The Role of Oxidized Carbides on Thermal-Mechanical Performance of Polycrystalline Superalloys
The Role of Oxidized Carbides on Thermal-Mechanical Performance of Polycrystalline Superalloys
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DOI：
10.1007/s11661-018-4709-x
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发表时间：
2018-09-01
影响因子：
2.8
通讯作者：
Gault, Baptiste
中科院分区：
文献类型：
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作者：
Kontis, Paraskevas;Li, Zhuangming;Gault, Baptiste
在850℃热机械疲劳(TMF)条件下，研究了氧化MC碳化物在多晶高温合金中的主要裂纹萌生位置。将TMF测试样品的微观结构观察结果与在没有任何外部载荷的情况下在850摄氏度的空气中等温暴露30小时的大块样品的结果进行了比较。发现碳化物在850摄氏度暴露30小时后迅速氧化，导致与氧化产物相对应的表面喷发，从那里开始微裂纹。受控电子沟道对比成像显示，由于多孔性氧化碳化物的体积膨胀而导致的塑性变形导致了相邻基质中高密度的位错。高的位错密度促进了γ‘相的溶解动力学，铬和钴通过管状扩散沿位错沿位错的偏聚和扩散，从而形成软质再结晶晶粒。原子探针断层扫描显示，再结晶颗粒和邻近未变形的伽马基质之间存在显著的成分差异。对于TMF测试的合金也有类似的观察结果。我们的观察为氧化MC碳化物在TMF下对多晶高温合金的裂纹萌生性能的真正有害作用提供了新的见解。
Oxidized MC carbides which act as main crack initiation sites in a polycrystalline superalloy under thermal-mechanical fatigue (TMF) conditions at 850 degrees C were studied. Microstructural observations in the TMF tested specimens were compared to findings from bulk samples exposed isothermally in air at 850 degrees C for 30 hours in the absence of any external applied load. Carbides were found to oxidize rapidly after exposure at 850 degrees C for 30 hours resulting in surface eruptions corresponding to oxidation products, from where micro-cracks initiated. Plastic deformation due to volume expansion of the often porous oxidized carbides led to high dislocation densities in the adjacent matrix as revealed by controlled electron channeling contrast imaging. The high dislocation density facilitated the dissolution kinetics of gamma' precipitates by segregation and diffusion of chromium and cobalt along the dislocations via pipe diffusion, resulting in the formation of soft recrystallized grains. Atom probe tomography revealed substantial compositional differences between the recrystallized grains and the adjacent undeformed gamma matrix. Similar observations were made for the TMF tested alloy. Our observations provide new insights into the true detrimental role of oxidized MC carbides on the crack initiation performance of polycrystalline superalloys under TMF.