Review paper	Received: October 10, 2013
Accepted: November 28, 2013
Interfaces in the magnesium-matrix composites
Mejna območja v kompozitih z magnezijevo osnovo
Matej Steinacher1, *, Primož Mrvar1, Franc Zupanič2
University of Ljubljana, Faculty of Natural Sciences and Engineering, Aškerčeva cesta 12, 1000 Ljubljana, Slovenia 2University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, 2000 Maribor, Slovenia Corresponding author. E-mail: matej.steinacher@gmail.com
Abstract
Processes at the matrix/reinforcement interfaces strongly influence the properties of the composites. The basic task of the interfaces is to assure the strong bonding between the composite's constituents. In addition, they must be mechanically and thermodynami-cally stable. Therefore, the understandings how the bonds at the interfaces are formed, as well as the related processes, are of crucial importance by designing and manufacturing of the composites. This review paper describes the interfaces in the magnesium-matrix composites reinforced with different types of SiC, Al2O3, and SiO2, and prepared by different methods.
Key words: magnesium-matrix, reinforcement, composite, interface, reaction product.
Izvleček
Procesi v mejnih območjih med osnovo in utrjeval-no sestavino močno vplivajo na lastnosti kompozitov. Osnovna naloga mejnih območij je zagotavljanje trdne povezave med sestavinami kompozita, prav tako morajo biti mejna območja mehansko in termodinamično stabilna. Zato je poznanje načina nastanka povezovanja v mejnih območjih in procesov, ki se zgodijo na njih, zelo pomembno za načrtovanje in izdelavo kompozitov. Pregledni članek opisuje mejna območja v kompozitnih materialih z magnezijevo osnovo, ki so utrjeni z različnimi oblikami SiC, Al2O3 in SiO2 ter izdelani z različnimi postopki.
Ključne besede: magnezijeva osnova, utrjevalna sestavina, kompozit, mejno območje, reakcijski produkt
Introduction
Composites are modern materials, which consist of at least two chemically, physically, and mechanically different materials. The properties of the composites depend on the properties of the matrix and reinforcing phase, shape, fraction, distribution, and orientation of the reinforcing phase, interactions at the matrix/reinforcement interface, processing parameters, and heat treatment conditions. Often, these properties can be predicted, using the rule of mixtures[1]:
L=L V +LV	(1)
c m m r r	J
where Lc is the property of the composite (e.g. Young's modulus, density, etc.), V the volume content, and indices m and r indicates the matrix and the reinforcement, respectively. The interface between matrix and reinforcement is a region with different physical and chemical properties compared to the properties of the composite's constituents. Interface bonding arises from the adhesion of the constituents, which depends on the wettability. In the composites, the wettability is defined as the ability of the liquid matrix to spread over the solid surface of the reinforcement. Physical and chemical processes at the interfaces can strongly influence the mechanical, thermome-chanical, and thermodynamic properties of the composites.[1]
Reaction products are formed at the matrix/reinforcement interfaces as a result of the chemical reactions. These interfacial reaction products are usually brittle and could be strongly or weakly bonded to the reinforcement. There exists the critical thickness of the reaction products beyond which the composite properties becomes deteriorated.[2] In the titanium-matrix
composites reinforced with SiC fibres, the critical thickness of the reaction products was 1 nm. The mechanical properties of the composite above this value were significantly decreased.[3] Magnesium-matrix composites are promising materials because of their low density and high strength/weight ratio. The specific strength and stiffness of the magnesium-matrix composites should be greater than those of the aluminium-matrix composites. The selection of the ceramic reinforcement (chemical composition, shape, and volume fraction) and magnesium (alloy) matrix can be used to tailor the thermal conductivity of the composites. Particles (p), fibres (f), whiskers (w), and recently also different preforms (e.g. ceramic foam (cf)) are usually used as reinforcements (Figure 1). An advantage of the magnesium, compared to the aluminium, is that it can wet most of the ceramic reinforcement. A disadvantage is its reactivity with the reinforcements. In many cases, the reinforcements are very prone to oxidation. The oxidation behaviour and further reactions could influence interfacial structure and composition, and hence the nature and strength of the interfacial bonding. Undesirable reaction products at or near the interface may lead to loss of the load-bearing ability and thus change the mechanical properties of the composite.[2]
Morphologies of the interfaces
Figure 2 schematically presents distinct types of the matrix/reinforcement interfaces. At the interface type I (Figure 2a), the interfacial reaction products (IRPs) are directly in contact with the reinforcement. For the interface type II (Figure 2b), the interfacial reaction zone (IRZ) consists of two distinct layers. The first layer consists of the IRPs, and this layer is in direct
Figure 1: Shapes of the reinforcements. a) particles (p), b) fibres (f), c) whiskers (w) and d) ceramic foam (cf).
