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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.

In this work, the as-cast Mg-rich Mg98.5Gd1Zn0.5 and Mg98.5Gd0.5Y0.5Zn0.5 alloys are prepared by the semi-continuous casting method, and their hydrogen storage performance and catalytic mechanisms are investigated by experimental and first-principles calculations approaches. The results show that the LPSO phases decompose and in-situ form the RE(Gd/Y)Hx(x = 2,3) nano-hydrides upon hydrogenation. These nano-hydrides not only serve as the in-situ catalysts to promote the hydrogen ab/desorption of Mg matrix, but also present the pinning effect to inhibit the growth of Mg/MgH2 grains during hydrogenation and dehydrogenation. Comparatively, the two alloys exhibit the similar hydrogen absorption kinetics, while the hydrogen desorption kinetics of Mg98.5Gd1Zn0.5 is superior to that of Mg98.5Gd0.5Y0.5Zn0.5. The first-principles calculations reveal that the GdH2 and YH2 hydrides exhibit different catalytic effects on weakening the bond strength of H–H within H2 and Mg–H within MgH2, which interprets well the differences in the hydrogen ab/desorption kinetics between Mg98.5Gd1Zn0.5 and Mg98.5Gd0.5Y0.5Zn0.5 alloys.

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.

In this work, the NiCu solid solutions with different Ni/Cu molar ratios were prepared and then doped into MgH2 through ball milling to modify its hydrogen sorption properties. The experimental results show that the Ni–25%Cu and Ni–50%Cu exhibit the superior catalytic effect over pure Ni, pure Cu and Ni–75%Cu on the dehydrogenation of MgH2. Among them, the 5h-milled MgH2/Ni–50%Cu system starts to desorb hydrogen at 205.8 °C, which is about 96.9 °C lower than that of as-milled pristine MgH2. Moreover, the MgH2/Ni-50% Cu system can release 5.14 wt% hydrogen at 300 °C within 15 min, while MgH2 can only release 2.92 wt% hydrogen under the same condition. More importantly, the dehydrogenated MgH2/Ni–50%Cu can reabsorb 4.37 wt% hydrogen even at 250 °C for 30 min. The enhanced hydrogen sorption kinetics of MgH2/Ni–50%Cu mainly attributes to the “hydrogen spillover” effect of in-situ formed Mg2Ni(Cu) phases. Further theoretical calculations reveal that the strong interactions between H and Ni/Cu result in the weakened stability of MgH2, decreased bond strength of Mg–H and enhanced dehydrogenation properties of MgH2. This research provides an important guidance for design multiple transition metals catalysts to improve efficiently hydrogen storage properties of magnesium-based and other metal-based hydrides.

Brazed diamond tools with active filler metals have attracted tremendous attention due to their high interfacial bonding strength, good formability and excellent mechanical performance. However, the intrinsic interfacial bonding mechanism and fracture behavior of metal/diamond remain unclear. Herein, we first investigate the effects of alloying elements Cr, B, and Si on the interfacial bonding strength and adhesive transfer of brazed diamond with Ni–Cr–B–Si filler alloy by first-principles calculations and experiments. The results show that the Cr exhibits a strong embedding ability at Ni/diamond interface relative to B and Si. Among them, the Cr effectively enhances the interfacial bonding strength due to the formation of strong Cr–C bonds and high work of separation of Ni/diamond interface, which is in good agreement with the well crystallized Cr–C carbides experimentally detected at Ni/diamond interface. The tensile simulation of Ni/diamond interface further verifies the strengthening effects of Cr on Ni/diamond interface since the fracture of diamond occurs prior to the debonding of Ni/diamond interface. The analysis of electronic structures indicates that the strong ionic bond interactions between Cr and C atoms at Ni/diamond interface intrinsically dominate the interfacial bonding properties of brazed diamond with Ni-based filler alloy.

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.

