Evaluating the effects of loading parameters on single-crystal slip in tantalum using molecular mechanics

Abstract

This study is aimed at developing a physics-based crystal plasticity finite element model for body-centred cubic (BCC) metals, through the introduction of atomic-level deformation information from molecular dynamics (MD) investigations of dislocation motion at the onset of plastic flow. In this study, three critical variables governing crystal plasticity mediated by dislocation motion are considered. MD simulations are first performed across a range of finite temperatures up to 600K to quantify the temperature dependence of critical stress required for slip initiation. An important feature of slip in BCC metals is that it is not solely dependent on the Schmid law measure of resolved shear stress, commonly employed in crystal plasticity models. The configuration of a screw dislocation and its subsequent motion is studied under different load orientations to quantify these non-Schmid effects. Finally, the influence of strain rates on thermal activation is studied by inducing higher stresses during activation at higher applied strain rates. Functional dependence of the critical resolved shear stress on temperature, loading orientation and strain rate is determined from the MD simulation results. The functional forms are derived from the thermal activation mechanisms that govern the plastic behaviour and quantification of relevant deformation variables. The resulting physics-based rate-dependent crystal plasticity model is implemented in a crystal plasticity finite element code. Uniaxial simulations reveal orientation-dependent tension–compression asymmetry of yield that more accurately represents single-crystal experimental results than standard models.

Keywords:

• crystal plasticity
• molecular dynamics
• tantalum
• non-Schmid effects
• strain rate
• thermal activation
• embedded atom method
• finite element method
• tension–compression asymmetry

Acknowledgments

The authors gratefully acknowledge the Advanced Simulations and Computing Program at Los Alamos National Laboratory (Dr. M. Schraad, project leader) for support of this work. They also express appreciation for fruitful discussions with Dr. I. Beyerlein of Los Alamos National Laboratory and Professor V. Vitek of University of Pennsylvania.