X-ray Microdiffraction Characterization of Deformation Heterogeneities in BCC Crystals

The deformation behavior of BCC metals is being investigated by x-ray microdiffraction measurements (mu XRD) for the purpose of characterizing the dislocation structure that results from uniaxial compression experiments. The high brilliance synchrotron source at the Advanced Light Source (Lawrence Berkeley National Lab) and the micron resolution of the focusing optics allow for the mapping of Laue diffraction patterns across a sample. These measurements are then analyzed in order to map the distribution of residual stresses in the crystal. An important finding is the observation of Laue spot ''streaking'', which indicates localized rotations in the lattice. These may represent an accumulation of same-sign dislocations. Theoretical modeling of the diffraction response for various slip systems is presented, and compared to experimental data. Preliminary results include orientation maps from a highly strained Ta bicrystal and a less highly strained Mo single crystal. The orientation maps of the bicrystal indicate a cell-like structure of dense dislocation walls. This deformation structure is consistent with previous OIM studies of the same crystal. The results suggest that mu XRD may be a particularly useful tool for microscale studies of deformation patterns in a multi-scale investigation of the mechanisms of deformation that ranges from macroscopic deformation tests to high resolution TEM studies of dislocation structures

Surface softening in metal-ceramic sliding contacts: An experimental and numerical investigation

This study investigates the tribolayer properties at the interface of ceramic/metal (i.e., WC/W) sliding contacts using various experimental approaches and classical atomistic simulations. Experimentally, nanoindentation and micropillar compression tests, as well as adhesion mapping by means of atomic force microscopy, are used to evaluate the strength of tungsten?carbon tribolayers. To capture the influence of environmental conditions, a detailed chemical and structural analysis is performed on the worn surfaces by means of XPS mapping and depth profiling along with transmission electron microscopy of the debris particles. Experimentally, the results indicate a decrease in hardness and modulus of the worn surface compared to the unworn one. Atomistic simulations of nanoindentation on deformed and undeformed specimens are used to probe the strength of the WC tribolayer and despite the fact that the simulations do not include oxygen, the simulations correlate well with the experiments on deformed and undeformed surfaces, where the difference in behavior is attributed to the bonding and structural differences of amorphous and crystalline W-C. Adhesion mapping indicates a decrease in surface adhesion, which based on chemical analysis is attributed to surface passivation

Mechanical characterization of oligo(ethylene glycol)-based hydrogels by dynamic nanoindentation experiments

© 2015 Elsevier Ltd. Oligo(ethylene glycol)-based (OEG) hydrogel samples of varying cross-link densities and degrees of swelling were characterized through dynamic nanoindentation testing. Experiments were performed using a non-standard nanoindentation method, which was validated on a standard polystyrene sample. This method maximizes the capability of the instrument to measure the stiffness and damping of highly compliant, viscoelastic materials. Experiments were performed over the frequency range of 1 to 50. Hz, using a 1. mm diameter flat punch indenter. A hydration method was adopted to avoid sample dehydration during testing. Values of storage modulus (E\u27) ranged from 3.5 to 8.9. MPa for the different OEG-hydrogel samples investigated. Samples with higher OEG concentrations showed greater scatter in the modulus measurements and it is attributed to inhomogeneities in these materials. The (E\u27) values did not show a strong variation over frequency for any of the samples. Values of loss modulus (E′) were two orders of magnitude lower than the storage modulus, resulting in very low values of loss factor (. E′/. E\u27 \u3c 0.1). These are characteristics of strong gels, which present negligible viscous properties

Plastic deformation mechanisms of magnesium matrix nanocomposites elaborated by friction stir processing

Magnesium based nanocomposites have attracted much attention over the past few years as a promising solution to lightweighting, energy saving and emission reduction, especially for automotive and aerospace applications. With a specific weight of 1.74 g.cm-3, magnesium is the lightest of all structural metals. However, the strength of magnesium needs to be improved in order to compete with other light metals such as aluminum and titanium. A solution for this would be the reinforcement of magnesium or its alloys with oxide nanoparticles. Several studies have been carried out on the deformation mechanisms of pure magnesium single crystals (Obara, Yoshinaga, & Morozumi, 1974) (Kelley & Hosford, 1967) (Lilleodden, 2010). However, to the author’s knowledge, there are no significant studies on the deformation mechanisms of magnesium based nanocomposites reinforced with oxide particles. The use of magnesium as a structural material demands hence a fundamental study of the role of nanoparticles on the deformation mechanisms of magnesium. Plasticity of magnesium, which has a hexagonal closed-packed crystalline structure, is characterized by a very strong plastic anisotropy as well as a complex twinning activity. It is then important to understand how these nanoparticles will interfere with slip and twinning. The aim of the present work is to investigate the single crystalline plastic behavior of magnesium based nanocomposites reinforced with Y2O3 nanoparticles. The choice of single crystalline specimens is motivated by the idea of eliminating the effects of grain boundaries and strain incompatibilities and thus isolating the role of particles in the strengthening mechanisms. The plastic deformation of these nanocomposites will be compared to the one of pure magnesium single crystals for different relevant crystallographic orientations