ma con grafito damping

8/8/2019 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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Y.W. Wu et al. / Materials Science and Engineering A 527 (2010) 68166821 6817

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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6818 Y.W. Wu et al. / Materials Science and Engineering A 527 (2010) 68166821

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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6820 Y.W. Wu et al. / Materials Science and Engineering A 527 (2010) 68166821

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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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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