Mechanism Of Columnar Grains Formation And Microstructure Evolution Of TiAl Based Alloys During Directional Heat Treatment

With the further development of new materials science and the deeply study on the blade materials of aerospace engines,TiAl based alloys gradually shifts from as-cast microstructure to directional even single crystal microstructure.Based on numerical simulation and experimental verification of temperature field generated by induction coils with high frequency alternating current,the directional heat treatment(DHT)technique is proposed,and its effects on controlling microstructure and improving mechanical properties of TiAl based alloys are demonstrated in this study.Ti44 Al,Ti44Al6Nb1Cr and Ti44Al6Nb1Cr2 V alloys are chosen as research materials,respectively.Temperature field is simulated under different parameters during DHT.The grain boundary migration and growth behavior of columnar grains are studied.The effects of DHT on directional microstructure and morphology of lamellar clusters and B2 phase are analyzed.The effect of cyclic DHT on directional microstructure and mechanical properties of TiAl based alloys is discussed.And the necessary conditions of DHT are obtained for TiAl alloys with different dimensions.In the part of temperature field simulation,it is found that effective heat treatment region(EHTR)across the section of ingot can be realized through controlling section size of ingot and loaded power.Consequently,the purpose of regional heat treatment is achieved.When the loaded power is 21.6 k W,the temperature of EHTR in Ti44 Al alloy can reach 1477 °C in single β phase domain.Grains conduct β transformation and grow up continuously during DHT.Finally,directional columnar grains are obtained and the feasibility of DHT technique is proved.Meanwhile,it is proved that growth of columnar grains is caused by curve grain boundary firstly at high temperature,according to theoretical analysis and experiments.Subsequently,the curve grain boundary migrates directionally under directional heat flow and directional movement of ingot.Consequently,columnar grains are obtained.The calculation results indicate that when pulling rate is 4.17 μm/s for DHT at 1477 °C,the migration rate of grain boundary ranges from 0.350 μm/s to 0.505 μm/s for Ti44 Al alloy and columnar grains would grow up continuously.The effect of cyclic DHT on directional microstructure controlling of as-cast Ti44 Al alloy has been studied.And it is found that columnar grains grow up obviously with increase of cyclic times.The directional properties of grains are improved gradually and thickness of lamellar phases decrease firstly and then increase.The 80% angles are less than 60° between lamellar phases and growth direction of columnar grains after cyclic DHT for 4 times.And 41% angles is 0°.The thickness of lamellar phases decrease gradually and the minimum is about 0.6 μm.Microhardness is up to 312.9 HV,increasing by 28% compared with that of as-cast alloy(244.8 HV).Compression strength is about1200 MPa at room temperature.The main reason is that the nucleation points of γ phase increase during DHT resulting in lamellar phases being refined.The compression test indicates that main crack prefers to initiate at grain boundary and propagate along it.The second cracks are initiated at the tip of main crack generally and propagate along lamellar phases.The effects of DHT on improving directional microstructure and mechanical properties of high Nb-TiAl alloys prepared through cold crucible directional solidification(CCDS)are studied.It is found that grains grow up along the axial direction and merge along the radial direction of Ti44 Al Nb1Cr alloy after CCDS(DS-Ti44Al6Nb1Cr)during DHT.Microstructure changes greatly as well.Lamellar clusters change from ellipsoidal to columnar and B2 phases change from reticular to banded.They are separated and parallal each other.The yield strength,ultimate tensile strength and total strain are 574 MPa,636 MPa and 2.28% for DHT alloy,respectively.Compared with that of DSTi44Al6Nb1 Cr alloy(422 MPa,523 MPa and 3.03%),the strengths increase by 36% and22%,respectively.Excellent mechanical properties are in DHT alloy at 700 °C.The ultimate tensile strength and total strain are 695 MPa and 5.8%,respectively.The similar results are obtained in DS-Ti44Al6Nb1Cr2 V alloy after DHT.Columnar grains grow up obviously and become more straight.The angle between growth direction of columnar grains and axial direction of ingot decreases from 17.55° to 10.67°.Lamellar clusters change from ellipsoid to columnar and B2 phases change from reticular to banded.And they are parallel and distributed along growth direction of columnar grains.The ultimate tensile strength and total strain of DHT alloy are 609 MPa and 1.86%,compared with that of DS-Ti44Al6Nb1Cr2 V alloy(505 MPa and 1.40%).Mechanical properties are improved further.The reason is directional microstructure is improved and thickness of lamellar γ phases decreases after DHT.Not only a large number of dislocations but also some stacking fault bands are formed in lamellar γ phases of DHT alloy in the process of tensile test,which also plays an important role in improving mechanical properties of DS alloy.