Abstract: By integrating an enhanced speckle pattern gold remodeling technique for high-resolution strain mapping of carbides with scanning electron microscopy, we have successfully mapped the strain localization of carbides with varying characteristics (size, aspect ratio, and roundness) during loading over numerous incremental strain steps. This study pioneers the use of high-resolution digital image correlation to track the strain evolutions within and outside carbides, providing qualitative and quantitative insights into the carbide fracture of the most widely used M50 aero-bearing steel at the microstructural level, revealing the relationship between the complex carbide characteristics and the fracture strains. The strain localization within the carbides (Type I), from the matrix (Type II), and at the interface (Type III) was found to govern distinct types of fracture modes, and a Type IV fracture was found to develop instantaneously without significant prior strain localization. Statistically, these four types of carbide fractures are dominated by the pre-existing defects, size, roundness, and aspect ratio, respectively. Nonetheless, these carbide characteristics factors, individually, fail to explain the distribution of the critical carbide fracture strain. We propose a composite factor that considers the combined effect of these individual influences, revealing a log-linear relationship with the applied strain to carbide fracture that is mechanistically meaningful: a small carbide that is round with smooth interface fractures at higher strain, whereas larger carbides that are elongated with sharp interfaces may fracture at lower strain. This paper suggests a robust quantitative in-situ approach to analyze the fracture of carbides, and the results may provide valuable insights for process optimization and carbide tuning for advanced bearing steel and other multi-phase alloys.

Key words: In-situ scanning electron microscopy, Gold remodeling, Strain localization, Carbide fracture, Composite factor for carbide characteristics