Acta Materialia 131 (2017) 57e64

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Concurrent strengthening of ultraﬁne-grained age-hardenable Al-Mg
alloy by means of high-pressure torsion and spinodal decomposition
Yongpeng Tang a, Wataru Goto a, 1, Shoichi Hirosawa a, *, Zenji Horita b, Seungwon Lee c,Kenji Matsuda c, Daisuke Terada daDepartment of Mechanical Engineering and Materials Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
Department of Materials Science and Engineering, Kyushu University, Fukuoka 819-0395, Japan
Graduate School of Science and Engineering, University of Toyama, Toyama 930-8555, Japand
Department of Mechanical Science and Engineering, Chiba Institute of Technology, Narashino 275-0016, Japanbca r t i c l e i n f o

a b s t r a c t

Article history:
Received 28 December 2016
Received in revised form
26 March 2017
Accepted 2 April 2017
Available online 3 April 2017

In this study, the age-hardening behavior and precipitate microstructures of severely-deformed and thenartiﬁcially-aged Al-13.4 wt%Mg alloy has been investigated by Vickers hardness test, X-ray diffraction(XRD) analysis, transmission electron microscopy (TEM) and atom probe tomography (APT). The combined processing of high-pressure torsion (HPT) and aging treatment at a temperature below spinodal
lines results in a higher attained hardness of ~HV296 with an age-hardenability (i.e DHV31 ± 2) comparable to that of the undeformed specimen without HPT (i.e. DHV33 ± 2). The corresponding TEM
microstructures consist of modulated structures associated with spinodal decomposition, and quantitative estimation of the amplitude, as well as the wavelength, of Mg ﬂuctuations was successfully conducted by APT for the ﬁrst time for this alloy system. The linear relationship between the increment ofVickers hardness and the estimated amplitude of the undeformed specimen supposed that Kato's
spinodal-hardening mechanism works even in the HPTed specimen with a high number density of grainboundaries. Therefore, our proposed strategy; i.e. taking advantage of spinodal decomposition, isregarded as a convincing approach to achieving concurrent strengthening by ultraﬁne-grained andprecipitation hardenings for the alloys that decompose via spinodal decomposition.
© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords:
High-pressure torsion
Spinodal decomposition
Concurrent strengthening
Atom probe tomography
Al-Mg alloy

1. Introduction
Among strengthening mechanisms of aluminum alloys, both of
precipitation hardening and hardening by grain reﬁnement areprimarily important, and thus many efforts have been done forachieving concurrent strengthening by the two strengthening
mechanisms [1e9]. Severe plastic deformation (SPD) such as equalchannel angular pressing (ECAP), accumulative roll bonding (ARB)
and high-pressure torsion (HPT) is now a well-established method
to obtain ultraﬁne grains at submicrometer or nanometer scale[10], but the induced hardening by the ultraﬁne grains (i.e.
ultraﬁne-grained hardening) is often diminished or spoiled by thesubsequent aging treatment because preferential precipitation at
dislocations and grain boundaries becomes predominant in place of* Corresponding author.
E-mail address: hirosawa@ynu.ac.jp (S. Hirosawa).
1
Present: Fuji Heavy Industries Ltd.
http://dx.doi.org/10.1016/j.actamat.2017.04.002
1359-6454/© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

reﬁned transgranular precipitates [4,6,11]. However, the authors
recently reported that the hardness of the HPTed specimen of
2091Al-Li-Cu alloy can be further increased to > HV290 by means ofaging treatment at temperatures below spinodal lines within themetastable a-Al þ d0 -Al3Li ﬁeld of Al-Li binary system [6,9]. From
the results of in-situ small-angle X-ray scattering (SAXS) measurement, their proposed strategy of “taking advantage of spinodal
decomposition” was considered to be applicable not only to the AlLi-Cu alloy but also to other alloys that decompose via spinodal
decomposition; e.g. highly concentrated Al-Mg alloys.
Although there exists different terminology regarding how toindicate the formed zones or phases [12], the phase decomposition
sequence of Al-Mg alloys with high Mg concentrations can be
written after Sato et al. [13,14] and Dauger et al. [15e17];a / modulated structure / ordered GP zone / b0
(Al2Mg3) / b(Al2Mg3)

