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 degrees 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 theta' (Al2Cu) precipitates contribute to strengthening in Al-Cu alloys, but above 250 degrees 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 degrees C, similar to 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 theta' precipitate-matrix interfaces provides the mechanism by which the precipitates are stabilized to a higher homologous temperature.