Effect of addition temperature on dispersion behavior and grain refinement efficiency of MgO introduced into Mg alloy

The effect of addition temperature of MgO particles(MgOp)on their dispersion behavior and the efficiency of grain refinement in AZ31 Mg alloy was investigated.In addition,the grain refinement mechanism was systematically studied by microstructure characterization,thermodynamic calculation,and analysis of solidification curves.The results show that the grain size of AZ31 Mg alloy initially decreases and then increases as the MgOp addition temperature is increased from 720 to 810℃,exhibiting a minimum value of 136μm at 780℃.The improved grain refinement efficiency with increasing MgOp addition temperature can be attributed to the reduced Mg melt viscosity and enhanced wettability between MgOp and Mg melt.Furthermore,a corresponding physical model describing the solidification behavior and grain refinement mechanism was proposed.

Effect of Elastic Strains on Adsorption Energies of C,H and O on Transition Metal Oxides

Platinum(Pt)-based noble metal catalysts(PGMs)are the most widely used commercial catalysts,but they have the problems of high cost,low reserves,and susceptibility to small-molecule toxicity.Transition metal oxides(TMOs)are regarded as potential substitutes for PGMs because of their stability in oxidizing environments and excellent catalytic performance.In this study,comprehensive investigation into the influence of elastic strains on the adsorption energies of carbon(C),hydrogen(H)and oxygen(O)on TMOs was conducted.Based on density functional theory(DFT)calculations,these effects in both tetragonal structures(PtO_(2),PdO_(2))and hexagonal structures(ZnO,CdO),along with their respective transition metals were systematically explored.It was identified that the optimal adsorption sites on metal oxides pinpointed the top of oxygen or the top of metal atom,while face-centered cubic(FCC)and hexagonal close-packed(HCP)holes were preferred for the transition metals.Furthermore,under the influence of elastic strains,the results demonstrated significant disparities in the adsorption energies of H and O between oxides and transition metals.Despite these differences,the effect of elastic strains on the adsorption energies of C,H and O on TMOs mirrored those on transition metals:adsorption energies increased under compressive strains,indicating weaker adsorption,and decreased under tension strains,indicating stronger adsorption.This behavior was rationalized based on the d-band model for adsorption atop a metallic atom or the p-band model for adsorption atop an oxygen atom.Consequently,elastic strains present a promising avenue for tailoring the catalytic properties of TMOs.

Microstructure and mechanical properties of continuous drive friction welded Ti_(2)AlNb alloy under different rotational rates

Ti_(2)AlNb-based alloy was joined in a continuous drive friction welding machine under different rotational rates(500,1000 and 1500 r/min).The microstructure and mechanical properties of the joints were investigated.It is shown that the weld zone(WZ) is fully composed of recrystallized B2 phase,and the grain size decreases with increasing rotational rate.The thermo-mechanically affected zone(TMAZ) suffers severe deformation during welding,due to which most of original precipitation phase is dissolved and streamlines are present.In the heat affected zone(HAZ),only the fine O phase is dissolved.The as-welded joint produced using 1000 r/min has the best mechanical properties,whose strength and elongation are both close to those of the base metal,while the as-welded joint obtained using 500 r/min exhibits the worst mechanical properties.Post-weld annealing treatment annihilates the deformation microstructure and fine O phase precipitates in the joints,consequently improving the mechanical properties significantly.Decomposed α_(2) phase is a weakness for the mechanical performance of the joint since microcracks are apt to form in it in the tensile test.

镍硼及镍钼硼合金镀层的组织和性能研究

通过化学镀方法制备高硬度Ni B及Ni Mo B合金镀层。经XRD分析确认Ni B合金镀态镀层组织以非晶态为主 ,并混有含硼过饱和镍的固溶体。热处理后 ,Ni B和Ni Mo B两合金镀层的硬度分别高达Hv12 0 0和 140 0以上。Ni Mo B镀层组织及性能随Na2 MoO4 ·2H2 O浓度而变化 ;当浓度为 0 .6 0 4 g L时 。

Compressive response and microstructural evolution of in-situ TiB_(2)particle-reinforced 7075 aluminum matrix composite

