查看更多>>摘要：The flow behavior and microstructure of Fe-Mn-Al-C austenitic steel and Nb-V microalloyed Fe-Mn-Al-C austenitic steel during uniaxial hot compression deformation are systematically studied in the present paper. The flow curves of both steels are analyzed, as well as constitutive equations and processing maps are established. The results reveal that the flow softening phenomenon of Nb-V microalloyed Fe-Mn-Al-C austenitic steel is more significant at low temperatures and low strain rates. Moreover, the hot deformation activation energy of FeMn-Al-C steel is marginally affected by the addition of Nb-V. The uniformly distributed nano-sized (Nb,V)C particles, which precipitate in Nb-V microalloyed Fe-Mn-Al-C steel, act as the nucleation sites for dynamic recrystallization (DRX). These particles promote the DRX and pin the grain boundaries to restrain the growth of DRX grains. Nb-V microalloying renders a positive influence on the hot deformation stability of Fe-Mn-Al-C steel and broadens the optimum hot processing window. Based on the processing map and microstructure, the optimum hot processing windows of Nb-V microalloyed Fe-Mn-Al-C steel at the strain of 0.7 are determined as 894-1025 degrees C/0.01-0.14 s(-1 )and 1050-1200 degrees C/0.03-0.95 s(-1).
查看更多>>摘要：The regulation of Ce valence and the optimization of intergranular phase can effectively improve magnetic properties, which is beneficial in promoting the application of high abundance rare earth in permanent magnets. In this work, [(La0.3Ce0.7)(x)Pr1-x](17)Fe78B6 (x = 0.1-0.7) melt-spun alloys are prepared. The results illustrate that, Ce shows the mixed valency consisting of Ce3+ and Ce4+ configuration, and when x = 0.3, the alloy possesses the highest Ce3+ ratio. Meanwhile, the highest Ce4+ ratio can be determined in x = 0.1 alloy. TEM characterization demonstrates that, the intergranular rare earth oxides including CeO2 phase, Pr2O3 phase, (Pr, Ce)(2)O-3 as well as La2O3 phase can be determined, among which CeO 2 phase, existed in x = 0.3 alloy, is more beneficial in elevating the coercivity. Thus, the optimum comprehensive magnetic properties with the remanence of 6.5 kG and the coercivity of 14.9 kOe can be obtained in x = 0.3 alloy. In addition, due to more uniform microstructure of melt-spun alloy with x = 0.3, higher thermal stability can be obtained. Elemental metallurgical behavior reveals that La, Ce and Pr elements are enriched in intergranular phase, and main phase region is rich for Fe element.
查看更多>>摘要：Determining displacement and/or strain fields at the nanoscale during material deformation can be instrumental in developing a multiscale understanding of material response. Full-field quantitative kinematic measurements based on transmission electron microscopy (TEM) are lagging behind other microscopy techniques. Here, we develop an experimental approach combining digital image correlation (DIC) and particle tracking (PT) for characterizing in situ microscale deformation of amorphous SiO2 in the TEM. Gold nanoparticles deposited on SiO2 provide both the speckle pattern required by DIC when averaged over a subset region and target particles for PT. To demonstrate and validate the feasibility of using DIC and PT in the TEM, micron sized SiO2 beam samples are machined using a focused ion beam (FIB) and loaded in the TEM via indentation. DIC and PT are then applied to measure in situ displacements from a sequence of TEM images taken during loading and creep of the beam. Results of the two measurement methods agree well with each other and with the applied displacement measurements, thus demonstrating their effectiveness in determining local displacements from TEM experiments. Sources of noise resulting from sample drift and image intensity variations are discussed.
