Aging Precipitation Behavior And Mechanical Properties Of Al-Zn-Mg-Cu-Zr-Sc Alloy

Al-Zn-Mg-Cu series aluminum alloys widely used in aerospace, transportation and otherimportant industries structural materials, are characterized by high strength and hardness, goodcorrosion resistance and toughness, excellent formability and weld ability. Having animportant strategic position in the military and the economy of nation in the future, also it is anindispensable material resource of many military, civilian aircraft and ships. Micro-alloyingand heat treatment can improve the comprehensive property of Al-Zn-Mg-Cu alloys. Sc is themost effective micro-alloying element to aluminum discovered so far. Using Sc tomicro-alloyed can improve strength, plasticity and toughness of aluminum, and significantlyimprove the corrosion resistance of the alloys. Therefore, as a new generation of high-propertyaluminum, research and development Al-Zn-Mg-Cu system Sc-containing alloy is important.In this investigation, rare earth element Sc affecting the mechanical properties of T6treatedAl-Zn-Mg-Cu-Zr alloys and the aging precipitation behaviors give the basic knowledge ofhow to make a better research on the new aluminum alloys with ultra-high strength.In this investigation the figures microstructure reveals that the addition of0.2%Sc canrefine the grains of the extruded Al-Zn-Mg-Cu-Zr alloy and eliminate the dendrite of castingAl-Zn-Mg-Cu-Zr alloy, increase recrystallization temperature of aluminum alloy, restrainrecrystallization of aluminum alloy in extrude process. The data of tensile tests gets us tounderstand that the adding the appropriate amount of Sc is able to increase ultimate tensile andyield strengths of the extruded Al-Zn-Mg-Cu-Zr alloys have an extraordinary impact on thetensile posterities of Al-Zn-Mg-Cu-Zr(-Sc) alloys Within2~66h aging time, the tensilestrength and yield strength of Al-Zn-Mg-Cu-Zr(-Sc) alloys increase first decrease and reachthe peak at48h. The observations on the fracture surfaces show that in under peak aging time,the tensile fracture surface of Al-Zn-Mg-Cu-Zr(-Sc) alloys observed a large number of dimples,performing ductile fracture characteristic. The strength models of Al-Zn-Mg-Cu-Zr-Sc alloyare η’ precipitates strengthening and a large amount of fine disperse Al3(Sc, Zr) particleshaving petal-like shape precipitate during homogenization which can pin dislocation andsub-boundaries also have strengthening effects.The results of low-cyclic fatigue tests show that the T6treated Al-Zn-Mg-Cu-Zr(-Sc)alloys exhibit the cyclic strain hardening, and stable cyclic stress response during fatigue deformation. For two kinds of alloys, the relationship between the elastic strain amplitude andreversals to failure is linear and can be well described by the Basquin equation, while therelationship between the plastic strain amplitude and reversals to failure also exhibits a linearbehavior and obeys the Coffin-Manson equation. Under the low-cycle fatigue loadingcondition with total strain control mode, the fatigue cracks initiate in a transgranular mode atthe free surface of fatigue specimens, and propagate transgranularly for the T6treatedAl-Zn-Mg-Cu-Zr(-Sc) alloys.