Dislocation creation and void nucleation in FCC ductile metals under tensile loading: a general microscopic picture.

Numerous theoretical and experimental efforts have been paid to describe and understand the dislocation and void nucleation processes that are fundamental for dynamic fracture modeling of strained metals. To date an essential physical picture on the self-organized atomic collective motions during dislocation creation, as well as the essential mechanisms for the void nucleation obscured by the extreme diversity in structural configurations around the void nucleation core, is still severely lacking in literature. Here, we depict the origin of dislocation creation and void nucleation during uniaxial high strain rate tensile processes in face-centered-cubic (FCC) ductile metals. We find that the dislocations are created through three distinguished stages: (i) Flattened octahedral structures (FOSs) are randomly activated by thermal fluctuations; (ii) The double-layer defect clusters are formed by self-organized stacking of FOSs on the close-packed plane; (iii) The stacking faults are formed and the Shockley partial dislocations are created from the double-layer defect clusters. Whereas, the void nucleation is shown to follow a two-stage description. We demonstrate that our findings on the origin of dislocation creation and void nucleation are universal for a variety of FCC ductile metals with low stacking fault energies.

Nucleation and growth mechanisms of hcp domains in compressed iron.

In our previous work, we have pointed out that the shock-induced phase transition in iron occurs with the help of interface energy which reduces the potential barrier between two phases. Here, through studying the nucleation and growth mechanisms of hcp domains in compressed iron, we find that the flatted-octahedral-structure (FOS) is the primary structural unit of the embryo nucleus and phase interface of hcp domains, and the interfacial energy is reduced via formation of FOSs. The phase transition process can be described by the following four stages: (i) Some atoms deviate from their equilibrium positions with the aid of thermal fluctuations to form FOSs with two different deformation directions in the local region; (ii) FOSs with different deformation directions aggregate to form a thin stratified structure like twin-crystal configuration; (iii) The thin stratified structure undergoes a relative slip to form the new hcp phase; (iv) The hcp phase domain grows up through the formation of new FOSs along the phase boundary. In addition, through comparing the time evolution curves of initial single phase domain, we find that the growth rate of single phase domain depends on the loading way and its occurrence time.

The different roles of Pu-oxide overlayers in the hydrogenation of Pu-metal: an ab initio molecular dynamics study based on van der Waals density functional (vdW-DF)+U.

Based on the non-local van der Waals density functional (vdW-DF)+U scheme, we carry out the ab initio molecular dynamics (AIMD) study of the interaction dynamics for H2 impingement against the stoichiometric PuO2(111), the reduced PuO2(111), and the stoichiometric α-Pu2O3(111) surfaces. The hydrogen molecular physisorption states, which cannot be captured by pure DFT+U method, are obtained by employing the vdW-DF+U scheme. We show that except for the weak physisorption, PuO 2(111) surfaces are so difficult of access that almost all of the H2 molecules will bounce back to the vacuum when their initial kinetic energies are not sufficient. Although the dissociative adsorption of H2 on PuO2(111) surfaces is found to be very exothermic, the collision-induced dissociation barriers of H2 are calculated to be as high as 3.2 eV and 2.0 eV for stoichiometric and reduced PuO2 surfaces, respectively. Unlike PuO2, our AIMD study directly reveals that the hydrogen molecules can penetrate into α-Pu2O3(111) surface and diffuse easily due to the 25% native O vacancies located along the ⟨111⟩ diagonals of α-Pu2O3 matrix. By examining the temperature effect and the internal vibrational excitations of H2, we provide a detailed insight into the interaction dynamics of H2 in α-Pu2O3. The optimum pathways for hydrogen penetration and diffusion, the corresponding energy barriers (1.0 eV and 0.53 eV, respectively) and rate constants are systematically calculated. Overall, our study fairly reveals the different interaction mechanisms between H2 and Pu-oxide surfaces, which have strong implications to the interpretation of experimental observations.

Morphology and growth speed of hcp domains during shock-induced phase transition in iron.

Emergence and time evolution of micro-structured new-phase domains play a crucial role in determining the macroscopic physical and mechanical behaviors of iron under shock compression. Here, we investigate, through molecular dynamics simulations and theoretical modelings, shock-induced phase transition process of iron from body-centered-cubic (bcc) to hexagonal-close-packed (hcp) structure. We present a central-moment method and a rolling-ball algorithm to calculate and analyze the morphology and growth speed of the hcp phase domains, and then propose a phase transition model to clarify our derived growth law of the phase domains. We also demonstrate that the new-phase evolution process undergoes three distinguished stages with different time scales of the hcp phase fraction in the system.

Terahertz wave generation from hyper-Raman lines in two-level quantum systems driven by two-color lasers.

Based on spatial-temporal symmetry breaking mechanism, we propose a novel scheme for terahertz (THz) wave generation from hyper-Raman lines associated with the 0th harmonic (a particular even harmonic) in a two-level quantum system driven by two-color laser fields. With the help of analysis of quasi-energy, the frequency of THz wave can be tuned by changing the field amplitude of the driving laser. By optimizing the parameters of the laser fields, we are able to obtain arbitrary frequency radiation in the THz regime with appreciable strength (as strong as the typical harmonics). Our proposal can be realized in experiment in view of the recent experimental progress of even-harmonics generation by two-color laser fields.

First-principles local density approximation + U and generalized gradient approximation + U study of plutonium oxides.

The electronic structure and properties of PuO2 and Pu2O3 have been studied from first principles by the all-electron projector-augmented-wave method. The local density approximation+U and the generalized gradient approximation+U formalisms have been used to account for the strong on-site Coulomb repulsion among the localized Pu 5f electrons. We discuss how the properties of PuO2 and Pu2O3 are affected by the choice of U as well as the choice of exchange-correlation potential. Also, oxidation reaction of Pu2O3, leading to formation of PuO2, and its dependence on U and exchange-correlation potential have been studied. Our results show that by choosing an appropriate U, it is promising to correctly and consistently describe structural, electronic, and thermodynamic properties of PuO2 and Pu2O3, which enable the modeling of redox process involving Pu-based materials possible.

Transitions and transport for a spatially periodic stochastic system with locally coupled oscillators.

In this paper, with a special model, we investigate the spatially periodic stochastic system with locally coupled oscillators subject to a constant force F. A nonequilibrium second-order phase transition is found when F=0. This phase transition is reentrant when the additive noise is weak. With varying the constant force F, a continuous or discontinuous transition between the states with positive and negative mean fields (mu>0 and mu<0) is observed, which is not a phase transition. The mean field or current sometimes exhibits hysteresis as a function of F. With the variation of the force F, when hysteresis of the mean field or current versus F appears, a nonzero probability current with definite direction will occur at the point F=0. The correlation between the additive and multiplicative noises has an effect on the transitions and the transport.