Study on GP-Zones Formation in the Sc Inoculated Ternary Al-Zn-Mg Alloys

Abstract

Mostly, ternary Al-Zn-Mg (7075) alloys are used in automotive and aerospace engineering due to readily age-hardening response and high specific strength-to-density ratio. Scandium (Sc) has great inoculation effect in Al-Zn-Mg alloys. The cast Al-Zn-Mg alloys system undergoes a complex transition sequence in order to gain maximum strength. This sequence consists of solutionizing then immediate quenching in water (T4), natural ageing and artificial ageing (T6); usually, in several temperature steps to results in a structure of finely dispersed precipitates (Al3Sc). Quenching in water from the solution temperature 465oC/1 h induces severe internal stresses in the material; and introduction of dislocations in the supersaturated solid solution (SSSS) modifies the precipitation process. They act concurrently as vacancy sinks during room temperature ageing, which preferred nucleation sites and as short-circuit diffusion paths during later precipitation steps. The microstructure and the peak hardening effects are shown to depend on the competition between GP-zones and nucleation of ή and/or nucleation of dislocations. According to the ageing, process starts with the diffusion of Zn to Mg atoms and GP-zones develop by the formation of Mg-Zn clusters. The GP-zones can act as prospective nucleation sites of more stable precipitates providing the basis of artificial ageing treatments applied to Al-Zn-Mg alloy systems. The kinetics of the activation energy have been studied on a number of three different Al-Zn-Mg alloys by Vicker’s hardness measurement between 120 and 180oC. A systematic investigation has been carried out to study the decomposition of the SSSS in the formation of GP-zones and subsequent transition phases in the wide temperature range in Sc inoculated Al-Zn-Mg alloys. The influence of various process parameters on the microstructure and mechanical properties of the Al-Zn-Mg alloys are studied by OM, EBSD, FESEM, resistivity, XRD, DSC, TEM, and tensile properties.

Cite this Article

Mandal PK. Study on GP-Zones

Formation in the Sc Inoculated Ternary

Al-Zn-Mg Alloys. Journal of Materials

& Metallurgical Engineering. 2016;

6(2): 53–69p.