The hot forming behavior,failure mechanism,and microstructure evolution of in-situ TiB_(2)particle-reinforced 7075 aluminum matrix composite were investigated by isothermal compression test under different deformation conditions of deformation temperatures of 300−450℃ and strain rates of 0.001^(−1)s^(−1).The results demonstrate that the failure behavior of the composite exhibits both particle fracture and interface debonding at low temperature and high strain rate,and dimple rupture of the matrix at high temperature and low strain rate.Full dynamic recrystallization,which improves the composite formability,occurs under conditions of high temperature(450℃)and low strain rate(0.001 s^(−1));the grain size of the matrix after hot compression was significantly smaller than that of traditional 7075Al and ex-situ particle reinforced 7075Al matrix composite.Based on the flow stress curves,a constitutive model describing the relationship of the flow stress,true strain,strain rate and temperature was proposed.Furthermore,the processing maps based on both the dynamic material modeling(DMM)and modified DMM(MDMM)were established to analyze flow instability domain of the composite and optimize hot forming processing parameters.The optimum processing domain was determined at temperatures of 425−450℃ and strain rates of 0.001−0.01 s^(−1),in which the fine grain microstructure can be gained and particle crack and interface debonding can be avoided.

Effect of double aging on mechanical properties and microstructure of EV31A alloy

To improve the ductility of a commercial Mg−rare earth alloy EV31A(Mg−3Nd−1.5Gd−0.3Zn−0.5Zr),a heat treatment method called double aging is explored,and its effect on mechanical properties and microstructure of the alloy is studied.Ultimate strength and elongation of the alloy can be increased to 288 MPa and 6.6%by the optimum double aging process,compared to 273 MPa and 4.9%after single aging.Time consumption of the aging process is also significantly decreased from 16 h(single aging)to 2 h.HAADF-STEM characterization shows that the primary precipitate isβ'phase,which is similar toβ'phase in Mg−Nd binary alloy.By double aging,theβ'phase is finer and more densely distributed compared with single aging,with approximately double density and half size,which explains the improvement in strength and ductility.

Decomposition kinetics of carbon-doped FeCoCrNiMn high-entropy alloy at intermediate temperature

Phase decomposition kinetics and the corresponding mechanical properties of the severe cold-rolled(SCRed) carbon-doped(1.3 at.%) equimolar FeCoCrNiMn high-entropy alloy(HEA) after being annealed at 500 ℃ were investigated. This single face-centered cubic(FCC) solid-solution HEA decomposed to M23 C6+L10, B2, and σ in chronological order. The formation kinetics of the L10, B2, and σ phases followed the Johnson-Mehl-AvramiKolmogorov(JMAK) equation. The yield strength of the HEA was 1520 MPa and increased to 1920 MPa after being annealed at 500 ℃ for 1 h, as a result of the formation of nanosized M23 C6 and L10. Both strength and ductility decreased after 2 d of annealing due to the increase of volume fractions and the coarsening of the M23C6 and L10 precipitates. From 4 to 32 d, the hardness was found to increase, which is ascribed to the rapid formation of the B2 and σ phases. From 32 to 64 d, the hardness increased further to finally reach about HV 760, with the FCC matrix almost exhausted to form the M23 C6, L10, B2, and σ phases. The results of this work may serve as a guide for the heat-treatment of carbon-doped HEAs.

Microstructure and corrosion behavior of as-homogenized and as-extruded Mg-xLi-3Al-2Zn-0.5Y alloys(x=4,8,12)

The microstructure and corrosion behavior of the as-homogenized and as-extruded Mg-xLi-3Al-2Zn-0.5Y alloys(x=4,8,12,wt.%)were studied.The results show that as the Li content increases from 4%to 12%,the matrix transfers from singleα-Mg phase,(α-Mg+β-Li)dual phase to singleβ-Li phase.A mixed corrosion feature of intergranular corrosion and pitting corrosion occurs in the Mg-4Li-3Al-2Zn-0.5Y and Mg-12Li-3Al-2Zn-0.5Y alloys.The former is related to the precipitated AlLi phase along the grain boundaries,and the latter is related to the high potential difference between the second phase and the matrix.The corrosion resistance of the as-extruded alloys is better than that of the as-homogenized alloys.The superior corrosion resistance of the as-extruded Mg-8Li-3Al-2Zn-0.5Y alloy with the lowest corrosion rate(P_(W)=(0.63±0.26)mm/a)is attributed to the more uniform distribution of second phases,the protectiveα-Mg phase via sacrificing theβ-Li phase and the relatively integrated oxide film.

