UC Riverside

Metallic silver has been extensively used by humans for millennia, with usage recorded as far back as 4000 BCE due to its abundance of useful material properties including corrosion resistance and high formability, making it an ideal metal for use by ancient civilizations. In more recent centuries other properties such as the oligodynamic effect, which gives rise to antimicrobial properties in silver, as well as the highest thermal and electrical conductivity of any metal, have helped silver retain the interest of scholars and scientists in numerous disciplines. Several issues with silver however have reduced its use in many applications in which it would otherwise be an ideal material choice. Some material properties of silver, in particular the mechanical strength and extremely low stacking fault energy, make silver a very poor choice for structural purposes, which require a high resistance to deformation. Pure silver possesses very low hardness and strength and has a tendency to gall under repeated loading and unloading. While it is possible to strengthen silver through a variety of strengthening mechanisms, including work hardening or reduction of the grain size (i.e., Hall-Petch strengthening), the low stacking fault energy of silver makes it energetically favorable for dislocations to dissociate into partial dislocations with a wide equilibrium splitting distance. While this tendency of dislocations to dissociate allows silver to obtain extremely high dislocation densities, it acts as a driving force for recovery, recrystallization and grain growth within the crystal lattice. The poor mechanical properties and low stacking fault energy in silver culminate in a material which self-anneals at room temperature, losing the increased strength and hardness imparted by materials processing techniques used to strengthen it.In this work, we explore two severe plastic deformation methods for strengthening of silver – mechanical alloying combined with spark plasma sintering, and high pressure torsion. Microstructural stabilization through dilute alloying additions and impurities is explored for prevention of self-annealing, producing ultrafine grained and nanocrystalline high strength silver and silver alloys with high thermal stability and resistance to grain growth.