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Plastic deformation of MgO : n Al2O3 spinels at temperatures below 1000 °C (0.5 Tm)
P. Veyssière, S. Kirby, J. Rabier
To cite this version:
P. Veyssière, S. Kirby, J. Rabier. Plastic deformation of MgO : n Al2O3 spinels at tempera- tures below 1000 °C (0.5 Tm). Journal de Physique Colloques, 1980, 41 (C6), pp.C6-175-C6-178.
�10.1051/jphyscol:1980645�. �jpa-00220083�
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 7, Tome 41, Juillet 1980, page C6-175
Plastic deformation of MgO : n A1
20
3spinels at temperatures below 1000 °C (0.5 T
m)
P. Veyssiere (*), S. H. Kirby (**) and J. Rabier (*)
(*) Laboratoire de Metallurgie Physique, Faculte des Sciences, 40, avenue du Recteur Pineau, 86022 Poitiers. France (**) U.S. Geological Survey, Menlo Park, CA 94025, U.S.A.
Résumé. — Du spinelle MgO:« A1203 a été déformé de 20 à 950 °C (0,1 à 0,5 TF) en compression sous une pression hydrostatique de 1,3 à 1,7 GPa. Le plan de glissement facile du spinelle équimolaire (n ^ 1,1) aux tem- pératures faibles et intermédiaires est { 110 } < 110 >. Aux températures supérieures à 600 °C, les scissions cri- tiques résolues (CRSS) pour ce système sont de moitié inférieures à celles correspondant aux deux autres systèmes activés { 111 } < 110 > et { 100 } < 110 >. Des échantillons fortement non équimolaires (« ~ 3,5) ne se déforment pas par glissement { 110 } < 110 >. La CRSS pour le glissement facile { 1 1 1 } < 1 1 0 > pour n - 3,5 est nettement plus faible que son homologue pour n ^ 1,1. Elles sont du même ordre pour le système { 100 } < 110 >. Au-dessus de 500 °C pour M ~ 1,1, la CRSS associée à { 110 } < 110 > augmente fortement, alors que celles relatives aux systèmes { l l l } < 1 1 0 > e t { 100 } < 110 > sont relativement insensibles à la température. Les mécanismes pou- vant être responsables de ce durcissement avec la température sont discutés.
Abstract. — We have achieved significant plastic strain of MgO: n A1203 from 20° to 950 °C (0.1 to 0.5 TJ by compression testing under superimposed hydrostatic pressures of 1.3 to 1.7 GPa. The easy slip system for nearly equimolar spinel (« a; 1.1) at low to intermediate temperatures is { 110 } < 110 >. At temperatures up to 600 °C, the critical resolved shear stresses CRSS for this slip system are about half those for { 111 } < 110 >
and { 100 } < 110 >, the other two slip systems which are activated. Strongly non-equimolar crystals (n ^ 3.5) do not deform by { 110 } < 110 > glide; the CRSS for { 100 } < 110 > is about the same. At temperatures below 500 °C, the CRSS for all of the slip systems in the nearly equimolar crystals decreases with increasing tempe- rature; At temperatures above 500 °C (n = 1.1), the CRSS for {110 } < 110 > slip dramatically increases, and the CRSS for both { 111 } < 110 > and { 100 } < 110 > slip are insensitive to temperature. A number of potential mechanisms could account for these temperature effects, including a strengthening mechanism predicted earlier by two of the present authors.
1. Introduction. — It is generally held that above 0.5 Tm the plasticity of crystalline materials is strongly influenced by point defect diffusion via dislocation climb. At low temperatures, plastic behavior is pro- moted by thermal activation over barriers to disloca- tion glide motion under shear stress. The intermediate temperature regime (0.25 to 0.5 T^ is very complex and poorly understood in both metals and simple ionic crystals. Earlier studies [1-3] have predicted a strengthening of MgO : n A1203 spinels in this intermediate regime, which we summarize below.
Stacking fault energy calculations, assuming a purely ionic model, have shown that faults parallel to { 110 } with fault vector R = 1/4 < 110 > normal to the plane of the fault should have minimum stacking fault energy [1-3]. This prediction has been experi- mentally verified on widely extended faults in several different materials with the spinel structure (see references in [3]). Since unit dislocations in spinel with Burgers vectors 6 = 1 / 2 < 1 1 0 > can dissociate into two partials with collinear Burgers vectors each with b = 1/4 < 110 >, climb dissociation of such unit dislocations according to such a vector reaction may be favored [1-3] and has been observed in nickel
ferrite [3, 4] and MgO : A1203 spinels [5-8]. Such climb dissociation should result in anomalous tem- perature effects on yield strength [3]. This process should be thermally activated and time dependent, since point defects are exchanged between the two partial dislocations : one partial climbs negatively while absorbing vacancies emitted by its companion, which climbs positively. An equilibrium width is not reached instantaneously since the climb dissociation rate is controlled by point defect diffusion rate; the lower the temperature or the higher the imposed strain rate, the longer the time to reach the equilibrium dissociation distance. At intermediate temperatures where plastic strain is accomplished primarily by glide motion of dislocations, but where the point defect diffusivity is significant, the wider the climb dissociation, the more difficult the activation of their glide motion. One should therefore expect a streng- thening with increasing temperature in spinels deform- ed at temperatures between 0.3 and 0.5 Tm [2-3].
