A Study On The Decomposition Of Supersaturated Solid Solutions In Al-Mg-Si-Cu Alloy

Due to their low density, excellent formability and medium to high strengthobtained after heat treatment, the Al-Mg-Si-Cu alloys have great application prospectsin lightweight vehicles, which are extremely important for energy-saving andemission-reduction. The properties of the Al-Mg-Si-Cu alloys are largely determinedby the nanometer-sized precipitates formed during aging, because the precipitatesembedded in aluminum matrix can obstruct the movement of dislocations when thematerials are subject to deformation and thus strengthen the materials. For this alloysystem, the decomposition pathway of the supersaturated solid solution, formed aftersolution treatment and subsequent quenching, varies with the aging temperature.Accordingly, the type, size and distribution of the precipitates as well as the alloyproperties will be different. In this thesis, the age-hardening behaviors along with theintragranular and grain boundary precipitation of the Al-Mg-Si-Cu alloys aged at180℃were studied using microhardness test, scanning electron microscopy andtransmission electron microscopy. Fine observation and analysis of Cu-containingquaternary Q’ phase were conducted by high-angle angular dark-field scanningtransmission electron microscopy. Besides, the hardening behavior during naturalaging and its effect on the following artificial aging were also investigated.The age-hardening potential of Al-Mg-Si-Cu alloy was closely related with itsCu content and Mg/Si ratio. When the Cu content was constant, the larger the Mg/Siratio the higher the peak hardness, while the peak hardness increased with the Cucontent if the Mg/Si ratio remained unchanged. Two types of Q′phase precipitates,lath shaped Q′phase having {100}Alor {510}Alhabit planes and rod shaped Q′phasepossessing no specific interface orientation with the matrix, were found coexisting inAl-Mg-Si-Cu alloys. After prolonged aging, rod shaped Q′transformed gradually intolath-like Q′with the cross-section elongated along <510>Al. For Al-0.5Mg-1.0Si-0.8Cu (wt.%) alloy, needle-like initial-β" precipitated after aging for0.5h, thenneedle-like β″and a small number of lath-like L phase formed after aging for5h,rod-like Q’ phases (accounts for about two thirds of the total number of precipitates)and lath-like Q′phases dominated after aging for120h. For Al-1.13Mg-1.33Si-0.8Cu(wt.%) alloy aged for120h, rod-like Q’ phase (accounts for about one third of the totalnumber of precipitates) and lath-like Q’ phase occurred. For Al-0.42Mg-1.28Si-3.1Cu (wt.%) alloy, rod-like Q’ phase, lath-like Q’ phase and plate-like θ’ phase precipitatedafter aging for48h.Two types of discontinuous grain boundary precipitates were existent duringartificial aging:(1) Si-rich particles of about1μm and (2) Q-phase particles of10-30nm. Coarse Si precipitates, possessing platelet, globule or rod morphologies, wereobserved after aging for36h. The intragranular precipitates continued growing andeventually form an observable precipitate free zone along the grain boundaries afteraging for120h.In Stage I (0-5h) of natural aging, the hardening rate increased with the Cucontent while decreased with the Mg/Si ratio, and vice versa in Stage II (after5h). Thepeak hardness of the Al-Mg-Si-Cu alloy obtained by artificial aging after naturalaging for1h reduced by3-4HV. After natural aging for24h,336h or720h, the agingkinetics of the Al-Mg-Si-Cu alloy during artificial aging decreased, while the peakhardness and the corresponding occurrence times varied with the Mg/Si ratio.