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INTERFACIAL DEFECTS IN NiO-ZrO2 (CaO) EUTECTIC
V. Dravid, M. Notis, C. Lyman, A. Revcolevschi, G. Bleris
To cite this version:
V. Dravid, M. Notis, C. Lyman, A. Revcolevschi, G. Bleris. INTERFACIAL DEFECTS IN NiO-ZrO2 (CaO) EUTECTIC. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-971-C1-978.
�10.1051/jphyscol:19901151�. �jpa-00230064�
COLLOQUE DE PHYSIQUE
Colloque Cl, supplement au n O l , Tome 5 1 , janvier 1990
INTERFACIAL DEFECTS I N Ni0-ZrO,(CaO) EUTECTIC
V . P . DRAVID* M. R . NOTIS* , C. E. LYMAN' , A . REVCOLEVSCHI' and G.L. BLERIS'**
'~epartrnent of Materials Science and Engineering, Lehigh University, P A 18015, U.S.A.
" ~ n i v e r s i t 6 de Paris-Sud, Orsay, France
"'~epartment of Physics, University of Thessaloniki, Greece RESUME
Des interfaces lamellaires dans I'eutectique obtenu par solidification dirigee (ESD) Ni0-Zr02(Ca0) ont ete etudiees par des techniques optiques electroniques. Plusieurs defauts caracteristiques a ['interface ont 6te analyses et interpretes I'aide de considerations geometriques. Une attention particuliere est portbe aux defauts topographiques et a leurs caracteristiques structurales et fonctionnelles.
ABSTRACT
Lamellar interfaces in the Ni0-ZrO:!(CaO) directionally solidified eutectic (DSE) have been investigated using electron optical techniques. Several characteristic interfacial defects have been analyzed and discussed invoking geometric arguments. Special attention is given to the topographical defects and their structural and functional characteristics.
INTRODUCTION
Directionally solidified eutectics (DSEs) in binary or pseudo-binary systems have been the subject of many investigations over the past three decadesl. A major motivation for these studies was the possibility of exploiting highly anisotropic properties of these materials which stem from their aligned microstructures. A typical aligned DSE may consist of rods of minor phase in a matrix or alternate lamellae of the two phases, dictated usually by the volume fraction of the minor phase.
DSE interfaces may also serve as model systems for the basic study of the nature and character of interphase interfaces23. Chadwick3 pointed out that crystallographic constraints have a dominant effect on the mechanism of eutectic solidification. However, it is now clear that uniqueness in crystallography is not necessarily a rule in metallic DSEs. Nevertheless, at least in Ni0-Zr02(Ca0) (hereafter referred to as NZ, N for NiO and Z for Zr02), we have shown that the lamellar interfaces are predominantly (1 11) N // (100) Z. In our earlier investigations4 we have shown that this interface orientation correponds to a definite minimum in interface energy. We have also been able to identify some relaxation mechanisms through experimental techniquess.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901151
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In this article we concentrate our examination on the oute radial regions of the DSE. These regions are prone to growth perturbations whic often force the lamellar interfaces away from their preferred orientation. At finer scale, such lamellar irregularities exhibit a rich variety of interfacc defects which are analogous to the interfacial defects i n solid-statl transformation interfaces. It is believed that a study of such defects and the1 characterization would contribute to our general understanding of the nature anc character of line andfor step defects and their structural consequences.
EXPERIMENTAL
Dhalenne & Revcolevschi6 have described the experimenta details concerning the DSE growth. The specimen preparation techniques for TEh are also described in our earlier publication4. Briefly, mechanical polishing a thin sections, followed by dimpling and ion beam thinning were used to obtai~
electron transparent foils. Conventional transmission electron microscopy (CTEM and high resolution TEM (HRTEM) were performed using Philips 400 and Philip!
430T microscopes.
RESULTS
The crystallography associated with the NZ DSE is described b:
Laroui et al.7 and Dravid et al.4 and can be summarized as:
Growth Direction // [l701 N /l [OOl] Z Interface Normal // [ l 1 l ] N N [ l 001 and
[i
721 N I/ [o i01 z
We have recently characterized the planar interfaces and havc been able to image at least one array of misfit dislocations perpendicular to tht growth direction. Other interface relaxation events pertaining to the plana interfaces in NZ have been discussed elsewhere.
Microstructural irregularities are common in the peripheria sections of the DSE. These coarser irregularities in fact exhibit remarkable fint structure at higher magnification and resolution. Three principle types of stef defects have been identified. These three step defects are cateogrized based or their distribution/character and are the subject of this study. Fig. 1 A appears a:
a distinct deviation of the interface away from its preferred plana configuration. However, it can be appreciated that this local curvature i:
accommodated by faceting through which the interface attempts to maintain 2
low energy configuration. The flat faces of the facets are precisely aligned alonl (1 11) N // (100) Z and the riser plane of the facets is roughly (001) N // (110) Z The risers of the facets are semicoherent and displav distinct strain contrast a:
may be seen in Fig. 16. The localized strain field (dislocation content) associated with each facet riser could be confirmed by imaging the tilted interface in the weak beam mode (Fig. 1C).
