| High Nb containing TiAl alloys have attracted much attention owing to low density, high specific strength, super high temperature strength, good high temperature oxidation resistance, high acid or alkali corrosion resistance and so on. Combining the benefits of high Nb containing TiAl alloys and porous materials, high Nb containing TiAl porous alloys can be fabricated by powder metallurgy method. High Nb containing TiAl porous alloys solve the worse oxidation and acid corrosion resistance of porous metals and the worse mechanical properties and weldability of porous ceramics, which would be applied in wider area. Therefore, this thesis studied the preparation process, phase transformation, pore-formation mechanism of Ti-48Al-6Nb porous alloys and skeleton surface modification. The main conclutions and innovations are listed as follows:(1) High Nb containing TiAl porous alloys were fabricated by powder metallurgy method and the 4-step heat-treatment process was confirmed. The air and vapor escaped from powder compact at 120℃ for lh; element Al reacted with Ti at 600℃ for 3hrs.; element Nb reacted with Al at 900℃ for 3hrs and also Ti-Al phase transformation happened; finally, Ti3Al/TiAl phase skeletons formed at 1350℃ for 3hrs. The pore structures were uniform with the diameter of 10-15um, and the porosity is about 36%.(2)Nb-Al diffusion reaction was studied by diffusion couples at relatively low temperatures (600-800℃). The results showed that there was not any reaction between Nb and Al at 600℃. Increasing temperature to 700℃, new phases with porous structures emerged at the interfaces. The interphase layers turned to be wider with the increase of time, and they were confirmed as NbAl3 phase. Raising temperature to 800℃, Nb-Al reaction became more tempestuous and interphase layer was much wider than that at 700℃. And also the interfaces were confirmed as NbAl3.(3)Phase transformations of Ti-48Al-6Nb porous alloys were studied at different temperatures. The results showed that there existed 5-step phase transformations. At 600℃, Ti reacted with Al forming TiAl3 phases with the pores emerging at the location of Al particles. Pore-diameter and total pore area increased. At 700-900℃, Nb reacted with Al forming NbAl3 phases with pores emerging. At 900-1100℃, TiAl3 phases transformed to TiAl phases and a few phases with the continuous increase of pore-diameter, and total pore area and porosity also increased. At 1100-1350℃, TiAl phases transformed to Ti3Al phases with the increase of pore-diameter. At 1350℃, NbAl3 transformed to Nb2A1 and Ti3Al/TiAl phases turned to be the main phases. Pore-diameter increased continuously but porosity decreased. At this temperature, total pore area increased obviously and pore-diameter distributed uniformly.(4)Acid corrosion resistance properties of Ti-48Al-6Nb porous alloys with different phase skeletons were studied. The results showed that Ti3Al/TiAl skeletons had the best acid corrosion resistance with the least weight loss. The pore structures changed a little that some pores diameter expanded slightly and porosity increased.(5)Ti-48Al-6Nb porous alloys tube was fabricated by cold isostatic pressing and hot sintering method. The surface was smooth with no cracks. The skeletons of the tube mainly consisted of Ti3Al/TiAl phases, and pore diameter shrunk to 8-10μm due to constrained sintering.(6)Nano-porous γ-Al2O3 layers with thickness of 200nm and pore-diameter of 60±10nm were fabricated successfully on Ti-48Al-6Nb porous skeleton surfaces after soaking in 2mol/L NaOH solution for 5 days. Total pore area increased sharply to 0.25m2/g due to nano-porous layers.(7)The formation mechanism of nano-porous layers was discovered.γ-Al2O3 layers formed initially on the skeletons and then transformed to nano-porous structures from the area with smaller radius of curvature, such as holes. Finally nano-pores extended to flat area making all the skeletons covered by nano-porous layers.(8)According to the electrochemistry test, nano-porous γ-Al2O3 layers improved the electrochemistry resistance of Ti-48Al-6Nb due to higher apacitance and impedance. |
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