Strengthening Al-Zn-Mg Alloys via Ultra-Fine Lamella Structures Containing a High Density of Dislocations and Clusters

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.

The Behaviours of Alloys under Thermo-Mechanical Treatment

“…The works of Q.L. Zhao [4], Z. Gao et al [5] and Y. Qin et al [6] deal with the processing of aluminium alloys. The first work [4] analyses the effect of the temperature before rolling on the microstructure and mechanical properties of a 6061 aluminium alloy.…”

“…The grain refined within the processed zone due to geometric dynamic recrystallization. In the work of Y. Qin et al [6], innovative thermo-mechanical processing was applied to an Al-Zn-Mg alloy, resulting in an increment in the yield strength of 100 and 200 MPa with respect to the T6 condition. The compared processes consisted of cold deformation after different heat treatments.…”