Fig. 2 displays another case of a faceted interface. The lei portion of the image (F) has a curvature to it and like Fig. 1 it is accommodatec by faceting. The right hand portion of this image also exhibits step defects, bu unlike region F, the spacing of these steps is not regular. We term these steps a!
ledges which presumably arise due to migration of facets from region F into the adjacent planar interface. One such ledge is imaged in Fig. 3 using HRTEM. Note that on the either side of the ledge the interface is exactly oriented along (111) N // (100)
Z.
The riser of the ledge is semicoherent and strain contrast was observed. There is one terminating (111) N plane at the riser. The intrinsic geometric misfit between (11 1) N and (200) Z is such that about 10 planes of ( l 11) N are required to match 9 planes of (200) Z. The 'core' of this defect lies well within the NiO phase, presumably due to the iower elastic modulus of NiO compared to cubic Zr02.Another interesting step/dislocation configuration is presented in Fig. 4 which appears as a dissociated interface. Although the deviation is substantial, the interface adopts a saw-tooth morphology such that at least one face of the steps is (111) N // (100) Z. The interstep distance is relatively constant. Such observation was also made by Dravid et al.2 in the case of MgO-ZrOz DSE.
DISCUSSION
It is now well established that crystalline interfaces may possess considerable order and many theories describing this order have been developed. Line defects such as dislocations appear to be an integral part of crystalline interfaces. Such line defects, in addition to their elastic strain, may also exhibit topographic details such as steps at the cores. This dual nature of interface dislocations has been the focus of many investigationsa. In fact Cahng has pointed out that for a planar interface (faceted, unlike a diffuse interface) some defect mechanism is necessary for the migration of interfaces. However, there is still some controversy surrounding the characterization and description of such defects. There appears to be no consensus over the description of various terminologieslo such as steps, growth ledges, structural ledges, facets etc. In view of these problems, we find the recent crystallographic theory of interfacial structure and defects by Pond1 1
particularly attractive. Rather than invoking just the translational symmetry as in the coincidence site lattice (CSL)l2 approach, it utilizes the full space group symmetry of the adjoining crystals to predict admissible line discontinuities and their character. Dependinq on the extent and type of broken symmetry, both the defect character and topological details can be described. We have recently developed a simple but generalized computer program which utilizes the expressions given by Pond to predict the admissible defects and the defect character in question. Some of the results obtained by this program are discussed in the later part of this section.
Figs. 1 and 2 (region F) are clearly the cases where the interface locally relaxes into lower energy configuration via faceting. However, when local curvature is accommodated, neither the number of facets nor the distance between succesive facets are constant or random but are definitely regular. In such cases, the interfacet distance decreases as the interface normal deviates from its preferred orientation. It thus appears logical to term Fig. 1 steps as orientational facets whose function is to change the interface orientation (which is a consequence of extraneous force such as growth perturbations as in
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our case) with minimum expense of interface energy. On the other hand, in Fig.
the interface does not possess any curvature but its unrelaxed orientation I
unfavorable (away from cusp position in energy against orientation plot) an thus it dissociates into multiple steps. If we consider the interface energy pc unit area for the facet faces and risers as a, and a2 respectively, the interfac should, in principle, exhibit only one large step as this corresponds to minimu~
interface energy. But the fact that such a large step readily dissociates int multiple steps implies that the interaction energy among the multiple steps ma have a local minimum. The increase in number of steps (and thus the interfac area) contributes to the increase in total interfacial enrgy but elastic and othc interactions among the steps may tend to reduce the total interface energy a described in some details by King & Smitha. The result thus necessitates certain number of steps per unit length (area) of the interface. It is therefor appropriate to term these steps as structural facets. The fact that thes interfaces facet implies that low energy interfaces are available via relaxatior The cause of faceting is attributed to unfavorable orientation of the interface i its unrelaxed state. A fine scale analog of structural facets may be structun ledge proposed by Hall et al.14 Dahmenl5 has recently discussed this aspect i the context of invariant line analysis in FCC-BCC and FCC-(near)HCP interfaces.
