| Abstract: | It is well-believed that below a certain particle size, grain boundary-mediated plastic deformation (e.g., grain rotation, grain boundary sliding and diffusion) substitutes for conventional dislocation nucleation and motion as the dominant deformation mechanism. However, in situ probing of grain boundary processes of ultrafine nanocrystals during plastic deformation has not been feasible, precluding the direct exploration of the nanomechanics. Here we present the in situ texturing observation of bulk-sized platinum in a nickel pressure medium of various particle sizes from 500 nm down to 3 nm. Surprisingly, the texture strength of the same-sized platinum drops rapidly with decreasing grain size of the nickel medium, indicating that more active grain rotation occurs in the smaller nickel nanocrystals. Insight into these processes provides a better understanding of the plastic deformation of nanomaterials in a few-nanometer length scale.The plastic deformation of conventional polycrystalline metals has been well-studied. The plastic behavior of coarse-grained metals (with particle size larger than 100 nm) is mainly controlled by the nucleation and motion of lattice dislocations. Plastic deformation by dislocation glide results in crystallite rotations, generating lattice preferred orientation or texture. The anisotropic physical properties of a polycrystalline material are strongly related to the preferred alignment of its crystallites. Texture studies are of interest in many fields. In material science and engineering, texture control is essential in improving the strength and lifetime of structural materials (1). In Earth science, understanding texture development of minerals is important for interpreting seismic anisotropy in the Earth’s interior (2–5).The plastic deformation of nanomaterials has attracted much interest in recent years (6–12), but many controversies still exist (6–17). Various mechanisms have been reported (8, 11–18). It has been proposed that below a critical length scale the strength of nanometals would exhibit an inverse Hall–Petch size dependence because in the plastic deformation of fine nanocrystals, dislocation activity gives way to grain boundary (GB) sliding, diffusion, and grain rotation (7). If GB-mediated mechanisms dominate plastic deformation, it would yield a d−4 dependence on grain rotation rate, where d is the grain size (9), i.e., grain rotation activity would be greatly enhanced in fine nanocrystals. Grain-rotation-induced crystallographic alignment has been observed in 2–3-nm ferrihydrite nanocrystals (15–17). In contrast, computer simulations suggest that GB mobility drops with decreasing grain size (19, 20). Although the observation of grain rotation during deformation of micrometer-sized crystals is feasible (21, 22), in situ probing of grain rotation of ultrafine nanocrystals is difficult, precluding the direct exploration of mechanics at nanometer scales. Whether grain rotation becomes more active and dominant in finer nanocrystals is not yet experimentally verified. In this work, radial diamond-anvil cell (rDAC) X-ray diffraction (XRD) experiments (2) are used to make in situ observation of the texturing of stressed polycrystalline platinum in nickel media of various mean particle sizes, from 500 nm down to 3 nm. The texturing change of platinum is expected to reflect some activity at the GBs of the nickel medium. |
