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Title: Mechanical properties of body-centred cubic nanopillars
Author: Yilmaz, Halil
ISNI:       0000 0004 7233 0287
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2018
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Understanding the mechanical properties and deformation characteristics of nanoscale metallic nanopillars and wires is a significant concern for designing reliable small devices that must resist loads in service. This thesis aims to extend understanding of the size dependent behaviour of nanopillars and wires in compression and tension by investigating their mechanical properties and deformation characteristics. Single crystal bcc pillars were fabricated by focussed ion beam (FIB) machining from Fe, Nb, V, Ta, Mo, W and Cr, as well as the ferrite (bcc) and austenite (fcc) components of a duplex stainless steel (DSS). These were tested in compression over a range of test temperatures from 193 K to 393 K using various types of nanomechanical devices. The effect of sample size (pillar diameter) on the strength was investigated and found to increase with decreasing pillar size. In bcc metals, the yield or flow stress, 􀀂􀀖, is inversely proportional with some power of the pillar diameter, d. In bcc metals tested, the power-law exponent, n, were found in the range of between -0.23 to -0.63, showing a less pronounced size effect than found for fcc pillars. The power-law exponent for bcc pillar deformation is also temperature dependent and was found to scale with the ratio of test temperature, Ttest to the critical temperature for screw dislocation mobility, Tc, of the bcc metal (T*= Ttest / Tc). It is notable that the size effect exponent weakens (approaches 0) as T* decreases. However, when the experiments are carried out at temperatures close to or just above Tc, the power-law exponents approaches the value reported in the literature for a range of fcc metals (-1 < n < -0.6). The variation in the power-law exponent observed for bcc metals can be explained by the change in mobility of thermally activated screw dislocations. Their mobility can be modelled by a threshold or lattice friction stress. If this friction stress is introduced into the empirical equation that relates the strength of fcc metal pillars to their diameter, a strong correlation between size effect exponent, the normalised test temperature (T*) and friction stress is obtained. It was found that the friction stress values (Fe, Nb and V) increase as Ttest decreases from 296 to 193 K. When the pillar diameter decreases, the friction stress would be more easily overcome due to the increase in surface-to-volume ratio. The contribution of lattice friction stress on the strength is higher at larger pillars than those for nanopillars. Thus, the divergence between best fit lines has become more apparent at micron-sized pillars, resulting in weaker size effects. Furthermore, the transition in deformation morphology from localized to wavy deformation was only found in Fe pillars, as the Ttest decreased from 296 to 193 K, further revealing that temperature has also strong influence on deformation behaviours of bcc pillars.
Supervisor: Derby, Brian ; Quinta Da Fonseca, Joao Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Nanowire Tension ; Temperature Dependency ; Nanopillar Compression ; Size Effect Dependency ; Deformation Characteristics ; Mechanical Properties