Metalloids and transition/refractory elements typically differ significantly in the electronic structure and atomic size, allowing for stronger solid-solution hardening in high-entropy alloys (HEAs) as well as improved work-hardening capability, which leads to exceptional strength-ductility synergy. In this regard, Si addition has opened up a new pathway for developing novel and high-performance HEAs, which has recently attracted considerable attention from the materials science community. Accordingly, the present review paper summarizes the recent progress in tailoring the mechanical properties and strengthening mechanisms of Si-added HEAs. After reviewing the general strengthening mechanisms of HEAs, the impact of Si addition is critically discussed, especially its effects on the (I) solid-solution hardening by local lattice distortion and chemical short-range order hardening, (II) second-phase strengthening by promoting the formation of disordered solid-solution phases, silicides, σ-phase, and other intermetallics, (III) structural refinement and facilitating the development of heterostructures, and (IV) work-hardening behavior by altering the dislocation arrangements, boosting the twinning-induced plasticity (TWIP) effect as well as HCP and BCC transformation-induced plasticity (TRIP) effect by reduced and variable stacking fault energy (SFE). Finally, the research gaps and future prospects are introduced, including metastability engineering, superplasticity, application of severe plastic deformation (SPD) techniques, and additive manufacturing. 

Tailoring the strengthening mechanisms of high-entropy alloys toward excellent strength-ductility synergy by metalloid silicon alloying: A review

NiTi alloy exhibits superior superelastic properties but its wide application is limited by its poor workability. In this study, Ni-rich NiTi wire was used as the feedstock and a wall-shaped NiTi component with height of 130 mm was deposited on Ti substrate by cold metal transfer (CMT) based wire arc additive manufacturing (WAAM) process. The microstructure and mechanical properties of the as-deposited NiTi alloy specimens in the horizontal and vertical directions, as well as their correlations were discussed in detail. In particular, the superelastic properties of the deposited materials were evaluated by cyclic loading-unloading tensile test. It was found that a very high superelastic recovery rate of more than 90% can be reached after 25 cycles for samples tested along both the horizontal and vertical directions. The mechanism that governs the high superelasticity in the alloy was explored in terms of microstructure, chemical composition and phase transformation analyses. 

High superelasticity NiTi fabricated by cold metal transfer based wire arc additive manufacturing

The toxic gas trimethylamine (TMA) with fishy odor can not only be used as an indicator for evaluating seafood spoilage, but also as a biomarker for trimethylaminuria and kidney disease. This paper constructs a ppb-level trimethylamine gas sensor that operates at room temperature based on MoO3/V2O5 nanocomposites, which were fabricated by hydrothermal synthesis process. The adsorption energy of TMA on sensitive materials was calculated through the density functional theory, and the sensing mechanism was explored by simulating the adsorption process of TMA on the sensitive materials. The response (11.37%) of MoO3/V2O5 composites to ultra-low concentration (20 ppb) TMA was better than that of pure MoO3 nanorods (5.49%) and V2O5 nanospheres (6.36%) at room temperature. Compared with pure MoO3 sensors, an n-n heterojunction is formed between MoO3 and V2O5, which enhances the sensing performance of pure sensing material for TMA. In addition, the detection of exhaled gas in simulated kidney disease patients indicates that this sensor has great potential as an effective TMA detection tool in the field of non-invasive disease diagnosis. 

Ppb-level detection of trimethylamine as biomarker in exhaled gas based on MoO3/V2O5 hierarchical heterostructure

Numerical simulation of arc-droplet-weld pool behaviors during the external magnetic field-assisted MIG welding-brazing of aluminum to steel

