| The Ti-Al-Nb ternary system includes three classes of alloys with great potential for application,which are y-TiAl alloys,O-Ti2AlNb alloys and σ+γdual-phase alloys.The y-TiAl alloys are mainly composed of a2 and y phases,which are promising for applications at 650-900 ℃.The O-Ti2AlNb alloys are mainly composed of O,α2 and β phases,which aiming to service at 500-800 ℃.The σ+γ two phase alloys are mainly composed of β,σ and γ phases,which are candidate materials for applications at temperatures higher than 1000 ℃.Sinceγ-TiAl alloys and O-Ti2AlNb alloys have excellent high temperature properties,such as high strength,good oxidation resistance,high temperature creep resistance,Ti-Al-Nb ternary system has become a hot research topic in recent years.In order to reduce costs and meet the higher operating temperatures,researchers must optimize the alloys composition and the fabrication process.However,the lack of knowledge about the Ti-Al-Nb phase diagrams has severly restricted the application and development of Ti-Al-Nb alloys.Most of the prior works are focused on 1100 ℃ or 1200 ℃,with very limited data or some controversial results for other temperatures.For example,the solidification sequences are not clear;the published 1400 ℃ isothermal sections are contradictory to the experimental results in recent years;no systematic study of 1300 ℃ isothermal section can be found in the published papers;the phase diagrams at lower than 1000 ℃ have not been systematically studied because of the difficulties in measurement.Moreover,the ωo phase can be formed in high Nb-TiAl alloys at their aiming service temperatures.However,the formation process and the phase equilibria relationships about ωo phase have not been clarilied.As the main constituent phase of O-Ti2AlNb alloys,the formation process and the phase equilibria relationships of O phase also have some controversies.In addition,there are different views on the phase equilibrium relationship of the Ti-rich side TiAl binary system.The present thesis uses a variety of methods to study the phase equilibria and phase transformation in Ti-Al-Nb ternary system,trying to clarify the phase stability and phase equilibria of the system,which can give some guides to the alloy design,heat treatment and hot processing and give the ideas for enhancing the microstructure stability at their service temperatures for the Ti-Al-Nb system.The main conclusions and innovations are listed as follows:(1)By quenching heat treated alloys into water,the 1300 ℃ and 1400 ℃isothermal sections for 0-70 Al(Unless otherwise stated,all components in this paper are atomic ratio)contents were constructed.There are four three-phase regions and nine two-phase regions in these isothermal sectons.The α+β+γ,σ+β+γ and σ+δ+β three phases regions move to a higher Al content and a lower Ti content with increasing temperature.In addition,α and β single phase regions expand with increasing temperature.(2)The solidication sequences of alloys with compositions in Ti-(40-60)Al-(0-23)Nb were detailedly studied by investigating the as-cast microstructures.It was found that the solidication sequence has little relationship with Nb content.The changes of the solidication sequences are mainly caused by the changes of Al contents.The primary phase could be β phase if the Al contents are lower than 51;the primary phase could be a phase if the Al contents are between 51 and 56;the primary phase could be γ phase if the Al contents are higher than 56.The range of a primary phase region expands to Ti-55.5 Al-22.5 Nb based on the present results,which is much larger than that of the prior calculation results.(3)The Ti-Al-Nb ternary system has some characteristics,which are high melting point,complex phase equilibria relationship and forming continuous solid solution by the Ti and Nb.Based on these characteristics,a diffusion couple named ’five diffusion-components couples’ was designed in the present work.Five diffusion-components couples were formed by adding two diffusion components to the traditional tripe diffusion couples using diffusion multiples method.Based on the results,the five diffusion-components couples are suitable for investigating the phase diagrams of the Ti-Al-Nb ternary system,which can solve the difficulties during studying the phase diagrams at half of the melting temperatures.(4)The 900 ℃,980 0C and 1010 ℃ isothermal sections of Ti-Al-Nb ternary system were identified successfully.The 900 ℃ isothermal section contains 10 single-phase regions,19 two-phase regions and 10 three-phase regions;the 980 ℃isothermal section contains 9 single-phase regions,19 two-phase regions and 10 three-phase regions(where ε+η+γ was not plotted at 980 ℃);the 1010 ℃isothermal section includes 8 single-phase regions,13 two-phase regions and 6 three-phase regions(where ε+η+γ was not plotted at 1010 ℃).The ωo phase,as well as the phase equilibria relationships,was identified in the present work.Theωo phase can be equilibria with α2 phase,γ phase,σ phase and O phase,and formωo+α2+O,σ+O+ωo,ωo+γ+σ and ωo+γ+α2 four three-phase regions.Moreover,the O phase is also stable at 900 ℃ and can form α2+O+β,δ+O+β,δ+O+σ,ω0+O+σand ω0+O+α2 five three-phase regions.The ω0 phase vanished at 980 ℃isothermal section.However,the O phase is still stable at 980 ℃ and the adjacent relationship has changed into α2+O+β,α2+O+β0 and two O+σ+β0 three-phase regions.The O phase vanished at 1010 ℃ isothermal section.In addition,the β0 phase region and β phase region are continuous at 1010 ℃.However,the β0 phase region divided into two parts,one part is in the shape of a small island in the middle of the phase diagram,and the other part is connected to the β phase.(5)The phase transformations in Ti-Al-Nb ternary system at intermediate temperatures were identified,based on the 900 ℃,980 ℃ and 1010 ℃ isothermal sections.Six invariant reactions(β+β0→α2+O,β+σ→δ+O,β0+γ→σ+ω0,β0+γ→ω0+α2,β0+σ→O+ω0 and β0→ω0+α2+O)and two three-phase reactions(β0+σ→O,β0+γ→ω0)occur between 1010 ℃ and 900 ℃.Based on these reactions,the formation processes of O phase and ω0 phase,as well as the disappearing process of β0 phase can be clarified.(6)The phase equilibria and phase stability of β0 phase,β phase,α phase andα2 phase in Ti-Al binary system were in-situ investigated,using neutron diffraction method.Based on the results,β0 phase is not stable in the binary system and there are two peritectoid reactions between β phase,α phase and α2 phase. |
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