Structural Characterization of Reaction Product

Solid State Phenomena Vols. 172-174 (2011) pp 1273-1278 Online available since 2011/Jun/30 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/SSP.172-174.1273

Structural Characterization of Reaction Product Region in Al/MgO and Al/MgAl2O4 Systems Rafal Nowak1,a, Natalia Sobczak1,2,b, Edmund Sienicki2,c, Jerzy Morgiel3,d 1

Foundry Research Institute, 73 Zakopianska St., Cracow, POLAND 2

Motor Transport Institute, 80 Jagiellonska St., Warsaw, POLAND

3

Institute of Metallurgy and Materials Science, PAS, 25 Reymonta St., Cracow, POLAND

a

b

c

d

[email protected], [email protected], [email protected], [email protected]

Key words: single crystals, MgO, MgAl2O4, redox reaction

Abstract. The reaction product region, formed between molten aluminium and MgO and MgAl2O4 single crystals of three different crystallographic orientations, was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) coupled with X-ray energy dispersive spectrometry (EDS). The Al/MgO and Al/MgAl2O4 couples were produced under ultra high vacuum at 800, 900 and 1000°C. The observations proved the redox reactions of Al with both MgO and MgAl2O4. Independently of crystallographic orientation of initial oxide single crystals, the reaction product region (RPR) was formed and it was built of oxide particles surrounded by continuous metallic phase. For Al/MgO couples, the RPR was composed of two layers, where in the first layer, the oxide phase was Al2O3 while in the second layer, the MgAl2O4 was identified. In the case of Al/MgAl2O4 couples, a single layer was distinguished and only the Al2O3 phase was recognized. Introduction Information on interaction in Al/MgO and Al/MgAl2O4 systems is of practical importance for understanding the reasons of degradation of MgO-rich refractories by Al-rich melts and for selecting suitable conditions for synthesis of metal-ceramic composites or for joining oxides to ceramics. This paper is focused on the analysis of structure, chemistry and phase composition of reactively formed interfacial regions in Al/MgO and Al/MgAl2O4 couples in order to identify the type of possible reactions and accompanying processes taking place at high temperatures. Experimental Procedure The Al/MgO and Al/MgAl2O4 couples were produced during the sessile drop wettability tests by heating of Al (99,999%) sample on MgO or MgAl2O4 single crystal substrates at 800, 900 and 1000°C for 60 or 120 minutes in a vacuum of 5×10-6 mbar [1,2]. MgO crystals were produced by arc melting while MgAl2O4 – by Czochralski method (MTI Corp., USA) [3]. The substrates were polished by the producer up to a roughness of 8 Ǻ. Structure and chemistry characterization of reaction product region in the solidified, cross-sectioned and mechanically polished Al/MgO and Al/MgAl2O4 couples was carried out by optical microscopy (OM) and scanning electron microscopy (JOEL 3036) coupled with energy dispersive X-ray spectrometry (EDS). Preparation of the thin foils for TEM was performed applying Quanta 3D (Fei). TEM lamellas from the particular location were obtained by Focused Ion Beam (FIB) technique. The TEM examinations were carried out using TECNAI G2 FEG super TWIN (200 kV) microscope equipped with High Angle Angular Dark Field (HAADF) detector and integrated with EDAX energy dispersive X-ray spectroscopy system.

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Solid-Solid Phase Transformations in Inorganic Materials

Results and discussion OM observations of cross-sectioned Al/MgAl2O4 couples revealed one layer of continuous reaction product region (RPR) formed inside the substrate under Al drop, particularly well distinguished in polarized light and showing a heterogeneous structure composed of small ceramic precipitates surrounded by thin metallic channels (Fig. 1), typical for C4 (Co-Continuous-Ceramic-Composite) structure reported in other reactive Al/MeO systems [4-11]. SEM+EDS analyses of RPR evidenced the presence of alumina and Al(Mg) phases (Fig. 2).

