ma con grafito damping
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Materials Science and Engineering A 527 (2010) 68166821
Contents lists available at ScienceDirect
Materials Science and Engineering A
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m s e a
Damping capacities and tensile properties of magnesium matrix compositesreinforced by graphite particles
Y.W. Wu , K. Wu, K.K. Deng, K.B. Nie, X.J. Wang, X.S. Hu, M.Y. Zheng
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e i n f o
Article history:
Received 6 March 2010
Received in revised form 22 May 2010Accepted 15 July 2010
Keywords:
Magnesium matrix composites
Graphite particles
Damping capacities
Tensile properties
Stir casting
a b s t r a c t
Magnesium matrix composites reinforced by graphite particles were fabricated using stir casting with
graphite particle size of 50m and graphite particle volume fractions of 5, 10, 15 and 20%, respectively.
Theas-cast composites were extruded at 300C with an extrusion ratio of 12:1. The experimental results
reveal that thegraphite particles play an important role on the tensile properties and damping capacities
of the composites. The strength increases with the addition of 5% graphite particles, but decreases with
further addition of graphite particles. The strain amplitude independent damping increases significantly
as the graphite particle volume fraction increases from 0 to 10%, but almost keeps constant when the
volumefractionexceeds 10%. Twodamping peaks arefoundat 150and 350C, respectively. The damping
peak around 150 C is considered to be caused by movable boundary slip, and the damping peak around
350 C is inferred to be recrystallization peak.
2010 Elsevier B.V. All rights reserved.
1. Introduction
Withdevelopmentof modern industryand transportation, noise
pollution caused by the vibration has become one of the seri-
ous environmental problems. And vibration decreases instrument
performance,such as stability, reliability andsecurity. So the damp-
ing capacity is a critically important material property from the
viewpoint of vibration suppression, noise control and instrument
performance enhancement [1]. Therefore, it is necessary to seek
for high damping capacity materials to eliminate or alleviate such
damage.
Among all commercial metallic materials, magnesium and its
alloys are the lightest structural metallic materials and have irre-
placeable properties compared with other metallic materials, such
as high specific strength and high specific elastic modulus [2,3].
It is well known that AZ91 magnesium alloy exhibits excellent
mechanical properties, but its dampingcapacities are relatively low
compared with pure magnesium [47]. In order to improve thedamping capacities of AZ91 magnesium alloy, magnesium matrix
composites are good candidates for realizing high damping, for
example, graphite particles (Grp) introduced into the magnesium
matrix are beneficial for damping capacities [8]. Graphite particles
are found to exhibit relatively high damping capacities when mea-
sured in its bulk form [9,10]. The addition of graphite particles of
Corresponding author at: 433# Harbin Institute of Technology, Harbin 150001,
PR China. Tel.: +86 451 86402291; fax: +86 451 86413922.
E-mail address: [email protected] (Y.W. Wu).
various micro-sizes to aluminum alloys has been investigated by
Rohatgi et al. [11], Zhang et al. [12], and Perez et al. [13]. Their
work has revealed that the micro-graphite particles may produce
a substantial increase in damping capacities.
Accordingly, the primary aim of this paper is to explore the
dampingcapacities of Grp/AZ91composites,fabricatedvia stircast-
ing. To this end, a dynamic mechanical analyzer (DMA) is used to
measure the damping capacities of the composites. The operative
dampingmechanismsin thecomposites are discussedin light of the
data obtained from damping measurements. It is expected that the
present study may give guidelines to improve damping capacities
and to understand correlated mechanisms. Moreover, the tensile
properties of the composites are investigated.
2. Experimental
A commercial magnesiumalloy AZ91was selected as the matrix,
and flake graphite particles with an average size of 50m were
employed as the reinforcement. The Grp/AZ91 composites were
fabricated by stir casting in a protective atmosphere of CO 2 and
SF6. The graphite particle volume fractions were 5, 10, 15 and 20%,
respectively. And then the as-cast composites were extruded at
300 C with an extrusion ratio of 12:1 after T4 treatment (415 C
for 24 h). For comparison, an unreinforced AZ91 alloy ingot was
also extruded under the same conditions.
