of 12
1
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RepoRts
| 7:
15628
| DOI:10.1038/s41598-017-15229-4
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Shock Synthesis of Decagonal
Quasicrystals
J.
Oppenheim
1
, C.
Ma
1
, J.
Hu
1
, L.
Bindi
2,3
, P. J.
Steinhardt
4,5
& P. D.
Asimow
1
The Khatyrka meteorite contains both icosahedral and decagonal quasicrystals. In our previous studies,
icosahedral quasicrystals have been synthesized and recovered from shock experiments at the interface
between CuAl
5
and stainless steel 304 alloys. In this study, we report a new shock recovery experiment
aimed at synthesizing decagonal quasicrystals similar to decagonite, natural Al
71
Ni
24
Fe
5
. Aluminum
2024 and permalloy 80 alloys were stacked together and shocked in a stainless steel 304 recovery
chamber. Abundant decagonal quasicrystals of average composition Al
73
Ni
19
Fe
4
Cu
2
Mg
0.6
Mo
0.4
Mn
0.3
with traces of Si and Cr were found along the recovered interface between the Al and permalloy. The
experiment also synthesized AlNiFe alloy with the B2 (CsCl-type) structure and the metastable Al
9
Ni
2
phase. We present chemical (scanning electron microscopy and electron microprobe) and structural
(electron backscatter diffraction and transmission electron microscopy) characterization of the
recovered phases and discuss the implications of this shock synthesis for the stability of quasicrystals
during high-pressure shocks and for the interpretation of the phase assemblage found in Khatyrka.
Unlike periodic crystals, which have 1-, 2-, 3-, 4-, or 6-fold rotational symmetries, quasicrystals are aperiodic
structures
1
,
2
that can exhibit previously forbidden rotational symmetries such as 5-, 8-, 10-, and 12-fold. Three
types of natural quasicrystal have been discovered in (and, so far,
only
in) the Khatyrka meteorite
3
7
. Two of these,
Al
63
Cu
24
Fe
13
icosahedrite (all formulas herein are on an atomic basis) and Al
62
Cu
31
Fe
7
i-Phase II, have icosahedral
symmetry, featuring six five-fold axes of rotational symmetry
4
. On the other hand, Al
71
Ni
24
Fe
5
decagonite displays
a single 10-fold rotational symmetry axis together with periodic patterns taken perpendicular to the 10-fold
direction
5
. The occurrence of natural quasicrystals in a meteorite demonstrating evidence of a high-pressure
shock and rapid post-shock cooling
8
suggests that the passage of a shock wave facilitates nucleation and growth
of quasicrystals and perhaps relaxes the constraints on precise ratios of starting materials required by static syn-
thesis methods. We have experimentally confirmed this idea in previous reports of successful laboratory shock
synthesis of icosahedral quasicrystals analogous to icosahedrite but with higher Al contents and five significant
components (Al, Cu, Fe, Cr, Ni)
9
,
10
.
The quasicrystals in Khatyrka coexist with a number of other exotic, but crystalline, metallic phases, including
aluminum (Al), stolperite (CuAl), kryachkoite (Al,Cu)
6
(Fe,Cu), hollisterite (Al
3
Fe)
11
, khatyrkite (CuAl
2
), cupal-
ite (CuAl), and steinhardtite
12
. Steinhardtite is a new polymorph of Al and exhibits a body-centered cubic (bcc)
structure, unlike the common phase of aluminum, which has a face-centered cubic (fcc) structure.
In this study, we used an experimental technique similar to Asimow
et al
.
9
to try to synthesize decagonal
quasicrystals (d-QC) similar to decagonite
6
in order to further confirm the hypothesis that natural quasicrystals
in the Khatyrka meteorite are explained by shock processing of unusual metallic alloys during collisions in the
asteroid belt. Because the goal of this work is primarily to show that synthesis of Al-Ni-Fe d-QC by shock is
straightforward, we did not go to extraordinary ends to match the starting materials to those present in Khatyrka.
Instead we selected readily available commercial alloys. An aluminum 2024 disc was loaded into a permalloy 80
cup within a stainless steel 304 (SS304) recovery chamber. The target was impacted by a Ta flyer at 1041
±
1 m/s
and then the chamber was recovered intact, sawn open, polished, and examined by scanning electron micros-
copy (SEM) in imaging, energy dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction
(EBSD) modes; by wavelength-dispersive electron probe microanalysis (EPMA); and by transmission electron
1
Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd. M/C170-
25, Pasadena, CA, 91125, USA.
