First-principles calculations of phase transition, elasticity, and thermodynamic properties for TiZr alloy

tructural transformation, pressure dependent elasticity behaviors, phonon, and thermodynamic properties of the equiatomic TiZr alloy are investigated by using first-principles density-functional theory. Our calculated lattice parameters and equation of state for $\alpha$ and $\omega$ phases as well as the phase transition sequence of $\alpha$$\mathtt{\rightarrow}$$\omega$$\mathtt{\rightarrow}$$\beta$ are consistent well with experiments. Elastic constants of $\alpha$ and $\omega$ phases indicate that they are mechanically stable. For cubic $\beta$ phase, however, it is mechanically unstable at zero pressure and the critical pressure for its mechanical stability is predicted to equal to 2.19 GPa. We find that the moduli, elastic sound velocities, and Debye temperature all increase with pressure for three phases of TiZr alloy. The relatively large $B/G$ values illustrate that the TiZr alloy is rather ductile and its ductility is more predominant than that of element Zr, especially in $\beta$ phase. Elastic wave velocities and Debye temperature have abrupt increase behaviors upon the $\alpha$$\mathtt{\rightarrow}$$\omega$ transition at around 10 GPa and exhibit abrupt decrease feature upon the $\omega$$\mathtt{\rightarrow}$$\beta$ transition at higher pressure. Through Mulliken population analysis, we illustrate that the increase of the \emph{d}-band occupancy will stabilize the cubic $\beta$ phase. Phonon dispersions for three phases of TiZr alloy are firstly presented and the $\beta$ phase phonons clearly indicate its dynamically unstable nature under ambient condition. Thermodynamics of Gibbs free energy, entropy, and heat capacity are obtained by quasiharmonic approximation and Debye model.Comment: 9 pages, 10 figure

Low frequency sea level variability on the Middle Atlantic Bight

Low-frequency sea level fluctuations on the Mid-Atlantic Bight, from Cape Cod to Cape Hatteras, and their relations to wind forcing were examined over a one-year (1975) period. The dominant sea level fluctuations occurred at time scales of 4 days, and they were coherent over the entire Bight. On the other hand, sea levels were not coherent between the southern (south of Kiptopeake B.) and northern part at shorter time scales...

Model of frontogenesis: Subduction and upwelling

A high-resolution, three-dimensional, primitive-equation model is used to study frontogenesis. The initial state includes a surface front and geostrophic jet. A small initial disturbance grows rapidly into a steepened backward-breaking wave, characterized by narrow wave trough and broad wave crest. Analysis of the energetics indicates that the unstable waves are generated by baroclinic instability. The wavelength scales as the baroclinic deformation radius, but the growth rate appears to be much faster than found in previous primitive-equation model studies. The predicted downward velocity also is an order of magnitude greater than found in previous model studies. As the amplitude of unstable wave becomes very large, a narrow density front whose width is less than the deformation radius, is formed in the wave trough. The frontal zone is marked by high cyclonic vorticity (relative vorticity \u3e f) and intense surface subduction (50–100 m day−1). The frontogenesis is caused by the interaction between synoptic-scale confluence and mesoscale convergence. The strong vertical circulation associated with frontal waves may play a major role in the material exchange and biological production in frontal zone

Generation and propagation of inertial waves in the subtropical front

A primitive-equation numerical model is used to examine the generation and propagation of internal-inertial waves in the Subtropical Front. The mesoscale variability in surface inertial currents is induced by radiation of internal-inertial waves out of the surface layer. On the warm side of the front, surface inertial energy is carried away by normal internal-inertial waves. A deep inertial energy maximum exists at the base of the thermocline where the effective local inertial frequency approaches the planetary inertial frequency. On the cold side of the front, the surface inertial energy is carried away by anomalously low frequency internal waves. A subsurface inertial energy maximum occurs at the top of the thermocline where the density slope becomes flat. The propagation of internal-inertial waves is consistent with the WKB approximation. On the other hand, since the upper-ocean response consists of a full spectrum of internal-inertial waves, prediction of inertial energy distribution based on ray theory is invalid. Comparison between model results and current profiler observations in the Subtropical Front is quite favorable