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Microstructural Design of Bulk Ultra-fine Grained and Nanostructured Materials for High Performance Applications

Final Report Summary - BUNSMAT (Microstructural Design of Bulk Ultra-fine Grained and Nanostructured Materials for High Performance Applications)

Bulk ultra-fine grained (UFG) and nanostructured (NS) materials continue to attract significant interest due to their improved features over conventional coarse-grained materials. This interest largely stems from the potential of obtaining a promising combination of enhanced mechanical behavior and physical properties with possible application in various industries ranging from automotive to aerospace and biomedical to energy. Conventional method for grain refinement is deforming a material to high plastic strains. Traditional deformation methods, such as rolling, drawing and extrusion can impart large strains on the workpiece. However, one or more dimensions of the sample are always gradually reduced during deformation, thus a foil with limited utilization is produced. Additionally, refining grain size down to sub-micron regime is challenging due to the extent of strain that can be applied. With the above limitations, novel plastic deformation techniques with severe straining capability without a change in the billet dimensions have made the synthesis of bulk UFG-NS materials a reality.
The ultimate goal of BUNSMAT is to elucidate the mechanisms that govern deformation and failure of ultra-fine microstructures obtained via severe plastic deformation (SPD). Findings of this research are instrumental in the development of improved materials with desirable properties including high strength, enhanced ductility and resistance to fatigue damage for service at ambient and elevated temperature conditions. The processing utilized for microstructural refinement encompasses a unique extrusion die consisting of two channels of equal cross-section intersecting at a right angle. Severe deformation is conducted in a state of simple shear with processing routes obtained via the rotation of the billet around its long axis between successive passes. Successful extrusions were performed on commercial purity (CP) titanium up to 8 passes accumulating a total strain of 9.24 in the as-processed materials.
Revelation of the severely deformed microstructure in light of monotonic and cyclic deformation with a special emphasis on thermal stability is aimed. Accordingly, post-SPD microstructural and mechanical characterizations are carried out, exhibiting the influence of high temperatures up to 900oC. Experiments on coarse-grained (CG) and processed samples demonstrated that SPD enhances the strength of CP titanium with close to 50% increase in steady flow stress levels at temperatures as high as 600oC. Thermal stability investigations encompassing controlled heat treatment experiments verified the retention of the UFG microstructure up to 600oC. Above this temperature, grain growth was apparent. This held true for both purity levels except displaying different rates of growth, with grade 2 showing more resistance up to 700°C and grade 4 displaying a relatively stable behavior at temperatures above 700°C. This contrast was linked with the effect of oxygen content and the difference in processing temperatures. The flow behavior was modeled utilizing three techniques including a dislocation density based approach, an Arrhenius type approach and a modified Johnson-Cook approach. Although all models showed decent predictions, the capability of each approach varied depending on the deformation rate and temperature.
Further monotonic and cyclic characterizations on grade 4 material revealed important findings. Accordingly, thermal stability against coarsening up to 500°C is obtained under monotonic conditions. The effect of loading was manifested with a significant tension-compression asymmetry that diminishes at lower deformation rates. Fracture surface studies indicated a noticeable amount of equiaxed dimples and micro-voids confirming the occurrence of ductile failure. The severely deformed microstructures subjected to fatigue loading exhibited stability up to 90000 cycles at 400°C in relation with its high fraction of high angle grain boundaries. This improved performance of the refined samples corresponds to a five-fold increase over the CG counterpart. Damping capacities of both CG and UFG titanium vary slightly with increasing temperature up to 300oC followed by drastic escalations with the onset observed at a comparably lower temperature for the latter, pointing to improved damping response of the severely deformed microstructure.
Severely deformed materials bring about dramatic mechanical performance improvement leading to competitive advantage in transportation, energy, automotive and biomedical applications with broad impact in science, technology and social welfare. Extension of the operational range of structural materials is the targeted impact with high prospect of attracting commercial interest. The field of metal forming also benefits from high temperature studies for the adjustment of warm and hot working process parameters. The outcome of improved workability supports the potential utilization of UFG-NS materials in various industrial applications.
With the support through project BUNSMAT, the PI established an active research group named MEMFIS conducting progressive research in the area of mechanics and manufacturing of advanced materials. Currently, the group has 8 researchers enabling activity in various projects. Hitherto, 1 PhD and 3 MS students have completed their graduate degrees. The work performed in two research laboratories are supported via additional governmental and industrial funding sources obtained over the course of this project. The project outcomes are disseminated in a timely manner in the form of five journal publications and eight conference presentations. Outreach activities included informatory talks at various high schools and summer term lectures. Another aspect of knowledge transfer is through consulting and teaching activities in the area of materials behavior. Most courses taught at the host institution include application based teaching elements and aid the PI share his vast experience gained in academic and industrial settings.