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Significance Statement
The plastic response of body-centered cubic (bcc) metals such as iron or tungsten is notoriously difficult to predict due to the existence of a strong temperature dependence and the so-called non-Schmidt effects. Non-Schmidt behavior manifests itself in the form of a tension/compression asymmetry in uniaxial tensile tests that is unique to crystals with bcc structure. For its part, temperature effects are such that bcc metals generally behave as brittle crystals at low temperatures and as ductile ones at higher temperatures. The origin of this behavior can be found in the particular features of screw dislocation cores, which –in contrast with other crystal structures– display highly nonplanar characteristics. Despite the fundamental nature of this issue, and the technological importance of bcc metals, no truly quantitative model for bcc plasticity existed in the literature. Cereceda et al. have developed a numerical approach that can simulate macroscopically relevant time and length scales and, at the same time, predict the temperature and tensile response of bcc crystals purely from first-principles physical information. Their approach links the atomistic properties of screw dislocations with a macroscopic representation of plasticity that allows the calculations to be compared one-to-one with experiments. Their results show excellent agreement for the case of tungsten, and their methodology provides the basis for quantitative explaining the anomalous plastic behavior of bcc metals and their alloys.
Journal Reference
International Journal of Plasticity, Volume 78, March 2016, Pages 242-265.
David Cereceda1,2,3, Martin Diehl4, Franz Roters4, Dierk Raabe4, J. Manuel Perlado3, Jaime Marian1
[expand title=”Show Affiliations”]- Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, CA 90095, USA
- Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA, USA
- Instituto de Fusión Nuclear, Universidad Politécnica de Madrid, E-28006 Madrid, Spain
- Max-Planck-Institut für Eisenforschung, Max-Planck-Straße 1, 40237 Düsseldorf, Germany. [/expand]
Abstract
We use a physically-based crystal plasticity model to predict the yield strength of body-centered cubic (bcc) tungsten single crystals subjected to uniaxial loading. Our model captures the thermally-activated character of screw dislocation motion and full non-Schmid effects, both of which are known to play critical roles in bcc plasticity. The model uses atomistic calculations as the sole source of constitutive information, with no parameter fitting of any kind to experimental data. Our results are in excellent agreement with experimental measurements of the yield stress as a function of temperature for a number of loading orientations. The validated methodology is employed to calculate the temperature and strain-rate dependence of the yield strength for 231 crystallographic orientations within the standard stereographic triangle. We extract the strain-rate sensitivity of W crystals at different temperatures, and finish with the calculation of yield surfaces under biaxial loading conditions that can be used to define effective yield criteria for engineering design models.
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