Hindawi Publishing Corporation
Advances in Materials Science and Engineering
Volume 2009, Article ID 628737, 8 pages
doi:10.1155/2009/628737

Research Article
Tribological Analysis of Mg2Si Particulates
Reinforced Powder Metallurgy Magnesium Alloy
Composites under Oil Lubrication Condition
Katsuyoshi Kondoh, Junko Umeda, and Kenshi Kawabata
Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaragi, Osaka 567-0047, Japan
Correspondence should be addressed to Katsuyoshi Kondoh, kondoh@jwri.osaka-u.ac.jp
Received 29 December 2008; Revised 3 April 2009; Accepted 1 May 2009
Recommended by Ming-Xing Zhang
For the evaluation of wear behavior of Mg composites under oil lubrication conditions, powder metallurgy Mg97 Y2 Zn1 alloy
reinforced with additive Mg2 Si particles were fabricated by the repeated plastic working (RPW) and hot extrusion. The RPW
process was eﬀective in refining both Mg2 Si reinforcements and α-Mg grains causing the matrix hardening. When increasing therepetition number of RPW process from 200 to 600 cycles, the particle size of Mg2 Si additives changed from 8 μm to 1∼2 μm, and αMg grain size was 1 μm or less. With regard to the defensive and oﬀensive properties of Mg alloys reinforced with Mg2 Si dispersoids,
the composite had superior adhesive wear resistance compared with the conventional Mg alloys because of its extremely highmicrohardness of 95∼180 Hv by RPW process. The uniform distribution of refined Mg2 Si particles was useful for improving bothdefensive and oﬀensive properties against AZ31B counter disk specimens. The Mg2 Si prominent dispersoids in the matrix werealso eﬀective in forming the oil grooves around them, and caused the low and stable friction coeﬃcient. On the other hand, in thecase of the composite containing coarse Mg2 Si particles, severely deep scratches were given on the counter face of the AZ31B disk,
and resulted in an unstable and high friction coeﬃcient.
Copyright © 2009 Katsuyoshi Kondoh et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.

1. Introduction
Magnesium alloys are applied to engineering components
in the automotive industries due to their low density, lessthan 2 g/cm3 , and eﬀective for weight reduction, assisting
the improvement of the fuel eﬃciency [1]. In addition,
they also possess a good damping capacity and superior
machinability to the other light metals such as aluminumand titanium alloys. Furthermore, Mg matrix composites
show outstanding thermomechanical properties becauseof the hard dispersoids. On the other hand, magnesium
silicide (Mg2 Si) intermetallic compounds is one of the useful
reinforcements of Mg alloys because of its low density of
1.91 g/cm3 , high melting point of 1358 K, 120G Pa Young’s
modulus, high micro-Vicker’s hardness of 600–700 Hv, andlow coeﬃcient of thermal expansion of 7.5 × 10−6 K−1 [2, 3].
Mg composites with in situ synthesized Mg2 Si dispersoids
were studied by a casting method and solid state reaction
process [4–6]. In the casting process, the needle like Mg2 Si

compounds distributed in the matrix caused the reductionof mechanical properties. On the other hand, fine Mg2 Si
particles via a solid state reaction between added siliconparticles and the Mg matrix were eﬀective for improving
the strength of the composites. By employing SiO2 particles
as additives, the microstructural and mechanical responsesof Mg alloy composites reinforced with Mg2 Si/MgO hybrid
dispersoids were studied [7]. The reduction of SiO2 particles
by Mg was completely conducted by controlling the reaction
temperature. It was set as an exothermic temperature of
the DTA profile, when using the elemental mixture of MgSiO2 powder. The Mg matrix composites revealed highmechanical properties and good wear resistance due toa good coherence of in situ synthesized Mg2 Si and thematrix [8]. In the previous study, Mg alloy composites
reinforced with some additives such as nano-sized alumina,
SiC, and feldspar particles were used to evaluate their
tribological properties and wear resistance under dry sliding
conditions [9–11]. This was because the weight reduction of

