STRUCTURE DETERMINATION OF MG5SI6 PARTICLES IN AL BY DYNAMIC ELECTRON-DIFFRACTION STUDIES

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 relatio n have made their structural determination difficult. Small precipitat es in commercial aluminum-magnesium-silicon alloys play a crucial role in increasing the mechanical strength of these alloys. The compositio n and structure of the beta '' phase in an aluminum-magnesium-silicon alloy, which occur as precipitates (typically 4 nanometers by 4 nanome ters by 50 nanometers) and are associated with a particularly strong i ncrease in mechanical strength, were determined. Element analysis indi cates that the composition is Mg5Si6. A rough structure model was obta ined from exit waves reconstructed from high-resolution electron micro scopy images. The structure was refined with electron nanodiffraction data (overall R value of 3.1 percent) with the use of a recently devel oped least squares refinement procedure in which dynamic diffraction i s fully taken into account.