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INFLUENCE OF THE STOICHIOMETRY ON THE MECHANICAL PROPERTIES OF SPINEL (Al2O3)nMgO SINGLE CRYSTALS

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INFLUENCE OF THE STOICHIOMETRY ON THE

MECHANICAL PROPERTIES OF SPINEL

(Al2O3)nMgO SINGLE CRYSTALS

N. Doukhan, R. Duclos, B. Escaig

To cite this version:

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C7-566 JOURNAL DE PHYSIQUE Colloque C7, suppKment au no 12, Tome 37, Dkcembre 1976

INFLUENCE OF THE STOICHIOMETRY

ON

THE MECHANICAL

PROPERTIES OF SPINEL (A120,),Mg0 SINGLE CRYSTALS

N. DOUKHAN, R. DUCLOS and B. ESCAIG

Laboratoire de Structure et PropriCtts de 1'Etat Solide (*) Universitt des Sciences et Techniques de Lille

B. P. 36, 59560 Villeneuve-d'Ascq, France

R6sum6. - Des monocristaux de spinelle de differentes compositions (MgO(Alz0 3) I , I et MgO(A1~03)1,8) ont et6 deformes par fluage a haute tempkrature : entre 1 580 OCet 1 740 "C pour le premier et a 1 550oC et 1 600oCpour le second. Nous trouvons une Bnergie d'activation d'environ 6 eV pour les deux compositions. Les sous-structures de fluage ont CtB observks par topographies de Berg-Barrett et par microscopic electronique par transmission. Les sous-structures de fluage qui presentent entre elles quelques similitudes diffkrent fortement de celles observQs sur des mono- cristaux de spinelle MgO(A120 3) 1 , s flu& en dessous de 1 520 oC.

Abstract. - Spine1 single crystals of various compositions (MgO(Al20 3) 1.1 and MgO(Alz0 3) I . 8)

have been crept at high temperature : between 1 580 OC and 1 740 OC for the first and at 1 550 OC and 1 600 oC for the second. An activation energy of about 6 eV is found for both compositions. Creep substructures have been investigated by Berg-Barrett topographies and by transmission electron microscopy. The creep substructures which present some similarities, strongly differ of the oneobserved on MgO(A1203)1.8 single crystals crept below 1 520 oC.

1. Introduction.

-

The spine1 MgA120, forms with Al,03 a large range of solid solutions (A120,),Mg0 with n varing from 1 to 5. The excess of alumina results in cation vacancies predominantly on the aluminium (octahedral) sites. The influence of these vacancies is important since slip planes and mechanical properties are known to vary with the stoichiometry n

(ratio A1203/MgO) [l, 21.

Single crystal creep properties have been investigated for two stoichiometries n = 1.1 and n = 1.8. In the first case the influence of stoichiometry can be eva- luated, being known that a unique solid solution is thermodynamically stable at temperatures above 1 000 OC. With the stoichiometry n = 1.8, the influence on creep of thermodynamical metastability can be studied in addition, since above 1 520 OC one solid solution phase (A1203),.,Mg0 is stable while below, two phases should occur, the solid solution A120,Mg and an exsoluted alumina phase. In a n earlier work [3], [001] compression creep has been studied for n = 1.8 below 1 520 OC. { 110 ) slip only is active, building up a well defined cellular substructure in two dimensions (in two { 110 ) slip systems) ; the creep law is consis- tent with a climb controlled dislocation slip. Note that no exsolution of alumina has been evidenced by trans- mission electron microscopy (T. E. M.) in this work. This paper presents single crystal creep results for n = 1.8 above 1 520 OC and some preliminary

observations for the nearly stoichiometric crystal MgO(A12O3)1 . l

Mechanical tests are supplemented to substructure observations, either by X-ray topography (Berg- Barrett technique, B. B. T.) at the sample scale, or by T. E. M. at a smaller scale. While activation energies are not so much different, a drastic change is observed in the creep substructure of both cases.

