From Metastable States to Functional Materials Properties: Novel Phase Transformation in Cu-Based Alloys

This study explores phase transformations, metastable states, and functional properties of technologically relevant Cu-based alloys. Integrating high-temperature in-situ imaging, high-energy X-ray diffraction, and nanoindentation offers insights into the (micro-)structural evolution and property development of Cu-Pd-Ag and Cu-Sn alloys under relevant thermal and mechanical conditions during service. Cu-Pd-Ag alloys play a crucial role in electrical contacts and hydrogen purification membranes, where controlled phase ordering significantly influences conductivity and mechanical performance. In-depth investigations reveal a complex interplay between ordered and disordered phases, showing that alloying with Ag modifies transformation kinetics and stabilizes phases with functional properties. Systematic variation of heating rates, initial states, deformation levels, and compositions demonstrates that both processing history and composition critically affect phase formation and stability. Additionally, a phase could be identified which has not been measured before, only simulated. Cu-Sn alloys, which have a long history in musical instruments and modern applications such as coatings and electronic components, exhibit intricate phase transformations and a wide range of stable and metastable phases. This study identifies previously overlooked metastable and martensitic phases, traces their formation pathways, and connects them to conductivity, acoustic, and local mechanical properties. Advanced characterization methods, including high-energy X-ray diffraction, transmission electron microscopy, atom probe tomography, and high-temperature nanoindentation, provide a comprehensive understanding of these phases' structural, chemical, and mechanical characteristics. This thesis bridges the gap between fundamental materials science and industrial applications by correlating in-situ imaging, structural analysis, functional, and mechanical property measurements. The findings contribute to the development of tailored alloys for high-performance uses and highlight the significance of metastable phases in tuning material properties. This interdisciplinary approach not only enhances the understanding of phase transformations in Cu-based alloys but also provides a methodological blueprint for future studies on complex alloys under real-world conditions.