Elucidating the kinetics of twin boundaries from thermal fluctuations
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esearch Letters
Elucidating the kinetics of twin boundaries from thermal fluctuations Dengke Chen and Yashashree Kulkarni, Department of Mechanical Engineering, University of Houston, Houston, Texas 77204 Address all correspondence to Yashashree Kulkarni at [email protected] (Received 5 June 2013; accepted 9 September 2013)
Abstract There is compelling evidence for the critical role of twin boundaries (TBs) in imparting the extraordinary combination of strength and ductility to nanotwinned metals. Here, we investigate the thermal fluctuations of TBs in face-centered-cubic metals to elucidate the deformation mechanisms governing their kinetic properties using molecular dynamics simulations. Our results show that the TB motion is strongly coupled to shear deformation up to 0.95 homologous temperature. A rather unexpected observation is that coherent TBs do not exhibit any capillarityinduced fluctuations even at high temperatures, in sharp contrast to other high-angle grain boundaries.
Nanotwinned metals are known to demonstrate a remarkable combination of mechanical properties, namely, ultra-high strength, enhanced ductility, and high strain rate sensitivity.[1–6] This is in contrast to nanocrystalline materials, which exhibit a loss of ductility, and grain stability with decreasing grain size, thereby offsetting the initial excitement generated by their very high yield strength (see[7] for review). It is well documented, through many experimental and theoretical studies, that this loss of stability of nanograined metals, which has severely limited their practical application, is associated with thermally-activated or stress-assisted grain growth caused by grain boundary (GB)-mediated processes such as migration and sliding.[8–11] It is natural then, that the grain growth and twin lamella stability in nanotwinned metals would also be intimately connected to the thermodynamic and kinetic properties of twin boundaries (TBs) and GBs. Although the prospect of the stability of nanotwinned structures is of vital concern, one which defines their ultimate utility and raises fundamental questions regarding the underlying physics, the issue has remained relatively unaddressed until recently.[12,13] In this Research Letter, we report our investigation of the motion of TBs by atomistic modeling of their thermal fluctuations over a range of temperatures. In the theory of statistical mechanics of interfaces, thermal fluctuations have been effectively used to elucidate the thermodynamic and kinetic properties of fluid and solid membranes and interfaces.[14] In the case of high-angle GBs, the capillary wave theory has been successfully applied to relate their long wavelength thermal fluctuations to important quantities such as the GB stiffness and mobility.[15] According to the capillarity theory, the energetic cost for these out-of-plane fluctuations is attributed to the surface tension, or in other words, the increase in the area of the interface to accommodate the bending due to fluctuations. The Fourier spectrum of these capillarity-induc
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