In Situ Characterization of the Effect of Twin-Microstructure Interactions on {1 0 1 2} Tension and {1 0 1 1} Contraction Twin Nucleation, Growth and Damage in Magnesium

William D Russell,Hongjoo Rhee,Sven C Vogel,Nicholas R Bratton,Shiraz Mujahid,Robert D Moser,Haitham El Kadiri,Christopher D Barrett,Yubraj Paudel,Bhasker Paliwal,Andrew L Oppedal,Wilburn R Whittington,Zackery B Mcclelland

doi:10.3390/met10111403

Abstract

Through in situ electron backscatter diffraction (EBSD) experiments, this paper uncovers dominant damage mechanisms in traditional magnesium alloys exhibiting deformation twinning. The findings emphasize the level of deleterious strain incompatibility induced by twin interaction with other deformation modes and microstructural defects. A double fiber obtained by plane-strain extrusion as a starting texture of AM30 magnesium alloy offered the opportunity to track deformation by EBSD in neighboring grains where some undergo profuse {1 0 1 2} twinning and others do not. For a tensile loading applied along extrusion transverse (ET) direction, those experiencing profuse twinning reveal a major effect of grain boundaries on non-Schmid behavior affecting twin variant selection and growth. Similarly, a neighboring grain, with its ⟨c⟩-axis oriented nearly perpendicular to tensile loading, showed an abnormally early nucleation of {1 0 1 1} contraction twins (2% strain) while the same {1 0 1 1} twin mode triggering under ⟨c⟩-axis uniaxial compression have higher value of critical resolved shear stress exceeding the values for pyramidal ⟨c + a⟩ dislocations. The difference in nucleation behavior of contraction vs. compression {1 0 1 1} twins is attributed to the hydrostatic stresses that promote the required atomic shuffles at the core of twinning disconnections.

Highlights

The reduction of greenhouse gases and the reduced dependence on hydrocarbon-based fuels are of key priority worldwide
Coupled electron backscatter diffraction (EBSD) and FEG-scanning electron microscopy (SEM) micrographic characterization at different strain levels revealed cracks nucleating at the intersection between two intersecting {1 0 1 2} twins, a twin and slip bands, and twin and grain boundaries
For the case of slip–twin interaction (Figure 7), one possible mechanism behind crack nucleation is the difficulty for the twin to facet or form a disconnection as a dislocation meets the interface [42,43,44,45,46]

Summary

Introduction

The reduction of greenhouse gases and the reduced dependence on hydrocarbon-based fuels are of key priority worldwide. The United States, Canada, China, and the Euro-zone have engaged in a plan to reduce the mass of CO2 emitted by passenger vehicles a full 30% to 50% below current standards. Such tremendous improvements will require extensive vehicle mass reduction with lightweight Metals 2020, 10, 1403; doi:10.3390/met10111403 www.mdpi.com/journal/metals. Two of the most attractive metals with potential to satisfy these conflicting demands, magnesium (Mg) and titanium (Ti) alloys, exhibit a hexagonal close-packed (HCP) crystal structure at room temperature [1]. The limitation in forming of Mg alloys is associated with the plastic anisotropy of a hexagonal lattice. The inability of Mg alloys to deform along the crystal hci-axis slip and satisfy the von Mises criterion of five independent deformation systems for polycrystalline plasticity stipulates plastic accommodation by deformation twins: {1 0 1 2} tension twins and {1 0 1 1} contraction twins

Methods

Results

Conclusion