The crystal structure of the β′ phase in Al–Mg–Si alloys

doi:10.1016/j.actamat.2007.02.032

Acta Materialia

Volume 55, Issue 11, June 2007, Pages 3815-3823

https://doi.org/10.1016/j.actamat.2007.02.032 Get rights and content

Abstract

The crystal structure of β′, one of the metastable phases formed during precipitation hardening of Al–Mg–Si (6xxx) alloys, has been determined using electron diffraction (ED). With the aid of high-resolution electron microscopyimages and the composition estimated from energy dispersive X-ray analysis, an initial model for the structure of the β′ phase was obtained. The data from digitally recorded ED patterns were then used for a least-squares refinement of the atomic parameters of the model with a software program package developed in Delft (MSLS), taking into account dynamic scattering. The β′ structure has a composition of Mg₉Si₅. The unit cell is hexagonal, space group P6₃/m, with unit cell parameters a = 0.715 nm and c = 1.215 nm. The composition and structure were confirmed by ab initio calculations.

Introduction

The attractive physical and chemical properties (in terms of corrosion, formability, weldability, etc.) of Al–Mg–Si (6xxx) alloys have led to them becoming widely used in industry. Their most striking feature is the large increase in strength during precipitation hardening of these alloys (from ∼60 HV to up to 130 HV). This process involves the formation of Mg–Si metastable phases during heat treatments at moderate temperatures (100–200 °C). The precipitation sequence has generally been reported as $supersaturated solid solution \to clusters with varying Mg and Si contents \to GP zones \to β^{″} \to β^{'} + B^{'} + U1 + U2 \to β (stable)$

Table 1 gives an overview of the different phases.

Upon rapid cooling of the alloy, from the homogenization temperature of 550 °C to room temperature, a supersaturated solid solution (SSSS) of Mg and Si in Al is formed. After storage at room temperature, the alloy is processed further by annealing, typically at temperatures of 100–200 °C. The hardness of the alloy will increase due to the formation of the precipitates listed in Table 1 (age hardening).

In the first stage of the precipitation process, Si and Mg will start to form clusters. Initially, the clusters will be rich in Si, due both to its poorer solubility in Al and its higher diffusion speed. Subsequently, Mg and more Si will diffuse into the Si-rich clusters to form Mg/Si clusters [9]. From these clusters, firstly the GP zones will form. Throughout the literature, there has been ambiguity regarding the term “GP zones” (or “GP-I zones”). In some cases the name is used for the highly coherent needles from which the β″ phase evolves (e.g. Ref. [1]), while elsewhere it refers to the spherical, fully coherent particles formed initially after the Mg/Si clusters (e.g. Ref. [10]).

Upon further age hardening, β″ particles (also referred to as GP-II zones) are formed. The β″ phase is fully coherent with the Al matrix along the b-axis and semi-coherent along a and c. It consists of needles typically 4 × 4 × 50 nm in size (with the b-axis along the needle axis). The mechanical strength of the Al–Mg–Si alloys originates mainly from the β″ and GP-I phases. The β′ phase discussed in this paper forms as rods of ∼10 × 10 × 500 nm. For β′, generally a hexagonal cell has been reported, with unit cell dimensions a = 0.705 nm and c = 0.405 nm (e.g. in Refs. [4], [5]). It is fully coherent with the 〈0 0 1〉_Al along the c-axis. A crystal structure with spacegroup $P {\bar{6}}_{2} m$ has been suggested [7]. However, the suggested structure was based on very limited data and its composition (Mg/Si = 0.74) does not match with the energy dispersive X-ray (EDX) data (Mg/Si = 1.39) presented. Also, first-principle calculations [11] have shown that the structure is energetically very unfavourable. Upon relaxation of the unit cell parameters, the unit cell was found to expand strongly along the c-axis, which would be in contradiction with the coherency of the β′ phase with the 〈0 0 1〉_Al.

The β′ phase coexists with the hexagonal B′ phase and at least two other phases: U1 [4] and U2 [4], [6] (also referred to as type A and type B [5]).

The β phase, the equilibrium phase, has been found to be cubic F centred CaF₂. It forms as plates with dimensions of several micrometres with composition Mg₂Si.

In this paper we report the complete crystal structure of the β′ phase. The structure has been refined using quantitative analysis of electron diffraction data. The β′ phase has a hexagonal unit cell, spacegroup P6₃/m, with a = 0.715 nm and c = 1.215 nm. This structure has been confirmed using first-principles calculations.

Section snippets

Material and sample preparation

A commercial 6082 Mg–Al–Si alloy (composition: 0.6 wt.% Mg, 0.9 wt.% Si, 0.5 wt.% Mn and 0.2 wt.% Fe) was homogenized in a salt bath at 540 °C for 55 min and subsequently quenched in water at room temperature. After 2 days storage at room temperature, the samples were annealed for 8 h at 260 °C. As the β′ particles’ structure is not preserved if conventional electropolishing is used, the specimens for high-resolution electron microscopy (HREM) and diffraction experiments were ion-milled instead, using

The initial model

An image of a typical β′ particle is shown in Fig. 1A. To propose an initial model, an exit wave reconstruction from a through focus series of HREM images was performed. The resulting reconstructed exit wave (of which a section is shown in Fig. 1B and C) was combined with knowledge of typical interatomic distances found in previously reported structures [1], [2], [3], [4], [5], [6], [8] of phases in the precipitation sequence to suggest a starting model.

For the initial refinement we used [0 0 1]_β′

Summary

The crystal structure of β′, one of the phases formed during precipitation hardening of Al–Mg–Si alloys, has been determined successfully using quantitative electron diffraction. The β′ phase has a hexagonal unit cell, spacegroup P6₃/m, with a = 0.715 nm and c = 1.215 nm. The model takes into account three (equivalent) ways to arrange the Si atoms at 00z positions along the c-axis. If the positioning of these Si atoms is randomly distributed among the three possible arrangements the model can be

Acknowledgements

This work is a part of the research programme of the “Stichting voor Fundamenteel Onderzoek der Materie (FOM)”, which is financially supported by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)”. C.D. Marioara and S.J. Andersen acknowledge financial support through the NorLight programme – financed through industry and the Norwegian Research Council. Industrial sponsors are the following Norwegian companies: Hydro Aluminium AS, Raufoss ASA and Elkem Aluminium. More

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The crystal structure of the β′ phase in Al–Mg–Si alloys

Abstract

Introduction

Section snippets

Material and sample preparation

The initial model

Summary

Acknowledgements

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