TECHNICAL PAPER

Synthesis and Characterization of Al–Mg–SiO2 ParticulateComposite Using Amorphous SiO2 from Rice Husk Ash

P. Y. Deshmukh • D. R. Peshwe • J. Bhatt •

S. U. Pathak

Received: 2 July 2011 / Accepted: 7 October 2011 / Published online: 1 December 2011

� Indian Institute of Metals 2011

Abstract This paper discusses the effect of the dispersion

of amorphous nano size (35–55 nm) SiO2 particles in

Al–Mg (5%) alloy. Amorphous SiO2 ([95% SiO2)

extracted from rice husk was added to the Al–Mg (5%)

alloy in the proportion of 5 wt% of the Al–Mg alloy. The

work aims to study the evolution of different phases like

MgAl2O4, Mg2Si, and MgO in Al–Mg–SiO2 composite

using amorphous nano SiO2. Experimental results of the

synthesized composite show presence of MgAl2O4 (Spinel

structure) and other phases like MgO and Mg2Si which

impart hardness of 126.82 HV10g to the composite. The

Al–Mg (5%) SiO2 composite microstructure appeared as a

typical lamellar structure. The XRD and energy dispersive

spectroscopy analysis display the presence of Mg2Si

formed along the grain boundary.

Keywords Hardness measurement � X-ray diffraction �Aluminum alloy � Composites � Casting

1 Introduction

Rice husk is one of the agricultural waste available in

abundance throughout the world. Rice husk forms a very

hard covering above the grain to protect the grain during its

growth. Husk contains 50–60% fixed carbon while the rest

is SiO2 and minor oxides. In recent years many efforts have

been made to utilize the silicon-rich rice-husk ash (a

product of the controlled burning of rice husk) for

manufacturing cement and a variety of other useful prod-

ucts [1]. The displacement reactions between liquid metal

and ceramic oxides are used to fabricate ceramic and metal

matrix composites (MMCs). Such reactions may be of the

type

4Mþ 3SiO2 ¼ [ 2M2O3 þ 3Si

where M is a trivalent metal [2]. Aluminum-Ceramic par-

ticle composites are generally prepared using conventional

technique of powder metallurgy. In recent study prepara-

tion of composites by casting using liquid metallurgy

techniques is being attempted. Pure SiO2 is found to be an

important oxygen source in the formation of MgAl2O4

reinforcement in Al–Mg alloy. The reactivity of SiO2 with

Al alloy is found to be very high as compared to other

sources such as Al2O3, MgO, TiO2, oxidized SiC, alumino

silicate, glass and mullite because of the amorphous nature

of SiO2 [3]. The major hurdles faced during the casting of

Aluminium-Ceramic particle composites by liquid metal-

lurgy techniques are: (1) Absence of wetting between

molten Aluminium and ceramic particles (oxides and car-

bides) at temperatures used in conventional foundry prac-

tice. It leads to rejection of ceramic particles when added to

the melt. (2) Dispersion of the ceramic particles in the melt

may not be uniform and they might get segregated due to

difference in the densities of Aluminium and the ceramic

particles [4]. MgAl2O4 (spinel) is considered to be com-

mercially important ceramic reinforcement in fabrication

of MMCs because of its attractive properties such as high

melting point, high mechanical strength at elevated tem-

peratures, high chemical inertness, and good thermal shock

resistance properties imparted with Al for many applica-

tions. Generally any oxygen source like dissolved oxygen,

pure oxygen or atmospheric oxygen is sufficient for the

formation of MgAl2O4 in Al–Mg alloy. Amorphous SiO2 is

P. Y. Deshmukh (&) � D. R. Peshwe � J. Bhatt � S. U. Pathak

Department of Metallurgical and Materials Engineering,

Visvesvaraya National Institute of Technology,

South Ambazari Road, Nagpur 440010, Maharashtra, India

e-mail: pallavi_yd@yahoo.com

123

Trans Indian Inst Met (December 2011) 64(6):575–581

DOI 10.1007/s12666-011-0083-8

highly reactive in nature as compared to crystalline SiO2

because of its more open structure and more number of

open bonds [3].

