PSI - Issue 69

Zeynab Aalipour et al. / Procedia Structural Integrity 69 (2025) 105–112

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3.2. Microstructure Evolution (FESEM-EBSD)

3.2.1. Electron Backscatter Diffraction

Figure 3(a,c,e) presents three EBSD-IPF (Electron Backscatter Diffraction–Inverse Pole Figure) maps acquired at sequential partitioning time intervals for H-Si steel samples, illustrating the evolving phase distribution and crystallographic orientations. In each image, the black regions represent the body-centered cubic (BCC) martensite phase, while the colored regions correspond to the face-centered cubic (FCC) RA. The diverse spectrum of colors in the RA reflects variations in crystal orientation, thus revealing the complexity of the developing texture. This limited presence of RA in 100s suggests that the thermodynamic and kinetic conditions, particularly carbon partitioning from supersaturated martensite have not yet progressed sufficiently to stabilize the austenitic phase in final quench. However, as time increases, there is a noticeable rise in the proportion of colored (FCC) areas, signifying enhanced stabilization of RA. This phenomenon can be attributed to diffusion-driven carbon enrichment in austenite, which effectively lowers the martensite start temperature and reduces the likelihood of further martensitic transformation upon cooling. By the second time interval (1000 s), the RA fraction has grown appreciably, forming more continuous regions interspersed among the shrinking black domains of martensite. Finally, in the longest partitioning time, the colored regions occupy a substantial portion of the microstructure, with RA grains exhibiting a range of crystallographic orientations. This clear trend of increasing RA fraction across the three partitioning times underscores the importance of holding time in regulating phase stability and microstructural development in H-Si steels. Prolonged exposure allows for more extensive carbon partitioning and diffusion, thereby favoring the maintenance of the austenitic phase. Such a transformation pathway has significant implications for mechanical properties, as higher RA content often correlates with enhanced ductility and toughness, owing to mechanisms like the TRIP effect.

Figure 3-Electron Backscatter Diffraction for H-Si during the final quench at holding times of (a) 100 s, (c) 1000 s, and (e) 10000 s, and for L-Si during the final quench at holding times of (b) 100 s, (d) 1000 s, and (f) 10000 s.