Phase transformations in novel hot-deformed Al–Zn–Mg–Cu–Si–Mn–Fe(–Sc–Zr) alloys

Abstract

Precipitation stages in the hot-deformed Al-5.28wt.%Zn-3.21wt.%Mg-1.48wt.%Cu-0.24wt.%Si-0.24wt.%Mn-0.14wt.%Fe(−0.23wt.%Sc-0.19wt.%Zr) alloys were investigated. Positron annihilation spectroscopy was employed for a study of vacancy assisted clustering of solutes on the atomic scale, type and concentration of open volume misfit defects and local chemical surroundings of defects. Microhardness testing, thermal analysis and microstructure observations were also used. The alloys contain the eutectic grain boundary phases: T-phase and α-Al (Mn,Fe,Si) phase. Positrons are trapped at Zn,Mg-containing Guinier-Preston (GP) zones with no open volume defects and at (precursors of) the η′-phase particles. Above ~150 °C precipitation of non-eutectic T-phase and coarsening of the S-phase particles are observed. Precipitate-matrix interfaces are characterized by vacancy-like misfit defects decorated by Si solutes. Binding energies of vacancies to various solutes (Si, Zn, Cu, Mn, Sc and Zr) obtained by ab-initio calculations were computed. Si solutes exhibit the highest vacancy binding energy among the constituents of the studied alloys. Other solutes (especially Mg, Sc, Zr) repel vacancies. The alloy with Sc,Zr-addition contains incoherent primary Al3(Sc,Zr)-phase. Layered primary particles with different morphological features such as square and polygonal shapes contain Sc,Zr-rich layers.

Keywords

• Aluminium 7xxx series
• Phase transformations
• Clusters
• Binding energy
• Annihilation of positrons
• Al3(Sc,Zr) particles