2
Mg alloy composites was eﬀective in reducing the friction
loss and saving the consumed energy in operating. The
conventional AZ91 alloys and pure Mg were employed as
the composite materials, and then the adhesion phenomena
mainly occurred on the sliding surface due to the poor heat
resistance of the matrix materials. In particular, with regard
to AZ91 alloy, the sliding wear map under dry conditions
was investigated by changing the applied load and sliding
speed [12]. It was concluded that high strength and hardnessof the matrix were required to control the adhesive wear
phenomena in addition to the intermetallic compounds ofthe alloy. Yttrium and Zinc additives to Mg alloys were also
useful for improving the mechanical properties, in particularhigh temperature strength due to their long-periodic stacking ordered (LPSO) structures [13, 14]. LPSO phases wereformed in not only rapid solidification but also conventional
ingot metallurgy process, and then Mg-Y-Zn cast ingots also
showed much higher tensile strengths than the conventional
AZ series alloys. In this study, the tribological property of
powder metallurgy (P/M) Mg97 Y2 Zn1 (at%) alloy reinforced
with additive Mg2 Si particles was investigated under oil
lubrication conditions. In particular, the microstructures of
the composites were refined by repeating the severe plastic
working on the elemental mixture of Mg alloy and Mg2 Si
powders. The eﬀects of the matrix hardness, Mg2 Si particle
size, and content of the composite on their wear behavior
were experimentally evaluated.

2. Experimental
2.1. Preparation of Mg2 Si Particles. Pure Mg powders, having
a mean particle size of 112 μm and purity of 99.9%, wereused as starting raw materials. Pure silicon powders with
21 μm diameter and 99.9% purity were prepared as anotherraw materials. The elemental mixture of Mg-33.33 at%
Si powders was consolidated by a pressure of 15 MPa in
vacuum (<4 MPa) by using the spark plasma sintering (SPS)
equipment (SPS SYNTEX Inc., SPS-1030). The sintering
conditions at 893 K for 600 seconds were suﬃcient for thesolid state reaction between Mg and Si powders to synthesize
Mg2 Si intermetallics [15]. XRD analysis on the sinteredmaterial indicated the only Mg2 Si peaks, and no peak of
Mg and Si of raw powders was detected in the XRD profile.
Fine Mg2 Si powder with a mean particle size of 8.2 μm was
obtained by ball milling (SEIWA GIKEN Co., RM-30) for 7.2
kiloseconds in air.
2.2. Powder Metallurgy Mg Alloy Composite with Mg2 Si via
RPW Process. Machined chips of Mg97 Y2 Zn1 (at%) castingot, containing 30 ppm Fe and 17 ppm Cu as impurities,
were used as input materials. The elemental mixture of theabove Mg-Y-Zn alloy powders and Mg2 Si additives (Mg2 Si
content; 0, 5 and 10 wt%) was prepared by ball-milling
process for 7.2 kiloseconds. The repeated plastic working(RPW) process, schematically illustrated in Figure 1 [16],was applied to the elemental mixture to refine Mg2 Si particles
and uniformly distribute them over the matrix of the green
compact. In RPW process, the alternate operation between

