Complex Metallic Alloys Research Articles

Editorial (Thematic Issue: The Usefulness of Complex Metallic Alloys: Secured Breakthroughs and Applications on the Market)

Editorial (Thematic Issue: The Usefulness of Complex Metallic Alloys: Secured Breakthroughs and Applications on the Market)

Passivation/precipitation mechanisms of Al-Cr-Fe Complex Metallic Alloys in acidic chloride containing electrolyte

The transition from corrosion product formation to protective passivation has been studied for several Al-Cr-Fe Complex Metallic Alloys (CMA) phases during long immersion time (269 days) in 0.01mol L−1 HCl+1M NaCl at pH 2.0, using ICPMS (Inductively Coupled Plasma Mass Spectrometry) and UV- Visible spectroscopy analysis. The different dissolution mechanisms of the investigated phases could be identified by the formation of either Fe(II) or Fe(III) species. In the case of the actively corroding 12.5%at Cr containing phase, Fe(II) complexes were detected in solution, whereas for the passive 27.2%at Cr gamma phase, Fe(III) species were mainly present due to the passive dissolution of the nm-thick protective oxyhydroxide film. Subsequent Raman characterisations of the surface indicated the presence of a Fe3O4 -type layer on the corroding low Cr containing phases surface which is in agreement with the formation of a Fe(II)/Fe(III) mixed oxide. Speciation simulations allowed predicting Fe(III)-Cl complexes as stable species in the studied environments and related precipitation phenomena controlling the pH evolution. These results can further be used to characterise the active to passive corrosion transition occurring also in different Fe,Cr- containing alloys (like stainless steels) in acidic domain.

Comprehensive model of metadislocation movement in Al13Co4

Metadislocations are highly complex defects mediating plasticity in several complex metallic alloys. Available models characterizing the atomic rearrangements during the movement of these defects are limited to two dimensions and heavy atomic species. Combining high-resolution scanning transmission electron microscopy, density functional theory and simulated annealing we develop a three-dimensional model of a metadislocation glide step of 12.3 A in the complex metallic alloy Al13Co4 including all atomic species. The rearrangements within the core are shown to involve maximum atomic jump distances of 3.4 A.

High temperature oxidation of AlCrFe complex metallic alloys

Isothermal oxidation of Al65Cr27Fe8 and Al80Cr15Fe5 was studied in the 600–1080°C range. Formation of transient alumina layers is obtained up to 900°C. On Al65Cr27Fe8 transient to α-phase transformations occur when performing oxidation at 1000°C, together with the possible appearance of (Al0.9Cr0.1)2O3. At 1080°C, direct formation of α-alumina is obtained. On Al80Cr15Fe5, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900°C, revealing thermal etching of the underneath alloy surface. At 1050°C the α-Al2O3 scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.

Lattice dynamics and macroscopic properties in complex metallic alloys

Complex metallic alloys are long-range ordered materials, characterized by large unit cells, comprising several tens to thousands of atoms [1]. These complex alloys often consist of characteristic, cluster building blocks, which in many cases show icosahedral symmetry. Numerous complex phases are known, that can be described in a rather simple way as the periodic or quasi-periodic packing of such atomic clusters. The lattice dynamics of CMAs has been the subject of both theoretical and experimental investigations in view of their interesting macroscopic properties such as low thermal conductivity. In aperiodic crystals in the higher wave-vector regime, theory predicts that the lattice modes are critical: they are neither extended as in simple crystals nor localized as in disordered systems [2]. Experimentally phonons have been studied in different CMAs systems like clathrates, approximant-crystals and quasicrystals. For all of them, acoustic modes are well-defined for wave-vectors close to Brillouin zone centres, but then broaden rapidly as the result of coupling with other excitations [3]. We will present a combined experimental and atomistic simulation study of the lattice dynamics of the complex metallic alloy Al13Co4 phase [4], which is a periodic approximant of the decagonal phase. Particular attention will be paid to the differences between the periodic and `quasiperiodic' directions. Inelastic neutron scattering measurements carried out on a large, single grain on a triple-axis spectrometer will be compared to simulations, focussing on the dispersion relations and the intensity distribution of the S(Q,ω) scattering function, which is a very sensitive test of the model [3]. Simulations are performed with DFT methods and empirical, oscillating, pair potentials [5]. In addition, thermal conductivity calculations, based on the Green-Kubo method, will be compared with measurements, which show a weak anisotropy [6-7]. In this way, the structure-dynamics-properties relation for CMAs is thoroughly explored.

