MgZn precipatates in Mg matrix - electron diffraction tomography study

M. Klementová, L. Palatinus, M. Němec, V. Gärtnerová

Institute of Physics of the AS CR, v.v.i., Na Slovance 2, 182 21 Prague 8, Czech Republic

klemari@fzu.cz

Low density, high specific strength and the ease of recycling make magnesium and its alloys potentially good candidates for numerous structural applications [1]. One of the most common alloying elements in magnesium is Zn. Besides remarkable improvement of mechanical properties via solid solution and/or precipitation strengthening, Zn is together with Mg classified as a biocompatible element. Thus, Mg-Zn based systems can also be considered as an attractive material for implants.

Binary magnesium alloy with nominal composition Mg-12 wt.% Zn was prepared by die casting under Ar atmosphere and subsequently annealed at 320 °C for 20 hours followed by warm water quenching. The goal of this work is to describe the crystal structure of Zn based particles present in the binary magnesium alloy.

Samples were studied by transmission electron microscopy performed on a Philips CM 120 (LaB6, 120 kV) equipped with a NanoMEGAS precession unit DigiStar, an Olympus SIS CCD camera Veleta (2048x2048), and an EDAX windowless EDS detector Apollo XLTW. Precession-assisted electron diffraction tomography (EDT) in microdiffraction setup was used to acquire data for structure determination of MgZn precipitates and their orientation within the Mg matrix.

Precipitates of several micrometers in size (Fig. 1a) correspond to Mg21Zn25 phase with rhombohedral structure, space group R-3c, lattice parameters a ~ 26 Å, c ~ 8.9 Å (Fig. 1b). The structure was determined from 1711 independent reflections (averaged from 13026 measured intensities, Rint = 24.09), and refined using kinematical approximation to R-value of 26.53 %. The structure model matches very well the previously reported structure of Mg21Zn25 [2]. The matrix is formed by hexagonal Mg, space group P63/mmc, lattice parameters a=3.2 Å, c=5.2 Å. Orientation relationship of MgZn precipitates in Mg matrix was observed as (10-1)Mg || (010)MgZn and [101]Mg close to [201]MgZn (Fig. 1c). However, this relationship might vary significantly as the precipitates are quite coarse and therefore loss of coherency is expected.

Figure 1: (a) Bright-field TEM image of MgZn precipitates in Mg matrix, (b) results of structure solution from EDT data of a MgZn precipitate viewed down [001] (top - map of electrostatic potential, bottom - structural model), (c) oriented SAED pattern of a MgZn precipitate in Mg matrix viewed down Mg [101].

1. T.M. Pollock, Science, 328, (2010), 986-987.

2. R. Černý & G. Renaudin, Acta Cryst., C58, (2002), 154-155.