UC Riverside

Nanocrystalline (NC) metals have sparked an increasing number of research interest due to their superior mechanical properties as compared to their coarse-grained (CG) counterparts. Such enhanced mechanical properties of NC alloys can vary significantly depending on their microstructures, which are affected by their processing routes and deformation mechanisms. As a result, it is more than significant to study the processing - microstructure - property relationship in novel binary NC alloys. In this dissertation, this relationship is addressed in both engineering perspective and science perspective. The engineering perspective involves the mechanical and bio performance of Fe-Mn alloys as a function of processing routes and microstructural evolution. Fe-Mn alloys have been recognized with a new application for making bioresorbable implants due to the combination of their promising mechanical strength and bioresorbability. However, although coarse-grained Fe-based alloys as a bioresorbable material have been well studied, there is a knowledge gap on how the microstructural evolution mechanisms affect corresponding mechanical/bio response, as well as how the porous structures of the material affect the degradation rate in Fe-Mn in nanoscale compared with their coarse-grained counterparts. Therefore, in this study we used high pressure torsion (HPT) to refine the grain size of Fe-30wt%Mn, and spark plasma sintering (SPS) to create a nano-porous structure. The advanced mechanical strength after HPT and improved degradation rate by porous structure in NC Fe-Mn alloys show that the material can be optimized by tailoring parameters during processing and fabrication. The second part of this dissertation, via a science perspective, focuses on the microstructural evolution and mechanical response as a function of shear strain and temperature in microstructurally stable, Cu-based immiscible NC alloys. In the field of thermal stability of NC materials, less effort has been devoted to the investigation of current-based sintering approaches for consolidation. More specifically, there is a lack of knowledge of the retention of thermal stability and resultant microstructures that occur during SPS. Also, the independent contribution of shear strain and temperature on the microstructural evolution in this type of microstructurally stable, Cu-based immiscible NC alloys have not been well addressed. Therefore, in my work, we studied the microstructural evolution in thermally stabilized Cu-Ta alloy powders during SPS consolidation with particular attention to the microstructural evolution and to the effects on strengthening, and we used a pin-on-disk tribometer to study the microstructural evolution of Cu-Nb alloys to shear strain and temperature separately. NC Cu-Ta alloys made by SPS exhibited a microstructural stability at elevated temperatures or under mechanical loading conditions, which is explained by observed bimodality of Ta segregates (Ta particles and Ta nanoclusters). Also, the effect of shear strain and temperature are decoupled in the tribometer test. Results have shown grain refinement caused by severe shear deformation, as well as grain growth/recrystallization and strain relaxation caused by external heating. In this way, this dissertation offers a more systematic understanding on how severe deformation and temperature affect the microstructures, thus the mechanical response, in thermomechanical stabile, immiscible Cu-based NC alloys. In summary, this dissertation aims to provide a better understanding of mechanical strength and degradation rate as a function of grain size and microstructural evolution of Fe-Mn alloys, thermomechanical stability and strengthening mechanism as a function of hierarchical microstructure in Cu-Ta alloys, as well as decoupled shear strain effects and temperature effects in Cu-Nb alloys.