H. CHEN et al.: EFFECT OF ZrO2 ADDITIONS ON FABRICATION OF ZrO2/Mg COMPOSITES VIA ...
193–197
EFFECT OF ZrO
2
ADDITIONS ON FABRICATION OF ZrO
2
/Mg
COMPOSITES VIA FRICTION-STIR PROCESSING
VPLIV DODATKA ZrO
2
NA IZDELAVO ZrO
2
/Mg KOMPOZITOV Z
VRTILNO-TORNIM POSTOPKOM
Hongmei Chen
1*
, Xiaowen Li
1
, Si’en Liao
1
, Jing Zhang
2
, Yunxue Jin
1
, Hongwei Cui
3
1
School of Materials Science and Engineering, Jiangsu University of Science and Technology, no. 2, Mengxi Road, Zhenjiang 212003, China
2
School of Metallurgy and Materials Engineering, Jiangsu University of Science and Technology, Changxing Road, Zhangjiagang 215600,
China
3
School of Materials Science and Engineering, Shandong University of Technology, no. 266, West Road of Xincun, Zibo 255000, China
Prejem rokopisa – received: 2018-01-09; sprejem za objavo – accepted for publication: 2018-11-05
doi:10.17222/mit.2018.004
ZrO2/Mg composites were prepared with friction-stir processing (FSP) using different ZrO2 amounts. The effects of FSP on the
microstructure, mechanical properties and damping capacities of ZrO2/Mg composites are discussed in this paper. After
friction-stir processing, the central region of the stir zone of a ZrO2/Mg composite obtained fine, uniform, equiaxed crystal
grains. An addition of ZrO2 can obviously refine the grains and improve the microhardness and mechanical properties of
ZrO2/Mg composites. The ZrO2/Mg composites prepared with friction-stir processing have better damping capacities. During a
strain-damping test, the damping behavior of ZrO2/Mg composites follows the Granato-Lücke dislocation damping theory (G-L
theory). The ZrO2/Mg composites with a groove width of 0.8 mm had the best damping capacities.
Keywords: ZrO
2
/Mg composites, friction-stir processing, damping capacity, G-L theory
Avtorji so pripravili ZrO2/Mg kompozite z vrtilno tornim postopkom (FSP, angl.: friction stir processing) z razli~no vsebnostjo
ZrO2. V ~lanku opisujejo vpliv FSP postopka na mikrostrukturo, mehanske lastnosti in sposobnost za du{enje izdelanih
ZrO2/Mg kompozitov. Po izvedenem FSP postopku je v centralnem delu ZrO2/Mg kompozitov (v coni me{anja oz. vrtenja)
nastalo homogeno podro~je drobnih enakoosnih kristalnih zrn. Dodatek ZrO2 je o~itno udrobnil mikrostrukturo kompozita,
zvi{al mikrotrdoto in izbolj{al ostale mehanske lastnosti ZrO2/Mg kompozitov. Izdelani FSP ZrO2/Mg kompoziti imajo tudi
bolj{e sposobnost du{enja. Deformacijsko-du{ilni preizkus je pokazal, da se karakteristika du{enja izdelanih ZrO2/Mg
kompozitov dobro ujema z Granato–Lücke dislokacijsko teorijo du{enja (G-L teorija). Kompoziti ZrO2/Mg s {irino brazde 0,8
mm so imeli najbolj{o sposobnost du{enja.
Klju~ne besede: ZrO2/Mg kompoziti, vrtilno-torni proces, sposobnost za mehansko du{enje, G-L teorija
1 INTRODUCTION
Magnesium and magnesium alloys are widely used
because of the advantages of their low density, high
specific strength, high damping capacity, noise reduction
a n ds oo n .
1–6
Pure magnesium has the best damping
capacity, but poor mechanical properties, such as low
hardness, low yield strength and tensile strength, which
limit the range of its application.
7–8
Several efforts to
improve its mechanical properties through various
severe-plastic-deformation (SPD) methods have been
made since 1990s.
9
On the basis of experimental
evidences, it is clearly believed that SPD could lead to a
significant grain refinement as well as a profound
influence on the precipitation processes.
