Effect of trace Er on the microstructure and properties of Al–Zn–Mg–Cu–Zr alloys during heat treatments

The effects of trace Er on the microstructural evolution and mechanical properties of newly designed Al–Zn–Mg–Cu–Zr alloys for drilling pipes were investigated. After homogenization, most of the eutectics dissolved into the matrix and some primary phases remained, including Al8Cu4Er, Al12(Mn,Cr), and Al3Ti. Dispersed nano-sized Al3(Er,Zr) precipitates were formed in the Er-containing alloys after homogenization. Following extrusion, solution and retrogression re-aging (RRA) treatment, the hardness and yield strength of the 0.10 wt% Er-containing alloy are increased by 14% (25.4 HV) and 12.7% (70 MPa) compared to the alloy without Er. This improvement could be ascribed to the precipitation strengthening of fine Al3(Er,Zr) particles, which complemented the conventional η' phase precipitation strengthening. The intergranular corrosion resistance of the RRA-treated alloy with the addition of 0.10 wt% Er was improved by 65% due to a lower amount of high angle grain boundaries. The fracture and intergranular corrosion mechanisms in alloys with and without Er addition were linked to the respective grain structures.