查看更多>>摘要：? 2022The reliable and convenient quantitative phase analysis (QPA) of titanium alloys is a meaningful work. In this study, the relative phase fraction of the bulk aviation serviced Ti-3Al-5Mo-4.5 V alloy was investigated by various methods, including Optical Microscope (OM), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Electron Back-Scattered Diffraction (EBSD) and X-ray diffraction (XRD). OM, SEM and TEM can scarcely do phase quantification. From the convincing results of EBSD test, the specimen was primarily constituted by fine, uniformly distributed and equiaxed α grains embedded in β matrix and α/β phase respectively accounted for 74.14% and 25.86%. The obstacles of Peaks overlapping and PO (Preferred Orientation) to XRD methods were greatly dealt with employing XRD whole pattern fitting method according to the 2θ diffraction pattern by careful scanning. Through initial fitting without PO correction, the calculated α/β phase fractions were 62.25% and 37.75%, respectively. The main PO could be inferred along the directions of (002)α, (101)α and (110)β. When modifying PO of these peaks stepwise utilizing March-Dollase function, the difference of the calculated profile to the observed one was gradually reduced to a low level and the calculated α/β phase components were respectively 73.04% and 26.96%. With the optimization of adding (200)β for PO correction or utilizing spherical harmonics function, the fitting quality was further improved and the quantitative results tended to stability. It was found that PO of α phase was suitable to be modified by eighth order spherical harmonics function because it has multiple diffraction peaks with complicated PO, while β phase adopting March-Dollase function. The final α/β phase contents determined were 73.65% and 26.35% respectively, which was greatly consistent with EBSD results.
查看更多>>摘要：? 2021Creep properties of a 4th generation nickel-based single crystal superalloy (Ni-SX) are investigated at 1150 °C and 1100 °C. Fracture mechanism is revealed as competitive microcrack initiation induced by topologically close-packed (TCP) phase formation and creep inconsistency on dendritic structures showing the strong temperature-and-time dependent relations. At 1150 °C, alloy displays the high nucleation rate of TCP phases. Dense TCP particles dissolve the surrounding γ/γ' microstructures forming large depleted zone where is easily stress-concentrated. Dislocations are found to emit from the interface between TCP and depleted zone that promotes dislocation shearing and microcrack initiation in the large depleted zone, as well as the fracture of alloy. On contrary, alloy crept at 1100 °C suffers to relatively restricted TCP nucleation. But the longer creep time gives to larger size of TCP precipitates. Compared to alloy crept at 1150 °C, the depleted zone is much smaller that is not prone for stress concentration. For this creep inconsistency induced by dendritic segregation plays the main role in determining the creep properties. The larger local strain rate forms microcracks primarily in interdendritic region causing the final fracture.
查看更多>>摘要：? 2022 Elsevier Inc.In this paper, CoCrFeNiMo0.2 high entropy alloy (HEA) particles reinforced titanium matrix composites were prepared by hot pressing sintering in the vacuum. The microstructure and composition of the composites at 700 °C ~ 900 °C were studied through X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Also, the mechanical properties of the composites were studied by microhardness tester, nano-indentation and uniaxial compression experiment. The results show that the interface layer of different thicknesses is formed between the particles and the matrix in the composites under different sintering temperatures. At 700 °C ~ 850 °C, the mechanical properties of the composites increase with the increase of temperature, which is related to the diffusion of HEA particles and the formation of the interface layer. When the sintering temperature is 850 °C, the hardness is 402.6 HV, the yield strength is 928.2 MPa, and the compressive strength is 2032.6 MPa. However, the high entropy alloy particles have a greater degree of dissolution and diffusion at 900 °C, the strengthening effect of the particles is weakened, so the mechanical properties of the composites are correspondingly reduced.
