Compressive Creep Behavior of Cast Al-Cu-Mn-Zr Alloys

Commercial Al-Cu-based alloys - which are used extensively in automotive engine applications because of their good castability and low cost - have high strength because of metastable θ′ (Al2Cu) precipitates, octagonal prisms in shape and with micron-size. However, above 200 °C, these precipitates coarsen rapidly and transform into the incoherent θ phase (Al2Cu), which is a much less efficient strengthener. To operate engines at higher temperature under more efficient conditions, there is a need to stabilize these metastable θ′ precipitates while also creating other high-temperature nanoprecipitates. Under the leadership of Dr. A. Shyam, this was achieved by additions of Mn and Zr to Al-Cu alloys, thus creating a new family of Al-Cu-Mn-Zr alloys which is under development at ORNL. A recent ORNL study elucidated the role of Mn and Zr in stabilizing the metastable θ′ Al2Cu precipitates formed on aging, via Mn and Zr segregation at the precipitate interfaces; this helps prevent θ′ overaging and maintain the strength of the alloy near peak values, even after long-term exposures to temperatures at 300 ºC and above. It was also demonstrated that Zr/Ti atoms segregating at the coherent θ′ interfaces create L12-Al3(Zr/Ti) nano-precipitates, thus forming a co-precipitate structure which is very thermally stable.