Effect Of Sc, La On The Microstructures And Mechanical Properties Of Al-Zn-Mg-Cu Alloys

Al-Zn-Mg-Cu based alloys are extensively used as structural materials in theaerospace industry for their high strength. Along with their wildly application inaerospace, improvement in their mechanical properties has become more and moreessential. Over the years, numerous studies have been dedicated to find out ways ofobtaining aluminum alloys with excellent strength. One access to that is improvingthe heat treatment technology, another （which is more favorite） way is developingnew alloys by adding some rare earth elements. This paper mainly dedicated to theinvestigation of Sc, La on microstructures and mechanical properties of Al-Zn-Mg-Cualloys. The results show that:1) The addition of Zr can refine the grains of the Al-8.5Zn-2Mg-1.2Cu alloy, but notmuch. Multiple additions of Sc and Zr can bring effectively refinement in the grainsof Al-8.5Zn-2Mg-1.2Cu alloy. The main precipitations along the grain boundaries areη（MgZn₂） and T（AlZnMgCu）, while precipitations in the grains are η（MgZn₂） phases.Some fine particles in bulk forms, with the size of3¹0μm, can be observed in somegrains. The lump-like phases are proved to be Al₃（Sc,Zr） phases, which are the mainreason of the great grain refinement of the alloy.2) Grains of the Al-8.5Zn-2Mg-1.2Cu-0.18Zr alloy remain nemaline withoutrecrystallization after solution treatment. The Al-8.5Zn-2Mg-1.2Cu-0.18Zr alloy has ahigher tensile strength than that of the Al-8.5Zn-2Mg-1.2Cu alloy, but a lowerelongation. Tensile strength of the Al-8.5Zn-2Mg-1.2Cu alloy was increased by43MPa when adding0.3Sc and0.18Zr to the alloy. Elongation of the Al-8.5Zn-2Mg-1.2Cu-0.3Sc-0.18Zr alloy is14.7%, which is also the highest.3) It can be observed that all the grains were effectively refined with theimprovement of La addition to the Al-Zn-Mg-Cu alloy, but not so obviously whenadding0.06%La to the alloy. Tensile strength of the Al-6.0Zn-2.6Mg-1.2Cu-La isincreased with the improvement of the addition of La. The Al-6.0Zn-2.6Mg-1.2Cu-0.3La got the highest tensile strength, which is70MPa higher than that of theAl-6.0Zn-2.6Mg-1.2Cu alloy. However, the tensile strength decreased when adding 0.5La to the alloy, but still higher than that of the Al-6.0Zn-2.6Mg-1.2Cu alloy.4) There are still lots of bulk phases were not dissolved into the matrix after singlesolution+single aging treatment. After single solution+two-stage aging treatment,the second phases became finer. Bulk phases along the rolling direction arecompletely dissolved into the matrix after the two-stage solution+single agingtreatment. Not only bulk phases along the rolling direction are completely dissolvedinto the matrix, but the precipitations are become much finer after the two-stagesolution+two-stage aging treatment.5) The selected-area diffraction patterns taken from the phase indicated that theneedle-like phase is Al₉₂La₈. The Al₉₂La₈is a kind of intermetallic compound in thebcc structure （a=6.635）.The Al₉₂La₈phases identified in the Al-8.0Zn-3.47Mg-1.2Cu-0.3La alloy were concluded to be precipitated after solution and peak agingtreatment. More compounds with bigger size will be formed within or along the grainboundaries with excess addition of La. The heterogeneous distribution of the phases ishard to be prevented and eliminated even after homogenization treatment mainly dueto the gross segregation generated in the process of casting. Therefore, theheterogeneously distributed phases are harmful to the mechanical properties of thealloys. That’s why the strength of the Al-8.0Zn-3.47Mg-1.2Cu-La is decreased whenthe addition of La excesses0.3.