Effects Of Trace Si On Microstructures And Properties Of Al-5.0Cu-0.3Mg (wt.%) Alloy

The effects of0.3wt.%Si addition on precipitation in Al-5.0Cu-0.3Mg (wt.%)alloy have been systematically investigated by high-angle annular dark-field scanningtransmission electron microscopy (HAADF-STEM), scanning electron microscopy(SEM) and energy spectrum (EDS) analysis method. The feasibility studies aboutimproving the combination performance, especially the corrosion resistance, ofSi-containing Al-5.0Cu-0.3Mg-0.3Si (wt.%) alloy by over-ageing treatment havebeen carried out via various corrosion tests, measurement of electric-conductivity andmechanical properties at room temperature. What’s more, in order to explore asuperior heat-treating process to improve the service properties of the Si-containingalloy further, the alloy samples have been annealing treated at different temperaturesbefore solid solution to adjust the grains of alloy, the information of grain has beencharacterized via electron backscatter diffraction (EBSD) analysis technique.The research results reveal the effects of0.3wt.%Si addition on ageingprecipitation at180℃in Al-5.0Cu-0.3Mg (wt.%) alloy. By contrast, it has beenfound out that due to the0.3wt.%Si addition, there has been the formation ofQ″-type precipitates in the Si-containing alloy. And the formation of Q″-typeprecipitates, on which the θ′precursors nucleate heterogeneously, contributes to theconsiderable increase in the ageing kinetics and higher strength at the early ageingstage. It has been detected that many Cu mono-layers though very short nucleatealmost simultaneously when they nucleate in the vicinity of Q″-type laths after detailobservations. Thus many short GP II zones or θ″phases form easily in theSi-containing alloy, and quickly evolve into the θ′phases eventually. Owe to thespecial precipitation of θ′phases in Al-5.0Cu-0.3Mg-0.3Si (wt.%) alloy, the θ′phases’ formations need a much shorter time and their thicknesses are reducedsignificantly,2cθ′is the most common thickness of θ′precipitates in Si-containingalloy during the entire investigated ageing period. The coarsening of θ′precipitates inSi-containing alloy is significantly impeded.The over-ageing treated Al-5.0Cu-0.3Mg-0.3Si (wt.%) alloy posses the sameexcellent mechanical performance as the alloy at peak-ageing stage, but a much bettercorrosion resistance than it, which caused by that0.3wt.%Si addition changes theprecipitates and their precipitation behaviors in Al-5.0Cu-0.3Mg (wt.%) alloy,including promoting the formation of θ′precipitates, impeding the coarsening of θ′precipitates and making the θ′precipitates remain large aspect ratio all the time, and so on.Annealing treatment at different temperatures before solid solution has beenadded to Al-5.0Cu-0.3Mg-0.3Si (wt.%) alloy. The studies find that grains size ofalloys decrease with addition of annealing treatments. By contrast, the amount ofhigh-angle boundary in the alloy with annealing treatment at100℃for1h beforesolution treatment at500℃for1h is the least, while that of alloys with otherheat-treatments are very close. What’s more, the textures of all the alloys withdifferent heat-treatments are recrystallization texture, mainly of which are{001}<110>R-Cube texture and {011}<100>Goss texture, and the {001}<100>Cubetexture which is a common recrystallization texture in the face-centred cubic metaldoesn’t exist in the alloys. Besides, some cold rolling textures still exist in the alloywithout annealing treatment before solution treatment at500℃for1h.