Acta Metallurgica Sinica-English Letters最新文献

Magnesium and its alloys have attracting rising attention as one of biodegradable metallic materials. However, the rapid corrosion and severe localized corrosion still hinder their extensive applications in clinics. In this study, micro-alloying of Ca (≤ 0.1 wt%) into Mg0.5Zn0.2Ge alloy developed in our previous work was explored to further enhance the corrosion resistance and alleviate the localized corrosion of the alloy. The results reveal that the addition of Ca leads to the transformation of the cathodic Mg₂Ge phase in Mg0.5Zn0.2Ca alloy into anodic MgCaGe phase in Ca-containing alloys, thereby changing the galvanic couples in alloys during immersion. The preferential dissolution of MgCaGe phase promotes the participation of Ca and Ge into the formation of corrosion products, resulting in the enrichment of Ca and Ge in the outmost of corrosion product layer, which stabilizes and passivates the corrosion product layer on Mg alloy surface. Additionally, the enrichment of Zn at the corrosion interface seems to further hinder the corrosion of Mg matrix. All of these factors confer a slower and more uniform corrosion on Mg0.5Zn0.2GexCa (x < 0.1 wt%) alloy, which provides favorable candidates for the further processing to gain suitable biodegradable Mg alloys.

In this work, a series of Co-based ternary Co–Er–B bulk metallic glasses (BMGs) with excellent soft magnetic properties and high strength were developed, and the local atomic structure of a typical Co_71.5Er_3.5B₂₅ metallic glass was studied through in situ high-energy synchrotron X-ray diffraction and ab initio molecular dynamics simulations. The results reveal that the BMG samples can be obtained in a composition region of Co_68.5–71.5Er_3.5–4B_25–27.5 by a conventional copper-mold casting method. The Co–Er–B metallic glasses possess stronger atomic bond strengths and denser local atomic packing structure composed of a higher fraction of icosahedral-like clusters but fewer deformed body-centered cubic and crystal-like polyhedrons, and they exhibit slower atomic diffusion behaviors during solidification, as compared to Co–Y–B counterparts. The enhancement in structural stability and the retardation of atomic-ordered diffusion lead to the better glass-forming ability of the Co–Er–B alloys. The smaller magnetic anisotropy energy in the Co–Er–B metallic glasses results in a lower coercivity of less than 1.3 A/m. The Co–Er–B BMGs exhibit high-yield strength of 3560–3969 MPa along with distinct plasticity of around 0.50%.

A refractory high entropy alloy Ti₆₂Nb₁₂Mo₁₂Ta₁₂W₂ was prepared by mechanical alloying and spark plasma sintering. The microstructure and mechanical properties of the Ti₆₂Nb₁₂Mo₁₂Ta₁₂W₂ alloy were analyzed. The experimental results show that the microstructure of the alloy is composed of two BCC phases, an FCC precipitated phase, and the precipitated phase which is a mixture of TiC, TiN and TiO. The alloy exhibits good room temperature compressive properties. The plasticity of the sample sintered at 1550 °C can reach 10.8%, and for the sample sintered at 1600 °C, the yield strength can be up to 2032 MPa, in the meantime the plasticity is 9.4%. The alloy also shows high strength at elevated temperature. The yield strength of the alloy exceeds 420 MPa at 900 °C, and value of which is still above 200 MPa when the test temperature reaches 1000 °C. Finally, the compressive yield strength model at room temperature is constructed. The prediction error of the model ranges from − 7.9% to − 12.4%, expressing fair performance.

La–Mg–Ni-based hydrogen storage alloys have excellent hydrogen storage properties. This work reports the hydrogen storage performance of a series of A₂B₇-type La_0.96Mg_0.04Ni_3.34Al_0.13 alloy and La_0.96-xY_xMg_0.04Ni_3.47–0.6xAl_0.6x (x = 0, 0.22, 0.33, 0.44) alloys, and explores the effect of Y and Al element combined substitution on the microstructure and hydrogen storage performance of A₂B₇-type La–Mg–Ni-based alloys. The alloys are composed of Ce₂Ni₇ phase and LaNi₅ phase. With the increase of x, the cell volume of Ce₂Ni₇ phase decreases, while that of LaNi₅ phase increases, indicating that Y atom mainly enters Ce₂Ni₇ phase and Al atom mainly enters LaNi₅ phase. An appropriate amount of co-substitution increases the hydrogen storage capacity and reduces the hydrogen absorption/desorption plateau pressure hysteresis of the alloy. When x = 0.44, the hydrogen storage capacity of the alloy is 1.449 wt%, and the hysteresis coefficient is 0.302. The cell volume of Ce₂Ni₇ phase and LaNi₅ phase expands to different degrees after 20 absorption/desorption cycles. With the increase of x, the volume expansion rate decreases, and the cycle capacity retention rate also gradually decreases. This is related to the amorphization of Ce₂Ni₇ phase. When x = 0.22, the capacity retention rate of the alloy is 91.4%.