查看更多>>摘要：The hot compression test of a Ti-47.5Al-2.5 V-1.0Cr-0.2Zr alloy was carried out at 1050-1200 degrees C and a strain rate of 0.001-1 s(-1) by a Gleeble3800 thermal simulation testing machine. The hot processing maps at the true strains of 0.2, 0.4, 0.6, and 0.8 were established, and the optimal thermal processing parameter region and the instability region were determined. The microstructures corresponding to different regions of the processing maps were systematically studied, and the validity of the established processing maps were verified. The results indicated that when the true strain is 0.8, the optimal hot working parameter region are 1132-1185 degrees C and 0.001-0.002 s(-1), respectively. The instability region are 1050-1125 degrees C/0.04-1 s(-1) and 1125-1200 degrees C/0.7-1 s(-1), respectively. The corresponding microstructures were lamellar microstructures with a bending and twisting deformation. At the same time, the instability phenomena, such as micro-cracks and voids, were found, and a large number of dislocation pile-ups appeared in the lamellar crystals. The dynamic recrystallization (DRX) degree of the microstructures corresponding to the optimal hot working parameter region is relatively complete, and the recrystallized grain size is relatively uniform. There is almost no residual lamellar microstructure, and no micro-cracks were found. The microstructures in the optimal hot working condition is obviously better than that in the instability region. The thermal processing safety region obtained in this work is authentic, and the processing map has a certain instructive role in the choice of thermal processing parameters.
查看更多>>摘要：A recrystallization-refined Ni-Fe-Cr-Nb-Ti-Al high-entropy alloy coating was prepared by supersonic particle deposition followed by laser irradiation. The surface-interface morphology, microstructure, grain orientation, and phase were examined by scanning electron microscopy, energy-dispersive spectrometry, electron backscatter diffraction, and X-ray diffraction analyses, respectively. The inverse pole figure of the coating showed that a large number of ultrafine grains and even nanograins filled the entire microscopic image. In addition, the refined grains did not show obvious preferred orientations and indicated isotropic features. Further, the fatigue limit of the recrystallized coating was 252 MPa, and the ability to realize uniform plastic deformation and stable deformation was enhanced owing to the high tensile strength and elongation ratio, which were 3000 MPa and 14%, respectively. The coating had a friction coefficient of 0.189, residual stress of -14.3 +/- 6 MPa, and microhardness of 720 kgf/mm2. The capacitance radius of the Nyquist plot indicated that passivation formation improved the shielding performance of the coating.
查看更多>>摘要：To further optimize the microstructure and enhance the mechanical properties of Inconel 718 alloy fabricated via laser additive manufacturing, the Nb-reinforced Inconel 718 composites were fabricated by laser directed energy deposition (LDED) and subsequently subjected to full heat treatment and direct aging treatment. The microstructure and the mechanical properties of the samples with various Nb contents (4.9, 7.0, 8.0, 9.0, 11.0 wt%) under three treated conditions were investigated. The results showed that increasing Nb content led to a more advanced dendrite structure in the as-deposited sample, accompanying with the emergence of fine grains. After undergoing the full heat treatment, the advanced dendrite structure was broken and corresponding recrystallization behavior occurred. The dendrite structure in the directly aged samples was similar to that in the as-deposited sample. Augmenting the Nb content could significantly alter the morphology, the amount, and the size of the Laves phase, gamma '' phase, and the delta phase in the three heat-treatment samples. Cracks appeared in the sample with 11.0 wt% Nb. With the increment of Nb, the tensile strength and the microhardness remarkably became enhanced while the elongation decreased in the three heat-treatment samples. The improvement effect of the direct aging on the tensile strength and the microhardness was superior to the full heat treatment. Grain fining and solid solution strengthening were mainly responsible for the enhancement of the strength and microhardness in the as-deposited samples. The variation of the gamma '' phase and the delta phase in amount and size jointly improved the heat-treated strength and the microhardness. The 9.0 wt% Nb sample with an excellent tensile strength (1598.3 MPa) which was superior to the currently reported strength, and the microhardness (543.2 HV0.2) can be obtained after direct aging. The fracture morphologies of the samples with various Nb contents were also discussed.
