Solving the Structure of the Phases in the Al-Mg-Si Alloy System with the Help of Ab Initio Modelling
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Solving the Structure of the Phases in the Al-Mg-Si Alloy System with the Help of Ab Initio Modelling A. G. Froseth, S. J. Andersen1, C. D. Marioara1, P. M. Derlet2, and R. Hoier Department of Physics, NTNU, 7491 Trondheim, NORWAY 1 SINTEF Materials Technology, Applied Physics, 7491 Trondheim, NORWAY 2 Present address: Paul Scherrer Institute, Nano-Crystalline Materials Group, CH-5232 Viligen PSI, SWITZERLAND
ABSTRACT The Al-Mg-Si (6xxx-series) alloy system is a precipitation hardened alloy gaining much of its strength from precipitate phases acting as pinning centers for dislocation movement. Five years ago, Zandbergen, Andersen and coworkers identified the crystallography of the so-called β'' phase, one of the main hardening phases, using solely electron microscopy techniques [1]. Later, several other phases have been identified using high resolution microscopy. To solve the crystallography of these phases and to get an increased understanding of the electronic structure and bonding, ab initio modeling has proven to be a valuable tool. We present results from calculations on two recently discovered phases and show how ab initio modeling can give insight into the bonding trends and electronic structure of the phases in this alloy system.
INTRODUCTION Precipitation hardened alloys is one of the most important alloy types in industry today. In the Al-Mg-Si alloy system, Mg-Si and Al-Mg-Si precipitates formed during heat treatment give rise to a very significant increase in strength. The precipitation sequence has generally been accepted to be: SSSS → Mg / Si Clusters → GP Zones → β ' ' → β ' → β
(1)
where SSSS means Super Saturated Solid Solution and GP Zones are Guinier Preston Zones. Very little is known about the early stages of the precipitation sequence. However, it is believed that due to the large amount of quenched in vacancies in the solid solution, Si and Mg quickly diffuse to generate Si and Mg clusters/co-clusters [2]. The first precipitate which can be resolved in a high resolution transmission electron microscope is the Guinier Preston zone. The next phase in the precipitation sequence, β’’, is a metastable phase closely related to the GP Zone, usually forming at temperatures above 150 ºC. This phase was long believed to have the same stoichiometry as the β phase - Mg2Si. In 1997, using solely electron microscopy techniques, the crystallographic structure of this phase was solved, revealing an Mg-rich phase – Mg5Si6 [1].
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Several studies have confirmed that this is the most important hardening phase, as it is more effective in pinning of dislocation movement [1]. Recently, several additional phases have been identified experimentally [3,4], giving the extended precipitation sequence: SSSS → Mg / Si Clusters → GP Zones → β ' ' → ( β ' + U 1 + U 2 + B ' ) → β
(2)
where β ’, U1, U2 and B’ are grouped together since not much is known about their interdependence. In ref. 3 the U1, U2 and B’ precipitates have the names type A, type B and type C respectively. Consequently, the precipit
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