Hardness environments around fatigued scratches in clad and unclad 2024 T351 aluminium alloy

The hardness distribution around fatigued surface scratches in clad and unclad 2024 Al alloys have been investigated via surface micro- and cross-sectional nano-hardness measurements. Results reveal that the area around the scratches can be divided into softened, hardened, and unaffected regions. There occurred a softening of the material on the original surface close to the scratches while a hardening effect occurred on the subsurface material to the side of the scratch for both the clad and unclad specimens. Also, the measurements did reveal a tendency of softening the material in the region of the scratch root for the clad alloy. However, a combination of hardening and softening occurred around the region of the interface between clad and substrate materials. This was attributed to a significant reduction of fatigue life. The occurrence of slight hardening in the material ahead of the scratch roots for unclad alloy was evident. Compared to the negative effects on fatigue life from stress concentrations stemming from scratch geometry, this positive factor is observed to be relatively small. The reduction becomes noticeable with increasing scratch depth. Also, hardness distribution maps are drawn around the scratch profiles. Thus, the study enhances the current understanding of the fatigue failure of scratched alloys.