Study On Phase Structure And Atomic Stacking Characteristic In High Nb-TiAl Alloy

The high Nb-TiAl alloy has been widely used in aerospace and automotive manufacturing and it is a model for the development of advanced high-temperature light alloys.The high Nb-TiAl alloy not only maintains the advantages of low density,high stiffness and good oxidation resistance under 750°C,but also increases the ordered temperature and melting point of the alloy and increases the resistance of the alloy,antioxidant and creep resistance.In this paper,42 kinds of TiAl-based alloy ingots were prepared by means of non-consumable vacuum arc smelting technology and water-cooled copper crucibles.Each alloy ingot undergoes 5 repeated melting and homogenization annealing.The main microstructures,phase parameters and alloy local characteristics of the alloys were studied.The characteristics of alloy solidification,equilibrium phase evolution,phase composition,lattice constant and alloy local structure were analyzed.The evolutionary features of the system have been systematically studied to pave the way for the performance analysis and application of high Nb-TiAl alloys.The optical micrographs and SEM photographs of 21 sets of TiAl-based alloy specimens with different Nb content were investigated systemically.It is found by metallographic analysis that the addition of Nb in the alloy is conducive to the uniform microstructure refinement of the alloy.In TiAl alloys with Al content lower than 35%（at.%,the same as below）,Nb addition greatly improves the microstructure refinement,as the grain size decreasing from about 100μm to 30μm.For TiAl alloys with 35%to 50%Al content,the microstructures are full lamellae ofα₂/γ.The inter-lamellar structure is decreased to less than1μm by Nb addition.As the Al content is higher（53%）,a large amount of twin crystals emerges,with the grain size increasing.In TiAl alloys with Al content of 35%⁵1.5%,the lamellar microstructure size increases with the increase of Al element.In the microscopic morphology,Nb element stabilizes the phase ofα₂ and inhibits the formation ofγ-phase.By comparing the binary phase diagrams of TiAl alloys and XRD diffraction results,it was found that the phase composition of the alloys also changed:Nb is a stable element of theβphase,and in the alloy with an Al content of 20%,the addition of 10 Nb improves the formation ofβphase in the alloy.It is found that that phase content and lattice parameter of the high Nb-TiAl alloy changed with the increase of Nb content.In the experiment,X-ray diffraction data of a total of14 alloys with different Al contents were collected,and the alloy phase content was calculated by adiabatic method.The results of quantitative analysis of high Nb-TiAl alloys shows that the content ofα₂ phase in the higher Nb content is higher than that in the lower part of Nb.With the increase of Nb element content in the alloy,the stability ofα₂ phase increases,and the change of theγphase is inhibited.The first-principles method was used to calculate the lattice constant ofγ-phase of high Nb-TiAl alloys with 12.25%and 3.12%Nb content respectively.It is found that the increase of Nb content resulted in the increase of theγ-phase lattice constant.By introducing the X-ray diffraction data into the software of Jade,the calculation of the lattice constant shows that with the increase of Nb,the lattice constant ofα₂ in the high Nb-TiAl alloy are decreased,and theγ-phase lattice constant is getting bigger.In the experiment,the results of theγ-phase lattice constant are consistent with the theoretical calculation,which confirms the reliability of the experimental data。It is found that that Nb element changes the atomic stacking structure of the alloy.It is shown that the change of the atomic position and coordination number in the atomic shell of the alloy can be obtained by the atomic pair distribution function（PDF）.Experimental results show that with the increase of Nb element content in the alloy,changes in the way atoms are bound and changes in atomic stack structure;the first peak of the PDF curve decreases,which means that the addition of high content Nb elements reduces the number of the nearest neighbor atoms in the structure of the capital alloy,and decreases the number of atoms in the structure.The decrease of peak sharpness in PDF curve indicates that the addition of Nb elements is not conducive to the ordering of the atomic structure of TiAl alloy.