【作者】 苏勇君；

【导师】 陈玉勇； 孔凡涛；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 博士

【摘要】 TiAl合金具有低密度、高强度、优异的阻燃能力和抗氧化性以及优良的抗蠕变性能和抗疲劳性能等优点，成为航空航天领域最具竞争力的结构材料。然而TiAl合金高温变形能力差以及室温塑性低等不足，制约了该合金实际工程化的应用。Beta-gamma TiAl合金具有优异的高温变形能力，拓宽了TiAl合金的热加工工艺窗口。本文围绕两种Beta-gamma TiAl合金—Ti-43Al-9V-Y和Ti-43Al-5V-4Nb （at.%），采用锻造的方法制备这两种合金锻坯，并系统研究了合金的组织和力学性能。采用真空自耗电极电弧炉熔炼技术，研制出大尺寸的Ti-43Al-9V-Y（at.%）合金铸锭，该铸锭尺寸为Ф220mm×800mm。通过特种包套锻造技术，在温度为1200°C，总变形量为80.8%，变形速率为0.01s^-1左右的条件下，制备出大尺寸的Ti-43Al-9V-Y合金锻坯，其尺寸达到Ф500mm×46mm，该锻坯组织均匀性较好。铸态Ti-43Al-9V-Y合金由块状的γ相和β/B2相以及少量的α₂和YAl2相组成。锻态Ti-43Al-9V-Y合金由γ/（β/B2）/α₂层片晶团以及分布在层片晶团界面处块状的γ相和β/B2相组成，为双态组织，层片晶团尺寸约为30-50μm。采用不同的热处理技术，获得了不同形态的Ti-43Al-9V-Y合金组织，随着热处理温度的升高和时间的增加，块状的γ相和β/B2相逐渐形成了γ/（β/B2）层片晶团，当热处理条件为1350°C/8h时，获得了γ/（β/B2）/α₂全层片组织。拉伸性能测试结果表明，锻态Ti-43Al-9V-Y合金室温抗拉强度为834MPa，屈服强度为680MPa，延伸率为2.0%。随着测试温度的升高，抗拉强度和屈服强度下降，延伸率上升，在700°C时，抗拉强度为693MPa，屈服强度为589MPa，延伸率为12.0%。锻态合金经过1350°C/8h热处理获得γ/（β/B2）/α₂全层片组织后，其室温抗拉强度为863MPa，屈服强度为698MPa，延伸率为1.5%，当测试温度为700°C时，抗拉强度为571MPa，屈服强度为539MPa，延伸率为2.0%。三点抗弯性能测试结果表明，锻态Ti-43Al-9V-Y合金和经过1350°C/8h热处理获得γ/（β/B2）/α₂全层片组织后的合金，其KIC值分别为22.50MPa·m^1/2和21.16MPa·m^1/2。原位拉伸测试结果表明，锻态Ti-43Al-9V-Y合金在裂纹扩展的过程中，当遇到β/B2相时，主裂纹会发生偏转；经过1350°C/8h热处理获得γ/（β/B2）/α₂全层片组织后的合金，在裂纹扩展的过程中，当层片晶面与主裂纹呈较小的角度时，则主裂纹优先向层片界面方向偏转并沿层片界面扩展，产生层间开裂，当主裂纹扩展至与其呈较大角度的层片晶面时，则主裂纹必然要发生大的偏转，继续扩展方式可能为穿层片或沿层片，具体的扩展路径依赖于β/B2层片的厚度，如果β/B2层片厚度较小时，则会发生穿层片断裂，如果β/B2层片厚度较大时，则会发生沿层片断裂。纳米硬度测试结果表明，在锻态Ti-43Al-9V-Y合金中，γ相、γ/（β/B2）层片和β/B2相的纳米硬度分别为4.44Gpa、4.78Gpa和5.25Gpa，γ相和β/B2相的硬度相近。高周疲劳性能测试结果表明，锻态Ti-43Al-9V-Y合金在测试温度为700°C和750°C时，其疲劳强度分别为481.9MPa和415.3MPa；经过1350°C/8h热处理获得γ/（β/B2）/α₂全层片组织后的合金，在测试温度为700°C和750°C时，其疲劳强度分别为437.1MPa和392.4MPa。疲劳断口形貌分析表明，这两种组织的合金，其疲劳裂纹都萌生于内部的夹杂缺陷处，疲劳裂纹扩展区有疲劳辉纹存在，疲劳瞬断区都有静拉伸断口的微观特征。高温蠕变性能测试结果表明，锻态Ti-43Al-9V-Y合金，在温度为700°C，应力为200MPa、250MPa和300MPa的条件下，其蠕变时间分别为371h、182h和74h。在700°C时，当应力为200MPa-250MPa时其应力常数为3.0088，应力为250MPa-300MPa时，其应力指数为3.943；在指定应力250MPa下的蠕变激活能为326.68kJ·mol-1。高温蠕变变形机制为稳态过程受原子力扩散过程控制，发生位错滑移和晶界滑移，而动态过程是因为孔洞和裂纹引起，最终导致蠕变失效。系统研究了元素粉末的快速烧结和无包套锻造技术，并在温度为1300°C两步锻，变形量分别为30%和60%，变形速率为0.01s^-1左右的条件下，研制出致密度为98.9%的Ti-43Al-5V-4Nb（at.%）合金锻坯。该锻坯主要由γ、α₂和β/B2相以及还没有完全扩散的Nb组成。拉伸性能测试结果表明，Ti-43Al-5V-4Nb合金锻坯的室温抗拉强度为442.96MPa，延伸率为0.27%。采用不同的热处理技术，获得了不同形态的Ti-43Al-5V-4Nb合金组织。在热处理条件为1300°C/1h时，获得了γ/α₂全层片组织。拉伸性能测试结果表明，全层片组织合金室温抗拉强度为535MPa，延伸率为0.52%。随着测试温度的升高，抗拉强度下降，延伸率上升，在700°C时，抗拉强度为582MPa，延伸率为11.00%。更多还原

【Abstract】 TiAl alloys are very promising structural materials for making structures andparts working at elevated temperatures in aerospace field, due to their low density,high specific strength, good high-temperature burning resistance and oxidationresistance, excellent creep and fatigue properties. However, the poor hotdeformability, low room-temperature ductility and fracture toughness limit theirpractical engineering applications. Beta-gamma TiAl alloys are deemed as a newgroup of TiAl alloys that attract scientists strong research interests because of theirexcellent hot deformability and wide hot processing condition window. In this paper,two types of beta-gamma TiAl alloys with nominal compositions of Ti-43Al-9V-Yand Ti-43Al-5V-4Nb （at.