查看更多>>摘要：An ultrafine-grained (UFG) and coarse-grained (CG) Ni2Al3 coating were prepared by aluminizing pure nickel substrate with and without nanocrystalline Ni film using an aluminization pack cementation process at 620 degrees C for 5 h. The oxidation kinetics and cyclic oxidation result show that the UFG-Ni2Al3 coating significantly enhances the oxidation-resistant with respect to the CG-Ni2Al3 coating in air at 900 degrees C. The SEM/EDS results confirmed that the microstructure of two 8-Ni2Al3 coatings is free of pores, cracks, and voids. The UFG-Ni2Al3 coating promotes the phase transformation with respect to the CG-Ni2Al3 coating after oxidation for various times were investigated by using photo-stimulated luminescence spectroscopy (PSLS) analysis. The experimental result also confirmed that the UFG-Ni2Al3 coating, on which an adherent alumina scale grew, can significantly control the formation of cavities at the scale/coating interface.
查看更多>>摘要：Filled Sb-based skutterudites are considered one of the most appealing thermoelectric materials in the mid temperature range. Even though Sb is not one of the most abundant elements in nature, the large thermoelectric figure of merit of these materials makes them attractive for applications such as thermoelectric generators. In order to get deeper insight into the fundamental physical mechanisms of thermal and electronic transport properties, we studied the temperature dependent electrical resistivity, Seebeck coefficient, thermal conductivity and specific heat. Three groups of skutterudites with excellent thermoelectric performance were investigated: (a) DDyFe4-xCoxSb12 (0 <= x <= 4; 0.08 <= y <= 0.7), to study the influence of Fe/Co substitution and the resulting filling level y as well as the influence of grain size, (b) DD0.7Fe3CoSb12 samples prepared from the same powder to study the effect of different synthesis nanostructuring techniques (hot-pressed, hot pressed and processed via high pressure torsion and cold-pressed and processed via high pressure torsion) and (c), a DD-filled skutterudite with and without Sb/Sn substitution before and after annealing. An overview of experimental investigations of the low-temperature transport is given and appropriate phenomenological models are adopted to elucidate the temperature-dependent features and the origin of high thermoelectric performance in these systems.
查看更多>>摘要：To explore the plastic deformation behavior of CoCrFeMnNi high entropy alloy (HEA) at extreme conditions, the effect of strain rate ranging from 1 x 10(8) s(-1) to 5 x 10(10) s(-1) on microstructural evolution and deformation mechanism was investigated via molecular dynamics (MD) simulations. Our results show that the alloy exhibits completely different deformation mechanism under lower and higher strain rates conditions. The deformation mechanism of the alloy is mainly the formation and evolution of stacking faults and twins driven by Shockley dislocations at lower strain rates, and a continuous phase transformation from FCC to HCP with the BCC phase as the transition at higher strain rates. Between the lower and higher strain rates, there are intermediate strain rates where the two deformation mechanisms coexist. A quantitative model was established to describe the strength of the alloy at intermediate strain rates by considering the contributions of four microstructural features that occur during the plastic deformation. Fitted empirical model with single adjustable parameters can agree well with those calculated by MD simulations. The present work can help researchers better understand the deformation mechanism of the CoCrFeMnNi HEA at different strain rates, thereby pointing out directions for alloy strengthening.
查看更多>>摘要：Cr-Mn system is very important for many applications. In the present work, the reported intermetallic compounds have been studied using in situ high temperature neutron powder diffraction. For the first time, site occupancies in a sigma phase are obtained as a function of temperature by Rietveld analysis. They are compared with calculated data obtained from DFT. The previously reported order-disorder transition in the sigma phase around 1000 degrees C is shown to be in fact a congruent phase transformation to the bcc Cr-Mn solid solution. This solid solution is probably continuous between pure Cr and delta-Mn. The second reported intermetallic alpha' phase (Cr2Mn3) phase is shown to be in fact a ternary nitride of the Cr-Mn-N system that should not appear in the binary phase diagram. The present work clarifies and explains all the features of the previous literature on this system and brings major revisions to the phase diagram. General conclusions are drawn concerning mistakes that can be done in the evaluations of phase diagrams.
Gul, Ali OktayKavaz, EsraBasgoz, OykumGuler, Omer...
11页
查看更多>>摘要：High-Entropy Alloys (HEAs) are regarded as potential structural materials for fusion and next-generation fission reactors, which will be required to fulfil growing nuclear energy demands. In this study, a HEA-composite was synthesized by adding B4C to an HEA containing Ni. The microstructure of the obtained HEA-composite was examined and the changes in its mechanical properties were revealed. Additionally, the nuclear radiation shielding properties of the Ni-containing HEA, and the HEA-composite are investigated using experimental and theoretical methods. Our initial findings showed that with the addition of 2.5% B4C to the alloy, the hardness increased more than two times. The addition of B4C to the HEA matrix resulted in a more than 90% and a nearly twofold increase in compressive strength. The shielding qualities of gamma-ray and neutron radiation were investigated using experimental and theoretical approaches. Our findings demonstrated that increasing the B4C reinforcement considerably enhanced the composite material's neutron attenuation capabilities. On the other hand, no significant change in the gamma-ray shielding characteristics of HEA and HEA-composite samples was observed. The gamma-ray shielding characteristics of HEA and HEA-composite samples were compared to those of other alloy shields and commercial products. Our findings indicate that both HEA and HEA-composite samples exhibit superior gamma-ray shielding characteristics when compared to the control samples. It can be concluded that increasing B4C reinforcement may be a multifunctional tool in terms of improving the mechanical properties as well as neutron attenuation properties for advanced applications in nuclear radiation facilities and next-generation fission reactors. Additionally, due to their promising material features and higher gamma-ray shielding capabilities compared to other kinds of alloys and commercial shields, HEAs may be beneficial materials.