Advances in Materials Science and Engineeringcold compaction and backward extrusion (an extrusion
ratio; 5.2) was carried out on the powder mixture filled in
the die, which was installed in a 1000 kN screw-driven pressmachine. The fragmentation of the additive Mg2 Si particles
occurred by the above severe plastic deformation during the
RPW process. At the same time, the refined particles wereembedded in the Mg alloy matrix. A lot of strains, whichwere eﬀective for the dynamic recrystallization during hot
extrusion, were also induced into the matrix of Mg-Y-Zn
alloy powder.
The maximum repetitive number was 600 cycles inthis study during RPW process. After RPW process witha suitable repetitive number, the columnar green compact
was fabricated, and served to hot extrusion process. The
green compact via RCP process was heated at 623 K for
300 seconds in nitrogen gas atmosphere, and immediately
consolidated by hot extrusion. The extrusion ratio was
30, and temperature of the extrusion container was 673 K.
The microstructure coarsening with large grains over 10 μm
occurred via dynamic recrystallization by heating the green
compact at 673 K. Therefore, the heating temperature of thepowder compacts was 623 K in this study. Micro-Vicker’s
harness measurement, optical microstructure, and scanningelectron microscope (SEM, JOEL JSM-6500F) observation
were carried out on the extruded Mg-Y-Zn alloy composites
reinforced with Mg2 Si particles. α-Mg grain and Mg2 Si
particle size distribution of the Mg alloy composite wasmeasured by the image analysis software (Image Pro-plus)
on optical microstructure photos.
2.3. Tribological Evaluation by Pin-on-Disk Type Wear Test.
Pin-on-disk type wear test equipment (Rhesca Co. Ltd., FPR2100) was used to evaluate the tribological properties of theabove Mg alloy composites under oil lubrication conditions.
Pin specimens, having 5 mm diameter and 10 mm length,were machined from the extruded Mg alloy composites.
The conventional AZ31B and ADC12 (die-cast Al-12 wt% Si)
alloys were used as the reference materials, and also
machined to the pin specimen. AZ31B disks were employed
as the counterpart materials. The surface roughness of thecounter face after polishing was measured by the surface
texture measuring instrument (Surfcom 1400D, TOKYOSEIMITSU CO). The Rmax value was 0.81∼0.93 μm. Wear
tests took place in 10W30 motor oil controlled at 308∼311 K.
The applied load on the pin specimen, sliding speed andtest time were 100 N, 1 m/s and 10 kiloseconds, respectively.
The changes in friction coeﬃcient during wear test were
evaluated. The volumetric wear loss of the pin specimens wasinvestigated by measuring the change of the pin length. The
sliding surfaces of the pin and disk specimens were observedby optical microscope to investigate wear behavior of each
Mg alloy composite.

3. Results and Discussion
3.1. Microstructural and Mechanical Responses of Mg
Alloy Composites via RPW Process. Figure 2 shows optical microstructures of hot extruded Mg97 Y2 Zn1 alloy

Advances in Materials Science and Engineering

3

Punch I Compaction Punch II
Green compact
via repeated plastic
working

Die

Drawback

Mixturepowder
Backward extrusion

Green compact
after drawback

Figure 1: Schematic illustration of repeated plastic working (RPW) process on elemental mixture of Mg-Y-Zn alloy powders and Mg2 Si
additives to fabricate columnar green compact.

Mg2 Si
10 μm(a) N = 200 (5 wt%)

10 μm(b) N = 400 (5 wt%)

10 μm(c) N = 600 (5 wt%)

Mg2 Si
10 μm(d) N = 200 (10 wt%)

10 μm

10 μm

Extrusion direction(e) N = 400 (10 wt%)(f) N = 600 (10 wt%)

Figure 2: Microstructure changes of powder metallurgy extruded Mg-Y-Zn alloy composites reinforced with additive Mg2 Si particles via
RPW process with N = 200, 400, and 600 cycles. Mg2 Si content of 5 wt%; (a)∼(c) and 10 wt%; (d)∼(f).

