Thermally stabilized interface structure of the θ′-precipitates in heat-resistant AlCu(ScZr) alloys

Abstract

AlCu alloy series are important and interesting heat-resistant materials widely used in aerospace and automotive industry. Up to date, the heat-resistance mechanisms of the alloys are still controversial. Here, we report a thermally stabilized interface structure of θ′-precipitate and propose a new mechanism by which the Sc element enhance thermal stability of the alloys. By utilizing state-of-the-art electron microscopy and elemental atomic-resolution imaging, it is shown that GP zones and θ′′ precipitates, which form during a preliminary low-temperature aging, do not transform into the θ′-precipitates. Instead, they rapidly dissolve upon subsequent thermal exposure at 300 °C, thereby accelerating the early formation and coarsening of θ′ and θ precipitates, ultimately reducing the heat resistance of alloys. On the contrary, the formation of Sc/θ′/Sc or Al₃Sc/θ′/Al₃Sc composite precipitates in the alloys via a high-temperature aging process results in the thermally stabilization of all interfaces of the strengthening θ′-precipitates, which greatly improve the heat resistance of Al-Cu alloys. There exists either an “in-phase” or an “anti-phase” relationship between the two sideward Al₃Sc layers within these Al₃Sc/θ′/Al₃Sc composite precipitates, which is determined exclusively by the number of Cu-layers in the central θ′-precipitate. This unique interface configuration necessitates the realignment of all Sc atoms in each Sc-containing layer whenever the θ′ precipitate increases in thickness by one Cu-layer. This mechanism effectively inhibits rapid thickening of θ′-precipitates at elevated temperatures. Our findings offer atomic-scale insights into the heat-resistant properties and optimal thermal processing of the alloys.