The damping tests were carried out by DMA (Model TA Q800,
USA) with single cantilever vibration mode. The dimensions of the
damping test specimens were 35 mm8 mm1 mm. Measure-
ments were made at various strain amplitudes () from 5.3106
0921-5093/$ see front matter 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2010.07.050
http://dx.doi.org/10.1016/j.msea.2010.07.050http://dx.doi.org/10.1016/j.msea.2010.07.050http://www.sciencedirect.com/science/journal/09215093http://www.elsevier.com/locate/mseamailto:[email protected]://dx.doi.org/10.1016/j.msea.2010.07.050http://dx.doi.org/10.1016/j.msea.2010.07.050mailto:[email protected]://www.elsevier.com/locate/mseahttp://www.sciencedirect.com/science/journal/09215093http://dx.doi.org/10.1016/j.msea.2010.07.050 -
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Fig.1. Optical micrographs: (a)as-extruded AZ91;(b) as-extruded 5% Grp/AZ91;(c) as-extruded 10% Grp/AZ91;(d) as-extruded 15% Grp/AZ91;(e) as-extruded 20% Grp/AZ91.
to 1.3103, the vibration frequency (f) was 1Hz, and the test
temperature (T) was room temperature. For the measurements
of temperature dependent damping capacities, the test conditions
were as follows: the strain amplitude () was 4105, the vibra-tion frequencies (f) were 0.5, 1.0, 5.0 and 10.0 Hz, the temperature
range (T) was from room temperature to 400 C and the heating
rate (T) was 5
C/min.The tensile tests were carried out by Instron-1186 tension
machine at roomtemperature andthe tensile ratewas 0.5 mm/min.
The microstructures of as-extruded Grp/AZ91composites andAZ91
alloy were examined under OLYMPUS-PMG3 type optical micro-
scope (OM).
3. Results and discussion
3.1. Microstructures of as-extruded Grp/AZ91 composites
The optical micrographs of as-extruded AZ91 alloy and
Grp/AZ91 composites with different graphite particle volume frac-
tionareshowninFig.1(a)(e). Withthe increaseof graphiteparticle
volume fraction, the grain size of as-extruded composites increases
significantly. And the grain size of as-extruded AZ91 alloy is larger
than that of 15% Grp/AZ91 composite, but slightly smaller than
that of 20% Grp/AZ91 composite. In addition, graphite particles are
elongated along the extrusion direction.
The variation of grain size can be attributed to the effect of
graphite particles on promoting recrystallization nucleation andgrowth during hot working. At relatively low volume fraction, the
graphite particles can promote recrystallization nucleation signif-
icantly, but have no obvious effect on promoting recrystallization
grain growth, so thegrain can be greatly refined. However, with the
increase of graphite particle volume fraction, the effect of graphite
particles on promoting recrystallization grain growth is strength-
ened, thusthe grainrefinement is weakenedgradually. Considering
that little previous experimental work has been reported on such
result, further research is needed to clarify this origin.
Fig.2(a)(e) shows the optical micrographs of as-extruded AZ91
alloy and Grp/AZ91 composites with different graphite particle
volume fraction after temperature dependent damping tests. With
the increase of graphite particle volume fraction, the grain size
decreases significantly. This indicates graphite particles can effec-
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Fig. 2. Optical micrographs aftertemperature dependentdamping tests: (a) as-extruded AZ91; (b) as-extruded 5% Grp/AZ91;(c) as-extruded 10% Grp/AZ91; (d) as-extruded
15% Grp/AZ91; (e) as-extruded 20% Grp/AZ91.
tively hinder grain growth during temperature dependentdamping
tests. For a detailed comparison, the grain size of as-extruded AZ91
alloy and Grp/AZ91 composites is measured before and after
temperature dependent damping tests, and the result is shown in
Fig. 3. It can be seen from Fig. 3, the grain growth is weakened sig-
nificantly during temperature dependent damping tests with the
increase of graphite particle volume fraction. When the graphiteparticle volumefraction is 0 (i.e. AZ91 alloy), thegrain growth is the
strongest. However, when the graphite particle volume fraction
reaches 20%, the grain growth is the weakest, even negligible.
3.2. Tensile properties of as-extruded Grp/AZ91 composites
The tensile properties of as-extruded Grp/AZ91 composites and
AZ91alloyareshowninFig.4. Itcanbeclearlyseenthattheultimate
tensile strength (UTS) and yield strength (YS) increase with the
addition of 5% graphiteparticles, but decrease withfurtheraddition
of graphite particles. Moreover, with the increase of graphite par-
ticle volume fraction, the elastic modulus increases monotonically,
and the ductility decreases monotonically.
Fig. 3. Grain size of as-extruded AZ91 alloy and Grp/AZ91 composites before and
after temperature dependent damping tests.
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Fig. 4. Tensile propertiesas functionsof graphite particle volume fraction:(a) ultimate tensile strengthand yieldstrengthand (b) elastic modulus and elongationto fracture.
The variation of the YS can primarily be attributed to grain size.
