A transmission electron microscopy study of precipitate phases that form during operation in a heat exchanger alloy

During manufacturing of heat exchangers, the core material is cladded with a lower-melting point alloy, rolled into thin strips before being formed and finally brazed at an elevated temperature. After a period of natural aging, the final product is operated at two different temperatures depending on the application: about 95 degrees C for radiators, and peaks up to about 250 degrees C for charge-air-coolers. For an Al-Mg-Si-Cu alloy type core material, this process translates into solution heat treatment, natural aging and aging during operation. High-resolution imaging with aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) revealed the presence of a complex mix of precipitates after 58 days at 95 degrees C, including a never before-reported phase which structurally is a mix between Al-Cu and Al-Mg-Si type precipitates. The stability of this phase is investigated with density functional theory (DFT). Q' is the main phase observed after 5 h at 250 degrees C, with most precipitates incorporating a certain type of stacking fault.