查看更多>>摘要:Single-phase high-entropy alloys and medium-entropy alloys (MEAs) with face-centered-cubic (FCC) structures have attracted considerable attention owing to their exceptional strength and ductility at cryogenic temperatures by performing twinning and martensitic transformations. In this study, a series of novel MEAs were developed (Cr12Ni12MnxFe76?x, x = 12, 16, 20) based on a thermodynamic approach. The Cr12Ni12Mn16Fe60 MEA was discovered to possess a unique combination of excellent strength (~ 1050 MPa) and ductility (~ 86%) at cryogenic temperatures. Through experimental assessment and thermodynamic modeling, the strength–ductility synergy was quantitatively demonstrated to be derived from the interplay among a series of plasticity mechanisms. In addition, a model was proposed for evaluating the stacking fault energy of Cr-Ni-Mn-Fe MEAs at different temperatures. This work demonstrates an efficient and novel strategy for developing high-strength and high-ductility alloys with single-phase FCC structures.
查看更多>>摘要:TiAlVN coatings gain extensive attention due to the self-lubrication effect with elevated temperatures. However, the poor oxidation resistance of TiAlVN limits their industrial application. In order to improve the oxidation resistance of TiAlVN, the TiAlSiN insertion layer is introduced into TiAlVN to synthesize nano-multilayered Ti0.40Al0.50V0.10N/Ti0.56Al0.34Si0.10N (ML_1) and Ti0.40Al0.50V0.10N/Ti0.46Al0.45Si0.09N (ML_2). Ab initio calculations combined with experiments prove that Si-doping reduces the solid solubility of Al in the cubic structure, while V-addition increases its solubility. The hardness of Ti0.40Al0.50V0.10N and Ti0.56Al0.34Si0.10N with cubic structure are 29.2 ± 0.9 and 36.5 ± 0.9 GPa, while Ti0.46Al0.45Si0.09N with mixed cubic and hexagonal structure exhibits low hardness of 29.1 ± 0.9 GPa. The coherently interfacial strengthen enhances the hardness to 33.6 ± 1.0 GPa of ML_1 and 35.2 ± 1.1 GPa of ML_2. Additionally, the Si-containing insertion layers improve the thermal stability of TiAlVN coating, where the onset temperature of w-AlN formation during annealing is increased by ~ 200–300 °C. After annealing at 1200 °C, Ti0.40Al0.50V0.10N, Ti0.56Al0.34Si0.10N and Ti0.46Al0.45Si0.09N possess the hardness values of 26.2 ± 1.5, 38.8 ± 0.8 and 29.5 ± 1.0 GPa, while ML_1 and ML_2 still manifest superior hardness values of 35.4 ± 1.1 and 35.1 ± 1.1 GPa. Furthermore, the oxidation resistance of our multilayers lies between that of TiAlVN and TiAlSiN. After oxidation at 700 °C for 10 h, Ti0.56Al0.34Si0.10N, Ti0.46Al0.45Si0.09N, ML_1 and ML_2 present oxide layers of ~0.1, ~0.1, ~2.3 and ~1.7 μm, respectively, while Ti0.40Al0.50V0.10N coatings has been completely oxidized.
查看更多>>摘要:Grain refinement to the sub-micron scale was found to enhance both strength and plasticity. In this size range, the micro-mechanisms of deformation are not yet sufficiently specified. In the present work, strain rate sensitivity was investigated in an Al-Mg-Si alloy processed by equal-channel angular extrusion. The microstructure was controlled by conventional transmission electron microscopy and orientation imaging-ASTAR. In the as-quenched state, deformation produces grains below 100 nm in size. The activation volume V was reduced to 70b3. Based on published theoretical models, we have shown that in such alloy, pure grain boundary sliding cannot be revealed in the low-stress range, due to the intervention of solute atoms. Beyond, the stress-dependence of V consists with an inverse Cottrell-Stokes behavior. This trend was confirmed by TEM which revealed dislocation activity and grain boundary emission. Ageing at 150 °C increases the contribution of GBS whereas the residual activation volume continues to be leveled.