composites with Mg2 Si particles via the RPW process with
N = 200 ∼ 600 cycles.
In both Mg2 Si contents of 5 wt% and 10 wt%, the refinement and uniform distribution of the additives obviously
occurs with increase in the repetition number of the RPW
process. For example, the initial mean particle size of Mg2 Si
additives is 8.2 μm, and drastically decreases to less than 2 μm
after the RPW process with N = 600 cycles as shown in
Figures 2(c) and 2(f). Macroscopic photos indicate that alarge plastic deformation of the matrix is observed, and someprimary particle boundaries also exist in the case of 200
cycles (seeing Figures 2(a) and 2(d)). Each material reveals alayer microstructure in the matrix, which is typically causedby hot extrusion. The distance between layers gradually
decreases with increasing the repetition number. Figure 3
shows optical microscope and SEM observation photos on
hot extruded Mg97 Y2 Zn1 alloy composites with additive
Mg2 Si particles via the RPW process with 400 cycles. Theircontent is (a) 5 wt% and (b) 10 wt%, respectively. The
grain size of both composites is 0.7∼1 μm or less via
dynamic recrystallization during hot extrusion. The optical
microstructures obviously reveal Mg2 Si spherical particles
with a diameter of 1∼3 μm. In the SEM photos, the whiteparticles correspond to refined Mg2 Si additives. Very fineparticles with 300 nm or less, which are impossible to be
detected by optical microscope observation, are uniformly

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Advances in Materials Science and Engineering

Mg2 SiMg2 Si

10 μm000104 5.0 kV ×20.0 K

1.50 μm(a) Mg2 Si content; 5 wt%

Mg2 SiMg2 Si
10 μm000009 5.0 kV ×20.0 K

1.50 μm

Extrusion direction(b) Mg2 Si content; 10 wt%

Figure 3: Optical microscope and scanning electron microscope observation on hot extruded Mg-Y-Zn alloy composites with Mg2 Si
particles of (a) 5 wt% and (b) 10 wt%. Repetition number in RPW process, N = 400 cycles.

3.2. Wear Phenomenon under Wet Sliding. Figure 5 shows
the comparison of the total volumetric wear loss of thepin specimens made of Mg97 Y2 Zn1 alloy composite withvarious contents of Mg2 Si additives. The loss was calculated
by measuring the changed height of the pin specimen after
the wet sliding test. When increasing the additive content

220200
Micro-hardness, Hv (0.25 N)

distributed inside α-Mg grains of the matrix. Figure 4
shows the hardening dependence of hot extruded Mg alloy
composites on the repetition number in the RPW process.
In each material, the hardness is proportion to the repetitive
number because α-Mg grain refinement by dynamic recrystallization occurs with increase in the repetition number. The
composite including 10 wt% Mg2 Si is harder than that withno additive when applying RPW process with 600 cycles.
In this study, the hardness was measured on the matrix ofthe composite, except for the Mg2 Si particles which could
be detected by optical microscope observation. The reason
for this increase of the hardness is considered as below. Veryfine Mg2 Si dispersoids with 100∼300 nm in diameter, shown
in SEM photos of Figure 3, eﬀectively cause the increaseof microhardness of the matrix. On the other hand, manystrains are induced into α-Mg grains around Mg2 Si hardfine particles prior to the others far from the particles by
the repetition of severe plastic deformation during RPW
process. They accelerate the formation of fine grains via
dynamic recrystallization during hot extrusion. Therefore,
the hardness of the Mg alloy via RCP process increases withan increase in the Mg2 Si content of the composite as shownin Figure 4.

180
160
140
120
100
80

Raw powder(without Mg2 Si)

60
40

0

200
400
600
Repetition number in RPW, N(n=30)
800

0% Mg2 Si
5% Mg2 Si
10% Mg2 Si

Figure 4: Hardness dependence of extruded Mg-Y-Zn alloy
composites reinforced with additive Mg2 Si particles on repetition
number in RPW process.

of Mg2 Si particles and the repetition number of the RPW
process, the wear loss of pin specimens is remarkably
reduced. This means that their wear resistance is obviously
improved by the hardening of the matrix and the uniform
distribution of fine Mg2 Si hard particles. For example, thetotal wear loss of the composite with 10 wt% Mg2 Si via
600 cycles RPW process is about 35% of that including noAdvances in Materials Science and Engineering