contact with the matrix. The second layer is in direct contact with the reinforcement, which consists of the matrix that extends along reinforcing surface. Thus, for interface type II the IRPs are not in direct contact with the reinforcement. The interface type III is very clean (Figure 2c) and the IRPs have not even formed at the interface at all.[4] If the matrix does not wet the reinforcement, the cracks and debond-ing free interfaces are present between them. This interface can be marked as the interface type IV (Figure 2d).
Figure 3 shows the interfaces in the composite with AZ91 matrix reinforced with SiC particles (SiCp), where three kinds of the interfaces were observed.[4] The reasons for existence of these three kinds of the interfaces in the present composite may arise from the conditions during stir-casting, which are very complicated. The friction between the melt and SiCp takes place during stirring and causes shearing during pouring. These actions can cause the formation of the interface III by breaking away the IRPs from the SiCp. However, the IRPs separated from the SiCp and SiCp are often pushed by the freezing front to the last solidified regions, and this leads to the formation of the interface II.[5] When the IRPs do not break away from the SiCp
during stirring and pouring, the interface I will be formed.[4]
Types of bonding at the interface
There are two types of bonding at the interface in the metal-matrix composites (MMCs): mechanical bonding and chemical bonding.
Mechanical bonding
Mechanical bonding is formed when the surfaces of the matrix and reinforcement are interconnected, and there are no chemical bonds between them. Interfaces in the MMCs are invariably rough, and the degree of the interfacial roughness increases the strength of the bond. In the MMCs reinforced with the ceramics, the metals generally have a higher coefficient of the thermal expansion than the ceramics. Thus, the metallic matrix in the composite will shrink more than the ceramic reinforcement on the cooling from a high temperature. This will lead to the mechanical gripping of the reinforcement by the matrix even in the absence of any chemical bonding. The matrix infiltrates into the cracks on the reinforcing surface, by the liquid flow or high temperature diffusion, which can also lead to some mechanical bonding.
matrix
matrix
reinforcement
• reaction products
Figure 2: Schematic representation of the distinct interfaces. a) type I, b) type II, c) type III141 and d) type IV.
The radial gripping stress, a, can be related to the interfacial shear strength, t., by the equation 2[6]:
= (2)
where / is the friction coefficient. It usually lies between 0.1 and 0.6. In general, the mechanical bond is a low energy bond vis-à-vis the chemical bond.
diffusion, which causes the change of chemical compositions of the constituent phases at the interface. Thus, chemical bonding includes the solid solution and/or chemical compound formation at the interface (Table 1). For the diffusion controlled growth in an infinite diffusion couple with a planar interface, the important relationship is valid[6]:
x2 = Dt	(3)
Chemical bonding
The metal/ceramics interfaces in the MMCs are generally formed at high temperatures. The diffusion and chemical reaction kinetics are faster at the elevated temperatures. Knowledge of the chemical reaction products and, if possible, their properties are needed. It is, therefore, imperative to understand the thermodynamics and kinetics of the reactions. In this way, the processing can be controlled, and optimum properties obtained. Chemical bonding in the MMCs involves atomic transport by the
where x is the thickness of the reaction layer, D the diffusivity, and t the time. The diffusivity, D, depends on the temperature in an exponential manner[6]:
D = D0eW{-§)	(4)
where D0 is a pre-exponential constant, AQ the activation energy for the rate controlling process, k the Boltzmann's constant, and T the temperature.
