Average crystal structure(s) of the embedded meta stable η′-phase in the Al–Mg–Zn system

Abstract

Meta stable embedded nano-sized -particles within a single grain extracted from an alloy having the nominal composition have been examined with X-ray diffraction. By applying the orientational and metric relationships that exist between the hexagonal unit cell of the -particles and the cubic unit cell of the Al-matrix, it has been proven possible to directly collect diffracted intensity data from the -particle ensemble. This has been done using synchrotron radiation and a -diffractometer having a scintillator point detector setup. The approach has resulted in improved data quality compared to previous experiments. The interpretation of the data set, based on a combination of Patterson syntheses, direct methods and geometrical restraints, yielded two possible average structural representations: one Al-rich with the approximate stoichiometric composition and one Al-depleted with approximate stoichiometric composition . Both structures are realized in the same space group, , and are most probably superimposed in the crystalline system examined. The geometries are discussed within the atomic environment approach where icosahedral or near-icosahedral configurations are encountered. Comparison with previous published models and the equilibrium structure reveals a main difference related to the distribution of the Zn-sites in the unit cell. A possible transformation path is also suggested. Various aspects and challenges regarding data collection, data reduction and data quality are specifically addressed.

Keywords:

• synchrotron X-ray diffraction
• Al–Mg–Zn system
• embedded nano-particles

Acknowledgments

The authors wish to express their thanks to dr. Atle Sjølyst-Kverneland and prof. Vidar Hansen for providing the material used in this investigation and for stimulating discussions. Furthermore we are grateful to the Swiss-Norwegian beamline consortium for providing synchrotron beamtime and general support during the experiment.