Fabrication, Microstructures And Mechanical Properties Of High Purity TiAl Alloys And TiAl-based Nanocomposites

TiAl alloys have the great potential application as structural materials under high temperature and cyclic load, such as low pressure turbine blades (LPTB) in aircraft engines and turbo chargers in automobiles. Because the application of Ti-48Al-2Cr-2Nb alloy in GEnx engines of Boeing 787 airplane, the study on the fundamentals and applications of TiAl alloys have attracted a great attention in the world. Whether developing new cast TiAl alloys or wrought TiAl alloys, an alloy ingot with a high purity is meaningful and base. Therefore, the main contents of this thesis are focused on the fabrication, microstructures and mechanical properties of high Nb-TiAl alloys by high purity vaccum levitation melting(VLM). Meanwhile, a stir-casting technique was used to fabricate TiA! based nanocomposites using electromagnetic stirring, and the effects of nanoparticles on microstructures and mechanical properties were also studied. The main conclusions and innovations are listed as follows:(1) Melting high Nb-TiAl alloys with the VLM method, an inappropriate stock distributing manner usually results in stock bridging and stock splash, which were harmful to the melting process and were difficult to control the alloy composition. An effective stock distributing manner preventing stock bridging is to put aluminium upon sponge titanium in the crucible. Reducing the volume of sponge titanium in crucible and increasing the furnace pressure are effective to prevent stock splash, which is resulted from that the pressure of Al vapor and gas in sponge is more than static pressure of the stock.(2) Optimizing twice VLM process, the composition and microstucture of high Nb-TiAl alloy are exact and homogenious. During the second melting process, a process of pumping-remelting-argon filling-melting can decrease the oxygen content to 300ppm.(3) If high Nb-TiAl alloys was melted in vaccum, the buring loss is mainly from splash of sponge titanium. A pressure of above 500Pa in the furnace through aflluence of Ar is effective to prevent aluminum evaporation, although the Al vapor pressure is higher.(4) An ingot mould was optimized to promote the sequence solidification of high Nb-TiAl alloys. The distribution of solidification defects was improved because the pores were totally eliminated, and shringkage cavities were distributed on the casting head of ingots. The best dimension of ingot mould is thickness of 20mm, inclination of 4.2°, h-d ratio of 3.3 and riser inclination of 30°.(5)The microstructure of high Nb-TiAl ingots prepared by VLM are exqiaxed grains with near lamellar microstructure. Decreasing Al content and boron addition can both refine the cast microstructures, however, the superpluous precipitation of borides are harmful to the tensile mechanical properties.(6) Coarse grain zone were observed if the high Nb-TiAl-B alloy was heat treated in the single a phase field. The formation of coarse grains was related to the boron content (>0.5%) and the cooling rate. Decreaing B content and increasing the cooling rate of ingot both result in the larger coarse grain zone. The temperature induced undercooling and the boron induced undercooling both contribute to the inhomogeneity of boride along the ingot scale, which results in various pinning effects on grain growth at high temperature. Meanwhile, flakes-like boride in cast alloy transforms to needles at high temperature, which was proved to weaken pinning effect on grain growth. The two types of boride are both rich in niobium.and with the same orthorhombic B27 structure.(7) A stir-casting technique was used to fabricate TiAl based nanocomposite, and the nanoparticles include α-Al2O3、γ-Al2O3、TiO2、Y2O3、TiC、Wand Mo. Among the particles,α-Al2O3、TiO2、TiC and W can distribute uniformly in TiAl matrix. Base on their distribution, the wettability of these nanoparticles sorted as W>TiO2>TiC>α-Al2O3.(8) During the solidification of TiAl alloy, the existence of nanoparticles, as the heterogeneous nuclei, can refine primary β dendrites. However, the addition of oxide nanoparticles leads to coarse lamellar colonies, which can be attributed to the dissolution of O, because O is a a phase stabilizing element; as the addition of TiC nanoparticle is above 1wt.%, the precipitation of Ti2AlC can refine the lamellar structure, meanwhile, the dissolution of carbon can reduce the lamellar spacing.(9)The addition of nanoparticles can increase the Vickers hardness of TiAl matrix at room and elevated temperature, especially TiC and W nanoparticles. However, the addition of TiC was harmful to the fracture toughness. W nanoparticles can both increase the hardness at room and elevated temperature and fracture toughness (40 MPa/m1/2).