremely low carrier density and small density of states at E_f in α-AlMnSi and perhaps in icosahedral quasicrystals. In β-rhombohedral boron, the electronic structure and properties depend on slight variation in atomic environment of the icosahedral multiple-shell structure of the crystals. As metallic atoms are doped into specific atomic sites of β-rhombohedral boron, the atomic structure, the electrical conductivity and its temperature dependence approach those of AlLiCu icosahedral quasicrystal and equal those of AlPdRe icosahedral quasicrystal. From the absolute densitymeasurement, it was revealed that the concept of rigid sphere packing, i.e. metallic bonding, is not applicable to AlPdReicosahedral quasicrystal. Various semiconductor-like behaviors of Al-based icosahedral quasicrystals were demonstrated by means of not only conventional conductivity and Hall coefficient measurements but also modulated photocurrent measurements.In addition, the thermoelectric properties of AlPdRe quasicrystal were estimated. The potential for application of the icosahedral quasicrystals as thermoelectric materials is also discussed.