2
Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via La Pira 4,
I-50121, Firenze, Italy.
3
CNR-Istituto di Geoscienze e Georisorse, Sezione di Firenze, Via La Pira 4, I-50121, Firenze,
Italy.
4
Department of Physics, Princeton University, Jadwin Hall, Princeton, NJ, 08544, USA.
5
Princeton Center for
Theoretical Science, Princeton University, Princeton, NJ, 08544, USA. Correspondence and requests for materials
should be addressed to P.D.A. (email:
Asimow@gps.caltech.edu
)
Received: 18 August 2017
Accepted: 23 October 2017
Published: xx xx xxxx
OPEN
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2
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RepoRts
| 7:
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| DOI:10.1038/s41598-017-15229-4
microscopy (TEM) on a section extracted by Focused Ion Beam (FIB) milling, in bright-field imaging, scanning,
EDS, and selected-area electron diffraction (SAED) modes.
Results
SEM and EPMA analysis.
Figure
1
displays a general view of the polished surface of the recovered sample
chamber, extending across the full width of the Al2024 layer, attenuated by the cratering flow to about half its
original thickness. In both backscattered electron contrast images and false-color EDS X-ray maps, the unreacted
SS304, Al2024, and permalloy 80 regions are evident along with newly-formed reacted zones incorporating ele-
ments from both the Al2024 and permalloy 80 starting materials. The SS304 does not appear to have participated
extensively in any reactions. The thickest observed mixed layer, about 100
μ
m wide, extends across the full rear
(down-range) contact between the starting materials. The thinner layer that appears, in this section, to occupy the
boundary between the SS304 and Al2024 is also formed by mixing Al2024 and permalloy 80, derived from the
side-wall of the permalloy cup around the Al2024.
Viewing the rear contact mixed layer at progressively higher magnification in Fig.
2
, it becomes clear that
this region contains mostly ~1
μ
m sized rounded grains with a few ~10
μ
m angular grains and numerous voids
ranging from 1 to 100
μ
m across. It is not clear whether these are representative of actual voids in the volume of
the recovered sample, formed upon decompression from the shock state, or whether they are only surface features
formed by plucking during polishing. However, the holes exposed on the surface were filled with epoxy in order
to allow better polishing of the surrounding metallic areas.
Phase identification of the various materials delineated by distinct backscatter contrast is achieved with point
EDS analyses and EBSD patterns to measure composition and structure at common points. The light gray, typi
-
cally micron-sized, often rounded grains display Kikuchi patterns with characteristic 10-fold symmetry (Fig.
3a
)
uniquely associated with decagonal quasicrystals. As expected, since the d-QC structure has only one 10-fold
symmetry axis, a relatively small fraction of grains show the 10-fold zone axis within the cone sampled by the
EBSD detector (in fact, the d-QC have a preferred orientation, see below). EDS analysis of these points shows
that they contain, above detection limit, observable concentrations of seven metals: Al, Ni, Fe, Cu, Mg, Mo, and
Mn. Quantitative analysis by electron microprobe shows an average composition for these regions, expressed on
a 100-atom basis, Al
73.3
Ni
19.3
Fe
4.3
Cu
1.8
Mg
0.6
Mo
0.4
Mn
0.3
(see Table
1
for ranges in composition based on standard
deviation among 8 analytical points).
Several other phases were identified in intimate contact with the d-QC in the mixed region. First, a cubic
structure (Fig.
3b
) was found, with two distinct groups of compositions. The average analyses of the two compo-
sitions are Al
63
Ni
29
Fe
5
Cu
2
Mo
1
and Al
50
Ni
37
Fe
8
Cu
2
Mo
1
(Table
1
; the sums of these formulas may differ from 100
due to rounding and minor components). The EBSD patterns of two candidate structures for this phase, the
Im
̄
3
m
(A2 or bcc) structure (like steinhardtite) and a
Pm
̄
3
m
primitive cubic unit cell structure (B2 or CsCl-type
structure), are quite similar. The phase diagram of the Al-Ni-Fe system
13
suggests that both compositions plot in
the stability field of the B2 phase. Careful examination of the EBSD patterns shows that the (111) reflection is
present (Fig.
3c
), which is a forbidden reflection in bcc (Fig.