The Al–Mg based Si alloy containing hard particles of

Mg2Si have received much attention due to their potential

properties such as low density (1.91 g/cm3), high melting

point (1358 K), 120 GPa Young’s modulus, high micro-

Vicker’s hardness (600–700 HV), and low coefficient of

thermal expansion (CTE) 7.5 9 10-6 K-1. The solubility

of Si in Mg is only 0.003 at.%. Hence, most of the Si reacts

with Mg to form the intermetallic compound Mg2Si, which

has a high melting temperature, low density, high hardness,

and a low CTE. Therefore, Mg–Si alloys have a great

potential as structural materials [5]. The main aim of this

paper is to introduce the rice husk amorphous silica in nano

size (35–55 nm) in the Al–Mg melt. The study of the

dispersion of SiO2 and it’s reaction with the Al–Mg alloy

with the formation of spinel structures, Mg2Si phase was

studied in correlation with the hardness with the help of

scanning electron microscopy (SEM) and energy dispersive

spectroscopy (EDS) analysis.

2 Experimental

The composition of the alloy used is as shown in the

Table 1.

The required amount of commercially pure Al–xMg

(x = 5 wt%) master alloy was melted in a resistance

heated muffle furnace in a graphite crucible of volume

capacity 130 cm3. The melt temperature was raised to

750�C. The Dross was removed by using the flux ‘Coverall

11’ (FOSECO) powder. The silica particles (particle size:

35–55 nm) were preheated to 500�C for 4 h to remove the

moisture. The preheating the SiO2 is very important as the

retention of the SiO2 particle in the Al matrix is more as

compared to the SiO2 at ambient temperature. If SiO2 is

preheated then only the retention over 2% SiO2 in Al

matrix can be possible. The appropriate stirring intensity

was maintained for forming the vortex in the melt. In the

center of the vortex the SiO2 particles (wrapped in an Al

foil) were added gently. Melt was stirred vigorously for the

uniform dispersion and to avoid agglomeration of SiO2

particles. After intense stirring the melt was poured into a

metallic mold for the hardness and structural character-

ization study. The metallic mold was used for the casting

Al–Mg–SiO2 MMC because of its higher solidification

rates, good surface finish, low surface, porosity and clean

surface (free from dirt, binders) as compared to the sand

mold. Liquid metallurgy technique is used for the casting

of MMC (Al–Mg–SiO2).

Samples for microstructure studies were taken from

castings in ‘as cast’ condition. They were polished, etched

with Keller’s reagent and examined under both optical and

scanning electron microscope (SEM) (JEOL JSM-6380A).

The surfaces of the composite were sputter coated with a

thin layer of platinum before SEM examination to avoid

charging effects.

The hardness of the composite was determined by

microindentation (Mitutoyo HM-112) taken on sample

surfaces under the load of 10, 25, 50, 100, and 200 g for

15 s dwell time in each quadrant. A minimum of four

indentations were measured per quadrant in order to nullify

the influence of the chemical and structural heterogeneity

of the material. Digital images of the indentation marks

were taken under optical microscope to study the nature of

its edges. The of the Vickers hardness was determined

according to the equation

HV ¼ 1:853 P=d2

where P was the applied load (in Kg) and d was the

diagonal length (in mm).

3 Results and Discussions

The silica used as an reinforcement was amorphous in

nature ant size range was in 35–55 nm. Figure 1 displays

the XRD pattern along with the TEM result of the silica

particles. XRD diffractograms of nanosilica (Fig. 1)

showed strong broad peaks between 22� and 23� (2h).

These strong broad peaks suggested characteristic of

amorphous SiO2.

This amorphous and nanostructure silica is dispersed in

Al–Mg(5%) alloy as explained in the experimental details.

SiO2 reacts with liquid Al–Mg to form MgAl2O4, Mg2Si,

MgO etc., and liberates free Si into the matrix (Fig. 2) [6].

