高铌TiAl合金高温疲劳性能原位研究

发布时间：2018-04-01 03:16

本文选题：高铌TiAl合金　切入点：疲劳　出处：《北京科技大学》2015年博士论文

【摘要】：具有高铌含量的TiAl合金,以其优异的高温力学性能和较低的密度被视为航空航天及汽车发动机等领域具有极大应用前景的高温结构材料。经过多年的研究,高铌钛铝合金的制备和加工技术,以及显微组织和常规力学性能的控制技术已经基本被掌握,但对于其高温疲劳性能以及安全寿命设计方面的研究还比较欠缺。本文利用SEM原位观察方法研究了高铌TiAl合金的高温疲劳性能中的一些关键问题,主要包括高铌TiAl合金的循环硬化/软化特征及其微观机制；疲劳裂纹萌生及扩展规律：显微组织和微观缺陷对其疲劳裂纹扩展速率及疲劳寿命的影响；疲劳寿命预测等。主要的结论与新的成果如下：近片层组织高铌TiAl合金在750℃时的拉仲过程中表现出一定塑性和应变硬化特征,拉伸过程中裂纹的扩展、片层的扭转以及位错的滑移是合金高温塑性的主要来源,而拉伸过程中发生的位错钉扎、缠结与塞积是合金高温拉伸过程中表现出一定的硬化特征的主要原因。透射分析表明合金在750℃时拉伸变形的主要机制是位错的滑移。近片层组织高铌TiAl合金在750℃循环变形时显示出明显的循环硬化特征；在循环变形过程中高铌TiAl合金呈现出先硬化后稳定的特征。结合透射观察结果分析得出,高铌TiAl合金在高温循环变形中先以产生孪晶的方式进行变形,而后以位错攀移的形式进行变形。由于循环初期出现的变形孪晶与位错发生交互作用,从而导致合金在循环初期发生循环硬化。 750℃下近片层组织高铌TiAl合金的疲劳裂纹萌生表现出多裂纹萌生特征。依据裂纹尖端微观组织的不同,疲劳裂纹的扩展过程呈现出两或三个不同扩展阶段：当疲劳裂纹平行于片层条带,扩展过程依次为快速扩展、稳定扩展和加速扩展三个阶段；而当疲劳裂纹垂直于片层条带,扩展过程则依次呈现为稳定扩展和加速扩展两个阶段。高铌TiAl合金在高温下具有良好的疲劳裂纹扩展抗力。对十高铌TiAl合金,高温使合金的片层团界面及片层间界而的强度降低,使得合金在高温下的疲劳过程相比室温条件下有更多的疲劳裂纹在片层团界面与片层间界而萌生,于是高温下也就相应地有更多的裂纹分支、偏转及桥接现象发生；同时,也正是由于高温下有更多的裂纹分支、偏转及桥接发生,使得高铌TiAl合金在高温下的疲劳裂纹扩展抗力远远高于室温条件下的。高温下全片层组织高铌TiAl合金比近片层组织高铌TiAl合金具有更高的疲劳裂纹萌生抗力和断裂韧性。高温下,全片层组织高铌TiAl合金疲劳断裂以沿片层及穿片层为主；近片层组织高铌TiAl合金疲劳断裂主要集中在γ晶粒处,较多的裂纹沿Y晶粒边界萌生和扩展,随着裂纹尺寸的增大,裂纹以沿片层、穿片层、沿γ晶及穿γ晶的混合方式进行扩展。借助SEM原位观察方法对高铌TiAl合金的疲劳裂纹扩展门槛值进行了测定。经测定,近片层组织Ti-45Al-8Nb-0.2W-0.2B-0.1Y合金在应力比P＝0.1的条件下,室温与750℃下的疲劳裂纹扩展门槛值分别为12.89MPa-m1/2与8.69MPa-m1/2。高铌TiAl合金在高温下的疲劳裂纹萌生寿命Ni与疲劳总寿命Nf的比值(N/Nf)是恒定的。对于带缺口试样,其疲劳裂纹萌生寿命在整个疲劳断裂寿命中所占的比例高达95%；对于无缺口试样,疲劳裂纹萌生寿命占整个疲劳断裂寿命的比例为43%。根据Lemaitre的损伤力学模型提出高铌TiAl合金疲劳裂纹萌生寿命和断裂寿命预测公式,近片层组织高铌TiAl合金在750℃下的疲劳裂纹萌生寿命和断裂寿命预测公式分别为和。
[Abstract]:TiAl alloy with high Nb content, high temperature structural material has great application prospect because of its excellent mechanical properties at high temperature and low density is regarded as the aerospace and automobile engine parts. After years of research, high Nb Aluminum Alloy preparation and processing technology, and control technology and mechanical properties microstructure has been basically mastered, but for the study of high temperature fatigue performance and safe life design is still relatively lacking. Some key problems of high temperature fatigue properties by SEM in situ observation method of high Nb TiAl alloy was studied. The cyclic hardening / softening including high Nb TiAl alloy and its microscopic mechanism and propagation characteristics; fatigue crack initiation: effect of microstructure and micro defect propagation rate and the fatigue life of fatigue crack; fatigue life prediction. The main conclusions and new The results are as follows:
Show some plasticity and strain hardening characteristics of nearly lamellar microstructure of high Nb TiAl alloy at 750 DEG C when the tensile process of crack propagation during tensile test, lamellar torsion and dislocation slip is the main source of high temperature plasticity, and the dislocation occurred during stretching pinning, entanglement and plug the main reason is to show some characteristics of the hardening tensile process. TEM analysis showed that the main mechanism of tensile deformation in the alloy at 750 DEG is dislocation.
Nearly lamellar microstructure of high Nb TiAl alloy exhibits cyclic hardening characteristic at 750 DEG C during cyclic deformation; cyclic deformation in high Nb TiAl alloy in the process of showing the first hardening characteristics of stability. After combining the transmission observation results obtained, the high Nb TiAl alloy in high temperature cyclic deformation to twinning mode deformation then, the dislocation climb in the form of deformation. As the deformation twinning and dislocation loops emerged in the initial interaction occurs, resulting in alloy cyclic hardening occurred in the initial cycles.
