Epitactic growth of a decagonal phase in an Al–Ni–Co alloy film deposited on a (0001) sapphire substrate

Thin films of Al–Ni–Co alloy with an average thickness of 15?nm were produced by means of conventional vacuum deposition technique on (0001) sapphire substrates heated at various test temperatures. The microstructures and textures of the films obtained were thoroughly investigated by atomic force microscopy, X-ray diffraction and transmission electron diffraction and imaging techniques. The diffraction measurements have evidenced that the vacuum deposition of Al₇₂Ni₁₅Co₁₃ alloy on the substrates heated above 400°C allows a homogeneous poly-quasicrystalline film, consisting of the Ni-rich basic decagonal phase to grow. It has been further indicated by in-plane XRD analysis that the film deposited at 550°C contains a considerable amount of the decagonal grains epitaxially grown on the sapphire substrate. Possible epitaxial relations occurring between the deposit and the substrate will be detailed on the basis of results obtained from electron diffraction measurements.     

Bridging room-temperature and high-temperature plasticity in decagonal Al–Ni–Co quasicrystals by micro-thermomechanical testing

Ever since quasicrystals were first discovered, they have been found to possess many unusual and useful properties. A long-standing problem, however, significantly impedes their practical usage: steady-state plastic deformation has only been found at high temperatures or under confining hydrostatic pressures. At low and intermediate temperatures, they are very brittle, suffer from low ductility and formability and, consequently, their deformation mechanisms are still not clear. Here, we systematically study the deformation behaviour of decagonal Al–Ni–Co quasicrystals using a micro-thermomechanical technique over a range of temperatures (25–500 °C), strain rates and sample sizes accompanying microstructural analysis. We demonstrate three temperature regimes for the quasicrystal plasticity: at room temperature, cracking controls deformation; at 100–300 °C, dislocation activities control the plastic deformation exhibiting serrated flows and a constant flow stress; at 400–500 °C, diffusion enhances the plasticity showing homogenous deformation. The micrometer-sized quasicrystals exhibit both high strengths of ~2.5–3.5 GPa and enhanced ductility of over 15% strains between 100 and 500 °C. This study improves understanding of quasicrystal plasticity in their low- and intermediate-temperature regimes, which was poorly understood before, and sheds light on their applications as small-sized structural materials.     

Growth and physical properties of the decagonal Al–Cu–Co quasicrystal grown from the ternary melt

Large faceted single-grain quasicrystals of the approximate composition Al_63.2Cu_19.5Co_17.3 and with a high degree of structural perfection are obtained through the slow cooling of a ternary melt with initial composition Al₆₅Cu₂₉Co₆. X-ray diffraction patterns of crushed single-grain samples are exceptionally sharp, indicating a high degree of structural order, with no evidence of secondary phases. Transmission electron micrographs also reveal sharp diffraction patterns in the even-n layers but diffuse scattering in the odd-n layers. Temperature-dependent magnetization, electrical resistivity and specific heat are measured using bars cut perpendicular and parallel to the c axis and show diamagnetic behaviour: γ?≈?0.5?mJ?mol^?1?K^?2, ρ _c(2?K)?≈?52?μΩ?cm and ρ _q(2?K)?≈?283?μΩ?cm.     

X-ray photoelectron studies of changes in the electronic structure upon phase transitions in Co–Ni and Co–Fe alloys

The changes in the electronic structure of Co–Ni and Co–Fe systems upon phase transitions are studied. X-ray photoelectron study of the valence-band spectra and the parameters of the multiplet splitting of Co, Ni and Fe 3s spectra is carried out at different temperatures. It is established that the ordering–separation phase transition in Co–Ni alloys takes place in the temperature range of 600–700°C. As opposed to Co–Ni alloys, in the Fe–Co alloy, ordering–separation–ordering phase transitions are observed. High-temperature ordering of the Fe₅₀Co₅₀ alloy is observed above 1200°C. The transition from ordering to separation is shown to lead to changes in the d electron spectra of the valence band and in the parameters of the multiplet splitting of the 3s spectra.     

On the electrical resistivity of the quasicrystalline decagonal Al–Mn system

A simple effective-medium approach is used to study anisotropy properties of the electrical resistivity in the quasicrystalline decagonal Al–Mn system.     

Dissolution patterns caused by chemical etching of Al–Co–Cu and Al–Co–Ni decagonal quasicrystals with a HF–HNO3–H2O solution

Dissolution patterns essential for Al–Co–Cu and Al–Co–Ni decagonal quasicrystals (d-QCs) have been investigated in detail by chemical etching using a HF–HNO₃–H₂O solution followed by scanning electron microscopy (SEM) observations. The chemical etching of definite surface areas of the samples, which are either normal or parallel to the tenfold axes, using a solution with HF–HNO₃–H₂O (1?:?5?:?4 in volume ratio; 0°C; 5–10?min), produces a large number of microfacet pits of decagonal prismatic shape. In addition, the same etching test in combination with SEM observations was carried out on a deformed sample of the Al–Co–Ni d-QC, which had been subjected to concentrated mechanical stress at an elevated temperature of 850°C by means of the Vickers indentation technique. The morphological features of the resulting micropits and their possible origins are discussed on the basis of results obtained by SEM observations.     