There are nanotwins in the shear band formed in a moment (about 10⁻⁵ s) in some NiCrFe-based medium-entropy alloys (MEAs), and these shear bands can be recognized as a special kind of materials due to their high strength and good plasticity. In this study, the single shear band of the NiCrFe MEA was prepared at 77 K. A series of characterizations were carried out to analyze the microstructures in the shear band. The strength of the shear band was investigated by the split Hopkinson pressure bar and in-situ compression. The micropillar in the shear band containing nanotwins exhibits excellent strength-plasticity synergy. The compressive yield strength of the shear band measured by in-situ compression is 175% higher than that of the matrix, reaching 1405 MPa, with the fracture strain exceeding 0.5. The strengthening mechanism of the shear band was revealed by the combination of the experimental results and molecular dynamics simulation. The synergistic effect of multiple strengthening mechanisms enhances the strength of the NiCrFe MEA containing nanotwins, in which the grain boundary strengthening of the ultrafine equiaxed grains and the dynamic Hall–Petch effect of the nanotwins dominate. In addition, the good plasticity of the shear band is ascribed to the stress concentration reduction of the twin boundaries of nanotwins and the activation of multiple slip systems due to the randomly oriented nanotwins. These findings provide theoretical guidance for the design of nanotwinned MEAs to realize excellent strength-plasticity synergy for structural materials.

For the evaluation of cavitation erosion resistance of metallic materials, it has always been a time-consuming technical problem. This paper innovatively proposed a parameter named cavitation hardening rate to characterize the plastic deformation and work hardening behavior of metallic materials during the cavitation process and then found a positive correlation between cavitation hardening rate and strain hardening index. Finally, the relationship between cavitation volume loss and strain hardening index was obtained, and a simple and rapid evaluation method for cavitation erosion resistance was established.

In this work, a good combination of strength and ductility is achieved in a Mg-13Gd-0.2Ni alloy by conventional extrusion and following aging treatment. The aged Mg-13Gd-0.2Ni alloy exhibits a yield strength (YS) of 363 MPa, an ultimate tensile strength (UTS) of 433 MPa, and an elongation of 11.9%. The aged Mg-13Gd-0.2Ni alloy contains a microstructure with a 95% proportion of dynamically recrystallized (DRXed) grains with a micron size, weak texture, and a high number density of prismatic β′ precipitates within grains. The bulk compounds enriched with Ni element which are mainly formed during casting and stable in the extruded and aged samples. Few dynamic compounds are formed during extrusion, and a high density of prismatic β′ precipitates are formed during aging. The high density of prismatic β′ precipitates results in a significant increase in the strength of the Mg-13Gd-0.2Ni alloy, and the YS and UTS are increased by 153 and 136 MPa, respectively. The high proportion of DRXed grains with a weak texture contributes mainly to the high ductility and the fine compounds with a low density at grain boundaries formed during aging have no significant adverse effect on ductility of the aged Mg-13Gd-0.2Ni alloy. These findings for the novel Mg-Gd-Ni alloy can provide guidance for the design of wrought Mg alloys with superior mechanical properties.

The corrosion behavior of the pre-oxidized GH4169 alloy was studied after 20 h of exposure under a solid NaCl deposit film in a wet O₂ environment at 600 °C by mass-gain measurements, X-ray diffraction, scanning electron microscopy, transmission electron microscopy methods. The results indicate that the pre-oxidized GH4169 alloy undergoes serious corrosion in the corrosive condition. The preformed Cr₂O₃ layer is gradually destroyed by deposit NaCl, and the inner Nb₂O₅ layer beneath the complete outer Cr₂O₃ layer also reacts with NaCl to form NaNbO₃. The electrochemical test results testified the existence of electrochemical reactions during this corrosion process. The corrosion behavior of pre-oxidized GH4169 under a solid NaCl deposit film in a wet O₂ environment at 600 °C has been discussed in detail.

Here, the composition of TaMoNbZrTiAl refractory high entropy alloy (RHEA) is optimized by increasing Ti content to improve its mechanical property especially the ductility, through comparing two RHEAs with different Ti content. The RHEAs contain two body-centered-cubic (BCC) phases. The BCC phase in the dendritic region is rich in Ta, Mo and Nb, and the BCC phase in the interdendritic region is enriched in Zr, Ti and Al. The as-cast RHEA with a higher Ti content remains dendritic microstructures, and Ti is mainly enriched in the interdendritic region. After annealing treatment at 1300 °C for 48 h, the dendritic microstructures change into equiaxed-grain morphology, accompanied by needle-like micron precipitates at grain boundaries in the RHEA with higher Ti content. For the as-cast RHEAs, the fracture strain increases by ~ 6.6% and the uniform plastic strain increases by ~ 5.9% at the compression test due to the increase of Ti content. Our work offers a reference for the composition design of RHEAs and makes a preliminary exploration of the optimization of the microstructures and mechanical properties.

Exfoliation corrosion (EFC) has rarely been reported for wrought magnesium alloys. Enhanced EFC of the as-extruded Mg–1Li–1Ca alloy was observed after the removal of outermost fine-grained skins. The microstructure and corrosion products were investigated using optical microscopy, scanning electron microscopy, electron backscattered diffraction, X-ray diffraction, and Fourier transform infrared spectrometry. The corrosion behavior was analyzed using electrochemical polarization, electrochemical impedance spectroscopy, and scanning Kelvin probe techniques. The results indicated that the surface of Mg–1Li–1Ca, with the superficial layer removed, was more susceptible to EFC. A linear correlation between grain boundary density and corrosion resistance was established. Additionally, the influence of factors such as grain size, intermetallic compounds, different alloy surfaces, and corrosion products on EFC were discussed, and the corresponding EFC mechanism was clarified.