Effect Of Nb Content On Damping Properties Of TiAl Alloy

In this thesis, through metallographic microscope(OM)、 scanning electron microscope(SEM)、transmission electron microscopy (TEM)、XRD、energy spectrum analysis (DSC)、dynamic mechanical thermal analyzer (DMA) and other experimental methods, the influence factors of damping properties of TiAl based alloy were investigated from three aspects of microstructure、composition and processing. The relationship between damping properties and TiAl based alloy’s microstructure and composition have been summarized. The main conclusions are presented as follows:Through appropriate heat treatment systems, TiAl based alloy(Ti-47Al-2Cr-2Nb-0.2W) can obtain different microstructures. The studies of the effect of microstructures on damping properties present some conclusions as follows:(1) Fine full lamellar microstructure which can obtain under the condition of heat treatment at1330℃for20min by AC has the best damping property. While duplex microstructure’s damping property is not that good, with the increase of the lamellar colonies, damping properties increase. Nearly y microstructure has the worse damping property relatively. The decisive factor of damping properties during the comparison of the three microstructures is the number of lamellar colonies.(2) In terms of fine full lamellar microstructure obtained at1330℃for20min by AC, the lamellar colonies are gradually enlarged with increasing temperature or holding time prolonged, which make damping performance decreased. It can be interpreted as related to how much of the phase interface. The more of the phase interface, the better of damping properties, so the decisive factor to full lamellar microstructure is the size of the lamellar colonies.(3) When temperature Ta equals1330℃, the changing principle of the damping properties of Ti-47Al-2Cr-2Nb-0.2W (at.%) alloy can be described by the following formula: HereA1=0.00801, B1=3.78E-4, A2=0.01195, B2=-9.35286E-4.In terms of different heat treatment, which means different Ta temperature, the value of A and B need to make appropriate modification.The damping properties of hot rolling TiAl alloy (Ti-47Al-2Cr-2Nb-0.2W) were studied from RD and TD directions. The conclusions are as follows:(4) The microstructures of ND、RD and TD direction are all duplex microstructure which composed of γ、a2and few β phase. By microstructure observation, there happens dynamic recrystallization in hot rolling TiAl alloy. The damping properties in RD and TD directions are the same. Therefore, the effect of hot rolling TiAl alloy can be contributed to the microstructures and the phase content.The effects of Nb content on TiAl based alloy’s damping properties were studied through as-cast TiAl-xNb(x=1,3,5,7) alloy. The conclusions are as follows:(5) In the microstructure of as-cast TiAl alloy, Nb element exists in the form of solid solution in the matrix. When the content of Nb is low, it distributes evenly in the matrix, when the content of Nb is higher, there exists phase segregation which is β phase.(6) With the increase of Nb content, damping performance of TiAl alloy decreased first and then increased, as the Nb content changed from1%to3%, damping performance decreased, and the peak value of tan changed from0.0041to0.0035.when the Nb content got5%, damping performance increased, the peak value of tan reached0.0046.Furthermore, the TiAl-7Nb’s peak value of tan got0.005. This is caused by the enlargement of the average size of lamellar colony, leading to damping performance decreased. β phase emerged by the adding of Nb element, damping performance was improved as (3phase provides Snoek relaxation mechanism structure foundation.(7) The Snoek relaxation mechanism damping performance can be described by the following formula: tanδ=K·tanδm [Tm/R(β)] Here, K is constant, tanδ is the value of damping peaks, Tm stands for peak temperature, R(β) is the damping peak width, which β accords with gauss normal distribution. The damping performance of TiAl alloy containing β phase has relationship with the ratio of peak temperature and peak width, and damping peak value.The greater the product,the better the damping performance.