Single crystals of Al-rich complex metallic alloys grown by Czochralski method

In quasicrystal-forming systems very often complex metallic phases exist which show similar structure motifs as the neighbouring quasicrystals but are periodic in all three directions. Therefore these phases are called approximants. They have similar chemical compositions as their parent quasicrystalline phases and can show the same periodicity of stacking of planes as for example found in decagonal quasicrystals. Using the Czochralski method cm3-size single-grain approximants of the Al4TM type, as orthorhombic Al4(Cr,Fe) and hexagonal Al4Cr and of the Al13TM4type, as monoclinic Al13(Co,Ni)4, orthorhombic Al13Co4, monoclinic Al13Fe4and its ternary extensions Al13(Fe,Cr)4and Al13(Fe,Ni)4have been grown from Al-rich solutions [1,2]. The Czochralski method has proven to be a suitable technique for growing single crystals from off-stoichiometric melts due to easy seeding, good mixing of the melt and by allowing good observation of the growth process. To meet the special needs of Al-rich melts a fully metal-sealed growth chamber is used to prevent almost any contact to traces of oxygen. Simultaneously to pulling the change of the liquidus temperature during the growth process has to be compensated by slightly decreasing the temperature using ramps of -(0.1 - 0.8) K/h. For seeding native seeds in different orientations were used. Pulling rates as low as (0.05 - 0.15) mm/h were necessary for growing because of matter transport limits in solution growth and kinetic reasons. Growing the approximant phases the same slow growth kinetics as known from the decagonal quasicrystals was found. Therefore, it might be concluded that the growth of quasicrystalline phases is primarily not limited by the absence of periodicity but by the large clusters that are a common feature in the crystal structures of both, quasicrystals and their approximants.

Electronic Properties and Complexity of Al-Based Complex Metallic Alloys

As a consequence of their structural complexity, the electron transport properties of Al-based complex metallic alloys scale as simple power laws of the number of atoms in the primitive unit cell. Furthermore, presence in energy space of localised, d-like states below the Fermi level is systematically observed, even in the absence of any transition metal constituting element. The relative intensity of this contribution to the total density of states correlates with the structural complexity of the compound. Thus, complexity plays a key role, via Hume-Rothery, hybridization effects and most probably hopping, in the selection, formation and stability of Al-based complex metallic alloys. The results are interpreted in terms of self-organized criticality. In order to promote discussion about the essence of the quasicrystalline state (''why are the atoms where they are?''), a speculative model is suggested.

Passive oxide film characterisation on Al-Cr-Fe and Al-Cu-Fe-Cr complex metallic alloys in neutral to alkaline electrolytes by photo- and electrochemical methods

The formation, chemical composition and especially electrochemical reactivity of the passive oxide formed on the Al64.2Cr27.2Fe8.1 (at. %) and Al66.9Cu11.6Fe11.6Cr10.6 (at. %) complex metallic alloys (CMAs) were investigated in borate buffer electrolyte of pH8.4 and 0.1M Na2SO4 electrolyte of pH 12. Open Circuit Potential evolution, potentiodynamic and potentiostatic anodic polarisation as well as electrochemical impedance measurements revealed high stability of the investigated materials in the alkaline domain relevant to cathodic areas. The high stability of these phases is attributed to the presence of a passive oxyhydroxide layer consisting of thin chromium oxyhydroxide as outermost layer and an inner aluminium oxyhydroxide layer close to the metal/oxide interface as found by XPS. The semiconducting properties characterised by means of photoelectrochemistry reveal further that the bandgap of the passive oxide on the CMAs corresponds to a less hydrated situation (closer to alumina) compared to the one grown on pure Al, thus supporting the higher stability of the CMAs phases.

