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.     

Orthorhombic rational approximants for decagonal quasicrystals

An important exercise in the study of rational approximants is to derive their metric, especially in relation to the corresponding quasicrystal or the underlying clusters. Kuo’s model has been the widely accepted model to calculate the metric of the decagonal approximants. Using an alternate model, the metric of the approximants and other complex structures with the icosahedral cluster are explained elsewhere. In this work a comparison is made between the two models bringing out their equivalence. Further, using the concept of average lattices, a modified model is proposed.     

Coincidence-site lattices as rational approximants to irrational twins

It is well known that sequences of crystals with Mackay icosahedral motif and increasing lattice parameters exist converging to the icosahedral quasicrystal in the limit. They are known as rational approximants. It has also been demonstrated that it is possible to create icosahedral symmetry by irrational twins involving five variants by 72° rotations around an irrational axis [τ 1 0] or an irrational angle of 44.48° around a rotation axis [1 1 1]. These twinned crystals do not share a coincidence site lattice. In this paper, it is demonstrated that the above twinning relationship arises in the limit of a sequence of coincidence site lattices starting with the cubic twins with Σ = 3 and extending through Σ = 7, 19, 49, 129, 337, …, ∞ created by rotation around [1 1 1] axis. It is also noted that the boundaries of higher CSL values (Σ > 7) are composed of a combination of structural units from Σ = 3 and Σ = 7 boundaries.     

Structure and tribological property of B2-based approximants

The present paper is concerned with a special group of approximants with B2 superstructures. In the first part, recent work on structural features of the B2 superstructure approximants is summarized. Experimental results obtained in Al-Cu-Mn and Al-Cu systems are presented, where a series of B2-based approximants are observed. These phases all have similar valence electron concentrations, in full support of thee/a-constant definition of approximants. Special emphasis is laid on the chemical twinning modes of the B2 basic structure in relation to the Al-Cu approximants. It is revealed that the B2 twinning mode responsible for the formation of local pentagonal atomic arrangements is of 180°/[111] type. This is also the origin of 5-fold twinning of the B2 phase on quasicrystal surfaces. Crystallographic features of phases B2, τ2, τ3,γ, and other newly discovered phases are also discussed. In all these phases, local pentagonal configurations are revealed. In the second part, dry tribological properties of some AlCuFe samples containing the B2-type phases are presented. The results indicated that the B2 phase having their valence ratio near that of the quasicrystal possesses low friction coefficient under various loads, comparable with the annealed quasicrystalline ingot. Such a result indicates that the B2-type phase withe/a near that of quasicrystal is indeed an approximant, which is in full support of the valence electron criterion for approximants.     

Low Temperature Electronic Transport in Al–Cu–Ru Quasicrystalline Alloys

Electrical resistivities of several quasicrystalline (QC) icosahedral Al–Cu–Ru alloys have been measured between room temperature and mK temperatures. None of the Al–Cu–Ru samples exhibited “insulating” behaviors in their resistivities. One sample had a large resistivity of ≈49,000 μΩ cm at room temperature. Its resistivity increased with decreasing temperatures by a factor of 3.2 and exhibited a maximum around 2 K. Fitting discrepancies between the experimental magnetoconductivity data and the weak localization and electron–electron interaction theories suggest that the weak localization theory poorly describes the electronic conduction behavior of this highly resistive quasicrystalline sample.     

Formation of a quasicrystalline phase in Al–Mn base alloys cast at intermediate cooling rates

Al-rich 94Al–6Mn and 94Al–4Mn–2Fe alloys were suction-cast to evaluate the feasibility of obtaining bulk quasicrystal-strengthened Al-alloys at intermediate cooling rates alloyed with non-toxic, easily accessible and affordable additions. The influence of different cooling rates on the potential formation of a quasicrystalline phase was examined by means of scanning and transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. Increased cooling rates in the thinnest castings entailed a change in sample phase composition. The highest cooling rates turned out to be insufficient to form an icosahedral quasicrystalline phase (I-phase) in the binary alloy. Instead, an orthorhombic approximant phase occurred (L-phase). The addition of Fe to the 94Al–6Mn binary alloy enhanced the formation of a quasicrystalline phase. At intermediate cooling rates of 10²–10³ K/s, various metastable phases were formed, including decagonal and icosahedral quasicrystals and their approximants. Rods (1 mm in diameter) composed of I-phase particles embedded in Al matrix exhibited a hardness of 1.5 GPa, much higher than the 1.1 GPa of 94Al–6Mn.     

Quasicrystalline materials

The report in late 1984 of phases in rapidly cooled aluminium alloys whose electron diffraction patterns exhibit icosahedral symmetry has stimulated intensive research into the structure and properties of quasicrystalline materials. Here I review our current understanding of quasicrystalline materials and quasiperiodic tilings, and in particular I discuss in detail the interpretation of transmission electron microscopy observations of quasicrystalline phases.     

