Hw. Zandbergen et al., STRUCTURE DETERMINATION OF MG5SI6 PARTICLES IN AL BY DYNAMIC ELECTRON-DIFFRACTION STUDIES, Science, 277(5330), 1997, pp. 1221-1225
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.