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The nucleation of vacancy condensation cavities on the (100), (111) and (110) metal-oxide interfaces of aluminum
THE
NUCLEATION
OF
(lOO), (111) AND
VACANCY
CONDENSATION
(110) METAL-OXIDE
CAVITIES
INTERFACES
ON
THE
OF ALUMINUM*
T. R. ANTHONY? The respective supercoolings required to nucleate vacancy condensation cavities on the (loo), the (111) and the (110) metal-oxide interfaces of aluminum were directly observed with 8 hot-stage microsoope. The critical supercooling was found to be significantly lower on the (110) metal-oxide interface then on either the (100) or the (111) metal-oxide interfaces. The relatively smell supercooling on the (110) metal-oxide interface is believed to be the result of the heterogeneous nucle8tion of v8cancy cavities on intersection points of screw dislocations with this interface. Cavity nucleation apparently occurs “homogeneously” on the other two interfaces, whose near-surfaoe regions contain only edge-like dislocations. FORMATION DES VIDES DE CONDENSATION DES LACUNES SUR LES INTERFACES METAL-OXYDE (loo), (111) ET (110) DE L’ALUMINIUM Les surfusions respectivement n&esseires pour donner naissence it des vides de condensation de lecunes sur les interfaces m&al-oxyde (loo), (111) et (110) de l’aluminium ont BtBobservbes directement avec un microsclopemuni d’un porte&hantillon chauffent. L’suteur a trouvb que la surfusion critique est nettament plus foible sur l’interface m&al+xyde (110) que sur lea interfaces m&al-oxyde (100) et (111). 11 pense que la surf&on relativement faible pour l’interfrtce m&al-oxyde (110) rbsulte de 18 germination inhomogene dea vides de leounes sur les points d’intersection des dislocations vis avec cette interface. La germination des vides se produit apparemment de faTon homogbne sur les deux autres interfaces, dont les regions voisines de la surface contiennent seulement des dislocations de la famille des dislocations coin. DIE KEIMBILDUNG HOHLRiiUME AUF
DER DURCH LEERSTELLENKONDENSATION ENTSTANDENEN DEN (lOO)-, (11I)- UND (1 lo)-METALL-OXID-GRENZFLACHEN IN ALUMINIUM
Die zur Keimbildung der durch Leerstellenkondensation entstendenen Hohlrhume auf den (IOO), (11 l)- und (1 lO)-Metell-Oxid-Grenzfltihen in Aluminium notwendigen Unterkiihlungen wurden in einem mit Heizpatrone ausgestatteten Mikroskop direkt beobaohtet. Die kritische Unterkiihlung fiir Kondensation auf (llO)-GrenzAiichen w8r betriichtlich kleiner als 8Uf den beiden anderen Metall-OxidGrenzfliichen. Die reletiv kleine notwendige Unterkiihlung auf der (1 IO)-Grenzflilche wird der heterogenen Keimbildung der Leerstellen-Hohlriiume an den DurohstoDpunkten der Schraubenversetzungen duroh diese Grenzfliiche zugeschrieben. Die Keimbildung fiir HohlrBume ist euf den anderen zwei Fl&chen, deren oberfliichennehe Bereiche nur Versetzungen mit Stufencharakter ent,hrtlten,anscheinend “homogen.” I. INTRODUCTION
During csncy rium
an anneal
concentration saturation
in aluminum
value.
has
Since the contiguous shown
a
metal-crystalline-oxide
super-
Since a preliminary
this deficiency
to
exists adjacent low
aluminum
to the free
variation
density.“)
appeared
dislocation
ineffective
of dislocations
with
and annealed aluminum,
interface vacancy
in
sink,(2*3)
effect
interface
measured
desirable
required
critical
This paper
to the
in this laboratory revealed
a wide
supercoolings,
it
this phenomenon
reports
orientation
to nucleate
on
of aluminum.(6~9J1)
also
to investigate
of crystallographic
toolings
implies that except for
is necessary cavities
investigation
polycrystals
in more detail.
has been
in the literature
that
condensation
of the
metal-oxide
to be a totally
vacancy
cooling
In electropolished which
nucleate
reaches an equilib-
disagreement
supercooling
become
saturated.
p thick
the critical
Subsequent
causes
surface,
this
over
the va-
solution
aluminum
a zone 200-300
There is a marked
at high temperatures,
vacancy
of the
on the supercondensation
an occasional intersecting grain boundary, there are relatively few vacancy sinks available to the near-
cavities on the metal-oxide
surface region. As a consequence, with a suitable cooling rate, the vacancy supersaturation in the zone
Aluminum single crystals (99.9999% purity) were grown from the melt in horizontal graphite boats by
neighboring
the free surface
will reach
II. EXPERIMENTAL
the critical
(100) and (111) faces were available. The particular orientations of the three crystals and their growth directions are shown in Fig. 2. The single crystals which were in the form of rectangular rods 6 mm x
* Received September 12, 1969; revised September 26, 1969. t General Electric Research and Development Center, Scheneatady, New York. 18, MAY
1970
PROCEDURE
described elsewhere.(13) Three senarate single crystals were grown so that samples with*(llO),
resulting vacancy condensation cavities (Fig. 1) have been the subject of a number of investigations(4-*Z) since they were first reported by Dougherty and Davis.(d)
VOL.
of aluminum.
a method
point where vacancy precipitates begin to condense heterogeneously on the metal-oxide interface. The
ACTA METALLURGICA,
interfaces
a study
6 mm were cut into four 2.5 cm lengths with a tine jeweler’s saw and etched severely in aqua regia to remove the strained material resulting from the
471
sectioning.
ACTA
472
METALLURGICA,
VOL.
18,
1970
interface introduced by preceding vacancy condensation cavities. III. condensation
‘. ;,_ ,Aluminum
__,
’
,, ‘;
,. ,, _’
matrix-~
,’
::
-,
: 7
* 5: .-<
FIG. 1. A vacancy condensation cavity on the metaloxide interface of aluminum formed by cooling from a high temperature anneal.