3d
) and indicates that the phase is B2. This result was
confirmed by TEM study, see below. We also observe an Al
9
Ni
2
phase with monoclinic
P
2
1
/
c
Al
9
Co
2
-type struc-
ture and typical composition Al
81
Ni
13
Fe
4
Cu
1
Mg
1
(Fig.
3e,f
and Table
1
). Al
9
Ni
2
is a metastable alloy that only
forms during rapid solidification
14
.
The cubic phase and the d-QC are closely associated and often form a core-and-petal flower texture (Fig.
2d
).
Oddly, the flowers appear to have an overall 10-fold symmetry in many sections, even though the phase at the
core of the flower is cubic and lacks such a symmetry element. This pattern resembles in some ways the texture
Figure 1.
General views of the polished surface of recovered specimen from shot S1235. First shock propagated
from bottom of images towards top. (
a
) Backscattered electron image, showing the initially 1 mm thick Al2024
layer attenuated by flow associated with impact deformation to ~0.5 mm in this area near the left side-wall of
the chamber. Regions of intermediate backscatter contrast mark a reaction zone up to ~0.1 mm wide with mean
atomic number intermediate between the Al-rich and Ni-rich starting materials. (
b
) X-ray intensity map of the
same general area, using the color scheme shown at lower left. Voids in the sample appear cyan due to Carbon in
the epoxy fill. The brown regions mark the mixed layers.
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3
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RepoRts
| 7:
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| DOI:10.1038/s41598-017-15229-4
of crystal clusters observed in synthetic Zn-Al-Cr alloys
15
, where multi-twins sharing a five-fold symmetry axis
led to the suggestion that crystals nucleated on a short-lived icosahedral cluster or nucleus. We speculate that the
cubic phase at the center of the core-and-petal structures in our experiment may be recrystallized from an initial
transient phase with decagonal symmetry, but this idea requires further investigation.
TEM analysis.
Two FIB sections were extracted from the reaction boundary area between the Al-alloy 2024
and permalloy (Fig.
1
). The reaction region has two distinct textures (Fig.
4a
). The first texture consists of coarse
grains of d-QC, the cubic AlNiFe phase, and Al
9
Ni
2
, with significant porosity. The other texture, closer to the
Al2024 side of the reaction zone, is denser in appearance and includes predominantly fine-grained d-QC. FIB sec-
tion A was extracted from the second textural area, because we presumed it would be more coherent during mill-
ing and liftout. The extracted and thinned FIB section A, taken from the area shown in a higher-magnification
secondary electron image in Fig.
4b
, is shown in a bright-field TEM image in Fig.
4c
, with three selected regions of
interest highlighted. FIB section B was later taken from the first textural area, displaying better-formed “flowers”,
and shown in secondary electron image Fig.
4d
and bright-field TEM image Fig.
4e
with two selected regions
of interest highlighted. The nearly uniform diffraction contrast of the d-QC grains in FIB section B indicates a
shared orientation, with most of their 10-fold axes in the plane of the section and roughly parallel to the direction
of shock propagation. One indicated grain in FIB section B is amorphous (Fig.
4e
) and has high gallium content,
inferred to be from Ga beam damage amorphization during sample preparation.
FIB Section A
,
Region of Interest 1
: This region includes an equant d-QC surrounded by irregularly shaped
QCs (Fig.
5
). SAED indicates the decagonal quasicrystal has a stacked-layer structure. Viewed along a zone axis
perpendicular to the 10-fold rotation axis, the measured spacing between layers is ~4.2
Å. This is identical to the
result obtained on natural decagonite by single crystal X-ray diffraction, an interlayer pacing of 4.208(2) Å
5
. The
composition of this d-QC grain by transmission EDS is roughly Al
66
Ni
19
Fe
4
Cu
x
(copper is not correctly measured
in TEM, which has Cu internal parts and the sample is held on a Cu TEM grid).
FIB Section A
,
Region of Interest 2
: This region (Figs
6
and
7
) contains a flower apparently similar to those
observed by SEM. The center phase, shown with moderately dark contrast in the bright field image in Fig.
6
and
very dark contrast in Fig.
7
, is clearly not bcc-structured steinhardtite. In Fig.
6
it is viewed along the [110] axis
and in Fig.
7
along the [111] axis and shows a SAED pattern matching the primitive cubic lattice of a CsCl-type
(B2) structure. Its cell parameter is ~3.0
Å. The [110] zone pattern is particularly useful for distinguishing the bcc,
face-centered (like
α
-Al) and primitive cubic lattices (Supplementary Fig. S1).
