Tungsten solubility in L12-ordered Al3Er and Al3Zr nanoprecipitates formed by aging in an aluminum matrix

Journal Pre-proof Tungsten solubility in L12-ordered Al3Er and Al3Zr nanoprecipitates formed by aging in an aluminum matrix A.R. Farkoosh, David C. Dunand, David N. Seidman PII:

S0925-8388(19)34629-8

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https://doi.org/10.1016/j.jallcom.2019.153383

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JALCOM 153383

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Journal of Alloys and Compounds

Received Date: 3 September 2019 Accepted Date: 11 December 2019

Please cite this article as: A.R. Farkoosh, D.C. Dunand, D.N. Seidman, Tungsten solubility in L12ordered Al3Er and Al3Zr nanoprecipitates formed by aging in an aluminum matrix, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153383. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Tungsten Solubility in L12-ordered Al3Er and Al3Zr Nanoprecipitates Formed by Aging in an Aluminum Matrix A. R. Farkoosh1, David C. Dunand1 and David N. Seidman1,2 1 Department of Materials Science & Engineering 2 Northwestern University Center for Atom-Probe Tomography (NUCAPT), Northwestern University, Evanston, IL 60208, USA

Abstract The partitioning behavior of tungsten is studied in two dilute Al-Er-W and Al-Zr-W alloys aged isochronally up to 400 and 475 ºC, to form L12-structured Al3Er and Al3Zr nanoprecipitates. Atom-probe tomography reveals low W solubility in the Al3Er and Al3Zr nanoprecipitates (0.071 and 0.234 at.%, respectively) and very low W solubility in the Al matrix (0.057 and 0.023 at.%, respectively), corresponding to precipitate/matrix partitioning ratios of ~1-10. First-principles calculations demonstrate that the formation energies of substitutional W atoms in the L12 lattices are high and positive (0.66-1.73 eV.atom-1), consistent with the small W solubilities measured experimentally in these nanoprecipitates.

Keywords: Aluminum alloys; zirconium; scandium; tungsten; atom-probe tomography

1. Introduction The coarsening resistance of L12-ordered nanoprecipitates (e.g., Al3Zr, Al3Sc or Al3Er) in aluminum alloys can be improved by microalloying with slow-diffusing transition metals that are soluble in the nanoprecipitates [1, 2]. Early efforts have focused on adding transition metals that form stable or metastable L12-ordered phases, assuming that such elements would have a high solubility in the Al3(Sc,Zr,Er) nanoprecipitates with an L12 structure. The solubility limit in the ternary L12-type Al3(Sc1-yXy) intermetallic phase, as measured in arc melted specimens, was high

for Group IIIb (Y) and Group IVb (Ti, Zr, Hf) transition metals (~12.5 at.%), and significantly lower for Group Vb transition metals (2.7, 2.2 and 1.8 at.% for V, Nb and Ta respectively) [3]. Many reports exist on modifying L12-nanoprecipitates with slow diffusers such as Ti, Hf, V, Nb, or Ta [4-16]. Titanium and hafnium are soluble in the L12 nanoprecipitates as anticipated, but they reduce the lattice mismatch of the precipitates with the matrix and thereby deteriorate creep resistance [4-8, 17, 18]. Additionally, Ti and Hf impart no significant coarsening resistance. Group V elements, however, partition to the L12-nanoprecipitates at small concentrations (with average concentrations of ~0.03-0.15 at.% for V, ~0.20 at.% for Nb and ~0.08 at.% for Ta); thus, their effect is modest [9, 11, 19]. Recently, De Luca et al. [20] have determined by atom-probe tomography (APT) [21-23] that Mo partitions to the L12-(Al,Si)3(Zr,Sc,Er) core-shell nanoprecipitates in a multicomponent Al-0.08Zr-0.014Sc-0.007Er-0.4Mn-0.08Mo-0.1Si alloy aged at 400 oC for 24 h to 11 days, achieving 0.60-1.00 at.%, and providing improved coarsening resistance at 400-425 oC. Tungsten, which like Mo belongs to Group V1b , has also been reported to partition to L12-nanoprecipitates in a similar aluminum alloy [24, 25], reaching ~0.05-0.40 at.% at 400-525 oC; chromium, as another Group VIb element, partitions to the L12nanoprecipitates at ~0.15 at.% in an isochronally peak-aged (450 oC) Al-0.11Zr-0.005Er-0.02Si0.08Cr alloy [26]. Tungsten, because of its very small diffusivity in aluminum (5 x 10-23 m2s-1 at 400 °C [1]), could potentially enhance the coarsening resistance of L12 nanoprecipitates if it partitions to that phase significantly; that is, if substantial solid-solubility of W exists in L12Al3(Zr,Sc,Er). Despite the low solid solubility of W in Al (0.025 at.% at 660 oC [27]), rapid solidification (107 to 108 K s-1) can achieve high supersaturations (up to 0.95 at.% W) in solidsolution by suppressing the peritectic reaction between liquid Al, α-Al (fcc) and Al12W (orderedbcc) [27, 28]. Thus, under conventional casting conditions, slightly supersaturating W in Al may

