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The effects of T4, T5, and T6 treatment on the microstructure and mechanical properties of the extruded Mg-4.3Gd-3.2Y-1.2Zn-0.5Zr (wt.%) alloy with a relatively low RE content (7.5 wt.%) were investigated. T4 treatment at 450–500 °C induces a gradual grain growth of α-Mg but an obvious transition of texture component from ⊥ED to ∥ED. Interdendritic LPSO phases are highly stable against annealing while intragranular ones experience dissolution and re-precipitation. After peak-ageing at 200 °C, the elongation of as-extruded and T4 samples is just slightly reduced or even increased due to the weak ageing hardening response. T5 sample exhibits an attractive combination of strength and ductility, with a tensile yield strength (TYS) of 303 MPa and elongation of 20.0%. The Hall–Petch relation for the alloys with or without ageing treatment has been estimated. Grain boundary strengthening rather than precipitation strengthening has the dominant contribution to TYS, and a modified equation is developed to predict grain boundary strengthening values for Mg-Gd-Y-Zn-Zr alloys which contain different Schmid factors for basal slip.

Magnesium hydride (MgH2) has been widely regarded as a potential hydrogen storage material owing to its high gravimetric and volumetric capacity. Its sluggish kinetics and high activation energy barrier, however, severely limit its practical application. Transition metal oxides (TMOs) have been extensively used as catalysts to improve the hydrogen storage performance of MgH2, but the low-valence transition metal (TM) ions, resulting from the reduction of TMOs accompanied by the formation of inactive MgO, have been demonstrated to be the most effective components. Herein, we theoretically and experimentally confirm that the doping of low-valence TMs into MgO could effectively weaken the Mg-H bonds and decrease the energy required for hydrogen desorption from MgH2, leading to superior catalytic activity compared to both TMOs and MgO. In particular, the apparent activation energy for the dehydrogenation of Mg(Nb)O-catalyzed MgH2 could be reduced to only 84.1 kJ mol-1, and the reversible capacity could reach around 7 wt.% after 5 cycles with a capacity retention of 96%. Detailed theoretical calculations confirm that the remarkable orbital hybridization between Mg(Nb)O and MgH2 promotes charge transfer from MgO to the MgH2 monomer, resulting in significantly weakened stability of MgH2, which could effectively enhance its hydrogen storage performance.

Compared to crystalline metals, metallic glasses (MGs) show an exceptional feature of improved ductility after being mechanically processed by wire drawing. However, its underlying mechanisms have not yet been fully elucidated. In this study, with the aid of atomistic simulations, wire drawing and subsequent tensile loading were performed on MG nanowires to systematically investigate the deformation mechanisms of MGs in wire drawing, the deformation-induced heterogeneities and their influences on the tensile ductility. The results revealed that the deformation mechanisms of MGs in wire drawing are closely associated with the area reduction ratio (R): at a small R of 4.7%, the area reduction is realized via shear transformations of atoms near the surface, leaving the core intact; while at a large R of 9.3%, it relies on the formation of multiple spatially distributed shear bands that redistributes the plasticity throughout the sample. The deformation-induced heterogeneities were understood through the detailed analysis of the resultant residual strain and stresses, the gradient rejuvenated amorphous structures, the unique free volume distribution and spatially distributed shear bands. Moreover, the tensile simulations revealed improved ductility synchronized with decreased yield strength of the drawn samples. The improved ductility is attributed to the synergistic effects of three beneficial factors: 1) The surface compressive residual axial stress leads to a shift of the yield sites from the surface to the core, suppressing the rapid formation of shear bands; 2) The rejuvenated structures near the surface constrain and accommodate the plastic deformation in the core; 3) The spatially distributed shear bands, generated at large R, serve as heterogeneous nucleation sites for highly dispersed plastic shearing. The findings provide a comprehensive elucidation of the deformation-induced heterogeneities of MGs in wire drawing and establish a physical relationship between these heterogeneities and mechanical properties, which can serve to interpret the experimental results.

In this paper, a novel vertically arranged double-electrodes A0 mode Lamb-wave AlN resonator on SOI substrate with a high electromechanical coupling coefficient and high figure of merit (FOM) is reported. The AlN resonator has a sandwich structure with aluminum and N-type doped silicon as electrode layers and a 500 nm thick AlN film as piezoelectric layer. The resonator has only two electrodes vertically arranged rather than horizontal interdigitated (IDT) electrodes which is common in conventional Lamb-wave resonators. The electrode gaps for the vertically arranged double-electrodes resonators are defined by AlN layer thickness rather than by photolithography for lateral field excitation resonators, which results in higher electric field strength and higher electromechanical coupling efficient ( k t 2 ). Compared with conventional thickness field excitation (TFE) resonators with floating bottom electrodes, the vertically arranged double-electrodes resonators have higher electric field strength as the potential difference is larger between the top electrode and bottom electrode than that between the IDT electrodes and floating electrode. As a result, a higher electromechanical coupling coefficient is achieved. Furthermore, the resonant frequency of the vertically arranged double-electrodes resonator presented in this work can be defined by photolithography by controlling the width of the silicon layer. The k t 2 of the vertically arranged double-electrodes resonator calculated from the measurement results of admittance versus frequency by numerically fitting with the Butterworth Van Dyke model shows an increase by 3.85 times, from 0.073% to 0.281% compared with conventional TFE resonators, and the FOM also increases by three times, from 2.66 to 7.99. This work provides a new structure to design future AlN Lamb-wave resonators on SOI substrate.