For grain strengthening, according to the Hall-Petch equation [14]:
= 0 + KD1/2 (1)
whereis the yield stress of materials, D is average grain diameter,0 is the yield stress of single crystal materials, and K is constant.
The smaller the grain size, the higher the YS. Besides grain size,the UTS is also related to the presence of graphite particles in
matrix, which serve as crack nucleation sites. With the increase
of graphite particle volume fraction, the ductility will decrease due
to more crack nucleation sites, which will result in the decrease of
the UTS. Addition of graphite particles improves the elastic modu-
lus of magnesium matrix, which can be attributed to the relatively
high modulus of graphite compared with the magnesium matrix.
3.3. Damping capacities of as-extruded Grp/AZ91 composites
Strain amplitude dependence of damping capacities in as-
extrudedGrp/AZ91compositesand AZ91alloyare shown in Fig. 5. It
indicates that the strain amplitude dependence of damping capac-
ities exhibit two regions. The damping can be divided into twocomponents [15]:
Q1() = Q10 +Q1H () (2)
In the first region, for lower strains, the damping values are
independent or only weakly dependent on the maximum strain
amplitude. In the second region, for higher strains, the damp-
ing capacities increase rapidly with the increase of the maximum
strain amplitude. According to Fig.5, the variations of critical strain
Fig. 5. Strain dependent damping capacities of as-extruded AZ91 alloy and
Grp/AZ91 composites at room temperature with f=1Hz.
(cr) and strain amplitude independent component (Q10 ) with the
increase of graphite particle volume fraction are shown in Fig. 6.
From Fig. 6, it can be seen, the Q10 increases significantly as the
graphite particle volume fraction increases from 0 to 10%, but
almost keeps constant when the volume fraction exceeds 10%.
However, with the increase of volume fraction, the cr has no obvi-
ous change. In addition, as shown in Fig. 5, the strain amplitudedependent component (Q1H ) increases significantly at high strainswith the increase of volume fraction.
The Q1H is related to dislocations by the following equationderived from the GranatoLcke (GL) model [16,17]:
Q1H =C1
exp(C2
) (3)
C1 =FBL
3N
6bEL2C
(4)
C2 =FBbELC
(5)
where is the strain amplitude; C1 and C2 are material constants;
is the dislocation density; FB is the binding force between dislo-cations and weak pinning points; E is the elastic modulus; LC andLN are average dislocation distance between weak pinning points
and strong pinning points, respectively; b is the Burgers vector. Eq.
(3) can be alternated as follows:
ln(Q1H ) = lnC1 C2
(6)
It can be noted from Eq. (6) thatthe GLplots should be straight
lines, whose intercept and slope are the values of ln C1 and C2,
Fig. 6. Critical strain and strain amplitude independent damping as functions of
graphite particle volume fraction.
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Fig. 7. GL plots for as-extruded AZ91 alloy and Grp/AZ91 composites at room
temperature.
respectively. From Fig. 7, it is shown that the Grp/AZ91 compos-
ites satisfy the GL model in limited strain amplitude range. We
attribute to that, besides dislocation damping, other factors, such
as intrinsic damping of graphite particles, particles/matrix inter-
face damping or grain boundary damping, also contribute to the
damping capacities of Grp/AZ91 composites at room temperature.
Temperature dependence of damping capacities in as-extruded
Grp/AZ91 composites and AZ91 alloy are shown in Fig. 8. It can
be seen from Fig. 8 that the damping capacities of Grp/AZ91
composites and AZ91 alloy are intensively dependent on testing
temperature, and they rise with increasing temperature. It also
shows that the damping-temperature curves of Grp/AZ91 com-
posites have two obvious peaks which occur at about 150 and
350 C (P1 around 150C and P2 around 350
C). The weakening
of the damping peak P1 in AZ91 alloy indicates that the modified
microstructures of the composites would be responsible for the
strengthening of the damping peak P1, and these modifications
are mainly due to the introduction of particles/matrix interfaces.
Moreover, the damping peak P1 is discovered firstly at low vibra-
tion frequency. In contrast, the damping peak P2
is discovered at
the same testing temperature (as shown in Fig. 9). Therefore, the
damping peak P1 is a relaxation process, but the damping peak P2is not a relaxation process.
