Surya Ganti

On the grain-size-dependent elastic modulus of nanocrystalline materials with and without grain-boundary sliding

A closed-form model was proposed to evaluate the elastic properties of nanocrystalline materials as a function of grain size. Grain-boundary sliding, present in nanocrystalline materials even at relatively low temperatures, was included in the formulation. The proposed analytical model agrees reasonably well with the experimental results for nanocrystalline copper and palladium.

Strain dependent facet stabilization in selective-area heteroepitaxial growth of GaN nanostructures

We report on the selective-area heteroepitaxy and facet evolution of submicron GaN islands on GaN-sapphire, AlN-sapphire, and bare sapphire substrates. It is shown that strain due to the lattice mismatch between GaN and the underlying substrate has a significant influence on the final morphology and faceting of submicron islands. Under identical metalorganic chemical vapor deposition growth parameters, islands with low or no mismatch strain exhibit pyramidal morphologies, while highly strained islands evolve into prismatic shapes. Furthermore, islands grown with relatively low compressive mismatch strain yield more uniform arrays of pyramids as compared to the nonstrained, homoepitaxially grown crystals. It is proposed that the strain dependency of Ehrlich-Schwoebel barriers across different crystallographic planes could potentially account for the observed morphologies during selective area growth of GaN islands.

The size-dependent elastic state of inclusions in non-local elastic solids

The elastic stress state of inclusions containing eigenstrains located in an infinite non-local media are discussed. Closed-form expressions are provided for the specific cases of spherical and cylindrical inclusions. The results indicate that non-locality reduces the stress jump across the inclusion interface when compared with the classical case (i.e. reduction in the so-called stress concentration). We also prove, using asymptotic analysis, the disappearance of stress singularities for polyhedral inclusions in non-local elastic solids. The obtained results are size dependent and asymptotically approach the classical size-independent solution for a ‘large’ inclusion size.

Prediction of Rate-Independent Constitutive Behavior of Pb-Free Solders Based on First Principles

This paper presents a methodology for the theoretical estimation of rate-independent plastic constitutive properties of Pb-free solders using three approaches. The first approach is based on a Nonlinear Effective Medium Theory (NEMT) that is scale independent. The second approach is based on the micromechanics and physics of plastic slip in heterogeneous alloys (henceforth called the physical model). This approach explicitly includes microstructural features such as grain size, particle size etc. The third approach is a combination of NEMT and the physical model. Our estimates involve no adjustable calibration parameters and are based on first principles and constituent properties. Parametric studies are conducted to show that the physical model is more effective for small particles sizes, small particle spacing and low volume fractions; while NEMT performs well for large volume fractions, large particle sizes and large particle spacing. The proposed hybrid approach, however, is valid for all particle sizes and volume fractions. Limited comparison with experimental data is also made and implications of our work in the economical design of novel Pb-free solders is discussed.Copyright