Table 1: Chemical reactions that can take place between magnesium and oxides, carbides, binding agents, and protective gases during the manufacturing of the MMCs
Phase	Chemical reaction	References
	2Mg(l) + SiO2(s) - 2MgO(s) + Si(s)	{5.1}	4, 7, 8
	2Mgm + 2SiO2(s) - Mg2Si°4(s) + Si(s)	{5.2}	8
	4Mgm + Si°2(s) - 2Mg°(s) + Mg2Si(s)	{5.3}	2, 8, 9
	SiO2(s) + Mg°(s) - MgSiO3(sl	{5.4}	10
	Si°2(s) + 2Mg°(s) - Mg2Si°4(s)	{5.5}	10
SiO2	4Al(ll + 3SiO2(s) - 2Al2°3 + 3Si(s)	{5.6}	4
	Mg(ll + 2Al(ll + 2SiO2(s) - MgAl2°4(s) + 2Si(s)	{5.7}	4, 9
	4Al(l) + 2Mg°(s) + 3Si°2(s) - 2MgAl2°4(s) + 3Si(s)	{5.8}	4
	2Mg°(s) + 5Si°2(s) + 2Al2O3(s) ( + CJ - Mg2Al4Si5°1R(s) ( + CJ	{5.9}	11
	2Mg(l) + SiO2(s) ( + CJ - 2MgO(s) + Si(s) ( + C(s))	{5.10}	11
	2Mgnl + Si(s) - Mg2Si(s)	{5.11}	4, 7
	2Mg(l) + SiC(s) - Mg2Si(s) + C(s)	{5.12}	4
SiC	4AL + 3C, - Al4C ,	{5.13}	4
	4Al(l) + 3SiC(s) - Al4C3(s) + 3Si(s)	{5.14}	4
	3Mg(l) + Al2O3(sl - 3MgO(s) + 2Al(l)	{5.15}	2, 12
AlA	3Mg(l) + 4Al2°3(s) - 3MgAl2O4(sl + Al(l)	{5.16}	11
	Mg°(s) + Al2O3(sl - MgAl2O4(sl	{5.17}	12
ai(P°3)3 (binding agent)	9Mg + Al(PO3)3 - 9MgO + Al + 3P	{5.18}	13
N2	3Mg(g) + N2(g) - Mg,N,	{5.19}	14
(protective gas)	Mg3N2 + 2Al(l) - 2A1N + 3Mg	{5.20}	14
Interfacial reaction products
The interfaces between magnesium-matrix and reinforcements are not thermodynamically stable thus some interfacial reaction products can be formed (Table 2] as a result of the chemical reactions.
Reactions at the Mg/SiC interface Magnesium and its alloys reinforced with the SiCp are very interesting because the reinforcement may lead to significant improvement of stiffness and strength.[2] Reaction products at the magnesium/SiC interface depend on the manufacturing method of the composite. Kaneda and Choh[15] found that the MgO and Mg2Si reaction products were formed at the pure magnesium/SiCp interface. The feature of this study was previous mixing of the SiO2 powder infiltration agent with the SiCp reinforcement which is necessary for spontaneous infiltration phenomenon. The Mg-RE3 alloy wets the SiCp well, therefore, in this composite the RE3Si2 interfacial reaction products were formed in the form of the needles[16] or thick reaction layer composed of the MgO, and Ce3Si2 fine particles.[17] On the other hand, the interfacial reaction products were not observed in the composites with the pure magnesium, Mg-Al5, Mg-Al8, and Mg-Zn6 matrices reinforced with the SiCp and prepared by the melt stir techni-
que[7, 17 18]. Also, Cao et al.[19 20] did not find the interfacial reaction products in the Mg-Zn4, and Mg-Zn6 alloys reinforced with the SiC nanoparticles and prepared by the ultrasonic cavitation.