The appropriate face of each crystal was ground and polished using successively finer grades of an aluminum-oxide abrasive. Following a three day anneal at 64O”C, this same face was then electropolished in a solution of 80% absolute methanol and 20% perchloric acid maintained at 0°C. Polishing was accomplished at 20 V with a surface current density of 1.2 amps/cm2 for approximately 10 min. After eleotropolishing, the samples were washed in fresh absolute methanol, rinsed in a jet of distilled water and dried with a blast of compressed air. The sample was then placed in a hot-stage microscope open to the atmosphere and annealed at a constant temperature for various periods of time. The annealing temperature for all of the experiments in this investigation lay in the range of 615 f 15°C. The length of the anneal varied from l-24 hr. Following the anneal, all of the samples were cooled at the same rate of O.B”C/sec. During cooling, the specimen was observed at 750 x with a light microscope and the temperature at which nucleation of the vacancy cavities commenced was recorded. Since a large number of such experiments were carried out in this investigation, the twelve single crystals were subjected to multiple tests. Before each individual experiment, the surface of the specimen was re-electropolished in the manner described above so as to remove any irregularities in the metal-oxide
I [r--YG--I-
-CRYSTAL
(100)
-
(110)
-
GROWTH iDIRECTION
FIQ. 2. Orientation and growth direction of the single orystal samples.
EXPERIMENTAL
RESULTS
The individual supercoolings required to nucleate vacancy cavities on the (lOO), (111) and (110) metaloxide interfaces of aluminum are shown in Fig. 3. Also the average and minimum values of these supercoolings are given for each interface. In a few cases, no vacancy cavities formed in the However, a search of area under observation. adjacent areas always revealed vacancy cavities. These few cases were not included in Fig. 3. From Fig. 3, it is seen that the supercoolings necessary to nucleate vacancy cavities on the (100) and the (111) metal-oxide interfaces of aluminum sre
xx
xx
0
“x
1
lxxx IX”
MO)
x x
-
Iloo)
ml)
METAL-OXIDE INTERFACE X.*..
= INDIVIDUAL EXPERIMENT
Fro. 3. The individual, minimum end average experimental supercooling8 required to nucleate vacancy condensation cavities on the (IlO), (100) snd the (111) metal-oxide interfaces. equal. In contrast, the requisite supercooling for the (110) metal-oxide interface is considerably smaller. For a particular interface, it was found that there w&s an inverse relation between the size of an individual vacancy cavity and the supercooling required for cavity nucleation. That is, large vacancy cavities were consistently associated with the smaller supercooling values for anyone of the three metal-oxide interfaces investigated. The initial cavity nucleation density also varied inversely with the recorded supercooling values on any particular interface. No attempt was made to compare quantitatively the initial nucleation density on the three different interfaces. As has been reported previously,‘B) the vacancy cavities exhibited a rectangular form on the (100) and (110) surfaces and usually sn irregular form on the (111) surface. Fimdly for the range of annealing times (l-24 hr) nearly
ANTHONY:
VACANCY
CONDENSATION
CAVITIES
used in these experiments, there appeared to be no relation between the length of the anneal and the me~ured su~reooling. IV. DISCUSSION
A. Variation of supercooling values on a particular metal-oxide bterface Single crystals of aluminum grown from the melt contain numerous subboundaries, the majority of which lay parallel to the growth direction of the crystal.05) These subboundaries with misorientations as low as several seconds of an arc have been shown to be effective vacancy sinks.(15) The drainage of vacancies from the near-surface region to such substructure sinks can appreciably As vacancies affect the measured supercooling. are absorbed by the substructure, the vacancy supersaturation is reduced and additional supercooling is needed to cause the nucleation of vacancy cavities. The depletion of vacancies from the supersaturated solution by the substructure is dependent on both the root-mean-square displacement R of a vacancy during supercooling and the substructure spacing. For the conditions appropriate to the (110) interface, R was approximately 1 x 1O-2 cm while for the (111) and (100) interfaces, R was about 3 x 1V cm. The substructure configuration was determined simply by measuring the spacing of the narrow zones depleted of vacancy cavities on the crystal surface, following each experiment.(is) It was found that the substructure spacing varied from 1 x lO-2 to 20 x 10M2cm in the crystals used in this investigation. The entire range of substructure spacing was found in all three types of crystals so that the difference in supercooling between the (110) and the (100) and (111) interfaces cannot be attributed to a difference in the substructure spacing of their respective crystals. Since the appropriate diffusion length of a vacancy was smaller than the substructure spacing on the (110) interface, the measured supercooling here was minimally affected by substructure drainage. This lack of any consequential vacancy drainage on the (110) interface is reflected in the small 6°C difference between the minimum value and the average supercooling value recorded on this interface. For the (100) and (111) interfaces, on the other hand, sufficient time elapsed before the critical supersaturation was reached so that significant vacancy drainage occurred in areas of the crystal where the substructure spacing was small. Because the substructure was not visible on the electropolished surface, the area under observation was always random
ON
METAL-OXIDE
INTERFACES
473
