New Al–AlN composites fabricated by squeeze casting: interfacial phenomena

Composites: Part A 33 (2002) 1421–1423 www.elsevier.com/locate/compositesa

New Al– AlN composites fabricated by squeeze casting: interfacial phenomena J. Vicens*, M. Che´dru**, J.L. Chermant LERMAT-ISMRA, FRE CNRS 2149, 6 Boulevard Mare´chal Juin, 14050 Caen Cedex, France

Abstract Three Al – AlN composites (2024, 6060, 5754 with , 45 vol% AlN) fabricated by squeeze casting were studied by TEM. Chemical reactions occurring at the matrix –AlN interfaces have been investigated. MgAl2O4 spinel crystals were found in 6060 and 5754 composites. The magnesium element of the matrix reacts with a very thin alumina layer which is deposited on the AlN surfaces during the liquid infiltration step. The 5754 composites exhibited a stronger reaction leading to the formation of MgO phases with the spinel. Degradation of the mechanical properties was clearly shown in that case. A faceting of the AlN surfaces was observed in all composites. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Metal-matrix composites (MMCs); B. Interface; AlN

1. Introduction A few studies have been performed on Al composites reinforced by AlN particles fabricated by melt infiltration of porous preform [1,2]. The Al– AlN system is expected to be favourable because aluminium does not react with AlN in contrast to Al –SiC composites for example. The Al4C3 phase in Al –SiC composites induces degradation of the mechanical properties. In Al-based composites reinforced by Al2O3 platelets fabricated by melt infiltration at low pressure [3], interfacial reactions were also observed at the platelet surface. This paper reports the observations of interfacial phenomena in as-produced Al– AlN composites fabricated by squeeze casting. Their incidence upon the mechanical behaviour was investigated by compressive tests on asproduced and heat treated specimens.

reactions is the spinel precipitation at the surface of the AlN particles. This was identified by EDX and electron diffraction patterns and clearly demonstrated in the AlMgSi0.5-composites. An example is presented in Fig. 1a where an AlN grain is decorated by spinel precipitates. An enlarged part of the interface is shown in Fig. 1b. MgAl2O4 crystals are faceted along {111} planes. The oxygen source for the spinel formation arises from the thin alumina layer which is deposited on AlN surfaces during the melt infiltration process. This very thin and probably amorphous layer has not been observed in HREM in Al –AlN composites. This mechanism has been already described in Al– Al2O3 composites [3] where an amorphous alumina layer (0.8 nm) was imaged at the alumina platelet interface in 6061-alumina composites. Two chemical reactions can explain the spinel formation: 3Mg þ 4Al2 O3 ! 3MgAl2 O4 þ 2Al

2. Results

and

The melt infiltration of a particulate preform was carried out using aluminium alloys 2024, 6060 and 5754 [4,5]. The compositions of the different matrices will be denoted as AlCu4Mg1, AlMgSi0.5 and AlMg3, respectively. The most peculiar result concerning interfacial

3Mg þ Al2 O3 ! 3MgO þ 2Al

* Corresponding author. **Present address: ISMANS, Le Mans, France.

ð1Þ

ð2Þ

In AlMgSi 0.5-composites, MgO phase was not observed and spinel formation is due to the reaction (1) between the magnesium element of the matrix and the alumina layer. Fig. 1b also reveals that the AlN crystal is strongly faceted with two main facets parallel   Faceting was not to the pyramidal {1011} and {101 1}:

1359-835X/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 8 3 5 X ( 0 2 ) 0 0 1 5 9 - 8

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Fig. 3. AlMgSi0.5 –AlN composite after T4. Faceted AlN interface exhibits Mg2Si precipitates.

Fig. 1. AlMgSi0.5 –AlN composite, as-produced state: (a) general view of a faceted AlN particle decorated by several MgAl2O4 crystals; (b) enlarged view of the Al–AlN interface. The facet indices have been noted in AlN and MgAl2O4.

observed along the lowest dense plane of the AlN wurtzite structure, i.e. the (0001)AlN basal plane as shown in Fig. 2. Therefore it can be deduced that a selective attack of AlN by the molten alloy did occur during the process. A more pronounced dissolution of AlN seems to take place for the high surface energy facets like {101 ‘}: As a consequence a lower surface energy can be achieved by means of a faceting mechanism along the  and ð101 1Þ  planes in case of Fig. 1. The melt ð1011Þ infiltration temperature (, 1123 K) makes this mechanism possible because solid state reactions between MgAl2O4 and AlN were observed from , 1273 K [6]. Similar considerations could be involved to explain faceting of

Fig. 4. AlCu4Mg1 –AlN composite as produced state. Example of a faceted Al –AlN interface.

Fig. 5. AlMg3 –AlN composite after a long term annealing treatment (823 K, 10 days). The sample has been tested by a compressive test and prepared for TEM. Microcracking took place mainly at the interface between two AlN grains where MgO and MgAl2O4 precipitations had occurred as confirmed by EDX.

Fig. 2. AlMgSi0.5 – AlN composite, as-produced state. The Al – AlN interface is parallel to the (0001) basal plane. This interface is not faceted and MgAl2O4 precipitation is observed.

the AlN particulates in AlMgSi0.5-composites after T4 heat treatment (Fig. 3) and in the as-produced AlCuMg1composite (Fig. 4). The precipitate imaged at the AlN interface in Fig. 3 was identified to Mg2Si. The AlMg3composite contains the highest amount of magnesium. After annealing at 573 K and 823 K (10 days), changes in the mechanical behaviour were observed. The yield stress and the compressive strength are lower (, 30%) than in the as-produced state. A decrease of the Young’s modulus measured by the ultrasonic method is about 15% compared to the value in the as-cast (, 150 MPa). TEM observations of annealed and compressed specimens reveal microcracking located at the Al – AlN

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interfaces where MgO and MgAl2O4 intergranular phases were found by EDX (Fig. 5).

interfaces was also observed. This was explained by surface energy consideration.

3. Conclusion

References

Interfacial reactions have been found in Al – AlN composites. Alumina enriched surfaces of AlN particles reacted with the magnesium element of the Al alloys. This gives rise to the crystallization of numerous spinel crystals at the AlN interfaces. This can be detrimental for the mechanical behaviour if the Mg content is more than 3% as in the AlMg3-composite. A selective attack of AlN by the molten alloys was found. As a result, faceting of AlN

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