Paper
Nanoparticle Formation in Magnesium Based Composite by Mg/Al2O3 Interfacial Reaction
2016 年 63 巻 7 号 p. 555-558
詳細
2016 年 63 巻 7 号 p. 555-558
Magnesium based composite containing 10 vol% Al2O3 particle was produced by a mechanical milling (MM) and spark plasma sintering (SPS) process and the nanoparticle formation by Mg/Al2O3 interfacial reaction was investigated in detail. The microstructural observation of the MM powder and SPS compact was carried out by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). TEM / EDS analysis and XRD result reveal that the nano Mg17Al12 and MgO phases are formed by Mg/Al2O3 interfacial reaction. The solute oxygen by Mg/Al2O3 interfacial reaction and magnesium of the matrix may immediately produce MgO, because magnesium barely dissolves oxygen. Therefore, many MgO particles are formed at the Mg/Al2O3 interface. On the other hand, magnesium can dissolve aluminum to some extent and the aluminum over the solubility limit may bond to magnesium, resulting in the formation of Mg17Al12.
Magnesium composites containing more than 10 vol% Al2O3, which are fabricated via mechanical milling (MM) followed by spark plasma sintering, demonstrate a good mechanical properties1–4). The authors reported that the Mg compact with more than 10 vol% Al2O3 consists of an Mg matrix, fine Al2O3 particles, nano MgO particles, and nano Mg17Al12 phase5). The hardness of the Mg/Al2O3 composite with nano MgO and Mg17Al12 increases compared to the composite MM powder containing the Mg and Al2O3. The hardness increase is attributed to the formation of the nano Mg17Al12 and nano MgO particles. However, the formation mechanism of nano MgO and Mg17Al12 phase is not yet clear. Therefore the formation mechanism of nano MgO and Mg17Al12 from Mg and Al2O3 phases is discussed in detail.
Commercially pure Mg powder (99.5 mass% purity; Kojundo Chemical Laboratory Co., Ltd.) and α-Al2O3 powder (99.9 mass% purity; Kojundo Chemical Laboratory Co., Ltd.) were used in this study. The initial particle sizes of the pure Mg and Al2O3 powders were approximately 180 and 0.3 μm, respectively. Mechanical milling was applied to these powders using a planetary ball mill (P-6; Fritch Co., Ltd.) with an SKD11 vial and SUJ2 steel balls in Ar atmosphere at room temperature. Mixed powders consisting of pure Mg and 10 vol% Al2O3 were mechanically milled for 173 ks at a rotation speed of 400 rpm. The composite MM powders containing the pure Mg and Al2O3 were then sintered using a spark plasma sintering (SPS) apparatus (SPS-510L; Sumitomo Coal Mining Co., Ltd.) at 713 K and 40 MPa for 0.6 to 7.2 ks subsequently to be sintered at 773 K and 40 MPa for 1.2 ks. The microstructural observation of the MM powders and SPS compacts containing Mg and Al2O3 was achieved using scanning electron microscopy (SEM; JSM7001FD; JEOL Ltd.), X-ray diffraction (XRD; Smart Lab; Rigaku Corporation), and transmission electron microscopy (TEM; JEM2100F; JEOL Ltd.)/energy dispersive X-ray spectroscopy (EDS). The samples for SEM and TEM analysis were prepared using an ion-beam polish. XRD profiles were measured under the condition of diffraction angle of 20–90 degree, tube voltage of 40 kV and tube current of 40 mA.
Fig. 1 shows the XRD profiles for the MM powder and the SPS compacts sintered at 713 K for 0.6 ks to 7.2 ks. Fig. 1 (a) corresponds to the profile of the MM powder. Figs. 1 (b), (c), (d) and (e) correspond to the profiles of the SPS compacts sintered at 713 K for 0.6, 1.8, 3.6 and 7.2 ks, respectively. The MM powder consists of Mg and Al2O3 phases, while the SPS compacts consist of Mg, Al2O3, MgO and Mg17Al12 phases. These MgO and Mg17Al12 phases are formed via the SPS process. The Mg17Al12 profile is more marked with increasing sintering time as shown in Figs. 1 (b), (c), (d) and (e).