Surface properties of AlCuFeB coatings with focus on resolving tribological issues of nanoimprint lithography

In recent years extensive research on CMAs (complex metallic alloys) were performed. Among many interesting qualities of complex metallic alloys are its low surface energy and high wear resistance, both making CMA compounds very appealing as a coating layer. The AlCuFeB compound is of particular interest since the addition of boron heps reduce friction forces. nano-imprint lithography as well as other manufacturing techniques need a solution to tackle problems related to stick-slip effects, and damage of molds and substrates, which are all effects of friction and adhesion forces on the interface of contacting materials. Complex metallic alloy compounds could be seen as a possible coating solution, reducing the surface forces, therefore ensuring lower failure rate during the manufacturing process as well as giving opportunity to create more complicated structures. This study focuses on properties of AlCuFeB CMA coatings, explores the possibilities of manufacturing such films, discusses their properties and points to the possible use of this compound in nano-imprint lithography. Most measurements in this study were performed with an atomic force microscope (AFM) acting as a profilometer, tribometer and tool to measure adhesion forces.

Friction and solid-solid adhesion on complex metallic alloys

The discovery in 1987 of stable quasicrystals in the Al–Cu–Fe system was soon exploited to patent specific coatings that showed reduced friction in ambient air against hard antagonists. Henceforth, it was possible to develop a number of applications, potential or commercially exploited to date, that will be alluded to in this topical review. A deeper understanding of the characteristics of complex metallic alloys (CMAs) may explain why material made of metals like Al, Cu and Fe offers reduced friction; low solid–solid adhesion came later. It is linked to the surface energy being significantly lower on those materials, in which translational symmetry has become a weak property, that is determined by the depth of the pseudo-gap at the Fermi energy. As a result, friction is anisotropic in CMAs that builds up according to the translation symmetry along one direction, but is aperiodic along the other two directions. A review is given in this article of the most salient data found along these lines during the past two decades or so.

Hard X-ray Photoelectron Spectroscopy (HAXPES) characterisation of electrochemical passivation oxide layers on Al–Cr–Fe complex metallic alloys (CMAs)

A Hard X-ray Photoelectron Spectroscopy (HAXPES) characterisation of the passivation layers formed by electrochemical polarisation of Al–Cr–Fe complex metallic alloys is presented. By employing X-ray excitation energies from 2.3 to 10.0keV, the depth distributions of Al- and Cr-oxide and hydroxide species in the (Al,Cr)-containing passive layers could be determined. Simultaneous analyses of the shallow Al 2s and deep Al 1s core level lines (respectively, more bulk- and surface-sensitive) provided complementary information to effectively determine the depth-resolved contributions of hydroxide and oxide species within the passivation layer. A Cr threshold concentration of 18 (at.%) was found for effective passivation at pH1.

Anomalous electronic transport in quasicrystals and related complex metallic alloys

We analyze the transport properties in approximants of quasicrystals alpha-AlMnSi, 1/1-AlCuFe and for the complex metallic phase lambda-AlMn. These phases presents strong analogies in their local atomic structures and are related to existing quasicrystalline phases. Experimentally they present unusual transport properties with low conductivities and a mix of metallic-like and insulating-like characteristics. We compute the band structure and the quantum diffusion in the perfect structure without disorder and introduce simple approximations that allow to treat the effect of disorder. Our results demonstrate that the standard Bloch-Boltzmann theory is not applicable to these intermetallic phases. Indeed their dispersion relation are flat indicating small band velocities and corrections to quantum diffusion that are not taken into account in the semi-classical Bloch-Boltzmann scheme become dominant. We call this regime the small velocity regime. A simple Relaxation Time Approximation to treat the effect of disorder allows us to reproduce the main experimental facts on conductivity qualitatively and even quantitatively.

