Structural modifications and electron beam damage in aluminium alloy precipitate θ′-Al2Cu

The theta'-Al2Cu phase in an Al-4Zn-2Cu-1Mg-0.7Si (wt.%) alloy was investigated by means of scanning transmission electron microscopy. With our specific alloy composition, the phase is often formed with stacking faults on {101}(theta') and {001}(theta') planes. The stacking faults on {101}theta' planes are often regularly spaced and create a previously unreported superstructure. Structural damage by electron irradiation is observed, even at a low acceleration voltage of 80 kV. The damage is more pronounced in the theta' precipitates with stacking faults, which agrees with theoretical calculations of knock-on scattering cross-sections. These two very different forms of disruptions of the theta' structure are linked to its spacious interstitial sites and the ease at which Cu atoms diffuse into and between them.