2. Experimental techniques. - Single Crystals boules of (AlZO3),MgO have been obtained from Cristal Tec/LETI (Grenoble). They are Verneuil grown with two stoichiometries : n = 1.1 and n = 1.8. They content only a few p. p. m. of impurities principaly CaZ + and Fe3

+.

Creep samples are cut with a diamond saw into 2.5 X 2.5 X 6 mm3. The compression axis is [OOl].

The specimens are tested in air inside a furnace with a graphite heating element.

After creep tests, creep substructure is observed by B. B. T. or T. E. M. Thin foils are prepared by ionic thinning (for more details and the understanding of the contrasts of B. B. T. see reference [3]).

3. Experimental results.

-

3.1 SPINEL MgO (AlZ0,),

.,.

-

Specimens of this composition have been crept in the stable solid solution phase at temperatures 1 550 OC and 1 600 OC.

(*) Associt au C. N. R. S. no 234.

3.1.1 Creep law.

-

Figure 1 shows creep curves obtained at 1 550 OC for several stresses. After a tran-

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MECHANICAL PROPERTIES OF (A120s)nMgO SINGLE CRYSTALS C7-567

FIG. 1.

-

Full lines : creep curves obtained at 1 550 OC at diffe-

rent stresses for the stoichiometry n = 1.8 ; dotted line : creep

curve obtained at 1 670OC at 9 kg/mrnz for the stoichiome-

try n = 1 . 1 .

sient of some hours, a steady state creep is reached which exhibits a behaviour apparently similar to the one observed at lower temperatures (1 300-1 450 OC, see ref. [3]).

E = Acn exp

-

U/kT

.

Conventional tests give n = 4.5 f 0.5. Temperature jumps (from 1 550 OC to 1 600 OC) give an approximate value U E 6 eV (compared with U 2: 5.2 eV below

1 520 OC).

3.1.2 Creep substructures. - In contrast to the similarity observed in the creep laws, a drastic change occurs in dislocation substructures when the tempera- ture crosses over the boundary of the phase diagram. Cellular substructures of the lower temperature type are no more formed in the higher temperature range. a) B. B. T. observations. - All B:rg-Barrett topo- graphies have been taken either polishing off the sample surfaces, or after cutting inside the sample itself. There- fore the observed dislocation substructures are repre-

sentative of the bulk.

These topographies show that the substructure formed above 1 520 OC consists of rough dislocation sheets in ( 110 ) planes parallel to the compression axis. The related lattice rotation axis has a mixed character, i. e. has a component both in the sheet planes (tilt) and along the sheet normal (twist). These rotations are alternate throughout the crystal.

Figure 2u shows the (440) reflecting planes corru- gated around the compression axis along the specimen side faces (black and white orientation contrast stripes). Figure 2b shows a (440) topography of the top face ; (1 10) dislocation sheets are clearly imaged here either by displacement or extinction contrasts (or both). Figure 3 shows a topography of the same face, but with a diffe- rent diffraction vector, g = [115], chosen to evidence the lattice rotation components normal to the sheet planes (seen horizontally in Fig. 3). The large misorien- t ations found through the whole specimen prevent us to complete the determination of the rotation axis.

FIG. 2. - a) B. B. T. of the (010) face of a MgO(A1203)1.8

single crystal crept at 1 550 QC at 4 kgjmmz ( E =0,23) g =

[a].

Note the existence of orientation contrast around 001 axis

(vertical black and white stripes) ; b) B. B. T. of the (001) face

of a MgO(A1203)1.8 single crystal crept at 1 550 O C at 7 kglmrnz

(E = 0.3) g = [404]. Extinction and/or displacement contrasts

reveal black lines in

<

110

>

directions. Note the different

spacing between the sheets as a function of the stress.

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C7-568 N. DOUKHAN, R. DUC :LOS AND B. ESCAIG

FIG. 5. - Stress dependence of the sheet spacing assuming an inverse stress dependence : the dotted line represents the size of the cellular substructure obtained at 10 kg/mmz at lower

temperature [3].