5

10
Total volumetric wear loss, V (mm3 )

0%

5 wt%
Mg2 Si content

10 wt%

8

6

4

3 μm
(a)

2

0

N = 600

N = 200
N = 400 N = 600
Repetition number in RPW, N

N = 600

Figure 5: Changes in total volumetric wear loss of pin specimens
including various contents of Mg2 Si additives.
3 μm

additives (0%). The discussion on damages of the sliding
surface of each specimen is given in below. First of all, Energy
Dispersive X-ray Spectroscopy (SEM-EDS, JEOL, JED-2300)
analysis was carried out on the sliding surface of the Mg
alloy composite reinforced with Mg2 Si additives. As shownin Figure 6, Mg2 Si dispersoids with 1∼3 μm diameter reveal
round shape, not angular by the severe plastic deformation
via RPW process. They are uniformly distributed in thematrix with no segregation. It also indicates no trace andno hollow of Mg2 Si particles detached from the matrix.
Figure 7 shows the comparison of sliding surfaces of thepin specimens of Mg-Y-Zn alloy with no Mg2 Si (a) and
10 wt% Mg2 Si additives. The repetition number of RPWprocess was 600 cycles. In general, the conventional Mg alloyshows the adhesive wear at the sliding surface when it directly
contacts the counter face in wear test because of its soft
matrix compared to the aluminum alloys and steels [9, 10].
As shown in Figure 7(a), however, the sliding surface of thepin specimen reveals very slight adhesion wear, and a smallchange of the surface roughness is observed. This is quite
diﬀerent from the previous results on tribological propertiesof the conventional AZ91D alloy and their matrix composites[17] as mentioned above. This means that high hardnessof about 140 Hv of the extruded Mg97 Y2 Zn1 alloy via RPW
process is enough to improve the defensive to the adhesion
wear due to the plastic deformation in sliding and contactingwith the counter disk specimen.
Therefore, the large volumetric wear loss of the Mg-YZn alloy without Mg2 Si additives shown in Figure 5 seems to
be mainly caused by the abrasive wear. On the other hand,in the case of the composite reinforced with 10 wt% Mg2 Si
particles, very slightly abrasive wear is observed on the sliding
surface as shown in Figure 7(b). In particular, the magnified
observation indicates that fine Mg2 Si particles are distributedin the matrix, and no particle is detached from the surface
after wear test. It is similar to the polished surface observedin the microstructure. This means that mild wear with noadhesion is formed at the sliding surface because in addition

(b)

3 μm
(c)

Figure 6: SEM-EDS analysis result on sliding surfaces of pin
specimens (Mg alloy composite reinforced with 10 wt% Mg2 Si
particles via RPW process with 600 cycles. (a) SEM image, (b)element mapping of Si and (c) Mg.

to the matrix hardening as mentioned above, fine Mg2 Si harddispersoids in the matrix are also eﬀective for the defensive toboth abrasive and adhesive phenomena in sliding.
With regard to the eﬀect of Mg2 Si particle size on thewear behavior, Figure 8 shows the sliding surfaces of the pin
specimens and AZ31B disks when employing Mg-Y-Zn alloy
composite reinforced with 5 wt% Mg2 Si particles via RPW
process with (a) 200, (b) 400 and (c) 600 cycles. In the caseof N = 200 cycles, coarse Mg2 Si additives with about 20 μm
are observed in the matrix, and their particle size gradually
decreases with increase in the repetition number of the
RPW process. This corresponds well to the microstructurechanges shown in Figures 2(a)–2(c). Every pin specimen
reveals slight scratches, not adhesive wear on the sliding
surface. In the case of N = 200 cycles, however, severely
deep scratches are observed at the counter face of AZ31B
disk specimen. This means coarse Mg2 Si hard dispersoids
attacked the counter disk material in sliding. With increasein the repetition number, the scratches become slight due tothe refinement of their hard particles of the pin specimens.

