The precipitation behavior of a novel Ni-Fe-based superalloy developed for advanced ultra-supercritical (A-USC) coal-fired power plant applications during high-temperature aging treatment was investigated. The results showed that the major precipitates in the novel alloy were randomly distributed MC carbides, M₂₃C₆ carbides at grain boundaries, and the γ′-Ni₃ (Al, Ti) phase in grain interiors after aging. MC remained relatively stable during both short-term and long-term aging. M₂₃C₆ quickly precipitated and exhibited a discrete distribution at grain boundaries during short-term aging, and partly developed into continuous films during long-term aging. After uniform precipitation, the shape of γ′ remained spherical, and the size kept increasing with aging time according to the Lifshitz–Slyozov–Wagner (LSW) model. The hardness of the novel alloy was mainly associated with the precipitation behavior of γ′; as γ′ gradually precipitated, the hardness steadily increased; after complete precipitation, as the size of γ′ increased, the hardness first increased and then decreased, reaching the peak hardness when the average radius of γ′ achieved the critical size. In addition, the novel alloy exhibited abnormal coarsening behavior at grain boundaries during both short-term and long-term aging. The coarsened grain boundaries were actually precipitate-free zones (PFZs) and the coarsened and elongated rod-like particles inside were identified as γ′ precipitates. The mechanism of strain-induced grain boundary migration and the discontinuous coarsening reaction is proposed for the formation of PFZs. Furthermore, PFZs were considered to be potential crack sources during the creep rupture test, leading to earlier failure of the material.

Nanoparticles agglomerate easily because of their high surface energy, which seriously reduces their tribological properties as lubricant additives. In this work, the core-shell nanoparticles Ag@polydopamine (PDA) were successfully prepared by the self-oxidation of dopamine hydrochloride on the surface of Ag nanoparticles and the dispersion of Ag nanoparticles in PAO6 was improved to promote anti-wear behaviors. The tribological properties of Ag@PDA nanocomposites as additives in poly alpha olefin (PAO) oil were studied under different concentrations, pressure and speed conditions by UMT-5 tribometer. It was demonstrated that the strong electrostatic repulsion of the PDA structure made the Ag nanoparticles better dispersed in PAO oil, thus playing a better lubricating role. When the concentration of the modified nanoparticles was 0.25 wt%, the friction coefficient of the lubricating oil decreased by 18.67% and no obvious wear was observed on the friction pair surface. When the Ag@PDA content was higher than 0.25 wt%, the tribological performance of the lubricating oil was weakened, which may be due to excessive Ag@PDA acting as an abrasive on the friction surface, thereby increasing friction and wear. The friction coefficient of the lubricating oil containing Ag@PDA decreased with the increase in load, but hardly changed with the increase in frequency.

In this paper, the precipitation behavior and its effect on resistivity in a new type of nickel–iron-based alloy during short-term aging were investigated. During the aging process, the ${\mathsf{\gamma }}^{\prime }$ phase increases in average size and decreases in number, with its area fraction fluctuating over time. This fluctuation is caused by the mismatch in the redissolution and growth rates of the ${\mathsf{\gamma }}^{\prime }$ phase. As the area fraction of the ${\mathsf{\gamma }}^{\prime }$ phase increases, the content of solute atoms in the matrix that scatter electrons decreases, lowering the resistivity of the alloy. Additionally, the continuous precipitation of ${\mathrm{M}}_{23}{\mathrm{C}}_6$ at grain boundaries during aging causes the resistivity to gradually increase. This paper explains the fluctuation in the total amount of ${\mathsf{\gamma }}^{\prime }$ phase during short-term aging and proposes a new method for characterizing the precipitation behavior of the ${\mathsf{\gamma }}^{\prime }$ phase in the novel alloy using the relative trend of resistivity changes.

Electromagnetic induction heating, which converts electromagnetic energy into thermal energy via electron-lattice collisions, and heat conduction heating, which transfers thermal energy through lattice vibrations, both have significant impacts on the solid-state precipitation behavior caused by atomic diffusion. This paper proposes a creep method based on heat conduction heating, which utilizes the turning point of negative creep to measure the isothermal transformation start curve of the γ′ phase in the alloy. The results are compared with the thermal expansion experiments under electromagnetic induction heating and simulations from the thermodynamic analysis software JMatPro. The results indicate that the nucleation incubation period of the γ′ phase in the creep experiment is longer, excluding the non-thermal effects of electricity, and more consistent with actual heat treatment conditions. The overlapping precipitation of other phases, such as M₂₃C₆ carbides at grain boundaries, reduces the γ′ phase’s fastest precipitation temperature determined by the creep and thermal expansion methods, thereby lowering the accuracy of the isothermal transformation curve. This study provides a reference for optimizing production processes and evaluating the service performance of precipitation-strengthened iron–nickel-based alloys.

The creep-rupture behavior and microstructural evolution of Ni–Fe-based alloy/Inconel 617 dissimilar welded joints were investigated by creep-rupture testing, interrupted creep testing, and microstructural characterization. Tests were conducted at 630–750 °C under stresses of 130–375 MPa. All joint specimens fractured in the heat-affected zone (HAZ) or base metal (BM) on the Ni–Fe-based alloy side, indicating that the Ni–Fe-based alloy side was the creep-critical region of the joint. Three failure regimes were identified according to the relative rupture lives of the joints and the Ni–Fe-based alloy BM. Fractographic observations revealed predominantly intergranular fracture, with creep cavities and cracks preferentially associated with grain-boundary discontinuous precipitation (DP) regions containing coarsened rod-like γ′ precipitates and precipitate-free zones (PFZs). Differences in DP evolution between the HAZ and BM were closely related to creep-damage localization and failure behavior. Interrupted creep tests showed that temperature was the dominant factor affecting DP formation and growth, while applied stress accelerated its evolution. DP developed rapidly during the initial exposure stage and subsequently grew more slowly. These findings clarify the relationship between grain-boundary DP evolution and creep-rupture failure in Ni–Fe-based alloy/Inconel 617 welded joints, and also provide microstructural guidance for assessing and mitigating creep degradation in dissimilar welded components used in high-temperature A-USC systems. Future work should combine longer-term creep testing, phase-resolved microstructural characterization, and predictive modeling to establish quantitative relationships among DP evolution, creep-damage accumulation, and rupture life.