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The enhanced theta-prime (θ′) precipitation in an Al-Cu alloy with trace Au additions
Yiqiang Chen1*, Zezhong Zhang1, Zhen Chen 2, Amalia Tsalanidis1, Matthew Weyland 1 3 , Scott Findlay2, Leslie J. Allen4, Jiehua Li5, Nikhil Medhekar1* , Laure Bourgeois1 3*
1. Department of Materials Science and Engineering, Monash University, VIC 3800,Australia
2. School of Physics and Astronomy, Monash University, VIC 3800, Australia3. Monash Centre for Electron Microscopy, Monash University, VIC 3800, Australia4. School of Physics, University of Melbourne, VIC 3010, Australia5. Chair of Casting Research, Montanuniversität Leoben, A-8700 Leoben, Austria
Abstract: Linking the atomic level kinetic precipitation pathways induced by
elemental additions to the resulting microstructure is fundamentally desirable for the
design of new classes of light alloys. Aberration-corrected scanning transmission
electron microscope (AC-STEM) and first principles calculations were used to
investigate the influence of trace Au (200 ppm) additions on precipitation in an Al-
Cu-Au alloy. These Au additions resulted in a significant enhancement of the low-
temperature age hardening, which was demonstrated to be associated with accelerated
precipitate nucleation and growth. Atomic-resolution annular dark field (ADF)
imaging showed the clearly reduced critical length and thickness of θ′ precipitates
with Au additions, therefore accelerating the nucleation of θ′. Agglomerated Au
clusters were observed in θ′ precipitates, which were demonstrated to correspond to a
localised energetically favourable state. These observations have been explained
through first-principles calculations and relevant thermodynamic modelling. This
work provides a potential way to refine the alloy microstructure for improving the
mechanical behaviour of light alloys.
1. Introduction
This is the preprint. The Published Journal Article can be found athttps://doi.org/10.1016/j.actamat.2016.12.012
Table 3. First-principles calculated formation energy per solute Es (eV/atom), per all
atoms Ea (eV/atom), and the corresponding lattice constants a (Å) and c (Å).
Structure Es Ea a c
Al-Cu solid solution -0.12 / 4.03 4.03
Al-Au solid solution -0.53 / 4.04 4.04
GP zone -0.21 -0.02 / /
GP zone12.5%Au-Cu
-0.17 -0.03 / /
θ′′ -0.24 -0.09 4.02 3.76
θ′′12.5%Au-Cu
-0.23 -0.10 4.07 3.89
θ′(Al2Cu) -0.42 -0.18 4.08 5.77
θ′12.5%Au-Cu
-0.44 -0.2 4.10 5.85
θ′25%Au-Cu
-0.48 -0.26 4.14 5.86
θ′50%Au-Cu
-0.56 -0.30 4.15 6.04
θ′75%Au-Cu
-0.65 -0.36 4.24 6.00
η′100%Au-Cu
-0.74 -0.42 4.29 6.06
θ′12.5%Au-Al
-0.18 -0.16 4.11 5.81
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Fig. 1. Vicker’s hardness response of the binary Al-1.7Cu and ternary Al-1.7Cu-
0.02Au alloys aged at (a) 160°C and (b) 200°C.
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Fig. 2. Typical ADF-STEM images showing the alloy microstructure for ageing at
160 °C for 3 hrs for the (a) binary and (b) ternary alloys; for ageing at 200 °C for 2
hrs for the (c) binary and d) ternary alloys, and for ageing at 200 °C for 24 hrs for the
(e) binary and (f) ternary alloys. These images were taken along the [100] zone axis.
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Fig. 3. Precipitate size distributions at different ageing treatment regimes for the
experimental binary and ternary alloys, (a) 160 °C for 3 hrs, (b) 200 °C for 2 hrs, (c)
200 °C for 24 hrs.
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Fig. 4. Early-stage precipitates found in the specimen aged at 160 °C for 3 hrs, viewed
along [100]: typical atomic-resolution ADF images for (a) and (b) incomplete θ′
precipitates or θ′ nuclei formed within a θ′′ precipitate, (c) θ′ precipitate featured in
the thickness of a single unit cell. The left inset displays the variation of the ADF
counts indicated by the green dashed line that was averaged over the half width of the
single unit cell to include the central column, while the right inset shows the overall
EDX spectrum of the dashed rectangular region θ′ precipitate with the thickness of (d)
1.5 unit cells and (e) 3 unit cells and (f) θ′ precipitate with two ledges. The inset in (f)
displays the normalised integrated line profile of the white rectangular region
indicated by the white dash arrow, where the blue and red are the Au-Lα (9.704 keV)
and Cu-Kα (8.04 keV), respectively.
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Fig. 5. An early-stage precipitate 4 unit cells thick found in the specimen aged at 160
°C for 3 hrs, viewed along [100]: (a) atomic-resolution ADF imaging, and the
background-subtracted X-ray maps (b) Cu-Kα (8.04 keV) and (c) the Au-Lα (9.704
keV) for the region in (a) indicated by the white dashed line.
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Fig. 6. (a) Typical atomic-resolution ADF images of a θ′ precipitate embedded in the
surrounding matrix. The inset shows EDX spectra for precipitate (black) and matrix
(red). (b) quantified atomic-resolution ADF image intensity across the indicated black
dashed line in (a), revealing a clear Au enrichment in the precipitate centre, (c) the
line profile of Au content across the precipitate. (d) lattice strain within the precipitate
indicated by the white dashed line, calculated using the geometrical phase approach
(GPA). This image was taken from a specimen aged for 24 hrs at 200°C.
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Fig. 7. Typical ADF images of the ternary alloy aged at 200˚C for 24 hrs, showing the
possibility of finding Au in θ′ precipitates. The bottom inset displays an even contrast
implying no Au in θ′ precipitate, whilst the top inset reflects a fine-scale contrast
demonstrating the atomic-scale Au agglomerations in the θ′ precipitate.
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Fig. 8. Typical atomic resolution ADF images of GP zones in (a) the binary alloy and
(b) the ternary alloy, for specimens aged at 160˚C for 3 hrs. The distortion in these
two images was due to specimen drift.
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Fig. 9. Calculated critical (a) length (L*) and thickness (t*), and (b) critical energy
barrier (ΔG*) for θ′ nucleus, as a function of Au substitution for Cu in θ′. The inset in
(b) schematically shows the thin cuboid model employed in this study, where L and t
are the length and thickness, respectively.
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Fig. 10. Two types of interfacial structure for alloys aged at 200°C for 24 hrs: (a)
direct θ′-α interface only found in the ternary alloy, and (b) complex interface θ′-θ′t-
θ′′-α interface found in both binary and ternary alloys.
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