Sinisa D. Mesarovic - US grants

ABSTRACT: Crystal Plasticity on the Nano and Micro Scales. Micromechanical Theory and Experiments.
PI: S. Dj. Mesarovic, School of Mechanical and Materials Engineering, Washington State University

A fundamental study of dislocation plasticity in small volumes is proposed. The proposed study will coordinate experiments, discrete dislocation (DD) simulations, and, development and implementation of a micromechanical continuum theory guided by experiments and DD simulations. The existing gaps - between theories and experiments, and, between a continuum theory and its micromechanical foundation (dislocation mechanics), will be closed.
The experiments on simple two-dimensional geometries with small number of active slip systems are proposed. In addition to the recently performed wedge microindentation, the constrained shear of a thin film will be performed with subsequent OIM measurements of lattice rotations, lattice curvature and densities of geometrically necessary dislocations.
By developing a rigorous thermodynamic foundation of the continuum theory based on dislocation mechanics. This will enable unambiguous continuum interpretation of experimental results and of discrete dislocation simulations.
The envisioned research program is integrated with graduate and undergraduate education at MME/WSU.

Recent developments in nanostructures have brought to light exceptional electromagnetic, thermal and optical properties of a class of foam-like nanostructures formed of disordered intertwined structural units (nanowires, nanobelts, nanotubes). Such disordered assemblies are named turfs. Applications include thermal switches, flat panel displays, hard discs drives, and, chemical and biological sensors.
Although the mechanical properties are usually not the primary service characteristic of turfs, they are nevertheless of paramount importance. Irrespective of application, the turfs are often subjected to mechanical loads, either as service load as in thermal switches, or, as accidental contacts. Under externally forced deformation, the nano-topology of the turf changes, which, in turn, affects all the other effective properties: electrical, thermal, optical, sensing and permeability.
We will develop an integrated approach to the problem: multiscale modeling, nanomechanical experiments, and, nanostructure characterization, with the following objectives:
Understanding and quantification of the behavior of turfs as materials on the basis of the physical and geometrical properties of the individual units and their collective behavior in the assembly.
Development of the nanoscale characterization methods that reveal the relevant parameters of the nanostructure.
Practical technological impact of the project is that the results will enable rational design of nanoturfs tailored for particular application in sensors, thermal switches and other devices. The REU component of the program is carefully structured and includes assessment methods, developed and proven at the Center for Teaching and Learning at WSU. Our pilot student mentoring program will provide graduate students with mentoring experience a skill that PhD graduates need, but is sorely missing in most graduate programs.

1235757 / 1234581
PI: Sekulic / Mesarovic

Spreading of liquids over solid surfaces is ubiquitous in nature and is the key aspect of many industrial processes, such as low temperature soldering, high temperature brazing, de-icing, organic liquid imbibing, etc. For example, in the case of brazing/soldering, the tight time scheduling of the processing combined with dissimilar wetting properties of materials being joined and a possible variability of temperature during spreading, require a precise control, which in turn requires quantitative prediction tools. The major difficulties in predicting such a complex flow include: (1) roughness of the spreading surface, (2) chemical reactions between the liquid/melt and the substrate, and, (3) inadequate models of the physical mechanism by which the triple line (a locus of points where solid, liquid and vapor/gas meet) propagates. This project will integrate experimental and modeling strategy through: (i) experiments on characterized virgin and designed surfaces, with in situ monitoring and measurements of the triple line motion, (ii) advances in theory and modeling based on the diffuse interface (phase-field) models, capable of representing propagation of phase- and chemical reaction fronts, as well as the diffusive nature of the triple line motion, and, (iii) integration of modeling and experiments. Real time in situ monitoring of the spreading at the micro scale around the triple line will be the source of data on kinetics of the liquid front propagation. To achieve projects objectives, the phase-field modeling for reactive and non-reactive spreading will be implemented within a versatile computational finite element framework, thus enabling studies of variety of problems with different geometries. The major experimental challenges are related to the fast evolution of the triple line and the presence of reactive substrates. The required time resolution will be achieved by developing hot stage microscopy techniques for in situ monitoring of the moving liquid front and the dynamics of the contact angle. Suppression of a chemical reaction will be accomplished by formation of intermetallics prior to spreading of a liquid metal. The major modeling challenges include: (a) implementation of the surface diffusion kinetics governing the motion of the triple line for the non-reactive model into the finite element framework to model rough surfaces, and, (b) implementation of the combined liquid-gas phase-field and the chemical reaction phase-field into the finite element framework.

The impact of this investigation will be felt in a broad set of applications, related to industrial and natural processes. The transformative nature of the research is that it will results in the ability to effectively control wetting by surface alterations and the selection of liquid system and solid substrates. This will enable rational design of industrial processes which depend on liquid spreading, and products whose function depends on wetting.