Performances of metal matrix composites (MMCs) rely strongly on the distribution of particles within the metal matrix but also on the chemical reaction which may occur at the liquid-solid interfaces. This paper presents the chemical reaction between aluminum based particles Al2O3 and Al2O3-AlOOH with magnesium alloys matrixes AZ91 and EL21, respectively, and studies the microstructure of these reinforced composites. Different methods such as transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and XRD were used to highlight these chemical reactions and to identify products. Results demonstrate the formation of MgO particles within the matrix for both composites and also the dissolution of aluminum in the eutectic region in the case of EL21.
Metal matrix composites have been catching industrials’ attention due to their mechanical properties. Their potential importance has increased in a variety of fields such as aerospace and automotive industries where the increasing fuel price is leading to the necessity to overcome the weight reduction issue, but they also find applications in electronic industries. Due to their low density, aluminum and magnesium alloys are very interesting as matrix in MMCs. Through the addition of submicron-sized particles it is possible to achieve an enhancement of properties with respect to base alloys, such as the yield strength, ultimate tensile strength, hardness, and stiffness. Based on total strengthening effect models [
There are various processing methods to elaborate MMCs such as liquid state processing, spray deposition techniques, and solid state processing routes. Each process has its own advantages, drawbacks, and applications. Besides, the processing route impacts directly on the mechanical properties and microstructures of these composites [
In the last years, many papers have been published on magnesium-matrix composites containing aluminum oxide particles, demonstrating some interesting properties of these composites regarding mechanical properties [
The aim of this work is to study the chemical interaction between aluminum oxide-based particles and the magnesium alloy matrix and to characterize the microstructure of AZ91 and EL21 alloys reinforced with such particles.
Materials were produced at the Helmholtz Zentrum Geesthacht (HZG), Germany. Chemical compositions of the AZ91 and Elektron 21 (EL21) Mg-alloy are listed in Tables
Chemical composition (in wt.%) of AZ91 Mg-alloy.
Al | Zn | Mn | Si | Fe | Cu | Ni | Mg |
---|---|---|---|---|---|---|---|
8.71 | 0.66 | 0.22 | 0.043 | 0.001 | 0.002 | 0.001 | Bal. |
Chemical composition (in wt.%) of EL21 Mg-alloy.
Nd | Gd | Zn | Zr | Mg |
---|---|---|---|---|
2.6–3.1 | 1.0–1.7 | 0.2–0.5 | Saturated | Bal. |
Microstructural characterization of both composites was carried out by transmission electron microscopy (TEM). The microscope was a JEOL JEM 2010 operating at 200 kV and equipped with a LaB6 filament. The incertitude of the EDS results obtained using this TEM is ±1%. Samples were prepared from 3 mm diameter disks (average thickness of 150
DSC reaction studies were carried out on a Perkin Elmer Pyris DSC 7 instrument, from room temperature at 700°C at the heating rate of 20°C/min in a flowing argon atmosphere. Steel crucibles were used to minimize the thermal effects linked to possible reactions between the magnesium alloy and the crucibles. The reaction was studied by mixing overnight powders of the metal and of the ceramic in a volume ratio 1 : 1 and then by cold pressing the powder mixture into a 3 mm diameter disc that was then analyzed with the DSC. All the operations concerning magnesium powders were carried out into a glove box, in order to minimize the oxidation of the magnesium.
X-ray diffraction (XRD) was also carried out to characterize both the starting powders and the product of the reactions after the DSC runs. A Panalytical X’Pert Pro MPD instrument was used, with CuKα radiation. Both aluminum oxide powders resulted in a mixture of several polymorphs, mainly α,
Figure
TEM BF picture of the microstructure of
(a) BF TEM picture of a eutectic region of
The detection of Al in the eutectic region does not however allow determining whether the Al detected comes from AlOOH or Al2O3. Nevertheless, the fact that strictly no Al2O3 particle could be detected neither in the matrix nor in the eutectic region suggests that Al detected in the eutectic region can come from both Al2O3 and AlOOH.
The absence of aluminum oxide particles coupled with the presence of elemental aluminum in the eutectic zone suggests that during the process aluminum oxide particles were not dissolved, but they underwent chemical reaction with magnesium to form MgO, as confirmed by the direct TEM observation of MgO particles, as shown in Figure
(a) BF TEM of MgO particle in
To better investigate the interaction between aluminum oxide particles and magnesium alloys, first thermodynamical calculation was performed. According to free Gibbs energies, the formation of MgO and MgAl2O4 is thermodynamically favorable as shown in the following reactions [
It is evident that the thermodynamics suggests that
In order to verify if the kinetics of these reactions is sufficient to guarantee the transformation of aluminum oxide into magnesium oxide, DSC measurements were carried out first on mixtures of pure magnesium and alumina and then on the mixtures of AZ91 or EL21 alloys with alumina. The results for pure Mg and EL21 alloy are shown in Figure
DSC of the reaction of pure Mg (blue) and Elektron 21 (red) with
From the DSC, it is evident that a strong reaction always occurs (the intense exothermic, down-heading peak), but the specific pathway of the reaction seems to change from the pure Mg case to the EL21 one. In the case of pure magnesium, an endothermic peak is observed at lower temperatures, corresponding to the Al-Mg eutectic melting. The reason for this different behavior is due to the fact that EL21 powders are rather coarse, while Mg ones are much finer. By changing the specific surface of the two powders, the contact surface between alloy and aluminum oxide changes, so that in the pure Mg case a significant interfacial reaction occurs before the main reaction peak. This reaction brings the formation of Al that dissolves in the Mg-alloy forming a Mg-Al alloy that melts at much lower temperature than pure Mg. Due to the low temperature, the interfacial Mg-Al2O3 reaction is relatively slow, so that the endothermic effect of the eutectic melting prevails on the exothermic effect of the reaction, and only an up-heading broad peak is observed. At higher temperature, the liquid Mg-Al alloy reacts strongly with the Al2O3 particles, causing the strong exothermic peak observed at 520°C.
In the case of EL21 alloy, the interfacial reaction is very small, due to the large size of Mg-alloy particles, and no evident thermal effect is observed. Only when the kinetics of the reaction is sufficiently fast, the reaction starts, and this brings a strong exothermic peak at 615°C that is so intense that covers also the endothermic melting peak of the Mg alloy.
The product of the reaction between Mg and Al2O3 was analyzed by XRD, as shown in Figure
XRD of the product of the reaction between pure Mg and
From the XRD analysis it is possible to observe the phases Mg, MgO,
Thus, the DSC and XRD analysis confirms that the reaction course for this composite material is correctly described by (
McLeod and Gabryel have studied the thermodynamic stability of several oxides with different magnesium concentrations [
The investigations of the AZ91 + Al2O3 lead to similar results. Whereas no alumina particle could be revealed in the Mg-matrix, MgO was observed. Also in this case, Al2O3 particles are hence proved to react in the melt, therefore leading to the formation of MgO which is shown in Figure
BF TEM of MgO particle in AZ91 + Al2O3 composite.
Our investigation demonstrates that incorporation of aluminum oxide particles in a molten Mg-based alloy leads to a systematic reaction of particles with the melt. For the EL21 alloy, the particle dissolution induces the segregation of Al to the eutectic region. On the opposite side, oxygen released by the reaction forms MgO particles inside the Mg-matrix. Moreover, the MgO particles identified in the TEM were systematically shown to range between several hundreds of nanometer and 1
The behavior of Al2O3 and
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research leading to these results has been carried out under ExoMet project under the European Community’s Seventh Framework Programme, Contract no. FP7-NMP3-LA-2012-280421.