Composite strengthening was attempted to improve the mechanical strength and toughness of the brittle near equiatomic Fe-Co alloy. The matrix alloy chosen for this research falls in the Fe-(30-50) Co group, which are known for their highest saturation induction (B-sat) and Curie temperature among the commercial soft magnetic alloys. The reinforcements, which exhibited a wide range of aspect ratios, included SiC particulates, SiC whiskers and carbon nanotubes (CNTs). In order to minimize the interfacial reaction between the reinforcements and the Fe-50Co alloy (matrix) and to realise higher compact density, spark plasma sintering (SPS) was selected for rapid compaction of materials. Reinforcements were coated using electroless deposition with Ni-P, copper and cobalt to modify the interfacial chemistry and thickness, and hence the final properties of the composites. A comprehensive study on the sintering variables found, within the range of examination and under constant heating and cooling rates, the optimum maximum temperature, soaking time and mechanical pressure of 900oC, 2-5 minutes and 80 MPa to rapidly consolidate the Fe- 50Co alloy to near-theoretical density. The volume fraction and size of the ordered regions in the monolithic alloy and hence the magnetic properties, were sensitive to the heating rate, cooling rate, temperature at which the mechanical pressure was applied and removed and post heat treatments. The influence of reinforcement coatings on the wetting characteristics, and in turn the properties, was compared using SiC particulate Fe-50Co composites. The introduction of bare and coarse (20 μm) SiC particulates negatively affected both magnetic and mechanical properties. Electroless Co coating of particulates improved both the flexural properties and magnetic characteristics such as permeability and coercivity by promoting the formation of narrower interfaces and better bonding. The addition of bare and coated whiskers in Fe-Co alloys enhanced densification and grain growth of the matrix. Copper coating over whiskers was found to be not helpful in realising uniform dispersion, whereas Co and Ni-P coating aided to achieve uniform dispersion of whiskers in the matrix. The amorphous Ni-P coating on whiskers was nanocrystallised during the rapid sintering process and resulted in a material with highly improved mechanical strength and ductility in comparison to the monolithic and other whisker reinforced composite materials. A novel attempt to prepare bulk Fe based alloy composites reinforced with CNTs was also undertaken. Both soft magnetic and mechanical property enhancements were observed in composites with lower vol% of CNTs (i.e. < 1.5%, in the range of examination up to 10%) due to the improvement of compact density by CNTs. An increase in the CNT vol% produced a negative effect on saturation induction and mechanical properties due to the agglomeration of CNTs and reduction in compact density. SPS helped to retain the structural integrity of CNTs during processing. Electroless Ni-P coating over the CNTs helped to reduce the structural damage of CNTs during processing and to improve the mechanical strength and ductility at a marginal cost of saturation induction, in comparison to the monolithic compacts and bare CNT reinforced composites. To date accurate temperature assessment of the compact in the SPS die has been difficult due to the remote position of the pyrometer within the body of the die. It has been found that the ferromagnetic Curie transition can be successfully employed to calibrate SPS pyrometer during processing.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:611056 |
| Date | January 2014 |
| Creators | Mani, Mahesh Kumar |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/60869/ |
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