PSI - Issue 61

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

50 4

Table 1: The values of coating thickness and coating hardness of aluminized samples

Coating Hardness (HV0.3) 1100-1200

Specimens

Coating Thickness (µm)

90-110

HDA

100-150

1100-1200

HDA+750

350-400

1000-1100

HDA+800

350-400

600-700

HDA+850

Cross sectional SEM micrograph of as-aluminized AISI 4140 specimen is shown in Fig. 3 with the corresponding EDS analysis in Table 2. The surface of the hot-dipped aluminide steel is composed of aluminide layers of different composition. There are two types of aluminide exists in the coating. Based on the EDS analysis, these are thinner layer of FeAl 3 (Cross 1 in Figure 3), and tongue-like and thicker Fe 2 Al 5 layer (Cross 2 in Figure 3) which is closer to the substrate. This is compatible with the previous results (Chen et al. 2016; Dey et al. 2023). The formation of these intermetallic phases by interdiffusion of the aluminium and iron atoms is the key factor in the bonding between aluminium and steel (Dey et al. 2023). Kishore et al. performed EBSD analysis of hot-dip aluminized Eurofer 97 steel and they found that the tongue-like morphology consists of mainly Fe 2 Al 5 because of crystallographic texturing in <001> direction (Kishore et al. 2021).

Aluminide layer

Table 2. EDS analysis of as-aluminized AISI 4140 specimen Point Al ( at % ) Fe ( at % ) 1 75 24.1 2 71 28.5

Substrate

Figure 3: Cross sectional SEM micrograph of as-aluminized condition of AISI 4140 specimen

Vertical and horizontal cracks were observed on the coating layer of HDA+750 specimen (diffusion annealing at 750  C after HDA) while only vertical cracks were observed on coating layer of HDA+800 specimen. Details of the initiation and propagation of cracks is attributed to formation of voids in the intermetallic layer due to the Kirkendall effect arising from the different diffusivities of Fe and Al atoms. Furthermore, vertical cracks are extended towards the steel substrate The cracks propagated through the coating/substrate interface because the adhesion strength between the aluminium coating and steel substrate might be lower than the yield strength of the steel substrate (Chen et al. 2016; Zhao et al. 2021a). In addition to the Kirkendall pores, tongue-like morphology of Fe 2 Al 5 , as being stress concentration sites, might play a role in crack initiation. Figure 4 shows the sharp-edge of the tongue-like morphology of Fe 2 Al 5 which is potential risk of regions for vertical cracks initiated.