Fig. 1. Optical microscopy images of cross-sectioned Al/MgAl2O4(111) couple (1000°C, 60 min). Al drop

Al drop

3

MgAl2O4

Point 1 2 3 4 5 6

Mg

Al at.% 14 38 2 55 2 54 - 100 2 46 13 36

O

Phases

48 MgAl2O4 48 Al(Mg); Al2O3 44 Al(Mg); Al2O3 Al 52 Al(Mg); Al2O3 51 MgAl2O4

Fig. 2. SEM images and EDS analysis of cross-sectioned Al/MgAl2O4(111) couple (1000°C, 60 min).

For Al/MgO couples produced at 900 or 1000°C, the RPR is heterogeneous and composed of two layers schematically shown in Fig. 3. SEM+EDS analysis (Fig. 4) evidenced that the 1st layer, formed at the drop-side interface, is composed of large crystals with a strong directional alignment. Its structure presents two mutually interpenetrating and continuous networks, similar to that already reported in Al/MgAl2O4 couples [12]. The 2nd layer, formed at the substrate-side interface, looks to be more dense with less visible grain boundaries. Similar two-layered structure of RPR was reported recently in Al/SiO2 and Al/mullite couples [11] but their EDS analysis did not show any variations in chemistry despite well distinguished differences in the structure of these layers. Therefore, it was concluded that the layered structure is caused by optical effect due to dissimilar dispersion of the phases formed in two layers. In order to verify the same effect in Al/MgAl2O4 couple, its detailed EDS analysis was also done (Fig. 4). It showed that the 1st layer is built of Al2O3 crystals separated by metallic channels filled with Al containing up to 4 at.% Mg. However, Mg

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was not detected in the drop, and this fact can be explained by rapid evaporation under dynamic vacuum at high temperature. EDS analysis of the 2nd layer suggests that it consists of fine MgAl2O4 precipitates surrounded by metal enriched in Mg, since Mg content in this layer is twice higher than that of pure spinel.

Fig. 3. Scheme of layered structure of reactive product region in Al/MgO couples.

Fig. 4. SEM images and EDS analysis of cross-sectioned Al/MgO(100) couple (1000°C, 60 min).

For Al/MgO couple, TEM examinations showed a good agreement with SEM+EDS analysis since in the 1st layer, Al2O3 and Al(Mg) phases were identified (Fig. 5), while the 2nd layer was composed mainly of MgAl2O4 phase (Fig. 6). Based on available literature data [13-17] and our observations that are similar for all crystallographic orientations of oxide substrates, the following two mechanisms of the formation of C4 structure can be proposed according to two reaction paths (Table 1) schematically shown in Fig. 7, taking into account the thermodynamically unstable interfaces, marked by double backslash (//), particularly in the initial Al//MgO contact system. For the first path, the phase transformation starts from the redox reaction (1). Under applied conditions (high dynamic vacuum and high temperature), the freshly formed Mg evaporates and its continuous removal from the reaction front takes place by working turbomolecular pump resulting in the shift of reaction (1) towards the formation of Al2O3. For this reaction, the calculated modified Pilling-Bedworth' ratio (PBR* [7]) indicates that the solid product formed (Al2O3) has a 24.15% less molar volume than initial oxide (MgO), thus creating cracking in the Al2O3 layer and the formation of the network of channels. Since at 1000 °C liquid Al wets Al2O3, the channels are filled with liquid metal to form a ceramic-metal network. These channels play an important role in rapid transfer of Al and Mg to and from the reaction front, respectively. Moreover, the wider are the channels, the larger are Al2O3 crystals, as it is evidenced by structural studies. Consequently, after reaction (1), the initial Al/MgO interface is replaced by two interfaces, i.e. stable Al/Al2O3 and unstable Al2O3//MgO.