查看更多>>摘要：Iron-chromium-aluminum (Fe-Cr-Al) alloys have great potential application as an accident-tolerant fuel cladding material in light-water reactors. It has excellent processing ability, resistance to steam oxidation at high temperatures (HTs), and irradiation swelling-resistance. It is necessary to improve the room- and hightemperature mechanical properties and reveal the strengthening mechanism. In this study, we analyzed the effect of Zr on the microstructure and room- and high-temperature tensile properties of hot- and warm-rolled FeCrAl alloys. The results show that the Laves phase could be effectively stabilized by Zr addition, and the proportion of low-angle grain boundaries increased. Moreover, the yield strength at room and high temperatures of the samples after Zr addition were improved, owing to the precipitation and sub-grain boundary strengthening by Zr. In addition, Zr changed the grain orientation, but had no relevance to the number of easier activated slip systems. The movable dislocation cannot slip during high-temperature tensile tests because of the pinning effect of the dynamic precipitated Laves phase, which could be the reason for the decrease in elongation at high temperatures.
查看更多>>摘要：The oxidation/carburization behavior and failure mechanism of 310S stainless steel (SS) and Alloy 800H exposed to supercritical carbon dioxide at 650 degrees C/20 MPa were investigated. The results show that the oxidation kinetics of 310S SS and Alloy 800H both approximately follow the parabolic oxidation law. The weight gain of Alloy 800H is higher than 310S SS for the first 2000 h exposure. While with the increase of exposure time to 3000 h, the weight gain of Alloy 800H decreases, and becomes lower than 310S SS. Cr2O3 is formed on the surface of 310S SS, while a multilayer structure mainly composed of outer MnCr2O4-Cr2O3 layer and inner MnCr2O4-SiO2 layer is formed on Alloy 800H. Carburization happens in the oxidation process of 310S SS and Alloy 800H. However, the carburization performances of the two kinds of materials are different. For Alloy 800H, a C layer is deposited nearby the O/M interface, which is because that the inner MnCr2O4 and SiO2 layer in the oxide scale acts as a barrier for the inward diffusion of C-containing substances. For 310S SS, a carburized region with more and bigger carbides is formed in the matrix alloy beneath the Cr2O3 layer. The oxide spallation occurs on Alloy 800H after 2000 h exposure, which explains the reason that the weight gain of Alloy 800H decreases with the further increment of testing time to 3000 h. The spallation of oxide film on Alloy 800H is likely to be attributed to the presence of the inner MnCr2O4 layer, continuous SiO2 layer and the deposited amorphous C nearby the O/M interface.
查看更多>>摘要：The practical applications of ZrC are restricted by its low fracture toughness and resistance to sintering densification. This study reports the in-situ precipitation behavior of (Zr,W)C solid solution precipitates induced by Ti. The (Zr0.7W0.3)C solid solution powder was prepared via carbothermic reduction using pressureless sintering at 2200 degrees C for 1 h. Varying quantities of TiH2 were reacted with the (Zr0.7W0.3)C matrix, to precipitate the second phase in-situ, via hot-pressing sintering (50 MPa) at 1600 degrees C for 1 h. When the content of TiH2 was 10 mol%, W and W2C were the second phases precipitated. As the TiH2 concentration increased, the precipitates converted into W2C and the (Ti,Zr)C solid solution. Furthermore, nano-scale dot-like W precipitates gradually increased inside the crystal grains of the (Zr,W,Ti)C matrix. The optimized comprehensive mechanical properties of (Zr,W, Ti)C-based multiphase ceramics were obtained by adding 40 mol% TiH2; the relative density was 99.7%, the Vickers hardness was 24.68 GPa, and the fracture toughness was 5.76 MPa.m(1/2). The precipitates (of varying type and concentration) exhibited distinct toughening effects relative to the (Zr,W)C solid solution.
NSTL
Elsevier