The relaxation process proceeds by atom diffusion, and the
relaxation time is content to the Arrhenius equation [18]:
1 = 0eH/kTor= 0e
H/kT (7)
where the 0 is frequency factor, the 0 is exponent factor, the kis constant and the H is activation energy. According to Eq. (7),
the relaxation time is a function of temperature. Therefore, the
damping-temperature peak can be gained by changing tempera-
ture relating to specific testing frequencythat satisfies the equation
= 1. Eq. (7) can be alternated as follows:
ln + ln 0 + H1000k
1000TP
= 0 (8)
where the TP is peak temperature, which is related to testing fre-
quency. The activation energy H can be calculated by the slope of
ln1000/TP. According to Fig. 9 andEq. (8), the relation betweenfrequency and peak temperature and their fit liner are described
in Fig. 10. The activation energy for the damping peak P1 of as-
extruded 10% Grp/AZ91 composite is 123kJ/mol between grain
Fig. 8. (a) Temperature dependent damping capacities of as-extruded AZ91 alloy and Grp/AZ91 composites with f=1Hz, = 4105 and T= 5 C/min and (b) amplification
at relatively low temperature.
Fig. 9. (a) Temperature dependent damping capacities of as-extruded 10% Grp/AZ91 composite with = 4105 , T= 5 C/min and testing frequencies (f) of 0.5, 1.0, 5.0 and
10.0 Hz and (b) amplification at relatively low temperature.
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Y.W. Wu et al. / Materials Science and Engineering A 527 (2010) 68166821 6821
Fig. 10. Arrhenius relation between testing frequency and peak temperature.
boundary diffusion energy (92kJ/mol) and lattice self-diffusion
energy (135 kJ/mol) of magnesium. Hence, the damping peak P1is considered to be caused by movable boundary slip controlled by
grain boundarydiffusionand lattice self-diffusion,such as interfaceslip and grain boundary slip.
With the increase of graphite particle volume fraction, the
damping peak P2 shifts to higher temperatures in as-extruded
Grp/AZ91 composites, and the height decreases. Moreover, the
damping peak P2 of as-extruded AZ91 alloy is higher than that
of as-extruded Grp/AZ91 composites, and the peak temperature
is slightly higher than that of as-extruded 15% Grp/AZ91 com-
posite. Combining Fig. 3 with Fig. 8, it can be estimated that the
peak temperature of damping peak P2 may be related to grain size
of as-extruded composites and alloy before temperature depen-
dent damping tests, and the peak height may be related to grain
growth during temperature dependent damping tests. Based on
above results, it canbe inferred that the damping peak P2 is recrys-
tallization peak.The peaktemperature shifts to lower temperaturesas the decrease of grain size, which is because that the nonequilib-
rium grain boundary of fine grain results in the decrease of driving
force that the recrystallization needs. The peak height decreases as
the increase of graphite particle volume fraction, which is because
that the graphite particles hinder grain growth during temperature
dependent damping tests, and thus decrease energy dissipation.
4. Conclusions
Magnesium matrix composites reinforced with graphite par-
ticles were successfully prepared using stir casting. The damping
capacities and tensile properties of as-extruded composites were
investigated. The following conclusions may be drawn from the
present study:
(1) Withthe increase of graphiteparticlevolume fraction, the grain
size of as-extruded composites increases significantly, which is
related to the effect of graphite particles on promoting recrys-
tallization nucleation and growth during hot working. Graphite
particles can effectively hinder grain growth during tempera-
ture dependent damping tests, the higher the graphite particle
volume fraction, the weaker the grain growth.
(2) The UTS and YS increase with the addition of 5% graphite
particles, but decrease with further addition of graphite par-
ticles. Moreover, with the increase of graphite particle volume
fraction, the elastic modulus increases monotonically, and the
ductilitydecreases monotonically. The variation of tensile prop-
erties is related to grain size and graphite particle volume
fraction, small grainsize leads to high YS, high graphite particle
volume fraction leads to high elastic modulus, butlow ductility
and UTS.
(3) The Q10 increases significantly as the graphite particle volumefraction increases from 0 to 10%, but almost keeps constant
when the volume fraction exceeds 10%. With the increase of
volume fraction, theQ1H increases significantly at high strains.The Grp/AZ91 composites satisfy the G-L model in limited
strain amplitude range, which indicates that, besides dislo-
cation damping, other factors, such as intrinsic damping of
graphite particles, particles/matrix interface damping or grain
boundary damping, also contribute to the damping capacities
of Grp/AZ91 composites at room temperature.
(4) Two damping peaks are found at 150 and 350 C, respectively.
The damping peak P1 is a relaxation process, and its activa-tion energy is 123kJ/mol between grain boundary diffusion
energy (92kJ/mol)and lattice self-diffusionenergy(135 kJ/mol)
of magnesium, which indicates the damping peak P1 is caused
by movable boundary slip. The damping peak P2 is not a relax-
ation process, and the peak temperature is related to grain size
before temperature dependent damping tests, and the peak
height is related to grain growth during temperature depen-
dent damping tests, so the damping peak P2 is inferred to be
recrystallization peak.
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