Bulk-nanocomposite synthesis through severe plastic deformation

Recently developed techniques for intense plastic deformation of metals, called Severe Plastic Deformation (SPD) are used primarily for the production of single-phase ultrafine-grained and nanocrystalline materials. In addition to this application these techniques offer new possibilities for generating microstructures similar to the ones generated by mechanical alloying. The product of these processes is, compared to classical mechanical alloying, a bulk material with a well-defined composition. The starting materials can be very versatile, such as coarse-grained two or multi phase alloys, a mixture of powders, sheets or wires or any other combination of solid starting materials. First feasibility studies with powders indicate the possibilities for generating dense nanostructured materials, novel nanocomposites and unexpected metastable materials which are impossible to generate in a bulk form by any other technique so far. The main goal is the development of a methodology that allows the prediction of the required SPD production steps to generate a desired pre- defined nanostructured bulk material or nanocomposite. The development of the novel methodology should be used to generate advanced nanocomposites for structural as well as functional applications with tailored properties. The systematic change of a large range of processing parameters and the application of different processing techniques along with their variation will be one approach to reach the defined goal. Another approach will be the use of recently developed in-situ techniques. These techniques will be used to analyze the deformation mechanisms over all length scales: from atomistic, over the nano and microscale up to the macroscale. In addition, the damage tolerance of the newly created materials will also a major part of the project. Fracture properties strongly determine the technical applicability of certain processing steps and so also the limits of the nanostructuring process. Furthermore, the damage tolerance will be a key factor for the structural but also functional applicability of the newly created materials. Model material systems, for example Fe-Cu or Fe-Mg to understand the structural fragmentation, the formation and stabilization of the nanocomposite will be selected for the investigations. Due to the vast variety of possible material combinations the intention is to study those model materials in detail leading to nanocomposites which address the needs for superior and sustainable material applications. As target groups for these new materials micromechanical, magnetic, electrical and even medical applications can be considered.

Composites are a material class consisting of at least two different constituents that lead to novel material properties that the single components do not possess on their own. Even though such structures have been known for many years, a lot of material development for new and unexpected properties is still possible in this field. Within this project such novel materials were synthesized by means of Severe Plastic Deformation (SPD) techniques and simultaneously further developed. Using SPD-techniques the structural size of the components can be made smaller and smaller which helps to generate unique material properties. This internal size is very important and can have a strong effect on structural (for example, how strong a material is) and functional (for example, how strongly magnetic a material is) properties. The starting materials for SPD-techniques can be very versatile and for this project high entropy alloys were frequently selected which represent a completely new material class on their own. It could be shown that the initial alloys, which were thought to be very stable, decompose into such nanocomposite or multiphase structures upon annealing. In this way novel nanocomposites, that are impossible to generate in a bulk form by any other technique, could be successfully created. The new materials have in common that they possess high strength that can be tuned in a wide range by changing the processing parameters. Besides strength, many material also require some deformability and a certain tolerance against flaws and cracks that mostly all materials possess. The newly created materials, however, often possess only limited deformability. With the aid of additional heat-treatments s it could be shown that in some of the synthesized materials the deformability can be enhanced. To learn more about ways how such materials in general can become less sensitive to cracks, another composite material, a steel wire that can be also found in suspension bridges or piano strings was investigated. Even though this composite does not belong to the most recent inventions in material science it still possesses the highest strength in all of our materials used for construction but still exhibits some deformability. To reveal its secret, the resistance against crack growth was evaluated. The presence of one very weak direction where cracks grow very easily, is the origin of high crack resistance in other loading directions, for example perpendicular to the wire axis, which is of primary importance for its use. A similar behavior is known from biological materials such as wood and bones. The underlying design concept found in the wire could stimulate further innovative ultra-strong and simultaneous tough metallic alloys and composites in future.