Effect Of Sc On Micro Structure And Mechanical Properties Of Al-Zn-Mg-Cu-Zr Alloy

Al-Zn-Mg-Cu series aluminum alloys have higher strength, hardness and bettertoughness, resistant to corrosion than other series aluminum alloys so that have morepromising applications in aerospace and aviation fields. In this investigation, the extrudedAl-6Zn-2.5Mg-2Cu-0.1Zr-xSc alloys were taken as the experimental materials, andthrough performing the tensile and low-cycle fatigue tests, the influence of rare earthelement Sc and T6treatment on the mechanical properties of hot-extruded Al-6Zn-2.5Mg-2Cu-0.1Zr-xSc alloys were determined in order to provide a theoretical reference fordeveloping new aluminum alloys with ultra-high strength.The microstructural observations reveal that the addition of0.1%Sc can refine thegrains of the extruded Al-6Zn-2.5Mg-2Cu-0.1Zr alloy. After solution plus aging (T6)treatment, the grains of the extruded Al-6Zn-2.5Mg-2Cu-0.1Zr alloy growth significantly,while the grains of the extruded Al-6Zn-2.5Mg-2Cu-0.1Zr-xSc alloys almost remain thesame size. The results of tensile tests show that the ultimate tensile and yield strengths ofthe extruded Al-6Zn-2.5Mg-2Cu-0.1Zr alloy significantly increase with adding theappropriate amount of Sc. Especially, the ultimate tensile and yield strengths of the alloysget obviously enhanced after T6treatment. The results of low-cyclic fatigue tests show thatduring fatigue deformation, the extruded Al-6Zn-2.5Mg-2Cu-0.1Zr alloy as well as the T6treated Al-6Zn-2.5Mg-2Cu-0.1Zr-0.1Sc and Al-6Zn-2.5Mg-2Cu-0.1Zr-0.3Sc alloys exhibitthe cyclic strain hardening, softening and stable cyclic stress response. For three kinds ofalloys, the relationship between the elastic strain amplitude and reversals to failure is linearand can be well described by the Basquin equation, while the relationship between theplastic strain amplitude and reversals to failure also exhibits a linear behavior and obeysthe Coffin-Manson equation. The observations on the fracture surfaces show that under thetension loading condition, as-extruded Al-6Zn-2.5Mg-2Cu-0.1Zr-xSc alloys and T6treatedAl-6Zn-2.5Mg-2Cu-0.1Zr-0.3Sc alloys exhibit a ductile fracture mode, while the extrudedAl-6Zn-2.5Mg-2Cu-0.1Zr-0.1Sc alloy subjected to T6treatment occur the mixed ductile- brittle fracture. Under the low-cycle fatigue loading condition with total strain controlmode, the fatigue cracks initiate in a transgranular mode at the free surface of fatiguespecimens, and propagate transgranularly for the extruded Al-6Zn-2.5Mg-2Cu-0.1Zr-xScalloys.