Direct observation of initial stages of precipitation hardening process in commercial Al 6061 alloy

Evolution of precipitates in aluminum 6061 alloy, quenched after 2 h hold at 550ºC (super-saturated solid solution state—SSSS) and aged at 145ºC (for up to 960 min), was studied by routine, ex-situ, transmission electron microscopy (TEM). In parallel, initial stages of the precipitation hardening process (after few-minute ageing) were studied by in situ TEM. The appearance of short-living (~ 8 min) loose, disordered, spherical ~ 6 nm in diameter precipitates was captured. These precipitates rearranged after relatively long interval of time (~ 100 min, corresponding to 240 min ageing in the ex-situ tests) into long-range ordered rod-like Guinier–Preston (GP) zones (presuming GPI). The dimensions of the latter, 20 nm × 2.1 nm, were determined based on the ex-situ TEM images. Longer, 960 min, ageing results in GPI → GPII transformation accompanied by an increase in dimensions and strength of the rod-like precipitates. Determined geometrical parameters are in a reasonable agreement with corresponding parameters assessed previously using shock-wave technique. This fact implies that shock-wave technique can be considered as a useful tool for studying dislocation/defect interactions in a wide variety of strengthened alloys. Graphical abstract: [Figure not available: see fulltext.].