%） were prepared and then undergone high temperatureforging. Microstructure and mechanical properties of the two TiAl alloys wereinvestigated systematically.A large size Ti-43Al-9V-Y ingot （Ф220mm×800mm） was prepared by vacuumconsumable electrode arc furnace remelting （VAR）. As-cast Ti-43Al-9V-Y alloyconsist of massive γ phase and β/B2phase, as well as a few α₂and YAl2phase. Alarge size Ti-43Al-9V-Y alloy pancake （Ф500mm×46mm） with a total deformationstrain of80.8%was prepared using specially canned forging technology carried outat1175°C and at a strain rate of0.01s^-1. The as-forged Ti-43Al-9V-Y alloy showeduniform duplex microstructure consisting of γ/（β/B2）/α₂lamellar crystal clusterswith the size of approximately30⁵0μm, and block-like γ phase and β/B2phaseprecipitating at the boundaries of the lamellar colonies. Different microstructures ofTi-43Al-9V-Y alloy were obtained by different heat treatment techniques. These γand β/B2phases gradually transformed into γ/（β/B2）/α₂lamellar clusters withincreasing of heat treatment temperature and duration. A fullly γ/（β/B2）/α₂lamellarcluster microstructure was obtained when annealing at1350°C/8h.Tensile properties of Ti-43Al-9V-Y alloy with different processing states weretested. The as-forged Ti-43Al-9V-Y alloy exhibits excellent room temperaturetensile properties, with an ultimate tensile strength of863MPa, yield strength of698MPa and an elongation of2.0%. The ultimate tensile strength and yield strengthincrease while the elongation decrease with increasing of the test temperature. At700°C, the ultimate tensile strength and yield strength are693MPa and589MPa,respectively, and the elongation reaches12.0%. After heat treatment of1350°C/8h,Ti-43Al-9V-Y alloy with fully-lamellar microstructure has an ultimate tensilestrength of863MPa, yield strength of698MPa and elongation of1.5%at roomtemperature. As the tensile temperature increases to700°C, the ultimate tensilestrength and yield strength decrease to571MPa and539MPa respectively, while the elongation increases to2.0%. By means of the three-point bending tests, it is foundthat the as forged Ti-43Al-9V-Y alloy with duplex microstructure shows the bestfracture toughness, its KICvalue is high up to22.5MPa·m^1/2, while forfully-lamellar microstructure after1350°C/8h heat treatment, its KICvalue is21.2MPa·m^1/2. In situ tensile test was performed on as-forged Ti-43Al-9V-Y alloy. Theresults show that cracks deflect when meeting β/B2phase during propagation in thisalloy. After the alloy was heat-treated at1350°C for8h, γ/（β/B2）/α₂fully-lamellarmicrostructure was obtained. It is found that when