Abstract
The effect of different precipitate microstructures obtained by different heat treatments on fatigue behavior of 7020 aluminum alloy was investigated. The fine Guinier Preston I (GPI) zones in the under-aged alloy can be repeatedly sheared by dislocations produced in cyclic loading, making the fatigue crack initiate difficultly and fatigue crack path propagate tortuously. Fatigue strength and fatigue crack propagation resistance of the alloy with shearable precipitates are much higher than those of the alloy with unshearable precipitates. The peak-aged alloy with continuous grain boundary precipitate (GBP) and narrow precipitate free zone (PFZ) is prone to initiate fatigue cracks and reduce fatigue strength. With the growth of unshearable precipitates, the fatigue strength of the alloy firstly increases and then decreases. Precipitates with moderate size in the over-aged alloy improve the roughness-induced crack closure (RICC) effect. Soft matrix with appropriate width between the precipitates can promote the slip reversibility and relax the crack tip stress. The fatigue strength of the moderately over-aged alloy reaches to 122.1 MPa at 107 cycles of loading, and the fatigue crack growth rate (FCGR) is 35.6% slower than that of the peak-aged alloy at ΔK of 10 MPa·m1/2.
Highlights
7020 aluminum alloy belongs to Al-Zn-Mg series heat-treatable aluminum alloy
Representative bright-field Transmission electron microscope (TEM) images and corresponding selected area electron diffraction (SAED) patterns were present in Figure 4 for the under-aged and peak-aged alloys
The effect of different precipitate microstructures on fatigue fracture behavior of 7020 aluminum alloy was investigated in this study
Summary
The alloy has been widely used in high-speed trains and new energy vehicles because of its high strength-weight ratio, good forming performance and excellent welding properties [1,2]. Precipitate characteristics in Al-Zn-Mg alloys are the main factors that determine the strength and corrosion performance [3,4,5], and strongly affect the fatigue performance [6,7]. It is necessary to investigate the precipitate characteristics and microscopic mechanism of 7020 aluminum alloy with optimal fatigue performance. The types, sizes and distribution characteristics of the precipitates of Al-Zn-Mg aluminum alloy can be adjusted through different aging processes. The precipitates in the alloy mainly include GP zone, η’ metastable phase and η (MgZn2 ) equilibrium phase [8]
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