Thermal stability and deformation of nanocrystalline nickel

• Main Author: Darnbrough, J. E.
• Published: University of Bristol 2014
• Subjects: 546
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.683939

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.