Microstructural characterization of intermetallic phases in a solution-treated Mg–5.0Sm–0.6Zn–0.5Zr (wt%) alloy

Abstract

Although Zn–Zr intermetallic phases have been frequently reported in solution-treated Mg–RE–Zn–Zr systems (RE presents rare earth), the types, crystal structures and influence on precipitation remain to be systematically investigated. In this work, the intermetallic phases formed during solution treatment in a Mg–Sm–Zn–Zr alloy were studied using transmission electron microscopy. Four types, namely the plate phase, the spindle-like phase, the nano-scale granuliform phase and the rod-like phase were revealed. Except the first one, the other three ones are Zn–Zr phase. In addition, the granuliform phase presents quadrate-shaped or lath-shaped and was identified as Zn2Zr3. However, these two Zn2Zr3 phases with different shapes follow quite different orientation relationships with Mg matrix. Furthermore, three kinds of rod-like phases were observed: one across the grain boundary but only coherent with one grain, one covered by jagged Mg3Sm precipitates and one with no surficial phase. The former twos are Zn2Zr3 while the later one is Zn2Zr. Finally, this work indicates that only the plate MgZn2 phase and the rod-like Zn2Zr3 phase in the grain interior will act as heterogeneous nucleation sites for the Mg3Sm precipitates, thus influencing the precipitation.

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Materials Characterization
journal homepage: www.elsevier.com/locate/matchar
Microstructural characterization of intermetallic phases in a solution-treated
Mg–5.0Sm–0.6Zn–0.5Zr (wt%) alloy
Kai Guan
a,b
, Baishun Li
a,b
, Qiang Yang
a,⁎
, Dongdong Zhang
a
, Xuhu Zhang
c
, Jingqi Zhang
c
,
Lei Zhao
c
, Xiaojuan Liu
a
, Jian Meng
a,⁎
a
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
b
University of Science & Technology of China, Hefei 230026, PR China
c
Aerospace Research Institute of Materials & Processing Technology, Beijing, 100076, PR China
ARTICLE INFO
Keywords:
Magnesium alloys
Zn–Zr phase
Orientation relationship
Transmission electron microscopy
Precipitation
ABSTRACT
Although Zn–Zr intermetallic phases have been frequently reported in solution-treated Mg–RE–Zn–Zr systems
(RE presents rare earth), the types, crystal structures and influence on precipitation remain to be systematically
investigated. In this work, the intermetallic phases formed during solution treatment in a Mg–Sm–Zn–Zr alloy
were studied using transmission electron microscopy. Four types, namely the plate phase, the spindle-like phase,
the nano-scale granuliform phase and the rod-like phase were revealed. Except the first one, the other three ones
are Zn–Zr phase. In addition, the granuliform phase presents quadrate-shaped or lath-shaped and was identified
as Zn
2
Zr
3
. However, these two Zn
2
Zr
3
phases with different shapes follow quite different orientation relation-
ships with Mg matrix. Furthermore, three kinds of rod-like phases were observed: one across the grain boundary
but only coherent with one grain, one covered by jagged Mg
3
Sm precipitates and one with no surficial phase.
The former twos are Zn
2
Zr
3
while the later one is Zn
2
Zr. Finally, this work indicates that only the plate MgZn
2
phase and the rod-like Zn
2
Zr
3
phase in the grain interior will act as heterogeneous nucleation sites for the Mg
3
Sm
precipitates, thus influencing the precipitation.
