Achieving Superior Strength and Ductility of AlSi10Mg Alloy Fabricated by Selective Laser Melting with Large Laser Power and High Scanning Speed

AlSi10Mg alloy was prepared by selected laser melting (SLM) in a high laser power range 300-400 W. The effects of energy density on the relative density, microstructure and mechanical properties of the SLMed AlSi10Mg alloy were studied. The results showed that the SLMed AlSi10Mg alloy fabricated at a laser power of 400 W and a scanning speed of 1800 mm/s had a relative density of 99.4%, a hardness of 147.8 HV, a tensile strength of 471.3 MPa, a yield strength of 307.1 MPa, and an elongation of 9.6%, exhibiting excellent comprehensive mechanical properties. The unique combination of the melt pool structure and microstructure caused by the large laser power and fast scanning was responsible for the excellent performance. The wide and shallow melt pool structure with few defects and proper overlapping between the continuous melt pools were obtained. The growth of columnar crystals was inhibited by a large proportion of equiaxed grains formed at the border of melt pools, and numerous sub-structures were observed within the α-Al grains. This study provided a more efficient process parameters for the preparation of the SLMed AlSi10Mg alloy. The enhanced mechanical property will help to broaden the application of the AlSi10Mg alloy in industry.     

Growth chemistry of amorphous silicon and amorphous silicon–germanium alloys

It is generally found that the optimum growth of a-Si:H and a-(Si,Ge):H films and devices depends critically upon the particular growth technique, and the particular growth parameters used to grow the film. Different techniques give very different results, which are sometimes contradictory. The standard model for growth assumes that the Si surface is mostly covered with H bonds, and that growth takes place primarily from silyl radicals. The model assumes that excess surface H is eliminated by a silyl radical, and that the adjacent or wing bonded H atoms are eliminated by a spontaneous bond breaking and H₂ formation. In this paper, we show that this model is thermodynamically incorrect, and that it does not explain many of the experimental data, such as why bombardment with inert gases reduces H content, and why material grown at higher growth rates is more unstable. Rather, we suggest that the fundamental limitation to the growth of a-Si:H and a-(Si,Ge):H is the elimination of excess H, both from the surface and from the bulk. The excess H is not eliminated by spontaneous reactions, nor by interactions with the silyl molecule. Rather, it is eliminated by interactions with free H radicals and ions. Inert ions, such as He and Ar, can accelerate the desorption of H from the surface. Atomic and ionic H can diffuse into the material, and also remove subsurface excess H, reforming the Si microstructure. We also show that the influence of ion bombardment is critical for growing high quality a-(Si,Ge):H alloys, and that deposition conditions that lead to low ion bombardment flux can produce poor materials.     

Chemical compatibility of Al2O3-added glass sealant with bare and coated interconnect in oxidizing and reducing environments

Chemical compatibility in oxidizing and reducing environments between sealants and interconnects for planar solid oxide fuel cells (SOFC) are investigated. (Co,Mn)₃O₄, Co-Mn, and Al coatings were prepared on the FeCr-based ferritic-stainless-steel (SUS430). The (Co,Mn)₃O₄ coating exhibited water vapor bubble formation at the outer sealing edges when exposed to H₂, indicating reactions between the coating and sealant. Co-Mn metal coating was found readily oxidized in an oxidizing environment. The glass with 5 wt% Al₂O₃ addition helped mitigate the Cr diffusion into sealant based on bare SUS430 substrate. Moreover, the Al metal layer effectively blocked the diffusion of Cr into the sealant at the interface and environment for 100 h, with a thickness of only 1 µm. At the constant discharge current of 24 A, the voltage is stabilized at about 4.2 V for 48 h using H₂ as fuel gas in 5-cell stack.     

Fast joining of α/β-SiAlON ceramic with WC-8Co cemented carbide using Ti-Cu foils by spark plasma sintering

The α/β-SiAlON ceramic was joined to WC-8Co cemented carbide with Ti-Cu by spark plasma sintering. Interfacial microstructure and phases of the joints were studied. The effects of joining temperature, holding time, and pressure on the shear strength of the α/β-SiAlON/WC-8Co joints were investigated. The typical interfacial microstructure of α/β-SiAlON/Ti-Cu/WC-8Co joint was α/β-SiAlON/TiN + Ti₅Si₃ + TiCu + TiCu₂ + Cu + Cu-Co solid solution/WC-8Co. In comparison with pressure, temperature and holding time had greater impacts on the degree of interfacial reaction during the joining process. When the joining temperature, holding time, and pressure were 900 °C, 5 min and 40 MPa, the highest shear strength (246.3 MPa) of α/β-SiAlON/WC-8Co joint was achieved, which is qualified for the high-speed machining of superalloy.     