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Y. Tang et al. / Acta Materialia 131 (2017) 57e64

where a is the original supersaturated solid solution, GP zone
stands for a L12-type ordered structure with a stoichiometriccomposition of Al3Mg (also indicated as b00 ), b0 is the semicoherent
hexagonal intermediate phase and b is the equilibrium phase with acomplex face-centered cubic (fcc) structure. Modulated structureassociated with spinodal decomposition is initially formed, andthen GP zones are developed in relationship to the modulations, if
aged at a temperature below spinodal lines within the metastable
a-Al þ Al3Mg ﬁeld of Al-Mg binary system [13]. Such modulated
structure can be observed along the [001]a direction by transmission electron microscopy (TEM) [13e16] and small-angle
neutron scattering (SANS) [17], but quantitative estimation of theamplitude of Mg ﬂuctuations has never succeeded, unlike in thecase of the wavelength, due to the drawbacks of their transmission
imaging and reciprocal space techniques (For example, overlappingof multi-wavelength developing toward different directions cannot
be avoided on TEM observation, whereas the smearing effect oflarge wavelength bears many uncertainties about compositional
evaluation by SANS).
In this study, therefore, an age-hardenable Al-13.4 wt%Mg (Al14.7 at%Mg) alloy subject to HPT and low-temperature aging has
been investigated as a model alloy to verify the effectiveness of our
proposed strategy to achieve concurrent strengthening; i.e. “taking
advantage of spinodal decomposition”. The amplitude as well as
the wavelength of modulated structures was successfully quantiﬁed by atom probe tomography (APT) for the ﬁrst time, and thencorrelated to the increment in Vickers hardness through a spinodalhardening mechanism proposed by Kato et al. [18]. By taking into
account the fact that the wavelength is sufﬁciently smaller than
average size of ultraﬁne grains, spinodal decomposition is supposed to proceed even in the HPTed specimen with a high number
density of grain boundaries.
2. Experimental
In this study, an Al-13.4 wt%Mg (Al-14.7 at%Mg) cast ingot with
impurity Cu, Fe, Mn and Zn less than 0.01 wt% has been utilized as amodel alloy after homogenization treatment at 718 K for 1210 ks,
followed by water quenching. Both of 10 mm square pieces and
10 mmf disks with 1 mm thick were taken from the ingot byelectrical discharge machining. All the specimens were solutiontreated at 723 K for 3.6ks and then water quenched in order toobtain the super-saturated solid solutions (Note that the maximum
solubility of Mg in Al is 18.6 at% at 723 K [19]). HPT was performedat room temperature within 3.6ks after quenching under an appliedpressure of 6 GPa for 5 revolutions (See the details in Ref. [5]). Aging
treatment was subsequently conducted on the square pieces of the
undeformed specimen without HPT as well as the disks of the
HPTed specimen at a temperature below the spinodal line (e.g.
343 K [13]) for various times.
The age-hardenability of those specimens was investigated by amicrohardness tester (Matsuzawa MMT-X1) with a load of 4.9N fora dwelling time of 15s. The hardness of each specimen was determined as an average value of ﬁve tested points out of seven ones
with an accuracy of ±HV2. X-ray diffraction (XRD) analysis was
conducted for both the specimens in the “before aging” condition
by a X-ray analytical instrumentation (Rigaku RINT-2000) using CuKa radiation (wavelength l ¼ 0.15418 nm) at 40 kV and 200 mA.
The foils for TEM observation were prepared by the electrolytic
twin-jet polishing technique with 15 vol% nitric acid in 85 vol%methanol at ~253 K. TEM microstructures and the corresponding
selected-area electron diffraction (SAED) patterns were observed
using a transmission electron microscope (JEOL JEM 2100F) at anaccelerating voltage of 200 kV. Microhardness test, XRD analysis
and TEM observation for the HPTed specimen were conducted in

the regions at a distance of 3.5 mm from the center of the original
10 mmf disks in order to avoid unsaturated levels of hardness, andthus undeveloped substructures being investigated [5].
Needle-shaped samples for APT were prepared by a standard
two-stage electropolishing method; i.e., a double layer techniquewith 25 vol% perchloric acid in acetic acid at 15 V and micropolishing with 2 vol% perchloric acid in 2-butoxyethanol [20]. APT
was carried out at a specimen temperature of 20 K with a pulse
fraction (pulse voltage/steady-state voltage) of 15e20% by an atom
probe instrument (CAMECA Instruments LEAP 3000HR) for the
undeformed specimen (Unfortunately, no successful analysis has
been accomplished for the HPTed specimen because of frequent
failure of needle-shaped samples due to the ultraﬁne-grained
structures). Visualization and quantitative estimation of the obtained three-dimensional atom-by-atom data were performed with
software packages (AMETEK IVAS3.6.0 and Oxford nanoScience
PoSAP) through several analyzing methods including isoconcentration surface, composition proﬁle, autocorrelation function and frequency distribution analyses [20].