查看更多>>摘要：? 2022 Elsevier Inc.Nickel-based single crystal (SX) superalloys are the key materials of turbine blades in aircrafts, that the microstructure evolution during high temperature creep has critical effects on the thermo-mechanical properties. Research on microstructure evolution and establishing the quantitative relationship between microstructure evolution and creep conditions are of great significance for service safety assessment of nickel-based SX superalloys. High generation nickel-based SX superalloys generally have high cost on account of precision casting and the addition of Re and Ru which are rare and precious metals. Hence, it is necessary to develop a rapid characterization and analysis approach in terms of microstructure features of nickel-based SX superalloys. In this study, we integrated high-throughput experiment, large-scale high-resolution characterization and high-throughput quantitative analysis techniques to establish an efficient method for investigating the microstructure evolution of nickel-based SX superalloys during high temperature creep. The high temperature interrupted creep tests were carried out on the variable section specimens with arc surface to acquire the microstructures which are changed with creep stress continuously. High-resolution SEM with ALTLAS module was employed to quickly characterize the large-scale microstructure throughout the universal stress scale. Based on U-Net deep learning algorithm, an automatic dendrite identification model was established to segment the dendrite region quickly and accurately. And then, the γ/γ’ microstructure parameters of dendrite region were continuously quantitated using a logical algorithm. The quantitative correlation between microstructure evolution and creep conditions of nickel-based SX superalloys could be established, which shows tremendous potential and significance in the service safety assessment of nickel-based SX superalloys.
查看更多>>摘要：? 2022 Elsevier Inc.A binary metastable β-type Ti-Nb alloy possessing large near-linear recoverable strain was designed to clarify the physical mechanisms responsible for high near-linear elastic deformability in near?oxygen-free Ti-based alloys. The in situ synchrotron X-ray diffraction (SXRD) investigation reveals that in addition to the inherent elastic deformation, a reversible β?α'' stress-induced martensitic transformation (SIMT) occurs during the near-linear elastic deformation. During the SIMT, the α'' martensitic variant 5 (V5) forms preferentially due to its maximum phase transformation strains among all 6 equivalent α'' variants. The orientation dependence of phase transformation strains of the preferential V5 leads to the appearance of different α'' diffraction peaks in the loading and Poisson's directions during the SIMT, which is beneficial for accommodating the external macroscopic deformation to the great extent. Our experimental results show that the high near-linear elastic deformability of present Ti-Nb alloy is attributed to a combined effect of the large inherent elastic deformation arising from low Young's modulus as well as a reversible SIMT that covers a wide stress range and exhibits small phase transformation strains limited by the lattice constants of β and α'' phases. These results could provide a comprehensive understanding of the mechanisms responsible for huge near-linear elastic deformability of β-type titanium alloys.
查看更多>>摘要：? 2022 Elsevier Inc.In this study, the effects of Cr addition and cooling rate on bainitic and martensitic transformations of 1000 MPa grade low carbon low alloy (LCLA) steels during continuous cooling were investigated, and the formation mechanism of C-rich martensite (CRM) was discussed. The appearance of CRM was closely related to the high concentration of C enrichment because of the incomplete formation of bainitic ferrite (BF). Most of CRM formed in the low temperature zone was distributed near the prior austenite grain boundaries (PAGB), with the maximum lattice imperfections, and the twinned substructure of high C martensite could be seen in it. With the same composition, the volume fraction and size of CRM were decreased as the cooling rate was increased. With the same cooling rate, the addition of Cr increased the volume fraction of CRM and reduced its size. Moreover, in the medium temperature zone, the addition of Cr and the increase in cooling rate lowered the transformation start temperature (TST), made the transformation after the formation of BF gradually transition to single martensitic transformation.
