【作者】 李翔；

【导师】 孔凡涛；

【作者基本信息】 哈尔滨工业大学 ， 材料工程（专业学位）， 2018， 硕士

【摘要】 TiAl合金具有低密度、高强度,良好的抗蠕变性能和抗氧化性能等优点,被认为是航空航天领域极具应用前景的轻质高温结构材料。但由于TiAl合金具有金属间化合物的本征脆性,限制了其发展应用。大量研究表明,TiAl合金经过热加工可有效改善其力学性能。本文选用高温变形能力优异的β-γTiAl合金为研究对象,名义成分为Ti-43Al-9V-Y（at.%）。研究了大尺寸Ti-43Al-9V-Y合金铸锭的组织与成分的均匀性;对锻态合金的显微组织与力学性能进行了研究。对锻态合金进行了不同的热处理,研究了热处理态合金组织的演变规律,分析了显微组织与力学性能之间的关系。采用真空自耗电极电弧熔炼（VAR）技术制备了大尺寸Ti-43Al-9V-Y合金铸锭,铸态合金主要为β/B2和γ两相组成的近层片组织,β/B2相所占体积分数达19.1%。铸锭从心部到边缘部位的成分均匀性良好,组织形态发生了从近层片组织—双态组织—近层片组织+双态组织的变化,Y₂O₃的形态发生了从断续的网络状—颗粒状—网络状的变化。铸锭在0.5r纵向处切取拉伸试样表明具有最佳的室温拉伸性能,其抗拉强度为536MPa,断裂应变为0.79%。铸锭心部的断裂韧性性能稍优于边缘和0.5r部位处,主要是由于心部含有粗大的片层组织,对裂纹扩展起到较好的阻碍作用。对锻态合金进行显微组织表征发现锻坯具有良好的组织均匀性。锻态合金由细小的γ晶、β/B2相以及少量α₂组成,经过锻造,合金发生了较为完全的动态再结晶,组织得到了明显的均匀细化,晶粒尺寸基本分布在10-25μm之间。经过锻造,合金的拉伸性能得到了显著提高。室温时,锻态合金的抗拉强度达806MPa,延伸率为2.9%。650℃时,合金的抗拉强度为702MPa,延伸率为5.8%,温度进一步升高,合金的抗拉强度降低,延伸率明显升高。800℃时,合金的延伸率达到了55%,合金表现出了明显的塑性断裂的特征。在垂直锻造方向的试样断裂韧性K_IC值为18.7 MPa?m^1/2,明显优于平行锻造方向的试样的断裂韧性。研究了热处理工艺对锻态合金组织与性能的影响。将锻态合金在1180℃-1350℃之间高温进行淬火,分析了锻态合金的高温相组成。锻态合金经过热处理工艺1250℃/30min/FC得到双态组织,双态组织的室温抗拉强度为710MPa,延伸率为1.27%。近层片组织的热处理工艺为1360℃/10h/FC,α₂/γ层片尺寸约为145μm,近层片组织的室温抗拉强度为678MPa,延伸率为0.87%。锻态合金经过热处理工艺1220℃/2h/FC得到等轴组织,其室温抗拉强度为663MPa,延伸率为1.06%。700℃时,双态组织也表现出了较好的综合力学性能,抗拉强度和延伸率分别为557MPa和23.5%。热处理并没有完全消除β/B2相,分布在层片晶周围的β/B2相有利于限制α₂/γ层片晶的长大。随着热处理温度的升高与时间的延长,α₂/γ层片的体积分数增加,合金显微硬度会增大。近层片组织表现出了较好的断裂韧性性能(24 MPa?m^1/2)主要是由于α₂/γ层片对裂纹萌生和扩展起到了阻碍作用。更多还原