A second objective of our study was to test Horn- stra's predictions [9] that the easy glide system in spinel should involve a synchro-shear mechanism which permits the dissociation of unit dislocations
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980645
C6-176 P. VEYSSIERE, S. H. KIRBY AND J. RABIER
into four nearly electrostatically neutral partials and should favor the { 11 1 ) ( 110 ) slip system. We have shown that MgO : 1.1 A1203 spinel crystals can be rendered plastic at 400 OC (0.28 T,) if the samples are compression tested under a superimposed hydro- static pressure of about 1.4 GPa [lo]. Contrary to Hornstra's prediction, { 111 ) ( 110 ) is the hardest of the three observed slip systems { 110 ) ( 110 ),
{ 100 } ( 110 ), and { 111 ) ( 110 ) ; the first of the three systems is clearly the easiest.
The aim of this paper is to provide an experimental contribution to the knowledge of the plasticity of materials with the spinel structure at intermediate temperatures. We report the influence of orientation, temperature, composition (n-value), and strain rate on the plasticity of MgO : n A1203 spinel crystals and outline the probable explanations of our observations.
2. Experimental details. - The experimental tech- nique will be described in detail in a later publication.
Briefly, Czochralski and Verneuil-grown single crystals of MgO : n Al,03 spinels were cut into oriented right rectangular prisms with dimensions 9.6, 3.7 and 3.7 mm. The crystals were pressurized to 1.3 to 1.7 GPa, heated to test temperature, and then compressed along the long axes of the prisms, which were oriented appro- ximately parallel to (001 ), ( 011 ), and ( 111 ).
The orientations were measured by Laue back reflec- tion to an accurary of better than f 10. The shortening rate was 2 x S- l, except for two tests at 2 x lop4.
The maximum stress difference o = (ol - 03), where a, is the maximum compressive stress parallel to the specimen axis and 0 3 is minimum principal compres- sive stress which is equal to the hydrostatic pressure.
We estimate that o is uncertain to about f 150 MPa.
The temporal and spatial variations in temperature are less than f 20 OC. Additional details may be found in [lo].
3. Results. - 3.1 GLIDE MECHANISM DEDUCED OM OPTICAL MEASUREMENTS. - Our earlier study of glide lines on nearly equimolar crystals (n
--
1.1)compressed approximately parallel to ( 001 ), ( 111 ), and ( 011 ) indicated that slip systems in the slip families { 110 } ( 0 1 ) { 100 ) ( 011 ), and { 111 } ( 101 ) were activated, respectively [lo]. The members of the above slip families with highest Schmid factors were activated to the exclusion of the symmetrically equivalent slip systems. Subsequent experiments at temperatures between 20 and 950 OC indicate that the same primary slip systems operated over the entire temperature range. Exploratory expe- riments on grossly non-equi-molar crystals (n
-
3.5)cut in about the same orientations indicate that { 110
1
( 101 ) does not occur in the ( 001 ) compres- sion orientation and that the indentities of the slip systems activated in the other two orientations are unchanged.3.2 MECHANICAL DATA. - All of the stress-strain curves exhibited a well-defined elastic stage followed by a usually prominent plastic yield point, followed by a strong work hardening stage. Some of the tests at 950 OC in the nearly equimolar specimens displayed yield stress drops. The variation of critical resolved shear stresses with temperature is shown in figure 1
Fig. 1. - Influence of' temperature on the CRSS (critical resol\cd shear stress) for the three primary slip systems which we havc oh\crv- ed in nearly equimolar MgO : n AI,O, spinel crystals (n
--
1.1).for the nearly equimolar samples. The CRSS stronp- ly decreases with increasing temperature up to 400 OC.
Above 400 OC (> 0.28 T,), the CRSS increases in the ( 001 ) experiments and is relatively temperature insensitive in the other orientations. The striking change in the temperature effect on yield strength is revealed in an Arrhenius plot of log, CRSS vs. (117") in figure 2. The ( 001 ) data, our most complete, fit a law of the form CRSS = to exp(E*/kT) where
t o = 0.57 GPa and E*
-
6 x J at tempera- tures below 500 OC ( < 0.3 T,). The prominent depar- ture from this conventional yield stress variation with temperature is suggestive of the onset of an important thermally-activated strengthening mechanism. Explo- ratory tests at 400 and 800 OC with compression parallel to ( 001 ) indicate no significant strain rate sensitivity of CRSS over the range of strain rates from 2 x to 2 x s-'.The principal effect of deviation from the approxi- mately equimolar composition on CRSS appears to be an increase in the CRSS for { 101 ) ( 101 ) slip such that it is not activated in the three representative orientations of compression (Table I).
