Atomistically informed continuum model for body centered cubic iron
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Atomistically informed continuum model for body centered cubic iron Aenne Koester1, Anxin Ma1 and Alexander Hartmaier1 1 Ruhr-University Bochum, Interdisciplinary Centre for Advanced Materials Simulation, Stiepeler Strasse 129, 44801 Bochum, Germany ABSTRACT Plastic deformation in body centered cubic iron is dominated by glide of screw dislocations with non-planar dislocation cores. This causes a strong strain rate and temperature dependence of flow stress, the breakdown of Schmid’s law and a dependence of dislocation mobility on shear stress components that do not contribute to the mechanical driving force for dislocation glide. Based on the framework of crystal plasticity, we developed a constitutive plasticity model that takes all these phenomena into account. To parameterize this continuum plasticity model molecular statics simulations using a semi-empirical potential have been performed. These atomistic calculations yielded quantitative relationships for the influence of all components of the local stress tensor on dislocation mobility. Together with experimental data from the literature on the kinetics of screw dislocations in bcc iron the constitutive relation presented here has been developed. As application example of the model, we calculated the tension compression asymmetry and the strain rate dependence of the hardening behavior within a bcc iron crystal. INTRODUCTION In bcc metals the plastic deformation is dominated by the glide of screw dislocations with non-planar core. Non-planar cores lead to a high Peierls barrier and to a strong dependence of flow stress on temperature and strain rate [1, 2]. Experimental and theoretical studies observed two phenomena, which are characteristic for bcc metals: The breakdown of Schmid’s law and the dependence of the yield stress on so-called non-glide components of the stress tensor. The term non-glide components mean shear stresses, which act in the direction of the Burgers vector, but on planes other than the glide plane or shear stresses perpendicular to the glide direction [3, 4]. Another characteristic feature of bcc materials is the absence of truly close packed planes. While in fcc materials the {111} planes show a close packed structure, such planes are missing in bcc materials. On the one hand based on experimental research of Brunner and Diehl [7] on bcc iron, the assumption is made that the choice of glide planes is temperature dependent. At low temperatures slip takes place on {110} planes and at high temperatures on {112} planes. On the other hand atomistic simulations stated that slip takes only place on {110} planes [4-6]. Recent results of experiments of bcc iron at low and room temperature help to clarify the question, on which glide planes screw dislocations moves. The results of those studies show that slip of screw dislocations takes place on {110} planes independent of the temperature regime [8,9]. Based on observations of the mechanical behavior of bcc metals an atomistically informed continuum model for bcc iron in the framework of crystal pl
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