Study Of Phase Transformation And Relation In TiAl-(6-10)Nb Alloys

High Nb-TiAl alloy has the advantages of higher strength,better creep resistance,corrosion resistance and lower specific gravity at 800°C,With the development of science and technology,high Nb-TiAl alloy has developed rapidly in the direction of high-temperature materials and is used in many fields such as aircraft engines and automobile exhaust valves.In recent years,researchers have discovered that high Nb-TiAl alloys will precipitate?_o phase in the service temperature range.At present,there are few reports on the precipitation process and phase transition path of?_o phase and O phase combined with the optimized vertical cross-sectional calculation phase diagram and experimental analysis of Ti-Al-Nb alloy.When the Al content is high,the Ti-Al-Nb alloy phase transition paths also have different views.In this paper,the combination of experiments and calculations is used to study the equilibrium transformation process and related microstructure characteristics in high Nb-TiAl alloys.Calculate the phase balance,phase relationship and phase transition path of the alloy during solidification by CALPHAD,in order to predict the structural characteristics of the alloy at room temperature and in use;The alloy structure under actual solidification conditions was studied by alloy method,and compared with the calculation results to optimize the state transition diagram of the alloy.Twenty-sevenTi-Al-Nb alloysampleswere prepared by vacuum solidification technology,each alloy was subjected to uniform annealing treatments.Scanning electron microscope?SEM?,energy dispersive spectrometer?EDS?,X-ray diffractometer?XRD?,and differential scanning calorimeter?DSC?were used to systematically study by the morphology,crystallography,and thermal analysis methods.The main conclusions and innovations are as follows:?1?When the content of Al in TiAl-?6-10?Nb alloy is?18-32%?,the phases of?₂,?and O are included,the?₂ phase accounts for the largest proportion,followed by?_o and O phases.With increasing Al content in the sample,the content of the?₂ phase increased,while the O and?_o phases decreased.When the TiAl-?6-10?Nb alloy has an Al content of?32-40%?,the included phases are?₂,?_o,?_o phases and a few analyzed?phases.In the SEM microstructure,the?_o phase is always accompanied by the?_o phase,and it can be speculated that the?_o phase precipitates from the?_o phase.When the Al content is?40-50%?,the alloy structure includes?_o phase,?₂ phase and?phase.?2?The vertical cross sections of the TiAl-?6-10?Nb ternary phase diagram were calculated using the Pandat software,and the phase transition paths of different Nb contents were obtained by combining analysis with tests.1)TiAl-?6-10?Nb alloy?₂ phase region solidification path:???+?_o??_o+?₂??_o+?₂+O.2)TiAl-?6-10?Nb alloy?_o phase solidification path:For alloys with an aluminum content of?30-39%?,the phase transition path is:???+?_o??_o+?₂??_o+?₂+???_o+?_o+?₂+?.For alloys with an aluminum content of?39-42%?,the phase transition path of the alloy is:?????_o?+???+?_o+???+?_o+?+???_o+?_o+?₂+?.3)TiAl-?6-10?Nb alloy?phase solidification path:For alloys with?42-47%?Al content,the phase transition path is:???+?????+???+?_o+?₂+???_o+?₂+?.For alloys with an Al content greater than 47%,the phase transition path is:?+???????+???₂+?.The phase transition path of Ti-?18-51?Al-?6-10?Nb partial alloy was clarified through experiments.It can be found that the phase transition path of the alloy is greatly affected by the Al content.?3?The phase equilibrium of the prepared Ti-Al-Nb alloy?Nb is 6,8,10%?was determined through experiments.There are some deviations between the thermodynamic calculation and the experimental detection,especially for the low temperature transformation.When the Al concentration is between 18-32%,The experimentally measured DSC curve has a particularly broad peak at about 850°C,which is the?₂?Otransition.At the concentration of 32-43%,It was experimentally determined that the?_o phase was formed from the retained?_o/?phase at about 850°C,rather than the?₂ phase mentioned in the thermodynamic calculations.?4?In the larger Al concentration range,some high-temperature stable phases?such as?/?_o phase?will remain at ambient temperature,and the orderly and disorderly transition???_o occurs at 1200°C-1400°C.In addition,under experimental conditions identified some phase transitions that have not been found in equilibrium,such as ?_o??_p transitions.