Chemical shifts of metallic and non-metallic Al–Re–Si approximant crystals studied by EELS and SXES

Chemical shifts of approximant crystals of 1/0-Al₁₂Re (1/0-metallic), 1/1-Al₇₃Re₁₅Si₁₂ (1/1-metallic) and 1/1-Al₇₃Re₁₇Si₁₀ (1/1-non-metallic) were examined by using electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Al L-shell excitation EELS spectra of these alloys showed an apparent chemical shift only for the 1/1-non-metallic alloy to the larger binding energy side by 0.2?eV. Al-Kα, Re-Mα and Si-Kα emission SXES spectra also showed a shift to the larger binding energy side only for 1/1-non-metallic alloy. 1/0-metallic and 1/1-metallic alloys did not show any chemical shift in EELS and SXES experiments. Chemical shifts were observed only in larger binding energy side compared with pure materials. This implies the decrease of valence charge at constituent atomic sites of 1/1-non-metallic alloy compared with 1/0-metallic, 1/1-metallic and pure materials. The decreased charges should distribute intermetallic sites, which should be related to a formation of covalent bonding among Al atomic sites reported by maximum-entropy method (MEM)/Rietveld analysis on this material. This relation between chemical shift and covalent bonding nature of this approximant alloy may support the presence of covalent bonding in Al-based quasicrystals.     

Characteristic chemical shifts of quasicrystalline Zn–Mg–Zr alloys studied by EELS and SXES

Chemical shifts of the constituent atoms of primitive icosahedral quasicrystal (P-QC), face-centred icosahedral quasicrystal (F-QC) and 1/1-approximant (1/1-AP) of F-QC Zn–Mg–Zr alloys were investigated for the first time using high energy-resolution electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Among Zn M-shell and Mg L-shell excitation EELS spectra of P-QC, F-QC and 1/1-AP alloys, only the quasicrystalline alloys showed a chemical shift towards the larger binding energy side. In Zn-L and Zr-L emission SXES spectra, the P-QC and F-QC alloys showed a chemical shift towards larger binding energy side. The magnitudes of the shifts in the Zn-L emission spectra of the quasicrystalline alloys were almost the same as for ZnO. These results strongly suggest a decrease in valence charge in quasicrystalline states. Therefore, it should be concluded that bonding in quasicrystalline states involves a characteristic increase in covalency compared with bonding in corresponding approximant and standard metal crystals.     

Scanning transmission electron microscopy investigation of an Al–Mg–Si–Ge–Cu alloy

Precipitation in a Mg-rich Al–Mg–Si–Ge–Cu alloy was investigated using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The precipitates were needle or lath shaped with the longest dimension parallel to ?001?_Al. The precipitates had no repeating unit cell when viewed along this direction. However, the precipitate structure in projection consisted of a hexagonal network of mixed Si and Ge columns, with Mg, Al, and Cu columns occupying specific sites in between the network columns. The Cu columns appeared with the same local arrangement of atomic columns as in Al–Mg–Si–Cu precipitates, and the Cu-free regions consisted of structural units with Mg and Al at specific sites. These structural units were often arranged in a locally ordered fashion, and in some cases the projected structure possessed and overall point symmetry. The amount of strain on the surrounding matrix was found to vary depending on the width of the precipitate cross section.     

The effect of Zn on precipitation in Al–Mg–Si alloys

Effects of addition of Zn (up to 1 wt%) on microstructure, precipitate structure and intergranular corrosion (IGC) in an Al–Mg–Si alloys were investigated. During ageing at 185?°C, the alloys showed modest increases in hardness as function of Zn content, corresponding to increased number densities of needle-shaped precipitates in the Al–Mg–Si alloy system. No precipitates of the Al–Zn–Mg alloy system were found. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the Zn atoms were incorporated in the precipitate structures at different atomic sites with various atomic column occupancies. Zn atoms segregated along grain boundaries, forming continuous film. It correlates to high IGC susceptibility when Zn concentration is ~1wt% and the materials in peak-aged condition.     