The effects of adding Mg improves wettability in the

following manner (i) Reduction in the interfacial energy

contributed by the negative free energy of the chemical

reaction between the reactive element such as SiO2 and the

substrate i.e., Al–Mg melt and (ii) The formation of a

reaction product at the liquid-substrate interface.

X-ray diffraction results of SiO2/Al–Mg composite

shown in Fig. 3 indicate that the main reaction products are

MgO, MgAl2O4 and Mg2Si.

Possible interfacial reactions between SiO2 particles and

Al–Mg matrix are as follows [3]:

Table 1 Chemical Composition of Al–5%Mg alloy taken for the

experiment

Mg Fe Si Al

Al–5%Mg 5.12 0.19 0.095 Balance

576 Trans Indian Inst Met (December 2011) 64(6):575–581

123

SiO2 sð Þ þ 2Mg lð Þ ! 2MgO sð Þ þ Si sð Þ;DG0

900 ¼ �172:6 kJð1Þ

3SiO2 sð Þ þ 4Al sð Þ ! 2Al2O3 sð Þ þ 3Si sð Þ;DG0

900 ¼ �210 kJð2Þ

2SiO2 sð Þ þ 2Al sð Þ þMg lð Þ ! MgAl2O4 sð Þ þ 2Si sð Þ;DG0

900 ¼ �220 kJ ð3Þ

Al2O3 sð Þ þ 3Mg lð Þ ! 3MgO sð Þ þ 2Al sð Þ;DG0

900 ¼ �43 kJð4Þ

4Al2O3 sð Þ þ 3Mg lð Þ ! 3MgAl2O4 sð Þ þ 2Al sð Þ;DG0

900 ¼ �13 kJð5Þ

2Mgþ Si! Mg2Si ð6Þ

MgO can be produced by reactions (1) and (4),

MgAl2O4 by reactions (3) and (5) and Mg2Si by reaction

(6). Si is a transitional product which react with Mg to form

Mg2Si (reaction 6). MgO, MgAl2O4 and Mg2Si are all

stable compounds. A12O3 can be formed by reaction (2),

but A12O3 has not been identified by X-ray diffraction

analysis, because A12O3 can react with Mg to form MgO

or MgAl2O4 by reactions (4) and (5). Thus reactions (1),

(3) and (6) are the probable reactions and are consistent

with the X-ray diffraction result [7]. Reactions (2), (4) and

(5) are transitional reactions.

The SiO2 particles appeared to have undergone reduc-

tion with the formation of Spinel Structures, resulting in

improved wettability between the particles and the melt.

Furthermore, during the stirring casting of SiO2/Al–Mg

composites, the addition of Mg into the aluminum melt

also helps to improve the wettability of reinforcement with

matrix, increasing interface bonding strength and prevent-

ing deleterious interfacial reaction. [8].

In the melting experiments, the SiO2 particles (density

2.63 g cm-3) did not settled or float even though they have

similar density of the melt (density 2.37 g cm-3), due to

the rapid stirring which breaks the oxide layer which acts

as a barrier between the two and the convection currents in

the melt.

The schematic illustration of the reaction of SiO2 par-

ticles in molten Al–Mg alloy is shown in Fig. 2. EDS of

the samples also confirmed the presence of spinel struc-

ture. The diffraction pattern shows peaks of newly formed

MgAl2O4 and Si, non-reacted SiO2 and Mg2Si caused by

Mg left in the matrix. Consequently, the reactions of SiO2

with the stable oxides were terminated within a short

period after the transfer of particles to the melt. The added

SiO2 particles were gradually transferred to the molten

Al–Mg alloy and they changed into MgAl2O4 by con-

suming Al and Mg present in the matrix. Then, the matrix

changed from the original Al–Mg to Al–Mg–Si alloy with

the reduction of SiO2 and consumption of Mg. Similar

results were obtained by Liu et al. [9] and Tsunekawaa

et al. [10].

