Research On Strain Fatigue Behavior Of Extruded Al-4Cu-1Mg-xGd Alloy

As the light-metal based structural materials with high strength and excellentcomprehensive performance, aluminum alloys are widely used in many filed such asaerospace industry. In practical applications, aluminum alloy components often worksunder alternating load, and thus fatigue is one of main failure forms. However, the presentstudy on fatigue behavior of aluminum alloys is obviously insufficient. Some researchessuggest that the addition of rare earth elements in aluminum alloys can effectively refinethe microstructure and improve their tensile properties. But the study concerning theinfluence of rare earth elements on fatigue behavior of aluminum alloys is few. Therefore,in this investigation, Al-4Cu-1Mg-xGd alloys with different Gd contents are prepared byhot-extruding technology, and the low-cycle fatigue (LCF) behavior of Al-4Cu-1Mg-xGdalloys with as-extrude and solution plus aging (T6) states is systematically investigated,especially focusing on discussion the influence of rare earth element Gd and T6treatmenton the LCF behavior, in order to provide a reliable theoretical basis for further applicationof this series of aluminum alloys.The microstructural observations reveal that the addition of appropriate amount ofrare earth element Gd can significantly refine the microstructure of the hot-extrudedAl-4Cu-1Mg alloy, and the microstructure of the alloy can be further refined after T6treatment. The results of LCF tests show that the Al-4Cu-1Mg-xGd alloys with differenttreated states can exhibit the cyclic strain hardening, cyclic strain softening and cyclicstability, which depends on the processed state and applied strain amplitude level. Theaddition of appropriate amount of rare earth element Gd can improve the cyclicdeformation resistance of hot-extruded Al-4Cu-1Mg alloy, while the T6treatment cansignificantly improve the cyclic deformation resistance of the extruded Al-Cu-Mg-xGdalloy. The addition of rare earth element Gd can prolong the LCF lives of as-extrudedAl-4Cu-1Mg alloy, but T6treatment decrease the LCF lives of the extruded Al-Cu-Mg-xGd alloy. For as-extruded and T6treated Al-Cu-Mg-xGd alloys, the relationships between both plastic strain amplitude and elastic strain amplitude with reversals to failure and areboth linear, and can also be described by the Coffin-Manson and Basquin equations,respectively. The analysis on the fatigue fracture surfaces of as-extruded and T6treatedAl-Cu-Mg-xGd alloys shows that the fatigue cracks initiate in a transgranular mode at thefree surface of fatigue specimens and propagate transgranularly under total straincontrolled loading mode.