1. Introduction
Magnesium alloys, owing to high specific strength, low density and
good damping property, have been growingly potential for aerospace
and transportation applications [1–3]. For the wide applications, many
investigations were conducted to further improve their mechanical
properties. Due to the outstanding strengthening effects of rare earth
(RE) elements, developing RE-containing high-strength alloy is one of
the most interesting topics in the worldwide [4–10]. Hitherto, several
traditional high-strength Mg–RE-based systems were developed, with
the room-temperature ultimate tensile strength (UTS) over 400 MPa or
even 500 MPa [11–13]. As an example, Homma et al. [11] reported that
the hot-extruded and artificially aged Mg–10Gd–5.7Y–1.6Zn–0.6Zr (wt
%) alloy is with UTS and yield strength (YS) of approximately 542 MPa
and 473 MPa, respectively, at room temperature. In addition, our pre-
vious work also indicates that the values of UTS and YS are approxi-
mately 461 MPa and 458 MPa, respectively, for the extruded
Mg–8Gd–1.2Zn–0.5Ce–0.5Zr alloy [12], and approximately 427 MPa
and 416 MPa, respectively, for the extruded Mg–3.5Sm–0.6Zn–0.5Zr
alloy [13].
It is well reported that zinc (Zn) can remarkably enhance the age-
hardening response of the Mg–RE-based alloys [14–17], and zirconium
(Zr) can significantly refine grains [18,19]. Therefore, except REs, the
high-strength Mg–RE-based alloys ordinarily contain both Zn and Zr.
Inevitably, quite a few Zn- and/or Zr-containing intermetallic phases
will form in the alloys during solidification or even during the following
solution treatments. Although infrequently observed in the as-cast
samples, the Zn–Zr intermetallic particles were always determined in
the solution-treated samples [19–25]. However, only few investigations
were performed to reveal their structures, much less to their influence
on extrusion ability, dynamic recrystallization and precipitation. For
example, Nie et al. [20] found a rod-shaped Zn–Zr particles in the so-
lution-treated Mg–1Ca–1Zn–1Zr (wt%) alloy, using transmission elec-
tron microscopy (TEM). Their selected area electron diffraction (SAED)
pattern analysis indicate that this phase is Zn
2
Zr
3
(primitive tetragonal
crystal structure, a= 0.768 nm and c= 0.699 nm). In addition, three
orientation relationships (ORs) between Zn
2
Zr
3
and Mg matrix were
revealed as follows:
(
110) //(0001) and [001] //[1120]
,
Zn2Zr3 Mg Zn2Zr3 Mg
(1)
https://doi.org/10.1016/j.matchar.2018.08.059
Received 10 July 2018; Received in revised form 31 August 2018; Accepted 31 August 2018
⁎
Corresponding authors.
E-mail addresses: qiangyang@ciac.ac.cn (Q. Yang), jmeng@ciac.ac.cn (J. Meng).
Materials Characterization 145 (2018) 329–336
Available online 01 September 2018
1044-5803/ © 2018 Elsevier Inc. All rights reserved.
T

(
110) //(0001) and [001] //[2350]
,
Zn2Zr3 Mg Zn2Zr3 Mg
(2)
and (130) //(1101) and [001] //[8179]
.
Zn2Zr3 Mg Zn2Zr3 Mg (3)
Afterwards, Sha et al. [21] examined the rod-like Zn–Zr inter-
metallic phase in a solution-treated Mg–6Zn–0.6Zr–0.5Cu (wt%) alloy.
They stated that the rod-like phase is not Zn
2
Zr
3
, but has a stoichio-
metry of Zn
3
(Zr
1-x
Mg
x
)
2
. Both of them have a similar primitive tetra-
gonal crystal structure. Then, Liu et al. [22] examined the rod-like in-
termetallic phase in a cast Mg–4.58Zn–2.6Gd–0.18Zr (wt%) alloy after
solution treatment at 505 °C for 16 h. They pointed out that the rod-like
phase is Zn
2
Zr
3
. Yet, with respect to an alloy with similar composition
(Mg–5Zn–2Gd–0.4Zr, in wt%), Li et al. [19] did not observe Zn
2
Zr
3
phase in the sample solution-treated at 500 °C for 18 h. Using trans-
mission electron microscopy and atom probe tomography, they iden-
tified the rectangular-shaped and the triangular-shaped phases as Zn
2
Zr
(face-centered cubic structure, a= 0.7397 nm) and ZnZr (primitive
cubic structure, a= 0.3336 nm), respectively. Furthermore, they also
found that the Zn
2
Zr phase is coherent with Mg matrix, following an OR
as:
(
0002) / /(111) and [1120] //[112]
.
