镍基单晶高温合金的低周疲劳裂纹萌生行为

Low-Cycle Fatigue Crack Initiation Behavior of Nickel-Based Single Crystal Superalloy

摘要:

对镍基单晶高温合金在530 ℃的低周疲劳裂纹萌生行为进行研究。结果表明：在530 ℃、应变比0.05时，疲劳裂纹的萌生行为与最大应变的变化关系密切。在最大应变为2.0%时，疲劳裂纹在试样表面的驻留滑移带位置萌生，且萌生平面位于｛111｝滑移面，萌生位置未见缺陷。当最大应变低于1.6%时，裂纹萌生于试样亚表面或内部的铸造疏松位置，且铸造疏松位于垂直轴向力的｛100｝滑移面。裂纹沿｛100｝滑移面萌生后经小阶段扩展后再沿不同｛111｝滑移面进行扩展。随最大应变减小，裂纹萌生位置逐渐由表面向内部转变。裂纹萌生区存在沿断口表面向内部延伸的二次裂纹，且二次裂纹附近存在明显应力集中。在裂纹萌生区的断口表面附近位错密度较高，大量位错切入γ'相。断口表面存在50~100 nm的氧化层，Ni、Al、Cr和Co元素存在向表面氧化层的偏析行为。

Abstract:

Low-cycle fatigue crack initiation behavior of nickel-based single crystal superalloy at 530 °C was investigated. Results show that the behavior of crack initiation is closely related to the maximum strain. When the maximum strain is 2.0%, the fatigue crack is originated at the position of persistent slip bands on the surface of specimen, which is located on the {111} slip plane. No defects are observed at the crack initiation position. When the maximum strain is lower than 1.6%, the cracks are initiated at the casting defects on sub-surface or at interior of the specimen. The casting defects are located on the {100} slip plane vertical to the axial force. The crack is initiated along the {100} slip plane and then expanded along different {111} slip planes after a short stage of expansion. As the maximum strain decreases, the position of crack initiation gradually changes from the surface to the interior. Moreover, the secondary cracks extending inward along the fracture surface appear in the crack initiation area, and there is obvious stress concentration near the secondary cracks. The dislocation density is high near the fracture surface in the crack initiation zone, where a lot of dislocations cutting into the γ' phase exist. An oxide layer of 50?100 nm is presented on the fracture surface, and Ni, Al, Cr and Co elements are mainly segregated into the oxide layer of the surface.