Narasimhan Swaminathan

Effect of Grain Boundary Stresses on Sink Strength

A fundamental understanding of the interactions between point defects and grain boundaries (GBs) is critical to designing radiation-tolerant nanocrystalline (nc) materials. An important consideration in this design is sink strength, which quantifies the efficiency of a sink to annihilate point defects. Contrary to the common belief that random high-angle GBs provide the upper limit for rate of defect annihilation, here we show that the sink strength of low-angle GBs can exceed that of high-angle GBs due to the effect of GB stress fields. This surprising finding provides a novel opportunity to enhance the radiation resistance of nc materials through GB engineering.

Radiation interaction with tilt grain boundaries in β-SiC

Interaction between grain boundaries and radiation is studied in 3C-SiC by conducting molecular dynamics cascade simulations on bicrystal samples with different misorientation angles. The damage in the in-grain regions was found to be unaffected by the grain boundary type and is comparable to damage in single crystal SiC. Radiation-induced chemical disorder in the grain boundary regions is quantified using the homonuclear to heteronuclear bond ratio (χ). We found that χ increases nearly monotonically with the misorientation angle, which behavior has been attributed to the decreasing distance between the grain boundary dislocation cores with an increasing misorientation angle. The change in the chemical disorder due to irradiation was found to be independent of the type of the grain boundary.

Yield behavior of porous nuclear fuel (UO2)

ABSTRACT Uranium dioxide (UO2) is one of the most common nuclear fuels. During burn-up, the fuel undergoes substantial microstructural changes including the formation of pressurized pores, thus becoming a porous material. These pores reduce the elastic modulus and alter the yield behavior of the material. In this work, a finite-element-based homogenization technique has been used to map the yield surface of UO2 with pressurized pores. Two scenarios are considered; in the first, the fuel matrix is a ductile material with a Von-mises type behavior, while in the second, the matrix is quasi brittle, which is simulated using the concrete damaged plasticity (CDP) model available in ABAQUS. For both of the scenarios, it is found that the yield strength decreases with an increase in porosity for a given internal pore pressure. For a given porosity, the yield surface shifts towards the negative hydrostatic axis in the Haigh-Westergard stress space with an increase in pore pressure. When the matrix is quasi brittle, the decrease in tensile hydrostatic strength is less than the increase in compressive hydrostatic strength, whereas in the case of a ductile matrix, the changes in the hydrostatic strengths are same. Furthermore, the shape of the yield surface changes from one deviatoric plane to another in both scenarios. Analytical equations, which are functions of pore pressure and porosity, are developed to describe the yield surface of porous UO2 while accounting for the changes in shape of the yield surface from one deviatoric plane to another. These yield functions can be used to predict the failure of porous UO2 fuel.

Effects of confinements on morphology of InxGa1−xAs thin film grown on sub-micron patterned GaAs substrate: Elastoplastic phase field model

An elastoplastic phase field model is developed to investigate the role of lateral confinement on morphology of thin films grown heteroepitaxially on patterned substrates. Parameters of the model are chosen to represent InxGa1−xAs thin films growing on GaAs patterned with SiO2. We determined the effect of misfit strain on morphology of thin films grown in 0.5 μm patterns with non-uniform deposition flux. Growth of islands inside patterns can be controlled by non-uniformity of deposition flux, misfit strain between film and the substrate, and also strain energy relaxation due to plastic deformation. Our results show that the evolution of island morphology depends non-monotonically on indium content and associated misfit strain due to coupling between the plastic relaxation and the confinements effects. Low indium concentration (0%–40%) causes formation of instabilities with relatively long wavelengths across the width of the pattern. Low surface diffusion (due to low indium concentration) and fewer islands...

Interfacial strength cross-over across silica- and graphite-cis-1,4-polyisoprene interfaces

A cross-over in the interfacial strength, with increase in the separation rate, is observed between graphite-cis-1,4-polyisoprene and amorphous silica-cis-1,4-polyisoprene interfaces. Molecular dynamics simulations are used to compare the traction-separation characteristics of the two interfaces in the opening mode of separation at various separation rates and temperatures above the glass transition temperature of cis-1,4-polyisoprene. It was observed that various parameters governing the interface strength, such as strength modulus (ratio of peak traction to the separation at peak traction), peak traction, and the work of adhesion are higher for the silica substrated interface at very low separation rates. However, at higher rates, the graphite substrated interface showed higher values for the strength parameters. The reasons for this interface strength cross-over are explained using the potential energy, mobility, entanglement strength, tensile stiffness, and densities of the polymer over both substrates and the interface cohesive binding energy. Based on these observations, it is concluded that silica filled rubber nanocomposites are suitable for normal automobile tire applications; however, graphite fillers may be more suitable for resisting very large impact loads.A cross-over in the interfacial strength, with increase in the separation rate, is observed between graphite-cis-1,4-polyisoprene and amorphous silica-cis-1,4-polyisoprene interfaces. Molecular dynamics simulations are used to compare the traction-separation characteristics of the two interfaces in the opening mode of separation at various separation rates and temperatures above the glass transition temperature of cis-1,4-polyisoprene. It was observed that various parameters governing the interface strength, such as strength modulus (ratio of peak traction to the separation at peak traction), peak traction, and the work of adhesion are higher for the silica substrated interface at very low separation rates. However, at higher rates, the graphite substrated interface showed higher values for the strength parameters. The reasons for this interface strength cross-over are explained using the potential energy, mobility, entanglement strength, tensile stiffness, and densities of the polymer over both substrate...