Microstructure and electrochemical corrosion behavior of selective laser melted Ti−6Al−4V alloy in simulated artificial saliva

Ti−6Al−4V alloy was fabricated via selective laser melting(SLM)to improve its corrosion resistance for implant.The microstructure and electrochemical corrosion behavior were investigated using scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),electrochemical test and contact angle test.It can be found that the as-selective laser melted(as-SLMed)Ti−6Al−4V alloys showβcolumnar microstructure in building direction and nearly circular checkerboard microstructure in scanning direction,while the wrought and wrought+HT samples exhibit equiaxed microstructure.The as-SLMed Ti−6Al−4V alloy exhibits better corrosion resistance than the wrought and wrought+HT samples due to hydrophobicity,high grain boundary density and uniform distribution of alloying elements in simulated artificial saliva at 37℃.

Effects of nano-sized TiB_(2) particles and Al_(3)Zr dispersoids on microstructure and mechanical properties of Al-Zn-Mg-Cu based materials

The effects of TiB_(2) and Zr on the microstructure,aging response and mechanical properties of hot-extruded Al-Zn-Mg-Cu based materials were investigated and compared by multi-scale microstructure characterization techniques.The results showed that proper addition of TiB_(2) particles could refine grain size during solidification,promote dynamic recrystallization during extrusion,and inhibit grain growth during solution treatment.Meanwhile,Zr addition had minor influence on the grain refinement during solidification,but could effectively suppress recrystallization and grain growth compared with the Zr-free alloy.Furthermore,the TiB_(2) addition could simultaneously enhance the aging kinetics and peak-aged hardness of the materials.Comparatively,Zr addition could also improve the peak-aged hardness with minor effect on the aging kinetics of the materials.Finally,the quench sensitivity,elastic modulus and tensile properties of the materials were compared and studied.Specifically,the relationship between the microstructure and mechanical properties,and the strengthening mechanisms were discussed in detail.

Enhanced thermal stability and mechanical properties of high-temperature resistant Al-Cu alloy with Zr and Mn micro-alloying

The high temperature(HT)thermal stability and mechanical properties of Al-5%Cu(AC)and Al-5%Cu-0.2%Mn-0.2 Zr%(ACMZ)alloys from 573 to 673 K were systematically studied.The results displayed that micro-alloying additions of Zr and Mn elements have presented a significant role in stabilizing the main strengthening metastableθ′precipitates at a temperature as high as 573 K.Simultaneously,the HT tensile test demonstrated that ACMZ alloy retained their strength of(88.6±8.8)MPa,which was much higher than that of AC alloy((32.5±0.8)MPa)after the thermal exposure at 573 K for 200 h.Finally,the underlying mechanisms of strength and ductility enhancement mechanism of the ACMZ alloy at HT were discussed in detail.

Microstructure and mechanical properties of as-cast and solid solution treated Mg-8Li-xAl-yZn alloys

The effects of Al/Zn ratio(mass ratio) on microstructure and mechanical properties of the Mg-8 Li alloy were investigated. The results indicate that in the as-cast Mg-8Li-xAl-yZn(x+y=5) alloys(LAZ alloys), when the Al/Zn ratio is 1:4 and 2:3, the secondary phases are mainly AlLi and MgLiZn phases;when the Al/Zn ratio is 3:2 and 4:1, the secondary phases are mainly AlLi and MgLi2Al phases. The decomposition temperature of MgLiZn phase is about 300 ℃ and the decomposition temperatures of AlLi phase and MgLi2 Al phase are higher(~350 ℃). Solid solution strengthening is the main factor for the improvement of strength of Mg-8Li-xAl-yZn alloys. The Mg-8 Li-3 Al-2 Zn alloy after solution treatment at 350 ℃ for 4 h has the best comprehensive mechanical properties(yield strength of 272.5 MPa, ultimate tensile strength of 315.0 MPa and elongation of 3.4%) among the studied as-cast and solid solution treated Mg-8Li-xAl-yZn alloys.

