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Characterization of the Tishomingo meteorite
Surface
292
Characterization
Science 246 (1991) 292-298 North-Holland
of the Tishomingo meteorite
K.F. Russell, E.A. Kenik and M.K. Miller Metals and Ceramics Division, Oak Ridge National Laboratory, Received
30 July 1990; accepted
for publication
27 August
Oak Ridge, TN 37831-6376,
USA
1990
A preliminary microstructural characterization of the Tishomingo meteorite has been performed with the combined techniques of atom probe field ion microscopy, analytical electron microscopy, scanning electron microscopy, optical microscopy, and nanoindentation. Eighty percent of this meteorite appears to have undergone a martensitic transformation; the remaining 20% being taenite (y). Field ion microscopy and transmission electron microscopy of the martensitically transformed region revealed a mixture of a face centered cubic phase, a body centered cubic phase and some small face centered cubic precipitates. The composition of these precipitates was found to be between 51 and 57 at% Ni and the surrounding matrix approximately 20% Ni.
1. i”trod”&Il
microscopy, microscopy,
The Tishomingo meteorite is unusual both in its high nickel content of 32.5 wt% Ni and in the relatively low concentration of trace elements. The only other meteorites with compositions in this range that have been studied are the Santa Catharina and Twin City meteorites. However, the microstructure of the Tishomingo meteorite is distinctly different. The martensitically transformed plates are extremely coarse and some have been reported to extend for several centimeters [l-3]. No carbides, graphite, phosphides or silicates have been observed. Some troilite (FeS) and daubreelite (FeCr,S,) inclusions have been observed [l-3]. Despite the unusual martensitic microstructure, this meteorite has been the subject of only a limited number of investigations since its discovery in 1965 [3]. Apart from its overall composition, no compositional information of the phases present has been reported. Indeed, even the phases present in this meteorite have not been fully established. In this paper, a preliminary microstructural characterization of the Tishomingo meteorite is reported with the combined techniques of atom probe field ion microscopy, analytical electron Elsevier Science Publishers
B.V. (North-Holland)
scanning electron microscopy, and nanoindentation.
optical
2. Experimental The material used in this investigation was a 0.4 g slice of the Tishomingo meteorite (USNM#5862). R.S. Clarke (Smithsonian Institution) has reported the bulk composition of this meteorite to be Fe-31.4% Ni, 1.09% Co, 0.018% P and 0.081% S. Microprobe analysis of this sample revealed a uniform composition of Fe-29.5% Ni, 1.28% Co. These results were similar to the microprobe data (31.6% Ni and 1.26% Co) reported by Ives et al. [3]. All compositions in this paper are quoted in atomic percent. The hardness and modulus parameters of the meteorite were determined with a Nano Instruments Nano Indenter [4,5]. This instrument permits the hardness to be measured on a finer scale than traditional microhardness instruments. Analytical electron microscopy was performed in a Philips EM400T equipped with a field emission gun and at 300 kV in a Philips CM30 equipped with a La$ gun. Atom probe field ion microscopy characterization was performed in the ORNL en-
K.F. Russell et al. / Characterization of the Tishomingo meteorite
ergy-compensated atom probe [6,7]. It should be noted that the atom probe field ion microscopy characterization was performed with a specimen temperature of 75 K and that some in-situ transformations were observed due to the combination of the low temperature and the stress on the specimen [ 81.
293
significantly across a plate, as shown in fig. 2. Since the microprobe composition determinations on a polished surface revealed a uniform composition, this anomalously high nickel measurement was due to the preferential etching out of certain constituents of the phase transformed martensite. 3.2. Hardness and modulus
3. Results 3.1. Optical and scanning electron microscopy The general microstructure of the Tishomingo meteorite is shown in the optical micrograph in fig. 1. Eighty percent of this meteorite appears to have undergone a martensitic transformation; the remaining 20% being face centered cubic taenite (y), in agreement with previous studies [l-3]. The average composition of the taenite was measured by X-ray analysis in a scanning electron microscope as 30.6% Ni. In an etched specimen, the composition of the martensite was found to be higher than the average composition and to vary
A series of 24 indentations approximately 4 pm apart were made across the taenite and martensite as shown in fig. 3. The test consisted of a displacement controlled loading segment to a depth of 250 nm, a hold segment, an unloading segment to 20% of the maximum applied load, another hold segment to determine the drift rate, and a complete unloading segment. The hardness and modulus were determined from this procedure using standard methods [5]. The hardness data for the 24 indentations are shown in fig. 4. The average hardnesses of the taenite and martensite phases were 3.6 and 5.7 GPa, respectively. The modulus was observed to be relatively consistent between these two phases; 190.8 and 195.1 GPa, for the taenite and martensite, respectively.