3. Results
Fig. 1 shows age-hardening curves of the undeformed and
HPTed specimens at 343 K. From the values of “before aging”
hardness, it is obvious that application of HPT signiﬁcantly
strengthens the undeformed specimen from HV120 to HV270. Sucha remarkable strengthening can be explained by ultraﬁne-grained
hardening because the corresponding TEM microstructure of the
HPTed specimen before aging consists of submicrometer-scale
grains (average grain size of ~100 nm) with high-angle grainboundaries (See the dark-ﬁeld image and the SAED pattern in
Fig. 2).
In the subsequent stage of aging, further strengthening can beachieved by prolonged aging treatment not only in the undeformed
specimen but also in the HPTed specimen. The age-hardenability of
DHV31 ± 2 for the HPTed specimen is almost identical to that of
DHV33 ± 2 for the undeformed specimen, suggesting that concurrent strengthening by HPT and aging treatment is activatedwithout diminishing the age-hardenability of this alloy system. It
should be also emphasized that the attained hardness of the HPTed
specimen reaches ~ HV296 after 2576 ks, comparable to > HV290 of
our previously reported 2091Al-Li-Cu alloy subject to HPT and lowtemperature aging [6,9].

Fig. 1. Vickers hardness change of the undeformed and HPTed specimens during aging
at 343 K. The conditions of aging treatment applied to the specimens for TEM and APTexperiments are indicated by arrows.

Y. Tang et al. / Acta Materialia 131 (2017) 57e64

59Fig. 2. Bright-ﬁeld and dark-ﬁeld TEM images with the corresponding SAED pattern for the HPTed specimen before aging.

Fig. 3 shows bright-ﬁeld TEM images and the corresponding
SAED patterns of the undeformed specimen aged at 343 K for 1555
and 2576 ks. In the earlier stage of aging, patchy pattern of phasemodulated contrast can be observed within the matrix of coarse
grains (See the magniﬁed microstructure in Fig. 3(a)). Such a
modulated structure is typical of spinodally decomposed Al-Mg
alloys, and thus the compositional ﬂuctuation of Mg is expected
to grow into GP zones eventually with the L12 ordered structure[13,14,16]. Fig. 3(b) conﬁrms this phase decomposition sequence
because spherical GP zones with Ashby-Brown contrast are uniformly developed after 2576ks aging with extra diffraction spots ofsuperlattice reﬂections as shown by the arrow.
Similar modulated structures can be observed in the HPTed
specimen. Fig. 4 shows bright-ﬁeld TEM images and thecorresponding SAED pattern of the HPTed specimen aged at 343 K
for 2576 ks. While average grain size of ultraﬁne grains remains at~120 nm (i.e. sluggish growth from ~100 nm in Fig. 2 even after the
prolonged aging treatment), it is clearly seen that phase-modulated
contrast arising from compositional ﬂuctuations is similarly present within the ultraﬁne grains. It is also obvious from Fig. 4 that no
grain boundary precipitates with less signiﬁcant role in strengthening are formed, irrespective of the ultraﬁne-grained structurewith a high number density of grain boundaries. This ﬁnding
conﬁrms that our proposed strategy of “taking advantage of spinodal decomposition” is effective in activating concurrent
strengthening by ultraﬁne-grained and precipitation hardenings
for the investigated Al-Mg alloy.
In this study, APT was performed on the underformed specimen

Fig. 3. Bright-ﬁeld TEM images with the corresponding SAED patterns for the undeformed specimen aged at 343 K for 1555 (a) or 2576 ks (b).

