Effect of Natural Aging on Artificial Aging of an Al-2.95Cu-1.55Li-0.57Mg-0.18Zr Alloy at 160℃
Due to their excellent combination of low density,high strength,and stiffness,third-generation Al-Cu-Li-Mg alloys are important lightweight materials in the aerospace industry.Precipitation strength-ening or hardening,which is controlled by precipitates,including the structure,size,morphology,distribu-tion,and volume fraction of precipitates,is mainly responsible for the alloy's excellent mechanical proper-ties.The precipitates in Al-Cu-Li-Mg alloys mainly include T1,S,δ',δ'/θ'/δ',θ',GPB,and various metasta-ble phases.In practice,the Al alloys are inevitably stored for a period at ambient temperature before sub-sequent processing during which natural aging occurs.Natural aging has an important effect on the pre-cipitation behavior of subsequent artificial aging in Al-Cu-Li-Mg alloys,but the related mechanism is still highly controversial.To solve this problem,the effects of natural aging treatment on the microstructure and mechanical properties of an Al-2.95Cu-1.55Li-0.57Mg-0.18Zr alloy treated by artificial aging at 160℃ were investigated using TEM,three-dimensional atom probe(3DAP),three-dimensional electron tomography(3DET),and mechanical property testing.It was discovered that natural aging significantly changed the artificial aging hardening behavior and caused two strengthening peaks in the alloy's hard-ness curve.The Mg-rich and Cu-Mg clusters and δ'precipitates formed during natural aging were first dis-solved at the initial stage of artificial aging,which resulted in a decrease in hardness.Then,large num-bers of GPB zones formed uniformly and dispersedly,followed by the formation of T1 precipitates with the increase of aging time,which caused the increase in hardness.The hardness reached its first peak value at 96 h.Following that,GPB zones dissolved and the hardness decreased again.When the aging time was continuously exceeded,the volume fraction of T1 precipitates and the number of lath-like S precipi-tates increased,so the hardness increased again and reached the second peak value at 192 h.In other words,the atomic clusters that form during natural aging can significantly modify precipitation behaviors and the evolution of mechanical properties during artificial aging.
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