be achievable. Given the lack of information on W micro-additions in aluminum alloys, we study herein two dilute Al-Er-W and Al-Zr-W alloys and present results of APT and first-principles calculations, which demonstrate the solubility and site preference of W within the L12 structure of their Al3Er and Al3Zr nanoprecipitates. 2. Experimental Two model alloys, Al-0.13Zr-0.03W and Al-0.006Er-0.09W (at.%, Table 1) were melted in a graphite crucible using 99.99% Al and Al-2W, Al-0.6Zr and Al-1Er (at.%) master alloys. The melt was maintained at 900 °C for 2 h, with periodic stirring and then cast into a graphite mold preheated at 200 oC, which was placed on an ice-cooled copper platen immediately prior to casting. Al-0.006Er-0.09W was homogenized at 640 oC for 24 h prior to aging but no homogenization step was performed on Al-0.13Zr-0.03W to avoid the decomposition of the supersaturated Zr-rich dendrite cores [29]. Alloys were aged isochronally to form L12nanoprecipitates. The Vickers microhardness and electrical conductivity were measured after each aging step (on different samples for each step) to monitor the precipitation of the L12nanoprecipitates. APT nanotips were prepared by a two-step electro-polishing technique [30]. A picosecond laser-pulsed local-electrode atom-probe (LEAP) 5000XS tomograph (Cameca Instruments Inc., Madison, WI) was utilized at a specimen temperature of 30 K, in ultrahigh vacuum (<10-8 Pa). Ultraviolet (UV) laser pulses (wavelength: 355 nm) were applied with an energy of 30 pJ/pulse and a pulse repetition rate of 500 kHz. Data analyses were performed using the program IVAS 3.8.2 (Cameca, Madison, WI). The proximity histogram methodology [31] was employed to display the compositional variations within the nanoprecipitates and the α-Al matrix, after performing background corrections to improve the accuracy of the compositional measurements. Details of the experiments are explained elsewhere [32].

3. Results and discussion Figure 1 displays the precipitation behavior of the alloys during isochronal aging as monitored by microhardness and electrical conductivity measurements. Al-0.006Er-0.09W reaches a peak microhardness of 292 ± 6 MPa at 300 oC due to the fast diffusion of Er in Al, while precipitation in Al-0.13Zr-0.03W commences at higher temperatures (350 - 375 oC) as evidenced by an increase in conductivity, and reaches a peak microhardness of 554 ± 22 MPa at 450 oC. The rootmean-square (RMS) diffusion distances, calculated for Er, Zr and W during isochronal aging steps, are plotted as a function of temperature in Fig. 1a for comparison. Erbium and Zr achieve an RMS value of ~10 nm at ~200 and ~400 oC, respectively, which coincide with significant increases in the microhardness values of the alloys. Tungsten diffuses at significantly slower rates in Al, achieving an RMS value of ~10 nm with a further increase in temperature to ~ 480 o

C. On the basis of these isochronal experiments and given the extremely small diffusivity of W

in aluminum, two different aging conditions were chosen to study the partitioning behavior of W: (1) Al-0.006Er-0.09W over-aged isochronally to 400 oC; and (2) Al-0.13Zr-0.03W over-aged isochronally to 475 oC, Fig. 1a. These aging conditions achieve considerable diffusion distances for W (~1 and ~7 nm, respectively), while maintaining the nanoscale size of the precipitates. It is apparent from Fig. 1 that W decreases significantly the electrical conductivity, consistent with strong electron-scattering by W atoms at the trace levels used herein (for example, the electrical conductivity of an Al-0.03Er alloy is ~35 MS.m-1 [33], which is ~5 MS.m-1 higher than the average electrical conductivity of Al-0.006Er-0.09W in Fig. 1b). Figure 2 displays APT reconstructions of both alloys containing the overaged L12-Al3Zr and L12-Al3Zr nanoprecipitates. A precipitate number density of 5 × 10 precipitate radius of <

and a mean

> = 3.1 ± 0.4 nm are measured in Al-0.13Zr-0.03W, Fig. 2a.

