Before privatisation of the electrical generation and distribution industries in the United Kingdom, the CEGB, owner and operator of the national grid, distributed electricity from electrical power stations to areas local to those stations. After the privatisation of the electrical industry the National Grid Company was formed and is obliged to supply the consumer with the cheapest supplier of electricity, irrespective of supplier and user, and this has caused an increased demand on the national grid. Due to the difficulty of obtaining the right of way for new overhead electricity transmission lines, the National Grid Company plc has focused its attention on developing an electrical conductor which has higher electrical conductivity than the present conductor alloy but with similar mechanical properties. To develop a new conductor it is important to gain an understanding of the factors that affect the electrical resistivity of aluminium conductors. The aim of the present research has been to investigate the effect of microstructural features on the electrical resistivity of aluminium and to measure the individual contribution of those features. The microstructural features examined in the present work are: intermetallic particles, dislocations, grain and sub-grain boundaries. The research involved manufacturing aluminium-based wires which contained a number of alloying additions that were subjected to varying levels of deformation up to true strains of 4. By using various thermo-mechanical treatments, it was possible to produce sets of aluminium wires of 1.18mm in diameter which contained varying volume fraction of intermetallic particles, varying dislocation densities and varying sub-grain sizes and grain sizes. The electrical resistivity of the wires was measured and related to their microstructure. Microstructural characterisation of the sets of wires was mainly carried out by transmission electron microscopy, scanning electron microscopy and optical metallography. This involved measuring dislocation density, grain/sub-grain size, sub-grain misorientation and volume fraction of intermetallic particles. The electrical resistance of the wires was measured using the four probe method. It was found that all microstructural features described above increased the resistivity of pure aluminium. The relationship between volume fraction of intermetallic particles and resistivity was found to be linear for volume fractions of less than 5%. The resistivity of the alloy could be calculated by considering the intermetallic particles and the aluminium matrix as two resistors in parallel. Cold working aluminium and aluminium alloys caused an increase in the resistivity. For the aluminium-iron alloys the rate of increase in resistivity was dependent on the volume fraction of intermetallic particles. The particles increased the rate of generation of dislocations as the alloy was deformed due to the plastic incompatibility between the matrix and the particles. For the aluminium-magnesium alloys the rate of increase in resistivity was dependent on the concentration of magnesium is solid solution. The rate of increase in resistivity when the wires were cold worked up to true strains of 1 was larger than when the wires were cold worked from true stains of 1 to 4. The microstructure of the wires that were cold worked to true strains of up to 1 was found to be dominated by dislocations. The relationship between dislocation density and resistivity was found to obey a power-law relationship. At low levels of deformation «0.3 true strain) the dislocations were randomly distributed and acted as individual electron scattering centres. As the dislocation density increased the dislocations became tangled and the degree of scattering of conduction electrons for each dislocation decreased. As the samples were deformed in the plastic strain range of 1 to 4, the dislocations formed dislocation cells / sub-grains. The increase in resistivity was found to be inversely proportional with sub-grain size due to scattering of conduction electrons by sub-grain boundaries. The relationship between grain size and resistivity was found to be inversely proportional to the grain size. This was the same relationship as that between sub-grain size and resistivity. The findings reported in this thesis provide an understanding of the way in which particles, dislocations and grain boundaries affect the resistivity of electrical conductors. The results show that, in general, factors which increase conductivity also increase strength. This information is vital when attempting to develop a conductor that combines high conductivity and high strength.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:630500 |
| Date | January 1998 |
| Creators | Barghout, Jeries Y. J. |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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