The AZ80/SiCp and AZ91/SiCp interfaces were without reaction products when the composites were prepared by the compocast-ing. Nevertheless, the particles of the Al12Mg17, and Cu5Zn8 compounds precipitated on the SiCp[21, 22], indicating that the SiCp acted as nu-cleation sites. Similarly, the Mg(Cu, Zn]2, and MgZn2 compounds precipitated at the SiCp in the ZC63 - SiCp composite prepared by the melt infiltration into the powder and the melt stir technique. In the ZE63 - SiCp composite, which was prepared by the same procedure, the ZrO2, and CeO2 interfacial reaction products were formed.[8] Further Wang et al.[4] found the Al4C3, MgO, and Mg2Si reaction products at the AZ91/SiCp interfaces when the composite was prepared by the melt stir technique. Directly at the reaction layer the carbon was present as a product of a chemical reaction between the magnesium and SiCp {5.12}. Magnesium does not have stable carbides but the aluminium, as an alloying element in the magnesium alloys, reacts with the carbon, and then the Al4C3 carbide can arises {5.13}. Also in this case, the SiCp acted as heterogeneous nucleation sites for Al12Mg17 and Al8Mn5 compounds. When the
Table 2: Interfacial reaction products formed at the interface between magnesium-matrix and different types of reinforcements
Matrix	Reinforcement	Interfacial reaction product
Mg		MgO, Mg2Si
Mg-RE3		MgO, RE3Si2 or Ce3Si2
ZE63	SiCp	CeO2, ZrO2
AZ91		Al4C3, MgO, Mg2Si, MgAl2O4, AlN
AZ91	SiCw	MgO
Mg		MgAl2O4, MgO, Mg2Si
AZ91		MgO
ZE41	Al2O3f	MgO
AS21		MgO
AE44		MgO
AZ31	SiO2cf	MgO, Mg2Si
AZ61	SiO2nano-p	MgO, Mg2Si
AZ31	SiO2-Al2O3cf	MgA^4, Mg2Al4Si5O18, Si, Mg2Si
AZ91	SiC-SiO2-C-Sicf	MgO, Mg2Si
AZ91-SiCp composite was prepared by the ultrasonic cavitation, the interfacial reaction products did not form.[23] Wu et al.[24] investigated the interfaces between the AZ91 alloy and SiC whiskers (SiCw) in the composite prepared by the squeeze casting. They determined the MgO interfacial reaction products, while Zheng et al.[25] did not find any interfacial reaction products in the same composite. When the Al(PO3)3 binding agent was added into the SiCw-preform, the MgO interfacial reaction products formed.[13] In the AZ91 - SiC nanoparticles composite prepared by the ultrasonic cavitation, Lan et al.[26] found the Mg2Si interfacial reaction products, which were broken away from the AZ91/SiC nanopar-ticles interfaces because of the intensive ultrasonic cavitation.
The MgO, MgAl2O4, and AlN interfacial reaction products and the Al12Mg17 compound were formed in the composite prepared by the melt infiltration of the AZ91 alloy into the premixed powder of the magnesium, aluminium, zinc, and SiC.[27, 28] The AlN reaction layer, which also contained magnesium, is the product of a chemical reaction between the Mg3N2 and aluminium {5.20}. The Mg3N2 layer around the particles of the powder was formed with reaction {5.19} between the magnesium and nitrogen, which was used as a protective gas.[14] The MgAl2O4 interfacial reaction product formed in the composites with the aluminium- matrix reinforced with the SiC, and Al2O3 when the magnesium content in aluminium was smaller than the mass fraction of Mg 4 % or 2 %.[9' 28] The reactions {5.6}, {5.7}, and {5.8} did not take place because of the large chemical affinity of the magnesium to oxygen and the large content of the magnesium in the magnesium - SiCp composites.
Reactions at the Mg/Al2O3 interface The composites, where the magnesium and its alloys are infiltrated into the reinforcing preform of the Al2O3 fibres (Al2O3f), are most often prepared by the squeeze casting[29-35]. The preform of the Al2O3f contains 3-4 % of the SiO2 binding agent.