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, the as-cast and extruded Mg-4.7Y-4.1Nd-0.5Zr alloys were prepared, and their microstructures, hydrogen storage properties and catalytic mechanisms were systematically studied. The results show that the as-cast Mg-4.7Y-4.1Nd-0.5Zr alloy is mainly composed of Mg, Mg24Y5 and Mg41Nd5 phases, while the Mg41Nd5 phase in the extruded alloy is dissolved into Mg matrix due to homogenization. After hydrogenation, the in-situ formed nano-particles of rare earth hydride REHx are uniformly distributed on the surface of MgH2, which exhibit remarkable catalytic effects on the subsequent hydrogenation and dehydrogenation of Mg matrix. According to the isothermal hydrogen absorption and desorption kinetic testing, the Mg-4.7Y-4.1Nd-0.5Zr alloys in the two states both exhibit good hydrogen sorption kinetics. Compared with pure Mg, the apparent activation energies of dehydrogenation reaction of the as-cast and extruded alloys are reduced to 114 kJ mol−1 H2 and 109 kJ mol−1 H2, respectively, which should be ascribed to the catalytic roles of YH2 and NdH2 nano-hydrides. Comparatively, the extruded alloy presents a more excellent dehydrogenation kinetics, and it can release more than 6 wt% of hydrogen within 15 min at 350 °C. First-principles calculations reveals that the rare earth hydrides YH2 and NdH2 improve the hydrogen storage performance of Mg alloy by weakening the bonding strength of the H–H bond within H2 molecule and the Mg–H bond within MgH2.

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

Mg-Zn-Mn (ZM) alloy shows great potential in biomedical applications due to its biocompatibility and bio-essential element composition, as well as its favourable mechanical and degradation properties. This paper aims to explore the effect of Zn on corrosion behaviour of ZM alloys via finite element (FE) prediction and in-vitro testing. Microstructure analysis showed that Zn had the grain refinement effect, and the second phase of Mg-Zn between grains increased with the increase of Zn content, which improved the mechanical properties of the alloy significantly at the cost of acceptable reduction in plasticity. After a continuum damage mechanics (CDM)-based degradation model was applied to the FE package, the corrosion process of the ZM alloys was predicted. The results indicated that the grain boundary had poor corrosion resistance while the second phase facilitated delaying corrosion expansion. Furthermore, in-vitro tests were carried out and consistent results were obtained, i.e., the grain refinement made the entire corrosion process more uniform and severe corrosion in local areas was avoided, and the intergranular second phase was beneficial to delay the corrosion process. This study suggested that Mg-Zn-Mn alloy has satisfactory mechanical strength and controllable corrosion rate, which should be a promising candidate for future biomedical applications.

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.

In this work, three different states (as-cast, homogenized and ECAP processed) of Mg98.5Y1Zn0.5 alloys with high theoretical hydrogen storage capacity (7.2 wt%) were prepared and comminuted into chips by filing. Then their hydrogen storage properties and underlying mechanisms were systematically investigated through experimental and first-principles calculations approaches. The results show that the activation properties of these alloys are enhanced in the order of homogenized, as-cast and ECAP processed states, which is closely associated with the size of filed chips. After activation, the as-cast and ECAP processed samples exhibit the superior hydrogen adsorption/desorption kinetics compared with the homogenized one. Interestingly, the as-cast sample presents the similar or even better kinetics relative to the ECAP processed one. This should be attributed to the more uniform dispersion of in-situ formed YH2 nanohydride in the as-cast alloy, which remarkably boosts the reversible hydrogen storage properties of Mg matrix by the synergistic pinning and catalytic effects. First-principles calculations reveal that the doping of Y not only reduces the activation energy of H2 dissociation on Mg surface, but also decreases the hydrogen removal energy of H atom from MgH2 bulk, which accounts well for the excellent hydrogen sorption kinetics of Mg98.5Y1Zn0.5 alloys.

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.

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

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.

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