Interface-dominated Growth of a Metastable Novel Alloy Phase

A new \textit{D0$_{23}$} metastable phase of Cu$_3$Au is found to grow at the interfaces of Au/Cu multilayers deposited by magnetron sputtering. The extent of formation of this novel alloy phase depends upon an optimal range of interfacial width primarily governed by the deposition wattage of the dc-magnetron used. Such interfacially confined growth is utilized to grow a $\sim$ 300 nm thick Au/Cu multilayer with thickness of each layer nearly equal to the optimal interfacial width which was obtained from secondary ion mass spectrometry (SIMS) data. This growth technique is observed to enhance the formation of the novel alloy phase to a considerable extent. SIMS depth profile also indicates that the mass fragment corresponding to Cu$_3$Au occupies the whole film while x-ray diffraction (XRD) shows almost all the strong peaks belonging to the \textit{D0$_{23}$} structure. High resolution cross-sectional transmission electron microscopy (HR-XTEM) shows the near perfect growth of the individual layers and also the lattice image of the alloy phase in the interfacial region. Vacuum annealing of the alloy film and XRD studies indicate stabilization of the \textit{D0$_{23}$} phase at $\sim$ 150$^{\circ}$C. The role of interfacial confinement, the interplay between interfacial strain and free energy and the hyperthermal species generated during the sputtering process are discussed.Comment: Accepted in Journal of Materials Researc

DEVELOPMENT OF A SELF-CONSISTENT COUPLED ATOMISTIC-CONTINUUM MODEL TO STUDY THE BRITTLE AND DUCTILE FRACTURE IN METALLIC MATERIALS

Modeling fracture and failure of material is a complex phenomenon that needs atomic-scale understanding of the kinetics and energetics of different deformation mechanisms. Several efforts has been made over the years to model the fracture at continuum scale e.g, cohesive zone model, phase-field model. The success of these continuum scale fracture models rely on the appropriate incorporation of the interaction between the crack and the different deformation mechanisms within the material such as interatomic decohesion, dislocation nucleation, mobility of the dislocations, dislocation reaction, twining etc. Hence, there is a need to develop a systematic framework to quantify these interactions and develop physics-based constitutive laws that can be used in continuum scale fracture models. This dissertation develops a concurrent coupled atomistic-continuum model to capture the interaction between different deformation mechanisms on the propagation of crack. The atomistic region is modeled using time-accelerated Molecular Dynamics(MD) and for the continuum region, the density-based Crystal Plasticity Finite Element(CPFE) model is used. Hyperdynamics is used for the time acceleration of the MD. The atomistic-continuum coupling is achieved by enforcing geometric compatibility and force equilibrium in the interface region. A sequence of steps is performed to characterize and quantify the dislocations at the interface and then transfer those dislocations from the atomistic to the continuum region in the density form. The propagation of the dislocations in the density form is modeled by solving the transport equation of a conserved quantity, also known as the advection equation. The mesh-less Reduced Kernel Particle Method(RKPM) is used to solve the advection equation over the continuum domain. The developed concurrent coupled atomistic-continuum model is used to study the brittle and ductile propagation of a crack in a nickel single crystal. A parametrized form of crack propagation law and the evolution of dislocation density is extracted from the model. The concurrent model has also been used to construct the free energy functional of the phase-field model where the evolution of different energy contributions during the fracture process is obtained. These evolution laws can be employed in full continuum scale models to study the fracture process at a larger spatial scale

Destabilization, stabilization, and multiple attractors in saturated mixotrophic environments

The ability of mixotrophs to combine phototrophy and phagotrophy is now well recognized and found to have important implications for ecosystem dynamics. In this paper we examine the dynamical consequences of the invasion of mixotrophs in a model that is a limiting case of the chemostat. The model is a hybrid of a competition model describing the competition between populations of autotroph and mixotroph for limiting resources, and a predator-prey type model describing the interaction between populations of autotroph and herbivore. Our results show that mixotrophs are able to invade in both autotrophic environments and environments described by interactions between autotrophs and herbivores. The interaction between autotrophs and herbivores might be in equilibrium or cycle. We find that invading mixotrophs have the ability to both stabilize and destabilize autotroph-herbivore dynamics depending on the competitive ability of mixotrophs. Moreover the invasion of mixotrophs can also result in multiple attractors. Therefore, our results reveal important consequences of mixotrophic invasions in ecosystems depending on environmental conditions.Comment: 51 pages, 9 figure

Modelling succession of key resource harvesting traits of mixotrophic plankton populations

Unicellular eukaryotes make up the base of the ocean food web and exist as a continuum in trophic strategy from pure heterotrophy (phagotrophic zooplankton) to pure photoautotrophy (‘phytoplankton'), with a dominance of mixotrophic organisms combining both strategies. Here we formulate a trait-based model for mixotrophy with three key resource-harvesting traits: photosynthesis, phagotrophy and inorganic nutrient uptake, which predicts the trophic strategy of species throughout the seasonal cycle. Assuming that simple carbohydrates from photosynthesis fuel respiration, and feeding primarily provides building blocks for growth, the model reproduces the observed light-dependent ingestion rates and species-specific growth rates with and without prey from the laboratory. The combination of traits yielding the highest growth rate suggests high investments in photosynthesis, and inorganic nutrient uptake in the spring and increased phagotrophy during the summer, reflecting general seasonal succession patterns of temperate waters. Our trait-based model presents a simple and general approach for the inclusion of mixotrophy, succession and evolution in ecosystem models

Effects of fertilizers used in agricultural fields on algal blooms

The increasing occurrence of algal blooms and their negative ecological impacts have led to intensified monitoring activities. This needs the proper identification of the most responsible factor/factors for the bloom formation. However, in natural systems, algal blooms result from a combination of factors and from observation it is difficult to identify the most important one. In the present paper, using a mathematical model we compare the effects of three human induced factors (fertilizer input in agricultural field, eutrophication due to other sources than fertilizers, and overfishing) on the bloom dynamics and DO level. By applying a sophisticated sensitivity analysis technique, we found that the increasing use of fertilizers in agricultural field causes more rapid algal growth and decreases DO level much faster than eutrophication from other sources and overfishing. We also look at the mechanisms how fertilizer input rate affects the algal bloom dynamics and DO level. The model can be helpful for the policy makers in determining the influential factors responsible for the bloom formation