|Thesis Advisor||金涛 ; 张哲峰|
|Place of Conferral||北京|
|Keyword||铼 镍基单晶高温合金 低周疲劳 Re Single-crystal Ni-based Superalloys Low-cycle Fatigue|
为此，本文选取含Re (3wt.%) 与不含Re两种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。
|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  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.
|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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