Mg Zn2Zr Mg Zn2Zr
(4)
Moreover, Fu et al. [23] reported that the globular Zn
2
Zr
3
phase and
the blocky ZrH
2
phase formed during solution treatment in the
Mg–3Nd–0.2Zn–0.4Zr (wt%) alloy.
According to the above mentioned analysis, several questions are
remained as: (i) How many types did the Zn–Zr intermetallic phases
form during solution treatment present? (ii) Which structure of each
typed Zn–Zr phase belongs to? (iii) How about the influence of the
Zn–Zr phases on the precipitation? Bearing these in mind, we system-
atically examined the morphologies and the crystal structures of the
intermetallic phases in the solution-treated Mg–5.0Sm–0.6Zn–0.5Zr (wt
%) alloy using TEM. The OR between various Zn–Zr phases and Mg
matrix and the influence of the Zn–Zr phases on precipitation were
revealed in this work.
2. Experimental Procedures
An alloy with the normal composition of Mg–5.0Sm–0.6Zn–0.5Zr
(wt%) was prepared using metal-mold casting by melting pure Mg and
Zn, Mg–30 wt% Sm and Mg–30 wt% Zr master alloys in an electric
resistance furnace under a mixed atmosphere (99 vol%CO
2
+ 1 vol%
SF
6
). Firstly, pure Mg and Zn were preheated to 200 °C in an electric
resistance furnace and then heated in the steel crucible. Until pure Mg
and Zn were fully melted and heated to approximately 755 °C, both
master alloys preheated to approximately 350 °C were added into the
melt. Subsequently, the melt was fully stirred for 8 ± 2 min, and then
was kept static for 30 ± 5 min. Finally, when the molten mixture was
cooled down to about 710 °C, it was poured into a mild steel (h13) mold
(pre-heated to 200° C) of 90 mm in diameter and 800 mm in length.
Cubic specimens with a side length of 10 mm were cut from the middle
segment of the cast ingots, and then were solution-treated at 520° C for
8 h followed by air cooling to obtain numerous Zn–Zr intermetallic
particles along with some heterogeneously nucleated fine precipitates.
Microstructural characterizations were performed using Olympus-
GX71 optical microscopy (OM), Hitachi S-4800 backscatter scanning
electron microscopy (BS-SEM), Bruker D8 FOCUS X-ray diffraction
(XRD) with CuKαradiation (λ= 0.15406 nm) operating at 40 kV and
40 mA with a scanning speed of 0.5°/min, and FEI Tecnai G
2
F20
transmission electron microscopy (TEM) equipped with EDAX energy
dispersive X-ray spectroscopy (EDS) operating at 200 kV. Specimens for
OM and SEM observations were prepared by mechanical polishing,
subsequently etched by a mixture of 5 g picric acid, 5 ml acetic acid,
100 ml ethanol, and 10 ml H
2
O. Thin foils with a diameter of 3 mm for
TEM observations were prepared using a Gatan 691 ion polishing ma-
chine.
3. Results and Discussion
Fig. 1 presents the backscatter SEM images of the as-cast (Fig. 1a
and b) and the solution-treated (Fig. 1c and d) Mg–5.0Sm–0.6Zn–0.5Zr
Fig. 1. Backscatter SEM images of (a, b) the as-cast and (c, d) the solution-treated Mg–5.0Sm–0.6Zn–0.5Zr (wt.%) alloy. (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of this article.)
K. Guan et al. Materials Characterization 145 (2018) 329–336
330

(wt%) alloy. Many large network-shaped intermetallic phases distribute
at grain boundaries (Fig. 1a). At the same time, no discernable fine
particles were observed in the αeMg grains (Fig. 1b). However, many
approximately circular patches consisted of fine intermetallic particles
appear near grain boundaries or in the grain center after solution
treatment (Fig. 1c). The corresponding magnified backscattered SEM
micrograph (Fig. 1d) of the region highlighted by a yellow box in
Fig. 1c presents that many fine particles with various morphologies
distribute unevenly in the αeMg grains or near the grain boundaries.
