Lamellar Orientation Controlling And Tensile Properties Of Directionally Solidified Beta-solidifying Gamma-TiAl Based Alloys Under High Withdrawing Rates

γ-TiAl-based alloy is a promising lightweight high-temperature structural material with excellent high temperature properties,which can be used to produce the low-pressure turbine blades in aero-engine.For TiAl alloy with full-lamellar structure,the lamellar orientation can be controlled by directional solidification to improve the mechanical properties.However,the current method for directional solidification uses a lower withdrawing rate to control the lamellar orientation of TiAl alloy and the melt is easy to be polluted in the directional solidification process.γ-TiAl alloy has been developed into the fourth generation.The addition of a large number ofβ-stabilized elements makes TiAl alloy solidify inβphase.It makes the lamellar orientation control of TiAl alloy involves the synchronous control of the solidification process and theβ/αtransition process.In this thesis,two kinds ofβsolidifying TiAl alloys,which are the third generation of TiAl alloy Ti-45Al-8Nb alloy（hereinafter referred to as 458 alloy）and the fourth generation of TiAl alloy Ti-43.5Al-4Nb-1Mo-0.1B alloy（hereinafter referred to as TNM alloy）,are used.First,the phase transition ofβ-solidifying TiAl alloy was in-situ studied by high temperature confocal laser microscope.Then,theβ-solidifying TiAl alloys were directionally solidified at different withdrawing rates respectively.The crystal growth and microstructures evolution of the directionally solidified alloys during solidification andβ/αtransition were analyzed A high-withdrawing-rate method of controlling the lamellar orientation of directionally solidifiedβ-solidifying TiAl alloys is proposed successfully.The influence of thermal stabilized treatment time and withdrawing rate on the microstructure of TiAl alloy was emphatically studied.Finally,the tensile properties of directionally solidifiedβ-solidifying solidification TiAl alloy were tested.The deformation and fracture behaviors of directionally solidified alloy at different withdrawing rates,as well as the relationship between withdrawing rates,microstructure and properties were discussed.β-solidifying TiAl alloys,TNM alloy and 458 alloy,exist singleβregion during the heating process,but the peritectic solidification occurs in both alloys due to Al segregation during the solidification process.The liquidus and solidus of 458 alloy meet the relationship T_l=1536.5V^-0.00282 and T_s=1519.0V^-0.00803 with cooling rate,the liquidus and solidus of TNM alloy meet the relationship T_l=1540.7V^-0.00436 and T_s=1522.1V^-0.00465 with cooling rate,respectively.For theβ/αtransition process of the twoβ-solidifying TiAl alloys,theαphase gradually changes from equiaxed growth to Widmannstatten growth with the increase of cooling rate,and theαphase growth direction gradually approaches to the lamellar direction of the lamellar structure formed subsequently.This provides the basis for the high withdrawing rate directional solidification to control the lamellar orientation of theβ-solidifying TiAl alloys.The growth rate ofαphase in the twoβ-solidifying TiAl alloys increases with the increase of cooling rate,but the maximum growth rate exists at the conventional cooling rate.For TNM alloy,the maximum growth rate ofα-phase is 25μm/s,and for 458 alloy,the maximum growth rate is 200μm/s.In theβ/αtransition,458 alloy has stronger ability to maintainαgrain continuous growth than TNM alloy.In the thermal stabilization treatment experiment,it was found that both kinds of alloys maintain the mushy zone composed ofβphase,and with the increase of thermal stabilization treatment time,the liquid phase channel and isolated liquid phase in the mushy zone reduced,and the solid-liquid interface gradually get smooth.Besides the alloy composition on both sides of the solid-liquid interface gradually gets stable.Theβphase near the solid-liquid interface coarsens which makes the solid-liquid interface gradually consistent with the direction ofβ（111）.Moreover,the solid-state transformation interface of the unmelted section gradually become smooth.The effect of the as-cast microstructure on the directional solidification structure is avoided,which provides a good basis for solidification and solid-state transformation in the directional solidification process.The columnar crystal to equiaxed crystal transformation（CET）of directionally solidified 458 alloy occurs at a lower withdrawing rate than that of the directionally solidified TNM alloy.With the increase of withdrawing rate,the solidification path of directionally solidified 458 alloy changes from hyper-peritectic solidification to hypo-peritectic solidification and then toβsingle-phase solidification;the solidification path of TNM alloy changed from hypo-peritectic solidification to metastable hypo-peritectic solidification and then toβsingle-phase solidification.The primary dendrite spacing of directionally solidifiedβ-solidifying alloys decreases with the increase of the withdrawing rate,which results in the decrease of segregation during solidification with the increase of the withdrawing rate.The increase of the withdrawing rate can significantly refine the size of lamellar colonies in directionally solidified TNM alloy,but it has no obvious effect on the coarse continuous lamellar colonies of directionally solidified 458 alloy.Theβ₀phase in directionally solidified 458 alloy increases with the increase of the withdrawing rate,which is higher than that in as-cast alloy.But for directionally solidified TNM alloy,the increase of the withdrawing rate can only increase the content ofβ₀ phase to the level of the as-cast alloy.Increasing the withdrawing rate can effectively refine the lamellar spacing of directionally solidified alloys.By analyzing the growth direction ofβdendrites during solidification andαphase duringβ/αtransition in directionally solidified alloys,the lamellar orientation controlling mechanism ofβ-solidifying TiAl alloy was summarized,and a high-withdrawing-method to control the lamellar orientation ofβ-solidifying TiAl alloy was successfully proposed.The tensile properties,deformation behavior and fracture behavior of directionally solidified alloys at different withdrawing rates were analyzed.The ultimate tensile strengths of directionally solidified 458 alloys at room temperature/700℃/900℃are improved to 512 MPa/852 MPa/504 MPa.The lamellar orientation of directionally solidified 458 alloy is well controlled with the increase of the withdrawing rate,and the lamellar bending deformation is difficult to occur,resulting in the increase of the brittle ductile transition temperature（BDTT）of the alloy.The ultimate tensile strength/ultimate elongation of directionally solidified TNM alloys are improved to 558 MPa/1.15%and the ultimate tensile strength at 800℃is improved to 639 MPa.With the increase of the withdrawing rate,the content ofβ₀ phase at the lamellar colony boundary of the directionally solidified TNM alloy is increased,and the lamellar colonies can deform integrally with the help ofβ₀ phase at the boundary of the lamellar colony,resulting in the decrease of the BDTT of the alloy.