HAADF-STEM study of β′-type precipitates in an over-aged Al–Mg–Si–Ag alloy

Coarse, rod-shaped precipitates growing along ?100?Al directions in an Al–1.0?wt% Mg₂Si alloy with 0.5?wt% Ag additions were investigated by high-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). All investigated precipitates had complex structures, being composed of domains separated by anti-phase resembling boundaries. The domains consist of a modified hexagonal β′-type structure that contains a considerable amount of Ag. Based on HAADF-STEM images, an average atomic model with space group P-62?m (189) and composition Al₃Mg₃Si₂Ag is proposed, having Al incorporation and Ag replacing certain Si atomic columns. Co-existence with the Ag-free β′-Mg₉Si₅ phase has been observed for some precipitates. The boundaries may be described as full or half units of the orthorhombic U2-AlMgSi precipitate phase. The HAADF-STEM images indicate partial replacements of Al atoms by Ag, in both the β′-type domains and the U2-type boundaries. Ag enrichment of the Al matrix near the precipitate/Al interface was observed for all the investigated precipitates     

Effective atomic numbers for CoCuNi alloys using transmission experiments

Effective atomic numbers for CuCoNi alloys against changing Ni contents were measured in the X-ray energy range from 15.746 to 40.930 keV. The gamma rays emitted a ²⁴¹Am point source have been send on absorbers to be used transmission arrangement. The X-rays were counted by a Si(Li) detector with a resolution of 160 eV at 5.9 keV. The compositions of the Ni films were determined to be 0.03, 0.47, 0.62, 1.23, 1.22 and 1.6 by a scanning electron microscopy in CuCoNi alloys prepared against changing Ni contents. CoCuNi alloy films were prepared with an electrodeposition technique. Also, the total effective atomic numbers of each alloy were estimated using mixture rule. The measured values were compared with estimated values for alloys.     

X-ray photoelectron spectroscopic studies on phase identification and quantification of nickel aluminides

Core-level XPS spectra for clean surfaces of Ni₃Al, NiAl, and NiAl₃ alloys were studied. The clean surfaces were obtained by fracturing in the ultra-high vacuum chamber. The positive chemical shifts of Ni 2p_3/2 peak for NiAl and NiAl₃ from Ni metal were 0.2 and 1.0 eV, respectively. The negative shift for Al 2p peak and the positive shift for Ni 3p peaks increased with the decreasing concentration of the corresponding elements. The peak position of the bulk plasmon loss peak for Al 2s peak shifted toward higher energy side, and further, the intensity ratio decreased with the decrease in aluminum concentration. Both the peak intensity ratios of Al 2p to Ni 3p determined by factor analysis and convenient separation are proportional to the atomic ratio of aluminum to nickel. The results indicate that the intensity ratio of Al 2p to Ni 3p determined by these two methods can be applied to the quantification for the surface of the nickel-aluminum alloys.     

Ab initio calculations of the ferromagnetic shape memory alloy Ni–Mn–Al

Results of the first-principles calculations of the magnetic shape-memory alloy NiMnAl are presented. The properties of a series of alloys in the composition range Ni₅₀Mn _x Al_{50? x} with 0 ≤ x ≤ 50 were deduced from the ab initio simulations. One essential result is that the alloy is ferromagnetic in the range from 14 to 31?at.% Mn. Furthermore, the martensitic phases 2?M, 10?M, and 14?M with long-periodic structure were calculated. They are metastable in the stoichiometric Ni₂MnAl alloy due to additional bonding between specific atomic sites. Their properties are discussed in terms of the density of states and their charge distribution.     

Ultraviolet photoelectron spectroscopy of Pd-Si glasses

In order to determine systematic changes in the density of states with alloy composition, photoelectron spectra at hv=21.2 eV were measured for several amorphous alloys based on the well-known Pd-Si glass system. Three binary alloys with 15, 20, and 25 at. % Si, two ternaries, Pd₈₀ Si₁₇ Cu₃ and Pd₈₀ Si₁₄ Cu₆, and polycrystalline Pd were analyzed. Compared to Pd, both the density of states at the Fermi energy and the d-band width are reduced in the glasses. The d-bands display an overall shift of 0.4 eV over the range of alloy compositions studied. Partial agreement with recent density of states calculations was obtained.     

Influence of impurity elements on the nucleation and growth of Si in high purity melt-spun Al–Si-based alloys

The nucleation and growth of Si has been investigated by TEM in a series of high purity melt spun Al–5Si (wt%)-based alloys with a trace addition of Fe and Sr. In the as-melt-spun condition, some twinned Si particles were found to form directly from the liquid along the grain boundary. The addition of Sr into Al–5Si-based alloys promotes the twinning of Si particles on the grain boundary and the formation of Si precipitates in the α-Al matrix. The majority of plate-shaped and truncated pyramid-shaped Si precipitates were also found to nucleate and grow along {111}_α-Al planes from supersaturated solid solution in the α-Al matrix. In contrast, controlled slow cooling decreased the amount of Si precipitates, while the size of the Si precipitates increased. The orientation relationship between these Si precipitates and the α-Al matrix still remained cube to cube. The β-Al₅FeSi intermetallic was also observed, depending on subsequent controlled cooling.     

