PSI - Issue 53

Luca Marchini et al. / Procedia Structural Integrity 53 (2024) 203–211 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 4 – Digital microscope image of a) AM and b) F samples after 1h of immersion; c) microstructure of F sample segregation bands.

The interaction layers formed on AM and F maraging steel samples after 30-minute of immersion test were analyzed in cross-section by SEM-EDS (Fig. 5). At this initial stage, EDS analysis confirms the presence of an Al 5 Fe 2 intermetallic layer, also called η phase, on both the samples (Fig. 5 a-b). The intermetallic layer has a thickness of about 10 μ m, but it is difficult to define, as several intermetallic particles detach from the intermetallic/Al interface and progressively dissolve into the molten Al. On the other hand, the steel/intermetallic interface is more continuous (except for few cracks developed upon cooling due to different thermal expansion coefficients), suggesting that the diffusion of Al towards steel is faster than the opposite diffusion of elements from steel to molten Al. The analysis carried out on the H11 sample reveals the formation of a layer of Al 3 Fe, with embedded several unreacted chromium carbide particles. Notably, the thickness of this layer is nearly half that of the maraging steel samples. The formation of these intermetallic compounds aligns with the existing literature (Nazari and Shabestari, 2009). Specifically, it has been reported that when the Si content in the aluminum melt ranged between 5 and 8 wt.%, the growth of the η phase was influenced by both interfacial reactions and diffusion processes. Moreover, it was observed that the apparent activation energy of the η phase decreased with increasing Si content (Zou et al. , 2021). The SEM images presented in Figures 5d-e-f show the interaction layer formed on different samples after 2 hours of immersion. It is evident that the intermetallic layer on the F sample has a thickness approximately double that of the AM sample. Furthermore, as suggested by the EDS analysis, the intermetallic layer of AM sample consists predominantly of two distinct layers, namely η in the inner part, and Al 8 Fe 2 Si – τ 5 phase (also identified as Al 7.4 Fe 2 Si) in the outer part, in contact with molten Al. Conversely, in the case of the forged sample, the intermetallic layer is primarily composed of the η phase. Probably, in case of the AM sample, the formation of a τ 5 intermetallic layer, contributed to reduce its corrosion kinetics. Actually, it was reported in literature that the