Fig. 1 a XRD spectra of amorphous nano structured silica (RHA burnt at 500�C for 6 h) the peak at 2h = 22.088, typical of silica, is seen in the

spectra. b TEM micrograph displaying the particles in the range of 35–55 nm of silica synthesized by burning rice husk at 500�C for 6 h

Fig. 2 Schematic illustration of mass transfer during the reaction

process of SiO2 with molten Al–Mg alloy

Trans Indian Inst Met (December 2011) 64(6):575–581 577

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There were two reaction routes through which spinel

particles were formed. The first reaction involved the

reduction of the Al2O3 by the Mg and the subsequent

reaction of the resulting MgO with the remaining Al2O3 to

form MgAl2O4. The second route was the reaction between

Mg and Al2O3 resulting in direct formation of MgAl2O4

and a small amount of excess Al [11]..The second route is

more likely to occur.

The ‘as cast’ Al–Mg-SiO2 alloy was examined under the

SEM and EDS to study the phases obtained. These phases

have correlated with the XRD results (Fig. 3). The mag-

nified microstructure images obtained are shown in the

Fig. 4a, b and c, which show the presence of Mg2Si along

the grain boundary. EDS analysis show that the Mg added

to the melts concentrates preferentially around the SiO2

particles improving the wettability of the Al–Mg–Si alloy

[10].

The SEM–EDS analysis of the material in the transition

zone between Al–Mg melt and amorphous SiO2 shown in

Fig. 5 reveals combined existence of high Al, O with Si

and Mg in the EDS spectrum, instead of only Si and O

peaks as in the spectra determined at the location of the

SiO2 particles that had not been reacted.

To illustrate the different morphological features due to

the phases formed along the grain boundary in the Al–Mg–

SiO2 alloy an EDS analysis was carried out. Figure 5

shows the EDS spectra of the unreacted silica particles in

an agglomerated form when the composite was removed

immediately from the graphite crucible. The bright region

of the micrograph is the SiO2 particle that had not been

reacted while the right hand side grey region is the syn-

thesized composite indicating that the reaction was directed

from right to left. An EDS spectrum displays the sudden

rise in Si content displaying the existence of a narrow band

Fig. 3 X-ray diffraction pattern

of Al–Mg (5%)–SiO2 alloy in as

cast condition displaying the

phases such as Mg2Si,

MgAl2O4, MgO, Al and

unreacted SiO2

Fig. 4 As-cast microstructure of the alloy Al–5%Mg–SiO2 etched by Keller’s reagent. a Magnification of 9100, b magnification of 9200 and

c magnification of 9500 displaying the formation of Mg2Si along the grain boundary

578 Trans Indian Inst Met (December 2011) 64(6):575–581

123

of material rich in Si indicating the presence of unreacted

SiO2 (Fig. 5). Similar results were observed by Wu and

Han [12]. The melt was removed immediately after adding

SiO2 in Al–Mg alloy to see the effect of reaction time span.

Figure 5 is the result of the microstructure of this alloy in

the ‘as cast’ condition. The reaction was allotted sufficient

time span (5 min) to complete the reaction. Later casting

was done followed by EDS study as shown in Figs. 6

and 7.

Figure 6 shows the EDS analysis image of the Al–Mg–

5%–SiO2 composite. The solidified microstructure of the

alloy contain three constituents: the dark phase is sur-

rounded by a white phase followed by polyphase structure

(Chinese script like structure) [13]. Moreover the dark

phase at the center show dendritic morphologies or

polygonal morphologies. The phases in the alloy are

Mg2Si, MgO, MgAl2O4 and Al–Mg intermetallics or

AlMgO are formed. Similar phases were also reported by

Fig. 5 EDS analysis of the Al–Mg (5%)–SiO2 alloy when cast immediately after adding amorphous silica (Etched by Keller’s reagent and

Magnification of 9700)

Fig. 6 EDS analysis of Al–Mg–5%SiO2, alloy at the magnification of 9700 and etched by Keller’s reagent showing presence of Al peak of

highest intensity in the matrix area

Trans Indian Inst Met (December 2011) 64(6):575–581 579

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Pan et al. [14] and [15], which follows bright areas those of

Mg2Si particles, dark areas of Mg particles and grey areas

represent Mg–Mg2Si eutectic. Moreover, Mg2Si particles

are surrounded by Mg particles and by Mg–Mg2Si eutectic

crystals.

