Low-Cycle Fatigue Behavior Of Extruded Al-Mg-Si-RE Aluminum Alloy

Aluminum alloys have such characteristics as low density,high specific strength and rigidity,and have been found a wide application in aeronautical and automotive industries. To extend the application field of aluminum alloys,some measures,which can improve the microstructure and mechanical properties of aluminum alloys,need to be taken.The rare earth elements Sc and Er are considered as two kinds of the most effective elements available to enhance the microstructures and properties of aluminum alloys.Thus,the research concerning the effect of Sc and Er in aluminum alloys is of both important theoretical referring value and practical directing significance for developing new aluminum alloys with high strength and ductility.In this investigation,the different amounts of Sc and Er were added into the Al-0.8%Mg-0.6%Si matrix alloy.And the extruded Al-0.8%Mg-0.6%Si-xRE alloys with different states were fabricated by melting,casting,hot extrusion and heat-treatment.Then, the observations on the microstructures and fracture surface morphologies as well as both tensile and fatigue tests were performed for the extruded Al-0.8%Mg-0.6%Si-xRE alloys. The microstructural observations reveal that the addition of Sc and Er with appropriate content can refine the grains of the extruded Al-0.8%Mg-0.6%Si-xRE alloys.The results of tensile experiments show that the addition of Sc and Er with appropriate content can significantly enhance the ultimate tensile strength,yield strength and elongation of the extruded Al-0.8%Mg-0.6%Si-xRE alloys.Especially,the ultimate tensile and yield strengths of the alloys get obviously enhanced after solid solution plus aging treatment. The results of low-cyclic fatigue tests show that during fatigue deformation,the extruded Al-0.8%Mg-0.6%Si-xRE alloys exhibit the cyclic strain hardening,softening and stable cyclic stress response,which mainly depends on the imposed total strain amplitude and the type of the added rare earth elements.For the extruded Al-0.8%Mg-0.6%Si-xRE alloys,the relationship between plastic and elastic strain amplitudes as well as reversals to failure can be described by Coffin-Manson and Basquin equations,respectively.And for the extruded Al-0.8%Mg-0.6%Si-0.2%Sc,a bilinear ralation between the plastic strain amplitude and reversals to failure has been noted.In addition,at the higher total strain amplitudes,a serrated flow can be observed in the cyclic hysteresis loop for the extruded Al-0.8%Mg-0.6%Si-xRE alloys.It means that the so-called dynamic strain aging takes place during fatigue deformation. The fractographic results reveal that under tensile loading conditions,the extruded Al-0.8%Mg-0.6%Si-xRE fractographic alloys show typical ductile fracture,while under low-cycle fatigue loading conditions,the fatigue cracks initiate transgranularly at the surface of fatigue samples and propagate in a transgranular mode.