Discrete breathers for understanding reconstructive mineral processes at low temperatures

Reconstructive transformations in layered silicates need a high tem- perature in order to be observed. However, very recently, some systems have been found where transformation can be studied at temperatures 600 C below the lowest experimental results previously reported, including sol-gel methods. We explore the possible relation with the existence of intrinsic localized modes, known as discrete breathers. We construct a model for nonlinear vibrations within the cation layer, obtain their parameters and calculate them numerically, obtaining their energies. Their statistics shows that although there are far less breathers than phonons, there are much more above the activation energy, being therefore a good candidate to explain the reconstructive transformations at low temperature.Comment: 27 pages, 11 figure

Born-Infeld with Higher Derivatives

We present new models of non-linear electromagnetism which satisfy the Noether-Gaillard-Zumino current conservation and are, therefore, self-dual. The new models differ from the Born-Infeld-type models in that they deform the Maxwell theory starting with terms like $\lambda (\partial F)^{4}$. We provide a recursive algorithm to find all higher order terms in the action of the form $\lambda^{n} \partial ^{4n} F^{2n+2}$, which are necessary for the U(1) duality current conservation. We use one of these models to find a self-dual completion of the $\lambda (\partial F)^{4}$ correction to the open string action. We discuss the implication of these findings for the issue of UV finiteness of ${\cal N}=8$ supergravity.Comment: 10 pages, revtex4, no figures, 1 table, new reference added, minor typos corrected, accepted for publication in PR

Review on the prediction of residual stress in welded steel components

Residual stress after welding has negative effects on the service life of welded steel components or structures. This work reviews three most commonly used methods for predicting residual stress, namely, empirical, semi-empirical and process simulation methods. Basic principles adopted by these methods are introduced. The features and limitations of each method are discussed as well. The empirical method is the most practical but its accuracy relies heavily on experiments. Mechanical theories are employed in the semi-empirical method, while other aspects, such as temperature variation and phase transformation, are simply ignored. The process simulation method has been widely used due to its capability of handling with large and complex components. To improve its accuracy and efficiency, several improvements need to be done for each simulation aspect of this method

Phase-Field Reaction-Pathway Kinetics of Martensitic Transformations in a Model Fe3Ni Alloy

A three-dimensional phase-field approach to martensitic transformations that uses reaction pathways in place of a Landau potential is introduced and applied to a model of Fe3Ni. Pathway branching involves an unbounded set of variants through duplication and rotations by the rotation point groups of the austenite and martensite phases. Path properties, including potential energy and elastic tensors, are calibrated by molecular statics. Acoustic waves are dealt with via a splitting technique between elastic and dissipative behaviors in a large-deformation framework. The sole free parameter of the model is the damping coefficient associated to transformations, tuned by comparisons with molecular dynamics simulations. Good quantitative agreement is then obtained between both methods.Comment: 4 pages, 3 figure

Displacive model of deformation twinning in hexagonal close-packed metals. Case of the (90 deg, a) and (86 deg, a) extension twins in magnesium

A crystallographic displacive model is proposed for the extension twins in magnesium. It is based on a hard-sphere assumption previously used for martensitic transformations. The atomic displacements are established, and the homogeneous lattice distortion is analytically expressed as a continuous angular-distortive matrix that takes the usual form of shear when the distortion is complete. The calculations prove that a volume change of 3 percents occurs for the intermediate states and that the twinning plane, even if untilted and restored when the distortion is complete, is not fully invariant during the transient states. The crystallographic calculations also show that the (90 deg, a) twins observed in magnesium nano-pillars and the (86 deg, a) twins observed in bulk samples come from the same mechanism, the only difference being the existence of a slight obliquity angle (+/- 3.4 deg) required to reduce the strains in the latter case. Continuous features in the pole figures between the low-misoriented (86 deg, a) twin variants are expected; they are confirmed by EBSD maps acquired on a deformed magnesium single crystal. As the continuous mechanism of extension twinning is not a simple shear, a "virtual work" criterion using the value of the intermediate distortion matrix at the maximum volume change is proposed in place of the usual Schmid's law. It allows predicting the formation of extension twins for crystal orientations associated with negative Schmid factors.Comment: 41 pages, 12 figures, 1 Appendix with 3 figures, 6 Suppl. Material