Effect of crystal orientation on the high temperature deformation of nearly stoichiometric spinel

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Effect of crystal orientation on the high temperature deformation of nearly stoichiometric spinel

N. Doukhan

To cite this version:

N. Doukhan. Effect of crystal orientation on the high temperature deformation of nearly stoichiometric spinel. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-139-C6-141. �10.1051/jphyscol:1980636�.

�jpa-00220074�

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JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 7, Tome 41, Juillet 1980, page C6-139

Effect of crystal orientation on the high temperature deformation of nearly stoichiometric spinel

N. Doukhan, R. Duclos and B. Escaig

Laboratoire de Structure et Proprietes de 1'Etat Solide (*), Universite des Sciences et Techniques de Lille, 59655 Villeneuve d'Ascq Cedex, France

Résumé. — On étudie l'influence de l'orientation des contraintes appliquées sur la déformation à haute tem- pérature de spinelle MgO.1,1 A1203 en comparant des échantillons d'orientations < 100 > et < 110 > pour les- quels les plans de glissement sont respectivement { 110 } et { 111 }. Dans les deux cas le glissement est transitoire, rapidement relayé par la montée des dislocations. On trouve que les plans de dissociation, différents des plans de glissement, sont également les• plans de déplacement des dislocations. Pour un axe de compression < 110 >

la vitesse de fluage croissante est en accord avec une augmentation de la densité de dislocations.

Abstract. — T h e influence of stress orientation on the high temperature deformation of MgO.1.1 A1203 spinel is investigated by comparing < 100 > and < 110 > samples forwhich slip planes are respectively { 110 } and { 111 }.

In both cases, glide is a transient phenomenon rapidly relieved by climb. The dissociation planes, different from the glide planes, are found to be the displacement planes of dislocations. For a < 110 > compression axis, the accelerated creep rate is consistent with an increase of the dislocation density.

1. Introduction. — In a previous paper [1] we have shown that : i) { 110 } < lTO > glide can be activated in spinel MgO .1.1 A1203 in creep tests with a compression axis (C.A.) parallel to < 100 >. Furthermore these glide systems seem to be easier than { 111 }

< 110 > glides which were previously assumed to be the only glide systems for this stoichiometry [2, 3].

ii) Glide on { 110 } <( 110 ) is a transient phenomenon (e < 1 %) rapidly relieved by pure climb.

o(»p.) 500.

400. / \

I ~ " _ _ ^ <uo>

300.

200. J

1 ^ ^ <ioo>

100.1

[ e(*)

1 2 3 4 5 6 7 8 9

Fig. 1. — Stress strain curves a = F/S versus s = A/// for a com- pression axis parallel to < 110 ) and < 100 >. Deformation tempe- rature T = I 630 °C and strain rate £ = 1.4 x 10"5 s"1 are the same for both curves. Note the important yield point followed by a continuously decreasing a for the < 110 > C.A. orientation.

(*) Associe au C.N.R.S.

In this paper we present the results obtained by creep and constant strain rate tests with a < 110 > C.A.

which activates { 111} < i l 0 > glide and we compare these new results with the previous ones.

2. Deformation curves. — Stress-strain curves <r-s for both C.A. orientations are reported in figure 1 while in figure 2 the creep curves for the same C.A.

orientations are shown. In these figures the deforma- tion conditions are comparable i.e. same T and s for a-z curves and same T and a for creep curves but a complete set of curves has been recorded for various values of T, i and a. For < 100 > C.A. the creep curves do not show any transient creep whatever the values of T and a be. Similarly no yield point is seen on the stress strain curves in contrast with what is observed on the curves corresponding to < 110 > C.A.

Yield point and transient creep indicate the initial glide stage is more difficult than for the former orientation (J). For larger deformations, the observed softening on as curves as well as the creep rate acceleration for < 110 > C.A. are consistent with an increase of the mobile dislocation density.

Temperature and stress sensitivity of strain-rate are very similar for both compressive stress orientations. One finds with an assumed deformation law (2)

e = AamQ-vlkt

(') Berg-Barrett topographs and T.E.M. investigations have clearly confirmed that the first deformation stage (e < 1 %) is effectively produced by dislocation glide for both orientations of the C.A. [4].

(2) With A slowly variable as the available mobile dislocation density for the < 110 > C.A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980636

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C6-140 N. DOUKHAN, R. DUCLOS AND B. ESCAIG

Fig. 2. - Creep curves ^E= A111 versus time for ( 110 ) and ( 100 ) C.A. Temperature T = 1 640 OC ; applied stress ( F / S ) = 120 MPa. Note that for ( 110 ) C.A. there is a transient creep which corresponds to the yield point observed on ^{0 - 8}curve.

( 100) C.A. : 3 < m < 4 ; 5 < U < 6.4eV

( 110

>

C.A. : 2.6 < m < 3.6 : 5.7 < U < 6.6 eV

of course, for the ( 110 ) C.A. where the deformation regime varies at the beginning of the deformation,

measurements of and have been performed at Fig. 3. -Typical dislocation substructure after 0.2% strain with ( 110 ) C.A.; the thin foil is cut parallel to the (111) glide

higher strains. This similarity of results indicates that _plane.

one

observes polygonal loops edge, screw and 600 in both cases the plastic deformation is controlled by segments. One also notes some straight edge dislocations with a

the same mechanism i.e. by climb.