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HAADF Detector

Al drop

1

EDX HAADF Detector Point 1

3

Counts

AlKα

2

CKα

First layer

b)

a)

Energy (keV)


EDX HAADF Detector Point 3 AlKα

Counts

Counts

AlKα

OKα

CKα CKα

c)

Energy (keV)

d)

Energy (keV)

Fig. 5. TEM image (a) and EDS analysis (b-d) of the drop-side interface in Al/MgO(100) couple (1000°C, 60 min): solidified drop (b) and RPR (c-d) in Al/MgO couple. HAADF Detector

First layer

EDX HAADF Detector Point 1 AlKα

1

Counts

2 3

CKα

OKα

Second layer

a)

b)

Energy (keV)



AlKα

AlKα

Counts

Counts

CKα OKα

MgKα CKα

c)

Energy (keV)

d)

Energy (keV)

Fig. 6. TEM image (a) and EDS analysis (b-d) of 1st (b-c) and 2nd (d) layers of RPR in Al/MgO(100) couple (1000°C, 60 min).

Next, Al2O3 and MgO, being in contact, react with each other to form spinel MgAl2O4 according to reaction (2) accompanied with ~6% volume increase, thus facilitating the densification of RPR by narrowing or even disappearance of the channels generated at the 1st stage. Finally, the RPR consists of two layers with different structure and composition of the thermodynamically stable interfaces such as Al/Al2O3, Al2O3/MgAl2O4 and MgAl2O4/MgO.

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Table 1. The proposed two paths of reactions in Al/MgO system. Sequence of interfaces

Reactions

PBR*

First path of reactions Al//MgO ↓ Al/Al2O3//MgO ↓ Al/Al2O3/MgAl2O4/MgO

2Al+3MgOAl2O3+3Mg↑

(1)

-24.15%

Al2O3+MgOMgAl2O4

(2)

+5.99%

Second path of reactions Al//MgO ↓ Al//MgAl2O4/MgO ↓ Al/Al2O3/MgAl2O4/MgO

2Al+4MgOMgAl2O4+3Mg↑

(3)

-13.28%

2Al+3MgAl2O44Al2O3+3Mg↑ (4)

-12.60%

Stage I PBR*=-24.15%

Stage I PBR*=-13.28%

Stage II PBR*=+5.99%

Stage II PBR*=-12.60%

Stage III Increasing thickness of layers

Stage III Increasing thickness of layers

a)

b)

Fig. 7. Scheme of phase transformations in Al/MgO by 1st path (a) and 2nd path (b) of reactions.

For the 2nd possible path of reactions, the interaction starts from the formation of MgAl2O4 by redox reaction (3) while freshly formed Mg diffuses into the Al drop and evaporates from its surface, creating favorable conditions for reaction (3) accompanied with 13.28% volume decrease and the formation of discontinuities in the reactively formed layer of MgAl2O4. Among the two interfaces formed (Al//MgAl2O4 and MgAl2O4/MgO), only MgAl2O4/MgO is thermodynamically stable. Therefore, at the next stage, the reaction (4) takes place leading to an additional 12.6% volume decrease accompanied with continuous evaporation of the freshly formed Mg.It is important that in the 2nd path of reactions, the required conditions for creating the C4 structure are fully satisfied only at the second stage because liquid Al does not wet MgAl2O4 while wetting in Al/MgAl2O4 couple takes place when the wettable reaction product Al2O3 is formed at the interface, as reported in [12]. Taking into account this fact as well as the structural observations showing more dense structure of the 2nd layer, one may conclude that the 1st path of reactions is preferable. It should also be noted that the above mentioned two factors (Mg evaporation and volume decrease) play an important role not only in the formation and growth of the RPR of C4 structure but also in the appearance of an interesting phenomenon called ‘substrate surface whiskering’ [12], observed during high-temperature wettability studies of both Al/MgO [15] and Al/MgAl2O4 [12] systems. Our TEM examinations evidenced that in both cases, the structure and chemistry of whiskers are similar to those of the corresponding single crystal substrates and no new reaction products were detected within each separated whisker. Moreover, the detailed observations of real-time movies clearly showed that high-temperature substrate whiskering is caused by substrate cracking and detachment of thin whisker-like crystals from the ‘mother’ oxide substrate. It presents experimental evidence of the important primary role of high stresses created in the substrate due to volume decrease accompanying the redox reactions in the examined systems.