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Y. Tang et al. / Acta Materialia 131 (2017) 57e64

Fig. 4. Bright-ﬁeld TEM images with the corresponding SAED pattern for the HPTed specimen aged at 343 K for 2576 ks.

to investigate when and how Mg concentration ﬂuctuates in three
dimensions. Fig. 5(a) and 6(a) illustrate elemental map of Mg atoms
in the analyzed volume of the undeformed specimen aged at 343 K
for 630 and 1555 ks. Although only Mg atoms were depicted as
green points with detected overall concentration of 13.1 at% or
14.6 at%, the excess overlap of Mg atoms visually restricts thedetermination of the features of those microstructures. Therefore,
isoconcentration map at speciﬁc isosurface levels of Mg was constructed to reveal the inhomogeneity of Mg distributions. From thecomparison of white-colored isoconcentration surfaces between
Figs. 5(b) and 6(b), it is obvious that compositional ﬂuctuations
develop to a great extent with increasing aging time.
To quantify the compositional ﬂuctuations, one-dimensional
composition proﬁle along the elastically soft [001]a direction of
selected boxes (e.g. rectangular parallelepiped in Figs. 5(b) and6(b)) has been drawn by taking into account crystallographic
orientation of the analyzed volume through Figs. 5(c) and 6(c)(Note that perfect match of stereographic projection of (111) a or(001) a with detection event histogram of Mg atoms on the X-Yplane allows X, Y and Z axes of the analyzed volume to be determined, and thus the longitudinal direction of the selected boxes canbe aligned to the [001]a direction precisely). From the values andpositions of maximum or minimum Mg concentrations on theauxiliary curves, it can be quantiﬁed that a ﬂuctuation of Mg concentration develops from 2.4 ± 0.5 at%Mg (630ks aging) to
5.2 ± 0.7 at%Mg (1555ks aging) in amplitude with a wavelength of~11 nm (Here, the amplitude is expressed as the average with a 2s
error, whereas the wavelength as divided Z distance by the numberof wavelengths traversing the volume in Figs. 5(d) and 6(d)).
Remember that no GP zones have been formed yet in the two aging
conditions, as suggested by the corresponding TEM microstructures(e.g. Fig. 3(a) for 1555ks aging), because the maximum Mg concentration on the auxiliary curves does not reach the metastableequilibrium of 25 at%Mg.
4. Discussion
4.1. Quantitative estimation of modulated structures
As described in 1. Introduction, spinodal decomposition ofhighly concentrated Al-Mg alloys has been characterized only by
TEM [13e16] or SANS [17], and thus our APT can provide new andunique information about the compositional ﬂuctuations eventhough only the undeformed specimen was investigated (To authors' knowledge, ATP has been only applied to the evaluation of
spinodal microstructures in iron-, copper- and titanium-based alloys [21e25], although the spatial distribution of the d0 eAl3Li phase
was depicted by the authors' APT, as evidence of spinodal decomposition (followed after congruent ordering) occurring in an Al-LiCu alloy [26]). For example, the above estimate of amplitude andwavelength of Mg ﬂuctuations (i.e. 2.4 ± 0.5 at%Mg or 5.2 ± 0.7 at%
Mg and ~11 nm) is based on intuitive real-space morphology ofFig. 5. Results of APT for the undeformed specimen aged at 343 K for 630 ks. (a) Elemental map of Mg atoms, (b) Isoconcentration map at isosurface level of 17.5 at%Mg (Five-point
weighted smooth and grid spacing of 0.8 nm), (c) Detection event histogram of Mg atoms on the X-Y plane matched with stereographic projection of (111)a, (d) Composition proﬁleof Mg along the [001] a direction of the selected box in (b).

Y. Tang et al. / Acta Materialia 131 (2017) 57e64

61

Fig. 6. Results of APT for the undeformed specimen aged at 343 K for 1555 ks. (a) Elemental map of Mg atoms, (b) Isoconcentration map at isosurface level of 18.0 at%Mg (Five-point
weighted smooth and grid spacing of 0.8 nm), (c) Detection event histogram of Mg atoms on the X-Y plane matched with stereographic projection of (001)a, (d) Composition proﬁleof Mg along the [001] a direction of the selected box in (b).

