Many-Scale Investigations of Deformation Behavior of Polycrystalline Composites: II-Micro-Macro Simultaneous FE and Discrete Dislocation Dynamics Simulation

The current work numerically investigates commercial polycrystalline Ag/17vol.%SnO2 composite tensile deformation behavior with available experimental data. Such composites are useful for electric contacts and have a highly textured initial material status after hot extrusion. Experimentally, the initial sharp fiber texture and the number of Σ3-twins were reduced due to tensile loading. The local inhomogeneous distribution of hardness and Young’s modulus gradually decreased from nanoindentation tests, approaching global homogeneity. Many-scale simulations, including micro-macro simultaneous finite element (FE) and discrete dislocation dynamics (DDD) simulations, were performed. Deformation mechanisms on the microscale are fundamental since they link those on the macro- and nanoscale. This work emphasizes micromechanical deformation behavior. Such FE calculations applied with crystal plasticity can predict local feature evolutions in detail, such as texture, morphology, and stress flow in individual grains. To avoid the negative influence of boundary conditions (BCs) on the result accuracy, BCs are given on the macrostructure, i.e., the microstructure is free of BCs. The particular type of 3D simulation, axisymmetry, is preferred, in which a 2D real microstructural cutout with 513 Ag grains is applied. From FE results, Σ3-twins strongly rotated to the loading direction (twins disappear), which, possibly, caused other grains to rotate away from the loading direction. The DDD simulation treats the dislocations as discrete lines and can predict the resolved shear stress (RSS) inside one grain with dependence on various features as dislocation density and lattice orientation. The RSS can act as the link between the FE and DDD predictions.

Ab initio and shell model studies of structural, thermoelastic and vibrational properties of SnO2 under pressure

The pressure dependences of the structural, thermoelastic and vibrational properties of SnO2 in its rutile phase are studied, as well as the pressure-induced transition to a CaCl2-type phase. These studies have been performed by means of ab initio (AI) density functional theory calculations using the localized basis code SIESTA. The results are employed to develop a shell model (SM) for application in future studies of nanostructured SnO2. A good agreement of the SM results for the pressure dependences of the above properties with the ones obtained from present and previous AI calculations as well as from experiments is achieved. The transition is characterized by a rotation of the Sn-centered oxygen octahedra around the tetragonal axis through the Sn. This rotation breaks the tetragonal symmetry of the lattice and an orthorhombic distortion appears above the critical pressure P(c). A zone-center phonon of B1g symmetry in the rutile phase involves such rotation and softens on approaching Pc. It becomes an Ag mode which stabilizes with increasing pressure in the CaCl2 phase. This behavior, together with the softening of the shear modulus (C11-C12)/2 related to the orthorhombic distortion, allows a precise determination of a value for Pc. An additional determination is provided by the splitting of the basal plane lattice parameters. Both the AI and the experimentally observed softening of the B(1g) mode are incomplete, indicating a small discontinuity at the transition. However, all results show continuous changes in volume and lattice parameters, indicating a second-order transition. All these results indicate that there should be sufficient confidence for the future employment of the shell model.

Namomechanical Imaging and Nanoscale Elastic Modulus Measurements of SnO2 Nanobelts

The relative surface contact stiffness of SnO2 nanobelts has been investigated via ultrasonic force microscopy (UFM). The nanobelt crystal structure, as determined via transmission electron microscopy, was indexed to the tetragonal rutile structure (with lattice constants identical to those of bulk SnO2) as reported previously. The atomic Sn:O composition of the nanobelts studied was at or near 1:2. Topographic imaging studies revealed the nanobelt surface to be atomically flat with the exception of surface nanodots, assumed to be local SnO2 crystallites. Preliminary reduced modulus measurements were carried out via differential UFM on both the flat and nanodot regions of the nanobelt. Using the underlying Si substrate as a calibration standard the SnO2 modulus was estimated at 157±12 GPa, significantly lower than corresponding bulk values for any of the observed crystal orientations. We speculate this discrepancy is due in part to a combination of the aspherical probe tip and unknown adhesive properties of nanobelt. An intrinsic reduction of the SnO2 nanobelt modulus cannot be ruled out.

Thermally driven interfacial dynamics of metal/oxide bilayer nanoribbons

Solid-solid interfacial processes greatly affect the performance of electronic and composite materials, but probing the dynamics of buried interfaces is challenging and often involves lengthy or invasive sample preparation. We show that bilayer nanoribbons-made here of tin dioxide and copper-are convenient structures for observing as-made interfaces as they respond to changing temperature in a transmission electron microscope (TEM). At low temperatures (<200 degrees C), differential thermal expansion causes the bilayers to bend when heated or cooled, with the motion determined by the extent of Cu-SnO(2) epitaxy. At higher temperatures, we are able to watch-in real time and with nanometer resolution-a progression of grain growth, interdiffusion, island formation, solid-state chemical reactions, and melting. This novel TEM geometry is readily applicable to other nanoribbon/coating combinations and is well suited to observing interfacial phenomena driven thermally or by the application of mechanical, electrical, or magnetic forces.