Novel design of multicomponent materials using severe plastic deformation methods
thesisposted on 02.03.2017 by Qi, Yuanshen
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Using severe plastic deformation (SPD) methods to design and fabricate multicomponent materials has been drawing increasing attention in recent years [1, 2]. In the present study, equal-channel angular pressing (ECAP), as the most highly developed SPD technique, is investigated to design and fabricate multicomponent materials for three applications. They are (i) high strength and low density materials for structural application, (ii) high strength, low stiffness and good biocompatibility materials for biomedical implant application, and (iii) high strength and high conductivity materials for electrical transmission line application. Accordingly, this project is divided into three sub-projects, which have the same processing principle of ECAP but different constituents in terms of material selection, shape, and scale of the constituents and the processing parameters. In the first sub-project, multicomponent materials having high strength and low density are aimed to be fabricated using ECAP. Aluminium and magnesium machining chips are used as raw materials. By blending them and consolidating the mixture using ECAP with back pressure of 175MPa, full dense metallic composite is fabricated. Testing results show that substantial improvement of mechanical properties, such as an increase of strength, strain-hardening capability and ductility, can be obtained. This is achieved by changing the strain path, processing temperature and post-ECAP thermal treatment, as well as by optimising the weight fraction of the constituent metals. Microstructure characterisation shows that the strain path can be optimised to achieve profuse breakage of oxide layer along interfaces. Moreover, processing or annealing at 300˚C can cause intermetallic phase formation at the interface resulted from enhanced interdiffusivity. In the second sub-project, multicomponent materials having high strength, low stiffness and good biocompatibility for biomedical implant application are targeted. Three components including titanium, magnesium and silicon powders are used as raw materials. They are blended and consolidated by ECAP with back pressure of 175MPa at 400˚C. By leaching out Si and Mg constituents, porous Ti/Mg and porous Ti having bone-like cancellous architecture are successfully fabricated. It is the first successful fabrication of porous Ti/Mg composite with 100% interconnectivity and high mechanical performance. After Mg constituents are leached away, porous Ti possesses excellent mechanical properties and good biocompatibility. Electron microscopy results show that the outstanding mechanical performance of the material is caused by the low processing temperature, elimination of post high temperature annealing and redistribution of constituents during processing. Moreover, biocompatibility is improved by the unique surface morphology resulting from etching of oxide-free surface and ultrafine-grained structure of the Ti substrate. The objective with the multicomponent material studied in the third sub-project is high strength and high electrical conductivity. The concentric jacket-core architecture is studied on bimetallic rods having an aluminium alloy 6201 out jacket layer and an austenitic steel 316L core, which has the potential application of overhead transmission line. Bimetallic rods with different geometry of constituents are deformed though ECAP with back pressure of 15MPa at 175˚C. Compared with as-received bimetallic rods, the deformed ones possess significantly higher strength due to grain refinement. More importantly, for one kind of geometry, the electrical conductivity is not sacrificed by the enhancement of strength. Microstructure characterisation results show that the co-deformation between core and jacket layer is critical in causing extensive dynamic ageing and microstructural refinement in Al component, which contributes to conductivity not being diminished when strength is raised.