Point defects and formation driving forces of complex metallic alloys: Atomic-scale study of Al4Cu9

A comprehensive atomic-scale investigation of γ1-Al4Cu9, based on density functional theory and the independent-point-defect approximation, is performed. It provides a detailed description of the point defect structure and metastable parts of free energy in this compound, revealing an unusual shallow shape around stoichiometry. Including the energetics of the fcc Al–Cu random solid solution on a wide composition range, our analysis allows to evaluate the chemical driving force for the formation of γ1, which is favourable in various Cu- but also Al-rich environments. Moreover, since the γ1 driving force is strongly enhanced by off-stoichiometry, Al excess may allow γ1-Al4+xCu9 to compete with the θ equilibrium phase in Al-rich solid solutions. These conclusions shed light on the persistence of γ1-Al4Cu9, instead of the expected θ phase, in various processes.

Flow microcapillary plasma mass spectrometry-based investigation of new Al–Cr–Fe complex metallic alloy passivation

Al-Cr-Fe complex metallic alloys are new intermetallic phases with low surface energy, low friction, and high corrosion resistance down to very low pH values (0-2). Flow microcapillary plasma mass spectrometry under potentiostatic control was used to characterize the dynamic aspect of passivation of an Al-Cr-Fe gamma phase in acidic electrolytes, allowing a better insight on the parameters inducing chemical stability at the oxyhydroxide-solution interface. In sulfuric acid pH 0, low element dissolution rates (in the µg cm(-2) range after 60 min) evidenced the passive state of the Al-Cr-Fe gamma phase with a preferential over-stoichiometric dissolution of Al and Fe cations. Longer air-aging was found to be beneficial for stabilizing the passive film. In chloride-containing electrolytes, ten times higher Al dissolution rates were detected at open-circuit potential (OCP), indicating that the spontaneously formed passive film becomes unstable. However, electrochemical polarization at low passive potentials induces electrical field generated oxide film modification, increasing chemical stability at the oxyhydroxide-solution interface. In the high potential passive region, localized attack is initiated with subsequent active metal dissolution.

Complex Metallic Alloys – Microstructure Characterization

The recent findings related to binary and ternary structurally complex phases in selected complex metallic alloys coming under Al-Pd-Co, Al-Cu-Co, and Al-Mn-Fe systems are presented. The phases were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, high-angle annular dark-field imaging, X-ray diffraction, and differential thermal analysis. There are highlighted some unusual features of phases D, U, T, and ε-family from both structural and compositional points of view.

Stabilization mechanism of γ-Mg17Al12 and β-Mg2Al3 complex metallic alloys

Large-unit-cell complex metallic alloys (CMAs) frequently achieve stability by lowering the kinetic energy of the electron system through formation of a pseudogap in the electronic density of states (DOS) across the Fermi energy εF. By employing experimental techniques that are sensitive to the electronic DOS in the vicinity of εF, we have studied the stabilization mechanism of two binary CMA phases from the Al–Mg system: the γ-Mg17Al12 phase with 58 atoms in the unit cell and the β-Mg2Al3 phase with 1178 atoms in the unit cell. Since the investigated alloys are free from transition metal elements, orbital hybridization effects must be small and we were able to test whether the alloys obey the Hume-Rothery stabilization mechanism, where a pseudogap in the DOS is produced by the Fermi surface–Brillouin zone interactions. The results have shown that the DOS of the γ-Mg17Al12 phase exhibits a pronounced pseudogap centered almost exactly at εF, which is compatible with the theoretical prediction that this phase is stabilized by the Hume-Rothery mechanism. The disordered cubic β-Mg2Al3 phase is most likely entropically stabilized at high temperatures, whereas at lower temperatures stability is achieved by undergoing a structural phase transition to more ordered rhombohedral β′ phase at 214 ° C, where all atomic sites become fully occupied. No pseudogap in the vicinity of εF was detected for the β′ phase on the energy scale of a few 100 meV as determined by the ‘thermal observation window’ of the Fermi–Dirac function, so that the Hume-Rothery stabilization mechanism is not confirmed for this compound. However, the existence of a much broader shallow pseudogap due to several critical reciprocal lattice vectors that simultaneously satisfy the Hume-Rothery interference condition remains the most plausible stabilization mechanism of this phase. At Tc = 0.85 K, the β′ phase undergoes a superconducting transition, which slightly increases the cohesive energy and may contribute to relative stability of this phase against competing neighboring phases.

