MARTA GAJEWSKA, JERZY MORGIEL
Characterization of reaction products
of liquid Al and Mg3N2
INTRODUCTION
Aluminium matrix composites (AMCs) have gained a considerable interest in automotive and aerospace applications due to the
light weight combined with higher stiffness, elastic modulus and
strength, as well as better thermal stability and wear resistance compared with conventional alloys [1÷9], So far, in this group of materials most of the attention has been paid to Al2O3 [1÷3] and SiC
[3÷5] reinforced composites. Some of them are now well developed
and have been already commercially applied [4, 5]. However, constant efforts are being made to improve existing materials and to design new ones in order to meet the increasing demands for advanced
structural and functional materials.
Recently, an increasing attention has been paid to aluminium nitride as a reinforcing phase in AMCs. The addition of AlN, due to its
good physicochemical, mechanical and thermal properties, allows
to enhance the modulus, strength, hardness, wear resistance and
high temperature performance of aluminium alloy matrix [6÷9].
The main advantage of the aluminium nitride over commonly applied in AMCs reinforcing phases is good bonding to aluminium
matrix, higher wettability in aluminium, as well as stability of aluminium/aluminium nitride interface [9÷12].
In conventional metal matrix composite production a reinforcing
phase is usually prepared separately, prior to the composite fabrication and introduced into the matrix via powder metallurgy [3, 6÷9],
spray deposition, casting techniques [1, 2, 5] etc. These, referred to
as ex-situ techniques, have one major disadvantage, which is generally weak bonding between the reinforcements and the matrix.
A possible solution to this problem are in-situ techniques, in which
the reinforcement is produced directly in the metallic matrix, e.g. by
chemical reactions between elements (or compounds) present in the
material, providing a strong reaction bonded matrix/reinforcement
interface [11÷17]. As a result, a good interface contact between matrix and the reinforcement can be achieved, which is a key factor for
the efficient load transfer in the composite material.
The aluminium nitride reinforcing phase have been obtained in
situ in metal matrix composites using ball milling of elemental metal powders in nitrating atmospheres (N2, NH3) or by so called reactive gas injection (RGI), which is a direct introduction of nitrogenbearing gas (N2, NH3) into the aluminium melt [14÷17]. However,
in most of the cases, the amount of the formed reinforcement and
its distribution in the matrix was not sufficient and hard to control
[14, 15].
The paper presents an attempt that has been made to produce aluminium matrix composite reinforced with dispersed aluminium nitride (AlN) by reaction of aluminium melt with magnesium nitride
(Mg3N2) compact dropped into the liquid metal. The Mg3N2 has
been selected as a nitrogen-bearing substrate due to two reasons:
firstly, it has been reported that in situ synthesis of AlN in liquid Al
is promoted by Mg addition in the Al melt and is realized through
the formation of an intermediate Mg3N2 phase [11, 12]; secondly,
M.Sc. Marta Gajewska ([email protected]), Prof. D,Sc. Ph.D. Jerzy Morgiel –
Institute of Metallurgy and Materials Science, Polish Academy of Sciences, Kraków
162 a reaction by product in the form of magnesium can be favourable,
as it serves as an alloying element which, via solution hardening,
markedly increases the strength of aluminium without decreasing
its ductility.
EXPERIMENTAL DETAILS
A cast aluminium (99.9%) charge was introduced into alumina
crucible placed in an induction furnace. Then, the melting process
was carried out under the protective argon atmosphere. In order
to introduce magnesium nitride particles into the aluminium melt,
a commercial Mg3N2 powder (Alfa Aesar, 99.7%, 325 mesh) with
addition of an aluminium powder (Alfa Aesar, 99.7%, ~11 µm)
was prepared in a form of green compacts (5 mm diameter, ~6 mm
height) at a pressure of ~400 MPa under Ar atmosphere which prevented decomposition of highly hygroscopic magnesium nitride. As
prepared compacts were then dropped into the aluminium bath and
held at the temperature of 900°C for 10 minutes.
A microstructure of the resulting casting was characterized using
Leica DM IRM metallographic microscope and FEI E-SEM XL30
scanning electron microscope with an integrated EDAX system.
