AZ91 magnesium alloy composite

Sctipta Materialia, Vol. 35, No. 4, pp. 529-534, 1996 Elsevier Science Ltd Copyright 0 1996 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359-6462/96 $12.00 + .OO

PII S1359-6462(96)00169-8

1:NTERFACIAL REACTION IN SQUEEZE CAST SICW/AZ91 MAGNESIUM ALLOY COMPOSITE Kun Wu, Mingyi Zheng, Min Zhao, Congkai Yao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P.R. China

Jihong Li Laboratory of Atomic Imaging of Solids, Institute of Metal Reseach, Academic Sinica, Shengyang 110015, P.R. China (Received January 3 1, 1996) (Accepted April 4, 1996) Introduction

It is well known that the interface between the matrix and the reinforcement plays an important role in the properties of metal matrix composites (MMCs) [ 11. Interfacial structure and its effect on the properties of aluminum matrix composites have been extensively studied [2-41. Compared Iwithaluminum matrix composites, only a few studies have been reported for magnesium matrix composites. Magnesium alloys reinforced with discontinuous phases, e.g. short fibers [5-61, particles [7-81 or whiskers [9] exhibit advantageous properties such as low density, high specific strength, high specific stiffness, high wear resistance and low coefftcient of thermal expansion, showing that magnesium matrix composites have increasingpotential in automotive, high perhormancedefence and aerospace application. The interface of Mg-MMCs is very different from that of Al-MMCs, due to high reactivity of magnesium. As magnesium have natural affinity for wetting or bonding to ceramic reinforcement, and furthermore, magnesium does not form any carbides, Sic is one of the most suitable reinforcement for magnesium matrix composite, SiCw/AZ9 1 magnesium matrix composites have excellent properties [9] compared with other discontinuously reinforced Mg-MMCs. The aim of the present study is to investigate the interfacial reactions in SiCw/AZ91 composites prepared by squeeze casting method, with particular emphasis on fhe morphology, structure and the formation mechanisms of the reaction products using electron microscopy, in order to provide a better understanding of the relationship between the interfacial structure and the properties of the composites. ExDerimental

and Material

The composites used in this investigation was fabricated by squeeze cast under COdSF, atmosphere. The matrix alloy is commercial heat-treatable AZ91 magnesium alloy (8.5-9.5%Al, 0.45-0.90°/aZn, 0.15-0.3O%Mn, 0.2O%Si, O.Ol%Ni, balance Mg), and the reinforcement is p-Sic whisker whose volume fraction in preform is 20% without binder.

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Microscopy specimens were sectioned from as-cast composites and ground into 0.3mm thick plate. Disc specimens of 3mm in diameter were dimpled with Gatan dimpler, then were ion milled to perforation using a liquid nitrogen cold stage to prevent the heating during the ion milling process. The interface of the composite was examined in detail by Philips EM420T analytical transmission microscope and JEOL2000EX-II high resolution transmission microscope. Results and Discussion

Figure l(a) is a typical TEM micrograh of the interface in as-cast SiCw/AZ91. It can be seen that there are small particles at the SiCw surface. Figure l(b) is the interface morphology at the higher magnification, showing the presence of discrete nanocrystalline particles with a size of 20nm or less at the SiCw/AZ91 interface. They grow from the Sic whisker surface into the magnesium matrix, and most of them have angular shapes. It is considered that these particles may be interfacial reaction products or the adhesion on the Sic whisker surface. Some researches [IO-l l] have reported that the adhesion on the p-SiCw surface usually are the variants of Sic, e.g.a-Sic. Figure 2(a) presents the interface morphology as viewed from the whisker growth axis [ 1lT] and Figure 2(b) shows the selected area diffraction patterns (SADPs) of the interfacial phase and the whisker. It can be seen that the interfacial phase has the same orientation relationship with SiCw. It has f.c.c. structure and a crystalline constant of 0.42nm which is slightly smaller than that of SiCw (a = 0.44nm), consistent with that of MgO (a = 0.42nm). Figure 3(a), (b) and (c) show the EDAX results for the matrix, the whisker-matrix interface without interfacial phase and the interfacial nanocrystalline particle, respectively. It can be seen that the nanocrystalline particle is a phase containing Mg, since the size of electron beam used is 50nm which is larger than the size of the interfacial nanocrystalline particles (usually less than 20nm), Al peak from matrix and Si peak from SiCw also appear. It can also be seen that Al atoms segregate to the whisker-matrix interface without interfacial phase. Considering the various phases which can possibly be present at the interface, such as a-Sic, Mg,Si, Mg,,A1,2, MgO, MgAI,O,, Al&, on the basis of composition and structure analysis, it can be identified that the interfacial phase is MgO. Figure 4 is the HREM image of area R in Figure l(b), showing a nanocrystalline particle present between SiCw and matrix alloy. Two sets of basic lattice planes with about 0.24 nm spacing at 70.5”C angle to each other can be seen in the nanocrystalline particle. It can be inferred that the nanocrystalline particles at interface might be of f.c.c. structure and orientated along [ 1011direction with the crystal constant of 0.42nm, which further confirmed the formation of MgO (a = 0.42nm) at the whisker-matrix interface.