Figure 2.
Progressive enlargement of backscattered electron images of the mixed layer (See Fig.
1a
for locations
of frames): (
a
) Within the ~100
μ
m wide mixed layer, porosity and 1–10
μ
m grains of distinct backscatter
contrast are visible. (
b
) Further enlargement shows angular ~10
μ
m grains of a cubic phase and rounded ~1
μ
m
grains identified as “d-QC”. (
c
) The dark spots are voids, the dark gray grains are Al
9
Ni
2
, the light gray are d-QC,
and the bright white areas are the cubic phase. (
d
) A “flower” with a core of cubic phase and petals of d-QC
radiating outward. Many of the flowers appear to display 10-fold symmetry.
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RepoRts
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| DOI:10.1038/s41598-017-15229-4
The composition of the Al-alloy is Al
55
Ni
29
Fe
4
Cu
x
in Region of Interest 2, comparable to the average of the
two cubic structure phases measured by SEM EDS. B2 is the expected structure for Al-rich AlNiFe alloy of this
composition
12
,
13
. In addition, the diffraction pattern shows clear superlattice structure. The weak diffraction spots
between the intense spots indicate a large periodicity in the structure (Figs
4
and
5
). It could be that the alloy is
not perfectly stoichiometric and each unit cell takes less than one vacancy; in such a case, ordering of the vacan-
cies is capable of creating the superlattice structure as shown by the example in Fig. A1
16
.
When the B2 Al-alloy grain at the center of the flower is rotated to view along the [110] zone axis, two of the
d-QC petals on opposite sides show exactly the same SAED pattern (Fig.
6
), indicating the two grains have the
same orientation. Moreover, their 10-fold rotation axis is likely parallel to the [001] zone of the Al-alloy. Other
quasicrystal petals of Region 2 share different zones at another orientation.
Figure 3.
Kikuchi patterns from Electron Backscatter Diffraction of selected phases in S1235. (
a
) d-QC
displaying the 10-fold zone axis. The pattern is clearly distinct from the 5-fold zone axes observed in icosahedral
quasicrystals by Asimow
et al
.
9
. (
b
) Unindexed pattern of cubic phase. Note the faint superposed double-band
running from upper-left to lower-center. (
c
) Indexing of pattern from (
b
) as
Pm
̄
3
m
B2 (CsCl) structured cubic
phase, with the green band representing the (111) plane highlighted as the narrow member of the double-band.
(
d
) Attempt to index pattern from (
b
) as
Im
̄
3
m
A2 (bcc) structure, with the red band representing the (222)
plane; note that (111) is forbidden in this structure. Hence the EBSD result favors the B2 structure for this
phase. (
e
) EBSD pattern of Al
9
Ni
2
phase. (
f
) Indexing of pattern (
e
) as monoclinic space group 14.
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FIB Section A
,
Region of Interest 3
: Silica-rich material occurs in the FIB section (Fig.
8
). Although some
globules appear to have grain boundaries, diffraction patterns collected from the region do not show crystalline
material. The silica is likely derived from the colloidal polishing medium used to prepare the section.
FIB Section B
,
Region of Interest 1
: This area (Fig.
9
) shows a cluster of d-QC grains with their ten-fold axes in
the plane of section. SAED including two of the grains shows that both are viewed perpendicular to the 10-fold
axis, both have the common 4.2
Å layer spacing, and their 10-fold axes lie at 30° to each other.
FIB Section B
,
Region of Interest 2
: This area (Fig.
10
) contains mottled regions of fcc aluminum interstitial to
the d-QC grains. SAED patterns of the fcc Al exhibit doubled spots, consistent with twinning being responsible
for the mottled contrast. Evidently, not all of the Al in the reacted region is transformed to d-QC, B2, and Al
9
Ni
2
forms; some interstitial fcc Al is present as well.
Discussion
The d-QC most likely formed by melting and mixing of the Al2024 and permalloy followed by rapid solidification.
However, simple impedance matching and Hugoniot equation of state calculations show that the temperatures
generated by shock compression in the target should not have exceeded 350
°C. Substantially higher temperature
is required to melt any of the layers, whether at pressure or upon release to 1
atm. We conclude that additional
heating mechanisms must have acted to locally raise the temperature at the interfaces between Al2024 and per
-
malloy 80. As we discuss in Oppenheim
et al
.
