Effects Of Ag, Mg, Er Elements On The Formation And Evolution Of Clusters And Mechanical Properties Of Al-Cu-Mg-Ag Alloys

Al-Cu-Mg alloys are widely used in aerospace appiliactions due to the combination of high strength, heat resistance and excellent fatigue crack propagation resistance. Trace addition of Ag is known to promote the dense precipitation of fine{111}α Ω plates in conventional Al-Cu-Mg alloys, thereby leading to the significant improvement of the strength and heat resistance. With the help of different mechanical properties tests, such as tensile strength test at various tempertatures, Vickers hardness measurement, creep performance test and fatigue crack propagation (FCP) rates test, as well as various microstructural characterization technequs, including metallography, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and three dimensional atom probe (3DAP) analysis, the effects of heat treatment, bulk alloy contents and applied tensile stress on the formation and evolution of various clusters and precipitates, as well as the mechanical propeties in Al-Cu-Mg-Ag alloys are deeply investigated in this paper. The conclusions are summarized as follows:(1) TEM and3DAP analysis on the microstructure within the plastic zone around the fatigue crack tip of underaged (170℃×0.5h)2524alloy indicated that the localized intense strain resulted in the dissolution of large co-clusters during fatigue.(2) At150℃, the age hardening effect of2524-T351alloy was stronger than that of2524-170℃×0.5h alloy. The age hardening rate of2524+Ag-170℃×0.5h alloy was greatly enhanced, whereas pre-stretching reduced the age hardening response of2524+Ag alloy. After aging at170℃for0.5h, both2524and2524+Ag alloys exhibited high FCP resistance as compared to corresponding T351alloys. This was attributed to the formation of large co-clusters during slight artificial aging process. Long term aging at150℃for500h was found to degrade the FCP resistance of both T351and170℃×0.5h alloys.(3) Quantitative analysis on the various clusters in Al-Cu-Mg-Ag alloys was firstly realized by3DAP technique. Results indicated a progressive evolution in the microstructure from small clusters to a large feature typical of GPB zones with increasing bulk Mg content. After aging at165℃for2h, the number density of Ω plates significantly increased from5.39×1022no./m3to10.67×1022no./m3as bulk Mg content increased from0.39%to0.81%but then dropped to8.1×1022no./m3as Mg content reached1.18%. Quantitative3DAP calculation results suggested that the nucleation of Ω phase during the initial aging stage were not only dominated by the number density of Mg-Ag co-clusters but also determined by the competition between Ω phase and other precipitates. The precipitation process of different alloys aged at165℃from5min to2h could be summarized as below:0.39Mg alloy:SSSâ†'G.P zones+Mg-Ag co-clusters->G.P zones+Mg-Ag/Cu-Mg co-clustersâ†'G.P zones+θ’+Ω;0.81Mg alloy:SSS->G.P zones+Mg-Ag/Cu-Mg co-clusters->GP zones+Mg-Ag/Cu-Mg co-clusters+GPB zonesâ†'G.P zones+Cu-Mg co-clusters+GPB zones+Ω;1.18Mg alloy:SSS->Mg-Ag/Cu-Mg co-clusters+GPB zonesâ†'Mg-Ag/Cu-Mg co-clusters+large GPB zonesâ†'Cu-Mg co-clusters+GPB zones+Q.(4) TEM and3DAP analysis firstly revealed that increasing bulk Ag content greatly increased the nucleation rates and numeber density of Ω plates, thereby leading to the improvement of the mechanical properties at various temperatures. Quantitative TEM calculation results on the microstructures after exposing at200℃for various times indicated the increasing Ag content was beneficial to the coarsening resistance of Ω phase because the progressive enrichment of Ag atoms within Mg-Ag segregation layer would help to stabilize the interfacial structure and increase the activation barrier for ledge nucleation and migration, thereby improving the coarsening resistance of Ω phase.(5) The preferred orientation of Ω precipitation in underaged Al-Cu-Mg-Ag alloys after creeping at200℃for various times with a tensile stress of240MPa indicated the stress-induced orienting effect occurred not only during the initial nucleation but also during the growth stage of Ω phase. The stress-induced accelerated thickening kinetics of Ω plates at200℃, as firstly revealed by HRTEM analysis, was attributed to the fact that the applied tensile stress faciliated the rapid nucleation and growth of ledges on the broad face of Ω phase.3DAP analysis suggested increasing Ag content was detrimental to the ledge nucleation under the applied tensile stress, thereby leading to the low thickening rate of Ω phase.(6) The precipitation of Ω phase in Al-Cu-Mg-Ag alloy was remarkably suppressed by trace addition of Er, whereas the formation of0’phase was greatly enhanced at165℃. FCP rates curves indicated the FCP resistance of Al-Cu-Mg-Ag alloys was significantly improved by addition of Er. Since Er-containing alloy possessed a larger scale in Paris regime, the fatigue crack was still observed to propagate in Paris regime asâ–³K reached40MPa·m0.5. The enhanced FCP resitance of Er-containing alloy was attributed to the large grains and the deflection of fatigue crack path.