Because of the low number density of the nanoprecipitates in Al-0.006Er-0.09W, only one L12Al3Er nanoprecipitate ( ~ 2

) was imaged in the nanotip, Fig. 2b. The corresponding

proximity histograms computed from the datasets (Fig. 3) display the concentrations profiles across the matrix/nanoprecipitate interfaces. The average composition of the nanoprecipitates, matrix composition (far-field) and the average composition of the nanotips calculated from these proximity histograms are reported in Table 2. In Al-0.13Zr-0.03W the average W concentration within the L12-Al3Zr nanoprecipitates is < <

!

>/<

#

> = 0.234 at.% (partitioning ratio of

>≈ 10), which is similar to values reported previously for the

concentration of other non-L12 forming Group Vb and VIb transition metals in the L12 nanoprecipitates [9, 10, 20, 26]. The tungsten distribution within the nanoprecipitates is nonuniform, decreasing from the interface towards the core of the precipitates (Fig. 3a) indicating that the W atoms are incorporated into the L12-Al3Zr phase progressively during different steps of isochronal aging (200 to 475 oC), due to the exponentially increasing diffusivity of W with increasing temperature; note the RMS diffusion distances of W as a function of temperature in Fig. 1a. The tungsten concentration in the α-Al-matrix in Al-0.13Zr-0.03W is 234 at. ppm, which is nearly equal to the average W concentration of the volume analyzed (238 at. ppm, Table 3). In Al-0.006Er-0.09W, W partitioning to the L12-Al3Er nanoprecipitates is significantly smaller, (<

!

> = 0.071 at.% , which is close to the detection limit, Fig. 3b). This is despite the

higher W content of this alloy, which can be due to: (i) slow diffusion of W at 400 oC and/or (ii) a smaller thermodynamic driving force for W partitioning to the L12-Al3Er nanoprecipitates; that is, a smaller solubility of W in the L12-Al3Er phase. The latter is addressed with densityfunctional theory (DFT) calculations below.

The average measured W concentration in the α-Al matrix in Al-0.006Er-0.09W is 568 at. ppm, which is about twice as high as the maximum solubility of W in aluminum (≈250 at. ppm at ~660 oC, decreasing to 100 at. ppm at 427 oC) reported for the Al-W system [27, 34]; this indicates that the matrix may still be supersaturated with W. Some Si was also detected in both L12-Al3Zr and L12-Al3Er nanoprecipitates. The average Si content of the L12-Al3Er nanoprecipitate is 0.626 at.%, an order of magnitude higher than that for the L12-Al3Zr nanoprecipitates (0.064 at.%), which is attributed to: (i) the higher expected solubility of Si in the L12-Al3Er phase [20, 32]; and (ii) ~ 25 times lower volume fraction of the L12-Al3Er nanoprecipitates in Al-0.006Er-0.09W. Despite the fact that Si is an impurity in these alloys at ~ 50 at. ppm, Table 1 (well below the solubility limit of Si in pure Al: 240 at. ppm at 250 oC and 2700 at. ppm at 400 oC [27]), its presence within the L12-nanoprecipitates highlights the significant role of Si, at. ppm levels, in the nucleation of the nanoprecipitates. The effect of Si additions at relatively high concentrations (up to 0.4 at.%) on accelerating the nucleation, growth and coarsening kinetics of the L12-Al3(M) nanoprecipitates has been studied previously [35, 36]. First-principles calculations were performed within the framework of DFT, utilizing the Vienna Ab initio Simulation Package (VASP) [37] and projector augmented wave (PAW) potentials [38]. The Perdew-Burke-Ernzerhof (PBE) parameterization of the generalized gradient approximation (GGA) [39] was employed. A periodic 2 × 2 × 2 supercell with 32 atoms was employed to determine the total energies of the cells, which converge to 1x10−5 eV atom−1. A plane wave cutoff energy of 520 eV and 11x11x11 Monkhorst–Pack k-point grids were found to be sufficient to yield fully converged results. All structures were relaxed with respect to the cellinternal degrees of freedom at a fixed volume. Then, the equilibrium supercell volume at 0 K and ground-state energies were obtained by calculating the total energy (% &'& ) as a function of a set

