The Quasi-Coarse-Grained Dynamics Method to Unravel the Mesoscale Evolution of Defects/Damage during Shock Loading and Spall Failure of Polycrystalline Al Microstructures.

A long-standing problem in modeling of shock response of metals is the ability to model defect nucleation and evolution mechanisms during plastic deformation and failure at the mesoscales. This paper demonstrates the capability of the "quasi-coarse-grained dynamics" (QCGD) simulation method to unravel microstructural evolution of polycrystalline Al microstructures at the mesoscales. The various QCGD simulations discussed here investigate the shock response of Al microstructures comprising of grain sizes ranging from 50 nm to 3.20 µm and correspond to system sizes ranging from 150 nm to 9.6 µm, respectively. The QCGD simulations are validated by demonstrating the capability to retain atomistic characteristics of the wave propagation behavior, plastic deformation mechanisms (dislocation nucleation, dissociation/recombination behavior, dislocation interactions/reactions), evolution of damage (voids), and evolution of temperature during shock loading. The capability to unravel the mesoscale evolution of microstructure is demonstrated by investigating the effect of grain size, shock pulse and system size on the shock response and spall failure of the metal. The computed values of spall strengths predicted using the QCGD simulations agree very well with the trend predicted by MD simulations and a strain rate dependence of the spall strength is proposed that fits the experimentally available values in the literature.

Origins of Ripples in CVD-Grown Few-layered MoS2 Structures under Applied Strain at Atomic Scales.

The potential of the applicability of two-dimensional molybdenum disulfide (MoS2) structures, in various electronics, optoelectronics, and flexible devices requires a fundamental understanding of the effects of strain on the electronic, magnetic and optical properties. Particularly important is the recent capability to grow large flakes of few-layered structures using chemical vapor deposition (CVD) wherein the top layers are relatively smaller in size than the bottom layers, resulting in the presence of edges/steps across adjacent layers. This paper investigates the strain response of such suspended few-layered structures at the atomic scales using classic molecular dynamics (MD) simulations. MD simulations suggest that the suspended CVD-grown structures are able to relax the applied in-plane strain through the nucleation of ripples under both tensile and compressive loading conditions. The presence of terraced edges in these structures is the cause for the nucleation of ripples at the edges that grow towards the center of the structure under applied in-plane strains. The peak amplitudes of ripples observed are in excellent agreement with the experimental observations. The study provides critical insights into the mechanisms of strain relaxation of suspended few-layered MoS2 structures that determine the interplay between the mechanical response and the electronic properties of CVD-grown structures.

Effects of Uniaxial and Biaxial Strain on Few-Layered Terrace Structures of MoS2 Grown by Vapor Transport.

One of the most fascinating properties of molybdenum disulfide (MoS2) is its ability to be subjected to large amounts of strain without experiencing degradation. The potential of MoS2 mono- and few-layers in electronics, optoelectronics, and flexible devices requires the fundamental understanding of their properties as a function of strain. While previous reports have studied mechanically exfoliated flakes, tensile strain experiments on chemical vapor deposition (CVD)-grown few-layered MoS2 have not been examined hitherto, although CVD is a state of the art synthesis technique with clear potential for scale-up processes. In this report, we used CVD-grown terrace MoS2 layers to study how the number and size of the layers affected the physical properties under uniaxial and biaxial tensile strain. Interestingly, we observed significant shifts in both the Raman in-plane mode (as high as -5.2 cm(-1)) and photoluminescence (PL) energy (as high as -88 meV) for the few-layered MoS2 under ∼1.5% applied uniaxial tensile strain when compared to monolayers and few-layers of MoS2 studied previously. We also observed slippage between the layers which resulted in a hysteresis of the Raman and PL spectra during further applications of strain. Through DFT calculations, we contended that this random layer slippage was due to defects present in CVD-grown materials. This work demonstrates that CVD-grown few-layered MoS2 is a realistic, exciting material for tuning its properties under tensile strain.