22 research outputs found

    Modeling dislocation sources and size effects at initial yield in continuum plasticity

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    Size effects at initial yield (prior to stage II) of idealized micron-sized specimens are modeled within a continuum model of plasticity. Two different aspects are considered: specification of a density of dislocation sources that represent the emission of dislocation dipoles, and the presence of an initial, spatially inhomogeneous excess dislocation content. Discreteness of the source distribution appears to lead to a stochastic response in stress-strain curves, with the stochasticity diminishing as the number of sources increases. Variability in stress-strain response due to variations of source distribution is also shown. These size effects at initial yield are inferred to be due to physical length scales in dislocation mobility and the discrete description of sources that induce internal-stress-related effects, and not due to length-scale effects in the mean-field strain-hardening response (as represented through a constitutive equation)

    Size effects in LiF micron-scale single crystals of low dislocation density

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    This study examines the deformation response of 20, 5, and 1μm diameter samples fabricated by FIB-milling from bulk ultrapure LiF single crystals. The bulk crystals have a very low initial dislocation density as revealed by an etch-pit technique. Two types of 〈001〉 microsamples were compressed preferentially by single slip inside a nanoindentation system. Similar to previously studied FCC-derivative metals, LiF microsamples demonstrate dramatic strengthening achieving the engineering flow stress o ≈ 650 MPa in 1-μm samples. The stress-diameter dependence obeys a power law, σ∼D -m, where m ≈ 0.8. Stochastic variation of flow stress, fast intermittent deformation events ( avalanches ) and highly localized slip bands after avalanches - all characteristic of size effects in metals, are also observed in LiF. Possible dislocation mechanisms of the observed size effects are discussed. © 2007 Materials Research Society

    Size Distribution of γ' Precipitates in Ni-Cr-Co-Al-Ti Alloys

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