of chosen lattice constants ((' ) and fitting the % &'& − (' data to the Birch–Murnaghan equation of state (EOS) [40-42]. For calculations involving Er, spin-polarized calculations were also performed and only small differences between the two sets of calculations were found. The lattice constants of stochiometric L12-Al3Zr and L12-Al3Er were calculated to be ao = 4.105 Ao and ao = 4.231 Ao, respectively, which are in good agreement with previous DFT calculations (ao = 4.1047 -4.1099 Ao for L12-Al3Zr [43-46] and ao = 4.232-4.234 for L12-Al3Er [47-49]); and the experimental results determined at room temperature are ao = 4.050-4.093 Ao for L12-Al3Zr [1, 44, 50, 51] and ao = 4.212-4.215 Ao for L12-Al3Er [1, 49]. Four substitutional structures were modeled by replacing Al or M (M = Zr or Er) atoms with W within the L12-Al3M superlattice and the corresponding substitutional formation energies (0 K) were calculated after performing the relaxation procedure, respectively as [36, 52-54]: = [.%/&'&012 +2 3

− % &'&4 5 + /7

− 7+ 3]/n

(1)

∆%+→4 = [.% &'&/4012 +2 3 − % &'&4 5 + /74 − 7+ 3]/n

(2)

∆%+→

4

where, % &'&4 represents the total energy of a 2 × 2 × 2 L12-Al3M supercell and %/&'&012 +2 3

4

or

% &'&/4012 +2 3 is the calculated total energy of the supercell after substitution. The quantity 79 is the DFT chemical potential of species Z calculated for a dilute solution Al31Z (Z = W, Zr or Er) and pure Al in the equilibrium state as 7 = % &'&

− 317 , where

is the number of

substitutional atoms, which corresponds to the fraction substituted, :. We utilized one substitutional atom,

= 1 and : = 1/32 corresponding to 3.125 at.% W in a 2 × 2 × 2 L12-

Al3M supercell. The substitutional formation energy for W dissolved in the α-Al matrix (fcc) was calculated using: ∆%+→

&'& = [/%;<

=

− % &'& 3 + /7

− 7+ 3]/n

(3)

In this case, 7+ is the chemical potential of pure W, calculated assuming the same cell symmetry. The results, Table 3, demonstrate that W strongly prefers to occupy the Al sublattice sites of L12-Al3Zr, as ∆%+→

is significantly (by 0.98 eV.atom-1) smaller than ∆%+→ . In the

case of the L12-Al3Er phase, however, W prefers to substitute at the Er sublattice sites, as ∆%+→! is slightly (by 0.08 eV atom-1) smaller than ∆%+→ . Tungsten is calculated to have a negative (-0.39 eV.atom-1) heat of solution in α-Al, which is consistent with theoretical models describing transition metal interactions with Al [55]. We note that all the substitutional point defects in the L12 phases have relatively large and positive formation energies. Thus, W is predicted to have a small solubility in the L12-nanoprecipitates. These substitutional formation energies are significantly higher in the case of the L12-Al3Er phase (1.73 eVatom-1 for Al3(Er1yWy)

as opposed to 0.66 eV.atom-1 for (Al1-xWx)3Zr), which suggests an even smaller solubility

of W in L12-Al3Er, as determined experimentally (Table 2, Fig. 3). These results indicate that micro-additions of W cannot alter significantly the thermodynamic driving force for the precipitation of L12-Al3Er, which constitute the cores of the L12-nanoprecipitates (with an Errich core and a Zr-rich shell) in multicomponent alloys [10, 56-58] and can be expected to influence very weakly the nucleation kinetics in these alloys. A similar effect was observed previously in the case of the group Vb transition elements (V [10, 11], Nb [9, 11] and Ta [9, 11]) microalloyed Al-Zr-Sc-Er alloys; these transition metals with limited solubility (0.1-0.3 at.%) in the L12-(Al,Si)3(Sc,Zr,Er) phase have minor effects on the nucleation, growth or coarsening kinetics of nanoprecipitates.

