Investigation On Arc Ion Plated Ti-Al-Cr High-temperature Protective Coatings On Ti-60 Alloys

Ti-60 alloy is a near-αhigh temperature titanium alloy belonging to Ti-Al-Zr-Mo-Si-Sn system.It can be used stably up to 600℃.However,when Ti-60 was used at high temperatures,oxidation and oxygen brittleness were the main reasons that affected heat stability of Ti-60 alloys.On the one hand,surface oxidation formed porous TiO₂ which had no protection;on the other hand,for titanium alloy had a good affinity towards oxygen at elevated temperatures in air,thereby oxygen penetrated into titanium alloys and formed oxygen-enriched layer which decreased the alloy's plasticity obviously. Thoese two factors severely limited the application of Ti-60 at high temperatures.Measures to improve one property often leaded to degradation of the other.Since oxidation attack was mainly limited to the outer region of a component,and mechanical properties were determined by the entire cross-section,hence,a promising approach to obtain both the best mechanical property and oxidation resistance was the use of surface modification technologies,particularly coating techniques.This thesis mainly concerns on two aspects,one is the oxidation behavior of Ti-60,and the other aspect,high temperature properties,hot corrosion behaviors,thermal shock cycling behaviors of the Ti-Al-Cr coatings deposited by arc ion plating(AIP) and interaction between coatings and Ti-60 base alloy were also investigated.The results showed that Ti-60 alloys followed positive effect rules of grain size at 650℃and 750℃,i.e.the smaller a grain size was,the lower oxidation rate was.The weight increased smallest for the Ti-60 alloy exposed at 600℃,slightly heavy at 700℃,and at 800℃destabilizing oxidation occurred.The as-deposited coatings prepared in AIP grew in layer and the peak of XRD diffraction pattern was obviously widened.After vacuum annealing,the layered microstructure disappeared and the inner of coatings became much denser.The peak of XRD diffraction patterns became narrow.Annealed Ti-48Al(atomic fraction,%) coatings were composed ofγphase,Ti₃Al phase and TiAl₃ phase;the amount of Ti₃Al phase and TiAl₃ phase were small.Annealed Ti-48Al-12Cr(atomic fraction,%) coatings were composed ofγphase,Laves phase,Ti₃Al phase and TiAl₃ phase;the amount of TiAl₃ phase was larger than that in Ti-48Al coatings.The oxide layer of Ti-48Al coatings and Ti-48Al-12Cr coatings remained intact and no spalling occurred in the process of isothermal oxidation and cycling oxidation at 800℃. Both of the two coatings could protect Ti-60 alloys effectively and keep a good binding force with matrixes at 800℃.In the process of oxidation at 900℃,Ti-48Al coatings spalled seriously,which could not protect Ti-60 matrix.While Ti-48Al-12Cr coatings formed lots of Al₂O₃ which could protect Ti-60 matrix effectively.This demonstrated that the addition of 12%Cr element could improve oxidation resistance of coatings effectively.Ti-Al-Cr-(Si,Y) coatings prepared in AIP could keep Ti-60 matrix from corrosion effectively in sulfate at 800℃and 850℃.Ti-Al-Cr-Si and Ti-Al-Cr-Si-Y coatings had better hot corrosion resistance than Ti-Al-Cr coatings in sulfate due to Y element could promote Al₂O₃ to form on the surface of the coatings;this could form more Al₂O₃ on the surface of coatings than that dissolved in sulfate,extend the hot corrosion incubation period of coatings.Si and Y elements could also make oxidation particles thinner which made the oxide layer denser and thus SO₄^2- and O diffused through the dense oxide layer difficultly. Therefore,the Ti-Al-Cr-Si and Ti-Al-Cr-Si-Y coatings could protect Ti-60 matrix effectively and improve the hot corrosion resistance of Ti-Al-Cr coatings.In addition to that, Ti-60 alloys and Ti-Al-Cr(Si,Y) coatings experienced more serious corrosion in 75% Na₂SO₄+25%K₂SO₄(weight fraction,%) than in pure Na₂SO₄.The AIP Ti-48Al-12Cr coatings provided excellent thermal shock protection on Ti-60 alloys at 900℃and 950℃.No obvious coatings spallings were found after 900℃/80 cycles and 950℃/30 cycles.While a large amount of coatings and oxidation layers were found to spall when Ti-48Al-5Cr coatings were thermal shocked at 900℃and 950℃,hence induced weight change curve descended.As 5%Cr was added into Ti-48Al coatings,it increased vacancies of oxygen in TiO₂;enhanced the atomic diffusion coefficient;promoted the growing rate of forming of TiO₂;oxide layers thicker and spalling occurred easily.12%Cr elements promoted the formation of Al₂O₃ oxidation layers in thermodynamics and kinetics points.Al₂O₃ was very thin and dense improved the thermal shock resistance of Ti-60 with coatings.Besides,thermal shock cycling experiments accelerated the oxides spalling and breakdown thereby they accelerated internal oxidation,crack forming and crack propagation.Diffusion experience between Ti-60 and Ti-48Al-12Cr coatings showed the relationship between the thicknesses of diffusion layers with time and temperatures, especially with temperatures.At 800℃,850℃and 900℃,the growth of diffusion layer between Ti-60 and Ti-48Al-12Cr coatings was controlled by diffusion,which followed parabolic rule.Growth activation energy of the diffusion layers between Ti-60 and Ti-48Al-12Cr coating was 113.71kJ/mol.