The similar solution-treated microstructures were frequently reported
elsewhere [22–25]. Liu et al. [22] identified the fine particles in a so-
lution-treated Mg–4.58Zn–2.6Gd–0.18Zr (wt%) alloy as Zn
2
Zr
3
phase.
In addition, two intermetallic phases were observed in the solution-
treated Mg–3Nd–0.2Zn–0.4Zr (wt%) alloy, namely the globular phase
and the blocky phase. They were confirmed to be Zn
2
Zr
3
and ZrH
2
,
respectively [23]. Also, Zr-containing fine particles were also observed
in the solution-treated Mg–6Gd–4Sm–0.4Zr (wt%) [24] and
Mg–3Zn–0.9Y–0.6Nd–0.6Zr (wt%) alloys [25]. However, their detailed
structural information is lacking. To examine the crystal structure of
fine intermetallic phases, XRD pattern of the solution-treated sample is
shown in Fig. 2. However, no clear diffraction peaks from intermetallic
phases except those from Mg matrix were detected. In other word, the
fine particles formed during solution treatment cannot be examined
using XRD analysis. This might be because of the too small volume
fraction of them and the measurement limitation of XRD [22].
In the following, we will examine the crystal structures of fine in-
termetallic particles in the solution-treated Mg–5.0Sm–0.6Zn–0.5Zr
alloy using TEM. Fig. 3a shows the representative bright-field TEM (BF-
TEM) micrograph of the rod-like phase stretched across a grain
boundary. Its length and diameter are 200–800 nm and 20–35 nm, re-
spectively. According to the high-resolution TEM (HR-TEM) image
(Fig. 3b) along with the fast Fourier transform (FFT) patterns from re-
gions indicated by A and B, the rod-like phase was determined to be
Zn
2
Zr
3
(primitive tetragonal crystal structure, a= 0.7633 nm and
c= 0.6965 nm). Additionally, it is coherent with the Mg matrix. The
corresponding OR was revealed as follow:
(
110) //(0001) and [111] //[1210]
.
Zn Zr Mg Zn Zr Mg
23 23
(5)
Rod-like Zn
2
Zr
3
phase with the same crystal structure has also been
reported in the solution-treated Mg–1Ca–1Zn–1Zr (wt%) alloy by Nie
et al. [20]. However, the OR between Zn
2
Zr
3
and Mg matrix revealed in
this work is quite different from those reported in Ref. [20]. In addition,
the above OR has not been reported in the previous open literature.
Fig. 4 illustrates the representative high-angle annular dark-field
scanning TEM (HAADF-STEM) image of the fine intermetallic phases in
grain interior. Four typed intermetallic phases with different
morphologies were observed, namely the large plate phase (denoted as
A), the nano-scale granuliform phase (denoted as B), the spindle-shaped
phase (denoted as C), and the rod-like phase. Detailed observations
reveal that the rod-like phase owns two different morphologies: one
with the surface being likely to be surrounded by jagged particles
(denoted as D), and the other one with the surface being very smooth
(denoted as E). Fig. 5a displays the HAADF-STEM micrograph of the
large plate phase (A in Fig. 4). Its uniform diameter is approximately
500 nm. Furthermore, some gibbous particles were observed on its
surface (Fig. 5b). According to the corresponding selected area elec-
tronic diffraction (SAED) pattern (Fig. 5c), there are two coexisted
phases: MgZn
2
(hexagonal structure, a= 0.5253 nm and
c= 0.8568 nm) [9] and Mg
3
Sm phase (face-centered cubic structure,
a= 0.7371 nm). Ordinarily, MgZn
2
phase was observed in the as-cast
Mg–Zn–Zr-based alloys and ordinarily dissolved into Mg matrix during
solution treatment [26–29]. However, the result in this work suggests
that the MgZn
2
phase could also form during solution treatment. The
corresponding formation mechanisms need much more elaborate ef-
forts to be revealed. Fig. 5d gives the HR-TEM image of the MgZn
2
/
Mg
3
Sm phase boundary. The corresponding FFT patterns from regions
labeled as A and B further confirm that the coarse plate phase is MgZn
2
and the gibbous phase on the coarse plate phase is Mg
3
Sm. In addition,
the Mg
3
Sm phase is coherent with the MgZn
2
phase. The corresponding
OR between them is as follow:
(
1120) //(110) and [0001] //[111]
.
