Deformation-induced grain growth in nanocrystalline copper

Nanocrystalline metals have demonstrated outstanding mechanical properties due to their small grain sizes. Grain instability in nanocrystalline metals has been of a long-standing concern since it influences the mechanical properties significantly; this has attracted new and increasing attentions with the recent observation of grain growth in nanocrystalline metals during plastic deformation. The mechanisms of this deformation-induced nanograin growth, which is observed much more in monotonic than in cyclic loading conditions, are still not well understood. Only part of this phenomenon is revealed from quite limited experimental and simulation works. The understanding of these phenomena is an essential explanation of the mechanical properties of nanocrystalline materials on one side, however, it is also important for the in service lifetime of components and the technologies to generate such nanostructures. This experimental project aims to develop our understanding on this deformation-induced microstructural instability phenomenon in NC metals, with emphasis on certain key issues which have not or only less been addressed - the instantaneous stress-strain response with respect to the microstructural instability, the influence of the strain (magnitude, cyclic versus monotonic) and the fundamental deformation mechanisms involved in grain coarsening. To achieve this goal, systematic, cyclic and monotonic experiments - bending, torsion and high pressure torsion - are to be executed on electrodeposited nanocrystalline Cu, a model material to study the mechanical and deformation behaviour at nanograin regime. The experiments cover various loading conditions and a wide scale over the specimen sizes (from sub-micrometers to several millimeters) as well as the applied strains (up to 1000). The unique combination of the techniques developed at Erich Schmid Institute for microstructural characterization and mechanical testing over all length scales from nano- to marco-scale will be utilized. This study is fundamentally important to understand the unique deformation mechanisms as well as technologically important to improve the application reliability of nanocrystalline materials.

The development of high strength and ductile structural materials is one of the major goals in material sciences. One opportunity to achieve high strength by sustaining moderate ductility is to refine the grain size, which is the size of the structural elements metallic materials are consisting of. Latest technologies allow to process nanostructured materials exhibiting grain sizes in the nanometre regime, which also bears difficulties. Extremely small scaled structures become unstable as they tend to coarsen upon mechanical loading, which causes a degradation of the materials strength. As the structural design of these materials is based on the initial strength parameters, such changes crucially reduce their reliability during in service use. To deepen the knowledge about underlying mechanisms and driving forces is therefore of utmost importance. By micro mechanical bending experiments conducted inside a scanning electron microscope it was possible to reveal the nature of this coarsening procedure as a continuous movement of the boundaries between the grains. Frequently boundaries move in such a direction, that the larger grains grow, whereas the smaller grains shrink and vanish. In conventional coarse grained materials such boundary movement is observed, when the temperature is raised to a certain amount. However, in case of nanostructured materials a thermal activation of the boundary motion must not be understood as a pre-requisite for grain growth anymore, which could be ascertained by grain growth procedures occurring at -196.15C during cyclic loading experiments. At lar ge cyclic deformation grain coarsening takes place preferentially in band like areas, where locally huge amount of deformation are realized. Thereby, it was observed that grains which were favorably aligned to accommodate cyclic deformation coarsened tremendously. This means, that the motion of the boundary being responsible for the coarsening is triggered by processes realizing the imposed deformation, which is the motion of dislocations. Although with increasing cyclic deformation grain coarsening continuously proceeds within the bands, eventually the strain amplitude, which is the imposed deformation per cycle, determines the coarsened grain size. It has been shown that these coarsening procedures predominantly occur in high purity materials and can be prevented by introducing a second phase with a lamellar arrangement.