Fig. 1. Optical micrograph of the Tishomingo meteorite showing a coarse martensitic (dark) microstructure and approximately 20% taenite (light regions).
294
Fig. 2. Scanning
K.F. Russell et al. / Characterization
of the Tishomingo meteorite
electron microscope composition profile across the microstructure of an etched specimen that showed high nickel content in the martensitically transformed region due to preferential etching of one phase.
Fig. 3. Optical
micrograph
of the indentations
across the martensite
and taenite
anomalously
295
K.F. Russellet al. / Characterizationof the Tishomingometeorite
-0
5
10
20
15
25
Indent Number Fig. 4. Hardness data for the indentations shown in fig. 3. The average hardnesses of the taenite and martensite were 3.6 and 5.7 GPa, respectively.
3.3. Transmission electron microscopy Transmission electron microscopy of the martensitically transformed regions revealed irreg-
Fig. 5. Transmission electron microscopy face centered cubic precipitates,
ularly-shaped precipitates, as shown in fig. 5. These small precipitates were typically lo-90 nm in size. Selected area diffraction indicated that the precipitates had a disordered face centered cubic crystal structure. Energy dispersive X-ray microanalysis of these precipitates indicated a composition of approximately 57% Ni. Electron diffraction of the martensitically transformed regions revealed the presence of a face centered cubic phase, yi. In-situ experiments with a liquid nitrogen cooled holder in the transmission electron microscope revealed that this yi phase transformed to martensite at cryogenic temperatures. Electron diffraction also indicated the presence of a body centered cubic crystal structure. However, compositional determinations were complicated by the preferential etching of this phase. The small face centered cubic precipitates were not observed in the retained y taenite regions which were characterized by a high density of dislocations, fig. 6. The average composition of
of a region that had undergone a martensitic transformation showing labelled p, a face centered cubic phase, labelled f, and a body centered
the presence cubic phase.
of small
296
K. F. Ruwell et al. / Characterization
of the Tishomingo
meteorite
Fig. 6. Transmission electron micrograph of the taenite showing a high density of dislocations.
the retained taenite was measured as approximately 30% Ni in agreement with the SEM results.
the transmission electron microscope. An atom probe composition profile from one of these precipitates into the surrounding matrix is shown in
3.4. Atom probe field ion microscopy Field ion micrographs of this meteorite, an example being shown in fig. 7, revealed a complex microstructure similar to that observed in Fe-NiC martensite [9-111. A field ion micrograph of a region that exhibited sets of parallel fine-scale bands or microtwins is shown in fig. 8. The average maximum width of these microtwins was estimated to be approximately 3.5 nm. An atom probe composition profile through this type of region is shown in fig. 9. The average composition of this region was found to be Fe-32.0 f 0.79% Ni, 1.42 f 0.20% Co, that is similar to the overall composition. Another type of region, shown in fig. 10, was evident in the field ion micrographs. In this type of region, small irregularly shaped brightly-imaging precipitates were observed in a matrix that was devoid of striations. The size and shape of these precipitates is in agreement with those observed in
Fig. 7. Field ion micrograph of the Tishomingo meteorite showing a complex microstructure.
K.F. Russell et al. / Characterization of the Tishomingo meteorite
Fig. 8. Field ion micrograph of a striated region of the meteorite.
fig. 11. The average compositions of this precipitate and the surrounding matrix were determined to be Fe-55.4 f 1.4% Ni, 0.26 f 0.15% Co, 0.17 f 0.12% S + 0 and Fe-19.7 2 1.26% Ni, 0.85 f 0.34% Co, 1.28 f 0.35% S + 0, respectively. Analysis of other precipitates revealed compositions ranging from 51% Ni to 57% Ni. The small size of either phase did not provide sufficient area in the field ion image to determine the crystal structure
291
Fig. 10. Field ion micrograph of brightly-imaging high-nickelcontent precipitates in a low-nickel-content matrix.
from the relative prominence atomic terraces.
of the low index
4. Discussion The complex multiphase microstructure that was observed in this meteorite indicates that its
Matrix
h n; 20 10 1
04 0
Distance - 25 Ion Blocks
Fig. 9. Atom probe composition profile through a striated region. The average composition of this region was found to be Fe-32.0 * 0.79% Ni, 1.42 f 0.20% Co.
