PSI - Issue 61

Aptullah Karakaş et al. / Procedia Structural Integrity 61 (2024) 47 – 52 Aptullah Karakas/ Structural Integrity Procedia 00 (2019) 000 – 000

51 5

Figure 4: Sharp-edge of aluminide layer of HDA+750 specimen

As given in Table 1, hardness decreased by diffusion annealing at 850  C with respect to higher hardness of the specimens annealed at 750  C and 800  C for the samples. The hardness decrement suggests that a phase transformation from Fe 2 Al 5 to FeAl occurs during annealing at 850  C, as mentioned earlier. This phase transformation has very important consequences. When a more brittle phase exists such as Fe 2 Al 5 at the interface, thermal expansion, elastic modulus or fracture toughness mismatch between the substrate and the coatings make the crack formation easier. However, when there is a more ductile phase such as FeAl at the interface, the ductile phase would tolerate these mismatch and provide endurance to crack initiation and propagation (Zhao et al. 2021b).

4. Conclusion

The following conclusions can be drawn from the present study. • The hot dip aluminizing process has been successfully applied to all surfaces by selecting appropriate process parameters such as bath composition of molten aluminum, immersion time and temperature. • After hot-dip aluminizing processes, cracks were observed in the coating layer during diffusion annealing at 750 and 800  C, while no cracks were observed during diffusion annealing at 850  C. This was attributed to the incompatibility of thermal expansion between the base metal and the coating layer at 750 and 800  C. In diffusion annealing at 850  C, the thermal expansion mismatch decreased due to the formation of a more ductile phase (FeAl). This resulted in a crack-free coating. On the other hand, while horizontal and vertical cracks were observed in diffusion annealing at 750  C, only vertical cracks were observed in diffusion annealing at 800  C. • While the coating layer has a tongue-like morphology in the samples that were diffusion annealed at 750  C and 800  C after hot-dip aluminizing, the interface does not have a tongue-like morphology but relatively flat after diffusion annealing at 850  C. This is attributed to the transformation of Fe2Al5 phase into FeAl phase during annealing at 850  C. Acknowledgements The authors would like to thank Repkon Machine and Tool Industry and Trade Inc. which financially supported this study.