MS-4-P-2744 Characterization of high strength Aluminium alloys with coherent precipitates

The present paper deals with a characterization of a class of aluminium alloys that are developed through a unique processing route that contains two different types of strengthening coherent precipitates in the aluminium matrix. The two types of precipitates are Al₂Cu (θ’, tetragonal structure) and Al₃Zr (L1₂ structure). The alloy belongs to Al-Cu binary system with minor additions of Zr, Nb and Zr, Hf. A unique three stage sequence of heat treatment was given to a chill cast alloy yielding final microstructure containing coherent finely dispersed L1₂ Al₃Zr precipitates that formed at high temperature and coherent disk shaped θ’ precipitates heterogeneously nucleated on the prior Al₃Zr particles. Therefore, tuning the heat treatment time and temperature for Al₃Zr precipitation, it is possible to achieve a finely spaced precipitate distribution of θ’ disks that are resistant to coarsening at high temperature. We show that this leads to remarkable high temperature strength with 0.2% proof stress of about 250 MPa at 250ºC, thus opening a new window for high temperature aluminium alloy development. The first stage of the heat treatment was done at three different temperatures that are 375ºC, 400ºC and 450ºC till the time the peak hardness was achieved. The precipitation and coarsening was monitored through hardness measurements (Vickers hardness, Hv) and STEM HAADF contrast images were used to calculate the change in length of θ’ disks, number of disks and their spacings for different temperatures and time. The final microstructures were also characterized by High Resolution Transmission Electron Microscopy (HRTEM) and atom probe tomography (APT).