10
, the geometry of the experiment is consistent with two distinct
heating mechanisms. Collapse of pore spaces created along the interfaces by imperfect machining and frictional
sliding due to shear flow across material boundaries with different particle velocities (parallel to the shock prop-
agation direction) both offer plausible local heating mechanisms. The reacted layer studied in detail here is found
along a boundary normal to the shock propagation direction and is therefore more likely related to pore collapse.
The d-QC formed in these experiments is very similar to natural decagonite
7
. Although we measure, above
detection limit, a number of minor components (Cu, Mg, Mo, and Mn) in the synthetic example, the ratios of
the major metals Al
76
Ni
20
Fe
4
(Table
1
) are quite similar to the natural case, Al
71
Ni
24
Fe
5
. Moreover, as we have
noted, the layer spacing perpendicular to the 10-fold axis, 4.2
Å, is identical to the more precise value obtained
by single-crystal X-ray diffraction on the natural specimen, 4.204(2) Å. Furthermore, like the natural decagonite,
our synthetic d-QC shows essentially no measurable phason strain. In the SAED pattern perpendicular to the
ten-fold axis (Fig.
5
, lower right), we do not observe any weak diffuse reflections, characteristic of imperfectly
ordered atoms along the ten-fold axis. Such perfection is surprising given the dynamic synthesis process and the
intergrowth with other phases. We made a similar observation in the case of our icosahedral shock-synthetized
quasicrystals
9
. In the case of natural decagonite, found in a shocked meteorite billions of years old, Bindi
et al
.
5
hypothesized that the low phason strain observed might be a side-effect of the original growth process or it might
reflect annealing over the very long age of the specimen. In the experimental product, however, such long anneal-
ing time has not been available and this supports the notion that shock synthesis grows essentially strain-free
highly perfect quasicrystals from the outset.
Al-Ni-Fe decagonal quasicrystals with the composition Al
70
Ni
15
Fe
15
were first discovered by Tsai
et al
.
17
. By
means of a convergent-beam electron diffraction (CBED) study, Saito
et al
.
18
found so-called G-M lines in
d-QC
B2 cubic phase 1
B2 cubic phase 2
Al
9
Ni
2
n
8
6
6
5
Weight percent
Al
56.2
±
0.7
45.3
±
0.6
31.0
±
0.5
63.9
±
0.6
Fe
6.8
±
0.5
6.8
±
0.5
9.6
±
0.5
6.1
±
0.4
Cu
3.2
±
0.6
3.3
±
0.5
3.0
±
0.5
1.5
±
0.4
Cr
nd
nd
0.16
±
0.08
nd
Ni
32.2
±
1.1
44.2
±
1.1
49.9
±
1.1
23.0
±
0.7
Mo
1.0
±
0.3
1.3
±
0.3
2.9
±
0.3
0.9
±
0.2
Mn
0.4
±
0.3
0.4
±
0.3
0.6
±
0.3
0.5
±
0.3
Mg
0.4
±
0.1
0.3
±
0.1
0.3
±
0.1
0.40
±
0.04
Si
nd
nd
0.14
±
0.06
nd
To t a l
100.3
101.7
97.4
96.3
Normalized atomic percent
Al
73.3
±
0.9
63.5
±
0.8
50.4
±
0.8
80.8
±
0.7
Fe
4.3
±
0.3
4.7
±
0.3
7.6
±
0.4
3.8
±
0.3
Cu
1.8
±
0.3
2.0
±
0.3
2.1
±
0.3
0.8
±
0.2
Cr
nd
nd
0.14
±
0.06
nd
Ni
19.3
±
0.7
28.5
±
0.7
37.4
±
0.9
13.4
±
0.4
Mo
0.4
±
0.1
0.5
±
0.1
1.3
±
0.1
0.3
±
0.1
Mn
0.3
±
0.2
0.2
±
0.2
0.4
±
0.2
0.3
±
0.2
Mg
0.6
±
0.1
0.4
±
0.1
0.5
±
0.1
0.56
±
0.06
Si
nd
nd
0.22
±
0.10
nd
Table 1.
Electron Probe analysis of S1235.
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odd-order reflections along the
c
*
direction, showing that these quasicrystals belong to the non-centrosymmetric
space group
P
̄
10
m
2. Their findings were confirmed in follow-up TEM studies by Tsuda
et al
.
19
. Tanaka
et al
.
