Acta Metallurgica Sinica-English Letters最新文献

In deaerated 0.05 M, 0.1 M, 0.2 M and 0.5 M K₂HPO₄ solutions with pH 9.5, AZ31 magnesium (Mg) alloy was subjected to potentiostatic polarization at − 0.8 V_SCE to deposit a phosphate conversion coating. The morphology, structure, chemical composition, and protective performance of the conversion coating during the formation process were characterized. The results showed that amorphous Mg(OH)₂ and MgHPO₄ and crystallized KMgPO₄·6H₂O (struvite–K) were successively deposited on the surface of AZ31 Mg alloy in the four solutions. MgHPO₄ was converted from Mg(OH)₂, while struvite–K was transformed from MgHPO₄. The distribution of Mg(OH)₂, MgHPO₄ and struvite–K along the thickness of the coating differs with K₂HPO₄ concentration. As the concentration of K₂HPO₄ increased, the coating time was gradually shortened, and the coating was gradually thinned. Meanwhile, the ratio of the nucleation rate to the growth rate of struvite–K crystal nuclei increased, resulting in a decrease in the size of struvite–K crystal. When the concentration of K₂HPO₄ increased from 0.05 to 0.1 M, the content of struvite–K increased, and the protective performance of the coating was enhanced. However, as the concentration of K₂HPO₄ continued to increase to 0.5 M, the content of struvite–K and the protective performance of the coating decreased.

This study develops novel Mg–Sn–In–Ga alloys as potential implant materials for orthopedic applications. The corrosion behavior of the Mg–Sn–In–Ga alloys was studied through mass loss measurements, hydrogen evolution measurements, electrochemical analysis, and corrosion morphology observations. The results show that the corrosion rate of the Mg–1Sn–1In–1Ga alloy was only 0.10 ± 0.003 mm/y after immersion in Hank’s solution for 15 days. This outstanding corrosion resistance was associated with the protective effect of the corrosion products. The increase in the Sn and Ga element content led to the precipitation of a large amount of Mg₂Sn and Mg₅Ga₂, which had a dominant effect on the corrosion rate in the Mg–5Sn–1In–2Ga alloy. These precipitates increased the current density and detached from the alloy surface during the corrosion process. This can lead to a weakened protective effect of the corrosion layer, and thus generate localized corrosion and an increase in the corrosion rate. The strength of the Mg–5Sn–1In–2Ga alloy was enhanced due to fine-grain strengthening and precipitation strengthening. The ultimate tensile strength and yield strength of the Mg–5Sn–1In–2Ga alloy were ~ 309 MPa and ~ 253 MPa, respectively.

The microstructure evolution and mechanical properties of Cu-bearing ferritic stainless steel with different Cr addition (Cr = 12, 15 and 17 wt%) were investigated. The phase transformation behavior under different cooling rate, Cu-rich precipitation behavior and its influence on the mechanical properties under different aging treatment are systematically characterized using dilatometry, differential scanning calorimeter (DSC) and transmission electron microscopy (TEM). The results indicated that the increase in Cr content narrowed the austenite phase region at high temperatures, affecting its microstructure under different cooling rates. The 12Cr-1.5Cu steel exhibited a fully austenitic phase region at high temperature and occurred apparent martensitic transformation after air cooling. Cooling rate significantly influenced the phase transition of the steels, and subsequently affected its mechanical properties. All three investigated steels showed higher strength and lower plasticity in air cooling condition compared to furnace cooling condition, due to the presence of martensite. After aging treatment, high number densities of Cu-rich precipitates were formed in steel matrix and the size of Cu-rich precipitates increased obviously with increasing aging temperature, while the tendency for number density was opposite. Fine and dispersed Cu-rich precipitates formed during low-temperature aging enhanced strength of the steels, while larger Cu-rich phases developed during high-temperature aging endowed greater ductility to the steels. Notably, the Cr content had no significant effect on the precipitation behavior of Cu-rich precipitation. These comprehensive results and analyses could provide a solid foundation for broader applications of Cu-bearing ferritic stainless steels.

Refractory high-entropy alloys (RHEAs) stand as promising candidates for the development of a new generation of high-temperature materials due to their exceptional mechanical properties. However, the inherent low ductility and high density of RHEAs have hindered their widespread adoption in industrial applications. In this study, an Nb₃₈Ti₃₅Al₁₅V₆Cr₄(TaHfMoW)₂ RHEA with a BCC/B2 dual-phase structure is successfully developed. The RHEA exhibits excellent specific yield strength of 162.4 MPa·g⁻¹·cm³ and 84.3 MPa·g⁻¹·cm³ at room temperature (RT) and 800 °C, respectively. It was found that the pinning effect of the B2 phase induces a gradual transition in dislocation slip mode from planar slip to cross slip, homogenizing the plastic flow and resulting in excellent compression plasticity (ε > 50%). Additionally, the B2 phase endows the alloy with excellent yield strength at high temperatures by precipitation strengthening. Moreover, the dominated a/2 < 111 > type dislocation contributes to the alloy's superior plasticity at high temperatures.

