Model of Enhanced Strength and Ductility of Metal/Graphene Composites with Bimodal Grain Size Distribution
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INTRODUCTION
DUE to their superior mechanical properties, last years single-phase but structurally inhomogeneous nanostructured materials have attracted tremendous attention (see, e.g., review).[1] Among such materials, one can distinguish metals and alloys with a bimodal grain size distribution,[2–10] gradient nanostructures,[11–19] bulk metals and alloys with nanoscale twins,[6,20–24] as well as metals containing nanograins dispersed inside coarse grains.[25] In particular, experiments[2–10,26] and simulations[26–31] of the mechanical behavior of metals and alloys with a bimodal grain size distribution demonstrated a combination of high strength with decent ductility, which is not achievable for structurally homogeneous nanostructured solids. In such materials, the hard nanocrystalline (nc) or ultrafine-grained (ufg) regions provide ultrahigh strength, A.G. SHEINERMAN is with Peter the Great St. Petersburg Polytechnic University, Polytechnicheskaya 29, St. Petersburg, Russia, 195251 and also with the Institute of Problems of Mechanical Engineering, Russian Academy of Sciences, Bolshoj pr. 61, Vasiljevskii Ostrov, St. Petersburg, Russia, 199178 and also with the Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg, Russia, 199034. Contact e-mail: [email protected] M. YU. GUTKIN is with Peter the Great St. Petersburg Polytechnic University and also with the Institute of Problems of Mechanical Engineering, Russian Academy of Sciences and also with the ITMO University, Kronverksky pr. 49, St. Petersburg, Russia 197101. Manuscript submitted July 5, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
while soft coarse grains are responsible for high enough strain hardening and resulting good ductility. Also, experiments[20–23] and modeling[32] demonstrated the possibility of achieving very high strength combined with good ductility using the conjunction of a bimodal grain size distribution and the formation of growth twins in the coarse grains of austenitic steels. In parallel with the use of single-phase materials, where the synergy of high strength and good ductility is achieved using the formation of inhomogeneous size and spatial distributions of grain and/or twin boundaries, recently, significant progress has been made in the fabrication of metallic materials reinforced by graphene platelets (see References 33 through 37 and references therein) or graphene nanoribbons[38] characterized not only by high strength but also by good ductility. The possibility of the simultaneous enhancement of strength and ductility in graphene-reinforced composites was attributed[37] to the trapping of dislocations by graphene platelets during plastic deformation. Due to the high total surface area of graphene platelets, the latter trapped a large amount of dislocations, thereby creating strong back stresses, enhancing strain hardening and improving ductility. Another approach to achieving a combination of high strength and satisfactory ductility was suggested by Xiang et al.,[39] who produced special bimodal
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