Precipitation in multicomponent, lean, Al-Mg-Si alloys: A transmission electron microscopy study
Doctoral thesis
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http://hdl.handle.net/11250/2414104Utgivelsesdato
2016Metadata
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- Institutt for fysikk [2702]
Sammendrag
Aluminium alloys have a number of versatile applications, and there are numerous alloys in
production today, where a variety of solute additions and heat treatments are applied. This
work focusses on lean, age-hardenable Al-Mg-Si alloys, which are light weight, have
excellent corrosion resistance and a moderate strength potential. In Al-Mg-Si alloys it is the
growth of nanosized needles, consisting of Mg, Si and Al itself, which leads to an increase in
strength during heat treatment of the alloys. The nanosized needles are metastable and grow
along the three <001> Al crystal directions. During heat treatment, several needle phases may
form, dependent on parameters such as the extent of the heat treatment, its temperature and
the alloy composition.
A major challenge in industry, and the first motivation behind this work, is to make extrusion
of lean Al-Mg-Si alloys easier and faster, while simultaneously not making any sacrifices in
strength. The second major motivational point of this thesis is to connect the macroscopic
material properties like strength and extrudability to the atomic scale characteristics. Only in
this way tailor-made alloys can be made for specific applications. Transmission electron
microscopy (TEM) serves as an excellent tool for microstructure characterization of Al. The
high spatial resolution and possibility of detecting a number of different electron-sample
interactions are obvious advantages.
High angle annular dark field scanning TEM (HAADF-STEM) with aberration corrected
lenses provides exceptionally high spatial resolution, down to the atomic scale. Aberration
corrected HAADF-STEM is particularly well suited for detecting elements with different
atomic numbers (Z) because of its Z contrast. However, in some cases the elements might be
too close in atomic number to separate intensity differences, which is where electron energy
loss spectroscopy (EELS) comes to aid. With EELS one obtains a ‘fingerprint’ of each
element based on the energy losses of the electrons, which are dependent on the elements
they have interacted with while passing through the sample.
For surface-specific dynamics in the material, information can be attained by photoelectron
spectroscopy (PES). PES enables us to do in-situ investigations of core levels during heat
treatment of the material. By making use of this method, the complications of detecting
elements with overlapping EEL edges, like e.g. Li and Mg, can be solved.
Two main approaches have been chosen to solve the extrudability vs strength issue here. The
first approach was based on lowering the amount of added Si and Mg, while adding back a
smaller or equal amount of Li, Cu, Ge or Ag. In the second approach, ‘disturbed ageing’ was
applied at various times during heat treatment. This was accomplished by elastic straining or
introducing small plastic deformations to the material.
It has been demonstrated how strength loss in an Al-Mg-Si alloy, caused by reduction in
solute, can be compensated by adding back smaller quantities of Li, Cu, Ag or Ge. These solute additions were added to Al-Mg-Si alloys alone or in different combinations. Ge was
discovered to be the most effective solute addition, significantly refining the precipitation and
strengthening the material in spite of lower total solute.
Cu, Ag and Ge additions have strong influence on the main hardening precipitate, β”. Ge in
particular changes its structure and promotes disorder, which seems to be favourable for
increased material strength. Cu and Ag both participate in the precipitation of needles and
change the precipitation sequence. HAADF-STEM investigations indicated that Li causes
modest structural changes to the main hardening precipitate β”.
DFT has been used to support the experimental results and better explain the observed
features in each alloy. Bonding energies, volume misfits, and formation enthalpies have been
calculated for solute additions, vacancies and structural variants of the β” phase. DFT and
HAADF-STEM support intensity variations suggesting Li to occupy Mg sites, and in
particular the Mg3 sites in β”.
The potential of atomic resolution EELS has been demonstrated by detailed investigation of
the elemental distribution in a precipitate cross section in a multicomponent Al alloy.
Furthermore, a correlative analysis of the EELS data was performed and connected to the
results. The EELS technique alone was able to resolve both the face centred cubic (fcc) Al
lattice along the <001> Al zone axes and the hexagonal Si-network in a precipitate cross
section. Some atomic columns were revealed to contain a mix of elements after combining
EELS and HAADF-STEM.
