Thesis Defense – Serita Sulzman

Title: Processing and Thermal Stability of Nanocrystalline Ag-Cu Alloys

Abstract: Nanocrystalline alloys offer multitudinous advantages over their larger-grained counterparts including increased strength, hardness, resistance to fatigue, and more. However a significant barrier to their implementation is their low thermal stability—they are prone to coarsening at very low homologous temperatures. Luckily, a thermodynamic approach to stabilizing the microstructures of nanocrystalline metals by adding an alloying element shows great promise. Recent improvements in computational models have facilitated identification of alloy systems in which solute segregation to the grain boundaries is energetically favorable. However, more experimental validation is needed to verify whether their predictions can translate to enhanced thermal stability of alloys in practice. In this work, computational calculations of segregation energies and various processing considerations provided guidance for the selection of the silver-copper system for further study. Procedures were developed to synthesize chemically homogenous nanocrystalline Ag-Cu alloys, and heat treatments with in-situ X-ray diffraction were designed to evaluate their resistance to grain growth at increasing temperatures. Examination of the microstructures of the heat treated samples with focused ion beam and scanning electron microcopy corroborated Scherrer grain size calculations which showed that the alloys with 5 at.% and 25 at.% copper maintained much smaller equilibrium grain sizes at all temperatures in the scope of study compared to pure silver. As was computationally predicted, these data show that the addition of copper can thermodynamically stabilize the nanocrystalline microstructure of silver. The experimental validation of these thermodynamic and other system selection criteria provides a framework for the development of novel thermally stable nanocrystalline alloys for countless engineering applications.
 
Thesis Supervisor
Christopher A. Schuh, Danae and Vasilis Salapatas Professor of Metallurgy, Massachusetts Institute of Technology