Rehman et al.[36] investigated the matrix/fibre interactions in the composites with the pure magnesium, AZ61, and AZ91 matrices reinforced with the different Al2O3f. A few large
Mg2Si particles were found in the pure magnesium reinforced with the S-Al2O3f (Safimax) with standard density. In the case of the reinforcing with the n-Al2O3f (Safimax) with low density, the fibres were reduced into the MgO and aluminium {5.15}. The fine MgO interfacial reaction products were observed in the AZ91 - S-Al2O3f (Saffil) composite. It is viable that increasing the aluminium content in the magnesium matrix may reduce the interfacial reactions. Also, Hach[37], Page[38], Hallstedt[39], Trojanova[40] and Sklenicka[41, 42] found the MgO particles at the matrix/fibre interfaces in the Mg - a-Al2O3f, Mg -S-Al2O3f (Saffil], ZE41 - a-Al2O3f, AS21 - S-Al2O3f (Saffil], and AZ91 - S-Al2O3f (Saffil) composites. The sizes of these particles were higher at the ZE41/a-Al2O3f interfaces than at the Mg/a-Al2O3f interfaces.[38] They were further increased by increasing casting temperature[43] and longer reaction times.[44] The presence of the spread MgO interfacial reaction layer in the AE44 - Al2O3 short-fibres (Saffil) composite has been reported also by Hu et al[45] Besides, they have also found the Al2RE particles. Similarly to the SiCp also the Al2O3f acted as nucleation agents because the p-Al12Mg17 compound at the AZ91/S-Al2O3f (Saffil) interfaces and the Al2Nd, Mg(Ag)12Nd, and Mg3Ag compounds at the QE/ S-Al2O3f (Saffil) interfaces were precipitated.[42] Shi et al.[12] found that in the Mg - Al2O3f composite the MgAl2O4 interfacial reaction product was formed, probably with reaction between the MgO and Al2O3 {5.17}. It should be noted that, in this study, the magnesium or aluminium powder was added into the preform of the Al2O3f and that the reaction time was 4 h at the temperature of 1123 K. Also in the study of wettability of the a-Al2O3f with pure magnesium the MgAl2O4 interfacial reaction product was found in addition to the MgO.[46-48] This shows that the MgAl2O4 reaction product is formed after very long reaction times.
Reactions at the Mg/SiO2 interface The SiO2 is seldom used as a reinforcing phase in the form of the particles or in any other form. Most often it is used as a binding agent by the manufacturing of the reinforcing preform of the Al2O3 fibres.
The melt is infiltrated into the pores and struts of the ceramic foam at the manufacturing of
interpenetrated phase composites. The Mg2Si, and MgO reaction products and Al12Mg17 compound were formed in the pores and struts of the SiO2 ceramic foam, which were filled with the AZ31 matrix.[49] Lee et al.[50] incorporated the SiO2 nanoparticles into the AZ61 matrix by the friction stir processing. The SiO2 nanoparti-cles reacted with the magnesium {5.3} and the Mg2Si, and MgO reaction products were formed.
Reactions at the Mg/SiC + Al2O3 + SiO2 interface
The reinforcing phase can consists of two or more carbides or oxides in the different preforms, e.g. the ceramic foam (cf). Zeschky et al.[11] found at the AZ31 /SiO2-Al2O3cf interface the MgAl2O4 {5.16}, and Mg2Al4Si5O18 {5.9} reaction products and the silicon, which further reacted with the magnesium and the Mg2Si {5.11} was formed. In the AZ91 - oxidized SiC-SiO2-C-Si(cf) composite, at the interfaces the MgO, and Mg2Si reaction products, into the struts of the ceramic foam very small content of the MgO, and in the centre of filled pores of the ceramic foam the MgO, Mg2Si, and y-Al12Mg17 were formed. In the case of reinforcing the AZ91 matrix with the non-oxidized SiC-SiO2-C-Sicf, the cracks and debonding free interfaces were obtained between the metal and ceramic skeleton.[10]
Conclusions
During the manufacturing of the magnesiummatrix composites, the strong bonding between the matrix and reinforcement, without the reaction products at the interfaces should be attained. However, most of the observed interfaces in the magnesium-matrix composites were covered with the interfacial reaction products. This means that the systems magnesium alloy - reinforcement (SiC, Al2O3, and SiO2) were thermodynamically unstable. The main interfacial reaction products were the MgO, and Mg2Si. Their size increased with increasing casting temperature and longer reaction time while the increasing the aluminium content into the magnesium matrix reduced the interfacial reactions. The types of the interfacial reaction products were also depended upon the
manufacturing method. Therefore, in order to obtain adequate properties of the magnesiummatrix composites, it is necessary to choose the appropriate combination of the constituents and suitable manufacturing process.
Acknowledgement
This work was partly financed by the Slovenian Research Agency (ARRS), projects 1000-09310152, and L2-2269.
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