Atomic resolution study of deformation-induced phenomena in nanocrystalline materials

Nanocrystalline (NC) materials exhibit exceptional properties such extremely high strength and high corrosion resistance, optical and magnetic properties as compared to their coarse grain counterparts. Several approaches have been applied to produce the NC materials, among which the severe plastic deformation is a very promising method for manufacturing the NC materials. NC materials prepared by sever plastic deformation have demonstrated outstanding mechanical properties due to the availability of small grain sizes. During the SPD, some unique features may occur, such as deformation-induced chemical intermixing of equilibrium-immiscible system, GB segregation, phase transformation and dissolution of precipitates in the immiscible systems etc... However, up to now, the atomic mechanism of these phenomena is not available, especially for chemical intermixing and segregation understanding at the atomic resolution. The atomic-scale understanding of these phenomena is an essential explanation of the properties that the NC materials demonstrate and helps optimizing the deformation process. On the other hand, it is also quite important that the insight into the atomic-scale process induced by deformation is expected to lead to the likely creation of new synthesis routes of NC materials. This project aims to develop our atomic-level understanding on the SPD deformation-induced solid state structural phenomena in NC materials, with emphasis on certain key issues which have not or only less been addressed up to now, e.g. the mechanism of interface chemical intermixing in the equilibrium-immiscible system, and grain boundary segregation occurred during the deformation of NC materials. To fulfill the goal, atomic resolution structural investigations, and quantitative atomic measurements, quantitative chemical composition analysis at the heterophase interface, and electronic structure measurements at the interface are to be performed on the immiscible binary model systems, e.g. Fe-Cu system. The experimental methods to be used include available modern advanced TEM techniques, including atomic resolution CS corrected HRTEM imaging, and atomic resolution chemical analysis via EELS, ELNES and EDXS, STEM imaging analysis. Moreover, in-situ experimental observation in a TEM will be used so that the evolution of the chemical mixed interface and segregation will be explored. The newly installed CS- corrected TEM, at the Erich Schmid Institute for atomic and electronic investigations of NC materials structure at the atomic level will be utilized. This study is of fundamental importance to understand the deformation-induced phenomena at the atomic level as well as technologically important to improve and well control the synthesis of NC materials and further explore the likely new synthesis routes of NC materials.

Severe plastic deformation (SPD) can produce novel nanostructured metals and nanocomposite alloys with superior and unique properties. Understanding the deformation induced phenomena, i.e., chemical intermixing, structure evolution and impurity effects, helps tailoring the microstructure and designing novel materials. In the present project, the fundamental issues generated in severely deformation were explored using atomic resolution transmission electron microscopy (HRTEM) combined with other approaches. To address the forced chemical intermixing issue, two phase nanocomposites (CuCr) can be deformed reaching the maximum solubility, which can be described by negative exponential trend. At the atomic level, the forced chemical intermixing although a large strain applied is inhomogeneous, and there exist numerous Cu atom clusters in Cr grains while less Cr clusters in Cu matrix. In this project, for the first time, the oxidation and decomposition processes of highly-strained single-phase Cu-Fe nanocrystalline alloys was studied at the atomic scale in details via in- situ heating experiments. It is found that, contrary to expectation, the oxidation process was initiated at relatively low temperature, forming nano-sized oxides such as CuO and Fe2O3 inside grains sequentially, and prior to Fe precipitating from Cu-Fe supersaturated solid solutions. ii) Via a comparison of high oxygen-content and low oxygen-content sample, it is found that oxygen significantly affects the microstructure and mechanical properties. Oxygen introduced during the powder pre-mixing can be partially dissolved into the matrix via severe plastic deformation, while the residual proportion of oxygen enriched at grain boundaries can effectively facilitate grain refinement. In this project, copper composites containing any of the three group VI refractory metals (Cr,Mo,W) via HPT at room temperature were developed, and the microstructure evolution with deformation was investigated. Via in-situ real time tracking the structure and chemistry of nanomaterials, we found that nanocrystallines are not only subjected to a structural change but also undergo a chemical evolution upon annealing. Some fundamental results obtained in the present project were published in 26 papers, 6 of those in refereed papers, including the most high ranking journals (Nature communication, Acta Materials, Script Materials etc..). The project leader was invited to talk at several international conferences about the results obtained in this project. Additionally, numerous further conference contributions were presented.