Nanocrystalline (NC) materials are of great interest to researchers due to their multitude of properties such as exceptional strength and radiation resistance owing to their high fraction of grain boundaries that act as defect sinks for radiation-induced defects, provided they are microstructurally stable. In this dissertation, radiation effects in microstructurally stable bulk NC copper (Cu)- tantalum (Ta) alloys engineered with uniformly dispersed Ta nano-precipitates are systematically probed. Towards this, both ex-situ and in-situ irradiations using heavy (self) ion, helium ion, and concurrent dual ion beams (He+Au) followed by isochronal annealing inside TEM were utilized to understand radiation tolerance and underlying mechanisms of microstructure evolution in stable NC alloys. With systematic self-ion irradiation, the high density of tantalum nanoclusters in Cu-10at.%Ta were observed to act as stable sinks in suppressing radiation hardening, in addition to stabilizing the grain boundaries; while the large incoherent precipitates experienced ballistic mixing and dissolution at high doses. Interestingly, the alloy exhibited a microstructure self-healing mechanism, where with a moderate thermal input, this dissolved tantalum eventually re-precipitated, thus replenishing the sink density. The high stability of these tantalum nanoclusters is attributed to the high positive enthalpy of mixing of tantalum in copper which also acted as a critical driving force against atomic mixing to facilitate re-precipitation of tantalum nanoclusters. Furthermore, these nanoclusters proved to be effective trapping sites for helium, thus sequestering helium into isolated small bubbles and aid in increasing the overall swelling threshold of the alloy. The alloy was then compositionally optimized to reduce the density of large incoherent precipitates without compromising on the grain size and nanocluster density (Cu-3at.%Ta) which resulted in a consistent and more promising response to high dose self-ion irradiation. In-situ helium and dual beam irradiation coupled with isochronal annealing till 723 K, also revealed a comparable microstructural stability and enhanced ability of Cu-3Ta in controlling bubble growth and suppressing swelling compared to Cu-10Ta indicating a promising improvement in radiation tolerance in the optimized composition. Overall, this work helps advancing the current understanding of radiation tolerance in stable nanocrystalline alloys and aid developing design strategies for engineering radiation tolerant materials with stable interfaces.

Materials have been the backbone of every major invention in the history of mankind, e.g. satellites and space shuttles would not exist without advancement in materials development. Integral to this, is the development of nanocrystalline (NC) materials that promise multitude of properties for advanced applications. However, they do not tend to preserve structural integrity under intense cyclic loading or long-term temperature exposures. Therefore, it is imperative to understand factors that alter the sub-features controlling both structural and functional properties under extreme conditions, particularly fatigue and creep. Thus, this dissertation systematically studies the tensile creep and fatigue behaviour of a chemically optimized and microstructurally stable bulk NC copper (Cu)-3at.% tantalum (Ta) alloy.

Strategic engineering of nanometer sized clusters of Ta into the alloy’s microstructure were found to suppress the microstructure instability and render remarkable improvement in the high temperature tensile creep resistance up to 0.64 times the melting temperature of Cu. Primary creep in this alloy was found to be governed by the relaxation of the microstructure under the applied stress. Further, during the secondary creep, short circuit diffusion of grain boundary atoms resulted in the negligible steady-state creep rate in the alloy. Under fatigue loading, the alloy showed higher resistance for crack nucleation owing to the inherent microstructural stability, and the interaction of the dislocations with the Ta nanoclusters. The underlying mechanism was found to be related to the diffused damage accumulation, i.e., during cyclic loading many grains participate in the plasticity process (nucleation of discrete grain boundary dislocations) resulting in homogenous accumulation rather than localized one as typically observed in coarse-grained materials. Overall, the engineered Ta nanoclusters were responsible for governing the underlying anomalous high temperature creep and fatigue deformation mechanisms in the alloy.

Finally, this study presents a design approach that involves alloying of pure metals in order to impart stability in NC materials and significantly enhance their structural properties, especially those at higher temperatures. Moreover, this design approach can be easily translated to other multicomponent systems for developing advanced high-performance structural materials.