I
I
!
5
10
15
I
20 Distance
1
25
I
30
I
35
40
45
- 50IonBlocks
Fig. 11. Atom probe composition profile from one precipitate shown in fig. 10 into the surrounding matrix. The average compositions of this precipitate and the surrounding matrix were determined to be Fe-55.4f 1.4% Ni, 0.26 rtO.lS% Co, 0.17*0.12% S+O and Fe-19.7*1.26X Ni, 0.85*0.34% Co, 1.28 f 0.35% S + 0, respectively.
298
K. F. Russell et al. / Characterization of the Tishomingo meteorite
thermal history comprised of several stages. Unfortunately, since the origin and the journey of the meteorite through space are unknown, its thermal history has to be deduced from the resulting microstructure. Possible conditions to which this meteorite had been exposed include being subjected to extreme cold or heat and shock due to collisions with other objects. As earlier suggested by Buchheit et al. [l] and Ives et al. [3], it is most probable that the coarse martensitically transformed region arose from the meteorite being subjected to cryogenic temperatures. After this cryogenic excursion, the meteorite must have been heated to a temperature of more than 200°C to produce the decomposition observed in the martensite. If the meteorite had been heated to a temperature above the taenite reversion temperature, A, = 3OO”C, then some of the martensite plates would have a face centered cubic crystal structure. This reversion could also possibly be accomplished at lower temperatures by shock. If the temperature was lower than the A, temperature (= 450°C) then some body centered cubic phase would remain. Up to this stage all the metallic phases have the same composition. If the meteorite was maintained in this intermediate temperature regime, then it is possible for the body centered cubic phase to decompose into the body centered cubic phase of low nickel content (20%) and the face centered cubic precipitates with greater than 51% Ni. Further experiments are required to ascertain whether this speculation is the case. The proposed decomposition schemes are also complicated by uncertainties in the low temperature regime of proposed equilibrium phase diagrams and by kinetic rather than thermodynamic considerations in reactions resulting in compositional changes.
Acknowledgments
The authors would like to thank Dr. R.S. Clarke, Jr., Curator, Division of Meteorites, Smithsonian Institution for providing the sample of the Tishomingo meteorite. The authors would also like to thank B. Lucas and Dr. W.C. Oliver for perfor~ng the Nano Indenter analyses, and W.S. Eatherly, T.J. Henson and R.R. Steele for their technical assistance. This research was sponsored by the Division of Materials Sciences, US Department of Energy, under contract DE-ACOS84OR21400 with Martin Marietta Energy Systems, Inc.
References PI R.D. Buchheit,
J.L. McCall and E.P. Henderson, Proc. Int. Metallographic Society Meeting, November 1968, Denver, Ed. K.A. Johnson and J.H. Bender (International Metallographic Society, Iowa, 1968) pp. 57-59. 121V.F. Buchwald, Handbook of Iron Meteorites, Vol. 3 (University of California Press, Berkeley. 1975) pp. 12011205. 131 L.K. Ives, M.B. Kasen, R.E. Schramm, A.W. Ruff and R.P. Reed, Geochim. Cosmochim. Acta 42 (1978) 1051. [41 M.F. Doerner and W.D. Nix, J. Mater. Res. 1 (1986) 601. and W.D. Nix, 151 M.J. Mayo, R.W. Siegel, A. Narayanasamy J. Mater. Res. 5 (1990) 1073. @I M.K. Miller, J. Phys. (Paris) 47 (1986) C2-493. [71 M.K. Miller and G.D.W. Smith, Atom Probe Microanalysis: Principles and Applications to Materials Problems (Materials Research Society, Pittsburgh, PA, 1989). PI M.K. Miller and K.F. Russell, Surf. Sci. 246 (1991) 299. [91 M.K. Miller, P.A. Beaven and G.D.W. Smith, Metall. Trans. 12 A (1981) 1197. PO1 M.K. Miller, P.A. Beaven, S.S. Brenner and G.D.W. Smith, Metall. Trans. 14 A (1983) 1021. WI K.A. Taylor, L. Chang, E.D.W. Smith, M. Cohen and J.B. Vander Sande, Metall. Trans. A20 (1989) 2717.
