| Taking the more and more urgent need for the energy from our society into consideration, a research concerning the material science is presented in this paper. A brief review about the emergence of nuclear energy, the pressing need for nuclear energy and the important application of the zirconium (Zr) and hafnium(Hf) alloys on the nuclear power industry is firstly introduced. In the nuclear facilities, zirconium alloys are widely used in nuclear cladding materials and hafnium alloys are mainly used for control rods. In the zirconium alloys or hafnium alloys, the emergence of hydrogen usually causes the materials brittle, which would influence the working life or even lead to nuclear leakage accidents. Though the interaction between hydrogen (H) and metal zirconium, hydrogen and hafnium alloys has raised great research interests among researchers during the past decades, the influence of H on the mechanical properties and phase transition is still unclear and has many controversies in the literature. Therefore, we have investigated the structural stabilities, mechanical stabilities, phase transition, as well as the intrinsic mechanism of Zr-H and Hf-H system by using first principle calculation.Results revealed that the HCP phase with H at tetrahedral (T) site was the most energetically favorable when0≤H/Zr<0.337, while the FCC phases with H at octahedral (O) and T sites were relatively more stable than other structures when0.337≤H/Zr<0.595and0.595≤H/Zr<1, respectively. Calculation also showed that H location had important effects on mechanical properties of various Zr-H phases, and that the HCP and FCC Zr-H phases with H at T site within the concentration ranges of0.2≤H/Zr<0.337and0.595≤H/Zr<1, respectively, were more brittle than pure HCP Zr, which probably gived a reasonable explanation to the brittleness and delayed hydride cracking of Zr alloys observed experimentally in the literature. In addition, two anisotropic indexes were used to express the elastic anisotropy of various Zr-H phases, and a strong correlation was found between structural stability and elastic anisotropy of various Zr-H phases. The ZrHx phases (x=1,1.25,1.5,1.75, and2) with the cubic fluorite-type (fcc,8phase) and face-centered-tetragonal (fct:ε phase, cla<1;γ phase, c/a>1) structures were all energetically favorable with negative heats of formation of-30to-56kJ/(mol·H) and very small structural energy differences, while mechanical stability played a more important role in determining the existence of various ZrHx phases. Calculation also showed that the intrinsic composition range of the δâ†'ε transition of ZrHx phases was x≥1.5, and that the fundamental mechanism of this transition was mechanical unstableness of the δ phase which would spontaneously transform into ε phase by means of the{110}<110> shear. Moreover, electronic structures showed that the co-function of van Hove singularities and degenerate bands along several directions brought about the high level of density of states at or near the Fermi level and fundamentally induced the mechanical unstableness of the δ phase.Results also revealed that the spin-orbit coupling (SO) should have an important effect on lowering the total energy of each HfHx (x=1,1.25,1.5,1.75, and2) phase with the FCC (δ) and FCT (γ and ε) structures, while has a negligible effect on structural stability of various HfHx phases as well as atomic structure and lattice constant of FCC HfHx phases. Calculations also showed that mechanical stability played a more significant role than thermodynamics in determining the existence of various HfHx phases as well as phase transitions between8, γ and, ε, and that the intrinsic composition range of the δâ†'ε transition was1.75≤r≤2, which could clarify the controversy of experimental observations in the literature. In addition, the splitting of degenerate bands and the change of density of states at Fermi level could bring about a deep understanding of the effect of SO on stability of various HfHx phases. |
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