Progress in the study of recrystallization defects in single crystal nickel-based superalloys

Single-crystal (SX) blades made of nickel-based superalloys represent a major technological breakthrough in the aerospace sector. As core, critical components, they enable advanced modern aero-engines to achieve high thrust and high thrust-to-weight ratios. The elimination of grain boundaries in SX blades provides excellent high-temperature performance. However, recrystallization (RX) can reintroduce grain boundaries, severely degrading in-service performance. Consequently, RX defects are a significant concern throughout the fabrication and service lifecycle of SX blades. Effective control of RX during fabrication is crucial for improving blade yield, and pre-existing and newly formed RX during service pose a severe threat to engine safety and reliability.

This paper reviews decades of studies on RX in single-crystal nickel-based superalloys. Based on the manufacturing procedures of SX turbine blades and the causes of RX formation, RX is classified into seven types. Three categories can be identified during solution heat treatment (following prior plastic deformation): structural RX induced by directional solidification shrinkage, spot-like surface RX caused by mold shell debonding, and RX induced by plastic strain at core pins due to core-pin fracture. Four categories are triggered by extrinsic mechanical strain during subsequent thermal processes (e.g., welding, coating, and service): sandblasting-induced surface RX, machining-induced surface RX, shot-peening-induced surface RX, and tension–compression deformation-induced RX. This classification provides a clear framework for targeted RX studies and control. Combined with microstructural characterization, numerical simulation, and machine learning, this paper further elaborates on the nucleation and growth mechanisms, formation laws, and critical conditions for each RX type. Nevertheless, monitoring RX nucleation sites in SX superalloys remains challenging. Existing study primarily focuses on grain growth at the micrometer scale, with insufficient atomic-scale imaging observations and in-situ studies to reveal its intrinsic nature. The paper also summarizes advancements and limitations in RX nondestructive testing techniques (e.g., synchrotron X-ray diffraction) and control strategies (e.g., fillet transition design and magnetic field treatment). Significant challenges persist in controlling structural RX for multibody SX blades and in inspecting and controlling inner-cavity structural RX in double-wall, ultra-cooled SX blades. The restriction of RX on blade structural design and performance improvement has become a growing area of concern. A new breakthrough integrating multiple engineering technical elements is urgently required for regulating spot-like surface RX. For core-pin fracture-induced RX, quantitative data criteria for core-pin material specification and process implementation are lacking, and reliable data guidance for evaluating the influence of rolled core pins in different blade regions on in-service performance is unavailable. Even when indispensable extrinsic mechanical strain is introduced during blade fabrication following solution heat treatment, its latent RX tendency during subsequent thermal operations and high-temperature service must be thoroughly assessed.

Finally, this paper explores and envisions the identification of the universal physical essence and specific laws of RX formation, as well as the construction of a systematic RX database. This is achieved through comprehensive analytical approaches, including cross-scale and in-situ characterization, thereby providing effective guidance for the precise control of all types of RX defects.