Entropy effects were not considered in this study, which are

expected to be in favor of W partitioning to the L12-Al3Zr nanoprecipitates with increasing temperature. For L12-Al3Zr, when a high-mass W atom from the matrix substitutes for a stiffly bonded, low-mass Al atom (with large amplitude vibrations) [59, 60]; within the L12-lattice, a

small increase in the vibrational entropy of the ordered phase is expected, which can lead to some solubility of W in L12-Al3Zr at high temperatures. 4. Conclusions We investigated the partitioning behavior of W in two dilute aluminum alloys, Al-0.13Zr-0.03W and Al-0.006Er-0.09W (at.%), during isochronal aging (25 oC / 3 h steps) experiments. Our APT measurements of the compositions of the nanoprecipitates and matrix demonstrate that W remains mostly in solid solution within the α-Al (fcc) matrix of these two alloys. While APT measurements yield a higher W concentration (0.234 at.%, with a partitioning ratio of ~10) within the L12-Al3Zr phase, the average concentration of W in the L12-Al3Er phase is negligible (0.071 at.%), and similar to that in the matrix. The results of supercell defect-energy calculations, Table 3, indicate that W prefers strongly to occupy the Al sublattice sites of L12-Al3Zr, while in L12-Al3Er, W weakly prefers to substitute at the Er sublattice sites. These substitutions, however, are associated with relatively large and positive formation energies, which is consistent with the limited solubility of W in the L12-phase observed experimentally. Our results provide supporting evidence for the low solubility of Group 5b-6b transition metals in L12-Al3M (M = Zr or Er) nanoprecipitates formed on aging in an Al (fcc) matrix. Acknowledgments This research was supported by the Office of Naval Research (N00014-18-1-2550). Atom-probe tomography was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The LEAP tomograph at NUCAPT was purchased and upgraded with grants from the NSF-MRI (DMR-0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781, N00014-1712870) programs. NUCAPT received support from the MRSEC program (NSF DMR-1720139) at the Materials Research Center, the SHyNE Resource (NSF

ECCS-1542205), and the Initiative for Sustainability and Energy (ISEN) at Northwestern University. This work made use of the Materials Characterization and Imaging Facility which receives support from the MRSEC Program (NSF DMR-1720139) of the Materials Research Center at Northwestern University. This work made use of the EPIC facility of Northwestern University’s NUANCE Center, which has received support from the MRSEC program NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); and the State of Illinois, through the IIN. DNS and DCD disclose financial interests in Braidy Industries, which could potentially benefit from the outcomes of this research.

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Table and Figure captions Table 1 Chemical composition of the experimental alloys, as determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Table 2 Average composition (as measured employing APT) of the L12-nanoprecipitates (43 nanoprecipitates for Al-0.13Zr-0.03W and one nanoprecipitate for Al-0.006Er-0.09W), matrix (far-field) and nanotips of the alloys. Table 3 Total energies (Etot) and substitutional formation energies (∆%+

3 calculated

,

utilizing equations 1-3.

Fig. 1 Evolution of: (a) Vickers microhardness; and (b) electrical conductivity of Al-0.006Er0.09W and Al-0.13Zr-0.03W during isochronal (3 h/25oC-steps) aging. The root-mean-square (RMS) diffusion distances in Al are plotted as a function of temperature for Er, Zr, and W during &

&

isochronal aging. They were calculated utilizing ?@A = 2 B&JQ CA /D3ED = 2 B&JQ CA' F:G HK

IJ

L /MJ NO&3

P ED, where

DR and DS are the initial and final times to achieve a specified temperature; CTR and CT , are the pre

exponential factor of the diffusivity and the diffusivity of element i in aluminum, respectively;

UT is the activation energy for tracer diffusion; and kB is Boltzmann’s constant. Values for CTR, CT and UT are taken from Ref. [1]. VA = 200 ° is the initial temperature and X = 8.33 Z ℎ for a 3 h / 25 oC step isochronal heat treatment. Fig. 2 3D-APT reconstruction of: (a) Al-0.13Zr-0.03W after isochronal aging to 475 oC; and (b) Al-0.006Er-0.09W after isochronal aging to 400 oC, showing L12-nanoprecipitates within the α(fcc) Al matrix. Only 5% of the Al atoms are displayed for clarity. The L12-Al3Zr nanoprecipitates are delineated in green using 2.5 at.% Zr iso-concentration surfaces. The apparent elongated morphology of the L12-Al3Zr nanoprecipitates is due to the higher evaporation field of Zr than Al and consequent local magnification effects.

Fig. 3 Proximity histograms showing elemental distributions in the L12-nanoprecipitates in: (a) Al-0.13Zr-0.03W isochronally aged to 475 oC; and (b) Al-0.006Er-0.09W, isochronally aged to 400 oC. The gray shaded areas represent the detection limit (DL) defined as one atom per proxigram bin (in at.%). The bin width was 0.3 and 0.5 nm for (a) and (b) respectively. The matrix/L12-interface (vertical dot-dash line) is defined as the inflection point of the Al concentration profile. The error bars represent the one-sigma statistical error [31]. Some error bars are smaller than the marker size. Tomographic 3-D reconstructions of representative nanoprecipitates are given as insets.