20
, by
CBED and high-resolution microscopy, revealed a transition from
P
10
̄
m
2 to the centrosymmetric
P
10
5
/
mmc
as a
function of the Ni/Fe ratio. In detail, along the join Al
70
Ni
x
Fe
30-x
in the range 10
<
x
<
20, these authors showed
Figure 4.
Extraction and preparation of FIB sections for TEM analysis. (
a
) Backscattered electron image
showing variation in texture across the mixed region. FIB section A was extracted from the denser-textured
area (red box) close to the Al2024 layer. FIB section B was taken from an area with two prominent “flowers”
(green box). (
b
) Secondary electron image of the area targeted for FIB section A liftout, shown by the dashed red
rectangle. (
c
) Bright-field TEM image of extracted and thinned FIB section A. Regions of Interest 1, 2, and 3 are
further imaged and described below (Figs
5
,
6
,
7
and
8
). (
d
) Secondary electron image of the area targeted for
FIB section B liftout, shown by the dashed green rectangle. (
e
) Bright-field TEM image of extracted and thinned
FIB section B. Regions of Interest 1 and 2 are further imaged and described below (Figs
9
and
10
) and the region
labeled “amorphous” is discussed in the text.
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Figure 5.
Region of Interest 1 from FIB section A (see Fig.
4
). (
a
) Bright-field image of a decagonal
quasicrystal (dark) along the 10-fold rotation zone axis. The surrounding fine grains are quasicrystals of
different orientations. (
b
) SAED pattern of the quasicrystal along the 10-fold rotation axis. (
c
) SAED pattern
perpendicular to the 10-fold rotation axis, showing the interlayer spacing corresponding to 4.2
Å.
Figure 6.
Region of Interest 2 from FIB section A. (
a
) Bright field image. (
b
) In this orientation, two “petals” of
dark diffraction contrast display identical SAED images perpendicular to the 10-fold axis of the d-QC. Other
petals of weak contrast are d-QC grains along minor zone axes. (
c
) The grain at the center shows the [110]
zone-axis orientation of a cubic primitive lattice, with weak spots in the SAED pattern showing superlattice
diffraction.
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that alloys with 10
<
x
<
17.5 crystallize in the
P
̄
10
m
2 structure, whereas alloys with 17.5
<
x
<
20 give rise to the
P
10
5
/
mmc
structure.
For the decagonal quasicrystal synthesized here, all the TEM-SAEDs we collected that have the
c
*
direction
display clear odd-order reflections, indicating the absence of the
c
-glide perpendicular to the 10-fold axis. This
likely indicates that the 10
5
screw axis is absent and the d-QC studied here exhibits the non-centrosymmetric
P
10
̄
m
2 structure. Another possibility is that it exhibits a different centrosymmetric space group such as
P
10
5
/
mmm
.
It is noteworthy that the composition of the d-QC reported here is close to that of the thermodynamically
stable decagonal phase in the Al-Ni-Fe system, Al
71
Ni
24
Fe
5
21
, which is known to exhibit the centrosymmetric
P
10
5
/
mmc
structure
22
. The same results were found for the natural analogue of Al
71
Ni
24
Fe
5
, the mineral decago
-
nite
5
, by single-crystal X-ray diffraction
6
. Although it is possible our shock-recovered specimen occupies a meta-
stable or preserved high-pressure structure, we speculate that the discrepancy in the space group could be due to
the presence of other minor elements in the chemical formula (i.e., Cu and Mg). Ordering of such minor elements
Figure 7.
A different orientation of Region of Interest 2 from FIB section A. (
a
) Bright field image; diffraction
contrast has changed due to rotation of view relative to Fig.
6a
. (
b
) The zone axis of the Al-alloy at the center in
this view is [111]. Again, the intermediate weak spots are superlattice diffraction.
Figure 8.
Bright field image of Region of Interest 3 in FIB section A. Details of globules in the area of
amorphous silica-rich material are visible.
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could definitely influence the atomic arrangements sufficiently to make the difference among these rather similar
space groups.
In the natural Khatyrka case, decagonite is associated with the bcc-structured steinhardtite phase. However,
in this experiment we did not find the bcc structure. Instead we found the stable CsCl-type or B2 structure of
AlNiFe, alongside small amounts of interstitial fcc Al and the metastable Al
9
Ni
2
phase. In fact, studies of various
AlNiFe alloys have not uncovered any stability region for steinhardtite either at ambient or elevated pressure
23
.