A possible crystallographic explanation for the five-fold diffraction symmetry in icosahedral phases

X-ray and electron diffraction data from the Al-Cu-Fe icosahedral phase are compared and analysed on the basis of the microcrystalline and multi-domain model developed by the author. It is shown that a crystallographic explanation is now possible for both the enigmatic five-fold symmetry and non-periodicity of reflections observed in electron diffraction patterns of icosahedral phases.     

Formation of stable and metastable phases in Al−Mn alloys by the use of a gravity chill casting technique

The structures present in taper-section chill casting of hypereutectic Al−Mn alloys containing between 5 and 18 wt% Mn have been studied by optical metallography, X-ray diffractometry and microanalysis, in order to relate intermetallic phases such as the Al₆Mn and Al₄Mn obtained at low freezing rates with metastable structures such as the decagonal and icosahedral phase obtained at high cooling rates.     

Electron diffraction and HREM image characteristics from quasicrystalline phases in alloys of Al-Mn and Al-Mn-Si

An investigation of the quasicrystalline phases in rapidly cooled alloys of Al₈₆Mn₁₄ and Al₇₄Mn₂₀Si₆ is carried out. The Al-Mn-Si alloy shows three different kinds of phase, the icosahedral phase, the crystalline α-type phase and a third phase which resembles the decagonal T phase found in alloys of Al-Mn. Both kind of compounds (Al-Mn and Al-Mn-Si) have icosahedral diffraction patterns along similar zone axes whose lowest order diffracted vectors are different in magnitude. In the past, these extra rings of reflections were associated with the presence of superstructures. The HREM images obtained along the five-fold axis display image contrast features of similar characteristics when the specimen thickness is increased. These kind of images also show high atomic density features which are curved and modulated in intensity. These features can be seen along the five-fold, three-fold and two-fold axis. Diffraction patterns along the main icosahedral axis give rise to spots with characteristic morphology. A qualitative insight on the nature of these effects can be obtained from simulated images of density waves in a field of phasons and phonons. The Fourier transform of these images can also give a qualitative understanding of some features in the experimental diffraction patterns.     

Role of Fe substitution and quenching rate on the formation of various quasicrystalline and related phases

We have investigated Fe substituted versions of the quasicrystalline (qc) alloy corresponding to Al₆₅Cu₂₀(Cr, Fe)₁₅ with special reference to the possible occurrence of various quasicrystalline and related phases. Based on the explorations of various compositions it has been found that alloy compositions Al₆₅Cu₂₀Cr₁₂Fe₃ and Al₆₅Cu₂₀Cr₉Fe₆ exhibit interesting structural phases and features at different quenching rates. At higher quenching rates (wheel speed ～ 25 m/sec) all the alloys exhibit icosahedral phase. For Al₆₅Cu₂₀Cr₁₂Fe₃ alloy, however, both the icosahedral and even the decagonal phases get formed at higher quenching rates. At higher quenching rate, alloy having Fe 3 at % exhibits twobcc phases,bccI (a = 8.9 å) andbccIIa = 15.45 å). The orientation relationships between icosahedral and crystalline phases are: Mirror plane ∥ [001] _bcc I and [351] _bcc II, 5-fold ∥ [113] _bcc II and 3-fold ∥ [110]inbcc II. At lower quenching rate, the alloy having Fe 6 at % exhibits orthorhombic phase (a = 23.6 å,b = 12.4 å,c = 20.1 å). Some prominent orientation relationships of the orthorhombic phase with decagonal phase have also been reported. At lower quenching rate (～ 10 m/sec), the alloy (Al₆₅Cu₂₂Cr₉Fe₆) shows the presence of diffuse scattering of intensities along quasi-periodic direction of the decagonal phase. For making the occurrence of the sheets of intensities intelligible, a model based on the rotation and shift of icosahedra has been put forward.     

Structure transformations of the decagonal and icosahedral phases in quaternary alloys

An investigation of the phase transformations experienced by the decagonal and icosahedral phases in two different quaternary alloys is carried out. The transformation in the decagonal phase of Al-Cu-Co-Si alloy is induced by the electron radiation in a transmission electron microscope. However, in the icosahedral phase of Al-Cu-Co-Fe alloy this transformation is induced by annealing. Electron diffraction patterns obtained from both phases suggest that the deformation mechanism involved in these kinds of transition is related to twinning.     