spinodal microstructures in Figs. 5 and 6, and thus there is no need
to worry about overlapping of multi-wavelength developing toward different directions, unlike in the case of transmission imaging technique such as TEM. As for the statistical reliability of theestimate, furthermore, the following two analyzing methods; i.e.
autocorrelation function and frequency distribution analyses, cancompensate for such a drawback of APT, as with the case of reciprocal space technique by SANS.
Fig. 7(a, c) shows one-dimensional autocorrelation function R(k)calculated from composition proﬁles of Mg in the undeformed
specimen aged at 343 K for 630 and 1555 ks (e.g. Figs. 5(d) and6(d)). Under this analyzing method, a positive value of R(k) is
generated at lag k if a modulated two-phase microstructure isformed with a wavelength corresponding to k, whereas random
solid solutions yield values of R(k) close to zero [21]. It is clearlyseen in Fig. 7(a,c) that the ﬁrst positive maximum of R(k) appears at
k ¼ 12.1 ± 0.1 or 12.8 ± 0.1, and thus wavelength of those ﬂuctuations is found to increase from 12.1 ± 0.1 nm to 12.8 ± 0.1 nm during
the period from 630ks to 1555 ks (Thus, a minor adjustment of themanually estimated wavelength of ~11 nm has to be made). Similarvalue of 12.5 nm is reported for an Al-10 wt%Mg alloy naturally
aged for 1980 ks, in which not only the periodicity of phasemodulated contrast along the [001]a direction but also the diffuse
appearance of satellite spots (or sidebands) near each Bragg
reﬂection was carefully investigated on the SAED patterns [13,14](N.B. Such satellite spots are characteristic of periodic modulations of composition, and from those spacing the wavelength of themodulation can be determined as average quantity through theDaniel-Lipson equation [27]).
As for the amplitude of compositional ﬂuctuations, on the otherFig. 7. (a, c) One-dimensional autocorrelation function calculated from the composition proﬁles of Mg in Figs. 5(d) and 6(d) and (b, d) frequency distribution curve of Mg concentrations estimated from overall APT data in Figs. 5(a) and 6(a) for the undeformed specimen aged at 343 K for 630 (a, b) and 1555 ks (c, d).

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Y. Tang et al. / Acta Materialia 131 (2017) 57e64

hand, neither TEM nor SANS could estimate quantitatively.
Fig. 7(b,d) shows frequency distribution curves of Mg concentrations, constructed by dividing overall APT data (e.g. Figs. 5(a) and6(a)) into equal sized blocks of 100 atoms and plotting theobserved number of blocks with each composition, for the undeformed specimen aged at 343 K for 630 and 1555 ks. Slight deviation of frequency distribution from binomial distribution(corresponding to random solid solution) can be seen after 630ks
aging, and then a further broadening of the frequency distribution
is found to occur after 1555ks aging. The corresponding c2 test also
proved such an inhomogeneity of Mg distributions because the null
hypothesis of random solid solution was rejected with >99.9% of
signiﬁcance level. This ﬁnding provides collateral evidence of thedevelopment of spinodal decomposition, and prolonged aging
treatment is expected to yield the frequency distribution with morethan one peak corresponding to the decomposed Mg-rich and Mglean regions [20].
4.2. Strengthening by spinodal decomposition
It is known that spinodal decomposition can enhance the mechanical properties of iron-, copper-, titanium-, aluminum-,
manganese-based and other alloys [14,28e33], and the hardening
mechanism (i.e. spinodal-hardening mechanism) has been proposed by several researches [34e38] including Cahn, who established the present spinodal decomposition theory [39,40]. Aftertaking those proposed mechanisms into consideration, Kato et al.
[18] conclusively demonstrated based on a force balance equationof mixed dislocations that the increment in yield stress caused bycoherent internal stress in a fcc alloy is proportional to the amplitude of compositional ﬂuctuation, but independent of wavelength,of the modulated structures;AjhjY
Ds ¼ pﬃﬃﬃ ;6(1)

where Ds is the increment in yield stress [ ¼ MDtCRSS], M is Taylor
factor (¼ 3.06 for fcc metals), DtCRSS is the increment in criticalresolved shear stress, A is the amplitude of the compositional
ﬂuctuation, h is lattice mismatch [¼(1/a) (va/vc)], va/vc is thecomposition variation of stress-free lattice parameter a withrespect to Mg concentration c, and Y is elastic modulus calculated
from elastic constants C. In the case of aluminum with fcc structure,
the values of

Y〈100〉 ¼ ðC11 þ 2C12 Þ ðC11  C12 Þ=C11
Y〈110〉 ¼ ðC11 þ 2C12 ÞðC11 þ 6C44  C12 Þ=½2ðC11 þ 2C44 þ C12 Þ 
Y〈111〉 ¼ 6ðC11 þ 2C12 ÞC44 =ð4C44 þ C11 þ 2C12 Þ

