Composition Optimization And Design Of TiAl Alloy Based On TNM

Ti Al base alloy is considered as one of the alternative materials for high temperature Ni-base alloy because of its low density,high strength and excellent oxidation and creep resistance.However,due to its intrinsic brittleness and poor thermo-plasticity,Ti Al base alloy has always restricted its application in engineering.Especially with the development of aerospace industry,the service environment of the material has become more demanding and complex,and the special requirements for the material have become very strict.The results show that doping the third group of elements is an effective means to improve the mechanical properties of Ti Al alloy.Therefore,Ti Al based alloys which meet the special requirements become the key point of industrial research.In this thesis,the composition of Ti Al alloy was re-optimized and redesigned,and the influence of the composition on the microstructure and phase transition temperature of Ti Al alloy was studied.According to the composition of TNM alloy,TNM alloy with different content of Al,TNM alloy with different content of Mo element,TNM alloy with Cr,Hf,Zr,Gd and different content of Sn element were designed.The effects ofαphase stable elements,βphase stable elements,high melting point elements and rare earth elements on the microstructure and phase transition temperature of TNM alloy were investigated by scanning electron microscopy（SEM）,X-ray diffractometry（XRD）and high temperature differential scanner（DSC）.The study of the influence variation of TNM alloy will provide guidance for the design of Ti Al base alloy for special application.The main research contents are described as follows:（1）It is found that with the increase of Al content,β_o phase aggregates at the grain boundary,which plays a certain refining effect on the grain,and the T-αtransition temperature increases obviously.The mechanical properties of Ti Al alloy can be improved by increasing Al content when the composition of Ti Al alloy is optimized.（2）Mo element,as a strongβphase stable element,has the effect of refining grain.It also has the effect of transforming columnar crystals to equiaxed crystals.Mo element can reduceα→βtransition temperature,but has little effect onγ→αtransition temperature and T_e transition temperature.Therefore,when optimizing the composition of Ti Al alloy,increasing the content of Mo element in a certain range can expand the processing window of Ti Al and improve the hot processability of the alloy.（3）Under the combined action of Cr and Nb,reducing the content of Nb can effectively refine grain size and prevent the precipitation ofβ_o phase particles.The combination of Cr and Nb can increase the eutectoid transition temperature of TNM alloy.Therefore,under the superposition of Cr effect,the high temperature stability of Ti Al alloy can be enhanced by appropriately increasing the content of Nb.（4）The doped Hf and Zr elements in TNM alloy have the effect of refining grain.Under the combined action of Hf,Zr and Nb,theβphase transition temperature in TNM alloy is greatly reduced,and theγphase transition temperature in TNM alloy is increased.Hf is a strongγphase stable element,and the stability effect is better than Zr element.（5）Trace doping of rare earth element Gd not only refines the grain,but also significantly reduces the width ofγ/α₂ layers in TNM alloy.Theα→βtransition temperature,Tαtransition temperature and Te eutectoid transition temperature of TNM alloy all decreased.（6）The change of Sn content has great influence on the microstructure of TNM alloy at room temperature.With the increase ofγ/α₂ content,the spacing ofγ/α₂ layers firstly increased and then decreased,and the whole layer structure gradually changed into bimodal structure.The T_αtransition temperature and T_e eutectoid transition temperature of TNM alloy both increase,and the difference between T_αtemperature and T_βtemperature of TNM alloy decreases gradually,narrowing the temperature range ofα₂+γtwo terms.