In situ image correlation measurement of grain-level deformation fields in twinning magnesium

Abstract

In this study, in situ digital image correlation (DIC) has been utilized for full-field strain measurements in a hexagonal close-packed (HCP) AZ31 magnesium alloy at both macroscopic and microscopic (crystallite) length scales. Emphasis has been on examining the influence of twinning in the deformation behavior of polycrystalline magnesium. Although there is an abundance of ex-situ studies, few studies have employed in-situ DIC to investigate the micromechanics of the HCP twinning at the grain length scale. The loading mode has been uniaxial compression and the experiment was carried out on a textured sample to promote twinning activity. The experimental setup contained two optical systems (micro-scale and macro-scale) that acquired images from two orthogonal faces of the sample, providing volumetric deductions with this surface technique. The microscopic measurement surface has been prepared with metallography to reveal grain boundaries prior to the experiment to relate grain morphology with measured deformation patterns. The microscopic DIC revealed grain-scale shear bands whose collaborative activity appeared at the sample-scale as two macro-scale shear bands families that are oriented to the loading axis. This shows that the strain heterogeneity is observed excessively at any length scale and a representative volume element cannot be defined for this material. The DIC analysis on the macroscopic imaging face has shown that macroscopic shear bands are not surface incidents, but volumetric formations. The orientation of these bands is associated with the texture of the material. At the micro-scale, the shear bands that extend over several grains follow grain boundaries rather than cutting through the grains, an observation commonly supported by literature. The DIC analysis is invalidated on twin bands when they sufficiently alter the DIC pattern with a surface step. On the bright side, this allows an automatic tracking of the twin formation. It is apparent that the zones of very high twin activity match with the zones of very high strain localization. Thus, twinning appears to play an important role in promoting strain localization..