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

Abstract

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 analysed 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 can not be dissolved during solution treatment, are detrimental to the fracture toughness of the alloys.

The Role of cobalt and nickel intermetallic phases on the mechanical properties and microstructure evolution of Al-Zn-Mg-Cu alloys

“…Addition of transition elements to high strength aluminum alloys enhances their mechanical properties. All the strengthening mechanisms of the high strength aluminum alloys depend primarily on precipitation hardening 10,11 . It is found that trace elements in aluminum can form a supplementary strengthening afforded in the most cases by dispersions which improve the recrystallization resistance and introduce a controlled amount of the strain 5,6 .…”

Effects of Sc and Zr microalloying additions on the microstructure and mechanical properties of new Al–Zn–Mg alloys

“…High-strength thermally strengthened weldable alloys based on the Al-Zn-Mg system are used extensively for structural applications in aerospace industry [16][17][18][19], and recently reported to be alloyed with a small amount of copper, which improves considerably the resistance of the base metal and especially that of welded joints to corrosion cracking and slow disruption. Though the presence of copper intensifies the formation of hot cracks in welding of aluminum alloys, this drawback is eliminated by the introduction of a combined scandium and zirconium additive [20].…”

Evolution of microstructure and properties in a new type 2mm Al–Zn–Mg–Sc–Zr alloy sheet

“…However, with the development of aerospace technology, it is urgent to develop new aluminum alloys with better performance. Material workers have found that rareearth elements are of great importance to improve the service performance of Al-based alloys [6][7][8][9][10]. On a per-atom basis, scandium (Sc) has the greatest strengthening effect of any existing alloying addition to Al [11][12][13].…”