Characterization and strengthening effects of different precipitates in Al–7Si–Mg alloy

Abstract

The microstructure of the nanoscale precipitates (especially the β phase) formed in the primary a-Al grains of a cast Al–7Si–Mg alloy during ageing at different temperatures, and the strengthening effects of them on the mechanical properties have been systematically investigated by transmission electron microscopy (TEM) and tensile test. The interactions of dislocations with different types of precipitates were analyzed in depth. It was found that the GP zones and β″ phase can be easily sheared by dislocations along the close-packed {111}Al planes, leading to good deformation compatibility and elongation. The step-like planar defects were found in the deformed β″ phase, indicating that this needle-shaped precipitate can be sheared not only along the short axis but also along the long axis. The β phase and nanoscale Si particles are usually bypassed by dislocations, showing a typical Orowan strengthening mechanism, leading to the highest strength. This may be related to their large size (about 100 nm in diameter), high shear modulus and modest number density. In addition, the moiré fringes and FFT pattern of β phase were discussed by calculation of diffraction vectors. The moiré fringes with about 2.389 nm and 2.664 nm spacing displayed in HRTEM image results from the interactions between the 004Al/006β and 2¯22Al/3¯33β diffraction respectively. The corresponding complex FFT pattern is caused by strong double diffraction of [110]β and [110]Al reflections. Furthermore, an interesting phenomenon that the β phase tends to nucleate and grow on the surface of Si particle was firstly observed and discussed in depth.