| 铼对镍基单晶高温合金低周疲劳性能的影响 | |
| 李芹芹 | |
| Subtype | 硕士 |
| Thesis Advisor | 金涛 ; 张哲峰 |
| 2012 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 北京 |
| Degree Discipline | 材料学 |
| Keyword | 铼 镍基单晶高温合金 低周疲劳 Re Single-crystal Ni-based Superalloys Low-cycle Fatigue |
| Abstract | " 镍基单晶高温合金具有优良的高温性能，广泛用于先进航空发动机和燃气轮机叶片。为了进一步提高单晶高温合金的服役温度，合金中加入了大量的W、Mo、Re等难熔元素进行强化，并且人们发现Re元素对合金高温性能的提高具有非常重要的作用。为了揭示Re的作用人们已经进行了一些研究，但Re的作用机理尤其对低周疲劳性能的影响机制尚不清楚。 为此，本文选取含Re (3wt.%) 与不含Re两种[001]取向单晶高温合金作为模型材料，首先从两种合金的相分布（包括相尺寸和分布均匀性）以及元素分布两方面研究了Re对单晶合金在900°C下应变控制疲劳性能的影响；随后从晶体变形形貌、微观位错结构方面研究了不同温度下两种高温合金的疲劳变形规律，以此探讨Re在不同温度下对单晶高温合金低周疲劳性能的作用机理。 Re对单晶合金900°C下低周疲劳性能的影响总结为以下几个方面。1) Re的加入强烈抑制g¢相的粗化并使其保持均匀分布，从而减小g基体相，对位错的运动产生不利影响；2) Re的加入增加了Cr、Co、Mo等元素向g相偏聚的趋势，Al、Ti、Ta等元素则向g¢相偏聚，基体相中难熔元素的增加使得高温加载时g/g¢两相界面的错配位错数量显著增加，进而阻碍位错在基体相中的运动；3) Re的添加增大了单晶高温合金中原子之间的结合能。这几方面的共同作用使得镍基单晶高温合金在900°C下的循环应力和疲劳寿命均得以提高。 两种合金在不同温度下进行低周疲劳实验后的变形形貌表明：在室温与250°C下，合金表面出现大量滑移带，侵入和挤出现象明显并且滑移条带笔直切割g¢相，g¢相基本没有强化作用，裂纹沿着滑移带萌生和扩展；温度逐渐升高到500°C和750°C，滑移带中侵入和挤出程度减弱，变形逐渐均匀，裂纹仍然沿着滑移带扩展；当温度升高到900°C，合金变形更加均匀，g¢相此时起到重要的强化作用，同时此温度下氧化加剧了疲劳损伤的程度。 两种合金不同温度下疲劳变形后的微观位错结构观察表明：室温下，层错能起着重要的作用，位错运动以平面滑移为主；随着温度的升高，层错能逐渐升高，位错运动开始进行交滑移和攀移，g¢相的强化作用逐渐显现，g/g¢两相界面错配度对合金的低周疲劳性能起着决定作用。 通过对两种合金的对比发现，Re提高了单晶高温合金在25°C、500°C、900°C时的循环应力和抵抗塑性变形的能力，延长了单晶高温合金在各个温度下的低周疲劳寿命；Re的加入使合金在室温和500°C变形时形成较为细密均匀的主滑移带，并且启动了多组次滑移带，多组滑移带相互作用，提高了高温合金的循环应力；微观位错结构分析表明Re的加入进一步降低了单晶高温合金在室温下的层错能值，使得位错运动更多地沿着特定滑移面进行，进而提高了高温合金室温下的低周疲劳性能。 " |
| Other Abstract | " Ni-based single-crystal superalloys have been widely used as turbine aerofoil materials in jet engines and industrial gas turbines for their superior high-temperature mechanical properties. Further improvement of the high-temperature performance can be obtained by adding refractory elements like Ta, W, Mo and Re, in which the element Re is found to play a special role. Some studies have been carried out to understand the effect of Re. Despite extensive investigations, it is still not clear how Re improves the high-temperature performances of the Ni-based superalloys, especially low-cycle fatigue (LCF) behaviors. Therefore, in this study, a series of strain-controlled LCF tests at 900°C were performed on Re-containing (3wt.%) and Re-free [001] Ni-based single-crystal superalloys in order to clarify the key role of Re in improving the high-temperature LCF performance by comparing the differences of the two alloys in the size and uniformity of g¢ phase and element distribution. Then, the deformation mechanisms of LCF behaviors in the two alloys at different temperatures were investigated by observation of surface deformation features and dislocation configurations in order to investigate the effect of Re on the LCF properties of single-crystal superalloys at different temperatures. The effects of Re on LCF behaviors of single-crystal superalloys mainly focus on the following aspects. First, the addition of Re strongly retards the coarsening process of g¢ phase and maintains the phase structure uniformity, and the narrower g channels will not facilitate dislocation gliding. Secondly, the Re addition can promote the segregation of Co, Cr, Mo into g matrix and Al, Ta, Ti into g¢ phase. More refractory elements partition into g matrix phase, which usually increases the lattice mismatch of the superalloy, further results in more dense dislocation networks during LCF tests at high temperature and eventually impedes any individual dislocation to move in the g matrix freely. Thirdly, Re increases the bonding energy of atoms in single-crystal superalloys. All these factors improve the LCF properties of single-crystal superalloys at 900°C under different strain amplitudes. The surface morphologies of fatigued single-crystal superalloys at different temperatures can be summaried as follows. At room temperature and 250°C, crystal surface exhibits lots of slip bands and seriously intrusive and extrusive states. The corresponding slip bands cut into g¢ phase straightly, indicating that then g¢ phase cann’t show strengthening effect; some cracks initiated and propagated along the slip bands. At 500°C and 750°C, the intrusion and extrusion on the crystal surfaces are not obvious and the deformation gradually becomes homogeneous; some cracks also nucleated and propagated along the slip bands. At 900°C, the slip character becomes much more homogeneous and the strengthening of g¢ phase is significant and the fatigue damage is accelerated because of oxidation. The fatigued dislocation structures of Ni-based single-crystal superalloys indicate that at room temperature, stacking fault energy (SFE) plays an important role and planar slip is the main plastic deformation mechanism. As the testing temperature increases, the SFE value increases; cross slip and climbing behaviors of dislocations in the matrix gradually become more obvious; meanwhile the interfacial mismatch of g/g¢ plays a key role in the LCF properties. The following conclusions can be made by comparing the LCF behaviors of the two alloys at different temperatures. Re improves the cyclic stress and increases the resistance to dislocation movement in single-crystal superalloys at 25°C, 500°C and 900°C; Re increases the LCF life of single-crystal superalloys at all testing temperatures. With the addition of Re, Ni-based single-crystal superalloys form more dense and uniformly distributed slip bands under cyclic loading at room temperature and 500°C. Meanwhile more secondary slip systems actuate and show better plastic deformation; these slip bands coordinate deformation and further improve the cyclic stress of the superalloy. The corresponding dislocation microstructures show that Re can effectively reduce the SFE value of Ni-based single-crystal superalloys, which will be conducive to the dislocation movement along the special slip plane, and eventually lead to improving the LCF properties of the superalloys. " |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/64521 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 李芹芹. 铼对镍基单晶高温合金低周疲劳性能的影响[D]. 北京. 中国科学院金属研究所,2012. |
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