Fig. 8. Relationship between the increment of Vickers hardness (Fig. 1) and the estimated amplitude of Mg ﬂuctuations (Figs. 5(d) and 6(d)) during aging of the undeformed specimen at 343 K.

specimen aged at 343 K for 630 and 1555 ks. From the good linearity through the two plots, it can be deduced that the developmentof coherent internal stress associated with modulated structures is
attributed to the strengthening by spinodal decomposition as
predicted by Kato et al. [18]. Note that the coherent internal stress isoriginated from the difference of atomic radii between Al and Mg,
and thus with increasing amplitude dislocation movement during
deformation is hindered.
The same spinodal-hardening mechanism is considered toactivate in the HPTed specimen with a high number density of grainboundaries. As described in 3. Results, the age-hardenability of
DHV31 ± 2 for the HPTed specimen was almost identical to that of
DHV33 ± 2 for the undeformed specimen (Fig. 1), and modulated
structure was similarly observed within the ultraﬁne grains in the
HPTed specimen without grain boundary precipitates (Fig. 4). It is
no surprise, therefore, that spinodal decomposition is supposed toproceed even in the HPTed specimen because the wavelength of
12.1 ± 0.1 nm or 12.8 ± 0.1 nm (Fig. 7(a,c)) is sufﬁciently smaller
than the average grain size of ~100 nm or ~120 nm (Figs. 2 and 4).
The sluggish growth of the ultraﬁne grains even after 2576ks aging
also maintains the contribution of ultraﬁne-grained hardening, but
unfortunately SPD induced GB segregation of Mg atoms [42] failed
to be detected in this study, leaving the reason why the ultraﬁne
grains grow so sluggishly as a subject of further investigation.

(2)
are estimated as 98.7, 110 and 114 GPa, respectively, from C11 ¼ 107,
C12 ¼ 60.8 and C44 ¼ 28.3 GPa [40], and thus the modulated
structures are conﬁrmed to develop along the elastically soft [001]a
direction, as observed in composition proﬁles of Mg in Figs. 5(d)and 6(d). Assuming that Vickers hardness is proportional to yieldstress with a proportionality constant of ~3 [41], therefore, theincrement of Vickers hardness can be correlated to the amplitude A
through

AjhjY
DHVf pﬃﬃﬃ :6(3)

Fig. 8 shows the relationship between the increment of Vickers
hardness (i.e. DHV ¼ 13.5 ± 2 and DHV ¼ 25 ± 2 in Fig. 1) and theestimated amplitude of Mg ﬂuctuations (i.e. A ¼ 2.4 ± 0.5 at%Mg
and A ¼ 5.2 ± 0.7 at%Mg in Figs. 5(d) and 6(d)) for the undeformed

4.3. Inﬂuence of severe plastic deformation on spinodal
decomposition
It is well known that ultraﬁne-grained structures contain a highdensity of defects such as dislocations and vacancies, and might
affect the mobility of solute atoms and thus at least the kinetics ofphase decomposition [43]. The application of HPT can achieve supersaturated solid solutions of excess amount of alloying elements[44], and in fact both the undeformed and HPTed specimens
possessed the super-saturated solid solutions in the “before aging”
condition, as suggested by the absence of any reﬂections from Mg
or other compounds in the obtained (but unshown) XRD proﬁles.
Fig. 9 shows determination procedure of lattice parameters of thesuper-saturated solid solutions from their XRD proﬁles throughCohen's least-squares method [45,46]. Almost equivalent lattice
parameters of 0.4101 and 0.4106 nm were estimated from

Y. Tang et al. / Acta Materialia 131 (2017) 57e64

63

elements [47e50], are effective in maximizing the effect of concurrent strengthening by ultraﬁne-grained and precipitation
hardenings, as previously reported for Al-Mg-Si(-Cu) alloys in
Refs. [6,9]. Therefore, our proposed three strategies are useful for
designing any concurrently strengthened severely-deformed agehardenable aluminum alloys.
5. Conclusions

Fig. 9. Determination procedure of lattice parameters of the obtained super-saturated
solid solutions from XRD proﬁles for the (a) undeformed and (b) HPTed specimens
through Cohen's least-squares method [45,46].