MgZn Mg Sm MgZn Mg Sm
2323
(6)
This demonstrates that the MgZn
2
particles will act as hetero-
geneous nucleation sites for the Mg
3
Sm precipitates. Furthermore, we
can deduce that the MgZn
2
particles formed during solution treatment
will influence the precipitation of the Mg–RE-based alloys.
Fig. 6a–c show three nano-scale granuliform phases (B in Fig. 4)ina
same Mg matrix. It can be seen that the granuliform phase mainly
presents two morphologies. One owns quadrate shape, with the size of
10 nm, and the other one is lath-shaped, with the width and length of
5–15 nm and 30–50 nm, respectively. According to the corresponding
FFT patterns (Fig. 6d–f), both the quadrate-shaped phase and the lath-
shaped phase are Zn
2
Zr
3
. Moreover, the quadrate-shaped phase is with
the (110) plane (indicated by blue lines) parallel to (0001)
Mg
plane
(indicated by yellow lines). However, the (110) plane of the lath-shaped
phase is deviated from the (0001)
Mg
plane by approximately 20°.
Therefore, the lath-shaped Zn
2
Zr
3
phase follows a different OR with the
matrix from that for the quadrate-shaped Zn
2
Zr
3
phase. Fig. 6d de-
monstrates an OR between the quadrate-shaped Zn
2
Zr
3
and Mg matrix
as:
(
110) //(0001) and [001] //[1210]
.
Zn2Zr3 Mg Zn2Zr3 Mg
(7)
This OR is well in line with that reported in Ref. [20]. On the other
hand, the OR between the lath-shaped Zn
2
Zr
3
and Mg matrix was re-
vealed as:
°
(
100) about 20.0 from (0001) and [001] //[1210]
.
Zn2Zr3 Mg Zn2Zr3 Mg
(8)
This is quite different from the ORs reported in the previous open
literature. In addition, it should be noted that different ORs correspond
to different shapes. Under OR7 and OR8 cases, the Zn
2
Zr
3
phase mainly
presents quadrate-shape and lath-shape, respectively. Ordinarily, the
major side facets of the precipitates are closely related to the corre-
sponding mismatches of the matching planes [30]. Under OR7 case,
(110)
Zn2Zr3
//(0001)
Mg
and (1
1
0)
Zn2Zr3
//(10
1
0)
Mg
act as the major side
facets. The corresponding mismatches are 3.6% and 2.9%, respectively.
The approximate mismatches result in approximately equal length of
the major side facets. Under OR8 case, (100)
Zn2Zr3
//(10
1
5)
Mg
and
(010)
Zn2Zr3
//(10
1
1)
Mg
act as the major side facets. The corresponding
mismatches are 2.2% and 3.7%, respectively. In addition, the
(010)
Zn2Zr3
plane always deviates from the (10
1
1)
Mg
plane by ap-
proximately 8.1°. Thus, the mismatch of (010)
Zn2Zr3
//(10
1
1)
Mg
is in-
deed greater than 3.7%. The relative higher mismatch difference may
result in more obvious growth speed difference. Subsequently, the
Zn
2
Zr
3
precipitate presents lath-shaped. On both surfaces of the
Fig. 2. XRD patterns of the solution-treated sample.
K. Guan et al. Materials Characterization 145 (2018) 329–336
331

quadrate-shaped and lath-shaped Zn
2
Zr
3
particles, there is no discern-
able fine precipitate. Therefore, these two typed Zn
2
Zr
3
particles
probably have no influence on precipitation.