Bindi
et al
.
12
hypothesized that metastable steinhardtite is formed and recovered in Khatyrka due to the unique
properties of shock synthesis, but our experiment provides no support for this particular hypothesis since our
experiment instead yielded only B2 AlNiFe alloy alongside the d-QC.
Figure 9.
FIB section B, Region of Interest 1. (
a
) Bright-field image of a cluster of d-QC grains showing similar
diffraction contrast (darker grey shades). (
b
) Corresponding electron diffraction (SAED) pattern encompassing
two of the grains, showing that both are oriented with the 10-fold zone axes in the plane of section and rotated
with respect to one another by 30°.
Figure 10.
FIB Section B, Region of Interest 2. (
a
) Interstitial aluminum alloy between the QCs has a mottled
contrast attributable to local strain and twinning. (
b
) [110] zone of the fcc Al-alloy. (
c
) [211] zone of the fcc Al-
alloy. The double spots indicate twinning in the lattice.
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The successful synthesis of decagonal quasicrystals by shock recovery reinforces the conclusion that the so-far
unique discovery of natural quasicrystals in the Khatyrka meteorite is explained by a collision experienced by
its parent body. Both icosahedral and decagonal quasicrystals can be found together in the same fragments of
Khatyrka, e.g., Grain 126,
5
,
6
,
12
,
24
. We have synthesized and recovered these two types of quasicrystals in sepa-
rate experiments, but the shock conditions achieved in the experiments were nearly identical and well within
the range of shock conditions that occur as a shock propagates through a heterogeneous object like a meteor
-
ite parent body. Although Khatyrka contains both metallic and silicate components and exothermic reactions
between them may have been important in the natural setting, here we successfully observed melting, mixing
and quasicrystal formation at interfaces between discrete bulk layers of all-metal starting materials, without any
reduction-oxidation reaction. The extraordinary ease with which unmixed metals mix to form quasicrystals
under shock stands in stark contrast to conventional metallurgical synthesis methods, which require intimate
mixing and controlled quenching of very specific bulk compositions.
Materials and Methods
Shock Recovery Experiment.
The recovery experiment was constructed as follows. A stainless steel 304
(SS304, Fe
71
Cr
18
Ni
8
Mn
2
Si
1
with traces of C, S, and P) retainer screw forms a 5 mm thick driver plate at the impact
surface. Behind this driver, a 1 mm thick, 4 mm diameter disk of Al2024 alloy (Al
94
Cu
4
Mg
1.5
Mn
0.5
, with no more
than 0.5% of any other element) was surrounded laterally by a 1 mm thick, 4 mm inner diameter, 5 mm outer
diameter ring of permalloy 80 (Ni
80
Fe
15
Mo
4.5
Mn
0.5
, with less than 0.3% Si) and backed by a 1 mm thick, 5 mm
diameter disk of permalloy 80. The permalloy parts were in turn surrounded and backed by a 2
cm thick SS304
inner screw with a 2 mm deep, 5 mm diameter counterbore machined into it to hold the sample. The inner and
outer screw were contained in a 3
cm thick, 5
cm diameter SS304 outer housing and momentum trap
25
. Machining
imperfections leave various cavities and grooves up to ~5
μ
m in size at contacts between the various parts; the
chamber is pumped to moderate vacuum before shooting but there is low-pressure gas in these void spaces.
The target was impacted by a 2 mm thick Ta flyer carried by a 20 mm diameter plastic sabot. The projec-
tile velocity, measured by dual laser interrupts, was 1041
±
1 ms
1
. Given this velocity estimate and the known
Hugoniots of Ta, SS304, and Al2024 and an estimate of the permalloy 80 Hugoniot from mixture theory and the
shock properties of Ni, Fe and Mo
26
28
, we estimated the states in each material after passage of the first shock with
analytical propagation of the uncertainties (Table
2
). However, given the thickness of the flyer and the duration
of supported shock, there were reverberations within the low-impedance Al2024 layer (Fig. S2). There was time
(~0.6
μ
s) for at least four reverberations, which would have raised the pressure in the Al2024 layer, stepwise, to a
pressure very close to the capsule pressure of 26.9
±
0.1
GPa and an estimated peak temperature
350 °C.