Study on oxygen vacancies in gallium oxide nanostructures

To improve the wear properties of magnesium alloys, a novel technique for preparing coatings using ultrasonic shot peening was developed, and tungsten carbide (WC) coatings were prepared for the first time on the magnesium alloys. The surface morphology, microstructure characteristics, and wear properties were investigated. The findings showed that this method deposited a WC coating with a thickness of 20 μm and a surface hardness of 3.24 GPa on the substrate. A 5 μm composite layer and a 425 μm thick hardened layer existed beneath the coating. The formation mechanism of the hardened layer can be interpreted as dislocation slip and grain refinement, resulting in a hardness of 1.96 GPa at the layer’s top. The treated sample exhibited excellent wear resistance due to the high hardness of the WC coating and hardened layer, with a wear volume of 0.737×108 μm3, representing a decrease of 49.52% and 35.29% compared to the untreated and ultrasonic shot peening one. The research provides a novel solution to improve the wear properties of magnesium alloys, and it contributes to enhancing magnesium alloys’ applicability in practical engineering fields such as aerospace, new energy vehicles, and biomedicine while also contributing to meeting the demand for lightweight manufacturing in these fields. 

Tungsten carbide coating prepared by ultrasonic shot peening to improve the wear properties of magnesium alloys

Microstructures and Mechanical Properties of Thick Ti–6Al–3Nb–2Zr–1Mo Joint by Magnetron-Controlled Narrow Gap TIG Welding

Ga2O3 with three types of surficial micro-nano structures were achieved by high-vacuum radio-frequency magnetron sputtering, i.e., thin film, nanowires and nano-hybrids containing nanorods, nanoparticles and tiny pores. All as-achieved Ga2O3-based gas sensors exhibited the peak responses at lower operating temperatures of 300–400 °C. The sensor fabricated on Ga2O3 with nano-hybrid structure performed the highest response of 19.95 at the ambient of 10% oxygen, the most stable cycling performance, and the fastest response speed of 0.7323 MΩ/s. The low-temperature response realized in this work was higher than those reported in previous literatures. The improved performances were owing to the greatest specific surface area and pore volume in the Ga2O3 nano-hybrids that increased the oxygen adsorption active sites and provided sufficient accesses for oxygen adsorption as well as desorption. This work revealed the potential applications of nanostructured Ga2O3 in low-temperature oxygen sensors. 

Enhanced low-temperature response of Ga2O3-based oxygen sensor by modulating the surficial micro-nano structures

In this study, we prepared TiAlCrNiSi high-entropy alloy films (HEFs) with different contents of four elements by multi-target magnetron co-sputtering with regulated sputtering power, and the elemental contents of the films all met the high-entropy range (5 at%−35 at%). Scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that the surface of the films was relatively smooth, the increase of sputtering power and Si element in the films reduced the roughness of the films. This demonstrates the effect of Si in refining the grains and reducing the roughness of the film. X-ray diffraction (XRD) showed that the average grain size of films decreases with the increase of Si content. This phenomenon proves that Si has the effect of refining grains and reducing the roughness of the film. The TC4 substrate and the films were treated at 700 ℃ for 50 h. It was found that TiAlCrNiSi HEFs could improve the high temperature oxidation resistance of TC4. The mechanism of oxidation resistance of the films was investigated by XRD, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), and it was found that the generation of dense oxides Al2O3, Cr2O3 and SiO2, the grain refining effect of Si and the appearance of amorphous structure were the main reasons for the increased oxidation resistance. 

Effect of Si content on the microstructure and high temperature oxidation resistance of TiAlCrNiSi high-entropy alloy films synthesized by multi-target magnetron co-sputtering

To recognize the specific safety problem brought by an increasing thickness, the layered microstructures, fatigue properties and the fractographies of a 100 mm-thick Ti–6Al–4V joint via electron beam welding were investigated. The results show that the seam zone has textural and mechanical heterogeneities in the layered thickness regions. The mid layer has the lowest fatigue life and different fracture-position tendency (more broken in seam zone) due to the hybrid influences of the grain size and the phase fraction of α′ and β phases. The fatigue cracks initiate on the surfaces of fatigue specimens and grow in the transgranular fracture mode. The fatigue deformation mode is adapted to the planar-slip mechanism with different degrees of dislocation blocking in distinct layers, which results in a “mid-layer favorite” crack initiation trend. Moreover, the mid layer has the highest crack propagation rate either.

https://link.springer.com/content/pdf/10.1007/s12540-020-00719-4.pdf

Inhomogeneous Fatigue Characteristics of Welded Ti–6Al–4V Thick Plate by the Electron Beam Method

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