DEVELOPMENT OF LOW COST 'IN-SITU' Ti/TiB AND Ti-6Al-4V/TiB COMPOSITES

摘要

High technology transport sectors are looking for higher performance materials, competitive in terms of cost, lighter than conventional titanium alloys and recyclable, which could improve efficiency of several components and reduce fuel consumption through weight reduction. Although novel anisotropic Titanium Reinforced Composites (TMCs) reinforced with continuous SiC CVD filaments were developed some years ago, and are commercially available, they are still very expensive in comparison with other MMCs or with conventional titanium alloys. During the fabrication of these SiC{sub}(CVD)/Ti composites, there is a chemical reaction at the filament/matrix interface which influences badly the mechanical properties of the composite. In order to decrease the cost of the composite material and to improve its chemical stability, it has been proposed to obtain the reinforcement during the preparation of the composite itself using a similar procedure that the preparation of alloys reinforced with a second phase. These composites materials can be called "in-situ" composites. The advantage is the chemical stability between the reinforcing phase and the matrix, the draw back is the difficulty to find the adequate reinforcing phase. More over, in order to have a composite effect, the second phase has to be stiff enough, the volume fraction has to be adjusted at minimum to the critical volume fraction of reinforcement and lastly the second phase must have a fibre shape in order to present a sufficient aspect ratio (Length/diameter) L/d higher than the critical value of 10 in order to permit a load transfer between the 2 constituents. Previous researches have proved that the titanium monoboride (TiB) full-fill these requirements [1]. The TiB has an orthorhombic crystal structure, an acicular shape, its stiffness is in the range of 450-550GPa, the analysis of the binary phase Ti-B phase diagram shows that the volume fraction can be adjusted as required and the TiB has a thermodynamic equilibrium with the titanium. The "in-situ" precipitation can be done in the solid or the liquid state of the matrix. A lot of papers have been published during the last decade on the fabrication of these composites through the powder metallurgy (PM) technique and some composites or even components are today commercially available [2]. The PM composites have to be forged and/or machined in order to achieve the right shape of the component. Few attempts were done with the casting routes where as the casting process can be an alternative way to reduce even more the cost and also to prepare more complicated shapes for example by using the investment casting process technology.