*3.4. Fractographic Analysis*

Optical stereo-micrographs of the fracture surfaces of polished and anodized AZ31B specimens after fatigue tests in air and PBS solution are shown in Figure 9. The specimens for optical stereoscopic analysis were chosen based on the number of cycles to failure. The aim was to identify the microfractographic features in the initial hours of the test, as it would lead to the most relevant information regarding the nucleation of fatigue cracks. Hence, specimens that failed in the first hours of test, at different applied stresses but with similar fatigue lives were chosen in order to investigate the effect of the PBS solution on the fractographic aspects of the AZ31B alloy. Table 5 displays the applied stress and corresponding number of cycles to failure of the polished and anodized specimens selected for fractographic analysis.

**Table 5.** Specimens chosen for fracture analysis after the corrosion fatigue tests.


The fatigue fracture surface may display distinguishable regions, denoting three different steps of fatigue failure: (i) crack nucleation; (ii) crack growth and propagation; (iii) catastrophic failure (final fracture) [45,46]. These regions are indicated in Figure 9 for the AZ31B specimens in the polished and anodized conditions. Fatigue fracture is often initiated in the surface or subsurface region [47]. However, identifying the exact point of crack nucleation is not an easy task [48]. These sites are indicated as A1 and A2 in Figure 9. Growth region generally expands radially for the surface of the specimen (region B in

Figure 9). The transition from region B to final fracture (region C) is accompanied by a change of the fractographic features to a darker, uneven region. Details regarding each one of these regions were obtained by SEM analysis.

**Figure 9.** Optical stereo-micrographs of the AZ31B alloy: (**a**) polished (air; 167.5 MPa); (**b**) anodized (air—105 MPa); (**c**) polished (PBS solution—140 MPa); and (**d**) anodized (PBS solution—92.5 MPa). Marks on the micrographs represent the following fracture regions: (A1 and A2) crack initiation; (B) crack propagation; (C) catastrophic failure.

Figure 10 shows the SEM micrographs corresponding to regions A (Figure 10A), B (Figure 10B) and C (Figure 10C) of Figure 9a. Green arrows point to possible crack nucleation sites in Figure 10A, denoted by discontinuities and small particles at the bottom right of the micrographs. However, the nucleation site was not clearly distinguished. A different aspect is seen in the crack growth region (Figure 10B). Faceted regions and shallow dimples are encountered over this region, suggesting an unstable crack growth mechanism was predominant. Conversely, a typical ductile dimpled fracture surface is seen in Figure 10C, suggesting that plastic deformation occurred in this region, before final fracture. Similar features of fractographic aspects of fatigue fracture surfaces of magnesium alloys were reported by other authors [48–51].

Regions A–C of the fracture surface of the air-tested anodized specimen shown in Figure 9b were further explored by SEM analysis. The corresponding micrographs are displayed in Figure 11. The nucleation region (A1 in Figure 9b) is shown in Figure 11A. Green arrows point to the interface between the anodized layer and the bulk AZ31B alloy, revealing different roughness at these sites. Moreover, subsurface particles are also observed and may have contributed to crack nucleation, as observed by other authors [24,45]. Yet, a gradual transition in the surface appearance from a smooth to a rough aspect is seen at the middle of the micrograph, indicating that crack propagation may have started at this site [52]. In Figure 11B, the crack growth region is further detailed. Its aspect resembles that shown in Figure 9b for the polished sample tested in air, being characterized by faceted regions and shallow dimples, typical of unstable crack growth. In the final fracture region (Figure 11C), the fracture surface is dominated by an intense dimpled structure, indicating plastic deformation at these sites, as also observed in Figure 10C for the polished sample.

It is clear, therefore, that anodization did not alter the microfractographic features of the AZ31B alloy subject to fatigue testing in air.

**Figure 10.** Fatigue fracture morphologies of the polished specimen tested in air at 167.5 MPa. The micrographs (**A**–**C**) correspond to the three different regions pointed out in Figure 9a.

SEM micrographs of regions A1, B, and C (Figure 9c) for the polished sample tested in PBS solution are displayed in Figure 12. The most probable crack nucleation sites (region A1, Figure 9c) are shown in Figure 11A where potential stress risers are pointed by the green arrows, such as discontinuities in the MgO/Mg(OH)2 layer spontaneously formed in the electrolyte and subsurface inclusions that may have facilitated fatigue crack nucleation. The transition between the smooth and rough aspects of the fracture surface is remarkable, as seen in the left part of the micrograph, in contrast with that of the polished samples tested in air (Figure 10A). According to the literature [23,53], this suggests a faster transition from the nucleation step to crack growth. Cracks would appear simultaneously at different sites, triggered by surface porosity or subsurface cracks. Hence, material plasticity is reduced, increasing the crack propagation rate, and leading to rough aspect of the fracture surface. It is likely that the concomitant action of corrosive environment and cyclic loading is responsible for such an enhanced propagation rate. In spite of the surface cleaning step before fractographic analysis, some oxide inclusions still remained in the fracture surface, as indicated by the arrows in Figure 12B,C. The final fracture region (Figure 12C) is similar to those of the specimens tested in air (Figures 10C and 11C), showing a dimpled structure.

**Figure 11.** Fatigue fracture morphologies of the anodized specimen tested in air at 105 MPa. The micrographs (**A**–**C**) correspond to the three different regions pointed out in Figure 9b.

SEM micrographs of the fracture surface of the anodized specimen tested in PBS solution are shown in Figure 13. In Figure 13A (region A1 in Figure 9d), green arrows indicate pores, subsurface defects between the substrate and the anodized layer, and inclusions that may be related to crack nucleation by acting as stress risers during fatigue loading. In the crack propagation region (Figure 13B), the fracture surface is quite flat and some micropores are indicated by green arrows. The aspect of the final fracture region (Figure 13C) is quite different from that of the polished sample fractured in PBS (Figure 12C). Instead of a dimpled structure, the surface is flat, indicating that plastic deformation was not as marked as observed for the polished sample. In this respect, it is evident that the presence of the anodized layer reduced plasticity at the final fracture step.

The main observations with respect to the microfractographic features of the fracture surfaces for the AZ31B alloy specimens described in Table 5 are synthesized in Table 6, along with possible causes of the final failure.

**Figure 12.** Fatigue fracture morphologies of the polished specimen tested in PBS solution at 140 MPa. The micrographs (**A**–**C**) correspond to the three different regions pointed out in the Figure 9c.


**Table 6.** Summary of characteristics of the fractured specimens submitted to the corrosion fatigue tests.

**Figure 13.** Fatigue fracture morphologies of the anodized specimen tested in the PBS solution at 92.5 MPa at room temperature. The images (**A**–**C**) correspond to the three different regions pointed out in the Figure 9d.
