Effect Of Isomorphous β-stabilizing Elements On Microstructures And Oxidation Resistance Of High Temperature Titanium Alloy

Compared with other kinds of metallic materials for aerospace applications, Ti-5.8Al-4.0Sn-3.5Zr-0.4Mo-0.4Si-0.4Nb-0.4Ta-0.05C (abbreviated as Ti-60A) alloy possesses much better creep resistance against high temperature, excellent thermo-stability and mechanical properties at the servicing temperatures of 600℃. As a nearαtitanium alloy, Ti-60A alloy is a new type titanium alloy considered for high temperature applications, such as for the manufacture of the aerofoil blades and discs in the aviation and aerospace industries. With the development of the high temperature titanium alloy for the engineering, the element content, especially the isomorphousβ-stabilizing elements such as Mo, Nb and Ta, should be adjusted. This thesis focuses on investigating the effect of Mo, Nb and Ta on elements distributing, microstructure evolution and oxidation behavior of the Ti-60A alloy.Elements distributing analysis by EPMA showed that the contents ofβ-stabilizing elements Mo, Nb and Ta increased in the each phase for three classic microstructure such as equiaxed structure, bimodal structure and lath structure with the increment of three elements, addition. It was found that Mo, Nb and Ta are enriched in theβ_T and Mo is much obvious than Nb and Ta. The contents of Mo, Nb and Ta inαphase in equiaxed structure is higher than that inα_p in bimodal structure and the contents of Mo, Nb and Ta inβ_T in bimodal structure is higher than that in lath structure. The addition of Mo leads to the increment of Al partition to theαphase. Sn and Zr keep almost constant in each phase and there is no obvious change in the element distribution in the alloys with different Nb and Ta contents.Microstructure analysis showed that the volume fraction and diameter ofαp reduces in bimodal structure and the lathαphase became more and more thinner with increase of Mo content. The effect of Nb and Ta on the microstructures of T-60A alloy is not obvious. Theβphase became thicker while the silicides change in shape and precipitation position because Mo content increase. It is revealed that the average grain size of Ti-60 alloy is smaller than that of the Ti-60A and IMI834 alloy. There is not obvious change in the average grain size of Ti-60 alloy with increasing temperature and time in theβ-phase zone, this can be attributed to the precipitation of the rare earth-rich phases on the boundaries. The average grain size of Ti-60A and IMI834 Alloy get obvious larger with increasing time in theβ-phase field. The grain growth kinetics at (β_T +20℃) shows that all of the isothermal growth curves of theβ-phase grains in the three alloys are approximately parabolic type with the growth exponents less than 1/2. That can be described as D_Ti-60=115.9+87.6t^0.15 , D_Ti-60A= 108.5+121.7t^0.34 and DIMI834 = 125.3+135.2t^0.35. Moreover, the grain growth activation energies of three titanium alloys have been calculated and discussed in the condition of heating temperature fromβ_T to (β_T +40℃) and keeping-temperature time of 10 min.The addition of isomorphousβ-stabilizing elements such as Nb and Ta can improve high temperature oxidation properties of the Ti-60A alloys. The improvement of oxidation resistance by Nb and Ta is mainly attributed to promoting the formation of dense and fine oxidation layer and increasing the adhesiveness between oxidation layer and substrate. There is no change in phase composition and alpha-case. The addition of isomorphousβ-stabilizing elements Mo reduces the high temperature oxidation properties of the Ti-60A alloys and the more Mo content, the worse oxidation resistance of Ti-60A alloy.