intersections of extrapolation lines through plots of a(¼(h2þk2þl2)1/2d) vs cos2q (Here, h, k, l are Miller indices of a Bragg
plane, and d is the lattice spacing calculated from l/2sinq), inagreement with reported lattice parameters of supersaturated Al(5e40)wt%Mg alloy subject to HPT [44]. Such an equivalent lattice
parameter, and thus equivalent Mg concentration in the matrix,
leads to the equal contribution of solid-solution hardening by Mg
atoms for both the undeformed and HPTed specimens. Therefore,
remarkable increase in “before aging” hardness in Fig. 1 can be
explained by SPD induced ultraﬁne-grained hardening alone.
In the subsequent stage of aging, furthermore, the effect ofultraﬁne-grained structures on spinodal decomposition might beinvolved in Fig. 1, but unfortunately the softening by the growth ofultraﬁne grains (i.e. from ~100 nm in Fig. 2 to ~120 nm in Fig. 4)
makes it impossible to evaluate the degree and kinetics of spinodal
decomposition in the HPTed specimen precisely. The quantiﬁcationof Mg ﬂuctuations in the fragile HPTed specimen is in progress bylaser-assisted APT.
4.4. Concurrent strengthening of ultraﬁne-grained age-hardenable
aluminum alloy
One of the signiﬁcant achievements in this study was to fabricate again novel Al-Mg alloy with a higher attained hardness (i.e.
~HV296) by means of concurrent strengthening by ultraﬁnegrained and precipitation hardenings. Following the success of
our previously reported 2091Al-Li-Cu alloy [6,9], it was veriﬁed thatthe HPTed specimen of the Al-13.4 wt%Mg model alloy can be also
strengthened by aging treatment at a lower temperature. Although
there are still problems to be solved, e.g. poor ductility and poor
corrosion resistance of such a highly concentrated Al-Mg alloy, theeffectiveness of our proposed strategy to achieve concurrent
strengthening; i.e. “taking advantage of spinodal decomposition”,
was proved to be applicable to the alloys that decompose via spinodal decomposition. In contrast, in the case of the alloys that
decompose via nucleation and growth process, the other two
strategies; i.e. lowering of aging temperature (within miscibility
gap, but above spinodal lines) and utilization of microalloying

In this study, concurrent strengthening of ultraﬁne-grained agehardenable Al-13.4 wt%Mg (Al-14.7 at%Mg) alloy has been successfully accomplished by combined processing of HPT and aging
treatment at 343 K below the spinodal line. Not only a higher
attained hardness of >HV 296 was obtained, but also the good agehardenability of DHV31 ± 2 was maintained almost comparable tothat of DHV33 ± 2 for the undeformed specimen without HPT. The
corresponding TEM images exhibited patchy pattern of phasemodulated contrast typical of spinodally decomposed microstructures, and APT could detect and quantify the ﬂuctuation of Mg
concentrations developing along the [001]a direction. The amplitude of Mg ﬂuctuations in the undeformed specimen was found toincrease from 2.4 ± 0.5 at%Mg to 5.2 ± 0.7 at%Mg during aging from630ks to 1555ks with wavelengths of 12.1 ± 0.1 nm and12.8 ± 0.1 nm, and the good linearity between the increment in
hardness and the estimated amplitude strongly supported Kato's
spinodal-hardening mechanism. By taking into account the factthat the wavelength is sufﬁciently smaller than average size of ultraﬁne grains (i.e. ~100 nm or ~120 nm), spinodal decomposition is
supposed to proceed even in the HPTed specimen. Therefore, theeffectiveness of our proposed strategy to achieve concurrent
strengthening; i.e. “taking advantage of spinodal decomposition”, isveriﬁed for the alloys that decompose via spinodal decomposition.
Acknowledgment
This study was supported by Japan Science and Technology
Agency (JST) under collaborative research based on industrial demand “Heterogeneous structure control: Towards innovative
development of metallic structural materials.” The authors would
like to thank Dr. T. Hamaoka and Mr. M. Kondo, Yokohama National
University, and Dr. Y. Miyahara, Central Research Institute of Electric Power Industry (CRIEPI), for the operation of some of TEM and
APT experiments. The cast ingot utilized in this study was generously supplied by Furukawa-Sky Aluminum Corp. (Present: UACJ
Corp.).
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