Anomalous hardening in magnesium driven by a size-dependent transition in deformation modes. (1st February 2018)

• Record Type: Journal Article
• Authors: Sim, Gi-Dong; Kim, Gyuseok; Lavenstein, Steven; Hamza, Mohamed H.; Fan, Haidong; El-Awady, Jaafar A.
• Is Part Of: Acta materialia. Volume 144(2018)
• Page Start: 11
• Page End: 20
• Publication Date: 2018-02-01
• Subjects: Hexagonal closed packed metals -- Deformation twinning -- Size effect -- Strain hardening; Materials -- Periodicals; Materials science -- Periodicals; Materials -- Mechanical properties -- Periodicals; Metallurgy -- Periodicals; Chemistry, Inorganic -- Periodicals; 620.112
• DOI: 10.1016/j.actamat.2017.10.033
• Languages: English
• ISSNs: 1359-6454

Abstract

Abstract: Here, we report a comprehensive study that combines in situ scanning electron microscopy experiments and atomistic simulations to quantify the effect of crystal size on the transformation in deformation modes in a-axis oriented Mg single crystals at room temperature. The experimental results indicate that the deformation is dominated by the nucleation and propagation of tensile twins. The stress required for twin propagation was found to increase with decreasing sample size, showing a typical "smaller is stronger" behavior. Furthermore, an anomalous increase in strain hardening is first reported for microcrystals having diameters larger than ∼18 μm, which is induced by twin-twin and dislocation-twin interactions. The hardening rate gradually decreases toward the bulk response as the microcrystal size increases. Below 18 μm, deformation is dominated by the nucleation and propagation of a single tensile twin followed by basal slip activity in the twinned crystal, leading to no apparent hardening. In addition, molecular dynamics simulations indicate a transition from twinning mediated plasticity to dislocation mediated plasticity for crystal sizes below a few hundred nanometers in size. A deformation mechanism map for twin oriented Mg single crystals, ranging from the nano-scale to bulk scale is proposed based on the current simulations and experiments. The current predicted size-affected deformation mechanism of twin oriented Mg single crystals can lead to better understanding of the competition between dislocations plasticity and twinning plasticity. Graphical abstract: Image 1