Effect of aging and dispersoid content on fatigue crack growth in Al–Mg–Si alloys

High cycle fatigue of AA6082 and AA6063 aluminum extrusions

“…The occurrence of intergranular failure in AA6XXX alloys [3,4,37,43,51,52,55], which was also present in nearly all fatigue crack initiation sites in this study, calls for some discussion of the nature of grain boundaries. The presence of grain boundary failure in aluminum alloys is generally attributed to precipitate free zones (PFZ) [3,4,43,51,52] but segregation of impurities to grain boundaries [56], the existence of high angle grain boundaries [57] or large slip gradients across grains [7,8,11,53] could also lead to failure at boundaries in polycrystalline materials. A schematic illustration of a PFZ at a grain boundary triple junction is shown in Figure 2-5.…”

“…The large strain accumulation near the surface of the extruded product usually results in a recrystallized surface structure. The dispersoid additions do not have a large effect on tensile properties, but could affect ductility, toughness and fatigue crack initiation and propagation [40][41][42][43].…”

“…The β phase is the stable FCC Mg 2 Si that will form after prolonged aging, but peak hardening of Al-Mg-Si alloys is attributed to the metastable β" phase. The β" precipitates are needle shaped with a preferred alignment of the needles along the <100> Al direction and a monoclinic crystal structure [43,47].…”

“…The peak hardness of precipitation hardened aluminum alloys occurs at the transition point between cutting/shearing of precipitates and bowing of dislocations around precipitates [6]. Studies have been conducted on the fatigue and deformation behavior of AA6XXX aluminum alloys with different heat treatments (over, under and peak aged), and specifically the interactions between dislocations, slip band structures and precipitates [42,43,51,52]. In one study, tensile and fatigue crack growth properties for under-and over-aged specimens were compared to peak aged specimen of Al-Mg-Si alloys [42,43].…”