Elevated temperature microstructural stability in cast AlCuMnZr alloys through solute segregation

Commonly used commercial cast aluminum alloys for the automotive industry are viable for temperatures only up to 250 °C, despite decades of study and development. Affordable cast aluminum alloys with improved high-temperature mechanical properties are needed to enable the next generation of higher efficiency passenger car engines. Metastable θ′ (Al₂Cu) precipitates contribute to strengthening in Al–Cu alloys, but above 250 °C coarsen and transform, leading to poor mechanical properties. A major challenge has been to inhibit coarsening and transformation by stabilizing the metastable precipitates to higher temperatures. Here, we report compositions and associated counter-intuitive microstructures that allow cast Al–Cu alloys to retain their strength after lengthy exposures up to 350 °C, ∼70% of their absolute melting point. Atomic-scale characterization along with first-principles calculations demonstrate that microalloying with Mn and Zr (while simultaneously limiting Si to < 0.1 wt %) is key to stabilization of high-energy interfaces. It is suggested that segregation of Mn and Zr to the θ′ precipitate-matrix interfaces provides the mechanism by which the precipitates are stabilized to a higher homologous temperature.