292
Characterization
Science 246 (1991) 292-298 North-Holland
of the Tishomingo meteorite
K.F. Russell, E.A. Kenik and M.K. Miller Metals and Ceramics Division, Oak Ridge National Laboratory, Received
30 July 1990; accepted
for publication
27 August
Oak Ridge, TN 37831-6376,
USA
1990
A preliminary microstructural characterization of the Tishomingo meteorite has been performed with the combined techniques of atom probe field ion microscopy, analytical electron microscopy, scanning electron microscopy, optical microscopy, and nanoindentation. Eighty percent of this meteorite appears to have undergone a martensitic transformation; the remaining 20% being taenite (y). Field ion microscopy and transmission electron microscopy of the martensitically transformed region revealed a mixture of a face centered cubic phase, a body centered cubic phase and some small face centered cubic precipitates. The composition of these precipitates was found to be between 51 and 57 at% Ni and the surrounding matrix approximately 20% Ni.
1. i”trod”&Il
microscopy, microscopy,
The Tishomingo meteorite is unusual both in its high nickel content of 32.5 wt% Ni and in the relatively low concentration of trace elements. The only other meteorites with compositions in this range that have been studied are the Santa Catharina and Twin City meteorites. However, the microstructure of the Tishomingo meteorite is distinctly different. The martensitically transformed plates are extremely coarse and some have been reported to extend for several centimeters [l-3]. No carbides, graphite, phosphides or silicates have been observed. Some troilite (FeS) and daubreelite (FeCr,S,) inclusions have been observed [l-3]. Despite the unusual martensitic microstructure, this meteorite has been the subject of only a limited number of investigations since its discovery in 1965 [3]. Apart from its overall composition, no compositional information of the phases present has been reported. Indeed, even the phases present in this meteorite have not been fully established. In this paper, a preliminary microstructural characterization of the Tishomingo meteorite is reported with the combined techniques of atom probe field ion microscopy, analytical electron Elsevier Science Publishers
B.V. (North-Holland)
scanning electron microscopy, and nanoindentation.
optical
2. Experimental The material used in this investigation was a 0.4 g slice of the Tishomingo meteorite (USNM#5862). R.S. Clarke (Smithsonian Institution) has reported the bulk composition of this meteorite to be Fe-31.4% Ni, 1.09% Co, 0.018% P and 0.081% S. Microprobe analysis of this sample revealed a uniform composition of Fe-29.5% Ni, 1.28% Co. These results were similar to the microprobe data (31.6% Ni and 1.26% Co) reported by Ives et al. [3]. All compositions in this paper are quoted in atomic percent. The hardness and modulus parameters of the meteorite were determined with a Nano Instruments Nano Indenter [4,5]. This instrument permits the hardness to be measured on a finer scale than traditional microhardness instruments. Analytical electron microscopy was performed in a Philips EM400T equipped with a field emission gun and at 300 kV in a Philips CM30 equipped with a La$ gun. Atom probe field ion microscopy characterization was performed in the ORNL en-
K.F. Russell et al. / Characterization of the Tishomingo meteorite
ergy-compensated atom probe [6,7]. It should be noted that the atom probe field ion microscopy characterization was performed with a specimen temperature of 75 K and that some in-situ transformations were observed due to the combination of the low temperature and the stress on the specimen [ 81.