查看更多>>摘要:In this work, the influences of laser shock peening (LSP) on the residual stress state, microstructures and mechanical properties of Ti-6Al-4V alloy fabricated by selective laser melting (SLM) were researched. Particularly, the microstructural evolution before and after LSP was clarified using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) observations. The results showed that the residual stress was adjusted from tensile to compressive state. Furthermore, high-density dislocations and a great number of parallel nano mechanical twins (nano-MTs) were promoted in the coarse lamellar α' structures, contributing to the grain refinement. The ultimate tensile strength and elongation of the SLM-manufactured (SLMed) Ti-6Al-4V alloy increased by approximately 14.3% and 18.3% after LSP treatment, respectively. Finally, the dominant mechanism of tensile property enhancement by LSP was revealed. High levels of compressive residual stress and grain refinement of the α' structures induced by LSP realized the excellent combination of strength and ductility.
查看更多>>摘要:In the present study, Ti and Y2O3 nano-powders were introduced in a base composition of Fe-42 wt% Ni invar alloy to investigate their effects on phase evolution and its thermal stability. Three compositions, namely, Fe-42Ni-2Y2O3-0.3Ti, Fe-42Ni-2Y2O3-1Ti, Fe-42Ni-2Y2O3-2Ti (all in wt%), were mechanically alloyed through high energy SPEX8000M ball mill with a varying amount of Ti (0.3, 1, and 2 wt%). The milled compositions were consolidated by spark plasma sintering at 1000 °C at 60 MPa pressure with a holding time of 5 min. The addition of Ti and Y2O3 nano-powders in the Fe-Ni metal matrix is expected to develop nanoclusters precipitates, thereby increasing its strength by Orowan strengthening. Consequently, submicron size grain was found to form in the case of 2 wt% Ti alloy (205 nm) as compared to 348 nm size grains in 0.3 wt% Ti alloy. The corresponding composition dictated higher nanoindentation hardness values (4.4 GPa and 5.2 GPa, respectively), which also validated Hall-Petch relationship. The Y2O3 nanoparticles break down into Y and O atoms through mechanical alloying and consequently dissolve into Fe-Ni metal matrix to form a supersaturated solid solution first. The addition of Ti atoms consolidates with Y and O and re-precipitates as Y-Ti-O based nanosize complex clusters within the matrix. XRD phase analysis and TEM-SAED study confirmed the formation of complex dispersoids (Y2Ti2O7 and TiO2), which played an important role in hindering matrix grain coarsening. The grain size evolution was confirmed by EBSD analysis in addition to TEM. The relative sintered density of 2Ti added alloy was found to be low (96.5%) compared to 1Ti (97.7%) and 0.3Ti (98.2%) added alloys, respectively. A ball on disk wear test showed a lower wear rate for the 2Ti added alloy due to the uniformly distributed dispersoids present within the metal matrix. Corrosion-resistant of the alloys (conducted in an electrolyte of 3.5% NaCl) were found to have insignificant effect of Ti variation in the said compositions. Orowan strengthening and Zenner pinning played pivotal roles to yield high thermal stability and ultrafine structure.
查看更多>>摘要:Nanocrystalline Fe-10Pb (wt%) powders were synthesised by mechanical alloying processes using the Vario-planetary high-energy ball mill PM 400 Retsch. The phase evolution and magnetic properties were investigated as a function of the milling time, t, (variation from 0 to 36 h range) by using the X-ray Diffraction (XRD) technique, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) and the vibrating sample magnetometer (VSM). The complete formation of FePb (bcc) was observed after a milling time of 16 h. When milling time increases, the lattice parameter decrease while the grain size decreases to 18.53 nm after a milling time of 36 h. Powder morphologies at different formation stages of the alloy was observed by SEM. The elemental maps of Fe and Pb done with EDX experiments, confirmed the results found by XRD about the evolution of the alloy formation. The results of vibrating sample magnetometer suggested that magnetic properties of Fe-10Pb (wt%) alloy nanoparticles were affected by composition, size and morphology of the particles.