Phase composition of the casting was investigated by XRD measurements performed on Bruker D2 PHASER diffractometer with
Cu Kα radiation (λ = 1.5406 Å). Vickers microhardness measurements were performed using a CSM-Instruments tester.
rESULTS AND DISCUSSION
The light microscopy investigations of a cross-section of the produced casting (Fig. 1) revealed heterogeneous structure of the material. Due to the fact that Al melt motion induced by eddy currents in
the electromagnetic field was not sufficient to distribute reinforcing
particles, introduced to the melt Mg3N2/Al green compacts stayed
on the surface of the melt. A sharp border between unreinforced
aluminium area and confined composite zone were observed. The
composite area was localized in the initial area of Mg3N2 compact
and its immediate vicinity. However, some regions of aluminium
interpenetrating through the compacts were present as well. Quantitative image processing allowed to estimate a concentration of
reinforcing particles in the composite region at a level of about
90 vol. %.
The microstructural examinations of the casting using scanning
electron microscopy (BSE – back-scattered electrons) combined
with X-ray microanalysis showed that particles present in the composite zone of the casting, characterized by a slightly darker than
the aluminium matrix contrast, consisted mainly of aluminium, and
nitrogen (Fig. 2, Tab. 1). Moreover, the Al matrix in the composite
zone, apart from some minor nitrogen and oxygen content, was
characterized also by average Mg content of ~3.5 at. %. The obtained results indicate that a reaction between Mg3N2 and liquid Al
occurred leading to the formation of AlN reinforcing particles and
aluminium/magnesium solution.
The average AlN particle size (longest dimension) in the composite zone, determined on the series of SEM BSE images, was
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Fig. 1. Microstructure of a cross-section of the casting; LM
Rys. 1. Mikrostruktura przekroju odlewu; mikroskop świetlny
~18 µm (Fig. 3). Some smaller Al/AlN composite regions outside
the composite zone were also observed, what suggests that some
portion of Mg3N2/Al compacts was incorporated deeper into the Al
melt. It was confirmed by a less significant but also noticeable Mg
presence in the areas distant from the composite zone (over 1 at. %).
The presence of oxygen in all of the regions of the casting is likely
due to the surface oxide contamination of the metallographic specimen or Mg3N2/Al raw powders.
The accompanying X-ray diffraction investigation of the produced casting (Fig. 3) confirmed presence of only two main phases:
aluminium and aluminium nitride. No diffraction line characteristic
for magnesium nitride phase can be distinguished in the XRD pattern. These results indicate that all Mg3N2 (or nearly all) was used
in the reaction with the liquid aluminium resulting in the AlN phase
formation in the produced casting.
Table 2 presents examples of Vickers microindentations
(HV0.02) of three different areas of the casting: Al away from the
composite zone, Al near the composite zone and the composite
zone. Microhardness tests showed significant differences in hardness between these areas. Higher hardness of the Al area near the
composite zone (39±14 HV0.02) than of the aluminium area away
from the composite zone (25±11 HV0.02) can be related to the
amount of magnesium dissolved in Al, which serves as an alloying
element leading to solution hardening and in this way improving its
mechanical properties. Morover, more than ten times higher hardness of the composite zone (430±86 HV0.02) is due to a very high
content of hard AlN particles in this area.
NR 3/2013 Fig. 2. Microstructure of a cross-section of the casting with marked
points of X-ray microanalysis; SEM (BSE)
Rys. 2. Mikrostruktura przekroju odlewu z zaznaczonymi punktami mikroanalizy rentgenowskiej; SEM (BSE)
Table 1. X-ray point microanalysis results of chosen casting crosssection regions
Tabela 1. Wyniki mikroanalizy rentgenowskiej w wybranych punktach
przekroju odlewu
Point
Elemental composition, at. %
Al
Mg
N
O
1
96.5
1
–
2.5
2
92.5
3.5
1
3
3
75.5
1
22.5
1
4
75.5
1.5
22
1
sUMMARY
The in situ Al/AlN composite material was successfully obtained by
reaction of aluminium melt with magnesium nitride (Mg3N2) introduced to the melt in a form of a green compact. The EDS chemical
analysis of the casting combined with X-ray diffraction technique
confirmed presence of AlN particles of the average size of about
~18 µm in the obtained material. Unfortunately, lack of applied
external mixing lead to formation of inhomogeneous microstruc-
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163
Fig. 3. AlN particle size distribution in composite zone
Rys. 3. Rozkład wielkości cząstek AlN w obszarze kompozytowym
Table 2. Vickers microhardness of different regions of the casting
Tabela 2. Mikrotwardość Vickersa różnych obszarów odlewu
Area in the casting
Example
of an indentation
Microhardness value
HV0.02
Al away from
the composite zone
25±11
Al near the composite
zone
39±14
Composite zone
430±86
ture of the casting: a composite zone with about 90 vol. % of AlN
particles and an area of unreinforced aluminium. Moreover, microhardness tests showed very high hardness values in the composite
zone, reaching up to 500 HV, as compared with ~25 HV in the unreinforced Al area.