293
significantly across a plate, as shown in fig. 2. Since the microprobe composition determinations on a polished surface revealed a uniform composition, this anomalously high nickel measurement was due to the preferential etching out of certain constituents of the phase transformed martensite. 3.2. Hardness and modulus
3. Results 3.1. Optical and scanning electron microscopy The general microstructure of the Tishomingo meteorite is shown in the optical micrograph in fig. 1. Eighty percent of this meteorite appears to have undergone a martensitic transformation; the remaining 20% being face centered cubic taenite (y), in agreement with previous studies [l-3]. The average composition of the taenite was measured by X-ray analysis in a scanning electron microscope as 30.6% Ni. In an etched specimen, the composition of the martensite was found to be higher than the average composition and to vary
A series of 24 indentations approximately 4 pm apart were made across the taenite and martensite as shown in fig. 3. The test consisted of a displacement controlled loading segment to a depth of 250 nm, a hold segment, an unloading segment to 20% of the maximum applied load, another hold segment to determine the drift rate, and a complete unloading segment. The hardness and modulus were determined from this procedure using standard methods [5]. The hardness data for the 24 indentations are shown in fig. 4. The average hardnesses of the taenite and martensite phases were 3.6 and 5.7 GPa, respectively. The modulus was observed to be relatively consistent between these two phases; 190.8 and 195.1 GPa, for the taenite and martensite, respectively.
Fig. 1. Optical micrograph of the Tishomingo meteorite showing a coarse martensitic (dark) microstructure and approximately 20% taenite (light regions).
294
Fig. 2. Scanning
K.F. Russell et al. / Characterization
of the Tishomingo meteorite
electron microscope composition profile across the microstructure of an etched specimen that showed high nickel content in the martensitically transformed region due to preferential etching of one phase.
Fig. 3. Optical
micrograph
of the indentations
across the martensite
and taenite
anomalously
295
K.F. Russellet al. / Characterizationof the Tishomingometeorite
-0
5
10
20
15
25
Indent Number Fig. 4. Hardness data for the indentations shown in fig. 3. The average hardnesses of the taenite and martensite were 3.6 and 5.7 GPa, respectively.
3.3. Transmission electron microscopy Transmission electron microscopy of the martensitically transformed regions revealed irreg-
Fig. 5. Transmission electron microscopy face centered cubic precipitates,
ularly-shaped precipitates, as shown in fig. 5. These small precipitates were typically lo-90 nm in size. Selected area diffraction indicated that the precipitates had a disordered face centered cubic crystal structure. Energy dispersive X-ray microanalysis of these precipitates indicated a composition of approximately 57% Ni. Electron diffraction of the martensitically transformed regions revealed the presence of a face centered cubic phase, yi. In-situ experiments with a liquid nitrogen cooled holder in the transmission electron microscope revealed that this yi phase transformed to martensite at cryogenic temperatures. Electron diffraction also indicated the presence of a body centered cubic crystal structure. However, compositional determinations were complicated by the preferential etching of this phase. The small face centered cubic precipitates were not observed in the retained y taenite regions which were characterized by a high density of dislocations, fig. 6. The average composition of
of a region that had undergone a martensitic transformation showing labelled p, a face centered cubic phase, labelled f, and a body centered
the presence cubic phase.
of small
296
K. F. Ruwell et al. / Characterization
of the Tishomingo
meteorite
Fig. 6. Transmission electron micrograph of the taenite showing a high density of dislocations.
the retained taenite was measured as approximately 30% Ni in agreement with the SEM results.
the transmission electron microscope. An atom probe composition profile from one of these precipitates into the surrounding matrix is shown in
3.4. Atom probe field ion microscopy Field ion micrographs of this meteorite, an example being shown in fig. 7, revealed a complex microstructure similar to that observed in Fe-NiC martensite [9-111. A field ion micrograph of a region that exhibited sets of parallel fine-scale bands or microtwins is shown in fig. 8. The average maximum width of these microtwins was estimated to be approximately 3.5 nm. An atom probe composition profile through this type of region is shown in fig. 9. The average composition of this region was found to be Fe-32.0 f 0.79% Ni, 1.42 f 0.20% Co, that is similar to the overall composition. Another type of region, shown in fig. 10, was evident in the field ion micrographs. In this type of region, small irregularly shaped brightly-imaging precipitates were observed in a matrix that was devoid of striations. The size and shape of these precipitates is in agreement with those observed in
Fig. 7. Field ion micrograph of the Tishomingo meteorite showing a complex microstructure.