10–11
Friction-stir
processing (FSP) is a solid-state SPD technique, invented
by The Welding Institute (TWI) in 1991.
12
The effects of
FSP on the microstructural evolution and mechanical-
property modification of various materials, in particular,
magnesium alloys have been thoroughly investigated.
13–15
The change of the grain size and grain orientation
during friction-stir processing is an important factor
affecting the damping performance of magnesium alloys.
Therefore, friction-stir processing provides a great possi-
bility to resolve the contradiction between the mechani-
cal properties and damping performance of magnesium
alloys.
16–20
ZrO
2
particles have a high melting point and their
doping to an alloy can inhibit the grain growth. When the
alloy is plastically deformed, ZrO
2
particles are uniform-
ly distributed along the grain boundaries, impeding the
movement of dislocations and bending the dislocations
deformed around them, promoting the formation of a
concentrated deformation zone. So ZrO
2
particles can be
used as a reinforcing phase to improve the hardness and
elongation of a material. Liu et al.
21
added M-ZrO
2
and
T-ZrO
2
particles into the AZ31 magnesium alloy to
obtain ZrO
2
/Mg composites with friction-stir processing,
and the microstructure and mechanical properties of the
composite was studied.
In this paper, ZrO
2
/Mg composites were prepared
with friction-stir processing. The effects of ZrO
2
amounts on the microstructure, mechanical properties
Materiali in tehnologije / Materials and technology 53 (2019) 2, 193–197 193
UDK 549.514:620.168:621.791 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(2)193(2019)
*Corresponding author e-mail:
hmchen@just.edu.cn

and damping properties of ZrO
2
/Mg composites were
investigated.
2 MATERIALS AND METHODS
The material used in this study was the AZ31 mag-
nesium alloy in the hot-rolled state with a composition of
(in w/%): Al–2.54, Zn–0.943, Mn–0.332, Si–0.0165 and
the Mg balance. The ZrO
2
-reinforced particles exhibit
monoclinic crystals with a particle size of 20–50 nm.
The base material was cut into a specimen of (300 ×
160 × 4) mm. A groove with a depth of 2.5 mm and
widths of (0.6, 0.8, and 1.0) mm was machined along its
centerline. The shoulder diameter, pin diameter and pin
height were (16.0, 4.0 and 3.8) mm, respectively. The
FSP tool was rotated at 1500 min
–1
in the clock-wise
direction. The traverse speed was 50 mm/min. The tilted
angle was 2.5°.
For an optical examination, samples were sectioned,
cold mounted, polished with the silicon dioxide paste
with a grain size of 1 μm and finally etched in a solution
of oxalic acid (2 g), nitric acid (2 mL) and distilled water
(98 mL), and then examined using a ZEISS optical
microscope. The microhardness of the ZrO
2
/Mg com-
posites was measured with a Vickers hardness tester with
a load of 100 gf for 10 s. Tensile testing was performed
on a SANS CMT5205 material testing machine at a
stretching rate of 2 mm/min at ambient conditions.
Damping samples were machined to dimensions of (35 ×
10 × 1) mm. The damping capacity was measured with a
dynamic mechanical analyzer (NETZSCH DMA-242C)
in the single cantilever deformation mode. For the
measurements of the strain amplitude dependence of
damping capacity, the range of the strain amplitude was
from 1 μm to 200 μm, and the measurement frequency
was 1 Hz.
3 RESULTS AND DISCUSSIONS
3.1 Microstructures
Figure 1 presents the microstructure of the central
region of the stir zone. Figure 1a shows the AZ31
magnesium alloy and Figure 1b shows the ZrO
2
/Mg
composite. The microstructure of the central region of
the stir zone was subjected to vigorous mechanical
stirring during the friction-stir process, which caused a
severe plastic deformation. Coarse grains were broken,
resulting in a noticeable refinement and fine, uniformly
equiaxed crystal grains. The grain size of the stir zone of
the AZ31 magnesium alloy was 16.91 μm and that of the
ZrO
2
/Mg composites was just 10.26 μm. It can be seen
that an addition of ZrO
2
can effectively reduce the grain
size. This is because after an addition of ZrO
2
reinforced
phase particles, the alloy grain boundaries are pinned,
thus greatly limiting the grain-boundary migration and
playing a significant role in the refinement of grains.
