Development and thermal stability of Cr10Mo25Ta25Ti15V25 refractory high entropy alloys

Refractory high entropy alloys (RHEAs) are promising materials for extreme environment applications, such as high-temperatures (> 1000 degrees C), corrosion and irradiation. In this study, a new CrMoTaTiV RHEA system has been designed using CALPHAD modeling and thermophysical parameter optimization, and produced by vacuum arc melting. Microstructure at room temperature as well as microstructural stability and me-chanical properties at high temperature have been investigated. It has been found that BCC alloys are formed with dendritic structures in which interdendritic regions are enriched with Ti, V and Cr, and de-pleted with Ta and Mo. Transmission electron microscopy analysis has shown dislocation tangles, loops and networks as well as the formation of nano-grains some of which are determined to be Ti-rich FCC phases. Besides, there is a secondary BCC formation below 1000 degrees C together with a Ti-rich FCC phase without any HCP or intermetallic phase formation. High strength and ductility of this alloy at the temperatures up to 1000 degrees C compared to the other RHEA systems have been attributed to its unique microstructure. This clearly shows the promise of this alloy system for high temperature applications due to its stability, high strength and possible oxidation resistance.(c) 2022 Elsevier B.V. All rights reserved

Formation Kinetics, Morphology and Magnetic Properties of BaHF Ceramics Synthesized in x wtNaCl- (100-x) wt KCl Molten Salts

Micron size barium hexaferrite (BaHF, BaFe12O19) platelets were prepared by molten-salt synthesis method in various weight proportions of NaCl-KCl salt mixtures as the liquid medium. The effects of calcination temperature and molten salt composition –x wt% NaCl and (100-x) wt% KCl-on the formation kinetics and amount of BaHF phase formation as well as on the morphology and magnetic properties of the final products have been discussed. Inductively coupled plasma-mass spectroscopy (ICP-MS) was used to determine the solubility of the starting materials in the salts to understand the formation mechanism of BaHF. X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM) were used to identify the characteristics of the synthesized BaHF platelets. ICP-MS analysis showed that Fe2O3 solubility is negligible, while BaCO3 has very high solubility in both molten KCl and NaCl. Quantitative XRD results showed that the usage of the molten salt containing 100 wt% NaCl results in the highest amount of conversion to BaHF at 900°C in 2 hours. On the other hand SEM results showed that KCl rich molten salt concentrations led to the formation of proper platelet morphology with sharp corners, while NaCl rich ones resulted in smoother platelet shapes. Flux compositions containing 70 to 90 wt% NaCl have led to the largest average platelet size. According to the magnetic measurements, as the content of NaCl in the molten salt increases, hysteresis loss of the ceramics become pronounced pointing out to the achievement of a more hard magnetic behavior in the synthesized BaHF ceramics. The magnetic saturation values of the synthesized ceramics change around 55 emu/g for all the flux compositions, where the highest saturation value is obtained with 44 wt% NaCl, which is 56.5 emu/g. The highest magnetic coercivity values were obtained with 90 wt% NaCl-10 wt% KCl and 100 wt% NaCl flux compositions. In conclusion, the results obtained from the experiments have shown that the composition of the NaCl-KCl molten salts plays an important role on the extent of BaHF formation, along with resulting platelet morphology and magnetic properties

In-situ observation of nano-oxide and defect evolution in 14YWT alloys

© 2020 Elsevier Inc.Nanostructured ferritic alloys (NFAs) are considered as candidates for structural components in advanced nuclear reactors due to their excellent radiation resistance as a result of a high density of nano-oxides (NOs) in the microstructure. Therefore, gaining an understanding on the stability of NOs under irradiation is crucial. In this study, we have investigated the evolution of defects and NOs in 14YWT NFAs under in-situ Kr ion irradiation at room temperature (RT) and 450 °C up to 10 dpa. It has been found that irradiations at 450 °C do not create any changes in the NOs, similar to the bulk irradiations. On the other hand, elemental mapping indicates that NOs dissolve mostly after 10 dpa irradiations at RT. Thus, while defects are both annihilated and pinned by NOs at low doses (before the dissolution of NOs), glissile loops start to escape to the foil surface at high doses (after the dissolution of NOs), justifying the significantly low fraction of loops compared to the literature values. High resolution transmission electron microscopy analysis has shown that the NOs are mostly coherent Y2Ti2O7 particles with pyrochlore crystal structure after both RT and 450 °C irradiations, similar to those observed before irradiation