We have investigated the coarsening mechanism of intergranular Cr-rich M2B borides after creep and annealing at 850C for approximately 3000 hours in a polycrystalline nickel-based superalloy. Borides were found to be coarser after creep, with measured thicknesses in the range of 800-1100nm, compared to borides annealed in the absence of external applied load (400-600nm). The borides had a thickness of 100-200nm before exposure at 850C. Transmission electron microscopy revealed that coarsened borides have either the tetragonal I4/mcm structure, or the orthorhombic Fddd, with those two structures coexisting in a single particle. The presence of a very high density of planar faults is systematically observed within the coarsened borides. The faults were correlated with chemical fluctuations of B and Cr, revealed by atom probe tomography. Our results allow us to suggest that borides coarsen by an epitaxy-like mechanism. In addition, partitioning of Ni and Co was observed at dislocations within the borides after creep providing insights into the deformation of borides. Consequences of coarsened intergranular borides on the creep performance of polycrystalline superalloys are discussed.
Understanding creep of a single-crystalline Co-Al-W-Ta superalloy by studying the deformation mechanism, segregation tendency and stacking fault energy
A Novel Strategy to Strengthen Directionally Solidified Superalloy Through Grain Boundary Simplified Design
Institute of Superalloys Science and Technology, School of Materials Science and Engineering, Zhejiang University（浙江大学材料科学与工程学院超合金科学与技术研究所）；State Key Laboratory of Silicon and Advanced Semiconductor Materials, School of Materials Science and Engineering, Zhejiang University（浙江大学材料科学与工程学院硅与先进半导体材料国家重点实验室）
High-Ti induced planar-fault transformation toward superlattice extrinsic stacking faults and microtwins in crept CoNi-based superalloys
Helmholtz-Zentrum Hereon（赫尔姆霍茨-海雷恩中心）；Tsinghua University（清华大学）；University of Stuttgart（斯图加特大学）；University of Twente（特文特大学）；Southeast University（东南大学）；Central South university（中南大学）
