查看更多>>摘要：? 2022The solidification process of Mg-15Gd(wt.%)alloys with different cross-section geometry is investigated in situ by synchrotron radiation imaging technology. The evolution of dendrites, including the nucleation and growth processes, has been recorded and analyzed for magnesium alloys with different cross-section geometry. It is shown that the cross-section geometry of the samples has significant effect on the number of dendrites, the growth rate of dendrites, and the solute distribution. We find that the cross-sectional change does not conducive to the nucleation of dendrites. The variable cross-section facilitates the growth of dendrites while the T-type section inhibits the growth of dendrites. The solute in the variable cross-section sample (VSS) is enriched at the shoulder while the solute in the T-type section sample (TSS) tends to concentrate at the bottom of the boss. Meanwhile, the fracture and drift of dendrites in VSS and TSS are observed. As a result, the grain fragments tend to form stray crystals
查看更多>>摘要：? 2022Inconel 718 (IN718) is a nickel-based superalloy which exhibits excellent tensile and impact resistant properties along with good corrosion resistance at high temperatures. However, due to the high toughness and work hardening, the machinability of this superalloy is low. Therefore, the selective laser melting (SLM) process has been adopted as an efficient technique to fabricate IN718 parts as it overcomes the problems associated with conventional manufacturing of superalloys. In the SLM process, various process parameters like scan strategy, laser power, scan speed, and energy density are defined for the fabrication to regulate the microstructure and thus control the mechanical properties like tensile strength, yield strength, impact strength, and hardness. In the SLM process, the thermal cycles induce residual stress into the part. These residual stresses can be detrimental to the microstructure and hence mechanical properties of the part. Residual stresses lead to warping of the part during the fabrication process, thereby leading to failure of the component. Although each process parameter has an independent and definitive effect on the overall mechanical and metallurgical properties, scan strategy is an independent process parameter which directly affects the level of residual stresses, microstructure, and mechanical properties of the SLM part, as the heat zones in part can be shifted from one location to another by varying the scan strategy. This variation in the area of the heat-affected zone determines the grain size ranging from equiaxed to elongated. Hence, the scan strategy is the only parameter that is varied for this study. The various scan strategies adopted here are chess, striped, flow-optimized, and customized scan strategies. In this study, the microstructural evaluation was deduced, followed by compositional analysis and hardness tests on the SLM fabricated parts. The residual stress of the parts was then determined using the hardness method. This effort was undertaken to identify the effect of scan strategy on residual stress and to discuss the metallurgical interactions between the mechanical and microstructural properties within the IN718 superalloy.
查看更多>>摘要：? 2022 Elsevier Inc.This paper presents studies concerning the influence of the Ga+ ion beam on the stability of retained austenite in K110 cold work tool steel (X155CrVMo12). The material was first austenitized at 1200 °C to dissolve a significant amount of the alloy's carbides to obtain the largest possible volume fraction of retained austenite, which was 93% after quenching in oil. Changes in the steel's crystal structure upon interaction with Ga+ ions were observed using a combination of SEM-FIB-EBSD techniques. Selected grains in the material were sputtered with Ga+ ions of various accelerating voltages (2 kV – 30 kV) using two different doses, namely 50 and 100 pC/μm2. It was found that the application of 5 kV Ga+ ion beam is enough to induce partial transformation of austenite to BCC phase. The use of higher voltages (8–30 kV) resulted in the complete transformation of austenite. Low acceleration voltages of 2 kV, even with a high dose of 500 pC/μm2, did not induce any phase transformation and only deformed austenite's crystal structure. Moreover, it was shown that the crystallographic orientation between austenite and the Ga+ ion beam induced BCC phase is not random.