Fig. 7a presents the BF-TEM image of a spindle-shaped phase (C in
Fig. 4), with long axis of approximately 360 nm. Intermetallic particles
with the similar morphology were also observed in the solution-treated
Mg–5Zn–2Gd–0.4Zr (wt%) alloy, and they were identified as ZnZr
phase (primitive cubic structure, a= 0.3336 nm) [19]. However, the
spindle-shaped phase in the studied alloy was identified as Zn
2
Zr
3
ac-
cording to the corresponding SAED pattern (Fig. 7b). Furthermore,
amounts of SAED characterizations and the corresponding HR-TEM
image (Fig. 7c) indicate that there is no identical OR between the
spindle-shaped Zn
2
Zr
3
and Mg matrix. Moreover, no obvious Mg
3
Sm
precipitate was observed on its surface. Thus, it can be deduced that
spindle-shaped Zn
2
Zr
3
phase did not act as nucleation sites for the
Mg
3
Sm precipitates. In other words, the spindle-shaped Zn
2
Zr
3
phase
Fig. 3. (a) BF-TEM micrograph and (b) HR-TEM micrograph of the rod-like phase across a grain boundary, (c) and (d) FFT patterns from the regions indicated by A
and B, respectively, in figure (b).
Fig. 4. HAADF-STEM image of the fine particles in the grain interior.
K. Guan et al. Materials Characterization 145 (2018) 329–336
332

Fig. 5. (a) HAADF-STEM micrograph, (b) BF-TEM micrograph, (c) the corresponding SAED pattern and (d) the corresponding HR-TEM image of the large plate phase
with some gibbous phases on its surface, (e) and (f) FFT patterns from the regions indicated by A and B, respectively, in figure (d).
Fig. 6. (a–c) HR-TEM micrographs along with (d–f) the corresponding FFT patterns of the nano-scale granuliform phases with (a) quadrate shape and (b and c) lath
shape.
K. Guan et al. Materials Characterization 145 (2018) 329–336
333

has no influence on the precipitation.
Fig. 8a shows the HAADF-STEM image of the rod-like phase with
jagged particles (D in Fig. 4). This rod-like phase is with diameter of
50–100 nm and length of 0.2–2μm. The corresponding HR-TEM image
of the phase boundary and the FFT pattern were shown in Fig. 8b and c,
respectively. Furthermore, the FFT patterns from local regions in-
dicated by A–CinFig. 8b are shown in Fig. 8d–f, respectively. The
results suggest that the rod-like phase is Zn
2
Zr
3
and the jagged phase on
the Zn
2
Zr
3
surface is Mg
3
Sm. Interestingly, both of them are coherent
with Mg matrix. The corresponding ORs are as follows:
(
101) // (0001) and [111] //[1210]
,
Zn Zr Mg Zn Zr Mg
23 23
(9)
and (101) //(0001) and [111] //[1210]
.
Mg Sm Mg Mg Sm Mg
33 (10)
The former one was frequently observed in this work, but it has not
been reported in the previous open literature. Additionally, the OR
between Mg
3
Sm and Mg matrix is fully consistent with that reported in
Ref. [8]. Therefore, the Mg
3
Sm phase on the Zn
2
Zr
3
surface is fully
same as the Mg
3
Sm precipitate that reported the in artificial aged
Mg–Sm–Zn–Zr system. Moreover, the Mg
3
Sm phase is also coherent
with the Zn
2
Zr
3
phase. The corresponding OR is as follow:
(
101) //(101) and [111] //[111] .
Zn2Zr3 Mg3Sm Zn2Zr3 Mg3Sm
(11)
Therefore, the long rod-like Zn
2
Zr
3
particles can act as hetero-
geneous nucleation sites for the Mg
3
Sm precipitates. In other words, the
long rod-like Zn
2
Zr
3
phase will influence the alloy's precipitation.