Phase transformations and structural characteristics of AL–Cu–Co(–Si) decagonal phase

The phase transformations and structural characteristics of the Al-Cu-Co-Si alloy have been studied by neutron diffraction and high-resolution electron microscopy. The Al₆₅Cu_17.5Co_17.5 decagonal phase is stable in the temperature range between 973 K and 1350 K. At the low-temperature end, it relaxes to a microcrystalline approximant structure. At the high-temperature end, it melts directly into liquid. Two distinct orthorhombic phases are identified in the Al₆₃Cu_17.5Co_17.5Si₂ microcrystalline structure. They are composed of several structure units that can also construct the Penrose tiling. Because of lack of units, a single orthorhombic phase cannot undergo the transformation towards the high-temperature decagonal phase. An analysis of the orientation relationships between the CsCl and orthorhombic phases leads to the definition of Penrose tiling-like subnetworks inside the orthorhombic unit cells so that these orthorhombic phases can be regarded as the periodic patchworks of quasiperiodic subnetworks.     

Origin of tweed in Au–Cu–Al alloys

The premartensitic tweed in Au–Cu–Al alloys, contrary to previous thought that resort to defects, is confirmed to be associated with the coherent embryos of an intermediate phase (I phase) embedded in parent phase. The parent?→?I phase transformation temperature was measured by differential scanning calorimeter and dynamic mechanical analysers, which shifts from 82.3 to 557.6?°C depending on the alloy composition. X-ray diffraction and transmission electron microscopes (TEM) results show that the parent?→?I phase transformation is a charge density wave transition that cannot be suppressed even by melt-spun method, which shows obvious compositional inhomogeneity between I phase and parent. The results imply that the parent?→?I phase transition is a fast displacive transformation coupled with diffusion. Moreover, accompanying the parent?→?I phase transformation, alloys demonstrate diversified microstructure revealed by TEM observation, from tweed to chessboard nanowires or twins. These findings provide the experimental evidence for that parent?→?I phase transformation in Au–Cu–Al alloys is originated from pseudospinodal decomposition as theoretically predicted.     

The Role of Transition Metals in Crystallization of Amorphous Al–Ni–Co–Yb Alloys

Technical Physics - Al86Ni4Co4Yb6 and Al86Ni6Co2Yb6 metallic ribbons have been obtained by a standard planar flow method. According to X-ray diffraction data, the ribbons are amorphous. Their...     

Structure of the B2 phase in Ti–25Al–25Mo alloy

The structure of the B2 phase has been investigated in Ti–25Al–25Mo alloy using Rietveld refinement of X-ray and neutron diffraction data in as-cast and solution-treated conditions. Different initial structure models have been used for the refinement. The site occupancy of the various chemical constituents in the B2 phase has been calculated and compared with earlier investigations. The relative merits of neutron diffraction over X-ray diffraction for structural refinement of the B2 phase in Ti–25Al–25Mo alloy have been demonstrated.     

Mechanical spectroscopy of decagonal Al–Cu–Fe–Cr quasicrystalline coatings

Mechanical spectroscopy measurements were performed on decagonal quasicrystalline Al–Cu–Fe–Cr coatings of three different thicknesses deposited on a mild steel substrate. The mechanical loss spectra indicate that the internal friction is mostly caused by the quasicrystalline coating and that the contributions of both the steel substrate and the interface are small. The shear modulus measured in torsion increases with temperature, while the Young’s modulus measured in flexion behaves normally. This shear modulus anomaly is interpreted as being due to solid friction between cracked segments of the quasicrystalline coating. This phenomenon also explains the broad athermal maximum found to occur in isochronal internal friction measurements. A quantitative model successfully reproducing the observed behaviour has been developed. Finally, the reversible high-temperature exponential background was interpreted as being due to the onset of the brittle-to-ductile transition in the quasicrystalline coating. The measured activation enthalpy is similar to the value that was deduced from compression tests performed at high temperatures on icosahedral Al–Cu–Fe bulk material.     

Structural origin of perpendicular magnetic anisotropy in Ni–W thin films

The magnetic properties and microstructure of electrodeposited Ni–W thin films (0–11.7 at% W in composition) were studied. The film structures were divided into three regions: an FCC nanocrystalline phase (0–2 at% W), a transition region from FCC nanocrystalline to amorphous phase (2–7 at% W), and an amorphous phase (>7 at% W). In the transition region, (4–5 at% W) films with perpendicular magnetic anisotropy (PMA) were found. The saturation magnetization, magnetic anisotropy field, perpendicular magnetic anisotropy and perpendicular coercivity for a typical Ni–W film (4.5 at% W) were 420 kA/m, 451 kA/m, 230 kJ/m and 113 kA/m, respectively. The microstructure of Ni–W films with PMA is composed of isolated columnar crystalline grains (27–36 nm) with the FCC phase surrounded by the Ni–W amorphous phase. The appearance of the interface between the magnetic core of Ni crystalline grains and the Ni–W non-magnetic boundary layer seems to be the driving mechanism for the appearance of PMA. The origin of PMA in Ni–W films is mainly attributed to the magnetoelastic anisotropy associated with in-plane internal stress and positive magnetostriction. The secondary source of PMA is believed to be the magnetocrystalline anisotropy of 〈1 1 1〉 columnar grains and its shape magnetic anisotropy. It is concluded that Ni–W electrodeposited films (4–5 at% W) may be applicable for perpendicular magnetic recording media.     

