Yong Jiang

First principles assessment of helium trapping in Y2TiO5 in nano-featured ferritic alloys

Nano-scale Y2Ti2O7 and Y2TiO5 oxides are the major features that provide high strength and irradiation tolerance in nano-structured ferritic alloys. Here, we employ density functional theory to study helium trapping in Y2TiO5. The results suggest that helium is more deeply trapped in Y2TiO5 compared to Y2Ti2O7. Helium occupies open channels in Y2TiO5, where it weakly chemically interacts with neighboring oxygen anions, and results in less volume expansion compared to Y2Ti2O7, reducing strains in the iron matrix. The corresponding helium mobility in these channels is very high. While its ultimate fate is to form oxide/matrix interface bubbles, transient deep trapping of helium in oxides plays a major role in the ability of NFA to manage helium distribution.

Predictions of thermal expansion coefficients of rare-earth zirconate pyrochlores: A quasi-harmonic approximation based on stable phonon modes

Rare-earth (RE) pyrochlores are considered as promising candidate materials for the thermal barrier coating. In this study, we performed first-principles calculations, augmented by quasi-harmonic phonon calculations, to investigate the thermal expansion behaviors of several RE2Zr2O7 (RE = La, Nd, Sm, Gd) pyrochlores. Our findings show that RE2Zr2O7 pyrochlores exhibit low-lying optical phonon frequencies that correspond to RE-cation rattling vibrational modes. These frequencies become imaginary upon volume expansion, preventing correct determination of the free energy versus volume relation and thereby quantification of thermal expansion using QH phonon calculations. To address this challenge, we proposed a QH approximation approach based on stable phonon modes where the RE-cation rattling modes were systematically eliminated. This approach is shown to provide accurate predictions of the coefficients of thermal expansion (CTEs) of RE2Zr2O7 pyrochlores, in good agreement with experimental measurements and data from first-principles molecular dynamics simulations. In addition, we showed that the QH Debye model considerably overestimates the magnitudes and wrongly predicts the trend for the CTEs of RE2Zr2O7 pyrochlores.

First-principles based calculation of the macroscopic α/β interface in titanium

The macroscopic α/β interface in titanium and titanium alloys consists of a ledge interface (112)β/(01-10)α and a side interface (11-1)β/(2-1-10)α in a zig-zag arrangement. Here, we report a first-principles study for predicting the atomic structure and the formation energy of the α/β-Ti interface. Both component interfaces were calculated using supercell models within a restrictive relaxation approach, with various staking sequences and high-symmetry parallel translations being considered. The ledge interface energy was predicted as 0.098u2009J/m2 and the side interface energy as 0.811u2009J/m2. By projecting the zig-zag interface area onto the macroscopic broad face, the macroscopic α/β interface energy was estimated to be as low as ∼0.12u2009J/m2, which, however, is almost double the ad hoc value used in previous phase-field simulations.

High-Temperature Deformation Behavior of Ti-6Al-2Sn-4Zr-2Mo Alloy with Lamellar Microstructure Under Plane-Strain Compression

High-temperature plane-strain compression of a Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloy with a lamellar structure was applied by a thermal simulation machine Gleeble 3800 at a temperature of 1223-1370xa0K and a strain rate of 0.01-10xa0s−1. Constitutive relations between the flow stress and strain at different temperatures and strain rates were constructed based on the Arrhenius equation. Processing maps based on the dynamic material model at strains of 0.5, 1.0, and 1.25 were also constructed to analyze the mechanism and instability of high-temperature deformation. Dynamic recrystallization was found to occur at 1223-1313xa0K and 0.01-0.1xa0s−1, with a peak efficiency of power dissipation of 70%. Observations of the microstructure demonstrated that α platelets and dynamic globularization were responsible for dynamic softening in the αu2009+u2009β phase field. Flow instability behaviors occurred at 1263-1343xa0K/1.78-10xa0s−1, 1233-1283xa0K/0.1-3.16xa0s−1, and 1353-1373xa0K/0.56-1.78xa0s−1, and adiabatic shear bands and microcracks were observed in these domains. Thus, for the lamellar Ti-6242 alloy, the recommended hot rolling parameters were 1243-1333xa0K and 0.01-0.1xa0s−1. Electron backscattered diffraction results revealed that the dynamic restoration mechanism in the β phase field was dynamic recovery when the strain rate was as high as 10xa0s−1.

The Roles of Oxide Interfaces and Grain Boundaries in Helium Management in Nano-Structure Ferritic Alloys: A First Principles Study

We report a theoretical study for assessing the possible roles of oxide interfaces in managing helium in nano-structured ferritic alloys (NFAs). Various candidate structures of the ferrite/Y2Ti2O7 interfaces are constructed and the associated energies are carefully evaluated. The calculated interface phase diagram predicts the interfaces to be Y/Ti-rich, for the wide temperature range of interest. Vacancies and helium both segregate to the interfaces, in preference to grain boundaries. Combined with our previous results on bulk-phase Y2Ti2O7, the profound implication of helium management in NFAs is discussed.

Surface Stabilites and Helium Trapping of Nano-Sized Oxide Phases in Nano-Structured Ferritic Alloys: A First Principles Study

Nano-sized Y2Ti2O7 and Y2TiO5 are primary precipitates in nano-structured ferritic alloys (NFAs). We report a first-principles study on the stabilities of polar Y2Ti2O7 surfaces, and also the selective helium trapping in NFAs. Results show that (1) all variants of Y2Ti2O7 (110) surfaces were found to be more stable than the (100) surfaces, and non-stoichiometric Y2Ti2O7 (110) surfaces are always more stable than their stoichiometric counterparts. (2) The radiation induced helium prefers the octa-interstitial sites in Y2Ti2O7 (or the open-channel along the b-axis of Y2TiO5) to vacancies in the iron matrix. (3) The He-He interaction in Y2Ti2O7 is essentially repulsive. Helium dispersion would largely depend on the number and dispersion of these nano-oxides in NFAs. (4) Charge transfer occurs only between helium and neighboring oxygen anions, suggesting the generality of trapping helium in oxides. Helium trapping at the Fe(100) /Y2Ti2O7(100) interface were also investigated.