Refining Grains of Metals through Plastic Deformation: Toward Grain Size Limits

CONSPECTUS: Refining grains of metals and alloys is an effective approach to tailor their properties and performance. Plastic deformation is routinely used for structure refinement, with which microstructures down to the submicron scale can be accessed in most metals and alloys. However, further reducing grain sizes below the submicron scale is challenging. Grain refinement induced by deformation is basically a process of generating grain boundaries (GBs) through interaction of dislocations and/or other defects. During plastic straining, however, the opposite processes may be triggered simultaneously: dislocation annihilation via their interactions and grain coarsening due to GB migration. These processes may intensify with decreasing grain sizes. The refinement may cease at a certain size (so-called "saturation grain size") when a balance is reached between the generation and annihilation processes. To refine grains further below the saturation size, one has to break the balance, either to suppress the GB annihilation or to enhance the GB generation, or both. In the past decades, several techniques have been developed by optimizing the processing parameters for promoting grain refinement. By increasing strain rates, decreasing deformation temperature, and enlarging strain gradient, grains of pure FCC metals like Cu, Ni, and Ag can be refined down into a few tens of nanometers. Within this grain size regime, an important phenomenon was discovered that structure relaxation of GBs is triggered through interactions of GB with partial dislocations. The relaxed GBs further stabilize the nanostructures, yielding an extraordinary stability of the nanograined metals under thermal and mechanical activations, even beyond that of their coarse-grained counterparts. In this Account, we summarize the development of plastic deformation induced grain refinement in metals and alloys in recent years, including the grain refinement mechanisms and advancement of the processing techniques. Then, GB relaxation in several nanograined FCC metals below the critical grain size induced by plastic deformation will be introduced. Changes of GB structures and the grain size effect, as well as the effect on thermal and mechanical stabilities of the nanograined metals will be emphasized. In addition, GB relaxation in nanograined and submicron-grained Cu induced by rapidly heating will be discussed. The effect of the GB relaxation, induced by either deformation or rapid heating, on stabilities of the nanograined samples will be analyzed in comparison with that of their coarse-grained counterparts. We conclude with a perspective on the future studies on this topic. On one hand, possibility of even further structure refinement in metals will be addressed. On the other hand, a novel strategy of materials development in contrast to traditional alloy is proposed, namely, material plainification by architecting stable boundaries. Aiming to promote the sustainability of materials, the "material plainification" strategy enables specific properties of metals by stabilizing and engineering interfaces especially at the nanometer scale instead of alloying, and therefore reducing the materials' compositional complexity, facilitating the recovery and reuse of materials.