4. Discussion. - Hornstra's prediction of easy glide on { 111 ) ( 110 ) is in conflict with our measu- rements of CRSS over the temperature range 20 to 800 OC, where { 110 } ( 110 ) is clearly the easiest
PLASTIC DEFORMATION Ol- MgO : ti A1203 SPINELS AT TEMPERATURES BELOW 1 000 O C ' C6-177
Fig. 2. - Arrhenius plot of the data of figure 1. Note strong depar- ture of { 110 } ( 110
>
data from a simple thermal activation law above 500 OC.Table I. - Critical resolved shear stresses for slip in MgO : n A1203 spinels (*).
Slip system CRSS, GPa
n = 1.1 n = 3.5
glide system. A crossover occurs at 800 OC above which { 110) ( 110) is harder than { 111 ) ( 110 ) slip. Except perhaps near 400 OC [lo] the { 100 ) ( 1 10 ) slip system is the hardest of the three observed sys- tems over the entire temperature range.
Westbrook [l 11 measured the Vickers indentation hardness of MgO : n A1203 spinels over about the same range of temperatures as the present compres- sion tests. At low temperatures, hardness decreases as n approaches 1. Comparison between indentation hardness measurements and the present data on yield strength are not straightforward since the localized plastic strains and stresses around an indentor are complex and general, since work hardening rates influence the hardness measurements, and since micro-fracturing around the indentor cannot be
(*) All tests conducted at a temperature of 400 V C , strain rate : 2 x 10-ss-' anda hyrostatic pressure of1.4 f 0.1 GPa.
(**) Not activated in our tests on these crystals.
ruled out. Our values of CRSS for n = 1.1 and n = 3.5 spinels at 400 OC are summarized in table I.
A surprising new feature of this comparative study is that { 110 ) ( 110 ) was not activated in the n = 3.5 spinel samples whereas this slip system was clearly the easiest slip system in the n = 1.1 crystals. The table shows that values of CRSS for n = 3.5 spinels are greater than, less than, and about equal to those for n = 1.1 spinels for the slip systems { 110 ) ( 110 ), { 111 ) ( 110 ) and { 100 ) ( 110 ), respectively. Since the deformation around an indentor probably induces a general strain involving several different slip sys- tems, it is unclear how our results are to be compared with the indentation hardness measurements. The systematic changes in CRSS for { 110 ) ( 110 ) and { 11 1 ) ( 110 ) slip with changes in molar ratio are not understood in fundamental terms.
The predicted strengthening effect of temperature is observed in only one orientation of compression which proniotes the { 110 ) ( 110 ) easy glide system.
The broad temperature range over which the strengths of ( 011 ) and ( 11 1 ) samples are relatively insensi- tive to temperature may reflect the same process responsible for strengthening in the ( 001 ) samples.
This anisotropy of anomalous temperature effects on CRSS could stem from several mechanisms which are to be tested by TEM observations :
A) Glide motion of { 110 ) ( 110 ) dislocations could show more orientations favoring sessile dissocia- tion than for glide motion of { 111 ) ( 110 ) and { 100) ( 110) dislocations. In fact, however, { 111 ) ( 110 ) slip has more geometrically distinct possibilities for sessile dissociations than the other two slip systems and therefore should show a streng- thening with temperature as well if this mechanism operates.
B) The situation could be analogous to the low temperature behavior of BCC crystals for which the microscopic yield strength detected by TEM is much lower than the macroscopic yield strength revealed by the stress-strain curve. In BCC crystals this beha- vior has been ascribed to the sessile dissociation of the screw segments, which is more easily surmounted as the temperature of the tests is increased. This explana- tion accounts for the observed strong softening with increasing temperature up to 500 OC, with the low apparent activation energy for macroscopic yield, and with the anomalously low Young's modulus measured in the
<
001 ) orientation (which may be a manifestation of microyielding at stresses below the macroscopic yield stress) [lo]. Preliminary TEM on a specimen loaded approximately parallel to ( 011 ), which activated { 111 } ( 110 ) with cross slip to { 100 ) ( 110 ), shows that screw dislocations are sessile in this orientation as well, and there is a ten- dency to dissociate in the (001) planes. Therefore, anomalous behavior should also have been observed in this orientation. The BCC analogy provides noC6- 178 P. VEYSSIEKE. S. H . K I R B Y AND I . RABIER
insight into the anomaly in temperature effects observ- ed above 500 OC.
