The crystallization of giant intermetallic compounds in 7075 aluminum alloys.

The formation mechanism of brittle and coarse giant intermetallic compounds in Al-Zn-Mg-Cu alloys, corresponding to the 7075 alloy, has been investigated. Giant intermetallic compounds are found to be a CrAl7 phase into which Mn and Ti dissolve, and they crystallize as primary crystals prior to the crystallization of α-Al. The primary crystallization field and the crystallization temperature, T (°C), of the CrAl7 phase are presented by [%Mn] + 3.25 [%Cr] = 0.682 (at 637.5°C) and T = 600.9+53.70 ([%Mn] + 3.25 [%Cr]), respectively. The influence of Zn, Mg and Cu on the -primary crystallization field of the Cr compounds is found to be rather small. Applying these equations, it is possible to predict and prevent the precipitation of the compounds during solidification. The addition of Zr to the 7075 alloy also causes the precipitation of the Zr primary compound of (Zr, Ti) Al3.