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Advances in Materials Science and Engineering

Adhesive

Abrasive
50 μm

10 μm

(a) Mg2 Si; 0% (N = 600)

Mg2 SiAbrasive
50 μm

10 μm

(b) Mg2 Si; 10% (N = 600)

Figure 7: Damages on sliding surfaces of pin specimens made of Mg-Y-Zn alloy reinforced with Mg2 Si particles via RPW process with 600
cycles. Mg2 Si content; (a) 0 wt% and (b) 10 wt%.

Fine Mg2 Si
Coarse Mg2 Si
20 μm

20 μm

20 μm

Deep scratch

Slight scratch
200 μm(a) Mg2 Si; 5% (N = 200)

200 μm(b) Mg2 Si; 5% (N = 400)

200 μm(c) Mg2 Si; 5% (N = 600)

Figure 8: Sliding surfaces of pin specimens (upper) and AZ31B disks (lower) when employing Mg-Y-Zn alloy composite reinforced with
5 wt% Mg2 Si particles via RPW process with (a) 200, (b) 400 and (c) 600 cycles.

When employing N = 600 cycles, very narrow abrasion
are detected on the counter face shown in Figure 8(c).
Figure 9 indicates changes in friction coeﬃcient during thewet sliding wear test, which correspond to the damage ofthe pin and disk specimens as mentioned above. When using
Mg97 Y2 Zn1 alloy with no Mg2 Si particle (N = 600 cycles),friction coeﬃcient was unstable. In particular, some sudden
increment of the coeﬃcient is detected. This is caused by the
stick-slip phenomena due to the repetition of adhesive andabrasive wear at the contacting surface between the pin anddisk specimen (shown in Figure 7(a)). With increase in the
Mg2 Si content (5 wt% and 10 wt%), friction coeﬃcient ofAdvances in Materials Science and Engineering

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0.08

Friction coeﬃcient (μ)

0.07
0.06
0.05
0.04
0.03
0.02

0

2

4
6
Wear distance (km)8105% (N = 200) attacked by detached Mg2 Si particles
10% (N = 600) oﬀensive by Mg2 Si particles
0% (N = 600) adhesive −→ abrasive −→ stick-slip
5% (N = 400)
5% (N = 600)

Figure 9: Friction coeﬃcient changes in Mg-Y-Zn alloy composites
containing various contents of Mg2 Si additives.

Volumetric wear loss, V (mm3 )108Without Mg2 Si
6

4

2

0
100

120

140
160
Micro-hardness, Hv

180

200

Figure 10: Dependence of volumetric wear loss on microhardnessof pin specimens made of P/M Mg-Y-Zn alloy composites reinforced with additive Mg2 Si particles.

both materials was constant and stable. As mentioned above,the sliding surfaces of both materials shown in Figures 7(b)and 8(c) revealed very slight damages with extremely narrowabrasion, and seemed as the polished surfaces. In particular,
Mg2 Si dispersoids are clearly observed as the prominent
dispersoids at the sliding surface because the matrix is softer
than Mg2 Si particles and its wear occurs much early. Thismeans that a mixed lubrication condition was stably formed,
and the sliding behavior was very mild due to the formationof the oil grooves around Mg2 Si prominent dispersoids.
This wear behavior is similar to that in the case of Al-Si
eutectic alloys under wet sliding conditions [18]. As a result,
friction coeﬃcient became stably lower when Mg2 Si additives