with respect to the substructure position and spacing. Thus, some observations were made on areas where there was substantial vacancy drainage to the substructure while others were made where there was little or no drainage. For this reason, there is a relatively large difference between the minimum supercooling and the average supercooling recorded on both the (100) and (111) surfaces. In some instances closely underlying substructure may drain away ao many vacancies that vacancy cavities do not form. Such a substructure configuration would explain the cases where no vacancy cavities were observed on certain sections of the crystal surface. In addition, substructure drainage can also account for the inverse relation between the measured supercooling and the size and density of vacancy condensation cavities. In cases where a fraction of the vacancies are absorbed by neighboring substructure, one would expect to have a larger meas~ed su~rcooling and a smaller final cavity size. Finally, the leakage of vacancies to the substructure can account for the large supercooling of 150°C recorded by previous investigators@) for single crystals of aluminum which had been grown and annealed in a manner essentially identical to that described in this paper. In a number of individual experiments in the present investigation, superooolings of this magnitude were measured. B. The dierence between the cr&ical su~~~wo~~~s 0% the (II@), (111) and (~~~) ~e~~~x~de ~~e~~a~s At the annealing temperatures used in this investigation, the aluminum-oxide film has a composite crystalline-amorphous structure which originates in the following manner.(16-20) During electropolishing, a thin film of amorphous Alz03 forms and covers the surface of the aluminum. On heating, aluminum ions diffuse from the bulk metal through the amorphous Also, layer and react with oxygen on the free surface to form additional amorphous Also,. At a temperature of approximately 500%, a crystalline form of Al,Os nucleates at the amorphous-oxide-metal interface. The nuclei of this crystalline oxide rapidly grow to a torminal thickness and thereafter grow only radially and parallel to the metal-oxide interface until coverage of this interface is complete. With a temperature-independent nucleation density of 3 x lo@/ cm2 and a radial growth rate of 7 x lo4 om~sec,(17) the crystalline A.&O3 completely covers the metaloxide interface after thirty minutes at 600°C. Since the minimum annealing period used in this investigation was one hour, all of the present experiments
474
ACTA
METALLURGICA,
were concerned with the nucleation of vacancy condensation cavities on a purely metal-crystallineoxide interface. Previous investigator@) using annealing times less than one hour in a tem~rature region where crystalline Al&$ was forming have noted a marked dependenoe of the critical supercooling on the time at which the specimen is kept at high temperature. This dependence is probably, in part, a result of the swiftly changing character (from amorphous to crystalline oxide) of the metal-oxide interface for such short anneals. In addition, in contrast to the growth process of the amorphous A&O,, the crystalline AlzOs grows by the diffusion of oxygen through the amorto the phous oxide layer(l’) or cracks therein underlying metal. This new growth process undoubtedly causes stresses in both the oxide film and the metal since the specific volume of the crystalline oxide is 40 per cent larger than that of the metal from which it forms. The rate at which these stresses are relieved may also be a contributing factor to thedependence of the measured supercooling on the time at temperature as reported previously.(ll) For the present experiments, however, the annealing period appeared to be sufficiently long to eliminate such time dependent factors since there appeared to be no relation between the measured supercooling and the duration of an anneal. The crystalline aluminum oxide which forms above 500°C is basically a face-centered cubic structure that nucleates epitaxially on the (111) and the (110) surfaces and nonepitaxially on the (100) surface. Transmission electron difFractiono6) has revealed that the oxide is polycrystalline on the (100) surface whereas the oxide exhibits a predominantly singlecrystal pattern on both the (110) and (111) snrfaces. This crystalline aluminum oxide grows as flat plates on the (111) metal surface but forms a faceted structure on the (110) metal surface. The faceting is such that the crystalline oxide on the (110) metal surface is also bounded by (111) metal planes. On the basis of the above evidence, one would anticipate that the surface energy of the metal-oxide interface would be a maximum on the (100) surface and would be smaller and approximately equal on the (110) and the (1ll)surfaces. Consequently, one would expect that the critical supercooling would be a minimum on the (100) face and be larger and approximately equal for the (111) and (110) metal The contrary observations in these interfaces. experiments (see Fig. 3) imply that the difference in critical su~rc~lings between the (llO), (111) and the (110) interfaces does not originate in surface
VOL.
18, 1979
energy differences between these particular metaloxide interfaces. If the orientations of the sample surfaces ooinoided precisely with one of the three low-index 7&b orientations examined in this study, then the metal surface should be atomistioally smooth except for single atoms or single vacancies. On the other hand, if the surface orientations deviated by a small angle 13from that of a low-index plane, a number of atomic steps proportional to 0 would appear on the surfaces. Since the orientations of the samples used in this investigation were only accurate to within one half of a degree and since the surfaces of the samples became very slightly rounded during eleotropolishing, all of the samples, without doubt, had atomic steps on their surfaces. Thus, a second possible explanation of the supercooling differences is that vacancy cavities may have heterogeneously nucleated at varying supercoolings on the different atomic steps associated with the respective metaloxide interfaces. In this regard, one would anticipate that those surfaces possessing the highest energy steps would exhibit the lowest critical supercoolings. The energy of atomic steps on the (lOO), (111) and (110) surfaces can be determined from the surface torques that strive to turn the surfaces into a perfect low-index orientations.(21) If the energy of the atomic step is high, the surface torque will be large and vice versa. Unfortunately, there have been no measurements of surface torques around low-index orientations in aluminum. However, theoretical calculations based on a nearest-neighbor mode1(22) and experimental studies of other f.o.o. crystaMz3) indicate that the surface torque is: largest near the (111) surface, somewhat smaller near the (100) surface and virtually zero near the (110) surface. Co~equently, for aluminum, one would expect that the step energies would be highest on the (111) surface and smallest on the (110) surface. Hence, if steps were serving as heterogeneous nucleation points for vacancy oondensation cavities, the critical supercooling should be smallest on the (111) interface and highest on the (110) interface. The diametri~lly opposite observations in these experiments (see Fig. 3) thus indicate that the variation in critical superooolings between the three low-index interfaces does not arise from differences in the atomic steps on these surfaces. The difference in critical supercoolings between the three interfaces can be explained, instead, by a model based on the different types of dislocations which intersect with the individual interfaces. Below each metal-oxide interface: image forces will cause
ANTHONY: Al,O,
film
VACANCY
CONDENSATION
CAVITIES
‘..
