Fabrication Of High Nb-TiAl Based Porous Materials And Their High Temperature Oxidation Resistance

TiAl porous materials are expected as next-generation high temperature structural filtration materials for the potential application in high-temperature gas/solid or liquid/solid two-phase separation fields at high temperature and corrosive environment,owing to their low density,high elastic modulus,excellent high-temperature strength,and good oxidation resistance.However,the poor high temperature oxidation resistance at elevated temperatures(above 800?)and poor filtration precision greatly limit their use in practice.Therefore,the improvement of high temperature oxidation resistance and filtration precision,as well as mechanisms analysis,are vital in practical applications of TiAl porous alloys.In this thesis,the effect of multiple alloying and nano-composite on the high temperature oxidation resistance and the filtration precision of TiAl porous materials were revealed by studying the oxidation behaviors,microstructure morphology and phase transformation of Ti-48A1,Ti-48Al-6Nb,Ti-40Al-10Nb-10Cr porous materials,and six kinds of porous nanocomposites with 8 wt%Nano-SiO2/Nano-NiO/Nano-ZrO2/Nano-Y2O3/Nano-MgO/Nano-ZnO additions before and after 900?/100 h isothermal oxidation,which will guide new porous materials design and their application.The main conclusions and innovations are listed as follows:(1)The oxidation kinetics curves of Ti-48A1 and Ti-48Al-6Nb porous materials after 900?/100 h isothermal oxidation showed that Ti-48Al porous material had a weight gain 28.39 g/m2 after 900?/100 h,while Ti-48Al-6Nb porous material showed a slower oxidation rate,and its oxidation weight gain was only 14.47 g/m2.This could be explained that Nb greatly hindered the O inward diffusion and Ti outward diffusion during high-temperature oxidation,resulting in promoting the formation of Al2O3 and suppress TiO2 on the surface of Ti-48Al-6Nb porous material.A large majority of micropore had remained after 900?/100 h isothermal oxidation.(2)Comparing the oxidation kinetics curves,microstructure morphology,phase contituent and pore parameter changes of the Ti-48Al-6Nb and Ti-40A1-lONb-lOCr porous material before and after 900?/100 h isothermal oxidation revealed that Ti-40Al-10Nb-10Cr porous material showed a slower oxidation rate than Ti-48Al-6Nb porous material,and its oxidation weight gain was only 8.52 g/m2 after 900?/100 h isothermal oxidation,which could be mainly attributed to the synergistic effect of Nb and Cr for suppressing the growth of inner layer TiO2 non-protective layer and promoting the formation of Al2O3 at the same time.In addition,the filtering accuracy was greatly improved through the surface modification of Ti-40Al-10Nb-10Cr porous material.The results of simulated flue/gas filtration showed that the filtration efficiency of modified samples was significantly higher than the original Ti-40Al-10Nb-10Cr porous alloy samples.The filtration efficiency of Ca,Mg and Si were greatly improved from the initial 44.29%,47.26%and 44.66%to 91.12%,95.21%and 93.20%,respectively.(3)The high Nb-TiAl-based porous nanocomposites with micro/nano scale pore size were successfully fabricated,and Al2O3 particles formed on the surfaces and matrix of porous nanocomposites by controlling the chemical reaction,which resulted in great improvement of high temperature oxidation resistance.For porous nanocomposite prepared with 8 wt%Nano-NiO addition,the high-temperature oxidation resistance was greatly improved by synergy formation of Al2O3 and N2Ti4O resulted from nano-NiO reaction with Ti,Al,Nb powders during the fabrication,and both Ni2Ti4O and Al2O3 as two diffusion barriers further hinder the O inward diffusion and Ti outward diffusion.As for porous nanocomposite prepared with 8 wt%Nano-SiO2 addition,the high-temperature oxidation resistance was greatly improved by synergy formation of Al2O3 and Ti5Si3 resulted from nano-SiO2 reaction with Ti,Al,Nb powders during the fabrication.The favorable presence of Ti5Si3 and Al2O3 particles in the porous nanocomposite can greatly hinder the O inward diffusion and Ti outward diffusion during high-temperature oxidation.(4)Five kinds of porous nanocomposites were successfully prepared through the preferential chemical reaction of Nano-SiO2/Nano-ZrO2/Nano-Y2O3/Nano-MgO/Nano-ZnO with Ti,A1 and Nb powders by adjusting the sintering temperature.Five porous nanocomposites showed better high temperature oxidation resistance than Ti-48Al-6Nb and Ti-40Al-10Nb-10Cr porous materials.In addition,five porous nanocomposites showed strong separation efficiencies in the application of the TiO2 nano-pollutant/water filtration.Comparing with the separation efficiency of TiO2 nano-pollutant by Ti-48Al-6Nb porous material(89,91%),five kinds of porous nanocomposites were achieved 99.91%(Nano-SiO2),99.80%(Nano-ZrO2),99.85%(Nano-Y2O3),99.83%(Nano-MgO),99.87%(Nano-ZnO)Ti02 Nano-pollutants effective separation,respectively,duo to a lot of Al2O3 particles formed and adhered to the substrate surface or inside the pore of porous nanocomposites,significantly improved the surface roughness and filtration precision of porous nanocomposites.(5)Combined with dual micro/nano scale size pores of high Nb-TiAl-based porous nanocomposite and the liquid phase plasma technology,an innovative "two-step" strategy was proposed for the efficient degradation of high-concentration organic dye wastewater,and the mechanism of the "two-step" strategy for the degradation of organic dye wastewater was revealed.The results showed that complete degradation of the 50 mg/L dye wastewater by using liquid phase electrolytic plasma processing synergistic with Nano-Au@SiO2/S2O82-only needs less than 6 min,and the degradation efficiency have greatly improved(the first step)compared to cyclic activated sludge process(CASS process,need 72 h);and then,the Nano-Au@SiO2 catalyst was effectively separated and recycled by using porous nanocomposite prepared with 8 wt%Nano-Y2O3 addition,and separation efficiency up to 99.35%(step 2).