Elsevier

Intermetallics

Volume 22, March 2012, Pages 62-67
Intermetallics

Crystal structure of a new quaternary Mg–Zn–Ca–Li phase

https://doi.org/10.1016/j.intermet.2011.09.013Get rights and content

Abstract

In course of a study of high strength, ductile alloys based on the Mg–Li system, we have identified a new quaternary phase with a composition of Mg31Zn26Ca13Li8. The structure of this phase was investigated by Transmission Electron Microscopy (TEM) using the Convergent Beam Electron Diffraction (CBED) technique. Full atomic model is proposed using direct methods applied to powder X-ray diffraction data. The phase has a cubic primitive unit cell with lattice parameter a = 9.386(8)Å and its crystal symmetry belongs to the Pm3¯ (No. 200) space group. The unit cell of the new phase exhibits a new structure type, which is a variant of the Mg2Zn11 structure. The reliability factors characterizing the Rietveld refinement procedure are: Rp = 8.6%, Rwp = 12.4%, RB = 6.9% and Rf = 4.4%.

Graphical abstract

Highlights

► The structure a quaternary phase Mg–Zn–Ca–Li was determined by a combination of electron crystallography and X-ray powder diffraction methods. ► Unit cell geometry was identified by electron diffraction methods. ► The general symmetry and point group was determined by CBED. ► Space group was determined combining X-ray powder diffraction and electron diffraction data. ► Full atomic model is proposed based on direct methods applied to X-ray powder diffraction data.

Introduction

It is well known that lithium has the ability to improve mechanical properties of magnesium alloys, such as ductility and formability [1]. On the other hand, this improvement leads to poor strength of Mg–Li binary alloys. In order to overcome this problem, additional alloying elements should be added to the alloy. Zinc is usually used from a wide range of potential alloying elements due to its effectiveness in strengthening of Mg–Li alloys [2], [3], [4], [5], [6]. However, in order to achieve the goal of high strength-ductile alloys, additional alloying with a fourth element should be made. Since the addition of calcium has the potential to achieve additional strengthening through formation of highly dispersed precipitates [7], [8], [9], we have chosen Mg–Li–Zn–Ca as the system of interest for the current study. This system was investigated previously by Li et al. [9], and the authors proposed that the improvement of mechanical properties in this alloy is due to the formation of fine microstructure of Mg6Zn3Ca2 precipitates at grain boundaries.

In our study, we used a reciprocating extrusion process (detailed explanation regarding the process can be found in [10]) in order to achieve even finer microstructure and homogeneously distributed precipitates. However, we have identified only one type of precipitates and it was not Mg6Zn3Ca2, as reported earlier. It was found that these particles are composed of magnesium, calcium, zinc and lithium. An attempt to identify these particles on the basis of the ternary phases known in the literature for the Mg–Ca–Zn–Li system has proved to be unsuccessful.

The present study was undertaken with the purpose of characterizing the full crystal structure of an unknown quaternary phase and determining its lattice parameters, symmetry and atomic structure. The main techniques used for this investigation were Transmission Electron Microscopy (TEM), in particular Convergent Beam Electron Diffraction (CBED), and X-ray Diffraction (XRD). The composition of the new phase was estimated by Electron Dispersive Spectroscopy (EDS) in the Scanning Electron Microscope (SEM) and TEM, and Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS).

Section snippets

Experimental

Several alloys with different compositions were prepared from pure Mg (99.8), Zn (99.9), and master alloys Mg–7.5wt%Li and Mg–35wt%Ca. The alloys were melted in a plain carbon steel crucible under protective argon atmosphere and cooled slowly to room temperature. The materials were examined by XRD, SEM and TEM.

The first alloy with nominal composition of Mg–7.5%Zn–6.5%Li–2.25%Ca (wt %) was analyzed by TEM. This sample contained some grains of the new phase but not enough for full structure

Microstructure of the alloys

SEM image in Fig. 2A shows particles of the unknown Mg–Zn–Ca–Li phase as it was revealed in the Mg–7.5%Zn–6.5%Li–2.25%Ca (wt%) alloy. The new phase forms as coarse and fine eutectic with pure α-Mg phase (HCP structure) and pure β-Li phase (BCC structure). In addition to the new phase, the alloy contained the Mg2Ca phase (within the eutectic) and Zn-rich precipitates in the α-magnesium and β-lithium grains.

In order to obtain a larger amount of the new phase, a second alloy was cast with

Summary and conclusions

A new quaternary Mg–Zn–Ca–Li phase was revealed in the current study. Its structure and composition were fully characterized using CBED, SAED, ToF-SIMS and EDS methods. This phase has a cubic primitive unit cell with lattice parameter a = 9.386(8) Å; its crystal symmetry can be described by the Pm3¯ space group. A complete structural model revealed a connection between the structure of the new phase and the known binary Mg2Zn11, however since this phase is quaternary and not all atoms share all

Acknowledgements

Authors would like to thank: Dr. Alex Berner and Catherine Cytermann for help with the composition analysis; and Dr. Tzipi Cohen-Hyams for assistance with the FIB sample preparation. Special thanks are addressed to Jacob Kinstler.

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