Atomistic study of metastable phases

Metastable phases are often the key-components of high-performance materials and are present in a variety of developments such as hardenable alloys (e.g., high speed steels, several light-weight alloys based on Al, Mg, and Ti), intermetallic titanium-aluminides, and hard coatings. Especially during rapid cooling and solidification, when atomic assembly kinetics are limited, metastable phases can easily form. Nevertheless, only little is known on structure, elastic constants, and phase stabilities of the metastable phases themselves. The decomposition of metastable and/or supersaturated phases into their stable constituents can occur themselves via the formation of other metastable phases if energetically favored. Plasma-assisted vapor deposition techniques with their ability of extremely high cooling rates can be seen as an `extreme processing` allowing the preparation of metastable and even unstable phases and materials with a high non-equilibrium density of structural built-in defects. This results in a broad range of unusual properties which are not possible via other processing methods. Plasma-assisted methods fostered the development of advanced hard coatings using materials science based design rules. The development of advanced coatings follows the major trend of synthesizing multicomponent and multiphase structures for special applications. Ternary transition metal nitrides provide a wide range of structure types. By composition variations, they allow fine tuning of mechanical and electronic properties. They provide the opportunity to adjust parameters like lattice constants, hardness, elasticity, or corrosion stability in order to optimize the overall performance of the coating. The main objective of the proposed research work is to understand the mechanisms behind the formation of metastable phases active in ternary material systems using Ti-Al-N, Cr-Al-N, and Zr-Al-N. In addition to their industrial relevance these three model systems are chosen based on the possibility to investigate also the influence of atom size, electron density, and binding character on structure, properties, and phase stability of AlN containing phases. Furthermore, the influence of additional alloying elements on the mentioned properties of Ti-Al-N, Cr-Al- N, and Zr-Al-N based materials will be studied. Here, also the differences of alloying elements on the properties of Ti-Al-N, Cr-Al-N, and Zr-Al-N shall be evaluated and explained. The strategy is to investigate the correlation between composition, structure, and supersaturation of thin film materials, using theoretical and experimental methods. Therefore, ab-initio calculations and continuum mechanical approaches in combination to advanced structural investigation methods like atom probe analyses and transmission electron microscopy are addressed.

The target for developing new materials can nicely be described by the Olympic motto faster, higher, stronger. But contrary to Olympic Games or any other Championships, doping- and even alloying is allowed in materials science. Thus, materials development is not just based on optimized microstructures (basically the architecture of materials, even at atomic-scale) but also on optimized chemical compositions. Modern materials science allows for their atomic-scale investigations and atomic-scale preparations. This allows for the development of materials, which combine properties that are extremely difficult (if not even naturally impossible) to combine. Industry is eagerly awaiting such supermaterials, for example to allow for faster machining (e.g., cutting or drilling operations) or higher application temperatures (e.g. the efficiency of jet-engines or gas-turbines increases with the combustion temperature). This all is based on the sufficient strength of the material against mechanical or corrosive attack, at the application temperature. New materials and their predicted properties based on their atomic-scale architecture can be calculated by means of supercomputers. Especially the metastable states, which can develop when using extreme process-conditions (such as diamond, which forms as a metastable state of carbon under extreme temperatures and pressures), significantly widen the property-spectrum of materials. Knowing and understanding these processes allows for the development of materials, tailor-made for specific applications. Thereby, the life-time and resistance of e.g., machining tools or components used in automobile or aerospace industry can be increased. For example, the bright gold-colored titanium-nitride, which is used as protective coating on cutting tools or decorative coating on high-quality jewelry: By a single additional atom (among hundred others), the properties of titanium-nitride (primarily consisting of titanium and nitrogen) can alter significantly. This can be calculated with supercomputers. The experiments proof if the new materials-design is successful. If not, additional calculations help to understand and better describe the problem. The high-level combination of theory and experiments form the core of this START-Project.