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A piecewise constitutive model, microstructure and fracture mechanism of a nickel-based superalloy 750H during high-temperature tensile deformation

Wang, Kaimeng, Jing, Hongyang, Xu, Lianyong, Han, Yongdian, Zhao, Lei, Xiao, Bo, Yang, Shangqing
Journal of materials science 2019 v.54 no.13 pp. 9775-9796
deformation, microstructure, models, superalloys, temperature, texture
In order to understand the high-temperature deformation behavior of a nickel-based superalloy, a range of tensile tests were carried out at 720, 750, and 780 °C with strain rates ranging from 5 × 10⁻⁵ to 5 × 10⁻³ s⁻¹. A piecewise constitutive model was applied to describe the work hardening-dynamic recovery and dynamic flow softening behaviors. The predicted flow stresses have a good agreement with the experimental results. Microstructures in the fracture frontier of the ruptured specimens were analyzed to further understand the fracture mechanism. Twinning and dislocation structures were surveyed at the tested conditions. Twin structure decreased as temperature increased. These two precipitates were characterized: M₂₃C₆ carbide located in the grain boundary and spherical γ′ phase in the matrix. Precipitates, twin and dislocation structures are the dominant strengthening mechanism of the superalloy during high-temperature deformation. Orientations < 111 >//RD and < 001 >//RD were detected as the main texture structure. Cavities formed near the precipitates and triple grain boundary. On the basis of fracture surface observations, the 750H superalloy shows both intergranular and transgranular fracture mode in the tested conditions. The dimples became small and shallow as the strain rate increased.