Improving the stability of nanocrystalline metals

Nanocrystalline materials have been a subject of extensive research over the past couple of decades due to their unique mechanical and functional properties. A variety of processing methods to obtain nanocrystalline materials already exist. Unfortunately, nanocrystalline structures are thermally and mechanically unstable due to the large amount of enthalpy stored in their large grain boundary area, and coarsening of the nanocrystalline structures alters material properties considerably. As a consequence, the stability of nanocrystalline metals is a fundamental issue for future potential applications. Adding solutes, impurity elements and second phase particles can stabilize a nanostructure against grain growth. The underlying mechanism for controlling the stability reduction of grain boundary energy by solute segregation or kinetic barriers for grain boundary migration by solute drag or second phase particles- is still under discussion and will be investigated in detail in the Co-Cu model system in this project. Synthesis of bulk nanocrystalline metallic materials, in this model system, with different types and amounts of solutes and impurity elements, will be conducted by severe plastic deformation and electrodeposition. The Co-Cu system has a positive heat of mixing, and the formation of non-equilibrium solid solutions can be studied. Furthermore, it exhibits extraordinary magnetic properties like giant magnetoresistance. The initial microstructures (e.g. texture, etc.) of the synthesized nanostructures are strongly dependent on the used processing method used. In this project, a direct comparison of the thermal stability of electrodeposited and severe plastically deformed nanomaterials and the influence of their initial microstructure regarding different defect density, different types of grain boundaries and distribution of solutes on stability, can be conducted. This will help to understand the relation of structure to stability and identify the mechanism controlling stability. The application of different thermodynamic or kinetic models to experimental results will help to identify the mechanisms of grain growth. Special attention will be drawn to the influence of solutes and impurity elements on the mechanical properties of the nanocrystalline materials. The aim of the project is to link the gained knowledge about optimum nanostructures for enhanced thermal stability to develop strategies to enhance grain size stability under thermal and mechanical load simultaneously. Possible formation of supersaturated solid solutions, decomposition processes, their influence on thermal stability and the mechanism behind bulk mechanical alloying will be investigated as well. The results of the project can be used as a basis for enhancing structural stability in nanocrystalline materials and creating stable bulk nanocrystalline metallic materials in the future, which will increase their potential applications.

Nanocrystalline materials have been a key area of research in materials science over the past couple of decades due to their extraordinary mechanical and functional properties. Nanocrystalline structures are, however, thermally and mechanically unstable due to the large amount of enthalpy stored in their large grain boundary area. Even at temperatures close to room temperature, significant coarsening of the nanocrystalline structures might occur. As a consequence, material properties might be considerably altered. For future potential technical applications, this is a fundamental issue, which must be solved. The goal of the applied project was to understand better the processes that govern, thermal, as well as mechanical, stability of nanocrystalline materials. To reach this goal various nanocrystalline structures were synthesized in the immiscible Cu-Co model alloy system by two different methods (electrodeposition and severe plastic deformation using high-pressure torsion deformation). Additionally, in-depth microstructural characterization of the nanocrystalline structures was carried out. Furthermore, the influence of the initial structures on thermal stability was investigated and the functional and mechanical properties of the structures were determined. With severe plastic deformation, single phase supersaturated Cu-Co solid solutions with different composition could be obtained. Subsequent phase separation during annealing resulted in complex microstructures. These structures are similar to conventional composite materials, which are made from two or more constituent materials. However, the Cu-Co composite materials processed during this project consist of constituents, which still have dimensions in the nanometer range. The structure is further stable even for very long annealing times at elevated annealing temperatures. Using different annealing treatments, the structure of the nanostructured composites could be further modified to tailor the mechanical and magnetic properties. The results of the project can be used as a basis for enhancing structural stability in nanocrystalline materials and creating stable bulk nanocrystalline metallic materials in the future, which will increase potential applications. The success of the project can be easily seen by the vast number of presentations at international conferences (more than 20 including 8 invited talks) and 8 publications in the renowned international journals, which have been published up to date.