Discrete dislocation dynamics simulations of twin size-effects in magnesium
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Discrete dislocation dynamics simulations of twin size-effects in magnesium Haidong Fan1,2*, Sylvie Aubry3, A. Arsenlis3, Jaafar A. El-Awady1* 1 Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218, USA 2 Department of Mechanics, Sichuan University, Chengdu, Sichuan 610065, P. R. China 3 Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, CA 94551-0808, USA * E-mails: [email protected] (H. Fan), [email protected] (J. A. El-Awady) ABSTRACT A dislocation- 1012 twin boundary (TB) interaction model was proposed and introduced into discrete dislocation dynamics simulations to study the mechanical behavior of micro-twinned Mg. Strong strain hardening was captured by current simulations, which is associated with the strong TB’s barrier effect. In addition, twin size effects with small TB spacing leading to a strong yield stress, were observed to be orientation dependent. Basal slip orientation produces a strong size effect, while prismatic slip does a weak one. INTRODUCTION As the lightest structural metal, magnesium (Mg) and its alloys have been of practical interest due to their potential use in automotive, aerospace and defense applications. Magnesium has a hexagonal closed packed (HCP) lattice structure with low crystal symmetry, thus, in addition to dislocation-mediated plasticity, twinning is very common and plays an important role in its plastic deformation [1]. However, a full understanding of the dislocation-twin boundary (TB) strengthening is still lacking. Jiang et al. [2] observed that intersections between primary and secondary 1012 twins result in significant grain refinement and strong hardening. In addition, Knezevic et al. [3] showed that compression twins in the tension-twinned grains attribute to the hardening behavior. Moreover, Barnett et al. [4] observed the formation of low angle boundaries arising from the dislocation-TB interaction, which act as a source of strengthening. Recently, Fan et al. [5] showed a competition exists between dislocation-TB induced hardening and twinning deformation induced softening. Over the past two decades, discrete dislocation dynamics (DDD) has been one of the most efficient methods to capture dislocation-mediated plasticity at the micro scale [6-10]. It has been successfully used to study crystal size effects [11, 12], grain size effects [13, 14] and intermittent behavior [15] of the FCC and BCC materials. More recently, DDD simulations of Mg investigated a number of important effects including: dislocation junction formation and strength [16, 17], orientation influence on the grain size effects [18], micro/nano-pillar plasticity [19], elastic anisotropy [20] and Peierls stress [21]. Nevertheless, in all these studies, twinning deformation was not considered. However, twinning plays an important and sometimes dominant role in the mechanical behavior of both single crystals and polycrystals. As a result, such DDD simulations without twinning may lead to inaccurate predication of the mechanical b
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