【Abstract】 TiAl alloys are considered as the most promising lightweight high-temperature structure materials in the filed of aerospace,owing to their low density,high strength,excellent oxidation resistance and outstanding creep properties.However,the intrinsic brittleness of intermatellic compounds in TiAl alloys limit s their development and applications.A large number of studies have shown that TiAl alloys can effectively improve their mechanical properties after hot processing.In this paper,we focus on the beta-gamma TiAl alloys with excellent high temperature deformation ability.The nominal component is Ti-43Al-9V-Y（at.%）.The uniformity on microstructure and composition of large size Ti-43Al-9V-Y alloy ignots were investigated.The microstructures and mechanical properties of forging billet were systematically investigated.Different heat treatment was performed on the forged alloy,and the evolution of microstructure in the heat-treated alloy was studied.The relationship between microstructure and mechanical properties were analyzed.The large size Ti-43Al-9V-Y alloy ignot was prepared by Vacuum Consumable Electrode Arc Furnace Remelting（VAR）.The as-cast alloy mainly containsβ/B2 andγphase,formingβ/B2+γnearly lamellar structure.The volume fraction ofβ/B2 phase accounts for 19.1%.Ingot has good composition uniformity from center to edge,the microstructure morphology changed from near-lamellar to duplex to near-lamellar+duplex.The morphology of Y₂O₃ gradually changed from discontinous network to granular to continous network.Longitudinal cut tensile specimens at 0.5r from ingot show the best tensile properties at room temperature,the ultimate tensile strength（UTS）and strain are 536 MPa and 0.79%.In the center,the ignot has better fracture toughness than that of the edge and 0.5r.This mainly owing to the coarse lamellar microstructure in the center of ignot can effectively hinder the propagation of the cracks.The microstructure of forged alloy was characterized reveals that the pancake has good microstructure uniformity.The as-forged alloy consistes of fineγphase,someβ/B2 phase and fewα₂ phase.The alloy undergoes relatively complete dynamic recrystallization during forging.The pancake exhibits very fine and uniform microstructure.The grain size is basically distributed between 10 and 25μm.The tensile properties of as-forged alloy were improved obviously by forging.The ultimate tensile strength and the elongation are 806 MP a and 2.9%at room temperature of the as-forged alloy.At 650℃,the ultimate tensile strength and the elongation are 702 MPa and 5.8%,respectively.With the increase of temperature,the ultimate tensile strength of the alloy will decrease and the elongation will increase significantly.When the temperature increases to 800℃,the elongation increases to55%,the alloy shows obvious plastic fracture characteristics.The fracture toughness of the as-forged alloy in the direction perpendicular to the forging is18.7MPa·m^1/2,higher than that of in the direction parallel to the forging.The effect of heat treatment process on microstructure and properties of as-forged alloy were reserached.The as-forged alloy was quenched at high temperature between 1180℃and 1350℃.The high temperature phase composition of the forged alloy was analyzed.Duplex microstructure can be obtained after heat treatment at1250°C/30min/FC,the ultimate tensile strength and the elongation at room temperature are 710 MPa and 1.27%,respectively.Near lamellar microstructure with dimensions of about 145μm can be obtained after heat treatment at 1360°C/10h/FC.The ultimate tensile strength is 678 MPa,the elongation reduces to 0.87%.Equiaxed microstructure can be produced after heat treatment at 1220/2h/FC,the ultimate tensile strength reduces to 663 MPa,the elongation is 1.06%at room temperature.Duplex microstructure also exhibits excellent comprehensive mechanical properties when the test temperature is 700℃,the UTS is 557 MPa,the elongation sharply rises to 23.5%.β/B2 phase is not completely eliminated after heat treatment.The β/B2 phase distributed around the lamellar contributes to limit the growth of the α₂/γ lamellar.With the increase of heat treatment temperature and time,the volume fraction of α₂/γ lamellar increases,and the microhardness of the alloy increases.Near lamellar microstructure exhibits good fracture toughness (24 MPa?m^1/2) due to the germination and propagation of cracks are hindered by the α₂/γ lamellar.更多还原