The aim of this study was to use degradation prediction and in-vivo evaluation to inspire a novel design of magnesium (Mg) implant having a coral-like open-cell porous interior and an outer solid casing. In this design, the porous interior acts as a bone-mimic channel for tissue infiltration and cell adhesion, while the solid casing enables better structural strength and integrity. Different porosities of porous interiors, combined with different wall thicknesses of outer casing, were designed. By implementing a continuum damage mechanics (CDM)-based biodegradation model into finite element simulations, the mechanical properties and degradation rates of the implant were predicted. The results showed that the implant with 70%–75% porosity and 0.5 mm wall thickness had the optimal structural strength and degradation rate. This implant structure was then fabricated. Compression tests and X-ray computed tomography (CT) scanning were carried out to investigate the material properties and the structural transformation of the implants respectively. Moreover, an in-vivo rabbit model was used to evaluate the degradation behaviours of the implant at different time points. The results showed that this novel Mg implant had a relatively sturdy material strength and the porous structure did benefit the ingrowth of bone tissue and expedite the healing process. Keywords: Open-cell porous Mg alloys, Continuum damage mechanics, Biodegradation model, Finite element simulation, X-ray computed tomography, In-vivo rabbit model

Bimetal-layer surfacing intensifying the working area of die by welding materials with gradient mechanical properties is an effective fabrication and refabrication technology for hot forging dies with low cost and a short cycle. Owing to the extremely high operating temperature and large stress of large hot forging dies, the selection of welding materials is quite important. In consideration of the excellent high-temperature performance of cobalt-based superalloy, a cobalt-based superalloy welding material, Co03 was designed in this study to be combined with ferrous alloys with outstanding low-temperature mechanical properties and low price to realized effective refabrication of large hot forging dies by bimetal-layer surfacing. Refabrication of dies and subsequent hot forging processes were simulated physically by bimetal-layer welding conducted on 5CrNiMo blocks and heat treatments. Compared with the combination of two ferrous alloys (JXHC30 and JXHC15), the combination of Co03 and JXHC15 exhibited more stable microstructure and mechanical properties in extreme operating conditions. To verify the feasibility of the combination of JXHC15 and Co03 in refabrication of hot forging dies through bimetal-layer surfacing, a pair of large hot forging dies were refabricated and employed in forging production. The service lives of refabricated dies were improved by dozens of times, compared with the former homogeneous 5CrNiMo dies.

The combined effects of LPSO orientation and α-Mg texture on tensile anisotropy were investigated for an extruded Mg-Gd-Y-Zn-Zr alloy with a typical bimodal microstructure, a strong basal fiber texture, and directionally arranged LPSO phases. Results indicate that the extrusion direction (ED) exhibits the optimal mechanical properties in all aspects. The 45° direction presents a high elongation, while the transverse direction (TD) was endowed with the inferior strength and elongation simultaneously. The LPSO orientation contributes to the strength anisotropy via different load-bearing strengthening effect along different tensile axis. Meanwhile, the deformed α-Mg grains, which exhibit stronger basal fiber texture and more prismatic slip when loaded along ED, account for the rest contribution to strength anisotropy via the stronger grain boundary strengthening effect. However, LPSO orientation rather than α-Mg texture accounts for the ductility anisotropy, since microcracks tend to initiate at the α-Mg/LPSO interface and LPSO orientation modifies the fracture mode via affecting the initiation and propagation of microcracks.

Inducing residual stress in metallic glasses (MGs) is recognized to be beneficial for plasticity but the mechanisms on how it affects shear band nucleation, propagation and multiplication remain poorly understood. With the aid of experimental and computational approaches, we address this issue by comparatively studying the deformation behavior of two types of MG samples, which were individually prepared by surface imprinting and photo-chemical etching but having similar surface patterns. Results showed that the imprinted MGs exhibit simultaneously enhanced plasticity and compressive strength, while the etched ones show limited plasticity improvement and reduced strength. The enhanced mechanical properties of the imprinted MGs are attributed to the compressive residual stresses generated near the surfaces, rather than the resultant geometrical pits. Finite element analysis revealed that the residual stress induces obvious stress gradient and inhomogeneous plastic deformation, which facilitate heterogeneous nucleation of multiple shear bands near the surfaces. Complementary atomistic simulations further revealed that the stress gradient resulting from the residual stress slows down the shear banding dynamics and causes deflection and branching, which consequently promotes shear band multiplication during propagation. This work uncovers the interactions between the residual stresses and shear bands, which are useful for processing MGs with desirable mechanical properties.

