Lattice Green's Function for Multiscale Modeling of Strain Field Due to a Vacancy or Other Point Defects in Graphene

A multiscale Green's function method, based upon a solution of the Dyson equation, is described for modeling the strain field due to a vacancy or any other point defect in graphene and other 2D materials. Numerical results are presented using a fourth-neighbor force-constant model for the purpose of illustration.

A Variational Principle of Dislocations Kinetic in Crystals and a Toughening Mechanism of BCC or HCP Metals

A dislocation kinetics-based analysis has been carried out on the toughening mechanisms of alloys. It is concluded that both improved strength and toughening can be achieved through adjusting the short range interatomic interactions between embedded solute atoms, or other point defects, that affect Peierls-Nabarro energy barrier, and the long range interactions between dislocation loops and heterogeneities such coherent precipitates, second phase particles, and crystallography; the latter determines dislocation loops’ patterns such as kink-jog formation. In order to quantify the effects of lattice heterogeneities, a variation principle that defines the energy minima of dislocation line configuration has been derived, which includes the effects of three-dimensional stress states and crystallography, instead of the conventional line energy-based Eular formulation that only considers the case under shear stress. This provides an analytical means and associated numerical tool to determine the favorite dislocation loop’s patterns in an alloy. The further analysis reveals that double-kinks within single slip-plane have limited effect on toughening while the corresponding bow-out solution may lead to a lower-bound estimate of precipitate strengthening. Therefore, a proposed strategy for toughening is to create dispersed softening centers in strengthened matrix that trap accumulated dislocation loops in the form of mixed double-kinks and jog-induced climbings, for example, helices. These kinds of dislocation patterns are able to spread out localized dislocations from single or close packed parallel slipping planes to many cross-over planes in multiple slip-systems, so as to delay the formation of shear bands while maximize the magnitude of bowing-out induced strengthening.

Formation of Defect Clusters in SrTiO3 Crystals Implanted with Xenon Ions

SrTiO3 crystals were implanted with 100 keV xenon (Xe+) ions at 673 or 1073 K up to 2.0 × 1020 ions m−2. Defect clusters formed in the ion-implanted samples were investigated with conventional and high-resolution transmission electron microscopy. Nanometer-sized clusters were formed in the samples. The clusters grew large in size after post-implantation annealing and with increasing the implantation dose. The clusters were faceted with {100}, or {110} of SrTiO3. Though the nano-sized clusters were expected to contain Xe atoms, they were not in crystalline state. The results suggest that even if the clusters contain Xe atoms, they also contain other point defects such as vacancies.

Electron irradiation and heat treatment of polycrystalline CVD diamond

Photoluminescence and Raman spectra have been used to characterize the properties of diamond films grown by microwave plasma assisted chemical vapour deposition. Measurements at 77 K and excitation wavelengths in the range 476.5 nm to 514.5 nm show the presence of two components, A and B, in the Raman spectrum in addition to the diamond Raman line. The A and B components are rather similar in appearance and show resonant Raman behaviour. Electron irradiation results in the removal of the A and B Raman components, but they return to their original strength after after heating at 600 °C. The Raman scattering species interact with other point defects in the CVD films during heat treatment, and may be related to the presence of silicon in the diamond film.