3. Dislocation substructures. - We only present here the creep substructures observed by T.E.M.

- [OOl] C.A. - At small strains (E

--

^0.2

^%)

^the

dislocation substructure consists in long straight edge dislocations lying in the four equally stressed { 110 ) glide planes, the dislocation density of the order of 10'' m-'. These dislocations are dissociated in the { 110 } plane perpendicular to their slip plane.

For larger deformation (i.e. for longer time at high temperature) the substructure is characterized by a rather uniform distribution of attractive junctions [I]. However some long edge dislocations are still observed even after 17

%

strain. By stereographic analysis one finds that they are no more straight although pure edge. They are smoothly wavy shaped in their dissociation plane which means that they move preferentially in this plane which is also their climb plane.

- ( 110 ) C.A. - At the beginning of the deformation (point A on the creep curve in figure 2) numerous polygonal loops are observed (Fig. 3).

They lie in the two most stressed { 111 ) glide planes.

The prefered orientations are edge, screw and f 60°

characters [4]. One also observes some straight edge dislocations lying along the intersection of both { 111 ) activated glide planes. They are probably formed by the reaction of two glissile dislocations leading to non stressed dislocations. The total dislocation den-

non activated Burgers vector. They lie at the intersection of the two { 11 1 } activated glide planes.

sity is also of the order of 10'' m-' for this C.A.

orientation at low deformation.

At higher strain (point B on figure 2, ^E= 10

%),

one still observes polygonal loops which seem to lie in the { 111 glide planes (Fig. 4). One also observes some junctions and long straight and isolated edge or 60° dislocations. The dislocation density is found higher, Precise stereographic analysis performed on many

-

^{2 x}^101'm-'.

loops show that, again in that case the straight sides of the loops, are slightly off their glide plane but are precisely confined in their dissociation plane (( 110 ) plane normal to the glide plane for the edge segments and { 100 ) planes for the 60° segments [4]). This clearly shows that, except for screws for which no dissociation could be observed in weak beam, dislocations move preferentially in their dissociation plane i.e. edge suffer pure climb while 60° experiment comparable amounts of climb and of glide (their dissociation plane is at 55O to their glide plane).

The dislocation substructure after 10

%

strain can thus be easily interpreted as the superposition of i) some small polygonal loops formed by slip ; with typical size of a few microns (GL on figure 4) and ii) long straight edge, or 600 dislocations which would be parts of larger polygonal loops intersected by the thin foil. These larger loops would simply stem from the lenthening by climb of the former loops.

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EFFECT O F CRYSTAL ORIENTATION ON THE HIGH TEMPERATURE DEFORMATION C6-141

Fig. 4. -Typical dislocation substructure after 10 % strain with ( 110 ) C.A. (111) thin foil. There are still polygonal loops but their edge or 600 segments are off the (lll).glide plane. They have moved in their dissociation plane. Long isolated edge or 6Q0 are supposed to be parts of larger loops (coming from small loops developed by climb) intersected by the thin foil.

4. Discussion. ^-In both cases (( 100 ) and ( 110 ) C.A.) plastic deformation begins by a first stage of dislocation glide and multiplication. This microplastic stage stops when dislocations reach sessile orientations and when the dislocation density is large enough to allow plastic deformation to be continued by climb.

Glide strain is relatively easy to produce in the first case (( 100 ) C.A.) because dislocations in their glide motion experiment only one sessile orientation (edge character) while in the second case (( 110 ) C.A.) there are at least three such directions (edge,

+

^60°

and - 600). The occurrence of a yield point in the stress strain curves (or a transient creep stage on creep curves) might be related for instance to screws which are observed as prefered orientation only in the second case. These might be slightly dissociated in { 110 ) planes ; they should then cross slip in { 111 } planes before they start gliding on them. The upper yield stress would thus correspond to the cross- slipping-stress.

After this microplastic stage (glide) dislocations move confined in their dissociation planes. This motion increases the length of octogonal loops thus leading to the observed increase in the density of mobile dislocations [5] and therefore also in the strain rate E: = pbv. A simple relation like E.

-

^p

-

^is

expected and seems to be consistent with T.E.M.

results which indicate clearly a higher dislocation content at ^E= 10

%

than at the beginning of the deformation.

DISCUSSION

Question. - T. E. MITCHELL. Reply. - R. D u c ~ o s .

Did say that the screw dislocations were No, the screw dislocations were not directly observed to be dissociated ? This is obviously of observed to be dissociated. This is an assumption interest because the dissociation would be by glide that we have made on the basis of the extension of rather than climb as is normal in spinel. the screw segments.

References

[I] D u c ~ o s , R., DOUKHAN, N., ESCAIG, B., J. Mat. Sci. 13 (1978) [4] DOUKHAN, N., DUCLOS, R., ESCAIG, B., J. Physique 40 (1979)

1740. 381.

[2] RADFORD, K. C., NEWEY, C. W. A., Proc. Br. Ceram. Soc. 9 [5] EDELIN, G., LE HAZE, R., DUPOW, J. M., Mem. Sci. Rev.

(1967) 131. Metall. LXVIII (1971) 43.

[3] LEWIS, M. H., Phil. Mag. 17 (1968) 481.