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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.

Zirconium (Zr)-based alloys, a new class of hard-tissue replacement materials, show lower strength compared to traditional medical metal materials such as stainless steel, cobalt alloy, and Ti-6Al-4V alloys, which may lead to premature fracture of the implant. Spinodal decomposition can increase the strength greatly without an increase in the elastic modulus of the alloy. In this study, a phase field method based on the Cahn–Hilliard equation was applied to the simulation of the spinodal decomposition in Zr–Nb alloys. The spinodal region on the Zr–Nb phase diagram was calculated by the phase field method by considering the interfacial energy and elastic strain energy contribution to the total Gibbs free energy. Furthermore, the effects of the Nb content and heat-treatment temperature on the morphology, amplitude, and volume fraction of the decomposition phases are discussed. Simulation results indicate that the morphology of the β′ phase is interconnected and regular with a preferential alignment in the ⟨110⟩ direction to reduce the strain energy, which may restrict the spinodal decomposition of the alloys. The two droplet phases merge, which can be attributed to the reduction in the elastic strain energy. The phase decomposition rate increases with an increase in aging temperature, but the aging temperature has only a small influence on the final volume fraction of the β′ phase.

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