As one aspect of LPSO morphology, the volume fraction of block LPSO phases (VFLPSO-B) was tailored by changing the total content of solute atoms in Mg-Y2x-Znx (x = 1, 0.5, and 0.25 at.%) alloys. The effects of VFLPSO-B on compression behavior, dynamic recrystallization (DRX), and workability were investigated. The results reveal that increasing VFLPSO-B leads to higher flow stress, hardness, DRX ratio, and better workability which is embodied in the lower deformation activation energy (Q), broader deformable conditions, and narrower improper processing domains. Kink bands and initial grain boundaries are ideal sites for the nucleation of DRX grains during compression. Block LPSO phases near the initial grain boundaries further promote the nucleation of DRX grains but inhibit their growth, which contributes to increasing the DRXed grain size exponent (m). This paper reveals the feasibility of improving the workability of LPSO-containing Mg alloys by modifying the LPSO morphology to broaden their applications.

Ni-based filler alloys are widely used in brazed diamond tools due to its high hardness, good wear resistance and favorable wettability for diamond. However, the fundamental understanding of microscopic interactions between Ni-based filler elements and diamond is still lacking. Herein, the adsorption and diffusion behaviors of Ni, Cr, B and Si elements contained in Ni-based filler alloy on diamond surface are systematically investigated through first-principles calculations. The results indicate that Ni and Cr atoms prefer to adsorb at hollow (H) site on diamond (111) surface, while B and Si atoms prefer top2 (T2) site. Among them, Cr atom presents the stronger adsorption and diffusion abilities on diamond (111) surface, which provides internal conditions for Cr to migrate towards interface between diamond and filler alloy, resulting in enhanced interfacial bonding. The co-adsorption of Cr, or B, or Si with Ni is conductive to the enhancement of adsorption ability for matrix element Ni on diamond surface. In addition, the diffusions of Ni, Cr, B and Si atoms into diamond (111) slab are shown to be much more difficult than those on diamond (111) surface, which may be originated from the dense surface structure of diamond and the stronger bonding interactions between the 1st C bilayer and adsorption atoms. Finally, the differences in adsorption abilities of Ni-based filler elements on diamond (111) surface are analyzed from aspect of electronic structures. The results could provide an important theoretical guidance for optimizing and designing filler alloy systems for brazed diamond tools.

This paper presents the concept of using a representative volume element (RVE) in a multiscale approach to predict the macroscopic stress–strain behaviour of a cast SS316L specimen under tension up to the point prior to necking. RVE models with various micro-void spatial configurations were built, and the effects of micro-voids and strain rate on the material properties (e.g., yield strength, ultimate tensile strength (UTS), ultimate tensile strain and strain hardening coefficient) were analysed. The spatial configuration of the micro-voids inside the cast SS316L specimen was acquired by the X-ray CT scanning system and each micro-void in the gauge length part was converted into a matching RVE model in the finite element (FE) analysis. Response surface methodology (RSM) was employed to investigate the effect of RVE configurations, i.e., the size of the RVE and the shape and spatial location of the micro-voids, on the material properties (yield strength and UTS) of the cast SS316L specimen at the macroscopic level, and then the optimal levels of the RVE configuration were determined. The stress–strain curve from the simulation did show a good agreement with the experimental results and hence the proposed concept was verified. Keywords: Multiscale approach, Representative volume element, X-ray computed tomography, Finite element method, Response surface methodology

Wrinkling defects commonly occur in tube hydroforming (THF) magnesium (Mg) alloy at elevated temperatures when the tube-end and axial-feeding regions of the workpiece are overheated. Most previously proposed methods for preventing such defects have been applied at room temperature and restricted by several limitations. Therefore, this paper presents a breakthrough in tool design through the appropriate control of temperature distribution of the Mg alloy AZ31B tubular material to minimise the wrinkling defects in THF at evaluated temperatures. The proposed cost-effective, simple and user-friendly collet-type device design was able to provide a non-isothermal condition for THF within an appropriate pre-heating time after die closing. An axisymmetric barrel-shaped component was taken as a prime example to demonstrate the methodology, in which various thermal potential differences between the axial-feeding and deformation regions were investigated using finite-element (FE) simulation so as to evaluate the wrinkling effects under various non-isothermal conditions. The results showed that the most satisfactory component could be obtained when the average temperatures of axial-feeding and deformation regions were around 240 and 330 °C, respectively. Subsequently, with the same approach, a wrinkle-free non-axisymmetric tubular bike-frame component was hydroformed successfully as a more realistic and practical application example. Keywords: Tube hydroforming, Wrinkling, Elevated temperatures, Magnesium alloy, Tool design, Non-isothermal condition