Effect of thermal exposure on microstructure and mechanical properties of Al-Si-Cu-Ni-Mg alloy produced by different casting technologies

The effect of thermal exposure at 350 ℃ for 200 h on microstructure and mechanical properties was investigated for Al-Si-Cu-Ni-Mg alloy, which was produced by permanent mold casting(PMC) and high pressure die casting(HPDC). The SEM and IPP software were used to characterize the morphology of Si phase in the studied alloys. The results show that the thermal exposure provokes spheroidization and coarsening of eutectic Si particles. The ultimate tensile strength of the HPDC alloy after thermal exposure is higher than that of the PMC alloy at room temperature. However, the TEPMC and TEHPDC alloys have similar tensile strength around 67 MPa at 350 ℃. Due to the coarsening of eutectic Si, the TEPMC alloy exhibits better creep resistance than the TEHPDC alloy under studied creep conditions. Therefore, the alloys with small size of eutectic Si are not suitably used at 350 ℃.

A review of refractory high-entropy alloys

This work reviews recent progress in the alloy design,microstructure,and mechanical properties of refractory high-entropy alloys(RHEAs).What’s more,the underlying strengthening mechanisms and deformation behavior are discussed.Composed mainly of near-equimolar refractory elements,RHEAs have superior mechanical properties,especially at high temperatures.However,many of them have limited room-temperature ductility.Much work has been done to solve this trade-off,and some of the RHEAs have the potential to be used for high-temperature applications in the future.In addition to their mechanical properties,RHEAs have other attractive properties,such as biocompatibility and wear resistance,which are discussed.Finally,current problems and future suggestions for RHEAs are discussed.

Microstructures and properties in surface layers of Mg-6Zn-1Ca magnesium alloy laser-clad with Al-Si powders

Laser surface cladding with Al-Si powders was applied to a Mg-6Zn-1Ca magnesium alloy to improve its surface properties.The microstructure,phase components and chemical compositions of the laser-clad layer were analyzed by using X-ray diffractometry(XRD),scanning electron microscopy(SEM)and energy dispersive spectrometry(EDS).The results show that the clad layer mainly consists ofα-Mg,Mg2Si dendrites,Mg17Al12and Al3Mg2phases.Owing to the formation of Mg2Si,Mg17Al12and Al3Mg2intermetallic compounds in the melted region and grain refinement,the microhardness of the clad layer(HV0.025310)is about5times higher than that of the substrate(HV0.02554).Besides,corrosion tests in the NaCl(3.5%,mass fraction)water solution show that the corrosion potential is increased from-1574.6mV for the untreated sample to-128.7mV for the laser-clad sample,while the corrosion current density is reduced from170.1to6.7μA/cm2.These results reveal that improved corrosion resistance and increased hardness of the Mg-6Zn-1Ca alloy can be both achieved after laser cladding with Al-Si powders.

Effects of Li content on microstructure and mechanical properties of as-cast Mg-xLi-3Al-2Zn-0.5Y alloys

The effects of Li content on the microstructure and mechanical properties of the as-cast Mg−xLi−3Al−2Zn−0.5Y(LAZx32-0.5Y)alloys were investigated by XRD,SEM,TEM,hardness tester and universal testing machine.The results show that the matrix of the alloy transforms fromα-Mg toα-Mg+β-Li and then toβ-Li when the Li content increases from 4%to 14%(mass fraction).All LAZx32-0.5Y alloys contain AlLi and Al_(2)Y,while MgLi_(2)Al appears only in the alloy containing theβ-Li matrix.As the Li content increases,the content of AlLi and MgLi_(2)Al gradually increases,while the content of Al_(2)Y does not change much.As the Li content increases from 4%to 10%,the ultimate tensile strength and hardness of the as-cast LAZx32-0.5Y alloys gradually decrease while the elongation gradually increases.The corresponding fracture mechanism changes from cleavage fracture to quasi-cleavage fracture and then to microporous aggregation fracture.This is mainly attributed to the decrease ofα-Mg and the increase ofβ-Li in the alloy.When the Li content continues to increase to 10%and 14%,the yield strength,ultimate tensile strength and hardness of the as-cast LAZx32-0.5Y alloys gradually increase,while the elongation decreases sharply,which is mainly attributed to the nano-scale MgLi_(2)Al uniformly distributed in theβ-Li matrix.

