Matrix-free assemblies of polymer-grafted nanoparticles (PGNs) enable mechanically robust materials for a variety of structural, electronic, and optical applications. Recent quasi-static mechanical studies have identified the key parameters that enhance canopy entanglement and promote plasticity of the PGNs below T g . Here we experimentally explore the high-strain-rate shock impact behavior of polystyrene grafted NPs and compare their energy absorption capabilities to that of homopolystyrene for film thicknesses ranging from 75 to 550 nm and for impact velocities from 350 to 800 m/s. Modeling reveals that the initial shock compression results in a rapid temperature increase at the impact site. The uniformity of this heating is consistent with observations of greater kinetic energy absorption per mass (E p *) of thinner films due to extensive viscoplastic deformation of molten film around the penetration site. Adiabatic heating is insufficient to raise the temperature at the exit surface of the thickest films resulting in increased strain localization at the impact periphery with less melt elongation. The extent and distribution of entanglements also influence E p *. Structurally, each NP acts as a giant cross-link node, coupling surrounding nodes via the number of canopy chains per NP and the nature and number of entanglements between canopies anchored to different NPs. Load sharing via this dual network, along with geometrical factors such as film thickness, lead to extreme E p * arising from the sequence of instantaneous adiabatic shock heating followed by visco-plastic drawing of the film by the projectile. These observations elucidate the critical factors necessary to create robust polymer-nanocomposite multifunctional films.
We report on an ab initio molecular dynamics study of the lattice parameters, thermal expansion coefficients, and elastic constants of ZrB2, TiB2, and HfB2 ceramics at ultrahigh temperatures (up to 2200 K). Equilibrium lattice parameters of the ceramics are determined at finite temperatures. A finite strain method is used to extract the stiffness tensor of the ceramics. The results obtained for ZrB2 and TiB2 agree well with experimental results reported in the literature. Our work demonstrate that accurate properties may be obtained from a statistical averaging of the lattice parameters alone neglecting phonon interactions.
Characterization of prior austenite grain size is important for understanding the microstructure-property relationships in steels. The prior austenite grain size plays an important role in defining the microstructural scale of low-temperature phases and the mechanical properties (e.g., strength, ductility, fracture toughness, etc.) of steels in the final product form. Moreover, in several failure analyses, the cracks are observed to propagate along the prior austenite grain boundaries (PAGBs). The delineation of PAGBs in steels of new composition can be quite challenging, as the response to a particular etching protocol is very sensitive to the chemical composition of steel. The objective of this study was to establish a methodology to delineate PAGBs in AF9628, a newly developed low-alloy high-performance steel. Several different etchants and etching techniques from the literature were evaluated. These methods were unsuccessful or had limited success in revealing PAGBs in AF9628. However, swab etching with a solution of 100 ml saturated aqueous picric acid and 0.5 g sodium dodecyl benzene sulfonate worked remarkably well for delineating the PAGBs in this steel. This etchant was found to have high selectivity, revealing PAGBs preferentially over packet, block, and sub-block boundaries.
Prior studies on martensitic steel microstructures have either delineated the prior austenite grain boundaries via chemical etching or reconstructed the prior austenite grains from crystallographic orientations measured with electron backscattered diffraction (EBSD). To appropriately validate the reconstruction algorithms, the EBSD data need to be collected on martensitic microstructures, where the prior austenite grain boundaries are delineated with techniques such as chemical etching that can serve as ground truth for comparison with the reconstructed prior austenite grains. In this article, the method of correlative microscopy is employed to collect scanning electron microscope (SEM) image and automated EBSD scan data from the same region of an appropriately etched steel specimen. The SEM images and automated EBSD scan data are presented for five different fields of view in the specimen. These datasets are analyzed and discussed in the accompanying article titled “Correlative microscopy for quantification of prior austenite grain size in AF9628 steel” [1].
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