Effect Of Nb Content On Solidification Characteristics And Microstructural Evolution Behavior Of TiAl Alloys

At present,casting Ti-48Al-2Cr-2Nb alloy of hyperperitectic solidification which is one of the few casting Ti-Al alloys has been applied.However,the working temperature of Ti-48Al-2Nb-2Cr?at.%?alloy is limited up to 650 ^oC and this is not enough for the low pressure turbine blade up to 800 ^oC.Thus,there is a urgent need to develop a novel high-temperature casting TiAl alloy.Alloying element Nb is added to improve high oxidation resistance,high temperature strength and lamellar structure stability.However,TiAl based alloys with high Nb addition show a bigger dendritic size and a more serious segregation,lead to the increasing cracking tendency and has an impact on fracture toughness.The casting Ti-48Al-x Nb?x=2,4,6,8?alloys were selected as the research objects and the effects of undercooling and cooling rate on microstructure,formation of metastable phase,and microsegregation were investigated.Meanwhile,this work is carried out to systematically research the effect of Nb content on the solidification path and solidification microstructure of the medium Nb containing TiAl cast alloys.In addition,the solidification characteristics and the high temperature performance,especially high-temperature yield strength,of a hyperperitectic TiAl cast alloy with a medium Nb content were discussed.The main contents and innovative conclusions are as follows:The variable temperature phase diagrams of ternary Ti-Al-?x=2,4,6,8?Nb alloys were calculated by Thermo-Calc software and the peritectic reaction temperature?T_p?,the maximum solid solubility point of?phase?C_??,the peritectic point?C_p?,and the maximum solid solubility point of?phase?C_??of TiAl-xNb alloy's pseudo-binary phase diagram were obtained.With the increase of Nb content,the solidification path changes gradually from hyperperitectic solidification to hypoperitectic solidification for the Ti-48Al-xNb alloys under near equilibrium conditions and the transition point of solidification path occurs at the Nb content of about 4 at%.Based on calculated phase diagram and solidification microstructure,the ratio of the content of the remaining liquid and the peritectic phase after peritectic reaction for Ti-48Al-2Nb alloy is 1:1.The globular?dendrites can be formed and the uniform solidification microstrucutre with less significant segregation can be obtained.Using the electromagnetic levitation method,?phase is always prior to nucleate from the undercooled melts for Ti-48Al-xNb?at.%?alloy in the achieved undercooling range.For Ti-48Al-8Nb alloy,when the undercooling is above a critical value,a remarkable morphological transition from typical hypoperitectic solidification to a sole solidification of the?phase resulting in the suppression of the peritectic reaction occurs.For hypoperitectic TiAl-?7,8?Nb alloy,the transition mode of solidification path is similar,but the critical value of transition is different.The critical value of morphological transition for the high Al content of hypoperitectic alloy is slightly higher than for the low Al content alloy.For the Ti-48Al-2Nb hyperperitectic alloy,a morphological transition occurs beyond a critical undercooling of 93 K,which can be attributed to the dendritic-cellular growth of primary?phase.The transition range of undercoolings for Ti-48Al-?2,4,8?Nb alloys are obtained.With the increase of Nb content,eliminating solidification segregation becomes increasingly difficult.Decreasing solidification segregation can be obtained by controlling Nb content for Ti-48Al-xNb alloys.Various substructures for Ti-48Al-?2,4,8?Nb alloys at high undercoolings were observed,which were composed of the deformation twins and small amounts of stacking faults.Microstress tend to increase with increasing undercooling.The foramtion of substructure can obsorb the stress of phase transformation well.With the increase of Nb content,the stacking fault energy can be reduced and the number of deformed twins can be increased.The microstructure of Ti-48Al-xNb?x=2,4,6,8?alloys at various cooling rate was investigated using the electromagnetic levitation method combined with copper mold casting.With decreasing of the cooling rate,the solidification path changes gradually from hypoperitectic solidification to a sole solidification of the?phase for the Ti-48Al-x Nb?x=4,6,8?alloys.In rapidly quenched solidification condition,the metastable?phase is formed as the leading phase.However,the?phase is still observed as the leading phase for Ti-48Al-?x=4,6,8?Nb alloys.The primary arm spacing corresponding to various cooling rates decreases with increasing cooling rates for Ti-48Al-?x=4,6,8?Nb alloys,.The primary dendrite arm spacing as function of cooling rate,can be described as:?₁=165.83?^-0.32,?₁=227.98?^-0.33 and?₁=278.05?^-0.34for Ti-48Al-?x=4,6,8?Nb alloys,respectively.For integrally cast TiAl alloy,the hyperperitectic solidification TiAl alloy should be adopted.The formation of coarse dendrite can be avoided and S-segregation can be decreased.More importantly,the hyperperitectic solidification TiAl alloy has lower melting point and better flowing property.The smaller crystallization temperature region and unobvious dendrite of the primary?phase which benefit the control of solidifying defects.Taking the Ti-48Al-2Cr-2Nb alloy composition as a design basis,the medium Nb-containing TiAl alloy with a homogeneous microstructure was developed by adjusting Al content and Nb content.The alloy is required to meet the following considerations:1.adjusting the Al content to ensure solidification through the region of hyperperitectic.2.The ratio of the content of the peritectic phase and the remaining liquid after peritectic reaction is 1⁰.95:1.The above two requirements are mainly based on the increse of crystallization temperature region with the increase of liquidus line slope.The liquidus line m_L increases with increasing the region of crystallization temperature.The liquid channel decreases with decreasing the region of crystallization temperature,and then decreases the tendency of cracking during solidification.Thus,the optimum condition is that the regions of crystallization temperature of?phase and?phase are approximately equal.The research on high-temperature tensile property of the Ti-48Al-2Cr-4Nb alloy has been made and the results show that the yield strength?YS?and ultimate tensile strength?UTS?of the novel TiAl alloy are 541 and 606 MPa at 800 ^oC,respectively,which are remarkably improved when comparing with Ti-48Al-2Cr-2Nb alloy.The Ti-48Al-2Cr-4Nb alloy after 100 h oxidation at 800 ^oC possesses superior oxidation resistance,which is greatly better than that of Ti-48Al-2Nb-2Cr alloy under the same condition.