查看更多>>摘要：? 2022 Elsevier Inc.The lengthening of bainitic ferrite laths in a model alloy containing a carbon gradient was observed in-situ using a Laser Scanning Confocal Microscope (LSCM). Results show that the observed lengthening rates increase with increasing temperature and decreasing carbon content. Crystallographic analysis was conducted by ex-situ electron backscattered diffraction to understand the relationship between growth kinetics and orientation relationships (OR). The OR of bainitic ferrite with respect to austenite deviates from the exact K–S and N–W models. The deviation between the close parallel planes, ?θCPP, increased with increasing transformation temperature and carbon content. Angle deviations between the close parallel directions,?θCPD, also increased with carbon content, while changes with transformation temperature were very small. The classical Zener-Hillert approach provided a reasonable first-order description of the global kinetics, but could not account for the observed effects of crystallography. A better fit of the experimental results was obtained by introducing finite mobility that varied with OR of the interface. It suggests that the lengthening kinetics of an individual bainitic lath may be dependent on its crystallographic orientation with respect to the parent austenite.
查看更多>>摘要：? 2022 Elsevier Inc.Ferromagnetic 3d transition metal nanowires based on the assembly of shape anisotropy seem apt candidates to replace materials based on rare-earth elements in current permanent magnets. Recently, great efforts have been devoted into enhancing the coercivity of Co nanowires, through accurate control of shape and morphology. Nevertheless, stacking faults can be formed relatively easily in close-packed metals like Co, and its effect on the magnetic properties was usually obscured by the geometrical effect and being ignored. In this work, the Co nanowires with different aspect ratios were synthesized by solvothermal chemistry processes. The effect of aspect ratio on the magnetic properties of Co nanowires was investigated, and a high magnetic property with a coercivity of 9 kOe was obtained in the Co nanowires with an aspect ratio of 10. The stacking faults were observed in the Co nanowires with an aspect ratio beyond 10, which may be a contributing factor to the lower coercivity than expected. The micromagnetic simulation was employed to reveal the effect of stacking faults on the magnetization reversal mechanism, based on which the coercivity degradation mechanism was discussed. This work is instructive for developing high-performance magnetic nanowires.
查看更多>>摘要：? 2022 The AuthorsEN AW-6016-T4 aluminum alloy sheets were joined with zinc-coated DC04 steel sheets using the Cold Metal Transfer (CMT) welding process. Si-rich and virtually Si-free commercial filler alloys Al-3Si-1Mn (AA-4020) and Al-4.5Mg-Mn-Zr (AA-5087) were used. The influence of silicon on the formation and morphology of intermetallic phases in the joints was studied. The intermetallic seam was investigated by scanning electron microscopy (SEM), electron microprobe microanalyzer (EPMA) using wavelength dispersive spectroscopy (WDS), and transmission electron microscopy (TEM) using selected area electron diffraction (SAED), energy dispersive spectroscopy (EDS), and electron energy loss spectroscopy (EELS). Short-range ordered (SRO) η-Al5Fe2, long-range ordered (LRO) η'''-Al5Fe2, and θ-Al13Fe4 phases formed in the Si-poor and Si-rich welds. The presence of Si in the filler metal leads to the additional formation of the αc-Al(Fe,Mn)Si phase. Grains containing η and η''' were identified. The continuous η seam was generally thinner for higher Si contents. It was found that Si contents higher than 3-3.5 at% in the η seam suppressed the formation of LRO η''' and favored the formation of η.
查看更多>>摘要：? 2022 Elsevier Inc.Radiation-induced defects are expected to change the corrosion kinetics when a material is exposed to a corrosive environment. Characterization of the defects forming under irradiation and their ability to move/diffuse is therefore key to understanding how they impact the corrosion kinetics and possibly mechanisms. For that matter, Fe/Fe3O4 interfaces were irradiated using 1 MeV Kr2+ at temperatures of 273 K (25 °C), 573 K (300 °C) and 773 K (500 °C) to a maximum dose of 10 dpa (as measured in the Fe matrix) in situ in a transmission electron microscope. The evolution of the microstructure was followed as a function of the dose and temperature. The formation of defects was followed in the metal and the oxide as well as at the metal/oxide interface and characterized in terms of size and density. The dynamic observation of the spatial distribution of the defects allowed for a qualitative assessment of the mobility of defects in the oxide as a function of temperature, revealing evidence for enhanced mobility even more so than in the Fe matrix. Mechanisms of induced defect mobility in the spinel are discussed to account for the observations.
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