The step defects at the RHS portion of Fig. 2 are termed ledge which are distributed quite randomly along the interface. Unlike region F, therl is no geometric constraint (curvature) on this portion of the interface for thl presence of any steps. One such profusely ledged plan-view NZ interface i:
shown in Fig. 5. It can be deduced that both the height and the distribution o such ledges are random. The migration of ledges necessitates normal growth a the interface. The interface migration occurs while most of its area is in thc cusp position. Such ledges have been investigated in several metallic system!
with regard to solid-state phase transformation processes. Following Pondll, a:
described earlier, we have made computations for a few simple cases o defects due to broken translation symmetry pertaining to Fig. 4. A reasonable agreement was obtained by taking t ~ i o as 1/2[110] and tzro2 as [100]. fhl expected dislocation Burgers vector is
gG
~,-,G,OJI and the step height correspond:to
',!+a
C 6 + 2 ).
When this vector is resolved into components parallel an(perpendicular to the interface, the resultant vectors have almost equa magnitude. Thus it is expected that this dislocation possesses about equal climt and glide components. It also follows. that movement of this defect along tht interface necessitates migration of the interface by the ledge mechanism11 provided the necessary driving force is available.
Assuming that a step defect mechanism for the migration of planar interface prevails, we can discuss the functional aspect of the abok mentioned step defects. Following Pond, it is possible to predict the dislocatic strain and topographical character of admissible line discontinuities. As long 2
the discontinuity has a step associated with it (may not possess dislocatic character, i.e. pure step), it can function as a growth step. This follows becaus if appropriate driving force for the movement of the step (and the associate dislocation, if present) is available, the movement of the step along th interface results in migration of the interface along its normal. A classic example of which is the propagation of Shockley partial dislocations alor
interphase boundary between Ag2AI and Alf7. The change in composition necessary for the transformation can be accomplished by diffusive processes which usually are the rate determining processes. It should be mentioned that even structural facets can migrate along the interface in a conservative manner inducing interface migrationls. With this in view, all of the aforementioned step discontinuities are potential growth steps.
CONCLUSION
Microstructural irregularities in NZ DSE are associated with a rich variety of interfacial defects analogous to those in solid-state transformation interfaces. The topographical defects observed in the study can be categorized as:
a) Orientational facets: These accommodate local curvature of the interfaces.
b) Structural facets: These are the dissociation products of a higher energy interface into multiple steps.
c) L e d g e s : These are randomly distributed step defects with or without dislocation character.
If a step mechanism for the migration of a planar interface prevails, there appears to be ample physical reasoning to assume all above defects as potential growth steps.
REFERENCES
1) See for example, A. Revcolevschi in Mat. Sci. res., Vol. 20 (Plenum Press, NY 1986), 115.
2) V.P. Dravid et al., in Phase Transformation 87 (Inst. Metals, 1988) 630.
3) G.A. Chadwick, Prog. Mat. Sci., Vol 12 (1963) 97.
4) V.P. Dravid et al., Ultramicroscopy, Vol. 29 (1989) 60.
5) V.P. Dravid et al., submitted for the Fall meeting of MRS, 1989.
6) G. Dhalenne & A. Revcolevschi, J. Cryst. Growth, Vol. 69 (1984) 616.
7) H. Laroui et al., J. Mat. Sci., Vol 24 (1989) 562.
8) A. King & D.A. Smith, Acta Cryst. Vol. A36 (1980) 335.
9) J.W. Cahn, Acta Met. Vol. 8 (1960) 556.
10) D.A. Smith
in
Proc MRS, EM of Materials, Vol. 31 (1984) 233.11) R.C. Pond in Dislocations in Solids (ed. F.R.N. Nabarro), Vol 8., Ch. 38.
12) W. Bollman, Crystal Defects & Crystalline Interfaces(Springer -Berlin 1970).
13) V.P. Dravid et al. unpublished research.
14) M.G. Hall et al., Surf. Sci., Vol 31 (1972) 257.
15) U. Dahmen, Scr. Met., Vol. 21 (1987) 1029.
16) H.I. Aaronson in Decomposition of ~ u s t e n i t e by Diffusional Processes (Interscience, NY 1962) 387.
17) J.M. Howe et al., Phil. Mag. Vol. A56 (1987) 31.
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Fig.
Fig.
1 (A) Orientational facets along a NZ interfacc The interfacet distance decreases as the interface normal deviates from (111) N /l (100) 2.
1 (B) Strong beam BF image of the faceted interface
exhibiting strain contras,
Fig. 1 (C) Tilted interface irnag in weak beam mode.
Fig. 2 Transition of orientational facets (F) into ledges (L).
Fig. 3 HRTEM image of an isolated ledge. The terminating (1 11) N plane is indicated with an arrow.
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Fig. 4 Dissociated NZ interface exhibiting saw-tooth morphology of structural facets. Note small kinks on some facets as indicated.
Fig. 5 Profusely ledged NZ interface in plan-view. Both heights and distribution of ledges are random.
