Nanoscale precipitation in aged Al–3.5Cu–0.4Mg–0.2Ge alloy

During ageing at 200 °C, a trace (0.2 wt%) addition of Ge to a base Al–3.5Cu–0.4Mg alloy resulted in anomalous precipitation processes by stimulating the formation of nanoscale precipitate and suppressing the formation of S′(Al2CuMg) precipitate. This behavior was demonstrated to be closely related to the preferential Mg–Ge cluster which formed in the early ageing stage. The increased hardening response and enhanced hardness in Ge-containing alloy were due to the rapid precipitation of high density of fine nanoscale precipitate. The transmission electron microscopy (TEM) investigation indicated that nanoscale precipitate, θ′ (Al2Cu), θ′II (Al2Cu) and σ (Al5Cu6Mg2) precipitates could coexist in Al matrix for prolonged ageing at 200 °C. And the nanoscale precipitate was demonstrated to own 〈001〉α needle-shaped morphology and be enriched with aluminum, copper, magnesium and germanium. The Mg/(Cu+Ge) atomic ratio in them is close to 1:1. High-resolution TEM observation indicated that the nanoscale precipitates appeared to be coherent with Al matrix and their orientation relationship is 〈100〉nano//〈100〉Al and {010}nano//{010}Al. A modified structure with composition of Al10Mg3Cu3−xGex (0<x<3) was proposed based on the results and simulation. The proposed structure has orthorhombic structure with lattice parameters a=0.405 nm, b=1.62 nm and c=0.405 nm and space group Imm2. The simulated diffraction patterns fit very well with experimental selected area electron diffraction (SAED) results.