the angle between the lamellarlayer interface and the crack is small, the crack deflects towards the direction oflayer interface and propagates along layer interface, resulting in interlayerdelamination. When a crack propagates to a layer interface which has a relativelylarge angle with the crack, the crack may propagate across the lamellae or along thelamellae interface, depending on β/B2lamellar size. If the size of β/B2lamellae issmall, fracture across the lamellar will occur; if the size is large, fracture along thelamellae interface will happen. Nano-hardness test shows that the nano-hardness ofγ phase, γ/（β/B2） lamina and β/B2phase is4.44GPa,4.78GPa and5.25GPa,respectively, in as-forged Ti-43Al-9V-Y alloy. The hardness of γ phase is close to thatof β/B2phase.High-cycle fatigue tests show that the fatigue strength of as-forgedTi-43Al-9V-Y alloy is481.9MPa and415.3MPa at700°C and750°C, respectively.After1350°C/8h heat treatment, the fatigue strength at700°C and750°C decreasesto437.1MPa and392.4MPa, respectively. Fatigue fracture morphology analysisshows that fatigue crack nucleates at the internal shrinkage pores, or inclusions.Fatigue crack propagation region has fatigue crack, whereas fatigue instantaneousfault zone has the micro characteristics of the static tensile fracture. Hightemperature creep performance test results show that the creep time of as-forgedTi-43Al-9V-Y alloy at700°C is371h,182h and74h, corresponding to stress of200MPa,200MPa and250MPa, respectively. At700°C, when the stress is200MPato250MPa, the stress constant is3.0088. When the stress is in the range of250MPa to300MPa, the stress index is3.943at this temperature. Creep activationenergy is26.68kJ·mol-1under the stress of250MPa. High temperature creepdeformation mechanism is dislocation glide and grain boundary sliding.The rapid sintering of elemental powders and forging technology without packwere studied systematically. In addition, as strain rate is about0.01s^-1, step forgingdeformation of30%and60%at1300°C was employed to fabricate Ti-43Al-5V-4Nb（at.%） alloy forging stock with density of98.9%. This forging stock is mainlycomposed of γ, α₂, β/B2phases and not fully diffused Nb. Test results show that thetensile strength and elongation of Ti-43Al-5V-4Nb alloy forging stock at roomtemperature are442.96MPa and0.27%, respectively. Many kinds of heat treatmentprocesses were performed on Ti-43Al-5V-4Nb alloy forging stock and different microstructures were obtained.1300°C/1h heat treatment leads to the formation ofγ/α₂fully-lamellar microstructure. The tensile strength of535MPa and elongationof0.52%in the alloy with fully-lamellar microstructure are obtained. With theincrease of test temperature, tensile strength declines and elongation increases. At700°C, tensile strength and elongation are582MPa and11.00%, respectively.更多还原