en.Wedoany.com Reported - As the global civil aviation industry faces severe pressure from greenhouse gas emissions, jet engine manufacturers are dedicated to developing lighter, smaller, and more thermally efficient engine cores. However, the reduction in core size has led to a significant increase in internal operating temperatures and stress levels. A recent in-depth review study on the microstructural evolution of nickel-based superalloys during manufacturing points out that precise control of thermomechanical processing and subsequent heat treatment can effectively balance the mechanical properties of components and reduce the risk of catastrophic failure during service.
Nickel-based superalloys, due to their excellent tensile strength, fatigue resistance, and environmental stability above 700°C, have become the preferred material for manufacturing critical safety components such as turbine discs. Research indicates that the ideal performance of these alloys highly depends on their microstructure, particularly the proportion and distribution of the $\gamma$ matrix phase and the $\gamma'$ strengthening phase. During thermomechanical processing, the central hub of the disc, which bears extremely high stress, requires a fine-grained structure for high strength; whereas the rim portion, operating in a high-temperature environment, needs a coarse-grained structure to enhance creep performance.
During the thermomechanical processing stage (such as isothermal forging), three major phenomena compete with each other: dynamic recovery, heteroepitaxial recrystallization, and discontinuous dynamic recrystallization. Among these, "heteroepitaxial recrystallization," first proposed in 2016, has become a recent research focus. This phenomenon involves the formation of a $\gamma$-like shell at the edges of primary $\gamma'$ particles, altering grain orientation and profoundly influencing the final component's mechanical behavior. Additionally, discontinuous dynamic recrystallization, through the nucleation and growth of a "necklace" structure, is a core pathway for achieving grain refinement.
The review specifically warns of the hazards of "abnormal grain growth." Within specific critical strain windows, a very small number of grains can rapidly coarsen by gaining an absolute growth advantage, forming abnormal structures with sizes more than 10 times that of adjacent grains. This significantly reduces the component's cyclic fatigue resistance. To avoid such defects, the study recommends that manufacturers adopt deformation processes with low forging temperatures and high strain rates, ensuring widespread recrystallization of the microstructure and thereby avoiding the critical range for abnormal growth.
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