13,767 research outputs found
A nanoindentation investigation of local strain rate sensitivity in dual-phase Ti alloys
Using nanoindentation we have investigated the local strain rate sensitivity in dual-phase Ti alloys, Ti-6Al-2Sn-4Zr-xMo (x=2 and 6), as strain rate sensitivity could be a potential factor causing cold dwell fatigue. Electron backscatter diffraction (EBSD) was used to select hard and soft grain orientations within each of the alloys. Nanoindentation based tests using the continuous stiffness measurement (CSM) method were performed with variable strain rates, on the order of 10−1 to 10−3s−1. Local strain rate sensitivity is determined using a power law linking equivalent flow stress and equivalent plastic strain rate. Analysis of residual impressions using both a scanning electron microscope (SEM) and a focused ion beam (FIB) reveals local deformation around the indents and shows that nanoindentation tested structures containing both α and β phases within individual colonies. This indicates that the indentation results are derived from averaged α/β properties. The results show that a trend of local rate sensitivity in Ti6242 and Ti6246 is strikingly different; as similar rate sensitivities are found in Ti6246 regardless of grain orientation, whilst a grain orientation dependence is observed in Ti6242. These findings are important for understanding dwell fatigue deformation modes, and the methodology demonstrated can be used for screening new alloy designs and microstructures
Multiscale Poromechanics of Wet Cement Paste
Capillary effects such as imbibition-drying cycles impact the mechanics of
granular systems over time. A multiscale poromechanics framework was applied to
cement paste, that is the most common building material, experiencing broad
humidity variations over the lifetime of infrastructure. First, the liquid
density distribution at intermediate to high relative humidities is obtained
using a lattice gas density functional method together with a realistic
nano-granular model of cement hydrates. The calculated adsorption/desorption
isotherms and pore size distributions are discussed and compare well to
nitrogen and water experiments. The standard method for pore size distribution
determination from desorption data is evaluated. Then, the integration of the
Korteweg liquid stress field around each cement hydrate particle provided the
capillary forces at the nanoscale. The cement mesoscale structure was relaxed
under the action of the capillary forces. Local irreversible deformations of
the cement nano-grains assembly were identified due to liquid-solid
interactions. The spatial correlations of the nonaffine displacements extend to
a few tens of nm. Finally, the Love-Weber method provided the homogenized
liquid stress at the micronscale. The homogenization length coincided with the
spatial correlation length nonaffine displacements. Our results on the solid
response to capillary stress field suggest that the micronscale texture is not
affected by mild drying, while local irreversible deformations still occur.
These results pave the way towards understanding capillary phenomena induced
stresses in heterogeneous porous media ranging from construction materials,
hydrogels to living systems.Comment: 6 figures in main text, 4 figures in the SI appendi
Deformation of grain boundaries in polar ice
The ice microstructure (grain boundaries) is a key feature used to study ice
evolution and to investigate past climatic changes. We studied a deep ice core,
in Dome Concordia, Antarctica, which records past mechanical deformations. We
measured a "texture tensor" which characterizes the pattern geometry and
reveals local heterogeneities of deformation along the core. These results
question key assumptions of the current models used for dating
Ellipticity loss analysis for tangent moduli deduced from a large strain elastic–plastic self-consistent model
In order to investigate the impact of microstructures and deformation mechanisms on the ductility of materials, the criterion first proposed by Rice is applied to elastic–plastic tangent moduli derived from a large strain micromechanical model combined with a self-consistent scale-transition technique. This approach takes into account several microstructural aspects for polycrystalline aggregates: initial and induced textures, dislocation densities as well as softening mechanisms such that the behavior during complex loading paths can be accurately described. In order to significantly reduce the computing time, a new method drawn from viscoplastic formulations is introduced so that the slip system activity can be efficiently determined. The different aspects of the single crystal hardening (self and latent hardening, dislocation storage and annihilation, mean free path, etc.) are taken into account both by the introduction of dislocation densities per slip system as internal variables and the corresponding evolution equations. Comparisons are made with experimental results for single and dual-phase steels involving linear and complex loading paths. Rice’s criterion is then coupled and applied to this constitutive model in order to determine the ellipticity loss of the polycrystalline tangent modulus. This criterion, which does not need any additional “fitting” parameter, is used to build Ellipticity Limit Diagrams (ELDs).ArcelorMittal Researc
Effect of impurities on morphology and growth mode of (111) and (001) epitaxial-like ScN films
ScN material is an emerging semiconductor with an indirect bandgap. It has
attracted attention for its thermoelectric properties, use as seed layers, and
for alloys for piezoelectric application. ScN or other transition metal nitride
semiconductors used for their interesting electrical properties are sensitive
to contaminants, such as oxygen or fluorine. In this present article, the
influence of depositions conditions on the amount of oxygen contaminants
incorporated in ScN films were investigated and their effects on the electrical
properties (electrical resistivity and Seebeck coefficient) were studied. The
epitaxial-like films of thickness 125 +-5 nm to 155 +-5 nm were deposited by
D.C.-magnetron sputtering on c-plane Al2O3, MgO(111) and r-plane Al2O3 at a
substrate temperature ranging from 700 to 950 degree C. The amount of oxygen
contaminants presents in the film, dissolved into ScN or as an oxide, was
related to the adatom mobility during growth, which is affected by the
deposition temperature and the presence of twin domain growth. The lowest
values of electrical resistivity of 50 micro-ohm cm were obtained on
ScN(111)/MgO(111) and on ScN(001)/r-plane Al2O3 grown at 950 degree C with no
twin domains and the lowest amount of oxygen contaminant. At the best, the
films exhibited an electrical resistivity of 50 micro-ohm cm with Seebeck
coefficient values maintained at -40 microV K-1, thus a power factor estimated
at 3.2 10-3 W m-1 K-2 (at room temperature)
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