Mechanism of Precipitate Microstructure Affecting Fatigue Behavior of 7020 Aluminum Alloy

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.

Quenching Sensitivity of Al-Zn-Mg Alloy after Non-Isothermal Heat Treatment

Type Al-Zn-Mg alloy has a wide ranges of application in vehicles, but corrosion resistance and mechanical properties of this alloy after heat treatment or heat straightening limits its utilization. This paper investigates the effect of quenching condition during non-isothermal heat treatment on corrosion behavior and mechanical properties of Al-Zn-Mg alloy. The corrosion resistance of Al-Zn-Mg alloy decreases after non-isothermal heat treatment and the sample after air quenching has the lowest corrosion resistance, the specimens can get better corrosion resistance when suffered 5 min air cooling followed by water quenching process. The evolution of mechanical properties and corrosion resistance of heat-treated specimens is caused by the modification of distribution of precipitates at grain boundary and the microchemistry of precipitates at grain boundary, precipitate-free zones and the matrix.

Effect of Mo Addition on the Grain Growth of IN718 Alloy

The effect of Mo addition on the grain growth of Inconel 718 alloy has been investigated. It was revealed that δ phase was precipitated at the grain boundaries when the Mo addition is in the range of 2.80% - 4.00%, while the Mo-bearing phase was precipitated at the grain boundaries when the Mo addition is in the range of 5.50% - 7.50%. By pinning the grain boundary, the grain boundary precipitate can effectively prevent the grain growths. So the grains grow rapidly with increase the solution temperature when the grain boundary precipitate is absent. The activation energy value for grain growth of IN718 alloys with different Mo addition is 210 - 255 kJ/mol and the grain growth mechanism is controlled by the self-diffusion. The grain growth exponent reduces with increasing Mo addition.