Strain Rate and Temperature Dependent Mechanical Behavior of Nanocrystalline Au Films
Nikhil Karanjgaokar (California Institute of Tech.), Ioannis Chasiotis (University of Illinois at Urbana-Champaign)
Eringen Medal Symposium in honor of G. Ravichandran
Wed 10:45 - 12:15
Salomon 001
Nanocrystalline Au films have outstanding yield strength but their long-term inelastic behavior is significantly affected by loading rate and temperature. Grain refinement leads to increased influence of grain boundary (GB) driven creep and GB based dislocation processes. The mechanical behavior of annealed Au films of 1-2 m thickness and grain size of 64 nm was studied over strain rates from 10^-5 to 10 /s between room temperature and 110 °C. These experiments were possible with the aid of DIC based strain measurements on thin films that were uniformly heated using a custom-built apparatus. Temperature profiles along the specimen gauge section were obtained with an Infrared camera. The yield strength was highly sensitive to both temperature and strain rate: at room temperature it increased by 100% within the range of strain rates, while it decreased by 50% in the given temperature range for most strain rates. The trends in ductility and activation volume pointed to two regimes of plastic deformation namely creep driven and dislocation mediated plasticity with the transition occurring at a higher strain rate for increasing temperature. The activation volume for creep-influenced deformation increased from 6.4 b3 at 298 K to 29.5 b3 at 383 K, signifying GB diffusion processes and dislocation mediated creep, respectively. The activation volumes calculated at high strain rates decreased from 19.7 b3 to 11.4 b3 between 298 K and 383 K. A model for thermally activated dislocation depinning was applied to explain the abnormal trend in activation volume. The strong indication from monotonic tensile tests for creep based deformation was validated by uniaxial creep experiments on Au films in the same temperature range. The creep experiments also indicated strong temperature sensitivity with an increase in the creep exponent from n=2.6 at room temperature to n=5 at 110 °C.