The synergistic role of Mn and Zr/Ti in producing θ′/L12 co-precipitates in Al-Cu alloys

Microstructural stability is a critical factor to consider when designing new alloys for high-temperature applications. An Al-Cu alloy with Mn and Zr additions has recently been developed to withstand extended exposures of up to 350 degrees C. The addition of Mn in combination with Zr and their segregation to precipitate interfaces play a significant role in stabilizing the metastable theta' precipitates responsible for the alloy's hardness; however, adding Zr and Mn separately only improves the stability to 200 degrees C and 300 degrees C, respectively. To this end, the effect of the synergistic additions on interfacial structure and chemistry was studied in detail using atom probe tomography (APT) and scanning transmission electron microscopy (STEM) for Al-Cu-Mn-Zr/Ti-containing alloys subjected to long-term annealing (up to 2,100 h) in the critical temperature range, 300 degrees C and 350 degrees C, to investigate the role of Zr/Ti in increasing the theta'-precipitate stability. The APT and STEM results reveal that Mn additions stabilize theta' long enough for the slower diffusing Zr atoms to segregate to coherent theta' interfaces that eventually create a theta'/L1(2)-Al-3 (Zr-x,Ti1-x) co-precipitate structure. The co-precipitate is highly stable, as shown by density functional theory calculations, and is a key factor that governs microstructural stability beyond 300 degrees C. This study reveals how solute additions with different stabilization mechanisms can work in concert to stabilize a desired microstructure, and the results provide insights that can be applied to other high-temperature alloy systems. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd.