Specimen Size Effect of Tensile Behavior of Al-4.0 wt pct Cu Alloy Sheets: Effects of Precipitates and Cyclic Predeformation

Aging hardened Al-4.0 wt pct Cu alloy sheets with thicknesses (t) of 0.15 to 1.0 mm were selected to explore the effects of the second phase and its interaction with the dislocations produced by cyclic predeformation on the extrinsic size effect of mechanical behavior. The dependence of tensile properties on t and the effect of cyclic predeformation were systematically studied. The results showed that with the decrease of t from 1.0 to 0.3 mm, the ultimate tensile strength sigma(UTS) of Al-4.0 wt pct Cu alloy sheets reduced continuously, but the uniform strain epsilon first increased and then decreased. When t was decreased to 0.15 mm, there was no noticeable change in sigma(UTS), but a slight increase in epsilon compared with the cases at t = 0.3 mm was observed, which is mainly related to the interaction between precipitates and dislocations. In contrast, a t-dependent yield strength sigma(YS) was not observed. A cyclic predeformation to 20 pct N-f (N-f: fatigue life) cycles promoted a notable increase in the strengths of various-t sheets, and a t-independent strength was found for the t in the range of 0.6-1.0 mm, which originates from the combined effects of the difficult initiation of dislocation sources in the material interior after predeformation, the enhanced interaction between the dislocations and precipitates and the strengthening of grains' surface layer caused by cyclic predeformation. Therefore, it is deduced that the precipitates inhibit the continuous decrease in the strength of thin sheets, and the cyclic predeformation and precipitatites greatly improve the strength and delay the occurrence of extrinsic size effect.