Identification and analysis of intermetallic phases in overaged Zr-containing and Cr-containing Al–Zn–Mg–Cu alloys

This paper investigates the effect of alloying elements on the characteristics of intermetallic phases in Zr-containing and Cr-containing 7xxx Al–Zn–Mg–Cu alloys at overaged conditions. Four Al–Zn–Mg–Cu alloy plates with different alloying element contents were studied by optical microscopy based image analysis, differential scanning calorimetry, scanning electron microscopy combined with energy disperse X-ray spectroscopy and transmission electron microscopy. The grain structures, recrystallisation, intermetallic phases and precipitates in the selected alloys have been analyzed and the presence of coarse intermetallic phases has been interpreted using established phase diagrams. The different effects of Zr or Cr addition to the alloys have been compared. The experimental results showed that the recrystallised area fraction of Zr-containing alloys is less than that of Cr-containing alloys, being attributable to Zr reducing recrystallisation more effectively than Cr. The detected particles are mainly S phase, Al7Cu2Fe, as well as dispersoids of Al3Zr for Zr-containing alloys and Cr-rich E phase for Cr-containing alloys. These coarse particles, especially the S phase which cannot be dissolved during solution treatment, are detrimental to the fracture toughness of the alloys.

Research highlights▶ Intermetallic phases in Zr-containing and Cr-containing Al–Zn–Mg–Cu alloys are analyzed. ▶ The recrystallised area fraction of Zr-containing alloys is less than that of Cr-containing alloys. ▶ The high Cu alloy has the highest volume fraction of coarse particles. ▶ The detected particles are mainly S phase, Al7Cu2Fe, as well as Cr, Fe-rich intermetallic particles.