激光定向能量沉积(laser directed energy deposition,LDED)凭借高效率与工艺柔性,正成为解决室温高脆性和高活性的TiAl4822(Ti-48Al-2Cr-2Nb)合金传统工艺难加工、难制备大型复杂构件问题的关键途径,以充分发挥其航空发动机等高温轻质...激光定向能量沉积(laser directed energy deposition,LDED)凭借高效率与工艺柔性,正成为解决室温高脆性和高活性的TiAl4822(Ti-48Al-2Cr-2Nb)合金传统工艺难加工、难制备大型复杂构件问题的关键途径,以充分发挥其航空发动机等高温轻质部件的理想材料潜力。然而,LDED过程中快速熔融-凝固循环会产生极大的温度梯度和残余应力,从而导致构件开裂,但目前尚无成熟手段能够完全抑制裂纹产生。本工作利用整体高温辅助LDED制备出30 mm×25 mm×6 mm致密无裂纹的TiAl4822合金薄壁构件,并对其宏观形貌、微观组织、孔隙率及显微硬度进行研究。研究结果表明:在常温条件下,LDED制备的TiAl4822合金薄壁样件易发生以解理为主的脆性断裂,显微组织以细小等轴晶为主;引入800℃整体高温辅助后,沉积层晶粒定向生长为自下而上倾斜的柱状晶,孔隙率从0.05%降至0.008%,孔径分布更均匀,表面未见宏观裂纹;与此同时,显微硬度由常温样件的390.46HV_(0.2)降至354.94HV_(0.2),这主要归因于在高温辅助条件下晶粒长大、晶界减少及析出相中γ相的含量相对增加。因此,整体高温辅助不仅有效抑制裂纹与大尺寸孔隙的产生,还优化微观组织均匀性,为TiAl4822合金的高致密、高性能制备提供新途径。展开更多
The elimination of the B2 phase in aβ-solidifying high Nb-containing TiAl alloy withβ/B2 andγphases was investigated using different heat treatments,with a focus on understanding the phase transformations and lamel...The elimination of the B2 phase in aβ-solidifying high Nb-containing TiAl alloy withβ/B2 andγphases was investigated using different heat treatments,with a focus on understanding the phase transformations and lamellae formation during the process.The phase transformation and lamellae formation during B2 phase elimination differs from that observed in conventional TiAl alloys.During the holding stage of heat treatment,theβ/B2 phase is replaced by theαphase through primary phase transformations ofβ→αandγ→α.Lamellae formation occurs within bothαandγgrains during cooling,initiating 30-40℃below the annealing temperature.This lamellar structure was formed via two main mechanisms:nucleation at grain boundaries followed by growth into the grain,and direct precipitation and growth within the grain.The orientation relationship between theγphase and its adjacentαphase is(111)_(γ)//(0001)_(α)and[011]_(γ)//[1120]_(α),with a coherency between the phases characterized by a misfit of approximately 1.7%.展开更多
High Al content inhibits the formation of B2 phase,which improves creep resistance in high Al/Nb-containing TiAl alloys.In this work,the microstructure evolution and creep behavior of TiAl based alloy Ti-46Al-8Nb(at.%...High Al content inhibits the formation of B2 phase,which improves creep resistance in high Al/Nb-containing TiAl alloys.In this work,the microstructure evolution and creep behavior of TiAl based alloy Ti-46Al-8Nb(at.%)with a high Al/Nb content,produced by the vacuum consumable electrode melting technology and the electromagnetic cold crucible melting technology,were studied.The microstructure of the Ti-46Al-8Nb alloy is composed ofα_(2)/ηphases arranged in layers with different orientations,which possesses smooth grain boundaries due to small-blocky segregation and irregular serrated grain boundaries caused by large-blocky segregation.Under conditions of 780-820℃and 125-175 MPa for 200 h,it exhibits typical power-law creep characteristics.The apparent activation energy of creep(Q)and apparent stress exponent(n)of the Ti-46Al-8Nb alloy are Q=274 kJ·mol^(-1)and n=1.97,respectively.The creep deformation mechanism is grain boundary sliding.Cracks easily form at the smooth boundary.The irregular serrated boundaries with small specific surface area hinder the dislocation movement,thereby improving the boundary creep resistance.When the stress concentration reaches a certain degree,the cracks will initiate between the lamellar structures within the grain.The crack usually propagates along the boundary perpendicular to or at an angle of 45 with the stress axis until creep failure occurs.展开更多
基金supported by the National Natural Science Foundation of China(Nos.51871012,52071021)Beijing Natural Science Foundation,China(No.2162024)+1 种基金Fundamental Research Funds for the Central Universities,China(No.FRF-GF-20-20B)the National Program on Key Basic Research Project of China(No.2011CB605502)。
文摘The elimination of the B2 phase in aβ-solidifying high Nb-containing TiAl alloy withβ/B2 andγphases was investigated using different heat treatments,with a focus on understanding the phase transformations and lamellae formation during the process.The phase transformation and lamellae formation during B2 phase elimination differs from that observed in conventional TiAl alloys.During the holding stage of heat treatment,theβ/B2 phase is replaced by theαphase through primary phase transformations ofβ→αandγ→α.Lamellae formation occurs within bothαandγgrains during cooling,initiating 30-40℃below the annealing temperature.This lamellar structure was formed via two main mechanisms:nucleation at grain boundaries followed by growth into the grain,and direct precipitation and growth within the grain.The orientation relationship between theγphase and its adjacentαphase is(111)_(γ)//(0001)_(α)and[011]_(γ)//[1120]_(α),with a coherency between the phases characterized by a misfit of approximately 1.7%.
基金supported by the National Natural Science Foundation of China(No.52425401)the Foundation of National Key Laboratory for Precision Hot Processing of Metals(No.JCKYS2021603C001)+1 种基金the Fundamental Research Funds for the Central Universities(No.2023FRFK06014)the Major Science and Technology Achievement Transformation Project of Heilongjiang Province(No.ZC2023SH0075)。
文摘High Al content inhibits the formation of B2 phase,which improves creep resistance in high Al/Nb-containing TiAl alloys.In this work,the microstructure evolution and creep behavior of TiAl based alloy Ti-46Al-8Nb(at.%)with a high Al/Nb content,produced by the vacuum consumable electrode melting technology and the electromagnetic cold crucible melting technology,were studied.The microstructure of the Ti-46Al-8Nb alloy is composed ofα_(2)/ηphases arranged in layers with different orientations,which possesses smooth grain boundaries due to small-blocky segregation and irregular serrated grain boundaries caused by large-blocky segregation.Under conditions of 780-820℃and 125-175 MPa for 200 h,it exhibits typical power-law creep characteristics.The apparent activation energy of creep(Q)and apparent stress exponent(n)of the Ti-46Al-8Nb alloy are Q=274 kJ·mol^(-1)and n=1.97,respectively.The creep deformation mechanism is grain boundary sliding.Cracks easily form at the smooth boundary.The irregular serrated boundaries with small specific surface area hinder the dislocation movement,thereby improving the boundary creep resistance.When the stress concentration reaches a certain degree,the cracks will initiate between the lamellar structures within the grain.The crack usually propagates along the boundary perpendicular to or at an angle of 45 with the stress axis until creep failure occurs.
