Tailoring the mechanical behavior of nanocrystalline thin films with alloying and heat treatments

Metals and alloys with grain sizes below a hundred nanometers exhibit very different mechanical and physical properties compared to their coarse grained counterparts. Unique nanoscale deformation mechanisms are triggered by the ubiquitous nature of grain boundaries in nanocrystalline (NC) materials. Microstructural instabilities can develop in NC materials during deformation due to stress-coupled grain boundary migration and global stress-driven grain growth. The presence or absence of these mechanisms can dramatically affect the attendant mechanical response of the material. The ability to control these grain boundary instabilities with impurity doping might make it possible to engineer nanostructured materials with desired properties. Motivated by this prospect, a collaborative effort was launched with scientists from The University of Pennsylvania, The University of Sydney and The Johns Hopkins University. This dissertation, in particular, describes efforts to tailor mechanical behavior of NC alloy systems by controlling global stress-driven grain growth through alloying and annealing treatments. NC aluminum and nickel, which have been shown to exhibit stress-assisted grain growth, were chosen as the parent materials for this study. NC aluminum was doped with oxygen, and NC nickel with phosphorus, to assess the role of grain boundary solutes in stabilizing grains against stress-assisted grain growth. Confocal co-sputtering techniques were employed to fabricate alloy thin films with precise control over chemistry and microstructure. Tensile properties were measured through microtensile testing and microstructural evolution associated with deformation was characterized using ex-situ and in-situ precession-assisted crystal orientation mapping in TEM. The critical global solute concentrations required to stabilize grain boundaries against applied stresses were identified. Local grain boundary pinning imparted mechanical stability to the microstructure and resulted in strengthening. Apart from solute additions, heat treatments can also modify NC grain boundaries. NC nickel exhibited anomalous strengthening upon moderate annealing below 300°C. This was attributed to the relaxation of the grain boundaries, leading to the suppression of overall stress-assisted grain growth. Taken as a whole, this study highlights the roles of impurity additions and grain boundaries in governing the mechanical stability of NC metals. Controlling the expression of this mechanism opens avenues to design NC materials with tailored mechanical properties