高铌TiAl合金疲劳—蠕变交互作用研究

发布时间：2018-03-31 12:29

本文选题：高铌TiAl合金　切入点：蠕变　出处：《北京科技大学》2016年博士论文

【摘要】：高铌TiAl合金以其优异的高温力学性能和较低的密度在航空、航天以及汽车发动机等领域显示了巨大的发展潜力,是Ni基高温合金潜在的替代材料。目前,高铌TiAl合金已被列为我国重点发展的航空发动机材料之一,受到了国家“973”及军工“863”项目的资助,其在成分设计、组织控制、制备成型以及加工焊接等方面取得了一系列进展,但在性能表征以及可靠性评估方面的研究还不够充分,尤其是在高温服役条件下,由疲劳和蠕变交互作用引起的损伤失效则更缺乏深入系统的研究,严重影响了该合金的进一步应用和设计开发。基于上述研究背景,本文围绕近片层组织高铌TiAl合金的高温疲劳—蠕变交互作用对其相关的高温力学性能展开了研究。具体内容包括高温拉伸及断裂韧性的研究、高温蠕变性能的研究、高温疲劳性能的研究以及高温疲劳—蠕变交互作用的研究。主要结论如下：高铌TiAl合金的高温拉伸性能和断裂韧性受显微组织和裂纹萌生及扩展行为的影响。SEM原位观察及断口观察表明,近片层组织的高铌TiAl合金在拉伸过程中裂纹主要在片层团界处萌生并且沿着片层团界扩展,相反,全片层组织在拉伸过程中裂纹主要在片层界面处萌生并且沿着片层界面扩展。由于裂纹沿晶界萌生和扩展降低了局部的应力集中,因此近片层组织的拉伸性能优于全片层组织。而由于裂纹沿晶界扩展的阻力小于裂纹沿片层界面或穿片层界面扩展的阻力,因此表现出近片层组织的断裂韧性低于全片层组织的特点。高铌TiAl合金的高温蠕变性能研究结果表明,随着温度或蠕变应力的增加,其最小蠕变速率(εmin)增加,蠕变寿命(Tr)降低。其蠕变的寿命预测公式为：logTr(h)+0.94×logεmin(%/h)=0.07SEM原位观察表明,其蠕变变形的三阶段与裂纹的萌生、扩展及相互连接相互对应。在稳态蠕变阶段主要表现为裂纹的萌生和扩展,而在加速扩展阶段则主要表现为裂纹的相互连接。微观机制分析表明,对应于不同的应力水平,其蠕变变形机制不同：低应力区为晶格扩散,中等应力区为位错滑移、高应力区为孪晶变形。高铌TiAl合金在高温疲劳变形时,应力比(R)对其疲劳寿命及变形机制有显著的影响。当0.1≤R≤0.4时,疲劳寿命(NT)受疲劳—蠕变交互作用控制,表现为极小值特征,其寿命预测公式为其中,σa为循环应力幅,σm为平均应力。当0.4≤R≤1时,疲劳寿命(Nf)由蠕变变形控制,并且随R增加N减小。其相应的寿命预测公式为：Nf=1.17×1020σm-5.46SEM原位观察表明,随着R的增加,疲劳断裂方式由R=0.1时的穿晶开裂转变为R=0.2和0.3时的穿晶和沿晶混合开裂,再到R≥0.4时的沿晶开裂。相应地,微观机制分析表明,疲劳变形机制由位错滑移和位错攀移转变为位错滑移和孪晶变形,再转变为孪晶变形。并且,加载频率对其疲劳性能也有一定的影响作用。随着加载频率(D的降低,疲劳断裂方式由f=10 Hz和1 Hz时的穿晶开裂转变为f=0.05 Hz和0.025 Hz时的沿晶和穿晶开裂；相应地,疲劳变形机制由位错滑移和位错攀移转变为孪晶变形和位错滑移。不同加载频率下的疲劳寿命公式为：Nf=118887.96(f)1.01高铌TiAl合金疲劳—蠕变交互作用研究表明,随着有效保载时间(△t/tp)的增加,其寿命(Nf)呈线性降低。其相应的寿命预测公式为：Nf=N10-Ktp/ΔtSEM原位观察表明,随着有效保载时间的增加,裂纹在片层团界面处的萌生几率明显增大,并且其裂纹扩展方式与纯疲劳和纯蠕变变形时的裂纹扩展方式显著不同,表现为混合的裂纹扩展特征。这种混合的裂纹扩展特征加速了裂纹的扩展速率,导致其寿命急剧下降。微观机制分析表明,位错滑移和孪晶变形共存是其疲劳—蠕变交互作用的典型特征。
[Abstract]:High Nb TiAl alloy with excellent mechanical properties at high temperature and low density in aviation, aerospace and automotive engine fields show a great potential for development, is the substitute of Ni based high temperature alloy potential. At present, the high Nb TiAl alloy has been listed as one of our focus on the development of aero engine materials by the national "973" and "863" military funded project, the organization control in the composition design, preparation, molding and welding processing has made a series of progress, but the study of characterization and reliability evaluation is not enough, especially the service conditions at high temperatures, caused by the action of fatigue and creep interaction the damage is more a lack of systematic research, has seriously affected the further application of the alloy design and development. Based on the above research background, this paper focuses on the high temperature nearly lamellar microstructure of high Nb TiAl alloy. High temperature fatigue creep interaction on the mechanical properties of the studied. The specific contents include research on high temperature tensile and fracture toughness, creep property research, study of high temperature fatigue properties and fatigue creep interaction. The main conclusions are as follows: the high temperature tensile properties and fracture toughness of high Nb TiAl alloy the microstructure and crack initiation and propagation behavior of the influence that.SEM in situ observation and fracture observation, the crack of high Nb TiAl alloy near lamellar structure during stretching mainly in the lamellar boundary of initiation and expand, lamellar along circles instead, lamellar microstructure under tensile deformation mainly in the lamellar interface the initiation and propagation along lamellar interface. The crack initiation and propagation along grain boundaries reduces the local stress concentration, so nearly lamellar