查看更多>>摘要：? 2022 Elsevier Inc.In this study, we examined the effect of stress relief annealing (SRA) temperatures on the microstructure and mechanical behavior of a Co-Cr-Mo alloy fabricated by selective laser melting (SLM). For the SRA, the as-built SLM samples were annealed at 750 °C and 1050 °C. In the as built SLM sample, the γ phase was mainly formed, but some ε phases were present together. The percentage of the ε phase increased significantly after SRA at 750 °C, and a microstructure with a 100% γ phase formed after SRA at 1050 °C. Changes in mechanical properties were characterized by an increase in both hardness and yield strength, while the elongation decreased after SRA at 750 °C. This was caused by the formation of the ε phase, which possesses high strength and hardness but brittleness. After SRA at 1050 °C, the hardness and yield strength decreased, and the elongation increased compared to the as-built SLM sample because of the decrease in the residual stress. In the γ phase of the Co-Cr-Mo alloy, strain-induced γ to ε martensitic transformation occurring during deformation, and it was more marked in the as-built SLM sample with high residual stress. Therefore, when the as-built sample was deformed, the brittle ε phase formed more rapidly than in the sample after SRA at 1050 °C, resulting in low elongation.
查看更多>>摘要：? 2022 Elsevier Inc.The effect of tungsten (W) on the precipitation and mechanical properties of 22Cr-25Ni austenitic heat-resistant steel during aging at 700 °C was systematically studied. The equilibrium phase diagrams of the steels containing 1.5 and 2.5 wt% W were calculated by thermodynamics software, and their microstructure was examined by SEM, XRD, and TEM. The results show that the coarsening rate of Cr23C6 precipitates at grain boundaries was significantly decreased, and Cu-rich phase and Z phase were uniformly distributed in intracrystalline regions of the steel when 2.5 wt% W were added during the aging process at 700 °C, compared to the addition of 1.5 wt% W. W atoms quickly diffused and accumulated, which led to 8.33 wt% W atoms diffused into Cr23C6 to form Cr23C6 which contains high W content (high W-Cr23C6) at the grain boundaries, the specific interface energy between Cr23C6 and austenite was reduced, and boundary binding strength was improved. In addition, the formation of nano-scale precipitate phases at grain boundaries by elements such as Cu and Nb were inhibited due to the formation of high W-Cr23C6 at these locations, which resulted in the increase and more even distribution of Cu-rich phase and Z phase within the grain. The stress concentration on the triple junctions caused by trapeziform Cr23C6 was decreased by plate-like high W-Cr23C6 precipitates resulting in increased ductility.
查看更多>>摘要：? 2022 Elsevier Inc.The (Nd10.23Pr2.56Fe68.32Co13.08B5.81)100-xNbx (x = 0, 0.5, 1, 2, 3) ribbons were prepared by melt-spinning at a wheel speed of 15 m/s. The effects of Nb addition on the microstructure and the magnetic properties of the ribbons were investigated. The X-ray diffraction (XRD) patterns of the ribbons show that the Nd2(Fe, Co)14B (2:14:1) phase is observed in all samples indicating that Nb addition does not change the phase constitution. The transmission electron microscopy (TEM) images show that Nb addition can dramatically reduce the average grain size from 52 ± 5 nm of the Nb-free alloy to 12 ± 2 nm of the Nb-containing x = 3 alloy. Based on the atom probe tomography (APT) result, we identify different mechanisms for grain refinement at different Nb content. When x = 0.5, the distribution of Nb is uniform at the intergranular phase and Nb addition promotes the segregation of rare-earth Nd and Pr at the thick ferromagnetic intergranular phase (≈ 10 nm), which leads to decreasing the average grain size to 38 nm. With further increasing Nb addition to x = 2, the existence of Nb-rich precipitations (2–4 nm in width) at the grain boundaries effectively reduces the grain size to 16 nm. The remanence (Br) and the maximum energy product ((BH)max) are obviously increased by Nb addition and the maximum of the Br and the (BH)max is obtained in the (Nd10.23Pr2.56Fe68.32Co13.08B5.81)98Nb2 (x = 2) alloy. The values of the Br and the (BH)max are 0.90 T and 138 kJ/m3, 8.4% and 22.1% higher than that of the Nb-free alloy. It ascribes to the grain refinement, the changes of the distribution and the composition of the ferromagnetic intergranular phase by Nb addition. Our findings provide a new method of designing prospective permanent alloys with increased magnetic properties by reducing the grain size and tuning the distribution and chemical composition of the intergranular phase.
查看更多>>摘要：? 2022 Elsevier Inc.The authors regret that the original version of this article contains an error for the author's affiliations. The correct affiliations are shown below. Tianzi Yanga, Junming Goua,?, Xiaolian Liub, Yiqun Zhanga, Tianyu Maa, Xiaobing Rena,c? aFrontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of Materials, and MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Xi'an Jiaotong University, Xi'an 710,049, China. bSchool of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310,012, China. cNational Institute for Materials Science, Tsukuba, Ibaraki 305–0047, Japan. The authors would like to apologise for any inconvenience caused.