Electronic structure of complex Hume-Rothery phases and quasicrystals in transition metal aluminides

Aluminum based alloys which contain transition metal elements with concentrations up to 25% and additional small quantities of silicon, have a wide variety of atomic structures with unusual physical properties. Among these materials are the famous quasiperiodic alloys which were discovered in 1984 in Al-Mn by Shechtman et al. One find also numerous complex phases which atomic structures bear resemblance with medium-range structure of quasicrystalline phases. Some of these complex phases, but not all of them, are crystalline approximants of quasicrystals. This review focuses on the effect of the transition metal elements on the electronic structure of these alloys. The case of Al(Si)-Mn alloys is considered as a benchmarking one for the present study. But many of the results and concepts developed here can still be applied to other Al-rich alloys containing TM atom from the 3d, 4d, and 5d series.On the one hand, it is now well accepted that the Hume-Rothery stabilization of the valence electrons plays a crucial role in these materials. On the other hand, it has been shown that the TM atoms are also very important for their stability and their physical properties. But, until recently, there has been no model taking into account these two aspects together. We present a model that unifies the classical Hume-Rothery stabilization for sp electron metals with the virtual bound state model for TM atoms embedded in a metallic matrix. This new formalism for “spd electron phases” is applied successfully to Al(Si)-TM alloys and it gives a coherent conceptual picture of their stability and physical properties. It is compared to accurate first-principles calculations of the electronic structure for these alloys, and to the most important experimental and theoretical results in the literature.We also investigate the transition metal aluminide alloys with a non-metallic character. This can be due either to the creation of a gap or to the localization of electrons by some atomic clusters. Again in that case, the scattering of sp states by d orbitals of transition metal plays a central role.     

Phase transformation reactions in rapidly solidified Al-6.5Fe-1.5V alloys

Phase transformation reactions, occurring during heating of as-atomised Al-6.5Fe-1.5V powders, extrusion of the powders, and heating of the as-extruded alloys produced from the powders, have been studied by DSC, XRD and TEM. The DSC studies of the as-atomised powders revealed several phase transformation reactions. The solid solution in zone A decomposed to form metastable phases at 360°C. These metastable phases further transformed to form equilibrium phases at 500°C. The microquasi-crystalline icosahedral (MI) phase particles present in zone A and zone B transformed to equilibrium phases at 500°C. The globular clusters of microquasi-crystalline icosahedral (GCMI) phase particles in zone C transformed polymorphously to icosahedral (I) phase particles at 450°C. These reactions were believed to occur during extrusion of the powders. During heating of the as-extruded alloys produced from coarse powder particles, I phase transformed polymorphously to hexagonal phase at 550°C. The hexagonal phase decomposed to monoclinic Al₄₅(V, Fe)₇ and Al₁₃Fe₄ phases upon heating for longer times.     

Metallic–covalent bonding conversion and thermoelectric properties of Al-based icosahedral quasicrystals and approximants

In this article, we review the characteristic features of icosahedral cluster solids, metallic–covalent bonding conversion (MCBC), and the thermoelectric properties of Al-based icosahedral quasicrystals and approximants. MCBC is clearly distinguishable from and closely related to the well-known metal–insulator transition. This unique bonding conversion has been experimentally verified in 1/1-AlReSi and 1/0-Al₁₂Re approximants by the maximum entropy method and Rietveld refinement for powder x-ray diffraction data, and is caused by a central atom inside the icosahedral clusters. This helps to understand pseudogap formation in the vicinity of the Fermi energy and establish a guiding principle for tuning the thermoelectric properties. From the electron density distribution analysis, rigid heavy clusters weakly bonded with glue atoms are observed in the 1/1-AlReSi approximant crystal, whose physical properties are close to icosahedral Al–Pd–TM (TM: Re, Mn) quasicrystals. They are considered to be an intermediate state among the three typical solids: metals, covalently bonded networks (semiconductor), and molecular solids. Using the above picture and detailed effective mass analysis, we propose a guiding principle of weakly bonded rigid heavy clusters to increase the thermoelectric figure of merit (ZT) by optimizing the bond strengths of intra- and inter-icosahedral clusters. Through element substitutions that mainly weaken the inter-cluster bonds, a dramatic increase of ZT from less than 0.01 to 0.26 was achieved. To further increase ZT, materials should form a real gap to obtain a higher Seebeck coefficient.     

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.     

Nanocrystallization of the Fd3m Ti2Ni-type phase in Hf-based metallic glasses

Three ternary and four quaternary hafnium-based alloys have been rapidly solidified, and the devitrification of the resultant metallic glasses has been studied to evaluate the influence of composition on the products. The formation of metastable and stable Fd3m (Pearson symbol cF96) Ti2Ni was evident whenever the alloy composition in the stable equilibrium diagrams showed this phase. The replacement of nickel by iron led to the appearance of this phase in preference to the icosahedral quasicrystal. Several common features of the amorphous alloys that form either nanoscale icosahedral or cF96 Ti2Ni-type phases on devitrification are discussed and summarized.     