It has been demonstrated how a commercial Al-Mg-Si alloy can get enhanced strength at
peak hardness conditions when elastic strain is applied at the beginning of natural ageing.
The strengthening effect is explained by enhanced formation of clusters, causing a higher
number density of precipitates at peak hardness. Applying 1 % plastic deformation increased
the material strength as compared to an un-deformed reference alloy; the strengthening effect
is in this case attributed to the introduction of dislocations to the material during deformation.
X-ray photoelectron spectroscopy (XPS) and X-ray photoemission electron microscopy
(XPEEM) were used to study Li, Mg and Si in the surface of an Al-Mg-Si-Li alloy. The
surface oxide layer was sputtered to a negligible thickness, and the relative abundance of
alloying agents (Li, Mg and Si) was recorded for different time-stamps during annealing. All
three elements were recorded to appear with a significant increase in surface concentration.
The concentration decreased again with further increase in annealing temperature and
duration of annealing. Si and Li both occur everywhere on the alloy surface, while Mg
migration is mostly restricted to grain boundaries. Li occurs at much higher temperature than
the two respective alloying agents.
Består av
Paper 1: Mørtsell, Eva Anne; Marioara, Calin Daniel; Andersen, Sigmund Jarle; Røyset, Jostein; Reiso, Oddvin; Holmestad, Randi. Effects of Germanium, Copper, and Silver Substitutions on Hardness and Microstructure in Lean Al-Mg-Si Alloys. Metallurgical and Materials Transactions. A 2015 ;Volum 46.(9) s. 4369-4379 - Is not included due to copyright available at http://dx.doi.org/10.1007/s11661-015-3039-5Paper 2: Mørtsell, Eva Anne; Marioara, Calin Daniel; Andersen, Sigmund Jarle; Røyset, Jostein; Reiso, Oddvin; Holmestad, Randi. TEM and HAADF-STEM investigations on the effect of Cu and Ge additions on precipitation in 6xxx Al alloys. Microscopy and Analysis 2016 ;Volum 30.(1) s. 14-16
Paper 3: Mørtsell, Eva Anne; Wenner, Sigurd; Longo, Paulo; Andersen, Sigmund Jarle; Marioara, Calin Daniel; Holmestad, Randi. Elemental electron energy loss mapping of a precipitate in a multi-component aluminium alloy. Micron 2016 ;Volum 86. s. 22-29 http://dx.doi.org/10.1016/j.micron.2016.03.006 The article in is reprinted with kind permission from Elsevier, sciencedirect.com
Paper 4: E. A. Mørtsell, S. J. Andersen, J. Friis, C. D. Marioara and R. Holmestad. Atomistic details of precipitates in lean Al-Mg-Si alloys with trace additions of Ag and Ge studied by HAADF-STEM and DFT
Paper 5: E. A. Mørtsell, C. D. Marioara, S. J. Andersen, I. G. Ringdalen, J. Røyset, O. Reiso and R. Holmestad. The effects and behaviour of Li and Cu alloying agents in lean Al-Mg-Si alloys
Paper 6: E. A. Mørtsell, I. Westermann, C. D. Marioara, K. O. Pedersen, S. J. Andersen, J. Røyset and R. Holmestad. The Effect of 1 % Plastic Deformation and Elastic Strain on a 6060 Aluminium Alloy during Natural and Artificial Ageing
Paper 7: S. Cooil, E. A. Mørtsell, F. Mazzola, M. Jorge, S. Wenner, M. Edmonds, L. Thomsen; H. G. Klemm, G. Peschel, R. Holmestad and J. Wells. Thermal migration of alloying agents in aluminium. Materials Research Express, Volume 3, Number 11 - This is an author-created, un-copyedited version of an article accepted for ppublished in Materials Research Express. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at http://dx.doi.org/10.1088/2053-1591/3/11/116501 Published 11 November 2016 • © 2016 IOP Publishing Ltd