Metastable phases in bulk intermetallics - TEM study

Severe plastic deformation of intermetallic compounds can be used to make metastable phases. The resulting metastable phases being far away from the thermodynamic equilibrium often exhibit properties that are superior to those of the corresponding crystalline or equilibrium phase. Consequently, metastable phases are a timely exciting research topic of materials science and there is a strong impetus to obtain a comprehensive understanding of the fundamental factors controlling the transformations and the stability of these new phases. It is the purpose of the present project to gain experimental findings of the formation, structure, properties and stability of deformation induced metastable phases in bulk intermetallic compounds. Mainly, L12 long-range ordered alloys (Zr 3 Al, Co3 Ti) and B2 long-range ordered alloys (FeAl, CoTi) will be studied. The metastable polymorphous phases will be produced using high-pressure torsion and accumulative roll bonding. Within the framework of the project the following innovative aspects will be studied: 1) the evolution and the characterization of the different metastable structures, 2) the fundamental deformation mechanisms in nanocrystalline intermetallics, 3) the cyclic occurrence of the amorphous phase in CoTi and Co3 Ti during SPD, 4) the thermal stability and recovery processes of the metastable structures. In order to achieve these challenging goals innovative experiments with well designed parameters will be carried out (e.g.: re-deformation of different nanocrystalline structures, re-ordering under different hydrostatic pressures). The systematic experimental standards will be based on TEM investigations including atomic resolution TEM and a spectrum of complementary experimental methods (e.g.: differential scanning calorimetry, scanning electron microscopy, and X-ray diffraction). The well-designed systematic study will lead to new experimental findings facilitating a deeper understanding of the fundamentals of deformation-induced metastable phases. These results can pave the way to design novel composite materials consisting of different metastable phases (e.g.: ordered coarse-grained and disordered nanocrystalline; nanocrystalline and amorphous) made by controlled severe deformation to tailor their properties.

The current project "Metastable phases in bulk intermetallics - a TEM study" aimed to clarify the evolution, characterization and stability of different metastable structures produced by severe plastic deformation of intermetallic alloys. Metastable phases are phases far away from thermodynamic equilibrium. They are of high current scientific and technological interest since they often exhibit novel physical and mechanical properties. In the present project bulk metastable phases in the form of nanocrystalline or amorphous materials were studied: nanocrystalline materials consist of variously oriented crystalline structural units (grains) with a size in the range of less than 100 nm; amorphous materials are solid materials that do not display a long-range translational periodicity of the atomic positions. These structures were studied by dedicated characterisation methods for nanomaterials with the main focus on Transmission Electron Microscopy (TEM) methods. The investigations within this project revealed that the final atomic structure and the saturation microstructure of the metastable phase after severe plastic deformation are strongly correlated to the atomic structure of the individual lattice defects (dislocations) carrying deformation. Depending on the type of dislocations that are stored in the material chemically disordered nanocrystalline or amorphous structures are tending to occur. One additional important output of the project is the clarification of the origin of the heterogeneous deformation-induced refined structure as a consequence of its mechanical property. The transition to a softer phase that occurs when the material approaches the amorphous state is accompanied by a change of the deformation mechanism. It could be also shown that in the course of deformation of the amorphous phase nanocrystals can evolve showing a different chemical composition compared to the surrounding. Another focus of the project - the study of the stability of the metastable phase - revealed both the process of restoration of chemical order and by heating under special conditions an unexpected grain-size reduction. This result is remarkable since it is contrary to the classical grain-growth process. To achieve the goals of the project novel evaluation methods and the corresponding software were developed. According to the scientific findings, the overall success of the project is demonstrated on the basis of publications in high-rank journals of materials science, the presentation of the results at numerous international conferences and the awarding of national and international prizes.