Structure determination of Mg5Si6 particles in Al by dynamic electron diffraction studies. Science

Precipitation hardening, in which small particles inhibit the movement of dislocations to strengthen a metal, has long been used to improve mechanical strength, especially of aluminum alloys. The small size of precipitates and the many possible variants of the orientation relation have made their structural determination difficult. Small precipitates in commercial aluminum-magnesium-silicon alloys play a crucial role in increasing the mechanical strength of these alloys. The composition and structure of the b " phase in an aluminum-magnesium-silicon alloy, which occur as precipitates (typically 4 nano-meters by 4 nanometers by 50 nanometers) and are associated with a particularly strong increase in mechanical strength, were determined. Element analysis indicates that the composition is Mg5Si6. A rough structure model was obtained from exit waves recon-structed from high-resolution electron microscopy images. The structure was refined with electron nanodiffraction data (overall R value of 3.1 percent) with the use of a recently developed least squares refinement procedure in which dynamic diffraction is fully taken into account. Pure metals are in general rather soft. For example, Al and Fe have a strength of 40 to 70 MPa, depending on grain size, stress level (which reflects, among other things, the dislocation density), and solutes in the matrix, whereas tool steel has a strength of 800 to 1400 MPa. For common applica-tions, most pure metals are too soft, and for many centuries research has been focused on methods to increase the strength of met-als. The methods used to obtain this result can be divided into a few groups, including cold working, solid solution hardening, and precipitation hardening. Cold working and solid solution harden-ing have been used since ancient times. Research on precipitation hardening started at the beginning of this century with the discovery of Al-Cu alloys and Wilm’s research (1) on the strength of Al-Mg al-loys. With precipitation hardening, small amounts of other elements are added that form precipitates during a specified temper-ature cycle. Commonly used precipitation-hardened Al alloys are the binary systems with Cu and Ag and the ternary system