Wednesday  |  Conference Center A  |  11:50 AM–12:10 PM

#13802, Coordinated Twinning Bands in Magnesium at the Existence of Stress Raisers via in situ Microscopic Image Correlation

Mechanical twinning underlies many of the challenges in the forming and structural utilization of Magnesium alloys, the class of engineering metals with the lowest density. Intense research activity has been devoted to the understanding and modeling of the abrupt twinning phenomenon. A particularly challenging aspect of twinning is its abrupt coordinated proliferation across the polycrystalline aggregate. Macroscopically, these events are akin to Lüders banding while the geometry and compactness of the coordinated twinning bands show pronounced dependence on microstructure (most notably on crystallographic texture) in Magnesium alloys. A series of systematic studies employed an in-situ area scanning variant of digital image correlation with optical microscopy to quantitatively characterize these bands. While the length-scale of optical microscopy is typically suited to investigate long-range strain structures over the polycrystalline aggregate, this instrument employs high optical resolution (high numerical aperture) objectives to also provide adequate intra-grain resolution even in medium-sized (~10 µm) grains [1]. The utilization of these objectives that also possess extremely small depth of fields is only possible through continual automated working distance corrections to fight off defocusing. The sharpness of the consequent imaging has further been used to introduce a novel microscopy mode, called residual intensity [1]. This mode isolates and presents the twins with an order higher resolution than the DIC strains, albeit using the same images. It has the further advantage of showing deformation structures (in this case, twins) that are activated in a specific load increment, namely, residual intensity is an imaging mode with a reference state.
Here, we will employ this OM-DIC technique over the sharp rolling texture (for which, the band strain reaches about one third the twin transformation strain [1] in an unnotched sample), but in a notched sample to guide and overlap macroscopic twin bands. The notch locations will be specifically designed for this regime of extreme plastic anisotropy. Microscopic DIC is again implemented in-situ to study formation, expansion and overlap of the coordinated twin bands. Both strain and residual intensity calculations will be performed across loads to investigate the effect of stress raisers to the twin coordination.
[1] N.A. Özdür, İ.B. Üçel, J. Yang, C.C. Aydıner, Exp. Mech., 61: 499-514 (2021)

Sefer Erman   Bogazici University
Cahit Aydıner   Bogazici University