Based on the observation, it is considered that Mg2Si and

Mg particles orderly precipitate from the melts and form the

Mg–Mg2Si eutectic upon solidification. It was seen that

Mg2Si primary particles were dendrites and the Mg sub-

primary particles were present as halos surrounding them.

In the Mg2Si dendrites, there were secondary branches that

were parallel to each other. Moreover, they grew in per-

pendicular directions and gently continued to develop into

minor branches. However, the minor branches were much

shorter in length than the major ones (Figs. 4, 6, 7).

Mg sub-primary particles nucleated and grew as halos

surrounding Mg2Si primary particles and thus arrested their

growth. To summarize, the primary crystals formed as

Mg2Si and Mg precipitates in sequence from the melt. It

can be seen clearly from Fig. 7 that the typical micro-

structure of the alloy contains three constituents: coarse

primary Mg2Si dendritic crystals, a-Mg dendritic halos and

Chinese script type Mg2Si ?Mg eutectic structures.

Moreover, the primary Mg2Si dendritic crystals is sur-

rounded by a-Mg halos, and then by polyphase eutectic

structure. This experimental result is in good agreement

with the works investigated previously by [16]. The for-

mation of microstructural feature is mainly attributed to the

solidification of the alloy under relatively high cooling rate,

leading to deviation from the equilibrium diagram during

solidification. After formation of primary Mg2Si, the liquid

phase surrounding them becomes enriched with Mg due to

the rejection of Mg solute atoms. Consequently, a-Mg

halos are formed and grow around primary Mg2Si.

Based on the above discussions, it is believed that

modification of the Mg2Si primary dendrites would be a

good way to improve the hardness of Al–Mg–Si alloys

because the transition of Mg2Si from coarse dendrites to

fine granules can simultaneously cause morphological

modifications of the Mg sub-primary particles. This leads

to refinement of the entire microstructure. The growth of

primary Mg2Si dendrites leads to the decrease in Si content

of the remaining liquid. It is well known that eutectic

solidification is affected by various factors such as crystal

characterization of the comprising phases, process inter-

actions and the specific freezing conditions. Moreover, the

eutectic structure can often be present in various mor-

phologies based on the principle of Lowest Interphase

Boundary Energy [14]. For the solidification of the

Mg–Mg2Si eutectic, the cooperative growth mechanism

yields a structure with a regular morphology having rod-

like Mg2Si distributed in the continuous matrix of Mg [14].

The hardness of the Al–Mg (5%)–SiO2 alloy was found

to be 126.82 HV(10g).

Vickers hardness at 10, 25, 50, 100, and 200 g are

measured for Al–Mg–SiO2 alloy. At each load the values

of microhardness are taken for 5–6 times and then the

average was found out. There was the ±error of 0.0012 for

each case approximately. The microhardness values and

the corresponding GPa values are tabulated in Table 2.

Figure 8 shows the hardness profile for the values tab-

ulated in Table 2. The hardness of the composite was found

to increase from 102.26 HV200 to 126.82 HV10. Indentation

images of the microhardness were taken at the various

loads under study using optical microscope to understand

the nature of the indent. This high value of hardness is

attributed to the formation of the hardening phase such as

Mg2Si. The Fig. 8 displays the indentation images of the

Fig. 7 EDS analysis of Al–Mg(5%)–SiO2 alloy at the magnification of 9700, and etched by Keller’s reagent indicating presence of Mg peak of

highest intensity and next lower intensity peak is that of Si, displaying the presence of the formation of Mg2Si along the grain boundary

580 Trans Indian Inst Met (December 2011) 64(6):575–581

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composite (Al–Mg(5%)–SiO2(5%) with the increasing

load. From this microhardness figure it is found that though

the applied load was increased from 10 to 200 g there is

increase in the indentation area only and there is no crack

developed along the corners of the image of the indenta-

tion. This means that the material is not only hard but it has

sufficient ductility to absorb the stresses developed during

the increasing load while taking microhardness. According

to the Ref. [10] the hardness of A1–2.5 wt% SiO2 partic-

ulate composites was four times that of pure Aluminium.

