This study focused on the crystal and electronic structures of a newly discovered phase in the Al-Cr-Sc system. The latter two species do not mix in a binary alloy, but can be alloyed with aluminium ...in the vicinity of the Al2−xCrxSc composition, where 0.3 < x < 0.5. After preparation of the pure constituents via arc melting, high-temperature annealing at 990 °C for 240 h was required to achieve full mixing of the elements. A detailed characterisation of the crystal structure, alloy microstructure and stability was obtained using single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD), in addition to transmission electron microscopy (TEM), especially in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mode, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDXS) and differential scanning calorimetry (DSC) measurements. The crystal structure was refined to a hexagonal unit cell of the MgZn2 type, space group no. 194, P63/mmc, which belongs to the Laves phases family. Special attention was paid to the occupancy of the crystallographic sites that were filled by both Cr and Al atoms. First-principles calculations based on the density functional theory (DFT) were performed to investigate the electronic structure of this ternary phase. The total density of states (DOS) exhibited a pronounced sp character, where a shallow pseudo-gap was visible 0.5 eV below the Fermi energy that brought a small but definite contribution to the thermodynamic stability of the compound.
The development of advanced materials and technologies based on magnetocaloric Gd and its compounds requires an understanding of the dependency of mechanical properties on their underlying ...microstructure. Therefore, the aim of the study was to characterize microstructural inhomogeneities in the gadolinium that can be used in magnetocaloric refrigeration systems. Microstructures of magnetocaloric gadolinium cylinders were investigated by light microscopy and FE-SEM (Field Emission Scanning Electron Microscopy), EDS (Energy-dispersive X-ray Spectroscopy), and BSE (Back-scattered Electrons) in both the extrusion and the extrusion-transversal directions. XRD (X-ray Diffraction) analyses were performed to reveal the presence of calcium- and fluorine-based compounds. Metallographic characterization showed an oxidized and inhomogeneous microstructure of the cross-sections. The edges and the outer parts of the cylinders were oxidized more intensively on the surfaces directly exposed to the processing tools. Moreover, a significant morphological anisotropy of the non-metallic inclusions was observed. CaF inclusions act as active nucleation sites for internal oxidation. The non-metallic, Ca- and F-containing inclusions can be classified as complex calciumoxyfluorides. The solubility of Er and Yb in the CaF was negligible compared to the Gd matrix and/or the oxide phase. Lower mechanical properties of the material are a consequence of the lower structural integrity due to selective oxidation of surfaces and interfaces.
Improved sample wettability was obtained by oxygen plasma functionalization of pyrolytic graphite. The samples were exposed to highly dissociated oxygen plasma with the density of 1 x 10(16) m(-3), ...the electron temperature of about 5.5 eV and the density of neutral oxygen atoms of 8 x 10(21) m(-3) for 20 s. The surface wettability was measured by a contact angle of water drop. The contact angle dropped from original 112 down to about 1 deg .The functional groups were detected by XPS analyses. The survey spectrum showed a substantial increase of oxygen concentration on the surface, while high-resolution analyses showed additional oxygen was bonded onto the graphite surface in the form of C-O polar functional group responsible for the increase of the surface energy.
The present study investigates the effect of shot peening (SP) on the mechanical properties and surface roughness of 7075 aluminum alloy during different stages and conditions of heat treatment. The ...mechanical properties were determined by measuring Vickers microhardness profiles and residual stress profiles, while the amount of alloying elements present in the solid solution of the samples under different heat treatment conditions was determined by measuring the electrical conductivity. The results show that the increase in microhardness near the SP surface and the maximum compressive residual stresses are mainly related to the content of alloying elements in the solid solution. Surface roughness increases with increasing SP Almen intensity, and samples with the highest microhardness and residual stresses have the lowest surface roughness.
This study confirms that crystalline TiB2 particles can act as a potent site for the heterogeneous nucleation of the metastable icosahedral quasicrystalline phase (IQC) formed in cast Al-Mn-Si-Cu-Mg ...alloys. The results show that a primary metastable IQC phase nucleates on facets of TiB2 particles. Electron backscatter diffraction patterns and selected-area electron diffraction patterns confirmed that at least five orientation relationships (ORs) exist between the stable TiB2 and the metastable IQC phase. The IQC adopts different ORs with the crystallineTiB2 particles to form low energy interfaces by matching close-packed planes at the interfaces. In the IQC phase, the close-packed planes are the 5-fold and 2-fold. Microstructural analyses show that inoculation has a noticeable influence on the size and distribution of the primary IQC phase, which in turn has a noticeable influence on the compressive properties of the investigated alloys. Due to their exceptional mechanical properties, these alloys could become a prime candidate for lightweight applications, especially in the automotive and aerospace industries.
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•This work confirms for the first time that crystalline TiB2 inoculants can serve as an effective substrate for IQC phase nucleation.•EBSD and SAED patterns confirm that at least five orientation relationships exist between the close-packed planes of the phases.•The microstructural analyses show that inoculation has a noticeable effect on the size and distribution of the primary IQC phase.•The size and distribution of the primary IQC phase clearly affect the compressive properties.•As-cast, a compressive yield strength of 380 MPa, a compressive strength of 680 MPa, and a compressive fracture strain of 20 % were achieved.
This work deals with the influence of gallium addition on the constitution and formation of the phases in the Al-rich corner of the Al–Mn system during casting. Al-rich binary Al–Mn and ternary ...Al–Mn–Ga alloys were cast into a copper mould with a cavity diameter of 5 mm. The microstructures of the produced alloys were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, electron backscatter diffraction and X-ray diffraction. The characterization revealed the presence of αAl, Al6Mn, λ-Al4Mn, L-Al4Mn, icosahedral quasicrystals (IQCs) and decagonal quasicrystals (DQCs). As the amount of gallium in the synthesized alloys increased, the proportion of the phases varied and IQCs and DQCs were formed. Microanalysis revealed that IQCs contained more gallium and less manganese than DQCs.
