Quantitative analysis of local deformation and direct correlation with microstructural and mechanical parameters during equal channel angular pressing in tools with variable geometries

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Equal Channel Angular Pressing (ECAP) is a processing technique that allows to subject metallic materials to severe plastic deformation which leads to massive grain refinement. The resulting ultrafine-grained materials are characterized by exceptional mechanical properties (high strength in combination with good ductility), making them potentially attractive for applications in, for instance, light-weight design. While the effect of ECAP on microstructure and properties of many alloys has been studied in great detail in recent years, there still exists a fundamental knowledge gap with respect to local deformation in the plastic deformation region of different ECAP tools: typically, equivalent strains are only estimated based on simplified models that do not consider, for example, the experimentally documented effects of material hardening or of a commonly applied back-pressure. A more detailed understanding of local (plastic) deformation, however, is needed in order to quantitatively describe the interrelationships between microstructural deformation mechanisms and grain refinement, which have only been analyzed qualitatively so far, and to fundamentally study and predict physical metallurgy processes as a function of material and processing parameters. The principal investigator has recently extended an analytical model that accurately describes the local deformation in 90°-ECAP tools along flow lines such that it can be also used to analyze arbitrary tool geometries. In combination with a high-resolution visioplastic technique, also developed in preliminary work, there now exists a tool-set that allows to study the key hypothesis of this proposal – the assumption that the actual equivalent strain (as expressed by the flow line exponent, its gradient in the work-piece, and the evolution of these parameters with increasing number of ECAP passes) represents a physically relevant measure that can be corelated with (or even used to quantitatively predict) many different phenomena, like microstructural and texture evolution, material properties, or damage accumulation. In this project, three model alloys and four different ECAP tools will be used to experimentally characterize the plastic deformation region under different processing conditions and how plastic deformation proceeds along different flow lines. Image recognition algorithms, based on artificial intelligence, will be developed and used to accurately fit local deformation on three complementary levels. This will for the first time also allow to evaluate the quality of different flow line models and thus to provide a benchmark test for different modeling approaches that are currently established in the ECAP community. Finally, the experimental and theoretical work will also provide the means to clearly identify relevant processing and material parameters, and to directly relate them with the resulting macroscopic properties.