Performance Comparison Between Isothermal Hot Corrosion And In Situ Cyclic Hot Corrosion of Nickel-Based Superalloys

Although a lot of work has been done to understand both major mechanisms of hot corrosion, namely type I (high-temperature hot corrosion) and type II (low temperature hot corrosion), there is very little information available on more representative cyclic performance in these regimes. This work addresses this by assessing the performance of isothermal (type I and type II) hot corrosion tests against combined (short and long) cyclic corrosion tests. Single-crystal alloy PWA 1484 and directionally solidified alloy MAR-M247 were assessed in all test regimes. Pre- and post-exposure dimensional metrology was used to quantify the corrosion damage and characterised using SEM/EDX. This paper highlights that the results of short cycle test conditions are more damaging compared to long cycle and standard isothermal type I and II test conditions. The cast nickel-based alloy MAR-M247 was found to be a better performer compared to PWA 1484 single-crystal alloy.

Effect of Stress on Creep Behavior of Single Crystal Alloy IC6SX at 980°C

Ni3Al-based single crystal alloy IC6SX was prepared by seed crystal method. The effect of different stress conditions on creep behavior of this alloy at 980°C was investigated. The results showed that the creep life of this alloy at 980°C decreased significantly with the increase of stress. When the stress increased from 180 MPa to 230 MPa, the creep life dropped from 245.5 h to 69.3 h, and the steady-state creep rate increased slightly but not significantly. Meanwhile, the morphology of γ ′ phase and dislocation after creep were studied. The results showed that with the increase of stress, the density of dislocations in the γ ′ phase increased gradually, the strength of this alloy decreased gradually, so the creep life decreased significantly. The Y-NiMo phase resolved from the γ phase decreased gradually as the creep life decreased. The creep experiment of the alloy was carried out at 980°C. Due to the higher temperature, the diffusion of atoms in this alloy became faster. Deformation was not only caused by the slippage of dislocations in the crystal but also by the diffusion of atoms. Therefore, the creep mechanism of single crystal alloy IC6SX at this temperature is a mixed mechanism of dislocation glide and diffusion.

Study on the Creep Behavior of a Ni3Al-Based Single Crystal Alloy at 850°C/450 MPa

The creep behaviors of Ni3Al-based single crystal alloy IC6SX with [001] and [111] orientations under the condition of 850°C/450 MPa were investigated. The effect of crystal orientation on the creep lives, fracture morphology, fracture mechanism, and dislocation evolution of the alloys with different orientations was analyzed systematically. The results showed that the creep lives of the alloy were closely related to the crystal orientation under the condition of 850°C/450 MPa. The creep lives of the single crystal alloys with [001] and [111] orientations were 56.3 h and 126.9 h, respectively. Moreover, the fracture morphologies of the two alloys with [001] and [111] orientations were different. The results showed that some holes formed at the fracture surface of the alloy with [111] rather than [001] orientation. Furthermore, the surface near the fracture of the two alloys with [001] and [111] orientations was serrated. Therefore, the fracture mechanism of the single crystal alloys with [001] and [111] orientations was ductile fracture. In addition, a large number of dislocations cut into the γ ′ phase. Therefore, the cutting mechanism of dislocations in the alloys with [001] and [111] orientations was the creep deformation mechanism.