FIG. 3. - Same specimen as figure 2b. B. B. T . of the (001) face g = [i15]. Orientation contrast around the [l101 vertical axis (horizontal black and white stripes) reveals the lattice rota-

tion component normal to the sheet planes.

FIG. 6.

-

Same specimen as figure 26. B. B. T . of the (010) face g = [440]. Note vertical { 110 ) sheets and horizontal (001)

sheets.

Fro. 4.

-

B. B. T. of the (100) face of a MgO(AIz03)1.8 single crystal crept at 1 550 OC at 5 kg/mrn2 g = [440]. Note the sequence of contrast stripes

...

blacklout of contrast/black/grey/out of

contrast/grey..

.

Note that generally, the sheet contrasts are faint (Fig. 2b) which means that they are probably very crudely formed.

The spacing between these sheets decreases with the applied stress. Figure 5 shows an approximate inverse relationship holds, d E 20 pblo. This is roughly in

agreement with the relation observed in the lower temperature range (shown in figure 5 as a dotted line). At higher deformations ( E E 0.2) other kind of

sheets are added in (001) planes, i. e. perpendicular to the compression axis (Fig. 6 ; see also horizontal cusps on stripes in Fig. 2a).

b) T. E. M. ob~eruations. - A relatively small and uniform dislocation density is found (p E 107 cm-')

as compared with lower temperatures. More interesting is the large number of attractive junctions. A careful examination of the involved slip planes is currently in progress. Figure 7 shows a typical view of these junctions.

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MECHANICAL PROPERTIES O F (A1203)nMg0 SINGLE CRYSTALS C7-569

FIG. 7.

-

T. E.

many junctions.

M. (010) foil : low dislocation density with The irregular back ground is due to a bad

ionic thinning.

FIG. 8.

-

T. E. M. same (010) foil as for figure 7 : subgrain boundaries lying approximatively in { 110 ) planes.

3.2 SP~NEL MgO(A120,),.,. - Only some speci- mens of this stoichiometry have been crept at 1 580 OC, 1 670 OC, and 1 740 OC, and we report below these preliminary results.

3.2.1 Creep curves. - Figure l shows the curve obtained at 1 6700C under an applied stress of 9 kg/mm2.

This curve shows first a nearly complete lack of transient creep. Secondly, the very low creep rate which is obtained has to be emphasized : for exemple, for having the same creep rate under the same applied stress, for the stoichiometry n = 1.8, a temperature as low as 1 400 OC has to be reached. Temperature jumps

(f 20 OC) yield an average value of the activation energy of about 6 eV (+ I eV), very close of the value measured at high temperatures for n = 1.8.

The ~ ( t ) curves measured at 1 580 OC and 1 740 OC

both show some accelerated creep. Therefore the stress exponent in the creep law has not been derivated ; experiments are now in progress for deciding of the possible existence of a sigmoidal creep.

3.2.2 Creep substructures. - (a) B. B. T. obser- vations;

-

No glide plane determination is apparent from the topographies. These show only the building

An exemple of these sheets is viewed by extinction (or displacement ?) contrast figure 9, after only 0.3 percent strain at 1 580 OC. Another exemple, slightly contrasted, is given figure 10, after 0.08 strain at

1 670 OC ; an orientation contrast is associated with the sheets, giving lattice rotation components around 001, and 010. After 0.17 creep strain at 1 740 OC, the same sheet formation is found, but the misorientation around 010 axis prevails, being as large as 40 between the center and the ends of the sample.

FIG. 9. -B. B. T. of the (010) face of a MgO(A1203)1.1 single

crystal deformed of 0.3 % at 11 kg/mm2 at 1 580 OC g = [6601.

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C7-570 N. DOUKHAN, R. DUCLOS AND B. ESCAIG

After creep, all specimens present an unusual concave shape (inverted barrel) with the ends more widened than the center. Moreover, a clear indication of a parasite (1 10) slip is apparent from the correspond- ing cellular substructure localised only at the corners of the sample (see Fig. 10). The stoichiometry analysis obtained with a microprobe gives nearly no variation across the specimen (n = 1 .l -+_ 0.1), which shows that

this observation cannot be ascribed to a possible diffu- sion of alumina from the alumina buttons used to prevent identations. We think rather more plausible to relate it to stress concentrations (triaxial stresses) at the corners.

