Structural Characterization of Reaction Product Region in Al MgO.pdf

8/10/2019 Structural Characterization of Reaction Product Region in Al MgO.pdf

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Structural Characterization of Reaction Product Region in Al/MgOand Al/MgAl2O4Systems

Rafal Nowak1,a, Natalia Sobczak1,2,b, Edmund Sienicki2,c, Jerzy Morgiel3,d1

Foundry Research Institute, 73 Zakopianska St., Cracow, POLAND2Motor Transport Institute, 80 Jagiellonska St., Warsaw, POLAND

3Institute of Metallurgy and Materials Science, PAS, 25 Reymonta St., Cracow, POLAND

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

Keywords: 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-rayenergy dispersive spectrometry (EDS). The Al/MgO and Al/MgAl2O4couples were produced

under ultra high vacuum at 800, 900 and 1000C. The observations proved the redoxreactionsof 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 MgAl2O4was identified. In the case of Al/MgAl2O4couples, a single layer

was distinguished and only the Al2O3phase was recognized.

IntroductionInformation on interaction in Al/MgO and Al/MgAl2O4systems 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/MgAl2O4couples in order to identify

the type of possible reactions and accompanying processes taking place at high temperatures.

Experimental ProcedureThe Al/MgO and Al/MgAl2O4couples were produced during the sessile drop wettability tests

by heating of Al (99,999%) sample on MgO or MgAl2O4single crystal substrates at 800, 900and 1000C for 60 or 120 minutes in a vacuum of 510-6 mbar [1,2]. MgO crystals were

produced by arc melting while MgAl2O4by Czochralski method (MTI Corp., USA) [3]. Thesubstrates were polished by the producer up to a roughness of 8 . Structure and chemistrycharacterization of reaction product region in the solidified, cross-sectioned and mechanically

polished Al/MgO and Al/MgAl2O4couples 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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Results and discussionOM 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 andAl(Mg) phases (Fig. 2).

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

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

For Al/MgO couples produced at 900 or 1000C, the RPR is heterogeneous and composed of

two layers schematically shown in Fig. 3. SEM+EDS analysis (Fig. 4) evidenced that the 1 st

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/MgAl2O4couple, its detailed EDS analysis was

also done (Fig. 4). It showed that the 1stlayer is built of Al2O3crystals separated by metallic

channels filled with Al containing up to 4 at.% Mg. However, Mg was not detected in thedrop, and this fact can be explained by rapid evaporation under dynamic vacuum at high

PointMg Al O

Phasesat.%

1 14 38 48 MgAl2O4

2 2 55 48 Al(Mg); Al2O3

3 2 54 44 Al(Mg); Al2O3

4 - 100 - Al

5 2 46 52 Al(Mg); Al2O3

6 13 36 51 MgAl2O4

Al drop Al drop

MgAl2O4

3


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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 (1000C, 60 min).

For Al/MgO couple, TEM examinations showed a good agreement with SEM+EDS analysis

since in the 1st layer, Al2O3and Al(Mg) phases were identified (Fig. 5), while the 2nd layer

was composed mainly of MgAl2O4phase (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 C4structure 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 theinitial 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 evaporatesand 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.

PointMg Al O

Phaseat.%

1 - 100 - Al

2 4 52 44 Al2O3+Al(Mg)

3 4 52 42 Al2O3+Al(Mg)4 43 18 40 Mg(Al)+MgAl2O4

5 46 13 41 Mg(Al)+MgAl2O4

6 52 5 43 MgO

Stand. 17 38 45 MgAl2O4

1

5

4

3

2

Al dropAl drop

6

RPR

First layer

Second layer

Al drop

MgO

1s

layer

2n

layer


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a)b)

c) d)

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

a) b)

c) d)

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 (1000C, 60 min).

Next, Al2O3and MgO, being in contact, react with each other to form spinel MgAl 2O4accordingto reaction (2) accompanied with ~6% volume increase, thus facilitating the densification of RPR

EDX HAADF Detector Point 1

AlK

OKCK

Energy (keV)

Counts

CK


OK

MgK

AlK


Counts

Energy (keV)Energy (keV)

Counts

CK

AlK

HAADF Detector

HAADF Detector


AlK

CK

Energy (keV)

Counts


AlK

CK

AlK

CK

OK

Energy (keV)Energy (keV)


C

ounts

Co

unts

1

23

1

2

3

First layer

Second layer

Al dorp

First layer


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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 stressescreated in the substrate due to volume decrease accompanying the redox reactions in the

examined systems.

SummaryOM, SEM and TEM observations coupled with EDS analysis of interfaces formed between

molten aluminium and MgO and MgAl2O4 single crystals at 800-1000C proved the redoxreactions leading to the formation of reaction product region. For Al/MgAl 2O4 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/MgAl2O4couples, while its 2nd layer is composed of MgAl2O4with small amount of thenarrow 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 phenomenaaccompanying high-temperature interaction between liquid Al and MgO or MgAl2O4that 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.

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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

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