Structural asymmetry in liquid Fe–Si alloys

The thermodynamic properties and microscopic structure of liquid Fe–Si alloys at 1873 K were studied by using the regular associated solution model. The model was utilized to determine the complex concentration in a regular associated solution of Fe, Si and Fe₂Si. The complex concentration was then used to calculate the integral excess free energy of mixing, activity, concentration fluctuations in the long-wavelength limit, S_CC (0), and the Warren–Cowley short-range parameter α ₁. The analysis suggests that heterocoordination leading to the formation of complex Fe₂Si is likely to exist in the liquid and is of a strongly interacting nature. The theoretical analysis reveals that the alloy is more ordered towards the Fe-rich region. The observed asymmetry in the properties of mixing of Fe–Si alloys in the molten state is successfully explained on the basis of the regular associated solution model.     

Half-metallicity and onsite Hubbard interaction on d-electronic states: a case study of Fe2NiZ (Z = Al,Ga, Si,Ge) Heusler systems

DFT-based structural optimisations of Fe₂NiZ (Z?=?Al, Ga, Si, Ge) Heusler compounds confirm the stability of these alloys in F-43m phase. While defining the electronic structure, onsite Hubbard approximation scheme for exchange correlations predicted better results than the generalised gradient approximation. Calculated band structure and densities of states together with spin magnetic moments designate the half-metallic character of these alloys. Indirect band gaps, 1.2?eV for Fe₂NiAl, 0.98?eV for Fe₂NiGa, 1.3?eV for Fe₂NiSi and 1.1?eV for Fe₂NiGe in spin-down states are observed. The ferromagnetic spin moments amount to an integral value of 5μB for (Al, Ga) and 6μB for (Si, Ge) systems with a maximum contribution from transition metal atom (Fe). To forecast the possible turnout of the thermopower, Seebeck coefficients, electrical and thermal conductivities are calculated, which directly hints the thermoelectric response of these materials. This study creates a possibility of these alloys in thermoelectrics and spintronics.     

A comparison between X-ray absorption spectroscopy and Bremsstrahlung Isochromat Spectroscopy: The empty states of Pd−Al alloys and Pd2Si

We present X-ray absorption spectra (XAS) obtained with synchrotron radiaton at the PdL _2,3 edges of Pd–Al alloys and Bremsstrahlung Isochromat Spectra (BIS) at 1,486.7 eV of Pd–Al alloys and of Pd₂Si. The XAS and BIS results for the alloys are very similar indicating that the effect of the core potential (e.g. many-body, electronhole excitation) is negligible. The BIS results on Pd₂Si show differences with respect to Pd–Al due to the more localized bonds. The comparison between XAS and BIS data in Pd₂Si is still an open problem and might indicate a more subtle role of the core-hole potential.     

Laser processing of bulk Al–12Si alloy: influence of microstructure on thermal properties

The feasibility of enhancing the thermal conductivity of an alloy via microstructural refinement was examined using Al–12%Si alloy as a model material. Al–12Si alloy samples were fabricated at different process parameters using laser engineered net shaping (LENS?) and the effect of microstructural features on the thermal conductivity was studied and compared with conventionally cast alloy. The large difference in melting points and laser light absorptivities of Si and Al as well as the low melt viscosity of Al–12Si alloy resulted in a very small process window to successfully fabricate bulk Al–12Si alloy samples using LENS?. Comparison of microstructural features of laser-processed samples with cast Al–12Si alloy showed significant refinement in eutectic Si for laser processed samples. Microstructural refinement not only improved the thermal conductivity of Al–12Si alloy but also compensated the detrimental effect of porosity on thermal conductivity. The thermal conductivity of cast alloy varied between 82 and 93?W/mK, which is ～21–76% lower than the values exhibited by laser-processed samples in the range 103–153?W/mK. The results of LENS? fabrication, microstructural evolution and thermal properties of Al–12Si alloy bulk structures can be extended to other immiscible alloys and metal matrix composites for a variety of engineering applications.     

