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A density functional theory (DFT) study of the 1/2 111 screw dislocation was performed in the following body-centered cubic transition metals: V, Nb, Ta, Cr, Mo, W, and Fe. The energies of the easy, hard, and split core configurations, as well as the pathways between them, were investigated and used to generate the two-dimensional (2D) Peierls potential, i.e. the energy landscape seen by the dislocation as a function of its position in the (111) plane. In all investigated elements, the nondegenerate easy core is the minimum energy configuration, while the split core configuration, centered in the immediate vicinity of a 111 atomic column, has a high energy near or above that of the hard core. This unexpected result yields 2D Peierls potentials very different from the usually assumed landscapes. The 2D Peierls potential in Fe differs from the other transition metals, with a monkey saddle instead of a local maximum located at the hard core. An estimation of the Peierls stress from the shape of the Peierls barrier is presented in all investigated metals. A strong group dependence of the core energy is also evidenced, related to the position of the Fermi level with respect to the minimum of the pseudogap of the electronic density of states.

The plastic behavior of the insensitive energetic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is investigated through molecular dynamics simulations.A recent method, built to follow any prescribed deformation path, is used to apply directional shear and compressive deformations to a TATB single crystal, leading to the tridimensional characterization of its nucleation von Mises stresses σ v (θ, φ), where θ and φ are the two angles (latitude and longitude, respectively) that define the loading direction. Furthermore, the local computation of the deformation gradient tensor helps to identify the mechanisms of the irreversible deformation. Two main types of plasticity mechanisms have been identified for the TATB single crystal: first, molecular dynamics simulations predict the existence of dislocations with an unusual local through-plane dilatancy process. Various slip systems among four different non-basal planes have been identified, namely ( 101), (101), (0 11) and (011) planes. Secondly, every deformation containing a basal-plane compressive component involves buckling deformation. A deformation path allowing a perfect twinning of the TATB triclinic cell has been found. This structure has been verified through molecular dynamics (MD) simulations. In order to understand the buckling mechanism, the TATB single crystal behavior under compression along its basal plane is studied in detail.

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is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. ABSTRACT: The dislocation core structures and elastic properties of the insensitive energetic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) are investigated as a function of pressure and temperature. A new method is proposed to compute the generalized stacking fault surfaces (noted γ-surfaces) and the complete second-order elastic tensor at finite temperature through molecular dynamics (MD) simulations. The energy landscapes in the two glide planes are shown to be similar between 0 and 300 K, thus leading to almost no modification on the dislocation evolution. A spreading of the dislocation cores over a hundred Burgers vectors is observed along the [100] and [010] directions for the edge and screw dislocations at 0 and 300 K, showing that dislocations should exhibit a very low friction for these glide systems at ambient pressure. For pressures varying between 0 and 10 GPa, the γ-surfaces' energy barriers that drive the width of partial dislocations follow the increase of shear elastic constants within the considered glide planes, thus limiting the changes of the dislocation core structure.

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An approximate equation of motion is proposed for screw and edge dislocations, which accounts for retardation and for relativistic effects in the subsonic range. Good quantitative agreement is found, in accelerated or in decelerated regimes, with numerical results of a more fundamental nature.Comment: 6 pages, 4 figures, LaTe