Precipitates often play key roles in improving the mechanical performance of structural materials. Using first-principles density functional theory method, we have calculated the geometry and energetics of small X n and X n He (X ¼ Cu, V, and Ta) clusters in bcc Fe matrix to investigate the effect of He on X precipitation on the initial stage in neutron-irradiated Fe alloys. Both substitutional and interstitial He attract solute atoms. The attraction of a substitutional He and a Cu atom is as strong as 0.30 eV (nearest neighbor) or 0.25 eV (next-nearest neighbor), even stronger than the vacancy-Cu pair. Such an attraction facilitates the clustering of Cu atoms. By comparison, the attraction of He to V (0.02 eV) or Ta (0.22 eV) is weaker than that of a vacancy. We find that one He can bind up to four Ta atoms to form a tetrahedron, despite the fact that in the absence of He, Ta atoms prefer to stay away from each other. The effect of He on the solute-solute and solute-matrix interactions can be understood from the facts that He behaves both as a free-volume filler and as a bonding insulator.

YTaO4 and the relevant modification are considered to be a promising new thermal barrier coating. In this article, phase stability and mechanical properties of the monoclinic (M), monoclinic-prime (M′), and tetragonal (T) REMO4 (M = Ta, Nb) are systematically investigated from first-principles calculations method based on density functional theory (DFT). Our calculations show that M′-RETaO4 is the thermodynamically stable phase at low temperatures, but the stable phase is a monoclinic structure for RENbO4. Moreover, the calculated relative energies between M (or M′) and T phases are inversely proportional to the ionic radius of rare earth elements. It means that the phase transformation temperature of M′→T or M→T could decrease along with the increasing ionic radius of RE3+, which is consistent with the experimental results. Besides, our calculations exhibit that adding Nb into the M′-RETaO4 phase could induce phase transformation temperature of M′→M. Elastic coefficient is attained by means of the strain-energy method. According to the Voigt–Reuss–Hill approximation method, bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio of T, M, and M’ phases are obtained. The B/G criterion proposed by Pugh theory exhibits that T, M, and M’ phases are all ductile. The hardness of REMO4 (M = Ta, Nb) phases are predicted based on semi-empirical equations, which is consistent with the experimental data. Finally, the anisotropic mechanical properties of the REMO4 materials have been analyzed. The emerging understanding provides theoretical guidance for the related materials development.

The phase relations in the ZrO2–Ta2O5–Nb2O5 system were experimentally investigated, and thermodynamic modeling was consequently performed in the present work. Samples with the selected compositions were synthesized using coprecipitation method, and then separately equilibrated at 1573 and 1673 K. The microstructural characterizations and phase analyses were carried out with the support of the X‐ray diffraction (XRD) and scanning electron microscopy (SEM/EDX). The phase transformations were measured through differential thermal analysis (DTA). No ternary compounds exist, while Ta2O5 and Nb2O5 phases substantially extend into the ternary system. Moreover, binary Ta2Zr6O17 and Nb2Zr6O17 phases form a continuous solid solution (A2Zr6O17) in the ternary system. A three‐phase equilibrium region β‐Ta2O5 + A2Zr6O17 + Nb2O5 was speculated. Based on the available literature, the ZrO2–Nb2O5, ZrO2–Ta2O5, and Nb2O5–Ta2O5 binary systems were critically evaluated and optimized by means of the CALPHAD (CALculation of PHAse Diagram) method. Combining with thermodynamic parameters of the binary systems, phase relations in the ternary system were extrapolated. However, the extrapolation cannot reproduce our experimental results, especially for the ternary solubilities of ZrO2 in the Ta2O5 and Nb2O5 phases and the melting behavior of selected samples. Finally, the ternary mixing parameters were introduced, and our experimental data were well reproduced by the improved thermodynamic descriptions.

No abstract