Low Cycle Fatigue Behavior Of The Extruded Al-4Cu-0.15Zr-0.15Sc Alloy

In this investigation, the hardness, tensile and low cycle fatigue tests were performed for the extruded Al-4Cu-0.15Zr-0.15 Sc alloy subjected to the solid solution plus single aging treatment, solid solution plus double aging treatment and solid solution plus retrogression and reaging treatment. The microstructures, tensile fracture surfaces and fatigue fracture surfaces of the extruded Al-4Cu-0.15Zr-0.15 Sc alloy with different aging states were characterized with optical microscope(OM), scanning electron microscope(SEM) and transmission electron microscope(TEM). The mechanical properties and microstructures of the alloy with different aging treatment crafts were investigated to provide the theoretical reference for being used in engineering practice.The observed results on the microstructures of the extruded Al-4Cu-0.15Zr-0.15 Sc alloy subjected to the solid solution plus single aging treatment show that the grain has the regular morphology. The observation and analysis results on the microstructures of the extruded Al-4Cu-0.15Zr-0.15 Sc alloys with different aging states by means of transmission electron microscope reveal that a large number of needle precipitated phase appear in Al alloys. For the extruded Al-4Cu-0.15Zr-0.15 Sc alloy subjected to the retrogression and reaging treatment, the size of the precipitate is the finest, the dispersion degree of the precipitate is the largest, and the volume fraction of the precipitate is the highest.The results of tensile tests for the extrude Al-4Cu-0.15Zr-0.15 Sc alloys with different aging treated show that with prolonging the aging time, the hardness, strength and elongation firstly increase and then decrease. Comparison shows that a single-stage aging, two-stage aging and retrogression and reaging treatment can effectively improve the hardness, ultimate tensile strength and yield strength of the extruded Al-4Cu-0.15Zr-0.15 Sc alloy. Among three aging treatment process, the extruded Al-4Cu-0.15Zr-0.15 Sc alloy subjected to the solution plus retrogression and reaging treatment exhibits the best comprehensive tensile properties.The results of low-cyclic fatigue tests show that the cyclic stress response behavior of the Al-4Cu-0.15Zr-0.15 Sc alloys with different aging states can be expressed as cyclic strain hardening and cyclic stability at low cycle fatigue loading conditions. Among the single-stage aging, two-stage aging and retrogression and reaging treatment, the extruded Al-4Cu-0.15Zr-0.15 Sc alloy subjected to retrogression and reaging treatment exhibits the best cyclic deformation resistance, and Al-4Cu-0.15Zr-0.15 Sc alloy subjected to single-stage treatment displays the longest low cycle fatigue life. The plastic and elastic strain amplitudes of Al-4Cu-0.15Zr-0.15 Sc alloys with different aging treated state as a function of reversals to failure show a linear relationship and obey the Coffin-Manson and Basquin equation, respectively.The fatigue cracks initiate at the surface of fatigue specimens in a transgranular mode, and propagate transgranularly.