/ Aluminum
matrix
FIG. 4(a). The heterogeneous nucleation of vacancy condensation csvities on the intersection points of the (110) met&oxide interface with the pure (110) screw dislocations of the near-surface region. AloO.
film
Aluminum
matrix
/
FIG. 4(b). The “homogeneous” nucleation of vacenoy condensation cavities on the (100) and (111) metal-oxide interfaces in areas away from the edge-like dislocations of the near-surface region.
dislocations remaining in the near-surface region to intersect perpendicularly with the interface.(“) As a consequence, immediately below the (110) surface, all of the dislocations will adopt a purely screw character since they are forced by image forces to lie in the (110) direction (Fig. 4a). In contrast, these same image forces will cause dislocations below the (100) and (111) surfaces to acquire a large edge component (Fig. 4b). The edge-like dislocations below the (100) and the (111) interfaces can easily absorb excess vacancies by climbing since their cores consist of the edge of an extra plane of atoms where vacancies may be readily incorporated. (25) Consequently, in the nesrsurface region around these edge-like dislocations, the vacancy supersaturation will be a minimum. Thus, the nucleation of vacancy condensation cavities on the (100) and the (111) metal-oxide interfaces will initially occur on areas of the interface farthest away from the edge-like dislocations (Fig. 4b). As a result, the critical supercoolings observed on the (100) and
ON
METAL-OXIDE
INTERFACES
476
the (111) metal-oxide interfaces will be relatively large and will depend only on the surface energy of these interfaces. In this regard, it is interesting to note that the critical supercooling appears to be smaller on the nonepitaxial (100) interface than on the epitaxial (111) interface as one would expect from surface energy considerations. According to the proposed model, the nucleation of vacancy cavities on the (110) metal-oxide interface occurs quite differently. Although a pure screw can be forced to absorb vacancies by breaking up the screw into a helical configuration, such a rearrangement will be strongly resisted by the image forces of the (110) interface as well as the line tension of the dislocation. Hence, for the pure screw dislocations which occupy the near-surface region adjacent to the (110) interface, there is no readily available mechanism for the absorption of vacancies.(25) The same image and line tension forces that resist the conversion of the pure screw into a. helix in the (110) near-surface region will, on the other hand, assist in the heterogeneous nucleation of vacancy condensation cavities at the intersection points of the pure (110) screw dislocations with the metaloxide interface. Accordingly, it is expected that the intersection points of these dislocations with the (110) metal-oxide interface will act as sites for the heterogeneous nucleation of vacancy condensation cavities during cooling (see Fig. 4a). Because of the relatively large dislocation core energy, the supersaturation required for the nucleation of vacancy cavities at these sites should be significantly reduced from that required on the unblemished met&oxide interface.c6) Consequently, the critical supersaturation for vacancy condensation should be very much lower on the (110) metal-oxide interface than on either the (111) or the (100) metal-oxide interfaces as was observed in these experiments. CONCLUSION
The supercooling required to nucleate vacancy cavities on the metal-oxide interfaces of aluminum was found to be significantly lower on the (110) metal-oxide interface than on either the (111) or (100) metal-oxide interfaces. On the (110) interface, nucleation of vacancy cavities probably occurs heterogeneously at the intersection points of (110) screw dislocations with the metal-oxide interface. In contrast, cavity nucleation apparently proceeds “homogeneously” and with a much larger supercooling on the (100) and (111) metal-oxide interfaces whose near-surface regions contain only edge-like dislooations.
ACTA
476
METALLURGICA,
The small difference in critical superooolings between the (100) and (111) interfaces is attributable to the fact that the crystalline aluminum oxide is predominantly single crystalline and epitaxial on the (111) metal surface while it is polycrystalline and nonepitaxial on the (100) metal surface. ACKNOWLEDGEMENTS
I would like to thank Dr. H. E. Cline for his critical review of the manuscript and for his suggestion that the anisotropy in supercoolings might originate in the types of dislocations beneath the respective metal-oxide interfaces. I would also like to thank Professor B. Chalmers for his comments and suggestions which helped to strengthen this manuscript. REFERENCES 1. A. AUTHIEE, C. B. RoDaERs rend A. R. LANG, Phil. Mag. la, 547 (1965). 2. J. E. -1s and B. C. MASTERS,Phi&. Mug. 17,(1968). 3. P. 5. DOR~ON, S. KRITZINGER and R. E. SYALLMAN, Phil. &fag. 17, 769 (1968). 4. P. E. DOUQEERTY and R. S. DAVIS, Acta Met. 7, 118 (1969). 6. M. B. KAREN and D. H. POLONIS, Acta Met. 10,821 (1962). 6. J. R. JA~PERSE end P. E. DOUGHERTY, PhiZ. Mug. 1, 636 (1964).
VOL.
18, 1970
7. B. K. BASU end C. ELBAUM, Acta Met. 18, 1117 (1965).
8. M. B. JXASEN, R. TAQ~ART and D. H. POLONIS, Phil.
Zag. I%, 453 (1966). 9. D. Foss and 0. H. HERBJORNSEN, Phd. Mw. 18,945 (1966). 10. G. A. CHADWICK, Phil. Msg. 14, 1295 (1966). 11. 0. H. HERBJORNSENand T. ASTRUP, Phil. Msg. 19, 693 (1969). 12. T. R. ANTHONY, Acta Met. 18,307 (1970). 13. B. CHALMERS, Proc. R. Sot. A175,100 (1940). 14. R. OSHIMA and R. E. FUJITA, Trcww. Japan Inat. Met& 10, 57 (1969). 16. P. E. DOUQEERTY and B. CHAKXERS, Trans. Am. Inst. M&z,. &&@-8 224, 1124 (1962). 16. P. E. DOVQHERTY &nd R. S. DAVIS, J. appZ. Phye. 84, 619 (1963). 17. A. F. BECK, M. A. HEINE, E. J. CAULE and M. J. PRYOR, Corros.Std. 7, 1 (1967). 18. 6. F. RANDALL tand W. J. BERNARD, J. appZ. Phys. 85, 1317 (1964). 19. H. A. FRANCIS, J. uppl. Phy8. 88, 715 (1967). 20. M. J. DIONAM, J. electrochem. Sot. 109, 184 (1962). 21. P. G. SHEWMONand W. M. ROBERTSONin MetaZSu~faees: Structurea, Energetic8 and Kinetics, Chapter 3. American Society of Xet& (1963). 22. J. K. MACKENZIE, A. J. W. MOORE end J. F. IYICHOLAS, J. Phye. Chem. SoZtia 28, 185 (1962). 23. F. WINSLOW tend P. G. SREWMON, Tram. Am. In&. Min. Engre 237, 1078 (1963). 24. A. H. COTTRELL, Dielocatione and PlaatM Plow in. CV#&, p. 54. Oxford Prcsa (1961). 2fi. R. M. THOMSON and R. W. BALLUFFI, J. appZ. Phy8. 38,803 (1962).