As indicated by E in Fig. 4, there is also another rod-like phase with
no jagged phase on its surface. Fig. 9a demonstrates the BF-TEM mi-
crograph of this phase (highlighted by yellow arrows). Its diameter and
length are of 40–120 nm and 150–1200 nm, respectively. From the
corresponding SAED pattern (Fig. 9b), this rod-like phase was
Fig. 7. (a) BF-TEM image, (b) SAED pattern and (c) HR-TEM image along with the FFT pattern of the spindle-shaped phase.
Fig. 8. (a) HAADF-STEM micrograph, (b) the HR-TEM image along with (c) the corresponding FFT patterns of the rod-like phase surrounded by jagged phases, and
(d-f) FFT patterns from local regions indicated by A-C in figure (b), respectively.
K. Guan et al. Materials Characterization 145 (2018) 329–336
334

confirmed as Zn
2
Zr (face-centered cubic structure, a= 0.7397 nm
[19]). In some cases, the rod-like Zn
2
Zr phase is crossed with another
rod-like phase. However, the corresponding SAED pattern analysis
(Fig. 9c) indicates that the crossed rod-like phase is Zn
2
Zr
3
. It is also
surrounded by Mg
3
Sm phase (Fig. 9d). Thus, the crossed rod-like phase
is the same as the long rod-like phase. However, no certain OR was
observed between the E-typed rod-like Zn
2
Zr phase and the crossed rod-
like Zn
2
Zr
3
phase. Fig. 9e presents the corresponding HAADF-STEM
image along with the EDS mappings of Mg, Sm, Zn and Zr, for the re-
gion highlighted by a yellow dotted box. The results indicate that al-
though both of the two rod-like phases are mainly composed of Zn and
Zr, distinct Sm enrichment was observed on the surface of rod-like
Zn
2
Zr
3
phase while no on the surface of the rod-like Zn
2
Zr phase. This
indicates that the rod-like Zn
2
Zr particles did not act as nucleation sites
for the Mg
3
Sm precipitates. Thus, the rod-like Zn
2
Zr phase has no ob-
vious influence on the alloy's precipitation. It is well documented that if
two phases have similar crystal structure with closer lattice parameters,
the one previously formed could act as an effective nucleation site for
the one formed later [31]. Both Zn
2
Zr and Mg
3
Sm phases have a face-
centered cubic structure. In addition, there lattice parameters are also
similar (0.7397 nm and 0.7371 nm for Zn
2
Zr and Mg
3
Sm, respectively).
However, the Zn
2
Zr phase did not act as the nucleation site for the
Mg
3
Sm phase in this work. Furthermore, it should also be noted that the
Zn
2
Zr phase is not coherent with Mg matrix while the Mg
3
Sm phase has
to be coherent with Mg matrix during precipitation to decreased system
energy. Therefore, whether the intermetallic phase formed during so-
lution treatment is coherent with Mg matrix is also the key to the
precipitation of Mg
3
Sm on its surface.
4. Conclusions
Intermetallic phases in the solution-treated Mg–RE–Zn–Zr alloy
were systematically investigated using TEM. Four types were revealed,
namely the plate phase, the spindle-like phase, the nano-scale granu-
liform phase and the rod-like phase. The former twos were identified as
MgZn
2
and Zn
2
Zr
3
, respectively. With respect to the granuliform phase,
two types were found, namely the quadrate-shaped one and the lath-
shaped one. They follow different ORs with Mg matrix. In addition,
three types were observed for the rod-like phase: one across the grain
boundary, one covered by jagged intermetallic phase and one with no
covered phase. Crystal structure analysis demonstrates that the former
twos are Zn
2
Zr
3
and the later one is Zn
2
Zr. Finally, only the plate MgZn
2
phase and the rod-like Zn
2
Zr
3
phase in the grain interior act as het-
erogeneous nucleation sites for the Mg
3
Sm precipitate, thus influencing
alloy's precipitation.
Acknowledgements
This work is supported by the National Natural Science Foundation
of China under grants no. 21521092, 51701200, and the Project for
Science & Technology Development of Jilin Province under grants no.
201602011004GX, 20170414001GH, 20180520004JH and
20180520160JH.
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