4 Conclusions

1. The locally available amorphous silica varieties were

found to be useful silica sources for the generation of

MgAl2O4, Mg2Si in Al–Mg alloy.

2. By constant stirring, both particle transfer and stable

oxide formation were promoted due to the forcibly

improved wettability between SiO2 particles and

molten Al–Mg alloy so that composite slurry contain-

ing no porosities was successively prepared within a

short stirring time.

3. Immediately after the transfer of SiO2 particles into

molten Al–Mg alloy, a SiO2–Al–Mg composite system

changes to an in situ composite slurry containing

MgAl2O4 and Al2O3 particles in the Al–Si matrix

through the exothermic reaction of SiO2 particles with

Mg and Al.

4. The metastable phase identified in the matrix consti-

tutes the final phases like Mg2Si and MgAl2O4.

5. The XRD studies have shown that simultaneous

formation of MgAl2O4 and MgO has happened at the

interface. The present study has detected MgAl2O4 and

MgO crystals at the Al–Mg alloy/quartz composite

interface.

6. The hardness value of the alloy was found to be

126.82 HV10.

7. The increase in the hardness did not developed cracks

in the composite (Al–Mg(5%)–SiO2(5%)) as the

stresses developed were absorbed within the matrix.

This implies that the material has sufficient ductility

along with the hardness.

Acknowledgments I am thankful to the Indian Institution of

Engineers for supporting the financial grant during this project under

the grant SCK/T-R&D/96/2009-2010.

References

1. Das S, Dan T K, Prasad S V, and Rohatgi P K, J Mater Sci Lett 5(1986) 562.

2. Hanabeand M, and Aswath P B, Acta Mater 45 (1997) 10.

3. Sreekumar V M, Pillai R M, Pai B C, and Chakraborty M,

J Mater Process Technol 192 (2007) 588.

4. Surappa M K, and Rohatgi P K, J Mater Sci 16 (1981) 983.

5. Liping W, Jun E M, and Baoxia A, J Rare Earths 26 (2008) 105.

6. Yoshikawa N, Hattori A, and Taniguchi S, Mater Sci Eng A 342(2003) 51.

7. Li G, Sun J, Guo Q, and Wu Y, J Mater Process Technol 161(2005) 445.

8. Geng L, Zhang H, Li H, Guan L, and Huang L, Trans NonferrousMet Soc China 20 (2010) 1851.

9. Liu Y L, Kang S B, and Kim H W Mater Lett 41 (1999) 267.

10. Tsunekawa Y, Suzuki H, and Genma Y, Mater Des 22 (2001)

467.

11. Hunti E M, and Hampikian J M, Acta Mater 47 (1999) 1497.

12. Wu C M L, and Han G W, Mater Charact 58 (2007) 416.

13. Srinivasan A, Pillai U T S, Swaminathan J, Das S K, and Pai B C,

J Mater Sci 41 (2006) 6087.

14. Pan Y, Liu X, and Yang H, Mater Charact 55 (2005) 241.

15. Chakrabarti D J, and Laughlin D E, Prog Mater Sci 49 (2004)

389.

16. Guo E J, Ma B X, and Wang L P, J Mater Process Technol 206(2008) 161.

Table 2 Vickers Microhardness at the different load values

Load (g) HV GPa

10 126.82 1.24

25 118.06 1.15

50 113.26 1.11

100 110.575 1.08

200 102.26 1.00

Fig. 8 Change in the load versus indentation

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