Dynamic Micro-Strain Analysis of Ultrafine-Grained Aluminum Magnesium Alloy Using Digital Image Correlation
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TRODUCTION
UNDERSTANDING deformation and fracture mechanisms is essential to improving mechanical properties of new materials. Microscopic investigations of new materials commonly rely on transmission electron microscopy (TEM), scanning electron microscopy (SEM), and focused ion beam (FIB). In recent years, new characterization techniques and methods have evolved to exploit the capabilities of these instruments. For example, in situ SEM observation of micro-strain evolution has become an attractive approach for studying the deformation mechanisms of advanced structural materials. Likewise, the use of digital image correlation (DIC) has been extended to small-scale deformation measurements. In this work, we focus on the direct observation of how micro-strain evolves among the grains of an ultrafine-grained (UFG) Al-Mg alloy. To our best knowledge, no study has been done on the in situ micro-strain measurement of an UFG Al-Mg alloy using DIC at the sub-micron level. DIC[1,2] is a non-contact, adaptable metrology technique for in-plane or out-of-plane strain field measurement that can be utilized on a variety of length scales ranging from civil engineering structures[3] to microstructures of metallic specimens.[4,5] The DIC algorithm
YUZHENG ZHANG, Ph.D. Candidate, and STEVEN R. NUTT, Professor, are with the Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-0241. Contact e-mail: [email protected] TROY D. TOPPING, Post Doctorate, and ENRIQUE J. LAVERNIA, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Manuscript submitted March 18, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
tracks a grayscale pattern on the deforming surface step by step in a small area called a subset. To track the fullfield surface deformation, an isotropic random speckle pattern is required on the specimen surface. This speckle pattern can be either intrinsic (from existing surface features) or extrinsic, as in a deposited pattern. The optimal feature size of a speckle pattern is reportedly 2 to 3 pixels for a recorded image.[6] Therefore, different patterning methods are needed to meet this requirement at different scales. While there have been multiple investigations reporting the use of DIC in making microscale strain measurements, one of the major challenges in such endeavors is producing a nanoscale, random, and isotropic speckle pattern required for DIC. In this study, the area of interest is only about 20 by 20 lm which requires a nanoscale speckle patterns for DIC analysis. Various approaches have been employed to generate extrinsic DIC patterns on different substrates at reduced scales. The most common patterning method for smallscale DIC involves using a high quality airbrush to spray microscale paint patterns on a substrate.[7,8] Another convenient approach is generating grid patterns using a grid mask.[9,10] Intrinsic patterns are also available when the surface of specimens exhibited small-scale f
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