查看更多>>摘要:Current experiments show that the ternary alloy SiyGezSnx has larger crystal lattice and bandgap control range than group IV binary alloy, and it is a popular material for the preparation of silicon-based high-efficiency light sources. However, the physical laws of the bandgap and luminescence properties of SiyGezSnx with concentration changes have not been fully elucidated because of high experimental costs and time-consuming research. Therefore, the band structures and optical gains of SiyGezSnx (x∈0–0.35, y∈0–1) were investigated in this study using a first-principles method based on density functional theory combined with special quasi-random structures and Heyd-Scuseria-Ernzerhof hybrid functional correction. The quantitative relationship between the bandgap and concentration change of SiyGezSnx was theoretically clarified. In addition, the optical gain coefficient of the SiyGezSnx with direct bandgap characteristics was studied. Compared with the structure of GeSn with the same Sn concentration, SiGeSn can maintain the direct bandgap when Si is introduced. The results show that the bandgap increases with increasing Si concentration whereas it decreases when more Sn is introduced. Thus, the best way to maintain a direct bandgap is to simultaneously increase the Si and Sn concentrations. To facilitate experimental research, a formula to determine the direct bandgap region of SiyGezSnx was realized. Regarding optical gains, the positive gain regions are consistent with the direct bandgap regions, and the optical gain values are related to fluctuations in electron and hole numbers in the different valleys. For Siy(y=0–0.21)GezSnx(x=0.1–0.31), there are two maximum optical gain regions around Ge0.9Sn0.1 and Si0.17Ge0.56Sn0.27. These results can directly provide theoretical guidance for experimental research, as well as accelerate the research and development of group IV alloy high-efficiency light sources.
查看更多>>摘要:Tm3+ and Tb3+ co-doped Y3Al5O12 (YAG) ceramics were fabricated by high temperature solid state reaction under oxygen sintering. The absorption spectra, fluorescence spectra and fluorescence decay curves of Tm,Tb:YAG ceramics were measured. The spectral parameters of Tm,Tb:YAG ceramic were calculated. The lifetimes of Tm3+:3F4 level of Tm,Tb:YAG ceramics were calculated to be 11.28, 2.12, 1.24, 0.63 and 0.49 ms for the Tb3+ doping concentration of 0, 0.1, 0.2, 0.35 at 0.5 at%, respectively. The co-doping Tb3+ ion is a productive deactivator that could reduce the filling of 3F4 mainfold and enhance the emission of 3H4→3F4 transition. These results indicate that the addition of Tb3+ ions in Tm,Tb:YAG ceramic could be a promising technique for laser operation around 1.5 μm.
查看更多>>摘要:With merits of large theoretical capacity and environmental benignity, Fe3O4-based materials are extensively researched as anodes in lithium-ion batteries (LIBs). However, they are faced with problems of low practical capacities and poor cycling durability. Herein, to achieve improved electrochemical kinetics and cyclic stability, the Fe3O4 is modulated to be dispersive nanoparticles and further encapsulated in a porous matrix by a well-designed strategy. Thanks to thus well-designed composite architecture, the as-prepared Fe3O4-based composite (Fe3O4@C) is given the improved electrochemical kinetics and structural stability, as demonstrated by the material and electrochemical characterizations. In addition to the synergistic effects of nanostructure and carbon coating that are commonly reported in Fe3O4/C composites, in our Fe3O4@C the unique space between dispersive Fe3O4 nanoparticles and the pores in carbon coating can improve electrolyte accessibility on the surface of the active material, enable rapid charges and ions transport and mitigate the change in volume, therefore achieving improved kinetics and superior structural stability. Consequently, the Fe3O4@C shows outstanding electrochemical properties, exhibiting high capacity of 864 mAh g?1 at 200 mA g?1 after 600 cycles as well as 514 mAh g?1 at 1000 mA g?1 after 600 cycles.
查看更多>>摘要:A series of LaInO3 bulk materials doped with Bi3+, Tb3+ and Eu3+ ions have been synthesized via a flux solid-state reaction technique, characterized by powder X-ray diffraction (PXRD) and investigated by luminescence spectroscopy. The compounds exhibit the pure crystal phase of LaInO3 (JCPDS No. 08-0148), and the structure is not affected by the presence of the dopants. The Bi3+ → Tb3+ and Tb3+ → Eu3+ energy transfer phenomena have been studied by means of luminescence spectroscopy and decay kinetics of Bi3+ excited state and of the 5D4 excited state of Tb3+. White light emission is attained via altering the concentration of dopant, and the obtained color coordinate (0.3248, 0.3030) is close to standard white light (0.313, 0.329). The energy transfer process is crucial, in order to tune the color of the emitted light.