Research progress in grain boundary serration in iron/nickel based austenitic polycrystalline alloys

Grain boundaries of high-temperature metallic materials, such as alloys, are often considered weak. At elevated temperatures, the strength of the grain boundary is relatively lower than that of the intragranular areas, and cracks often initially form on the grain boundary and then develop along it, which leads to premature failure and significantly degrades the mechanical performance of the material at high temperature. Therefore, how to optimize the morphology and improve the strength of the grain boundary is key to improving the properties of alloys at high temperatures. A serrated grain boundary is a type of grain boundary with a wave shape evolving from the bending of the flat grain boundary during special heat treatments. For iron/nickel-based austenitic polycrystalline alloys, grain boundary serration has been viewed as an effective method for strengthening their grain boundaries and enhancing their properties. Here, the research progress of serrated grain boundaries was reviewed based on the aspects of formation method, formation mechanism, and their influence on the properties of materials. The methods of formation of serrated grain boundaries for different types of alloys, such as controlled cooling heat treatment, isothermal heat treatment, mechanical heat treatment, and alloying, were summarized. The interactions between the grain boundary and intergranular precipitates, such as M₇C₃ carbide, M₂₃C₆ carbide, and γ′ phase, were discussed in detail to understand the formation mechanism of the serrated grain boundary and how it improving the properties of materials and reveal the driving force of grain boundary migration. In addition, the influences of the serrated grain boundary on the mechanical (rupture, creep, fatigue, and tensile) properties, corrosion properties (hot and stress corrosion), and heat-affected-zone (HAZ) liquefying cracking behavior of different alloys were analyzed. Last, based on the abovementioned details, development directions for future work on serrated grain boundaries were outlined.