Disorder broadening of E2 optical spectra in GeSi alloys

he E₂ transitions in GeSi alloys are analyzed quantitatively using low-field electroreflectance data. The model of M₁ saddle point transitions is found to be in fair agreement with differentiated electroreflectance spectra in the whole composttion range. The broadening parameter of pure Ge and Si increases by 0.035 eV in the alloy with equal content of both components.     

Microstructure and texture development of 7075 alloy during homogenisation

The microstructure evolution of Al–Zn–Mg–Cu alloy during homogenisation was studied by optical microscope, field emission scanning electron microscope, energy dispersive X-ray Spectroscopy, differential scanning calorimetry and X-ray diffraction in detailed. It has been found that primary cast structure consisted of primary α (Al), lamellar eutectic structure η Mg(Zn, Cu, Al)₂ and a small amount of θ (Al₂Cu) phase. A transformation of primary eutectic phase from η Mg(Zn, Cu, Al)₂ to S (Al₂CuMg) was observed after 6 h of homogenisation treatment. The volume fraction of dendrite network structure and intermetallic phase was decreased with increase in holding time and finally disappeared after 96 h of homogenisation, which is consistent with the results of homogenisation kinetic analysis. Crystallographic texture of this alloy after casting and 96 h of homogenisation was also studied. It was found that casting process led the development of strong Goss, Brass, P and CuT components, while after homogenisation Cube, S and Copper components became predominant. Mechanical tests revealed higher hardness, yield strength and tensile strength for cast materials compared to homogenised alloys due to the presence of coarse micro-segregation of MgZn₂ phase. The significant improvement of ductility was observed after 96-h homogenisation, which was attributed to dissolution of second phase particles and grain coarsening. Fracture surfaces of the cast samples indicated the presence of shrinkage porosity and consequently failure occurred in the interdendritic regions or grain boundaries with brittle mode, while homogenised alloys failed under ductile mode as evident by the presence of fine dimple surfaces.     

Relative intensities in X-ray photoelectron spectra.: Part VIII

The photoionization cross-sections for the 3s and 3p shells of atomic Si, P, S, and Cl and the S²⁺ ion, and for the 2s and 2p shells of atomic F have been calculated using the random-phase approximation with exchange (RPAE) for the average-configuration term. Using the theoretical atomic cross-section values, the partial cross-sections for photoionization of the SF₆ molecule have been calculated for hv ? 54 eV and the photoelectron spectra have been interpreted. The calculation of relative intensities in the photoelectron spectra of H₂S is presented. The influence of the effective charge of an atom on the photoionization cross-section value for a molecular level is shown.     

Disorder in liquid Cu–Pd alloys

The disorder in thermodynamic and microscopic structure of liquid Cu–Pd alloy at 1350?K has been studied using regular associated solution model. For this, we have calculated free energy of mixing (G_M ), activity (a), concentration fluctuation in long wavelength limit [S_CC (0)] and chemical short-range order parameter (α ₁) of liquid Cu–Pd alloy at 1350?K. The energetic and structural asymmetry of liquid Cu–Pd alloys has been successfully explained on the basis of regular associated solution model.     

Normal and excess nitrogen uptake by iron-based Fe–Cr–Al alloys: the role of the Cr/Al atomic ratio

Upon nitriding ferritic iron-based Fe–Cr–Al alloys, containing a total of 1.50 at. % (Cr?+?Al) alloying elements with varying Cr/Al atomic ratio (0.21–2.00), excess nitrogen uptake occurred, i.e. more nitrogen was incorporated in the specimens than compatible with only inner nitride formation and equilibrium nitrogen solubility of the unstrained ferrite matrix. The amount of excess nitrogen increased with decreasing Cr/Al atomic ratio. The microstructure of the nitrided zone was investigated by X-ray diffraction, electron probe microanalysis, transmission electron microscopy and electron energy loss spectroscopy. Metastable, fine platelet-type, mixed Cr_{1? x} Al _x N nitride precipitates developed in the nitrided zone for all of the investigated specimens. The degree of coherency of the nitride precipitates with the surrounding ferrite matrix is discussed in view of the anisotropy of the misfit. Analysis of nitrogen-absorption isotherms, recorded after subsequent pre- and de-nitriding treatments, allowed quantitative differentiation of different types of nitrogen taken up. The amounts of the different types of excess nitrogen as function of the Cr/Al atomic ratio are discussed in terms of the nitride/matrix misfit and the different chemical affinities of Cr and Al for N. The strikingly different nitriding behaviors of Fe–Cr–Al and Fe–Cr–Ti alloys could be explained on this basis.