E/a determination for 4d- and 5d-transition metal elements and their intermetallic compounds with Mg, Al, Zn, Cd and In

The present work is devoted to the determination of the effective electrons per atom ratio e/a by means of first-principles full-potential linearized augmented plane wave-Fourier method for elements from Rb to Ag in Period 5 and from Cs to Au in Period 6 of the periodic table and is regarded as a continuation of the preceding work done for elements from K to Cu in Period 4. The value of e/a was determined by reading off the square of the Fermi diameter, from the dispersion relation for electrons outside the Muffin-Tin spheres. A straightforward reading of the ordinate at the Fermi level, i.e. local reading method was validated for Rb and Cs in Group 1, Sr in Group 2, Y in Group 3, Pd and Pt in Group 10 and Ag and Au in Group 11. Instead, the nearly free electron (NFE) method was found to be indispensable for TM elements from Zr to Rh in Period 5 and those from Ba to Ir in Period 6. The composition dependence of e/a values for intermetallic compounds in X–TM (X = Mg, Al, Zn, Cd and In) alloy systems was also studied. The new Hume–Rothery electron concentration rule was established by constructing e/uc, the number of electrons per unit cell, vs. square of critical reciprocal lattice vector, , diagram for structurally complex metallic alloys having a pseudogap at the Fermi level. A proper use of either the local reading- or the NFE-e/a for the elements as indicated above is found to be essential.

Metadislocations: The case of pure glide

ABSTRACTMetadislocations are highly complex and pivotal defects mediating plastic deformation in complex metallic alloys. Here, we review recent results on the structure of metadislocations in the phases T-Al-Mn-Pd, T-Al-Mn-Fe and o-Al13Co4. In these materials, metadislocation motion is of particular interest as it takes place by pure glide in contrast to most other complex metallic alloys. Recently, novel metadislocations were found in the T-phase [1]. They have Burgers vectors${\rm{\vec b}} = \pm \tau ^{ - n} c\left( {\matrix{ 0 & 0 & 1 \cr } } \right)$(n = 2, 3, 4) and are associated to two, four and six planar defects, respectively. The type of planar defect depends on the deformation geometry. Metadislocation glide creates (1 0 0) stacking faults and climb creates (0 0 1) phason planes. Metadislocation glide was observed in the o-Al13Co4phase, as well [2]. The close structural relation of metadislocations in the phases T-Al-Mn-Pd, T-Al-Mn-Fe, Al13Co4and ε6-Al-Pd-Mn is discussed.

Atomic Dynamics in Complex Metallic Alloys

ABSTRACTComplex Metallic Alloys (CMAs) are metallic solids of high structural complexity, consisting of large numbers of atoms in their unit cells. Consequences of this structural complexity are manifold and give rise to a variety of exciting physical properties. The impact that such structural complexity may have on the lattice dynamics will be discussed. The surprising dynamical flexibility of Tsai-type clusters with the symmetry breaking central tetrahedron will be addressed for Zn6Sc, while in the Ba-Ge-Ni clathrate system the dynamics of encaged Ba guest atoms in the surrounding Ge-Ni host framework is analysed with respect to the experimentally evidenced strong reduction of lattice thermal conductivity. For both systems experimental results from neutron scattering are analyzed and interpreted on atomistic scale by means of ab initio and molecular dynamics simulations, resulting in a picture with the respective structural building blocks as the origin of the peculiarities in the dynamics.

Electrical Resistivity and Magnetoresistance of the δ-FeZn10 Complex Intermetallic Phase

ABSTRACTThe δ-FeZn10 phase possesses high structural complexity typical of complex metallic alloys: a giant unit cell comprising 556 atoms, polyhedral atomic order with icosahedrally-coordinated environments, fractionally occupied lattice sites and statistically disordered atomic clusters that introduce intrinsic disorder into the structure. The electrical resistivity is large and exhibits a maximum at about 220 K. The magnetoresistance is sizeable, amounting to 1.5 % at 2 K in 9 T field. The temperature–dependent resistivity is discussed within the frame of the theory of slow charge carriers, applicable to metallic systems with weak dispersion of the electronic bands, where the electron motion changes from ballistic to diffusive upon heating. A comparison to the theory of weak localization is also made.