"Microstructure Evolution and Mechanical Behavior of Cu-Based Immiscible Systems with Metastable Phases" 

Debargha Paul, MSE PhD Candidate 

Advisor: Professor Xinghang Zhang

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ABSTRACT

Nanostructured materials have attracted tremendous interest in a wide variety of technological and structural applications. Binary Cu-BCC alloys are being identified as promising materials due to their potential to combine superior mechanical, magnetic, and electrical properties. These systems are predominantly immiscible, with a positive energy of mixing, which prevents them from forming intermixed alloys. In this thesis, we have reported the formation of metastable phases in supersaturated solid solutions using non-equilibrium sputtering technique. Tuning the process parameters in this technique offers unique opportunities to engineer the phase stabilities and composition limits of metastable phases with generations of nanolaminate structures in co-sputtered systems. In immiscible Cu-Fe systems with equal atomic fractions of Cu and Fe, we have shown the transition of BCC-FCC dual structures to BCC dominated structures of Cu-rich and Fe-rich alloy phases by changing the substrate temperatures. In situ micropillar compression experiments coupled with transmission electron microscopy (TEM) studies revealed the superior mechanical properties with high flow stress of 2.4 GPa and good deformability of this nanolaminate BCC dominated microstructure with the presence of stacking faults. The composition-dependent study in the Cu-Nb system, showing similar BCC dominated microstructures with Nb atomic fractions above 50%, confirms the critical role of the deposition conditions in stabilizing the metastable structures in the immiscible systems. In addition, the radiation behavior of the supersaturated Nb-Cu system has been tested using in situ TEM Kr ion irradiation, which revealed significant microstructural evolution, including grain boundary void shrinkage and phase transformations with variations in radiation dosage. Altogether, these findings highlight the opportunity to obtain desirable mechanical and radiation tolerance properties through tailoring the deposition kinetics and compositions of non-equilibrium microstructures in these immiscible systems.