Using θ′ interfaces as templates for planar L12 precipitation in AlCuMnZr alloys

Abstract

• Al 3 Zr L1 2 / θ' co-precipitates form faster in additively manufactured Al-Cu-Mn-Zr (ACMZ) alloys • Rapid solidification allows for higher matrix solute contents • Interfacial Zr segregation rate is dependent on the Zr matrix content • Theta prime precipitates are perfect templates for planar Al 3 Zr L1 2 formation Controlled Mn and Zr additions to Al-Cu alloys have allowed for the improved retention of mechanical properties after extended 350°C exposures by stabilizing the main strengthening θ' (Al 2 Cu) phase. Ultimately, θ' /L1 2 (Al 3 Zr) co-precipitate formation stabilizes θ' most effectively; however, Zr diffuses sluggishly and has low solubility in aluminum castings. Increasing the Zr segregation rate would allow for faster and more effective θ' /L1 2 co-precipitation. It is demonstrated that the Zr segregation rate is faster when the Zr matrix content is higher. A much higher Zr matrix content facilitated by rapid cooling during additive manufacturing (AM) was achieved that produces faster θ' /L1 2 co-precipitation, which is shown by scanning transmission electron microscopy and atom probe tomography experiments. It was also found that Zr continuously segregates to θ' interfaces up to the most aggressive heat treatment studied such that planar L1 2 precipitates remain after the metastable θ' dissolves. In this manner, we demonstrate that θ' coherent interfaces serve as perfect templates to form stable planar L1 2 precipitates that can provide strength at higher temperatures than traditional θ' strengthened AlCu alloys. This work introduces an alloy design strategy that uses metastable precipitates to quickly nucleate and grow co-precipitates with a desired geometry that contain slow diffusing elements. These ideas can be applied to engineer more heat resistant alloys by taking advantage of high solute matrix contents enabled by rapid cooling during additive manufacturing.