microstructure tensile performance is better than that of FL group Fabric. Because the cracks propagate along the grain boundaries of the resistance is less than crack along lamellar interface or lamellar interface extended wear resistance, thus showing near the fracture toughness of microstructure than lamellar microstructure characteristics. The creep properties of high Nb TiAl alloy shows that with the increase of temperature and creep stress, the minimum creep rate (min) increased, the creep life (Tr) decreased. The creep life prediction formula: logTr (H) +0.94 * log e min (%/h) =0.07SEM in situ observation shows that the three stage and the crack initiation of creep deformation, expansion and mutual connection mutual corresponding in steady-state creep stage. Mainly for the initiation and propagation of cracks in the accelerated expansion stage is mainly connected to crack. The microscopic mechanism analysis showed that correspond to different levels of stress, the creep deformation mechanism is different: the low stress region for lattice diffusion, medium Stress is dislocation slip, high stress area is twin deformation. The high Nb TiAl alloy in high temperature fatigue deformation, stress ratio (R) has a significant effect on the deformation mechanism and its fatigue life. When R = 0.1 ~ 0.4, the fatigue life (NT) under the control of fatigue creep interaction. For the minimum feature, the life prediction formula for the sigma a cyclic stress amplitude, sigma m is the average stress. When R = 0.4 ~ 1, the fatigue life (Nf) controlled by creep deformation, and with the increase of R N decreases. The corresponding fatigue life prediction formula: Nf=1.17 * 1020 m-5.46SEM in situ observation showed that with the increase of R, the fatigue fracture mode by R=0.1 transgranular cracking into R=0.2 and 0.3 of transgranular and intergranular cracking mixture, and then to R more than 0.4 of the intergranular cracking. Accordingly, the micro mechanism analysis showed that the mechanism of fatigue deformation by dislocation slip and dislocation transfer to climb dislocation slip and twinning And then to the deformation, deformation twinning and loading frequency have certain effect on the fatigue performance. With the loading frequency (D decreased by f=10 Hz and fatigue fracture mode 1 Hz transgranular cracking into crystal cracking; f=0.05 Hz and 0.025 Hz in intergranular and accordingly. The mechanism of fatigue deformation by dislocation slip and dislocation climb into the fatigue life formula of deformation twinning and dislocation slip. Under different loading frequencies were as follows: Nf=118887.96 (f) on the interaction of fatigue and creep - 1.01 high Nb TiAl alloy shows that, with the effective holding time (t/tp) increased linearly with its life (Nf) reduced. The corresponding fatigue life prediction equation: Nf=N10-Ktp/ tSEM in situ observation showed that with the increase of effective holding time, crack initiation at the lamellar interface rate increases obviously, and the crack and deformation of pure fatigue and pure creep When the crack propagation of different performance for mixed crack propagation characteristics. This kind of mixed crack propagation characteristics of accelerated crack propagation rate, resulting in a sharp decline in life expectancy. The microscopic mechanism analysis showed that the dislocation slip and deformation twinning is a typical feature of the coexistence of fatigue creep interaction.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146.2

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