3.2 Mechanical properties
Figure 2 shows the microhardness distribution along
the cross-sections of a ZrO
2
/Mg composite. Figure 2a
presents the microhardness test position on the cross-
section of the ZrO
2
/Mg composite. The distance between
the vertical and horizontal lines of a mesh unit is
H. CHEN et al.: EFFECT OF ZrO2 ADDITIONS ON FABRICATION OF ZrO2/Mg COMPOSITES VIA ...
194 Materiali in tehnologije / Materials and technology 53 (2019) 2, 193–197
Figure 2: Microhardness distribution in the friction-stir zone of a
ZrO
2
/Mg composite a) microhardness test position (distance between
the vertical and horizontal lines: 0.5 mm) and b) microhardness cloud
map (unit of X- and Y-axes: mm)
Figure 1: Metallographic microstructural comparison of the central
regions of the stir zones: a) FSPed AZ31, b) FSPed ZrO
2
/Mg com-
posites

0.5 mm. The intersection points are the test points for the
microhardness measurement, forming a matrix of 31 ×
15 points, which was plotted as the microhardness distri-
bution in the Matlab software, as shown in Figure 2b.
The microhardness of the ZrO
2
/Mg composites was
related to the distribution of the ZrO
2
reinforced phase in
the magnesium-alloy matrix. As can be seen from Fig-
ure 2b, the microhardness was in a range of 75–79 HV
and the fluctuation was small. However, the microhard-
ness values in the boundary region of the friction zone
were relatively high, more than 80 HV, indicating a large
fluctuation. It can be seen that after the friction-stir pro-
cessing, the organization in the central region was very
uniform, while there was still a certain degree of agglo-
meration in the boundary region. This is because during
the friction-stir processing, the stirring force could not be
involved in the boundary region, which was not the
effective area of the composite material. Nevertheless, it
showed that the FSPed ZrO
2
/Mg composite material was
uniform and dense.
The addition of ZrO
2
enhanced the tensile strength
and yield strength of the ZrO
2
/Mg composites. Figure 3
shows the yield strength (YS), ultimate tensile strength
(UTS) and elongation (EL) of the FSPed AZ31 magne-
sium alloy and ZrO
2
/Mg composites with groove widths
of (0.6, 0.8 and 1) mm, respectively. It can be seen that
the addition of ZrO
2
can effectively improve the tensile
properties, exhibiting a very good enhancement effect.
With an increase in the ZrO
2
amount, the tensile
properties of the ZrO
2
/Mg composites first increased
inconspicuously and then decreased. This was because
the distribution of the ZrO
2
-rich phase was not uniform
and there was a large degree of agglomeration after the
friction-stir processing where the distribution of
agglomerates in the AZ31 magnesium alloy matrix was
also not homogeneous. When the ZrO
2
/Mg composites
were subjected to tensile loads, cracks were first gene-
rated around these agglomerates and they prematurely
cracked. This indicated that an excessive addition of
ZrO
2
cannot improve the tensile properties of ZrO
2
/Mg
composites because of the agglomerates.
3.3 Damping capacity
Figure 4 shows the damping-capacity dependence on
the strain amplitude of the ZrO
2
/Mg composites with
groove widths of (0.6, 0.8 and 1) mm, showing a typical
dislocation strain/damping spectrum. The damping
values Q
0
–1
( =10
–4
) of the ZrO
2
/Mg composites with
different ZrO
2
amounts at a low strain amplitude were
0.0111, 0.0117 and 0.0101, respectively. When the
damping capacity is higher than 0.01, it is generally
considered that the alloy exhibits a high damping
capacity.
22
Therefore, the ZrO
2
/Mg composites prepared
with the friction-stir method showed a good damping
performance.
