Title:
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Thermal stability and deformation of nanocrystalline nickel
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This work is conducted on nanocrystalline nickel using a range of novel techniques
in order to investigate the thermal stability of the microstructure and
the mechanical properties of subsequent microstructures. Work is focused
on two sources of nanocrystalline nickel produced via electrodeposition with
varying levels of impurity sulphur. Impurity sulphur alters the thermal stability
of the material; a low sulphur system is stable up to a temperature of
400°C and then coarsens normally to produce a micro-polycrystalline structure,
a high sulphur material coarsens at 300°C and the coarsening quickly
stagnates leaving a stable microstructure of rv550nm grains. Above 485°C,
the high sulphur material is observed to undergo abnormal growth of large
planar faceted grains. Observation of the growth post heat treatment via
standard electron microscopy techniques gave statistical data complemented
by in-situ heat treatment scanning electron microscopy and high temperature
resistivity measurement tests to elucidate the stages of growth. A range
of macro and micro scale tests were undertaken to investigate the physical
properties of the materials; from Vickers hardness and four point bend, to
novel in-situ micro-cantilever bend tests. The effect of the increased grain size
from heat treatment leads to a reduction in the hardness and yield strength
of the materials and in the case of the high sulphur material, segregation of
the impurity to the grain boundary led to a ductile to brittle transition.
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