Effect of grain architecture on ductility in SPD materials

Ductility and strength are very important material properties for structural and even for functional application, because a certain damage tolerance is always required. Beside this importance for the final application for processing and machining ductility is essential also. The strength of nanocrystalline materials is usually very high. However, the ductility and the fracture toughness - the property, which describe the ductility in the presence of cracks - is often very low, which exclude them from applications. A recent developed new class of techniques - severe plastic deformation - permits the generation of ultrafine grained (grain size between 100nm to 1m) and nanocrystalline materials, which offer the hope to overcome this drawback - the brittleness - of nanocrystalline materials. Despite the enormous increase in research activities on SPD materials in the last decade the understanding of ductility and especially fracture toughness controlling phenomena remains poor. One of the reasons is the limited amount of material and limited size of samples. The upscaling of SPD technologies allows now to overcome this problem. An important step in the last decade in the field of SPD was the improvement of the high pressure torsion technique. This technique is relatively simple, allows the application of very high strains, which are difficult or impossible to apply by other techniques, permits the deformation of relatively brittle materials and offers the simple variation of processing parameters like strain rate or deformation temperature. Furthermore, it enables also the consolidation of different powders and transforms them into novel nanostructured bulk materials, which can often not be generated by other techniques. Due to the upscaling of the HPT - the world largest HPT has been developed in the group of the applicant - it is now possible to determine the ductility and fracture toughness in all directions, even for materials deformed to very large strains. The pre-experiments exhibited unexpected large variations in the fracture resistance between the two investigated model materials ultrafine grained iron and ultrafine grained nickel and an extraordinary large orientation dependence. To explain the ductility of SPD material in tension experiments in the SPD community different phenomena are discussed, for example, bimodal grain size distributions, the grain boundary structure ("non- equilibrium boundaries"), special grain boundary associated deformation mechanisms, etc. The performed feasibility studies show, however that grain shape and arrangement seem to play a more important (maybe the dominant) role. The goal of the applied project is to understand better the effect of grain architecture (size, shape, arrangement, texture, and boundary structure) on the ductility and fracture resistance. This knowledge will help to develop nanocrystalline materials, which have intrinsically high strength with good ductility - a dream of material scientists. To obtain the goal, both the fracture phenomena and the architecture controlling parameters will be analyzed. The unique combination of the techniques developed at the Erich Schmid Institute to study the deformation and fracture processes over all length scales from the nano- to macro will be used to obtain progress in the design of damage tolerant nanocrystalline materials.

Ductility and strength are very important material properties for structural and even for functional application, because even for functional application a structural integrity is always required. Beside this importance for the final application for processing and machining ductility is essential also. A way to increase the strength of metals is to decrease their grain size, especially if we go to the nanocrystalline regime. The strength of nanocrystalline materials is usually very high. However, the ductility and the fracture toughness the property, which describe the ductility in the presence of cracks is often very low, which exclude them from applications. A recent developed new class of techniques severe plastic deformation permits the generation of ultrafine grained (grain size between 100nm to 1m) and nanocrystalline materials, which offer the hope to overcome this drawback the brittleness of these new materials. Despite the enormous increase in research activities on SPD materials till 2012 the understanding of ductility and especially fracture toughness controlling phenomena remains poor. Due to the upscaling of the HPT the world largest HPT has been developed in the Leoben group it is now possible to determine the ductility and fracture toughness in all directions, even for materials deformed to very large strains. The experiments exhibited unexpected large variations in the fracture resistance in the different ultrafine grained and nanocrystalline materials and an extraordinary large orientation dependence. To explain the ductility of SPD material in tension experiments in the SPD community different phenomena are discussed, for example, bimodal grain size distributions, the grain boundary structure (non-equilibrium boundaries), special grain boundary associated deformation mechanisms, etc.. The performed studies show, however that grain shape and arrangement seem to play a more important and often the dominant role. The goal of the applied project was to understand better the effect of grain architecture (size, shape, arrangement, texture, and boundary structure) on the ductility and fracture resistance. To obtain the goal, both the fracture phenomena and the architecture controlling parameters have been analyzed. The detailed analyses of the change of the grain architecture by changing the type of deformation delivered a deep understanding of phenomena controlling the size and shape of the grains, their arrangement, texture, and their boundary structure. This allows now to control significantly better the grain architecture of ultrafine grained and nanocrystalline metals. The unique combination of the techniques developed at the Erich Schmid Institute to study the deformation and fracture processes over all length scales from the nano- to macro were used to obtain an improved understanding of the fracture controlling processes in many different nanocrystalline materials. One of the main findings was the large anisotropy in the fracture resistance induced by the grain shape. It could be shown that this anisotropy offers new possibilities to design microstructures with ultrahigh strength and relative good ductility, a dream of material scientists. The success of the project can be easily seen by the vast number of presentations (more than 40) and publications (more than 20) in the best journals in this area.