After each shot, the chamber was recovered intact, sawn open along a plane parallel to the shock direction,
polished with abrasives down to 0.25
μ
m and then by 24
hours of vibrational polishing in 30
nm colloidal silica,
and examined by scanning electron microscopy, including EDS and EBSD maps and point analyses, by EPMA,
and by extracting a thin section by Focused Ion Beam (FIB) milling for TEM analysis.
Scanning Electron Microscopy.
Scanning electron microscopy was carried out with the California
Institute of Technology (Caltech) Geological and Planetary Sciences Division (GPS) analytical facility’s Zeiss
1550VP field-emission SEM. Imaging and EDS analyses used 15
kV accelerating potential and a 60
μ
m beam
aperture. EBSD analyses used 20
kV accelerating potential and a 120-
μ
m beam aperture in high-current mode.
EDS spectra were on an Oxford X-max Si-Drift Detector. EBSD patterns were collected with an HKL system.
Both were analyzed using the Oxford Instruments AZtec software.
Electron Probe.
Selected areas of the recovered samples were reanalyzed for Al, Cu, Fe, Cr, Ni, Mo, Mn, Mg,
and Si with the five spectrometer JEOL 8200 electron microprobe in Caltech’s GPS analytical facility, using 12
kV
accelerating potential, a focused 5
nA beam, 20
s counting times on peak and 10
s on each background position,
and pure metal standards.
Transmission Electron Microscopy.
The FIB and TEM facilities used are in the Kavli Nanoscience
Institute at Caltech. FIB section A was milled and lifted out from quasicrystal-bearing regions using an FEI Nova
600 Nanolab DualBeam FIB and SEM using a 30
kV Ga-ion beam for initial milling. After placement on a copper
TEM grid, this sample was thinned and finalized with an 8
kV 19
nA Ga-ion beam. FIB section B was thinned
to 700
nm on the Nova 600 with 30
kV Ga beam and then transferred to a. Zeiss Orion NanoFab to finalize the
sample thinning to ~100
nm, with a 5
kV Ga beam. Analytical transmission electron microscopy (ATEM) anal-
ysis was performed on a FEI Tecnai TF20 instrument with super-twin objective lenses, operated at 200
kV. The
energy dispersive spectroscopy (EDS) data were collected in TEM mode using an EDAX SiLi detector with 10
eV/
SS304
Al2024
Permalloy 80
SS304
P
first
(GPa)
26.9
±
0.1
15.5
±
0.1
22.2
±
0.1
23.0
±
0.1
T
first
(°C)
386
142
116
121
P
peak
(GPa)
26.9
±
0.1
26.9
±
0.1
23.0
±
0.1
23.0
±
0.1
T
peak
(°C)
386
314
235
121
Table 2.
First and peak shock states in experiment S1235. Estimates from successive impedance matches. A 2
mm thick Ta flier launched at 1041
±
1
ms
1
impacted, in sequence, a 5 mm thick SS304 driver, a 1 mm Al2024
disk, a 1 mm thick permalloy 80 disk, and a 2
cm thick SS304 inner screw.
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channel and 51.2
μ
s processing time, to achieve 500 counts per second signals and 20–50% dead time. The SEAD
patterns were integrated using Gatan DigitalMicrograph
to refine the
d
-spacings of the studied quasicrystals.
Data and materials availability.
The recovered specimen is archived in the Lindhurst Laboratory
at Caltech and requests for further study may be directed to the first author. Original images and spectra are
archived in the Caltech GPS Division Analytical Facility and Kavli NanoScience Institute and are available upon
request.
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Acknowledgements
JO was supported by Dr. George R. Rossman Summer Undergraduate Research Fellowship. The Caltech
Lindhurst Laboratory of Experimental Geophysics and its staff members M. J. Burns and R. Oliver are supported
by the National Science Foundation through award EAR-1426526. JH is supported by a grant from the Caltech/
JPL President and Director’s Fund. We gratefully acknowledge support and infrastructure provided for this work
by the Kavli Nanoscience Institute at Caltech. The Caltech GPS Division Analytical Facility is supported, in part,
by NSF Grants EAR-0318518 and DMR-0080065.
Author Contributions
J.O. and P.D.A. conceived and executed the experiments. J.O., J.H., and C.M. analyzed the run products. J.O., J.H.,
and P.D.A. wrote the manuscript and all co-authors edited the manuscript.
Additional Information
Supplementary information
accompanies this paper at
https://doi.org/10.1038/s41598-017-15229-4
.
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Competing Interests
:
The authors declare that they have no competing interests.
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