Figure 1. TEM micrographs of the interfacial reaction products in the cast composite: (a) low magnification and (b) high magnification.

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Figure 2. TEM micrographs of interface in the cast composite: (a) triangle transverse section whisker and (b) SADP of the interface.

It can be seen from Figure 4 that MgO grows along the SiCw surface, without forming the voids at the whisker-matrix interface. MgO is perfectly bonded to the whisker and there is no amorphous transition layer at the surface of the whisker. It is shown from many HREM images of the interface that MgO have the almost same morphology. With the growth direction of ~11 l> and <200>, MgO grows preferably along the Sic-h4g interface which may be due to high energy of Sic-Mg interface. Growth of MgO along interface may reduce the energy.

Figure 3. EDAX spectrum for (a) the matrix, (b) the whisker-matrix interface without interfacial phase and (c) the interfacial nanocrystalline phase.

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The following crystallographic orientation relationship can be obtained from SADP and HREM image: {lll}MgO

I( {lll}SiCw

MgO I(SiCw The fact that there exists an exact orientation relationship between the two phase indicates good lattice matching between the two phases along the interface. The {1 1 1 } planes of MgO and the {1 11} planes of SiCw bond directly with little strain along the interface as shown in Figure 5. The mismatch between the two lattice planes, the ( 1111 planes of p-Sic (0.25nm) and the ( 111) planes of Mg (0.24nm), is about 4% with the former as a reference. The MgO formation as an interfacial reaction product in Al,O,JMg composite had already been studied [ 121, with the reaction: Al,O,(s) + 3Mg(l) - 3MgO. Nutt [ 131 found that small crystalline MgO were distributed singly and in cluster along Al-Sic interface, they also concluded that the abundance of MgO at Al-Sic interface may be attributed to the reduction of A&O, during high temperature processing of the composites. In the present composite system, it can be seen that MgO was formed during the fabrication of the composites by chemical reaction of Mg with oxygen and /or other oxides, so the presence of oxygen will play an important role in the formation of MgO. The melting and pouring processes were conducted under the C02/SF6 atmosphere, so the content of oxygen in the molten alloy was very little. It can be concluded that the possible supply of oxygen was from the oxygen absorbed on the surface of the whisker. Since the ability for magnesium to form the oxide is about 10’ times higher than that of aluminum at any temperature [14], and the content of oxygen in the composite system was very little, it appears that MgO formed immediately during the fabrication process.The morphology of MgO did not change appreciably during the following solution treatment (380%/2h, 415W24h) and aging treatment (175%/70h), which further confirms the above conclusion. Figure 5 shows the interfacial reaction products formed at the surfaces of a uneven whisker. It can be seen that the amount and size of MgO are larger on the bent whisker surface and the surface with stacking faults or twins than on the flat whisker surface and the surface without defects. As mentioned above, the absorption of oxygen to the whisker surface played a key role in the interfacial reaction. It was associated with the structure of the whisker surface, the higher the surface energy, the more oxygen absorbed on the surface. So more and larger reaction products could form on these high energy surface, such as the bent surface and the surface with crystal defects. Although the whisker did not make a direct contribution to

Figure 4. HREM image of area R in Figure l(b). The incident beam is along the [IOT] axis of the whisker.

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Figure 5. The inter-facial reaction products on the different sites of the uneven Sic whisker surface. Figure 6. Tensile fractograph of the cast composite.

the interfacial reaction (because Sic did not take part in reaction), the structure of the whisker surface had a significant i:nfluence on the amount and size of the reaction product. In the fabrication process of magnesium matrix composites, the inter-facial reaction can be modified by the quality of the whisker. The discrem nanocrystalline MgO may be beneficial to the interfacial strengthening. Figure 6 shows the fracture surface of the composite, few evidence of whisker/matrix debonding and whisker pull-out can be observed, indicating a strong whisker/matrix bond. Conclusion

Interfacial reaction occurred at the Sic whisker-matrix interface during the fabrication of the composites. The interfacial reaction product is nanocrystalline particle MgO and it is formed by the following reaction: 2Mg(l) + O,(g)1- 2MgO(s), and/or Mg(1) + 0 - MgO(s). The absorption of oxygen on the SiCw surface supplies the oxygen for the reaction. The crystallographic orientation relationship between MgO and SiCw are {1 ll}MgO II { 111)SiCw and MgO 11
This work was supported by the National Natural Science Foundation of China. The authors would like to thank Dr.W.D.Fei for stimulating discussions. References 1. 2. 3. 4. 5. 6. 7. 8.

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