Microstructure and mechanical properties of as-cast Mg-8Li-xZn-yGd(x=1,2,3,4;y=1,2) alloys

The as-cast Mg-8 Li-xZn-yGd(x=1 2, 3,4;y=1,2;wt.%)alloys were prepared in a vacuum induction furnace and their microstructure and mechanical properties were investigated. The results show that the increase of Zn content results in the volume fraction of W-phase(Mg3 Zn3 Gd2) increasing while that of Mg3 Gd phase decreasing. The strength of Mg-8 Li-xZn-1 Gd alloys is improved with the increase of Zn content,which is ascribed to the second phase strengthening of fine strip-like W-phase and the solid solution strengthening of Zn element.For Mg-8 Li-4 Zn-yGd alloys,the increase of Gd content leads to the appearance of coarse and discontinuous net-like W-phase, which decreases the strength. The Mg-8 Li-4 Zn-1 Gd alloy exhibits an optimum comprehensive performance with the yield strength, ultimate tensile strength and elongation of 154.7 MPa, 197.0 MPa and 12.4%, respectively. In addition,the aging behavior of the typical alloys was also investigated.

Structural evolution of Pt‐based oxygen reduction reaction electrocatalysts

The commercialization of proton exchange membrane fuel cells(PEMFCs)could provide a cleaner energy society in the near future.However,the sluggish reaction kinetics and harsh conditions of the oxygen reduction reaction affect the durability and cost of PEMFCs.Most previous reports on Pt-based electrocatalyst designs have focused more on improving their activity;however,with the commercialization of PEMFCs,durability has received increasing attention.In-depth insight into the structural evolution of Pt-based electrocatalysts throughout their lifecycle can contribute to further optimization of their activity and durability.The development of in situ electron microscopy and other in situ techniques has promoted the elucidation of the evolution mechanism.This mini review highlights recent advances in the structural evolution of Pt-based electrocatalysts.The mechanisms are adequately discussed,and some methods to inhibit or exploit the structural evolution of the catalysts are also briefly reviewed.

Effect of NaOH concentration on microstructure and corrosion resistance of MAO coating on cast Al-Li alloy

Ceramic coating was prepared on cast Al-Cu-Li alloy by micro-arc oxidation(MAO) in a silicate electrolyte solution with additives including Na2WO4 and Na OH. The effect of NaOH addition varying from 1 to 9 g/L on the microstructure and corrosion behavior of the coatings was investigated. The coating was characterized by XRD, XPS and SEM with respect to the composition and microstructures. The results show that NaOH promoted the formation of oxides in the reaction of MAO and contributed to the increase of layer thickness. Electrochemical tests(polarization curve and EIS) and mass loss test indicate that the corrosion resistance of the coated Al-Cu-Li alloy was improved with the increased addition of NaOH. Moreover, the results prove that the alumina coating formed in the electrolyte with 7 g/L NaOH showed the best corrosion resistance. The examination of the full immersion corrosion test presents the same result of corrosion resistance.

Effect of refining processes on inclusions and mechanical properties of cast Al-2Li-2Cu-0.2Zr alloy

The effect of different refining processes on inclusions and mechanical properties of cast Al-2Li-2Cu-0.2Zr alloy was investigated, including two-stage hexachloroethane (C2Cl6) refining process, two-stage rotating gas bubbling refining process and two-stage composite refining process. It was found that the two-stage composite refining process, which combined C2Cl6 and rotating gas bubbling, can significantly improve the melt purity and mechanical properties of cast Al-2Li-2Cu-0.2Zr alloy. Compared to the unrefined alloy, the volume fraction of gas porosity defects and slag inclusions decreased from 1.47% to 0.12%, and the yield strength, ultimate tensile strength and elongation of as-quenched alloy increased from 113 MPa,179 MPa and 3.9% to 142 MPa, 293 MPa and 18.1%, respectively. C2Cl6 was first utilized to degas and remove large size slag inclusions before lithium addition, and then the rotating gas bubbling was utilized to do the further degassing and remove the suspended fine inclusions after lithium addition. The two-stage composite refining process can take advantage of two methods and get the remarkable refining effect.