Reinforcement Size Dependence of Load Bearing Capacity in Ultrafine-Grained Metal Matrix Composites
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PARTICULATE-REINFORCED metal matrix composites (MMCs) are well known to have enhanced mechanical properties—such as stiffness, strength, hardness, and fracture toughness—compared to monolithic metal alloys.[1–11] In a drive to further enhance the properties of MMCs, recent research has focused on reducing the reinforcement particle size from the micrometric regime to submicron and nanometric length scales.[6,9] The strength and hardness of nanoparticle-reinforced composites have been extensively studied and the operative strengthening mechanisms have been thoroughly described[6,9,12,13] with the general finding that as the reinforcement particle size decreases, the strength of the composite increases. The reduction in the reinforcement particle size also reduces stress concentrations at the reinforcement particle corners and leads to a consequential increase in work hardenability due to reinforcement-dislocation interactions, which results in enhanced ductility.[6,7,9]
HANRY YANG is with the Department of Chemical Engineering & Materials Science, University of California Davis, Davis, CA 95616, and also with the School of Mechanical & Materials Engineering, Washington State University, Pullman, WA, 99163. LIN JIANG and JULIE M. SCHOENUNG are with the Department of Chemical Engineering & Materials Science, University of California Davis, and also with the Department of Chemical Engineering & Materials Science, University of California Irvine, Irvine, CA 92697. Contact e-mail:[email protected] MARTIN BALOG and PETER KRIZIK are with the Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Racianska 75, 83102 Bratislava, Slovakia. Manuscript submitted January 28, 2017. Article published online July 5, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
However, nanoscale effects on the stiffness of MMCs have not been well documented. For example, B4C is an excellent candidate as a stiffening component in MMCs due to its high elastic modulus of 460 GPa[14] compared to 70 GPa for Al. Even relatively small additions of micron-sized B4C will result in substantial predicted increases in the composite’s stiffness. However, as the development of MMCs pushes the reinforcement size to the nanometric regime, it is unclear whether the nanometric B4C particles will continue to be effective stiffeners. Therefore, a systematic study into the dependence of elastic modulus on reinforcement particle size is required to solve this puzzle. Generally, because elastic modulus is an intrinsic property of a pure substance, dictated by its atomic bonding characteristics, it is often also considered to be a structure independent material property in more complex materials, and therefore, particle size is generally not considered when predicting the elastic modulus of a composite. Typically, only the volume fraction of the reinforcement phase is considered, such as in the traditional rule-of-mixtures (ROM) approximation. With this traditional approach, for a constant volume fraction of reinforcement particles, the elastic
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