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Theses Doctoral

Investigation of Plastic Strain Recovery and Creep in Thin Film Nanocrystalline Metals

Ghazi Esfahani, Nastaran

In this study an automatically controlled plane-strain bulge test system is used to characterized
the mechanical behavior of nanocrystalline thin film samples. The free-standing
thin film of copper with average grain size of 35nm is fabricated with thermal evaporation or
sputtering. The tests are performed to measure Young's modulus, determine the strain rate
for creep and monitor plastic strain recovery at room temperature and at 100°C Based on
the experimental strain rate during the creep, The value for diffusion coefficient of copper is
obtained. This value is in agreement with the diffusion coefficient resulted from numerical
simulation for nanocrystalline copper film in another work and is about 4 orders of magnitude
more than the value for conventional coarse grain one. By monitoring the plastic
strain recovery, it is observed that it occurs in two rates, a fast temporary one follow by a
slower rate. This phenomena can be explained due to grain boundary based deformation
mechanisms for this grain size.
We also develop a continuum model to prescribe grain boundary diffusion as the dominant
deformation mechanism for nanocrystalline thin film with a preexisting void. The
model is implemented using FEA software Abaqus. The numerical result indicates that plastic
strain recovery occurs and it has two rates. A parametric study on different factors which
can affect the recovery is performed and the strain recovery rates obtained from each parameter
are compared with the experimental one.

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More About This Work

Academic Units
Mechanical Engineering
Thesis Advisors
Kysar, Jeffrey W.
Degree
Ph.D., Columbia University
Published Here
September 30, 2014