NUCLEATION
OF
(lOO), (111) AND
VACANCY
CONDENSATION
(110) METAL-OXIDE
CAVITIES
INTERFACES
ON
THE
OF ALUMINUM*
T. R. ANTHONY? The respective supercoolings required to nucleate vacancy condensation cavities on the (loo), the (111) and the (110) metal-oxide interfaces of aluminum were directly observed with 8 hot-stage microsoope. The critical supercooling was found to be significantly lower on the (110) metal-oxide interface then on either the (100) or the (111) metal-oxide interfaces. The relatively smell supercooling on the (110) metal-oxide interface is believed to be the result of the heterogeneous nucle8tion of v8cancy cavities on intersection points of screw dislocations with this interface. Cavity nucleation apparently occurs “homogeneously” on the other two interfaces, whose near-surfaoe regions contain only edge-like dislocations. FORMATION DES VIDES DE CONDENSATION DES LACUNES SUR LES INTERFACES METAL-OXYDE (loo), (111) ET (110) DE L’ALUMINIUM Les surfusions respectivement n&esseires pour donner naissence it des vides de condensation de lecunes sur les interfaces m&al-oxyde (loo), (111) et (110) de l’aluminium ont BtBobservbes directement avec un microsclopemuni d’un porte&hantillon chauffent. L’suteur a trouvb que la surfusion critique est nettament plus foible sur l’interface m&al+xyde (110) que sur lea interfaces m&al-oxyde (100) et (111). 11 pense que la surf&on relativement faible pour l’interfrtce m&al-oxyde (110) rbsulte de 18 germination inhomogene dea vides de leounes sur les points d’intersection des dislocations vis avec cette interface. La germination des vides se produit apparemment de faTon homogbne sur les deux autres interfaces, dont les regions voisines de la surface contiennent seulement des dislocations de la famille des dislocations coin. DIE KEIMBILDUNG HOHLRiiUME AUF
DER DURCH LEERSTELLENKONDENSATION ENTSTANDENEN DEN (lOO)-, (11I)- UND (1 lo)-METALL-OXID-GRENZFLACHEN IN ALUMINIUM
Die zur Keimbildung der durch Leerstellenkondensation entstendenen Hohlrhume auf den (IOO), (11 l)- und (1 lO)-Metell-Oxid-Grenzfltihen in Aluminium notwendigen Unterkiihlungen wurden in einem mit Heizpatrone ausgestatteten Mikroskop direkt beobaohtet. Die kritische Unterkiihlung fiir Kondensation auf (llO)-GrenzAiichen w8r betriichtlich kleiner als 8Uf den beiden anderen Metall-OxidGrenzfliichen. Die reletiv kleine notwendige Unterkiihlung auf der (1 IO)-Grenzflilche wird der heterogenen Keimbildung der Leerstellen-Hohlriiume an den DurohstoDpunkten der Schraubenversetzungen duroh diese Grenzfliiche zugeschrieben. Die Keimbildung fiir HohlrBume ist euf den anderen zwei Fl&chen, deren oberfliichennehe Bereiche nur Versetzungen mit Stufencharakter ent,hrtlten,anscheinend “homogen.” I. INTRODUCTION
During csncy rium
an anneal
concentration saturation
in aluminum
value.
has
Since the contiguous shown
a
metal-crystalline-oxide
super-
Since a preliminary
this deficiency
to
exists adjacent low
aluminum
to the free
variation
density.“)
appeared
dislocation
ineffective
of dislocations
with
and annealed aluminum,
interface vacancy
in
sink,(2*3)
effect
interface
measured
desirable
required
critical
This paper
to the
in this laboratory revealed
a wide
supercoolings,
it
this phenomenon
reports
orientation
to nucleate
on
of aluminum.(6~9J1)
also
to investigate
of crystallographic
toolings
implies that except for
is necessary cavities
investigation
polycrystals
in more detail.
has been
in the literature
that
condensation
of the
metal-oxide
to be a totally
vacancy
cooling
In electropolished which
nucleate
reaches an equilib-
disagreement
supercooling
become
saturated.
p thick
the critical
Subsequent
causes
surface,
this
over
the va-
solution
aluminum
a zone 200-300
There is a marked
at high temperatures,
vacancy
of the
on the supercondensation
an occasional intersecting grain boundary, there are relatively few vacancy sinks available to the near-
cavities on the metal-oxide
surface region. As a consequence, with a suitable cooling rate, the vacancy supersaturation in the zone
Aluminum single crystals (99.9999% purity) were grown from the melt in horizontal graphite boats by
neighboring
the free surface
will reach
II. EXPERIMENTAL
the critical
(100) and (111) faces were available. The particular orientations of the three crystals and their growth directions are shown in Fig. 2. The single crystals which were in the form of rectangular rods 6 mm x
* Received September 12, 1969; revised September 26, 1969. t General Electric Research and Development Center, Scheneatady, New York. 18, MAY
1970
PROCEDURE
described elsewhere.(13) Three senarate single crystals were grown so that samples with*(llO),
resulting vacancy condensation cavities (Fig. 1) have been the subject of a number of investigations(4-*Z) since they were first reported by Dougherty and Davis.(d)
VOL.
of aluminum.
a method
point where vacancy precipitates begin to condense heterogeneously on the metal-oxide interface. The
ACTA METALLURGICA,
interfaces
a study
6 mm were cut into four 2.5 cm lengths with a tine jeweler’s saw and etched severely in aqua regia to remove the strained material resulting from the
471
sectioning.
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472
METALLURGICA,
VOL.
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1970
interface introduced by preceding vacancy condensation cavities. III. condensation
‘. ;,_ ,Aluminum
__,
’
,, ‘;
,. ,, _’
matrix-~
,’
::
-,
: 7
* 5: .-<
FIG. 1. A vacancy condensation cavity on the metaloxide interface of aluminum formed by cooling from a high temperature anneal.
