Abstract
A new method of thermo-mechanical processing has been designed by introducing pre-aging before general cold rolling for an Al-Zn-Mg alloy. This process results in an increase of 200 MPa in yield strength compared to that of the peak-aged samples. The microstructures were examined by transmission electron microscope and X-ray diffraction. It has been found that the enhanced strength is mainly contributed to by ultra-fine lamella structures containing a high density of dislocations pinned by nanoprecipitates. Extra strength is provided by the “interlocking” of precipitates and dislocations. Fractographic features analysis shows that crack propagation along the interface of the lamella structures is the direct reason for resulting in fracture, due to intra-granular strength exceeding grain boundary cohesion.
Highlights
Al-Zn-Mg alloys are widely used as structural materials in aerospace and other transportation applications for their high strength and low density [1]
The thermo-mechanical processes used were designed as solid solution treatment + severely plastic deformation (SPD) + aging, promoting heterogeneous nucleation of precipitates employing dislocations as the nucleation site, resulting in grain refinement via precipitates pinning the sub-grain boundary
A large number of coarse voids along the interfaces between the lamella structures are observed on the fracture surface of the AR sample (Figure 6b), suggesting intergranular cracking, which is probably fracture surface of the AR sample (Figure 6b), suggesting intergranular cracking, which is probably due to a high density of dislocations sticking at the boundaries after rolling
Summary
Al-Zn-Mg alloys are widely used as structural materials in aerospace and other transportation applications for their high strength and low density [1]. Zhao et al found that the coupling of dislocations and precipitates offers a pleasant surprise in microstructures [15,19,20,21], i.e., nanosized grains, which provide extra strength for 7075 alloy (one of the Al-Zn-Mg alloys) In these works, the thermo-mechanical processes used were designed as solid solution treatment + severely plastic deformation (SPD) + aging, promoting heterogeneous nucleation of precipitates employing dislocations as the nucleation site, resulting in grain refinement via precipitates pinning the sub-grain boundary. An Al-Zn-Mg alloy (7N01) with the lowest Zn content in all commercial Al-Zn-Mg series, widely applied as a structure material in high-speed trains, was employed to investigate the coupling of precipitates and dislocations This time, types of sheared precipitates (GPη0 zones [2,3]) by dislocations were introduced before conventional deformation (cold rolling), in other words, aging was ahead of deformation. 7N01 alloy, which was contributed to by ultra-fine lamella grains, phase transformation and a high density of evenly distributed dislocations
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