A differential cluster variation method for analysis of spiniodal decomposition in alloys

A differential cluster variation method (DCVM) is proposed for analysis of spinoidal decomposition in alloys. In this method, lattice symmetry operations in the presence of an infinitesimal composition gradient are utilized to deduce the connection equations for the correlation functions and to reduce the number of independent variables in the cluster variation analysis. Application of the method is made to calculate the gradient energy coefficient in the Cahn-Hilliard free energy function and the fastest growing wavelength for spinodal decomposition in Al-Li alloys. It is shown that the gradient coefficient of congruently ordered Al-Li alloys is much larger than that of the disordered system. In such an alloy system, the calculated fastest growing wavelength is approximately 10 nm, which is an order of magnitude larger than the experimentally observed domain size. This may provide a theoretical explanation why spinodal decomposition after a congruent ordering is dominated by the antiphase boundaries.Comment: 15 pages, 7 figure

Keynote – Mechanics of cellular uptake of one- and two-dimensional nanomaterials

With the rapid development of nanotechnology, various types of nanoparticles, nanowires, nanofibers, nanotubes, and atomically thin plates and sheets have emerged as candidates for an ever increasing list of potential applications for next generation electronics, microchips, composites, barrier coatings, biosensors, drug delivery, and energy harvesting, and conversion systems. There is now an urgent societal need to understand both beneficial and hazardous effects of nanotechnology which is projected to produce and release thousands of tons of nanomaterials into the environment in the coming decades. This discussion aims to present an overview of some recent studies conducted at Brown University on the mechanics of cell-nanomaterial interactions, including the modelling of nanoparticles entering cells by receptor-mediated endocytosis and coarse-grained molecular dynamics simulations of nanoparticles interacting with cell membranes. The discussions will be organized around the following questions: Why and how does cellular uptake of nanoparticles depend on particle size, shape, elasticity, and surface structure? In particular, we will discuss the effect of nanoparticle size on receptor-mediated endocytosis, the effect of elastic stiffness on cell-particle interactions, how high aspect ratio nanomaterials such as carbon nanotubes and graphenes enter cells and how different geometrical patterns of ligands on a nanoparticle can be designed to control the rate of particle uptake. REFERENCE Gao, H.J. Probing mechanical principles of cell-nanomaterial interactions. J. Mechanics Phys. Solids. 2014; 62(1), 312–339

Modeling fracture in nanotwinned matetrials

The author reports on the results obtained so far through a combination of advanced experimental and statistical techniques as well as constitutive modeling based on a continuum dislocation dynamics and viscoplastic model. The implementation relies on a finite element code to perform simulations of DP980 steels which can be compared to experiments. Design guidelines will be in the form of a microstructure map that relates thermo-mechanical processing conditions to desired properties and cost constraints. The aim will be to guide manufacturers in selecting a series of processing steps to transform the original material to a final material with specific properties

Effects of interfacial friction on flaw tolerant adhesion between two dissimilar elastic solids

Flaw tolerance refers to a state in which a pre-existing crack-like flaw does not propagate even as the material is stretched to failure near its theoretical strength. Such an optimal scenario can be achieved when the characteristic length scale is reduced to below a critical value. So far, the critical conditions to achieve flaw tolerance have been discussed mostly for homogeneous materials or for two dissimilar materials in frictionless or perfectly bonded adhesion. In this paper, we consider the role of friction in flaw tolerant adhesion between two dissimilar elastic solids. We adopt a frictional contact model in which slip is allowed wherever the shear stress along the interface reaches a threshold value defined as the friction strength. The critical length scale for flaw tolerance is derived analytically for a penny-shaped crack and for an external circular crack. Compared to the cases of frictionless contact, we find that interfacial friction can reduce the critical length scales for flaw tolerance by up to 12.5%

Deformation mechanisms in nanotwinned metal nanopillars

Nanotwinned metals are attractive in many applications because they simultaneously demonstrate high strength and high ductility, characteristics that are usually thought to be mutually exclusive. However, most nanotwinned metals are produced in polycrystalline forms and therefore contain randomly oriented twin and grain boundaries making it difficult to determine the origins of their useful mechanical properties. Here, we report the fabrication of arrays of vertically aligned copper nanopillars that contain a very high density of periodic twin boundaries and no grain boundaries or other microstructural features. We use tension experiments, transmission electron microscopy and atomistic simulations to investigate the influence of diameter, twin-boundary spacing and twin-boundary orientation on the mechanical responses of individual nanopillars. We observe a brittle-to-ductile transition in samples with orthogonally oriented twin boundaries as the twin-boundary spacing decreases below a critical value (~3–4 nm for copper). We also find that nanopillars with slanted twin boundaries deform via shear offsets and significant detwinning. The ability to decouple nanotwins from other microstructural features should lead to an improved understanding of the mechanical properties of nanotwinned metals