The appropriate face of each crystal was ground and polished using successively finer grades of an aluminum-oxide abrasive. Following a three day anneal at 64O”C, this same face was then electropolished in a solution of 80% absolute methanol and 20% perchloric acid maintained at 0°C. Polishing was accomplished at 20 V with a surface current density of 1.2 amps/cm2 for approximately 10 min. After eleotropolishing, the samples were washed in fresh absolute methanol, rinsed in a jet of distilled water and dried with a blast of compressed air. The sample was then placed in a hot-stage microscope open to the atmosphere and annealed at a constant temperature for various periods of time. The annealing temperature for all of the experiments in this investigation lay in the range of 615 f 15°C. The length of the anneal varied from l-24 hr. Following the anneal, all of the samples were cooled at the same rate of O.B”C/sec. During cooling, the specimen was observed at 750 x with a light microscope and the temperature at which nucleation of the vacancy cavities commenced was recorded. Since a large number of such experiments were carried out in this investigation, the twelve single crystals were subjected to multiple tests. Before each individual experiment, the surface of the specimen was re-electropolished in the manner described above so as to remove any irregularities in the metal-oxide
I [r--YG--I-
-CRYSTAL
(100)
-
(110)
-
GROWTH iDIRECTION
FIQ. 2. Orientation and growth direction of the single orystal samples.
EXPERIMENTAL
RESULTS
The individual supercoolings required to nucleate vacancy cavities on the (lOO), (111) and (110) metaloxide interfaces of aluminum are shown in Fig. 3. Also the average and minimum values of these supercoolings are given for each interface. In a few cases, no vacancy cavities formed in the However, a search of area under observation. adjacent areas always revealed vacancy cavities. These few cases were not included in Fig. 3. From Fig. 3, it is seen that the supercoolings necessary to nucleate vacancy cavities on the (100) and the (111) metal-oxide interfaces of aluminum sre
xx
xx
0
“x
1
lxxx IX”
MO)
x x
-
Iloo)
ml)
METAL-OXIDE INTERFACE X.*..
= INDIVIDUAL EXPERIMENT
Fro. 3. The individual, minimum end average experimental supercooling8 required to nucleate vacancy condensation cavities on the (IlO), (100) snd the (111) metal-oxide interfaces. equal. In contrast, the requisite supercooling for the (110) metal-oxide interface is considerably smaller. For a particular interface, it was found that there w&s an inverse relation between the size of an individual vacancy cavity and the supercooling required for cavity nucleation. That is, large vacancy cavities were consistently associated with the smaller supercooling values for anyone of the three metal-oxide interfaces investigated. The initial cavity nucleation density also varied inversely with the recorded supercooling values on any particular interface. No attempt was made to compare quantitatively the initial nucleation density on the three different interfaces. As has been reported previously,‘B) the vacancy cavities exhibited a rectangular form on the (100) and (110) surfaces and usually sn irregular form on the (111) surface. Fimdly for the range of annealing times (l-24 hr) nearly
ANTHONY:
VACANCY
CONDENSATION
CAVITIES
used in these experiments, there appeared to be no relation between the length of the anneal and the me~ured su~reooling. IV. DISCUSSION
A. Variation of supercooling values on a particular metal-oxide bterface Single crystals of aluminum grown from the melt contain numerous subboundaries, the majority of which lay parallel to the growth direction of the crystal.05) These subboundaries with misorientations as low as several seconds of an arc have been shown to be effective vacancy sinks.(15) The drainage of vacancies from the near-surface region to such substructure sinks can appreciably As vacancies affect the measured supercooling. are absorbed by the substructure, the vacancy supersaturation is reduced and additional supercooling is needed to cause the nucleation of vacancy cavities. The depletion of vacancies from the supersaturated solution by the substructure is dependent on both the root-mean-square displacement R of a vacancy during supercooling and the substructure spacing. For the conditions appropriate to the (110) interface, R was approximately 1 x 1O-2 cm while for the (111) and (100) interfaces, R was about 3 x 1V cm. The substructure configuration was determined simply by measuring the spacing of the narrow zones depleted of vacancy cavities on the crystal surface, following each experiment.(is) It was found that the substructure spacing varied from 1 x lO-2 to 20 x 10M2cm in the crystals used in this investigation. The entire range of substructure spacing was found in all three types of crystals so that the difference in supercooling between the (110) and the (100) and (111) interfaces cannot be attributed to a difference in the substructure spacing of their respective crystals. Since the appropriate diffusion length of a vacancy was smaller than the substructure spacing on the (110) interface, the measured supercooling here was minimally affected by substructure drainage. This lack of any consequential vacancy drainage on the (110) interface is reflected in the small 6°C difference between the minimum value and the average supercooling value recorded on this interface. For the (100) and (111) interfaces, on the other hand, sufficient time elapsed before the critical supersaturation was reached so that significant vacancy drainage occurred in areas of the crystal where the substructure spacing was small. Because the substructure was not visible on the electropolished surface, the area under observation was always random
ON
METAL-OXIDE
INTERFACES
473
