Strain localization during fatigue of precipitation-hardened aluminium alloys

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Abstract
Fatigue tests of A1-3.8 wt% Cu containing θ″ precipitates have been carried out at two plastic strain amplitudes, 9.0 × 10−4 and 9.2 × 10−5 corresponding to the high- and low-strain regions of the plateau in the cyclic streas-strain curve. The tests were interrupted at points in life both before and after the occurrence of peak hardening. During interruptions the specimens were repolished and retested for a quarter cycle in tension. The slip offsets thus produced were measured by interferometry and the strains revealed were compared with the overall value imposed during the quarter cycle of straining. Persistent slip bands (PSBs) start to form just before the hardening peak and the strain was observed to localize within the PSBs. With more cycles, the localization intensifies both in existing micro-PSBs and by the creation of new micro-PSBs. After the peak stress is attained, zero or minimal nucleation of micro-PSBs was observed. Nearly all of the plastic strain after the peak is localized in the PSBs and is manifested as surface slip steps. The average value of localized strain ranges from 0.3 to 0.6, two orders of magnitude higher than that observed in copper, and the volume fraction of PSBs was observed to be extremely low throughout the plateau. A gradually increasing level of strain localization was shown by all existing macro-PSBs (i.e. groups of micro-PSBs) during softening. It is believed the strain localization causes the degradation of the hardening precipitates and the consequent cyclic softoning.