were contained in the Mg alloy. In the case of the Mg2 Si
content of 5 wt% via RPW process with 200 cycles, coarse
Mg2 Si hard particles have a severely oﬀensive property on the
counter disk surface (shown in Figure 8(a)), and the friction
coeﬃcient gradually increases due to the abrasive wear ofthe disk specimen. 10 wt% fine Mg2 Si additives of the Mg
composite (N = 600 cycles) cause the remarkably oﬀensive
eﬀect on the counter material (shown in Figure 7(b)), andresult in the stable but slightly high friction coeﬃcient. The
distribution of refined Mg2 Si particles and matrix hardeningof Mg97 Y2 Zn1 alloy composites are suitable to keep thebalanced wear resistance. That is, both of the defensive andoﬀensive properties are given to the Mg composites when
adding 5 wt% Mg2 Si particles and applying the RPW processwith 600 cycles on Mg97 Y2 Zn1 alloy powder. The referencematerials of AZ31B and ADC12 alloys were used as a pin typespecimen to evaluate their tribological properties under thesame sliding conditions in this study. In the case of AZ31B
alloy, the total volumetric wear loss was about 37 mm3 , andextremely large compared to the Mg-Y-Zn alloy composite
with no Mg2 Si particles shown in Figure 5. The friction
coeﬃcient gradually increased from 0.03 to 0.08, and was
unstable. The microhardness (Hv) of the matrix of AZ31B
was about 46, and quite smaller than that of Mg-Y-Zn alloys
via RCP process (microhardness; 97). It was concluded thatthe poor wear resistance of AZ31B alloys was mainly causedby the extremely low hardness. The large wear loss was also
due to no additive of hard particles, such as Mg2 Si and Si,
contained in the conventional AZ31B alloy. On the other
hand, when ADC12 alloy was used as a pin specimen, thewear loss was about 14 mm3 , and its friction coeﬃcient was
0.03∼0.04, and very stable. This means coarse Si particles
are eﬀective to improve wear resistance, and also have an
important role to make a stable sliding condition by oil
grooves around Si dispersoids as mentioned in the previous
reports [18]. Figure 10 shows the dependence of volumetricwear loss on microhardness of pin specimens. This indicatesthat wear resistance strongly depends on the matrix hardness.
That is, the abrasion is a major wear phenomenon in thecase of Mg97 Y2 Zn1 alloys via RPW process. Furthermore,
Mg2 Si fine hard dispersoids are also very eﬀective to improve
the wear resistance and prevent the abrasion. The above
results are quite diﬀerent from wear behavior in the case of
the conventional Mg alloys resulting in the adhesive wear
[10, 19, 20]. This is because Mg97 Y2 Zn1 alloys have superiormechanical properties to the conventional ones, and preventthe plastic deformation, causing at the sliding surface. It is
concluded that the addition of Mg2 Si particles is also eﬀective
for improving the abrasive wear resistance. It is however,important to handle their suitable particle size and content
of the composite by microstructure control.

4. Conclusion
The wear behavior of powder metallurgy Mg97 Y2 Zn1 alloy
composites reinforced with additive Mg2 Si particles via RPW
process was investigated under oil lubrication conditions.
When increasing the repetition number of RPW process,8
the severe plastic deformation by RPW process was eﬀective
for the uniform distribution of refined Mg2 Si prominent
reinforcements with 1∼3 μm. It also accelerated the matrix
hardening (microhardness, Hv = 95∼180) by α-Mg grain
refinement of the composite. The former was useful for
improving the wear resistance and oﬀensive property against
AZ31B counter disk specimens. It also caused a low andstable friction coeﬃcient less than 0.03 by the oil groove
formation around the Mg2 Si particles, making mixed lubrication conditions. The hardening of the matrix was usefulto prevent the adhesion wear phenomenon when contacting
the AZ31B counter face. When no Mg2 Si hard particle
was dispersed in the matrix of the Mg alloys, the adhesive
was the main wear mechanism. On the other hand, whencoarse Mg2 Si additives were contained in the composite, they
severely attacked the counter face with deep scratches of
AZ31B disk, and the abrasive was the main wear mechanism.

Acknowledgment
A part of this study was financially supported by New
Energy and Industrial Technology Development Organization (NEDO), and helped in the experiments by Dr.
Yoshikazu Seki, KOBELCO Research Institute, Japan.

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