Optical and luminescent properties of quasi-stoichiometric YAG: Cr3+ ceramics

The impact of minor stoichiometric variations on the microstructure, optical characteristics, and luminescent properties of YAG:Cr ceramics, synthesized from chemically precipitated ceramic powders, was assessed for the first time. Transparent ceramics with over 70% transparency was produced with a nominal yttrium excess ranging from 0.47 to −1 mol.%. The phase composition, microstructure, and luminescent properties of quasi-stoichiometric YAG:Cr ceramics were examined, and the impact of stoichiometric deviations on the crystal lattice parameter and average grain size in ceramics was outlined. An examination of the optical characteristics of the ceramics revealed a specific absorption band in the case of yttrium excess. The effect of stoichiometry deviation on the luminescent properties of YAG:Cr ceramics was investigated. A change in stoichiometry from −1–0.47 mol.% excess yttrium resulted in a broadening of the luminescence R-line and a decrease in the lifetime of the excited state of Cr³⁺ from 1.91 to 1.81 ms.     

Grain-size effect on Cr3+ and F-centres photoluminescence in nanophase MgAl2O4 ceramics

Photoluminescence (PL) spectroscopy of transparent MgAl₂O₄ spinel ceramics with grain size between 100 and 300 nm was studied at 7 K temperature in the near-IR-VUV range of spectrum with synchrotron radiation excitation. The PL spectra were composed of optical transitions from spatially different regions of the ceramics, which analysis evidenced grain size effect on the emission line-shapes and intensities. In particular, emission of impurity Cr³⁺ ions, being structured in the crystalline bulk, became broad-band in the grain boundary regions, which was associated with respectively strong and weak local crystalline fields. It was observed that (i) excitons and F centres transfer energy to Cr³⁺ and (ii) Cr(²E_g)/Cr(⁴T_2g) and F-centres/Cr³⁺ PL intensity ratios underwent a linear dependence on the grain size.     

High-efficiency electromagnetic wave absorption of TiB2-SiCnws-SiOC synthesised using PDCs

A designed conductive nanoheterogeneous structure can boost interfacial polarisation and effectively enhance the wave absorption properties. Herein, maze-shaped nanoheterogeneous TiB₂-SiCnws-SiOC ceramics were synthesised via a polymer-derived ceramics (PDCs) approach for constructing a three-dimensional reticular structure. The addition of TiB₂ not only compensates for the lower conductivity, but also facilitates the growth of curved SiCnws. The results show that the addition of 20 wt% TiB₂ leads to favourable microwave absorption performance, with a minimal reflection coefficient of − 55.1 dB at 8.9 GHz and an effective absorption bandwidth of 4.2 GHz. This can be ascribed to the synergistic effect among polarisation loss, conduction loss, and microwave multiple reflection and scattering due to the unique structure. This study may contribute towards establishing multi-loss mechanisms and controllable dielectric properties in PDCs.     

Microstructure and high-temperature resistance of Al2O3/CoNiCrAlY coatings by laser cladding

This work reports on systematic investigation of the microstructure and oxidation behavior of Al₂O₃/CoNiCrAlY coatings in-situ synthesized on a K438 nickel-based superalloy substrate via laser cladding. Scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction were used to evaluate the microstructure evolution, phase transition and high-temperature resistance. Results indicated that, as the Al₂O₃ fractions increased from 0 to 25 wt%, the average grain size of CoNiCrAlY-xAl₂O₃ coatings decreased from ∼100.23 μm to ∼42.56 μm, and the high-temperature resistance was effectively improved due to the formation of (Co,Ni)(Al,Cr)₂O₄. However, the other deleterious effect that shielding layers easily to be exfoliated with the excessive formation of (Co,Ni)(Al,Cr)₂O₄, caused a degradation in high-temperature resistance.     

Investigation on crystal orientations in EB-PVD YSZ coatings under different substrate rotation mode

The crystal orientations in YSZ coatings produced by electron-beam physical vapor deposition (EB-PVD) were analyzed. Two different coatings have been prepared by distinct substrate rotation mode, including S1 prepared by the conventional method and S2 prepared by controlling the rotation and oscillation of the substrate. The substrate rotation modes were implemented by a rotating holder. Based on investigations of crystallographic microstructure and texture characterization methods indexed with reference to be tetragonal lattice symmetry, it concluded that S1 has an [001] in-plane orientation, whilst S2 has an [001] out-of-plane orientation.