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

Abstract

Meta stable embedded nano-sized η -particles within a single grain extracted from an alloy having the nominal composition Al 0.88 Zn 0.10 Mg 0.02 have been examined with X-ray diffraction. By applying the orientational and metric relationships that exist between the hexagonal unit cell of the η -particles and the cubic unit cell of the Al-matrix, it has been proven possible to directly collect diffracted intensity data from the η -particle ensemble. This has been done using synchrotron radiation and a κ-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 Mg 2 Zn 5 Al 2 and one Al-depleted with approximate stoichiometric composition Mg 2 Zn 7 Al. Both structures are realized in the same space group, P6 3 /mmc, 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.

Crystallography of embedded particles in Al–Mg–Zn alloys. Symmetry analysis

“…3 of Larsen et al (2014) for an actual example]. These are the governing quantities regarding the observability of a node in a diffraction experiment.…”

“…A hitherto unknown family, 12 , has been included in this presentation, given in the last line of Table 1. It was discovered in a reciprocal space reconstruction based on CCD data obtained, as a part of the assessment of crystal quality, in connection with a project aimed at 0 structure determination from synchrotron diffraction data (Larsen et al, 2014). Just a weak indication of this precipitate phase was found, as only reflections with the two largest values of structure factors were observed.…”

“…For its lattice constant a Al ¼ 4:055 Å is used. The structures of the variants within the 0 family (Larsen et al, 2014) and the families (Komura & Tokunaga, 1980) are all associated with the hexagonal space group P6 3 =mmc and the point group 6=mmm (G ). The metric relations fulfilled by the 0 lattice constants, leading to coherence with the Al lattice, are…”

“…The colour codes [green (g), blue (b), red (r) and yellow (y)], suitable for display purposes, are adopted following Sjølyst-Kverneland et al (2011) and Larsen et al (2014).…”

“…Degischer et al (1980) have given a summary of the variety of incoherent types with respect to their shapes and ORs. Further information concerning the alloy system in question has been reported by Gjønnes & Simensen (1970), Sjølyst-Kverneland et al (2011), Larsen et al (2014) and references therein. Guinier-Preston (GP) zones are not explicitly considered here.…”