Initiation showed fatigue crack initiation characteristics of more than 750 DEG C nearly lamellar microstructure of high Nb TiAl alloy. On the basis of crack tip microstructure, fatigue crack propagation process shows two or three different growth stages: when the fatigue crack parallel to the sheet strip, followed by the rapid expansion of the expansion process, stable growth and accelerate the expansion of three stages; and when the fatigue crack perpendicular to the lamellar strip, the expansion process are presented for stable growth and accelerate the expansion of the two stages.
High Nb TiAl alloy has good fatigue crack growth resistance at high temperature. The ten high Nb TiAl alloy, high temperature alloy to reduce lamellar interface and lamellar boundaries and the strength of the alloy at high temperature compared with the fatigue process under the condition of room temperature fatigue crack more in lamellar interface and tablets layer boundaries and the initiation, so high temperature corresponding to the crack branch more, deflection and bridging phenomenon; at the same time, it is because there are more high temperature crack branching, deflection and bridging occurs, the high Nb TiAl alloy fatigue crack growth resistance at high temperature is much higher than that at room temperature..
High temperature lamellar microstructure of high Nb TiAl alloy has the fracture toughness and fatigue crack initiation resistance higher than the nearly lamellar microstructure of high Nb TiAl alloy at high temperature, lamellar microstructure of high Nb TiAl alloy in fatigue fracture along the layers and wear layers; nearly lamellar microstructure of high Nb TiAl alloy fatigue fracture mainly in the austenite grain, more cracks along the grain boundary of Y initiation and propagation, with the increase of crack size, crack along the sheet layer, wear layer, extending along the Y and Y in mixed mode.
With the help of the fatigue crack of high Nb TiAl alloy SEM method in situ observation of growth threshold was determined. After the test, nearly lamellar microstructure of Ti-45Al-8Nb-0.2W-0.2B-0.1Y alloy in the stress ratio of P = 0.1 under the condition of fatigue crack growth at room temperature and 750 DEG C and the threshold value of 12.89MPa-m1/2 and 8.69MPa-m1/2. respectively.
The ratio of the initiation life of Ni and Nf of the total fatigue life of fatigue crack under high temperature high Nb TiAl alloy (N/Nf) is constant. The band notched specimens, the initiation life in the whole fatigue life of up to 95% the proportion of the fatigue crack; for unnotched specimens, fatigue crack initiation life total fatigue fracture the life of the ratio of 43%.
According to the damage mechanics model of Lemaitre, the prediction formula for fatigue crack initiation life and fracture life of high Nb TiAl alloy is put forward. The prediction formula of fatigue crack initiation life and fracture life of near niobium TiAl alloy at 750 temperature is respectively.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.23

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