## 3 Research Results and Discussion
### Fatigue Fracture Surfaces  
Figure 7 shows the fracture surface images of uncoated and film-coated samples of AA6063 Al and AZ91 Mg alloys after fatigue testing. A fractographic study was carried out for uncoated and multilayered Cr/CrN/TiCrN/TiCrCN nitride-coated AZ91 and AA6063 specimens with the objective of determining the fatigue life behavior in Figure 7. In literature, fatigue cracks started in cracks, surface/subsurface defects, and notches on the specimen surface in uncoated specimens and were merged with secondary cracks as the crack progressed [37; 53; 54].  
[PERSON] et al. reported similar results on the beginnings of cracks for AZ91 materials [55]. At very low cycle counts, pores act as a fatigue initiation point. The increased number of cycles controls crack beginnings at low strain amplitude values. Roughness adversely impacts fatigue life in both cases [56]. When the literature is examined, [PERSON] and [PERSON] (1993) concluded that fatigue cracks in AZ91-T6 alloy began from surface defects [57]. [PERSON] et al. (2004) observed that fatigue beginning in TiN-coated 316L steel nucleated from surface cracks and propagated along the coating-substrate interface. They emphasized that the fracture of the substrate was dominated by the nitride coating  
Figure 7: Fatigue fracture surface images of the specimens.  
since the fatigue cracks began at the coating surface. It was emphasized that the increased fatigue strength of the substrate in most of the max. alternating strain range was mainly due to the residual compressive stresses in the coating and the good adhesion of the coating to the substrate. It was concluded that TiN coating on 316L effectively improves corrosion, rust and wear as well as fatigue properties [30].  
Since the Cr/CrN/TiCrN/TiCrCN multilayer nitride coating displayed a columnar and dense microstructure, the fatigue limit value and strength were better in comparison with uncoated AZ91 materials because there were fewer cracks, notches, and defects on the surface that could lead to crack initiation in SEM image results. It was revealed that the fatigue fracture of the substrate coating composite was dominated by the fracture of the Cr/CrN/TiCrN/TiCrCN coating because fatigue cracks first formed on the coating's surface and then propagated toward the substrate [30]. Additionally, the lower value of surface roughness supports the said result.  
The studies by [32; 33], in which TiN and CrN single-layer coatings were applied separately to Al alloys, showed that the fatigue limit decreased.  
In the current study, Cr/CrN/TiCrN/TiCrCN multilayer nitride coating was applied to increase the fatigue strength of the AA603 Al alloy. Nevertheless, contrary to expectations, fatigue strength decreased with combined TiN and CrN coating. Assuming that fatigue cracks nucleate on the coating surface and rapidly propagate toward the substrate after consuming the full coating thickness provides metallographic support for the reason for the reduced fatigue strength, whereas the presence of Ti and Cr and the production of multilayer coating led to the increased fatigue strength of AZ91, causing a reduction in AA6063.