Whatever it may be, it demonstrates that (1 10) slip is unfavoured for this stoichiometry, except under very special stress conditions.

b) T. E. M. observations.

-

Only the sample crept to 0.08 strain at 1 670 O C has been examined. No sub-

grain boundaries is apparent. On the other hand, numerous attractive junctions are found, uniformly distributed throughout the thin foil (Fig. 1 l), connected with a generally low dislocation density. At least a cer- tain number of junctions have been shown to involve (111) slip for one of the reacting dislocations, and sometimes for both of them. This important observa- tion, if supported by further examinations, would be in favour of prevailing (1 11) slip systems.

phase in the phase diagram, as compared with creep of the metastable solid solution at temperatures lower than 1 520 O C for stoichiometry n = 1.8. In this later case, a very fine scaled micro-exsolution (not visible by T. E. M.) of alumina is plausible, i. e. on the form of the expulsion of Mg2+ ions from small clusters, and their replacement by a corresponding number of A13 + tetrahedral ions for electrical neutrality, some cation vacancies, probably octahedral, being created in the process. Although the creep law does not vary very much as temperature crosses over the phase diagram boundary (for n = 1.8), the creep mechanisms as featured by the dislocation substructure varies drasti- cally. Because the diffusion processes are not expected to change that much when heating up to a few tens degrees (nevertheless creep energies are slightly increased : from 5.3 eV to 6 eV), thechanges observed for n = 1.8 should be ascribed to the one re-mixing of AI3+ and Mg2+ ions.

Surprisingly enough, a number of similarities are found in the creep substructures observed for n = 1.1 and for n = 1.8 at higher temperatures :

-

a relatively low mobile dislocation density ;

-

the appearance of numerous attractive junctions, consistent with multiple slip conditions ;

-

the same kind of (001) dislocation sheets, formed perpendicularly to the compression axis.

Although some differences do exist (( 110 ) sheets form only in the case n = 1.8, and the creep rate is markedly lower in the case n = 1.1 owing to a pre- exponential factor at least ten to fifty times lower), this likeness supports some similarity in the active slip systems. On the other hand, some evidence for a (1 11) slip is expected and has been evidenced in the case n = 1.1, as it has been shown above.

Therefore the question of a possible change in slip plane from (1 10) to (1 11) rises in the case n = 1.8, when the temperature crosses over the phase diagram boundary. Further experimental evidences are of

Fw. 11. - T. E. M. a) (111) foil : low dislocation density with course still needed in order to solve this point.

many junctions ; b) same foil : weak beam image af a typical Other points deserve also attention as for exemple junctions ; the fault ribbons are well visible. Analysis of Burgers the exact shape of creep curves E ( t ) for = 1.1, and the vectors and dislocation directions gives the corresponding

glide planes { 11 1 ) and { 110 }. role of freshlaged dislocations. For n = 1.8, the creep

law resembles the one usually obtained for a climb controlled glide of dislocations, with a climb energy of 4. Discussion. - Significant changes in the creep the order of the one needed for matter transport by behaviour have been found as soon as creep is per- volume diffusion, as it is measured in sintering of formed in the stability range of the one solid solution MgA1,0, powder by Bratton [4].

References

[l] LEWIS, M. H., Phil. Mag. 14 (1966) 1003 ; Phil. Mug. 17 [3] DOUKHAN, N., DUCLOS, R., ESCAIG, B., J. Physique Collog.

(1968) 481. 34 (1973) C 9-379.

[2] RADFORD, K. C., NEWEY, C . W . A., Proc. Brit. Ceram. Soc.

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MECHANICAL PROPERTIES OF (A1203)nMe SINGLE CRYSTALS

DISCUSSION

H. STRUNK. - According to your X-ray topogra- microscopic structure of these dislocation layer struc- phies, in some of the crystals layer-like dislocation tures ?

structures form during creep exhibiting twist character.

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