查看更多>>摘要：? 2022 Elsevier Inc.Eutectic growth kinetics and microstructure evolution of highly undercooled Ti86.33Si13.67 alloy were systematically investigated using electrostatic levitation technique, and the maximum undercooling achieved was up to 343 K (0.21 TE). The experimental undercooling is rather low if the overheating degree is below a critical value, while that is high at the overheating degree beyond this value. The critical overheating degree is in the range from 60 to 86 K. The average eutectic growth velocity increases dramatically with the undercooling. The microstructure was proven to be consisted of α-Ti phase and Ti5Si3 phase. A small amount of metastable β-Ti phase inside the α-Ti phase was found for the alloy solidified at 343 K undercooling, and the orientation relationship between α-Ti phase and β-Ti phase is (110) β-Ti // (002) α-Ti. For the primary Ti5Si3 phase, it can be obtained in the microstructure solidified at low undercoolings, but cannot be found when the undercooling is larger than 200 K. The cluster like eutectic gradually occupied the whole alloy space with the increasing undercooling, and the irregular eutectic microstructure is confirmed to be directly generated from the undercooled liquid phase at high undercoolings.
查看更多>>摘要：? 2022 Elsevier Inc.Iron-based superalloys are alloys produced for use in corrosive environments as an alternative to high-cost nickel-based superalloys. However, their average strength and hardness, attributed to their austenitic structures, limit their use in tribological applications. In an attempt to counter these drawbacks, boriding was applied to an iron-based A286 superalloy having an initial surface hardness of 320 HV. Boriding kinetics, some mechanical properties, and tribo-wear (ambient air and 3.5 NaCl environment) behaviors of the formed boride layers were investigated. Multicomponent boride layers (consist of FeB, Fe2B, CrB, NiB, Ni4B3) were formed on the surface of the alloy, with hardness and thickness values of 1498–1961 HV and 20–130 μm, respectively, depending on the boriding temperature and the treatment time. The integral diffusion model was adopted to deal with the kinetics of monoboride and hemiboride layers formed on the surface. The boron activation energies of FeB, Fe2B, and DZ layer were estimated as equal to 175.86, 198.7, and 205.73 kJ mol?1, respectively. As a result of increased surface hardness, all of the borided samples displayed reduced friction coefficients and higher wear resistance compared to the untreated alloy, in both ambient air and 3.5% NaCl. However, the increase in wear resistance was not proportional to the increase in hardness; while the best wear resistance was obtained in samples borided at 850–950 °C for 6 h, the lowest wear resistance was obtained in samples borided for 4–6 h at 1050 °C. This situation was caused by the Kirkendall effect and residual stresses in the structure of alloying elements with different diffusion rates due to the high-temperature effect of the boriding process.
查看更多>>摘要：? 2022 Elsevier Inc.Both heterogeneous grain structure and dual nanoprecipitates (B2 and L12) have been designed and obtained in a FCC-based Al0.5Cr0.9FeNi2.5V0.2 high entropy alloy (HEA). The volume fraction of B2 phase is nearly unchanged, while the average size and volume fraction for L12 particles become larger after aging, resulting in a more severe heterogeneity. The aged samples display a better synergy of strength and ductility than the corresponding unaged samples. The aged samples show a transient up-turn strain hardening behavior and a higher hardening rate as compared to the corresponding unaged samples. The hetero-deformation-induced hardening plays a more important role in the aged samples than in the unaged samples, producing higher density of geometrically necessary dislocations for better tensile properties. Orowan-type bowing hardening and shearing hardening mechanisms are observed for B2 and L12 nano-particles, respectively. The size and interspacing of B2 and L12 particles are at nanometer scale, which should be very effective on hardening and strengthening by accumulating dislocations at phase interfaces. A theoretical analysis based on dislocation strengthening, grain boundary strengthening, Orowan-type bowing strengthening of B2 nano-particles, shearing strengthening of L12 nano-particles and strengthening of chemical short-range order has been found to provide well prediction on strength.
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