•Binary Al–Mn and ternary Al–Mn–Ga alloys were cast in a copper mould and characterized by SEM, EDS, EBSD and XRD.•Characterization of the alloys shows that αAl, Al6Mn, λ-Al4Mn, L-Al4Mn, IQCs and DQCs are present.•The EDS microanalysis shows that more gallium is present in the IQCs than in the DQCs.•The results reveal that with the addition of gallium to the binary Al–Mn system, L-Al4Mn, IQCs and in some cases DQCs form.
Electrochemical water-splitting is a promising green technology for the production of hydrogen. One of the bottlenecks, however, is the oxygen evolution half-reaction (OER), which could be overcome ...with the development of a suitable electrocatalyst. Recently, non-noble metal, high-entropy oxides (HEO) have been investigated as potential OER electrocatalysts, but complex synthesis approaches that usually produce the material in powder form limit their wider utilization. Here, an innovative synthesis strategy of formulating a nanostructured (CoFeNiMnCr)
3
O
4
HEO thin film on a CoFeNiMnCr high entropy alloy (HEA) using facile electrochemical and thermal treatment methods is presented. The CoFeNiMnCr HEA serves as exceptional support to be electrochemically treated in an ethylene glycol electrolyte with ammonium fluoride to form a rough and microporous structure with nanopits. The electrochemically treated CoFeNiMnCr HEA surface is more prone to oxidation during a low-temperature thermal treatment, leading to the growth of a spinel (CoFeNiMnCr)
3
O
4
HEO thin film. The (CoFeNiMnCr)
3
O
4
HEO exhibits a superior overpotential of 341 mV at 10 mA cm
−2
and a Tafel slope of 50 mV dec
−1
along with remarkable long-term stability in alkaline media. The excellent catalytic activity and stability for the OER can serve as a promising platform for the practical utilization of (CoFeNiMnCr)
3
O
4
HEO.
Graphical abstract
•Revealed dominant core-shell mechanism for grain boundary diffusion processed permanent magnet.•A structure-chemistry-magnetic-property analysis of Nd-Fe-B magnet used for electric vehicles and wind ...turbines.•Gaining high efficiency of Nd-Fe-B via grain boundary engineering.•Importance of high coercivity permanent magnets for production of electric components on a macro and nanoscale.
We propose a dominant core-shell formation mechanism for grain-boundary-diffusion-processed (GBDP), Tb-treated, Nd2Fe14B sintered magnets. A depth-sensitive analysis of Tb-treated samples, relative to a non-GBDP Nd2Fe14B magnet, showed a 30% increase of the coercivity in the central part of the magnet. A structure-chemistry-magnetic-property analysis revealed the dominant GBDP mechanism. On the surface of the Tb-treated magnet, the Tb is released from the starting precursor following a cascade of chemical reactions between the Tb oxide and the Nd and/or the Nd-Fe-B. The released Tb diffuses along the grain boundaries, forming a core-shell structure. The calculated optimum concentration for a 30% increase in the coercivity was 50 ppm of Tb. Off-axis electron-holography measurements were used to quantitatively map the characteristic magnetic states of the samples, confirming a different magnetic domain structure in the shell than in the core. The magnetic induction in the core was found to be 26% higher than that of the shell, which has a lower magnetic saturation due to the presence of Tb. The results show that the measured increase in the coercivity is due to a structural effect, and not the magnetic contribution of the Tb. Our results pave the way towards grain-boundary-engineering studies that can be used to increase the coercivity of Nd-Fe-B magnets for e-mobility and eco-power applications.
This investigation deals with the influence of calcium, strontium and zinc on the formation of primary, metastable quasicrystals in Al-Mn-Si alloys as these can enhance the mechanical properties of ...alloys. The synthesized alloys were cast into a copper mould. The castings were characterized using standard tools for studying microstructure. The Al-Mn-Si-Zn-Ca-Sr alloy contained more of the primary icosahedral quasicrystalline (iQc) phase and less of the approximant α-Al-Mn-Si phase than the Al-Mn-Si alloy. The higher volume fraction of the primary iQc phase in the Al-Mn-Si-Zn-Ca-Sr alloy is explained by the reduction of the critical radius size for nucleation and by an increase in the number of nucleation sites for the iQc phase. Increased number of nucleation sites and higher volume fraction of the primary iQc are both related to a reduction of the surface tension for the Al-based melt caused by the presence of calcium and strontium. These two elements do not become incorporated into the iQc phase but have a large effect on the course of the solidification of the investigated alloys and their constitution at room temperature.
We prepared three Al–Mn-based alloys with different copper, magnesium and silicon contents by casting into cylindrical copper molds. All the alloys exhibited primary metastable quasicrystals (QCs). ...In order to confirm the presence of either primary decagonal QCs (dQCs) or icosahedral QCs (iQCs) and to determine their compositions, the castings were characterized by means of light microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDS), electron-backscatter diffraction and X-ray diffraction. The dQCs are present in the Al–Mn-based alloys containing copper. In the case of the combined presence of copper and magnesium, iQCs are present in the edge region and dQCs are present in the central region. In the alloy containing copper, magnesium and silicon, iQCs are present in the casting. The average metallic radius (AMR) and electron-to-atom ratio of these primary phases were calculated by taking into account the composition of these primary phases, as determined by EDS. The AMR shows different values in the cases of dQCs and iQCs. Equal mean values of the AMR were found in iQCs with markedly different compositions. Furthermore, all the metastable QCs in this work show electron concentrations close to 2.6.
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