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Summary OM, SEM and TEM observations coupled with EDS analysis of interfaces formed between molten aluminium and MgO and MgAl2O4 single crystals at 800-1000°C proved the redox reactions leading to the formation of reaction product region. For Al/MgAl2O4 couples, it presents one single layer composed of separate Al2O3 particles surrounded with Al(Mg) phase. In Al/MgO couples, independently of substrate crystallographic orientation, the RPR is composed of two layers, where the 1st layer has a structure similar to that recorded in the Al/MgAl2O4 couples, while its 2nd layer is composed of MgAl2O4 with small amount of the narrow Mg(Al) channels. Two paths of possible reactions have been proposed taking into account the thermodynamic stability of particular contact systems (interfaces) formed at each step of the interaction. Structural analysis suggests that the following sequence of phase and interface transformations is preferable Al//MgO → Al/Al2O3//MgO → Al/Al2O3/MgAl2O4/MgO. Volume decrease and Mg evaporation are two phenomena accompanying high-temperature interaction between liquid Al and MgO or MgAl2O4 that play a key role in the formation of interpenetrating (C4) structure of reactively formed interfacial layers as well as in the substrate surface whiskering observed during wettability studies. References [1]

N. Sobczak, J. Schmidt, A. Kazakov, Patent PL-166953, 26.07.1991

[2]

N. Sobczak, M. Ksiazek, W. Radziwill, J. Morgiel, W. Baliga, L. Stobierski, Proc. High Temperature Capillarity, Foundry Research Institute, Poland (1997) 138

[3]

H.J. Scheel, J. Cryst. Growth 211 (2000) 1

[4]

D.R. Clarke, Interpenetrating Phase Composites: Report of the Snowmass Workshop, J. Amer. Cer. Soc. 75 [4] (1992) 739

[5]

M.C. Breslin, J. Ringnalda, L. Xu, M. Fuller, J. Seeger, G.S. Daehn, T. Otani, H.L. Fraser, Mater. Sci. Eng. A, 195[1] (1995) 113

[6]

W. Liu, U. Köster, Scripta Mater. 35[1] (1996) 35

[7]

M.C. Breslin, J. Ringnalda, J. Seeger, A.L. Marasco, G.S. Daehn, H.L. Fraser, Cer. Eng. Sci. Proc. 5[4] (1994) 104

[8]

R.E. Loehman, K.G. Ewsuk, A. Tomsia, J. Amer. Cer. Soc. 79[1] (1996) 27

[9]

N. Sobczak, in Bulk and Graded Nanometals, K.J. Kurzydlowski and Z. Pakiela (Eds.), Solid State Phen., 101-102 (2005) 221

[10] N. Sobczak, L. Stobierski, M. Ksiazek, W. Radziwill, R. Nowak, A. Kudyba, Ceramika 80 (2003) 831 [11] N. Sobczak, in Innowacje w odlewnictwie, Instytut Odlewnictwa, vol. 1 (2007) 187 [12] R. Nowak, N. Sobczak, W. Radziwill, A. Kudyba, E. Sienicki, in Innowacje w odlewnictwie, Instytut Odlewnictwa, vol. 2 (2008) 225 [13] L. Ceschini, G. Minak, A. Morri, Comp. Sci. Tech. 66 (2006) 333 [14] S. Antolin, A.S. Nagelberg, D.K. Creber, J. Amer. Cer. Soc. 75[2] (1992) 447 [15] K. Grjotheim, O. Herstad, J.M. Toguri, Canad. J. Chem. 39 (1961) 443 [16] D.R. Giese, F.J. Lamelas, H.A. Owen, R. Plass, M. Gajdardziska-Josifovska, Surf. Sci. 457 (2000) 326 [17] R. Nowak, N. Sobczak, A. Kudyba, W. Radziwill, E. Sienicki, in Innowacje w odlewnictwie, Instytut Odlewnictwa, vol. 3 (2009) 89

Solid-Solid Phase Transformations in Inorganic Materials doi:10.4028/www.scientific.net/SSP.172-174 Structural Characterization of Reaction Product Region in Al/MgO and Al/MgAl2O4 Systems doi:10.4028/www.scientific.net/SSP.172-174.1273