Performance of Alloy 709 under creep-fatigue at various dwell times
Performance of Alloy 709 under creep-fatigue at various dwell times
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DOI：
10.1016/j.msea.2019.138028
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发表时间：
2019-07-22
期刊：
影响因子：
6.4
通讯作者：
Rabiei, Afsaneh
中科院分区：
文献类型：
--
作者：
Lall, Amrita;Sarkar, Siddhartha;Rabiei, Afsaneh
本文报道了709合金在750 ℃下的蠕变-疲劳行为的综合实验评价。709合金是一种20 Cr-25 Ni奥氏体不锈钢，具有高温蠕变强度和耐腐蚀性，可用于核电站的结构部件。采用原位加热加载和扫描电子显微镜（SEM）及电子背散射衍射（EBSD）探测器进行了蠕变疲劳裂纹扩展（CFCG）试验。为了研究合金709在750摄氏度下在真空中具有不同停留时间的“实时”CFCG行为，制备扁平狗骨样品。通过增加起始缺口，在室温下进行高频疲劳循环，引入预裂纹。在加载和加热之前，使用EBSD绘制裂纹尖端之前的整个区域。这些地图被用来生成一组重合的网站晶格（CSL）的裂纹尖端的前面的区域的边界图。在完成EBSD和CSL映射后，在SEM中进行样品的加热和加载。在实验过程中，使用SEM成像在样品表面上监测裂纹生长，并将数据转移到CSL图中，以突出显示相对于样品中的晶界和沉淀物排列的裂纹路径。一些样品在加热和加载前沿着进行EBSD-CSL绘图，并在加热和加载后进行透射电子显微镜（TEM）成像。比较裂纹扩展前后的CSL图提供了有关裂纹路径和裂纹扩展模式对微观结构（主要是晶界特征）和停留时间的依赖性的更多详细信息。裂纹扩展后的微观结构的TEM分析，以验证原位加热和加载SEM数据的结果。实时监测的微观结构现象，如空洞成核，晶界空化，滑移激活，和电阻的孪晶界开裂CFCG试验过程中，揭示了一个新的光的裂纹扩展机制。结果表明，在较低的停留时间，裂纹主要以穿晶方式扩展，与滑移线的帮助。在较高的停留时间，晶间空穴占主导地位的裂纹扩展。然而，由于超过50%的晶界是共格孪晶界，其是抵抗空穴的低能量边界，当达到这种边界时，裂纹生长被延迟，因此孪晶界在高温下赋予合金709中的裂纹扩展一定的阻力。
A comprehensive experimental evaluation of the creep-fatigue behavior of Alloy 709 at 750 degrees C is reported in this study. Alloy 709 is a 20Cr-25Ni austenitic stainless steel, with high temperature creep strength and corrosion resistance, which can potentially be used in structural components of nuclear power plants. Creep-fatigue crack growth (CFCG) experiments were conducted using an in-situ heating-loading and Scanning Electron Microscope (SEM) equipped with Electron Backscatter Diffraction (EBSD) detector. To study the "real-time" CFCG behavior of Alloy 709 at 750 degrees C with varying dwell times in vacuum, flat dog bone samples were prepared. A starter notch was added, and a pre-crack was introduced by high frequency fatigue cycles at room temperature. Prior to loading and heating the entire area ahead of the crack tip was mapped using EBSD. These maps were utilized to generate a set of Coincident Site Lattice (CSL) boundary maps from the area ahead of the crack tip. Upon completion of the EBSD and CSL mapping, the heating and loading of samples took place in the SEM. During the experiment, crack growth was monitored on the surface of the sample using SEM imaging and data was transferred over to the CSL maps, to highlight the crack path with respect to the grain boundary and precipitations arrangement in the sample. Some samples went through EBSD-CSL mapping before heating and loading along with Transmission Electron Microscopy (TEM) imaging post heating and loading. Comparing the CSL maps before and after crack growth provided additional details about the dependence of crack path and crack growth mode on microstructure, primarily grain boundary character, and dwell time. TEM analysis of the microstructure after the crack growth was employed to validate the findings of the in-situ heating and loading SEM data. Real-time monitoring of microstructural phenomena, such as void nucleation, grain boundary cavitation, slip activation, and resistance of twin boundaries to cracking during CFCG tests, sheds a new light on the crack growth mechanism. The results indicated that at lower dwell times, the crack mainly propagates in a transgranular fashion, with the aid of slip lines. At higher dwell time, intergranular cavitation dominates the crack growth. However, as more than 50% of grain boundaries are coherent twin boundaries, which are low energy boundaries resistant to cavitation, crack growth is delayed when reaching such boundaries and hence twin boundaries impart some resistance to crack propagation in Alloy 709 at high temperatures.