Bakulin, Alexander, VChumakova, Lora S.Kulkova, Svetlana E.
8页
查看更多>>摘要：The oxygen absorption energy and its migration barriers in titanium silicide with composition Ti5Si3 and hexagonal structure have been calculated by using the projector augmented-wave method within the electron density functional theory. We have derived analytical expressions for the temperature-dependent diffusion coefficients along two crystallographic directions using Landman's method and calculated the activation energies and the pre-exponential factor. The mechanism of oxygen diffusion in the bulk Ti5Si3 is discussed. It is shown that the obtained diffusion coefficient of oxygen in this silicide is comparable with experimental ones for metal oxides such as Al2O3, SiO2, Cr2O3.
查看更多>>摘要：We report here a thorough study on the effect of 10 at.% Al addition into the ternary equimolar Ti0.33V0.33Nb0.33 alloy on the hydrogen storage properties. Despite a decrease of the storage capacity by 20%, several other properties are enhanced by the presence of Al. The hydride formation is destabilized in the quaternary alloy as compared to the pristine ternary composition, as also confirmed by machine learning approach. The hydrogen desorption occurs at lower temperature in the Al-containing alloy relative to the initial material. Moreover, the Al presence improves the stability during hydrogen absorption/desorption cycling without significant loss of the capacity and phase segregation. This study proves that Al addition into multi-principal element alloys is a promising strategy for the design of novel materials for hydrogen storage.
查看更多>>摘要：Alloys of Ti-46Al-6Nb-2.5C-xTa (at. %) were prepared to adjust gamma/alpha(2) phase, refine microstructures, and improve mechanical properties at room and elevated temperatures simultaneously. Phase contents, lamellar colony size, Ti2AlC morphology, compressive and tensile properties, and related mechanisms were discussed. The results show that the relative content of the gamma phase increases and the relative content of the alpha(2) phase decreases when Ta content increases from 0 to 2.0 at. %. The solid solution effect of Ta in reinforcing phases contributes to the absence of brittle B2 phase. The content of small length-diameter ratios of Ti2AlC particles increases. The lamellar colony size decreases from 29.9 to 21.6 mu m. The constitutional undercooling and heterogeneous nucleation are increased by Ta during the solidification process. Compressive strength increases from 1974 to 2330 MPa when Ta content increases to 0.8 at. %, and the strain increases from 19.8 to 28.2% when Ta content increases to 1.2 at. %. The tensile strength of the Ti-46Al-6Nb-2.5C-2.0Ta alloy increases from 460 to 503 MPa and its strain increases from 4.1 to 8.3%, when temperature increases from 750 to 950 degrees C. The refinement of lamellar colonies and Ti2AlC, solid solution of Ta, and the ratio of gamma/alpha(2) are the main reasons for improving mechanical properties of Ti-46Al-6Nb-2.5C-2.0Ta alloy.
查看更多>>摘要：This work aims to elucidate the hot deformation behavior and microstructure evolution of a non-equimolar TiZrHf-rich refractory high-entropy alloy (RHEA) with a composition of Ti2ZrHfV0.5Ta0.2 (at. %). The as-cast Ti2ZrHfV0.5Ta0.2 alloy composed solely of a body-centered cubic phase showed superior tensile mechanical properties with a yield strength of 865 MPa and a total elongation of 22.5%. The uniaxial compression tests were performed at temperatures ranging from 900 to 1100 degrees C and strain rates varying from 10-3 to 10-1 s-1. The relation among the flow stress, deformation temperature, and strain rate was represented by the Arrhenius-type constitutive equations. The calculations show a comparatively low apparent activation energy of 144-119 kJ/ mol over the entire range of strains, revealing a noticeable contradiction between the concentration of group IV elements (Ti, Zr, Hf) and the high-temperature strength. The excessive addition of Group IV elements tends to deteriorate the high-temperature strength due to the relatively low melting points and self-diffusion activation energies of Group IV elements. The misorientation analysis based on electron back-scattered diffraction indicated that the dynamic recrystallization mechanism depends strongly on the thermo-mechanical processing conditions. A low deformation temperature and high strain rate results in an abnormal discontinuous dynamic recrystallization mechanism. Additionally, the dominant role played by the edge dislocation in RHEAs during deformation is supposed to be the reason for the abnormal discontinuous dynamic recrystallization mechanism. A series of stress drops were observed under low deformation temperatures and high strain rates. Based on an interrupted hot-compression experiment and a strain aging experiment, the stress drop can be attributed to the unlocking of dislocations pinned by the Cottrell atmosphere or short-range ordering.
Rocha, Valeria RosaCesare, John-PaulMessina, Troy C.
8页
查看更多>>摘要：High entropy alloys are a burgeoning area of study. These alloys have the potential to fit many applications due to their high stability, mechanical strength, tunable magnetism, and other beneficial properties. Recent theoretical predictions of magnetic properties in the CoCrFeNi system prompted the work we present here. We prepared alloys CoCrxFeNiQy, where Q is a transition metal. We aimed to create alloys with Curie temperatures near room temperature due to their applicability to magnetic refrigeration. Agreement between theoretically predicted and experimentally measured Curie temperatures is within 10-20% for most alloys studied here. Alloys tend to phase separate when atomic percentages, atomic radius, or enthalpy of mixing for the fifth element, Q, are high.
NSTL
Elsevier