C) Climb dissociation could be anisotropic, i.e., the climb dissociation motion of partials normal to the glide plane could be favored for the { 110 ) glide plane (climb motion parallel to ( 110 )) than for climb dissociation of { 11 1 ) ( 110 ) and { 100 ) ( 110 ) dislocations. All of the observed slip systems have the same Burger's vector. Therefore, the extra half plane for edge components is the same; only the orientation of the edge of the extra plane (the dislo- cation line) varies. Since the diffusivity tensors for point defects in cubic crystals should be isotropic, the detailed structure of the edge of the extra slice must have a strong influence on rates of climb dissociation if this mechanism is a viable explanation of the aniso- tropy in thermal effects on CRSS. Although the spinel structure introduces considerable complexities to modeling dislocation core structure [2], we are cur- rently exploring possible jog structures along these lines.
D) Our earlier study [lo] revealed that the Young's modulus of MgO : 1.1 A1203 spinel is anomalously low for compression approximately parallel to ( 001 ) at 400 OC compared to the modulus predicted from the elastic compliance coefficients deduced from ultra- sonic measurements. We also note that the molar ratio n also affects the magnitude of the Young's modulus anomaly in the (001 ) direction. Solid solution alloys often show similar anisotropic ano- malies in modulus, and these observations are inter- preted in terms of stress-induced ordering of metal atoms [12]. The discrepancy between the Young's modulus predicted from ultrasonic measurements and the observed modulus at high stress may be caused by short-range ordering of magnesium and aluminium in the tetrahedral and octahedral sites in spinel. Such short-range ordering could strongly affect dissociation reactions and kinetics and could be associated with the contrasts in behavior of n = 1.1 and n = 3.5 crystals, since the molar ratio may affect the ordering process.
E) The observed strengthening with temperature in the ( 001 ) specimens could be due to the hardening effects of precipitation of A1203 at high temperature.
The lack of evidence for such unmixing in even the higher temperature tests on equimolar crystals cited earlier and the anisotropy in the temperature effects on CRSS casts doubt on this possibility. TEM obser- vations on our high temperature samples should resolve this point.
Lastly, the insensitivity of CRSS to changes 111
strain rate in the range 2 x to 2 x s-' is puzzling since the strengthening process is appa- rently thermally activated.
5. Conclusions. - We have explored in controlled experiments the plasticity of MgO : n A1203 spinels in the temperature range 0.1 to 0.5 T,, over which only indentation hardness measurements were pre- viously available. We have proved that { 110 ) ( 110 ) is the easy glide system at temperatures up to 0.45 T , for n = 1.1 and that this easy glide system is suppres- sed for crystals with n = 3.5. We have discovered an anomalous temperature interval (400 to 950 OC), in which a strengthening witli increasing temperature is observed in crystals in which { 110 ) ( 110 ) slip is activated and in which a strength plateau occurs in crystals which deform by { 11 1 ) and { 100 ) slip.
No strain rate sensitivity is observed in the anomalous region for crystals which deform by { 110 ) ( 100 ) slip. Several mechanisms are adduced which may account for these anomalous properties. Data and insight which could permit us to exclude the incorrect explanations are lacking.
Acknowledgements. - This work was partly sup- ported by the Division of Materials Sciences, Office of Basic Sciences, U.S. Department of Energy.
Jacques Castaing catalysed this research and J.P. Poi- rier pointed out the possible analogy of the behavior of BCC metals with that of spinel.
References
[l] V E Y S S I ~ E , P., RABIER, J., GRILHB, J., Phys. Status Solidi (a) [6] VEYSSIERE, P., RABIER, .I., GRILHE, .I., J. Mat. Sci. 12 (1977)
31 (1975) 605. . , 670 ---
[2] V E Y S S I ~ E , P., Thkse d'Etat, Universite de Poitiers (1977). [7] D u c ~ o s , R., DOUKHAN, N., ESCAIG, B., ibid. 13 (1978) 1740.
[3] VEYSSI~E, P., RABIER, J., GAREM, H., GRILHE, J., Phil. Mag. [8] DONLON, W. T., MITCHELL, T. E., HEUER, A. M., Phil. Mag.
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[9] HORNSTRA, J., J. Phys. Chem. Sol. 15 (1960) 311.
[4] VEYSSIERE, P., RABIER, I., GAREM, H., GRILHB, J., Ibid. 33 1101 KIRBY, S. H., V E ~ S S I ~ ~ R E , P., Phil. Mag. 41 (1980) 129.
(1976) 143. 1111 WESTBROOK, .I.H., Rev. Int. Htes. Temp. RPfract. 3 (1966) 47.
[5] WELSCH, G., HWANG, L., HEUER, A. M., MITCHELL, T. E., [12] HUNTINGTON, H. B., The Elastic ~onsiants-of Crystals ( ~ c a -
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