Crystallographic features of the approximant H (Mn7Si2V) phase in the Mn–Si–V alloy system

The intermetallic compound H (Mn₇Si₂V) phase in the Mn–Si–V alloy system can be regarded as an approximant phase of the dodecagonal quasicrystal as one of the two-dimensional quasicrystals. To understand the features of the approximant H phase, in this study, the crystallographic features of both the H phase and the (σ → H) reaction in Mn–Si–V alloy samples were investigated, mainly by transmission electron microscopy. It was found that, in the H phase, there were characteristic structural disorders with respect to an array of a dodecagonal structural unit consisting of 19 dodecagonal atomic columns. Concretely, penetrated structural units consisting of two dodecagonal structural units were presumed to be typical of such disorders. An interesting feature of the (σ → H) reaction was that regions with a rectangular arrangement of penetrated structural units (RAPU) first appeared in the σ matrix as the initial state, and H regions were then nucleated in contact with RAPU regions. The subsequent conversion of RAPU regions into H regions eventually resulted in the formation of the approximant H state as the final state. Furthermore, atomic positions in both the H structure and the dodecagonal quasicrystal were examined using a simple plane-wave model with 12 plane waves.     

Thermal expansion anisotropy of β-TlInS2 crystals in the course of phase transitions

Basing on the thermal expansion studies performed for β-TlInS₂ crystals in the course of their phase transitions, we have found that the crystals aged for one year are composed from at least two polytypes, which undergo phase transitions at different temperatures. The detailed analysis of anisotropy of the thermal expansion demonstrates that all of the diagonal components of the thermal expansion tensor associated with the eigensystem have almost the same modules but differ by their signs. This leads to appearance of an elliptical conical surface of zero thermal expansion. The orientation of this surface does not depend on the temperature in the temperature interval under study. We have also found that the thermal expansion coefficients along the crystallographic a and b axes are close to zero.     

Magnetically induced phase separation in the Co–Cr binary system

Direct evidence of phase separation in the Co-rich corner of the Co–Cr binary system, the transformation of high-temperature FCC α-Co into a ferromagnetic α_f phase and a paramagnetic α_p phase, has been experimentally obtained by using diffusion couple technique, scanning electron microscopy with energy dispersion analysis of X-ray (SEM-EDX), analytical transmission electron microscopy, optical microscopy and X-ray diffraction. Thermodynamic calculation, on the basis of the presently obtained phase equilibrium data, has verified that this phase separation arises from magnetic ordering, and that a similar phase separation appears in the HCP phase below 469°C which is transformed into a ferromagnetic ε_f phase and a paramagnetic ε_p phase. It is therefore concluded that magnetically induced phase separation must be responsible for the microscopic composition modulation of Cr in the CoCr thin film magnetic recording media.     

Unusual magnetization anisotropy in amorphous Nd–Fe–Al ribbons

Nd₆₀Fe₃₀Al₁₀ ribbons has been prepared by chill-block melt-spinning with different wheel speeds from 5 to 30 m/s. Fully amorphous ribbons were obtained at wheel speeds of 25 and 30 m/s. These ribbons exhibited an unusually large anisotropy in magnetization. The effect of the magnetic anisotropy decreased with decreasing wheel speed, and nearly disappeared at the wheel speed of 5 m/s, at which the ribbon consisted of a mixture of a more stable Fe-rich amorphous phase and a crystalline Nd phase with a strong crystallographic texture.     

Origin of the noncubic anisotropy in Mn∶ (Y,Gd)3Fe5O12 thin films

Thin Mn YIG films exhibit a large noncubic anisotropy. We show that this anisotropy may be understood on the basis of a strong Jahn-Teller effect of octahedrally coordinated Mn³⁺ ions. The corresponding mechanism is simple — the strain caused by the film/substrate misfit orders the local Jahn-Teller distortions and, as a consequence, noncubic anisotropy appears. In particular, due to the nonparallel local coordinate systems of octahedral sites, an uniaxial anisotropy is predicted also for the (111) growth plane. It is pointed out that the interaction between the Jahn-Teller centres and/or their interaction with the lattice defects lower the magnitude of the anisotropy.The temperature dependence of the uniaxial anisotropy was measured on the (001) and (111) oriented films using the FMR method. The results are in qualitative agreement with the prediction of theory. Quantitative comparison of theory with the experiment is difficult, as little is known about the distribution of the strains due to the lattice defects and about the interaction between the Jahn-Teller centres.The authors are indebted to Dr. S. Krupika for many helpful discussions, to Dr. J. Kub for measuring the misfits and to Dr. J. imová for the analysis of the films used in our experiments.