with respect to the substructure position and spacing. Thus, some observations were made on areas where there was substantial vacancy drainage to the substructure while others were made where there was little or no drainage. For this reason, there is a relatively large difference between the minimum supercooling and the average supercooling recorded on both the (100) and (111) surfaces. In some instances closely underlying substructure may drain away ao many vacancies that vacancy cavities do not form. Such a substructure configuration would explain the cases where no vacancy cavities were observed on certain sections of the crystal surface. In addition, substructure drainage can also account for the inverse relation between the measured supercooling and the size and density of vacancy condensation cavities. In cases where a fraction of the vacancies are absorbed by neighboring substructure, one would expect to have a larger meas~ed su~rcooling and a smaller final cavity size. Finally, the leakage of vacancies to the substructure can account for the large supercooling of 150°C recorded by previous investigators@) for single crystals of aluminum which had been grown and annealed in a manner essentially identical to that described in this paper. In a number of individual experiments in the present investigation, superooolings of this magnitude were measured. B. The dierence between the cr&ical su~~~wo~~~s 0% the (II@), (111) and (~~~) ~e~~~x~de ~~e~~a~s At the annealing temperatures used in this investigation, the aluminum-oxide film has a composite crystalline-amorphous structure which originates in the following manner.(16-20) During electropolishing, a thin film of amorphous Alz03 forms and covers the surface of the aluminum. On heating, aluminum ions diffuse from the bulk metal through the amorphous Also, layer and react with oxygen on the free surface to form additional amorphous Also,. At a temperature of approximately 500%, a crystalline form of Al,Os nucleates at the amorphous-oxide-metal interface. The nuclei of this crystalline oxide rapidly grow to a torminal thickness and thereafter grow only radially and parallel to the metal-oxide interface until coverage of this interface is complete. With a temperature-independent nucleation density of 3 x lo@/ cm2 and a radial growth rate of 7 x lo4 om~sec,(17) the crystalline A.&O3 completely covers the metaloxide interface after thirty minutes at 600°C. Since the minimum annealing period used in this investigation was one hour, all of the present experiments
474
ACTA
METALLURGICA,
were concerned with the nucleation of vacancy condensation cavities on a purely metal-crystallineoxide interface. Previous investigator@) using annealing times less than one hour in a tem~rature region where crystalline Al&$ was forming have noted a marked dependenoe of the critical supercooling on the time at which the specimen is kept at high temperature. This dependence is probably, in part, a result of the swiftly changing character (from amorphous to crystalline oxide) of the metal-oxide interface for such short anneals. In addition, in contrast to the growth process of the amorphous A&O,, the crystalline AlzOs grows by the diffusion of oxygen through the amorto the phous oxide layer(l’) or cracks therein underlying metal. This new growth process undoubtedly causes stresses in both the oxide film and the metal since the specific volume of the crystalline oxide is 40 per cent larger than that of the metal from which it forms. The rate at which these stresses are relieved may also be a contributing factor to thedependence of the measured supercooling on the time at temperature as reported previously.(ll) For the present experiments, however, the annealing period appeared to be sufficiently long to eliminate such time dependent factors since there appeared to be no relation between the measured supercooling and the duration of an anneal. The crystalline aluminum oxide which forms above 500°C is basically a face-centered cubic structure that nucleates epitaxially on the (111) and the (110) surfaces and nonepitaxially on the (100) surface. Transmission electron difFractiono6) has revealed that the oxide is polycrystalline on the (100) surface whereas the oxide exhibits a predominantly singlecrystal pattern on both the (110) and (111) snrfaces. This crystalline aluminum oxide grows as flat plates on the (111) metal surface but forms a faceted structure on the (110) metal surface. The faceting is such that the crystalline oxide on the (110) metal surface is also bounded by (111) metal planes. On the basis of the above evidence, one would anticipate that the surface energy of the metal-oxide interface would be a maximum on the (100) surface and would be smaller and approximately equal on the (110) and the (1ll)surfaces. Consequently, one would expect that the critical supercooling would be a minimum on the (100) face and be larger and approximately equal for the (111) and (110) metal The contrary observations in these interfaces. experiments (see Fig. 3) imply that the difference in critical su~rc~lings between the (llO), (111) and the (110) interfaces does not originate in surface
VOL.
18, 1979
energy differences between these particular metaloxide interfaces. If the orientations of the sample surfaces ooinoided precisely with one of the three low-index 7&b orientations examined in this study, then the metal surface should be atomistioally smooth except for single atoms or single vacancies. On the other hand, if the surface orientations deviated by a small angle 13from that of a low-index plane, a number of atomic steps proportional to 0 would appear on the surfaces. Since the orientations of the samples used in this investigation were only accurate to within one half of a degree and since the surfaces of the samples became very slightly rounded during eleotropolishing, all of the samples, without doubt, had atomic steps on their surfaces. Thus, a second possible explanation of the supercooling differences is that vacancy cavities may have heterogeneously nucleated at varying supercoolings on the different atomic steps associated with the respective metaloxide interfaces. In this regard, one would anticipate that those surfaces possessing the highest energy steps would exhibit the lowest critical supercoolings. The energy of atomic steps on the (lOO), (111) and (110) surfaces can be determined from the surface torques that strive to turn the surfaces into a perfect low-index orientations.(21) If the energy of the atomic step is high, the surface torque will be large and vice versa. Unfortunately, there have been no measurements of surface torques around low-index orientations in aluminum. However, theoretical calculations based on a nearest-neighbor mode1(22) and experimental studies of other f.o.o. crystaMz3) indicate that the surface torque is: largest near the (111) surface, somewhat smaller near the (100) surface and virtually zero near the (110) surface. Co~equently, for aluminum, one would expect that the step energies would be highest on the (111) surface and smallest on the (110) surface. Hence, if steps were serving as heterogeneous nucleation points for vacancy oondensation cavities, the critical supercooling should be smallest on the (111) interface and highest on the (110) interface. The diametri~lly opposite observations in these experiments (see Fig. 3) thus indicate that the variation in critical superooolings between the three low-index interfaces does not arise from differences in the atomic steps on these surfaces. The difference in critical supercoolings between the three interfaces can be explained, instead, by a model based on the different types of dislocations which intersect with the individual interfaces. Below each metal-oxide interface: image forces will cause
ANTHONY: Al,O,
film
VACANCY
CONDENSATION
CAVITIES
‘..