ACKNOWLEDGEMENTs
The work was supported through project No. POKL. 04.01.0000-004/10 co-financed by the European Union within the European
Social Fund.
164 Fig. 4. X-ray diffraction pattern of a cross-section of the casting
Rys. 4. Dyfraktogram rentgenowski z powierzchni przekroju odlewu
rEFERENCES
[1] Sajjadi S. A., Torabi Parizi M., Ezatpour H. R., Sedghi A.: Fabrication of
A356 composite reinforced with micro and nano Al2O3 particles by a developed compocasting method and study of its properties. J. Alloy Compd.
511 (2012) 226÷231.
[2] García-Romero A. M., Egizabal P., Irisarri A. M.: Fracture and fatigue behaviour of aluminium matrix composite automotive pistons. Appl. Compos. Mater. 17 (2010) 15÷30.
[3] Ozdemir I., Ahrens S., Muecklich S., Wielage B.: Nanocrystalline
Al-Al2O3p and SiCp composites produced by high-energy ball milling. J.
Mater. Process. Tech. 205 (2008) 111÷118.
[4] Ouyang Q. B., Gu H. L., Wang W. L., Zhang D., Zhang G. D.: Friction and
wear properties of aluminum matrix composites and its application. Key
Eng. Mat. 351 (2007) 147÷150.
[5] Rehman A., Das S., Dixit G.: Analysis of stir die cast Al-SiC composite
brake drums based on coefficient of friction. Tribol. Int. 51 (2012) 36÷41.
[6] Fogagnolo J. B., Robert M. H., Torralba J. M.: Mechanically alloyed AlN
particle-reinforced Al-6061 matrix composites: Powder processing, consolidation and mechanical strength and hardness of the as-extruded materials. Mater. Sci. Eng. A 426 (2006) 85÷94.
[7] Ahamed H., Senthilkumar V.: Role of nanosize reinforcement and milling
on the synthesis of nanocrystalline aluminium alloy composites by mechanical alloying. J. Alloy Compd. 505 (2010) 772÷782.
[8] Abdoli H., Saebnouri E., Sadrnezhaad S. K., Ghanbari M., Shahrabi T.:
Processing and surface properties of AlAlN composites produced from
nanostructured milled powders. J. Alloy. Compd. 490 (2010) 624÷630.
[9] Gajewska M., Dutkiewicz J., Lityńska-Dobrzyńska L., Morgiel J.: TEM
investigation of metal/ceramic interfaces in AA7475/AlN or Al2O3 nanocomposites. Sol. St. Phen. 186 (2012) 202÷205.
[10] Liu Y. Q., Cong H. T., Wang W., Sun C. H., Cheng H. M.: AlN nanoparticle-reinforced nanocrystalline Al matrix composites: fabrication and mechanical properties. Mater. Sci. Eng. A 505 (2009) 151÷156.
[11] Sreeja Kumari S. S., Pillai U. T. S., Pai B. C.: Synthesis and characterization of in situ Al-AlN composite by nitrogen gas bubbling method. J. Alloy
Compd. 509 (2011) 2503÷2509.
[12] Hou Q., Mutharasan R., Koczak M.: Feasibility of aluminium nitride formation in aluminum alloys. Mater. Sci. Eng. A 195 (1995) 121÷129.
[13] Zhang C., Fan T., Cao W., Ding J., Zhang D.: Size control of in situ formed
reinforcement in metal melts-theoretical treatment and application to in
situ (AlN + Mg2Si)/Mg composites. Compos. Sci. Technol. 69 (2009)
2688÷2694.
[14] Durai T. G., Das K., Das S.: Synthesis and characterization of Al matrix
composites reinforced by in situ alumina particulates. Mater. Sci. Eng.
A 445-446 (2007) 100÷105.
[15] Yu S. H., Shin K. S.: Fabrication of aluminum/aluminum nitride composi­
tes by reactive mechanical alloying. Mater. Sci. Forum 534-536 (2007)
181÷184.
[16] Naranjo M., Rodrıguez J. A., Herrera E. J.: Sintering of Al/AlN composite
powder obtained by gas-solid reaction milling. Scripta Mater. 49 (2003)
65÷69.
[17] Goujon C., Goeuriot P., Delcroix P., Le Caer G.: Mechanical alloying during cryomilling of a 5000 Al alloy/AlN powder: the effect of contamination. J. Alloy. Compd. 315 (2001) 276÷283.
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