Icosahedral and decagonal quasicrystals,crystalline phases,and multiple twins in rapidly solidified Al13Cr4Si4

Both the icosahedral and decagonal phases have been found in Al₁₃Cr₄Si₄ melt-spun ribbons. A definite orientation relationship existing between these two phases is determined by SAD which is the same as that obtained previously by Schaefer and Bendersky. During the icosahedral quasicrystal-crystal transformation in this Al−Cr−Si alloy, the Al₁₃Cr₄Si₄ crystalline phase forms at low temperature and an unknown phase at high temperature. Both crystalline phases have definite orientation relationships with the icosahedral phase. The five-fold rotational twins of Al₁₃Cr₄Si₄ around the [110] direction formed from the icosahedral phase follow the group-subgroup relationship. The unknown phase has a hexagonal lattice,a=0.73 andc=1.62nm.     

Fe-Cr-C系硬面合金及其硬质相的研究进展

针对工件表面的磨损破坏,常通过熔覆、喷涂等手段,在失效位置得到高耐磨性的硬面合金层来进行修复强化。该方法不仅经济方便,还可有效提高工件服役寿命。在表面修复工艺中,硬面层的成分选用极为重要。Fe-Cr-C系硬面合金即一类典型的Fe基表面修复材料,目前正广泛应用于各类工矿耐磨部件的表面修复及强化中,与其他成分的硬面合金相比,它具有几大显著优势:(1)成本低廉;(2)强度、韧度、耐磨性优异且较平衡;(3)性能可调节范围广,能满足于多种磨损工况的修复强化。 传统的Fe-Cr-C系硬面合金主要依靠其凝固时产生的M₃C、M₂₃C₆、M₇C₃、高碳马氏体等几种高硬度物相来获得一定的耐磨性。然而在实际磨损工况中,通常会出现硬度更高的SiC、Al₂O₃等磨料,且近年来,随着各种高新技术的竞相出现,大量机械设备规格的转型升级成为大势所趋,这使得各类耐磨部件需要满足于更为苛刻的服役条件。因此,Fe-Cr-C系硬面合金的耐磨性有待进一步改善。 针对这一问题,目前国内外的研究焦点多集中于合金微观组织调控,尤其是硬质相的引入及其尺寸形态改善等,且取得了一系列可观的成果。在硬质相引入方面,探索出各有优缺点的两种引入手段——原位合成法与外界加入法。其中,原位合成法一方面可在熔池反应中得到高热力学稳定性的陶瓷硬质相,另一方面也可在一定程度上强化合金组织。然而,由于熔池高温停留时间短,某些高熔点硬质相生成效率较低。虽外界加入法可有效解决这一问题,但是也需注意硬质相溶解烧损、硬质相与基体界面稳定性差等现象;在硬质相形态控制方面,不少学者探索出合金成分、熔覆制备工艺对硬质相含量、尺寸形态、生长方向的影响。对于合金成分,调整Cr和C的质量比(以下均简称为Cr/C值)或增加C含量可提高碳化物的体积分数,适量合金元素的添加也可通过异质形核作用细化硬质相;在熔覆工艺方面,提高焊后冷却速率可抑制合金凝固初期C原子的扩散,使初生M₇C₃碳化物呈细小及高密度形态,控制焊后热梯度方向也可使M₇C₃垂直于堆焊面生长。此外,在焊接熔池中适当引入磁场也可诱导液态金属的一次枝晶臂分离,增加碳化物的形核质点,从而起到细化组织的作用。 基于近年来的最新研究成果,本文归纳了Fe-Cr-C系硬面合金中硬质相调控的研究进展。首先介绍了合金的凝固行为与组织结构,然后着重综述了硬质相的引入方式、形态控制手段,最后对Fe-Cr-C系硬面合金未来可能的发展趋势提出见解,并围绕硬质相拟定了潜在的研究方向,以期为进一步改善Fe-Cr-C系硬面合金的耐磨性提供参考。     

Characterization of silicon phases in spray-formed and extruded hypereutectic Al–Si alloys by image analysis

The silicon phases in the spray-formed and extruded hypereutectic Al–Si alloys (AlSi18, AlSi25 and AlSi35) have been quantitatively evaluated by means of image analysis technique. The influence of silicon content in the alloys, thermal conditions during spray forming of the alloys and hot extrusion of the spray-formed alloys on the size, shape, dispersion and orientation of the silicon phases have been studied and discussed. In general, the silicon phases are greatly refined and uniformly distributed in the spray-formed Al–Si alloys. This improvement in the silicon phases is further facilitated by low thermal input as well as fast cooling conditions during spray forming. The silicon particles in the as-extruded Al–Si alloys appear more homogeneous and regular than those in the as-deposited Al–Si alloys but exhibit a certain amount of anisotropy and a tendency to preferred orientation. The silicon particles, depending on the particle size and shape, may fracture or coarsen during extrusion.