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Abstract Ni‐based single crystal (SX) superalloys are important high‐temperature materials used for manufacturing turbine blades in aero‐engines. During service under combinational impacts of temperature and stress, the SX superalloy may reach its life due to plastic deformation, which normally accompanies time‐dependent microstructural degradation. To reveal this dynamically mechanical response, tensile tests at 1050°C are carried out to record stress‐strain curves at five stain rates as well as creep curves at four applied stresses. Deformed microstructures and defects have been analyzed to understand mechanical behaviors and the underlying mechanism by using advanced scanning electron and scanning transmission electron microscopes. Results show that the deformation mode of the alloy strongly depends on the strain rates/applied stresses under mechanical loading. The dislocation density inside the γ phase is extremely low at all tests, indicating that the γ phase is relatively weak and ready to flow at this temperature even at a very fast strain rate. The deformation behavior of the γ′ phase is much complicated. At fast strain rates or high applied stresses, the dislocation density in the γ′ phase is very high, contributing to high‐stress requirements to deform the material. At slow strain rates or low applied stresses, rafting microstructures develop and the deformation mode becomes directional coarsening/diffusion‐dominated. Our results demonstrate a comprehensive understanding of the deformation mechanism of Ni‐based SX superalloys, which may provide lifetime prediction of the mechanical failure, as well as the database for superalloy applications in mechanical systems.

To investigate influences of Ta content on microstructural stability during thermal exposure at 900 °C (up to 3000 h) and creep properties at 980 °C/163 MPa in Ni-based SX superalloy, alloys with 7Ta, 8Ta and 9Ta (wt.%) were studied utilizing DSC, SEM, TEM, HR-XRD and APT. The results shows that Ta addition enhanced the partitioning behavior of alloying elements. Besides, lattice misfit was controlled by both alloy composition and temperature. Ta addition increased absolute misfit both room temperature and 980 °C. The difference was that the γ/γ′ lattice misfit changed from positive to negative at room temperature. The γ′ coarsening indicated that Ta addition reduced the effective diffusion coefficient and exacerbated W segregation at γ/γ′ interface, which was beneficial to improve the microstructural stability. Merging of several adjacent γ′ precipitates in preferred direction was observed to be delayed with increasing Ta content, which occurred at 500 h for 7Ta alloy, 800 h for 8Ta alloy and 1000 h for 9Ta alloy, respectively. Ta addition affected the diffusion kinetics and greatly prolonged the steady creep duration, and then resulted in longer creep rupture life. It was discussed based on the γ matrix channels, γ′ raft thickness, the perfection degree of γ′ rafts, residual γ′ precipitates, interfacial dislocation networks and solid solution strengthening. [ABSTRACT FROM AUTHOR]

The accelerated creep failure of Ni-based single crystal superalloy by hot corrosion is confirmed by creep tests at 900 °C/550 MPa–400MPa in air and salt environments. The analysis of creep curve, microstructure and fracture type shows that there are obvious differences in hot corrosion-creep behavior under high, middle and low creep stress conditions, indicating that there is the stress dependence between the influence of hot corrosion on the creep properties of the alloy. Hot corrosion accelerates creep failure of Ni-based single crystal superalloy by decreasing the deformation resistance, reducing the effective stress area, and affecting the creep fracture mode. [ABSTRACT FROM AUTHOR]

Abstract The morphologies and distributions of dislocation networks in Ni-based single crystal superalloys crept at 1100 °C and a large stress range from 120 to 174 MPa were investigated. The γ/γ′ lattice misfit at 1100 °C was calculated using the Vegard's law, where the γ- and γ′-compositions were quantified by 3D Atom Probe Tomography. The results indicated that the distribution of dislocation network showed a strong stress dependence. Two parts of dislocation networks, namely close- and sparse-spaced configurations, were observed. Further, the dislocation network spacings' analysis revealed that the close-spaced network (about 100 nm) mainly depended on the γ/γ′ lattice misfit (−0.2910 ± 0.0564)%, which was almost independent of stress. However, the sparse-spaced network (above 120 nm) might be attributed to the larger creep strain flows under higher stresses, which presented a strong stress dependence. It was demonstrated that the γ/γ′ lattice misfit played a key role in elevated temperature and low stress creep. [ABSTRACT FROM AUTHOR]

During the manufacturing process of Ni-based single crystal superalloy components, unexpected plastic deformation can be introduced in the material at different stages. A room-temperature plastic deformation was introduced to AM1 superalloy in between solution and aging heat treatments to mimic one case of unintentional plastic deformation. Our previous study revealed that the prior plastic deformation causes faster precipitation coarsening in the vicinity of the slip bands and decreases creep life, especially at temperatures above 950 °C. In this study, microstructures of creep ruptured and creep interrupted specimens were further analyzed to understand the origin of the early creep failure induced by plastic deformation. The results show that the plastic deformation enhances the nucleation and growth of deformation-pores during aging treatments. During creep deformation, the microstructure coarsened bands act as creep damage accumulating path for further void growth. Local stress concentration around coarsened voids activates recrystallization at the very last stage of the creep failure. [ABSTRACT FROM AUTHOR]