Y–Zr–O nano-oxide dispersion-strengthened (ODS) ferritic alloys have attracted increasing research efforts in recent years, for the enhanced nucleation and refinement of nano-oxides. Here, we report a first-principles exploration on the possible roles of Y₂Zr₂O₇ in helium management in Y + Zr and Y + Ti + Zr co-alloyed ODS ferritic alloys. Bulk phase calculations suggested that similarly as Y₂Ti₂O₇, Y₂Zr₂O₇ has a comparably strong capability for trapping helium at its interstitial sites. The equilibrium Fe/Y₂Zr₂O₇ interface structure was further predicted as the top-coordinated O-rich at the temperature range of interest. Vacancy and helium both segregate to the Y₂Zr₂O₇ interface, in preference to the Y₂Ti₂O₇ interface, the bulk interior of Y₂Zr₂O₇ and the grain boundaries. In this regard, Y₂Zr₂O₇ can be more effective than Y₂Ti₂O₇ in preventing vacancies and helium from reaching GBs. Based on these results, the profound implications for the helium tolerance of Zr-alloyed ODS ferritic alloys were discussed.

The complex stresses experienced by medical-grade porous metals in the physiological environment following implantation as bone repair materials necessitate a comprehensive understanding of their mechanical behavior. This paper investigates the effects of pore structure and matrix composition on the corrosion behavior and mechanical properties of pure Zn. Porous Zn alloys with varying pore sizes were prepared via vacuum infiltration casting. The results showed that addition of Mg elements and an increase in pore size were observed to enhance the strength and elastic modulus of the porous Zn alloy (41.34 ± 0.113 MPa and 0.58 ± 0.02 GPa of the C-Z3AM). However, corrosion tests indicated that specimens with smaller pores and the addition of Mg elements exhibited accelerated corrosion of porous Zn alloys in Hank’s solution. Electrochemical test results show the corrosion resistance rank in order of C-Z5A > C-Z3AM > N-Z5A > N-Z3AM. Additionally, the mechanical retention of porous Zn alloys in simulated body fluids was found to be significantly reduced by the incorporation of Mg elements and smaller pore sizes, the yield strength declines rates of C-Z5A, C-Z3AM and N-Z3AM after 30 days of immersion were 16.7%, 63.7% and 78.2%, respectively. The objective is to establish the role of the material-structure-corrosion-mechanics relationship, which can provide a theoretical and experimental basis for the design and evaluation of Zn and its alloy implanted devices.

In this study, the effects of defect, mean stress and lower loading are investigated for high cycle (HCF) and very high cycle fatigue (VHCF) behavior of Ti-6Al-4V alloy. It indicates that the S–N curve of Ti-6Al-4V alloy exhibits a linear decreasing trend or a plateau characteristic in HCF and VHCF regimes, which depends on the defect size and stress ratio. VHCF strength decreases with increasing the defect size, and it is irrespective of stress ratios. The fatigue crack initiates from specimen surface at R = −1 in both HCF and VHCF regimes. While the fatigue crack initiates from the subsurface or the interior of the specimen at R = 0.1 in VHCF regime. A sequence of lower stress amplitude below the fatigue strength at 10⁹ cyc has no or negligible influence on the fatigue life of 10⁵–10⁹ cyc. The lower stress amplitude in variable amplitude loadings does not affect the failure mechanism. The residual compressive stress relaxation is not observed after a large number of lower loadings under ultrasonic frequency fatigue test. Gerber formula and Goodman formula give dangerous predictions of VHCF strength for both smooth specimens and specimens with defects.

The strong corrosion resistance and corrosion behavior of the FeNiCoCrW_0.2Al_0.1 high-entropy alloy in 3.5 wt% NaCl solution was investigated. In order to explain the Cl⁻ induced degradation of different metal oxides on the surface of the passivate film, the energy required for the interaction of the corrosion oxidation products NiO, CoO, Fe₂O₃, and Cr₂O₃ surfaces with Cl⁻ is compared and calculated based on the assumptions of the point defect model and the density functional theory by using the electrochemical impedance spectroscopy and the X-ray photoelectron spectroscopy for the analysis of the mono-double-layer structure and elemental compositions of passivate film in the corrosion process. The combined experimental and simulation results showed that the alloy passivates naturally in air, forming a single passivation layer. The compositional layering of the passivation film in 3.5 wt% NaCl solution occurred with the increase of the contact time with NaCl. A double-layer passivation with a two-layered combinatorial structure was formed due to the imbalanced depletion of Co and Fe during corrosion, and that the dense outer structure of this high-entropy alloy, which was made up of NiO and Cr₂O₃, provided the predominantly high corrosion resistance. This paper provided a new perspective to study the strong corrosion resistance of FeNiCoCr-based high-entropy alloys.

Mg–1.2Y–1.2Ni (at.%) alloy was extruded at 400 °C with an extrusion ratio of 16:1 and different rates from 1 to 6 mm/s. The effect of extrusion rate on microstructure and mechanical properties of the Mg–1.2Y–1.2Ni alloy was systematically investigated. With the increase of extrusion rate, the average recrystallized grain size of Mg–1.2Y–1.2Ni alloy and mean particle diameter of Mg₂Ni phase were increased, while the density of geometrically necessary dislocation and the intensity of the basal texture were decreased. When extrusion rate increases from 1 to 6 mm/s, the tensile yield strength (TYS) of as-extruded Mg–1.2Y–1.2Ni alloy decreases from 501 to 281 MPa, while the elongation to failure increases from 1.5% to 6.2%. The Mg–1.2Y–1.2Ni alloy extruded at 3 mm/s obtained TYS of 421 MPa, the ultimate tensile strength (UTS) of 440 MPa and elongation to failure of 2.6%, respectively, exhibiting comprehensive mechanical properties with relatively good plasticity and ultrahigh strength. The ultrahigh TYS of 501 and 421 MPa was mainly due to the strengthening from ultrafine recrystallized grains, high volume fraction long period stacking ordered (LPSO) phases and high density dislocations.