Table 1 Chemical composition of the experimental alloys, as determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Composition (at.%) Zr Er W 0.0058 0.093 0.125 0.032

Alloy Al-0.006Er-0.09W Al-0.13Zr-0.03W

Fe 0.0047 0.0049

Si 0.0040 0.0058

Al Bal. Bal.

Table 2 Average composition (as measured employing APT) of the L12-nanoprecipitates (43 nanoprecipitates for Al-0.13Zr-0.03W and one nanoprecipitate for Al-0.006Er-0.09W), matrix (far-field) and nanotips of the alloys. Aging Temperature (oC)

Alloy

Precipitate Composition (at.%)

(nm)

Si2+

Matrix (far-field) Composition (at. ppm)

Nanotip Composition (at. ppm)

Zr

Er

a

W

Zr

Er

Si2+

W

Zr

Er

Si2+

W

Al-0.13Zr-0.03W

475

3.1 ± 0.4

26.97

-

0.064

0.234

318

-

32

234

1547

-

35

238

Al-0.006Er-0.09W

400

~2.0

-

24.69

0.626

0.071

-

<3*

52

568

-

152

58

580

a

atomic concentration of 28Si2+ in the LEAP tomographic mass spectrum. * below detection limit

Table 3 Total energies (Etot) and substitutional formation energies (∆ utilizing equations 1-3. (eV)

∆

Phase

Substitutional site

α-Al matrix (fcc)

AlxW

-129.15

-0.39

Nanoprecipitates (L12)

(Al1-xWx)3Zr

-181.45

0.66*

Al3(Zr1-yWy)

-174.68

1.64

(Al1-xWx)3Er

-146.62

1.81

Al3(Er1-yWy)

-145.34

1.73*

*Preferred sublattice site



,



,



)

) calculated

(eV.atom-1)

Fig. 1 Evolution of: (a) Vickers microhardness; and (b) electrical conductivity of Al-0.006Er0.09W and Al-0.13Zr-0.03W during isochronal (3h/25oC-steps) aging. The root-mean-square (RMS) diffusion distances in Al are plotted as a function of temperature for Er, Zr, and W during isochronal aging. They were calculated utilizing 2 2 , where and are the initial and final times to achieve a specified temperature; ! and ! , are the preexponential factor of the diffusivity and the diffusivity of element i in aluminum, respectively; "! is the activation energy for tracer diffusion; and kB is Boltzmann’s constant. Values for ! , ! and "! are taken from Ref. [1]. # 200 °& is the initial temperature and ' 8.33 + , - for a 3 o h / 25 C step isochronal heat treatment.

Fig. 2 3D-APT reconstruction of: (a) Al-0.13Zr-0.03W after isochronal aging to 475 oC; and (b) Al-0.006Er-0.09W after isochronal aging to 400 oC, showing L12-nanoprecipitates within the α(fcc) Al matrix. Only 5% of the Al atoms are displayed for clarity. The L12-Al3Zr nanoprecipitates are delineated in green using 2.5 at.% Zr iso-concentration surfaces. The apparent elongated morphology of the L12-Al3Zr nanoprecipitates is due to the higher evaporation field of Zr than Al and consequent local magnification effects.

Fig. 3 Proximity histograms showing elemental distributions in the L12-nanoprecipitates in: (a) Al-0.13Zr-0.03W isochronally aged to 475 oC; and (b) Al-0.006Er-0.09W, isochronally aged to 400 oC. The gray shaded areas represent the detection limit (DL) defined as one atom per proxigram bin (in at.%). The bin width was 0.3 and 0.5 nm for (a) and (b) respectively. The matrix/L12-interface (vertical dot-dash line) is defined as the inflection point of the Al concentration profile. The error bars represent the one-sigma statistical error [31]. Some error bars are smaller than the marker size. Tomographic 3-D reconstructions of representative nanoprecipitates are given as insets.

•

L12-nanoprecipitates formed upon aging the dilute Al-Zr-W and Al-Er-W alloys.

•

A limited solubility of W in the L12-phase was observed experimentally.

•

W prefers strongly to occupy the Al sublattice sites of L12-Al3Zr.

•

W weakly prefers to substitute at the Er sublattice sites of L12-Al3Er.

•

These substitutions are associated with large and positive formation energies.