Experimental characterization of deformation mechanisms in nanocrystalline thin films using in situ techniques
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Date
2017-03-22
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Johns Hopkins University
Abstract
The properties of a material depend immensely on its microstructure. The ability
to characterize a material’s microstructure and develop predictive models for how
the material will respond when, for example, load is applied is a key component of
designing with engineering materials. Metals that are nanocrystalline (i.e. grain
size < 100 nm) have unique properties owing to the ubiquity of grain boundaries
within the microstructure. This dissertation presents new methods of testing and
characterizing materials at the nanoscale.
In situ experiments were performed to measure the velocity of mobile grain
boundaries utilizing conventional transmission electron microscope (TEM)
imaging. The average velocity of migrating grain boundaries was calculated to be
on the order of 0.1 nm s)*, significantly higher than the velocity predicted from
diffusion-based processes. Additional in situ experiments were conducted that
utilize straining in combination with orientation imaging microscopy are utilized to
determine the character of boundaries migrating in response to high stresses.
From these experiments, no correlation between grain boundary character and
mobility was found. Also, by utilizing through these experiments, deformation
twinning was observed in nanocrystalline copper thin films. It was observed that
twin nucleation and growth proceeds from grain boundaries.
Additionally, many physical phenomena are affected by local stress state within a
crystal. A new technique is presented that provides the capability to map elasticiii
strains in polycrystalline materials with nanoscale resolution. This technique,
initially developed for analysis of semiconductor devices, was applied successfully
to engineering metals and ceramics for the first time. The strain resolution for this
technique was calculated for polycrystalline copper (0.15%) and hot-pressed boron
carbide (0.078%) specimens. The elastic strain near grain boundary facets was
measured and compared to a simple model, finding that the measured strain
values generally agree with the residual strains expected from thermal anisotropy.
Strain values were also measured in polycrystalline magnesium near deformation
twins and a low-angle tilt boundary.
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Keywords
Mechanical testing, nanocrystalline, grain boundary character, in situ straining