According to the G-L theory:
23,24
QQQ
−−−
=+
1
0
11
() () 
H
(1)
Q
BL f
Gb
0
1
4
2
36
−
=
 C
(2)
Q
C
C
H
−
−
=
1
1
2
exp
(/ )   (3)
C
FL
bEL
1
3
2
6
=
 BN
C
(4)
C
F
bEL
2
=
B
C
(5)
where C
1
and C
2
are the material constants;  is the
dislocation density; F
B
is the binding force between
dislocation and weak pinning points; L
N
and L
C
are the
average dislocation distances between strong pinning
points and weak pinning points, respectively; b is the
Burgers vector; E is the elastic modulus.
Taking the logarithmic transformation of Equation
(2), we get Equation (6):
H. CHEN et al.: EFFECT OF ZrO2 ADDITIONS ON FABRICATION OF ZrO2/Mg COMPOSITES VIA ...
Materiali in tehnologije / Materials and technology 53 (2019) 2, 193–197 195
Figure 3: Yield strength (YS), ultimate tensile strength (UTS) and
elongation (EL) of ZrO
2
/Mg composites with different ZrO
2
amounts
Figure 4: Damping-capacity dependence on the strain amplitude of
ZrO
2
/Mg composites with different ZrO
2
amounts

ln( ) ln QC
C
H
−
=−
1
1
2
  (6)
The damping-capacity dependence on the strain
amplitude of the stir zone at different ZrO
2
amounts is in
good accordance with the G-L model, as shown in Fig-
ure 5. It can be seen that the G-L model curves for the
ZrO
2
/Mg composites with different ZrO
2
amounts follow
a linear relationship. Therefore, the damping behavior of
the ZrO
2
/Mg composites with different ZrO
2
amounts
under a high-strain condition was consistent with the
G-L theory. For further analyses, the values of C
1
and C
2
calculated in accordance with the G-L plots
25
are shown
in Table 1.
Table 1: Values of C
1
and C
2
according to G-L plots
Contents 0.6 mm 0.8 mm 1 mm
C 1
×10
–5
10.820 6.403 6.705
C 2
×10
–3
1.66 1.28 1.42
(C 1/C 2)
1/3
3.399 3.394 3.216
From Equations (4) and (5), the (C
1
/C
2
)
1/3
value is
proportional to L
N
. With an increase in the ZrO
2
amount,
the (C
1
/C
2
)
1/3
value of the ZrO
2
/Mg composites became
smaller, showing that the distance from the strong pinn-
ing point L
N
gradually became smaller, but the change
was not obvious. This was because with the increase in
the ZrO
2
amount, the density of the dislocations or other
defects (such as grain boundary, phase boundary, etc.) in
the ZrO
2
/Mg composites and the concentration density of
impurity atoms on the dislocation line were increased,
reducing the average dislocation distance between strong
pinning points (L
N
). The increase of the distance of L
N
was beneficial for the improvement of the high-strain
damping capacity. It showed that the damping capacities
of ZrO
2
/Mg composites at the high-strain amplitude
decreased with the increase of ZrO
2
.
Combined with the damping values at the low-strain
amplitude, the ZrO
2
/Mg composites with groove widths
of 0.8 mm had the best damping capacities.
4 CONCLUSIONS
(1) After friction-stir processing, fine uniform
equiaxed crystal grains were obtained in the central
region of the stir zone of ZrO
2
/Mg composites. An
addition of ZrO
2
refined the grains and improved the
microhardness and mechanical properties of ZrO
2
/Mg
composite samples.
(2) The ZrO
2
/Mg composite samples prepared with
friction-stir processing had better damping capacities. In
the strain-damping test, the damping behavior of
ZrO
2
/Mg composites followed the G-L theory. The
ZrO
2
/Mg composites with groove widths of 0.8 mm had
the best damping capacities.
Acknowledgment
This work was supported by the National Natural
Science Foundation (Project no. 51301077), Natural
Science Foundation of the Jiangsu Province (Project no.
BK20130470), China Postdoctoral Science Foundation
(2017M611748) and Priority Academic Program Deve-
lopment of Jiangsu Higher Education Institutions.
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196 Materiali in tehnologije / Materials and technology 53 (2019) 2, 193–197
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Materiali in tehnologije / Materials and technology 53 (2019) 2, 193–197 197