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Development of soft magnetic alloy by nanomaterials

Albaaji, Amar 2017. Development of soft magnetic alloy by nanomaterials. PhD Thesis, Cardiff University.
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Abstract

Equiatomic FeCo alloy which has the highest saturation magnetisation of any soft magnetic material is limited in application due to it being very brittle. The development of composite carbon nano-particulate filled alloys has achieved little or no overall benefit due to difficulties with agglomeration in the mixing process. In this research, improvement in mechanical and magnetic properties has been achieved in FeCo alloy composite with a very low fraction carbon nanotube (CNT) reinforcement prepared only by low energy ball milling, with notable improvement as compared to composites prepared previously by pre-mixing in dimethylformamide, followed by low energy ball milling. Significant improvement in hardness and magnetic properties have been obtained in FeCo alloy by reinforcement with a small addition of graphene nanoplatelet (GNP) alone and prepared by low energy ball milling. However, significant overlapping between the nanoplatelets deteriorated the strength and elongation. Inserting CNT among nanoplatelets was found to reduce the overlapping and enabled refinement of the microstructure, however, at the expense of some increase in porosity in the spark plasma sintered materials. The spark plasma sintering conditions have been optimised at relatively low sintering pressure by use of high temperature and controlling the heating rate and dwelling time to enable near-full densification in base FeCo alloy. By careful selection of sintering parameters, it was found to be possible to increase yield strength, elongation and suppress the intergranular fracture in FeCo alloy, combined with good magnetic properties. This optimised SPS process was developed following the above-described solvent dispersion studies and applied subsequently to following alloy-composite developments. The ultimate strength, yield strength and elongation were significantly improved by embedding GNP in flaked FeCo powder, which enabled uniform dispersion up to 6 vol. % in contrast to very poor dispersion in spherical powder. The increased coercivity from dispersion in flaked FeCo powder was reduced by quenching the composites from the disordered region, which led to further improvement in elongation. However, the highest tensile strength of the heat treated samples was obtained after quenching from the ordered region. Interface bonding was improved in CNT- and GNP- filled FeCo alloy composites by high energy ball milling, which increased the properties of the composites. However, significant damage to the structure was imparted by ball milling of carbonaceous nanomaterial when the time of ball milling was extended. By nature of their two-dimensional form, dispersion of GNPs was found to be far more challenging in FeCo alloy compared with the CNTs. However, the mechanical properties of the GNP composite exceeded those of their counterpart CNT composite even at a lower volume fraction

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Spark Plasma Sintering, Soft Magnetic, Nano Composite, Graphene nanoplatelet, carbon nanotube
Date of First Compliant Deposit: 16 October 2017
Last Modified: 09 Jan 2018 17:57
URI: http://orca-mwe.cf.ac.uk/id/eprint/105527

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