XRD results for the MM powder (a) and the SPS compacts (b, c, d, e) of the Mg composites with 10 vol% Al2O3. The profiles of (b), (c), (d) and (e) correspond to the SPS compacts sintered at 713 K for 0.6, 1.8, 3.6 and 7.2 ks, respectively.
Fig. 2 shows the SEM micrograph of the SPS compact sintered at 713 K for 7.2 ks. The fine Al2O3 particle is distributed homogeneously within Mg matrix. It is noted that the different contrast is observed in an interface between Mg matrix and Al2O3 particles. It is difficult to find the MgO and Mg17Al12 particles in SEM observation.
SEM micrograph of the SPS compact of the Mg composite with 10 vol% Al2O3.
Fig. 3 shows the TEM micrograph and its EDS results in the SPS compact sintered at 713 K for 7.2 ks. Fig. 3 (a) shows the micrograph in the vicinity of the Al2O3 particle and Figs. 3 (b), (c) and (d) show the EDS elemental mapping result for Mg, Al and O, respectively. A nano particle region with the size of about 10 nm in diameter is observed in the interface between Mg matrix and Al2O3 particle as shown in Fig. 3 (a). The size of this Al2O3 particle including the nano particle region is about 160 nm as shown in Fig. 3 (a). The size of the region without Mg and with Al is about 120 nm as shown in Figs. 3 (b) and (c), while the size of the region with O is about 160 nm as shown in Fig. 3 (d). This EDS result indicates that the nano particle region between Mg matrix and Al2O3 particle consists of Mg and O. Therefore, it is revealed that many MgO particles are formed at the interface between Mg matrix and Al2O3 particle.
TEM micrograph (a) and EDS elemental mapping results (b, c, d) in the vicinity of the Al2O3 particle. The EDS elemental mapping result of (b), (c) and (d) correspond to Mg, Al and O, respectively.
Fig. 4 (a) shows the TEM micrograph of a precipitate in Mg matrix at several hundred nm from Al2O3 particle and Fig. 4 (b) shows the results for the EDS analysis of a horizontal line of (a). This precipitate with a habit plane is nano particle of the size of about 20 nm. The EDS profiles of Fig. 4 (b) indicate that aluminum concentration increased and magnesium concentration decreased at the position of the precipitate of Fig. 4 (a), whereas the oxygen concentration remained unchanged at this position. The ratio of Mg to Al in this precipitate is roughly similar to that of Mg17Al12 as shown in Fig. 4 (b). Therefore, it is concluded that this precipitate corresponds to Mg17Al12 phase.
TEM micrograph (a) and EDS line analysis results (b) for the precipitate in the Mg matrix.
Many MgO particles are observed at the interface between Mg and Al2O3, while the Mg17Al12 phase is observed at the position of several hundred nm from the Al2O3 particle. These results indicate that the solute oxygen by the Mg/Al2O3 interfacial reaction and magnesium of the matrix may immediately produce MgO at the Mg/Al2O3 interface, because magnesium barely dissolves oxygen. Therefore, many MgO particles are formed at the interface between Mg matrix and Al2O3 particle. On the other hand, magnesium can dissolve aluminum to some extent and the aluminum over the solubility limit may bond to magnesium. As the result, the Mg17Al12 phase is formed at the position of several hundred nm from the Al2O3 particle. The schematic illustration for the mechanism of formation of the MgO and Mg17Al12 is shown in Fig. 5. Such an interfacial reaction would occur as follows: 1,2,5)
Schematic illustrations for the formation mechanism of the MgO and Mg17Al12 by Mg/Al2O3 interfacial reaction.
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Magnesium composite containing 10 vol% Al2O3 was fabricated via mechanical milling followed by spark plasma sintering. The Mg/Al2O3 composite consists of Mg matrix, fine Al2O3 particles, nano MgO particles and some Mg17Al12 precipitate. Such MgO and Mg17Al12 are formed by the interfacial reaction between Mg matrix and Al2O3 particle. The MgO with the size of about 10 nm in diameter is formed at the interface between Mg and Al2O3, while the Mg17Al12 with the size of about 20 nm in Mg matrix is formed at the position of several hundred nm from the Al2O3 particle.
