Micro-strain Evolution and Toughening Mechanisms in a Trimodal Al-Based Metal Matrix Composite
A trimodal metal matrix composite (MMC) based on AA (Al alloy) 5083 (Al-4.4Mg-0.7Mn-0.15Cr wt pct) was synthesized by cryomilling powders followed by compaction of blended powders and ceramic particles using two successive dual mode dynamic forgings. The microstructure consisted of 66.5 vol pct ultrafine grain (UFG) region, 30 vol pct coarse grain (CG) region and 3.5 vol pct reinforcing boron carbide particles. The microstructure imparted high-tensile yield strength (581 MPa) compared to a conventional AA 5083 (242 MPa) and enhanced ductility compared to 100 pct UFG Al MMC. The deformation behavior of the heterogeneous structure and the effects of CG regions on crack propagation were investigated using in situ scanning electron microscopy micro-tensile tests. The micro-strain evolution measured using digital image correlation showed early plastic strain localization in CG regions. Micro-voids due to the strain mismatch at CG/UFG interfaces were responsible for crack initiation. CG region toughening was realized by plasticity-induced crack closure and zone shielding of disconnected micro-cracks. However, these toughening mechanisms did not effectively suppress its brittle behavior. Further optimization of the CG distribution (spacing and morphology) is required to achieve toughness levels required for structural applications.
KeywordsDigital Image Correlation Metal Matrix Composite Equal Channel Angular Pressing Coarse Grain Dynamic Strain Aging
The authors gratefully acknowledge J. Curulli and M. Mecklenburg for their valuable advice. The images and data used in this article were generated at the Center for Electron Microscopy and Microanalysis (CEMMA), University of Southern California. The authors wish to acknowledge the financial support provided by the Office of Naval Research under the guidance of Rod Peterson and Bill Golumbfskie (ONR Contract N00014-12-C-0241).
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