/ Aluminum
matrix
FIG. 4(a). The heterogeneous nucleation of vacancy condensation csvities on the intersection points of the (110) met&oxide interface with the pure (110) screw dislocations of the near-surface region. AloO.
film
Aluminum
matrix
/
FIG. 4(b). The “homogeneous” nucleation of vacenoy condensation cavities on the (100) and (111) metal-oxide interfaces in areas away from the edge-like dislocations of the near-surface region.
dislocations remaining in the near-surface region to intersect perpendicularly with the interface.(“) As a consequence, immediately below the (110) surface, all of the dislocations will adopt a purely screw character since they are forced by image forces to lie in the (110) direction (Fig. 4a). In contrast, these same image forces will cause dislocations below the (100) and (111) surfaces to acquire a large edge component (Fig. 4b). The edge-like dislocations below the (100) and the (111) interfaces can easily absorb excess vacancies by climbing since their cores consist of the edge of an extra plane of atoms where vacancies may be readily incorporated. (25) Consequently, in the nesrsurface region around these edge-like dislocations, the vacancy supersaturation will be a minimum. Thus, the nucleation of vacancy condensation cavities on the (100) and the (111) metal-oxide interfaces will initially occur on areas of the interface farthest away from the edge-like dislocations (Fig. 4b). As a result, the critical supercoolings observed on the (100) and
ON
METAL-OXIDE
INTERFACES
476
the (111) metal-oxide interfaces will be relatively large and will depend only on the surface energy of these interfaces. In this regard, it is interesting to note that the critical supercooling appears to be smaller on the nonepitaxial (100) interface than on the epitaxial (111) interface as one would expect from surface energy considerations. According to the proposed model, the nucleation of vacancy cavities on the (110) metal-oxide interface occurs quite differently. Although a pure screw can be forced to absorb vacancies by breaking up the screw into a helical configuration, such a rearrangement will be strongly resisted by the image forces of the (110) interface as well as the line tension of the dislocation. Hence, for the pure screw dislocations which occupy the near-surface region adjacent to the (110) interface, there is no readily available mechanism for the absorption of vacancies.(25) The same image and line tension forces that resist the conversion of the pure screw into a. helix in the (110) near-surface region will, on the other hand, assist in the heterogeneous nucleation of vacancy condensation cavities at the intersection points of the pure (110) screw dislocations with the metaloxide interface. Accordingly, it is expected that the intersection points of these dislocations with the (110) metal-oxide interface will act as sites for the heterogeneous nucleation of vacancy condensation cavities during cooling (see Fig. 4a). Because of the relatively large dislocation core energy, the supersaturation required for the nucleation of vacancy cavities at these sites should be significantly reduced from that required on the unblemished met&oxide interface.c6) Consequently, the critical supersaturation for vacancy condensation should be very much lower on the (110) metal-oxide interface than on either the (111) or the (100) metal-oxide interfaces as was observed in these experiments. CONCLUSION
The supercooling required to nucleate vacancy cavities on the metal-oxide interfaces of aluminum was found to be significantly lower on the (110) metal-oxide interface than on either the (111) or (100) metal-oxide interfaces. On the (110) interface, nucleation of vacancy cavities probably occurs heterogeneously at the intersection points of (110) screw dislocations with the metal-oxide interface. In contrast, cavity nucleation apparently proceeds “homogeneously” and with a much larger supercooling on the (100) and (111) metal-oxide interfaces whose near-surface regions contain only edge-like dislooations.
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476
METALLURGICA,
The small difference in critical superooolings between the (100) and (111) interfaces is attributable to the fact that the crystalline aluminum oxide is predominantly single crystalline and epitaxial on the (111) metal surface while it is polycrystalline and nonepitaxial on the (100) metal surface. ACKNOWLEDGEMENTS
I would like to thank Dr. H. E. Cline for his critical review of the manuscript and for his suggestion that the anisotropy in supercoolings might originate in the types of dislocations beneath the respective metal-oxide interfaces. I would also like to thank Professor B. Chalmers for his comments and suggestions which helped to strengthen this manuscript. REFERENCES 1. A. AUTHIEE, C. B. RoDaERs rend A. R. LANG, Phil. Mag. la, 547 (1965). 2. J. E. -1s and B. C. MASTERS,Phi&. Mug. 17,(1968). 3. P. 5. DOR~ON, S. KRITZINGER and R. E. SYALLMAN, Phil. &fag. 17, 769 (1968). 4. P. E. DOUQEERTY and R. S. DAVIS, Acta Met. 7, 118 (1969). 6. M. B. KAREN and D. H. POLONIS, Acta Met. 10,821 (1962). 6. J. R. JA~PERSE end P. E. DOUGHERTY, PhiZ. Mug. 1, 636 (1964).
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18, 1970
7. B. K. BASU end C. ELBAUM, Acta Met. 18, 1117 (1965).
8. M. B. JXASEN, R. TAQ~ART and D. H. POLONIS, Phil.
Zag. I%, 453 (1966). 9. D. Foss and 0. H. HERBJORNSEN, Phd. Mw. 18,945 (1966). 10. G. A. CHADWICK, Phil. Msg. 14, 1295 (1966). 11. 0. H. HERBJORNSENand T. ASTRUP, Phil. Msg. 19, 693 (1969). 12. T. R. ANTHONY, Acta Met. 18,307 (1970). 13. B. CHALMERS, Proc. R. Sot. A175,100 (1940). 14. R. OSHIMA and R. E. FUJITA, Trcww. Japan Inat. Met& 10, 57 (1969). 16. P. E. DOUQEERTY and B. CHAKXERS, Trans. Am. Inst. M&z,. &&@-8 224, 1124 (1962). 16. P. E. DOVQHERTY &nd R. S. DAVIS, J. appZ. Phye. 84, 619 (1963). 17. A. F. BECK, M. A. HEINE, E. J. CAULE and M. J. PRYOR, Corros.Std. 7, 1 (1967). 18. 6. F. RANDALL tand W. J. BERNARD, J. appZ. Phys. 85, 1317 (1964). 19. H. A. FRANCIS, J. uppl. Phy8. 88, 715 (1967). 20. M. J. DIONAM, J. electrochem. Sot. 109, 184 (1962). 21. P. G. SHEWMONand W. M. ROBERTSONin MetaZSu~faees: Structurea, Energetic8 and Kinetics, Chapter 3. American Society of Xet& (1963). 22. J. K. MACKENZIE, A. J. W. MOORE end J. F. IYICHOLAS, J. Phye. Chem. SoZtia 28, 185 (1962). 23. F. WINSLOW tend P. G. SREWMON, Tram. Am. In&. Min. Engre 237, 1078 (1963). 24. A. H. COTTRELL, Dielocatione and PlaatM Plow in. CV#&, p. 54. Oxford Prcsa (1961). 2fi. R. M. THOMSON and R. W. BALLUFFI, J. appZ. Phy8. 38,803 (1962).