K.F. Russell et al. / Characterization of the Tishomingo meteorite
Fig. 8. Field ion micrograph of a striated region of the meteorite.
fig. 11. The average compositions of this precipitate and the surrounding matrix were determined to be Fe-55.4 f 1.4% Ni, 0.26 f 0.15% Co, 0.17 f 0.12% S + 0 and Fe-19.7 2 1.26% Ni, 0.85 f 0.34% Co, 1.28 f 0.35% S + 0, respectively. Analysis of other precipitates revealed compositions ranging from 51% Ni to 57% Ni. The small size of either phase did not provide sufficient area in the field ion image to determine the crystal structure
291
Fig. 10. Field ion micrograph of brightly-imaging high-nickelcontent precipitates in a low-nickel-content matrix.
from the relative prominence atomic terraces.
of the low index
4. Discussion The complex multiphase microstructure that was observed in this meteorite indicates that its
Matrix
h n; 20 10 1
04 0
Distance - 25 Ion Blocks
Fig. 9. Atom probe composition profile through a striated region. The average composition of this region was found to be Fe-32.0 * 0.79% Ni, 1.42 f 0.20% Co.
I
I
!
5
10
15
I
20 Distance
1
25
I
30
I
35
40
45
- 50IonBlocks
Fig. 11. Atom probe composition profile from one precipitate shown in fig. 10 into the surrounding matrix. The average compositions of this precipitate and the surrounding matrix were determined to be Fe-55.4f 1.4% Ni, 0.26 rtO.lS% Co, 0.17*0.12% S+O and Fe-19.7*1.26X Ni, 0.85*0.34% Co, 1.28 f 0.35% S + 0, respectively.
298
K. F. Russell et al. / Characterization of the Tishomingo meteorite
thermal history comprised of several stages. Unfortunately, since the origin and the journey of the meteorite through space are unknown, its thermal history has to be deduced from the resulting microstructure. Possible conditions to which this meteorite had been exposed include being subjected to extreme cold or heat and shock due to collisions with other objects. As earlier suggested by Buchheit et al. [l] and Ives et al. [3], it is most probable that the coarse martensitically transformed region arose from the meteorite being subjected to cryogenic temperatures. After this cryogenic excursion, the meteorite must have been heated to a temperature of more than 200°C to produce the decomposition observed in the martensite. If the meteorite had been heated to a temperature above the taenite reversion temperature, A, = 3OO”C, then some of the martensite plates would have a face centered cubic crystal structure. This reversion could also possibly be accomplished at lower temperatures by shock. If the temperature was lower than the A, temperature (= 450°C) then some body centered cubic phase would remain. Up to this stage all the metallic phases have the same composition. If the meteorite was maintained in this intermediate temperature regime, then it is possible for the body centered cubic phase to decompose into the body centered cubic phase of low nickel content (20%) and the face centered cubic precipitates with greater than 51% Ni. Further experiments are required to ascertain whether this speculation is the case. The proposed decomposition schemes are also complicated by uncertainties in the low temperature regime of proposed equilibrium phase diagrams and by kinetic rather than thermodynamic considerations in reactions resulting in compositional changes.
Acknowledgments
The authors would like to thank Dr. R.S. Clarke, Jr., Curator, Division of Meteorites, Smithsonian Institution for providing the sample of the Tishomingo meteorite. The authors would also like to thank B. Lucas and Dr. W.C. Oliver for perfor~ng the Nano Indenter analyses, and W.S. Eatherly, T.J. Henson and R.R. Steele for their technical assistance. This research was sponsored by the Division of Materials Sciences, US Department of Energy, under contract DE-ACOS84OR21400 with Martin Marietta Energy Systems, Inc.
References PI R.D. Buchheit,
J.L. McCall and E.P. Henderson, Proc. Int. Metallographic Society Meeting, November 1968, Denver, Ed. K.A. Johnson and J.H. Bender (International Metallographic Society, Iowa, 1968) pp. 57-59. 121V.F. Buchwald, Handbook of Iron Meteorites, Vol. 3 (University of California Press, Berkeley. 1975) pp. 12011205. 131 L.K. Ives, M.B. Kasen, R.E. Schramm, A.W. Ruff and R.P. Reed, Geochim. Cosmochim. Acta 42 (1978) 1051. [41 M.F. Doerner and W.D. Nix, J. Mater. Res. 1 (1986) 601. and W.D. Nix, 151 M.J. Mayo, R.W. Siegel, A. Narayanasamy J. Mater. Res. 5 (1990) 1073. @I M.K. Miller, J. Phys. (Paris) 47 (1986) C2-493. [71 M.K. Miller and G.D.W. Smith, Atom Probe Microanalysis: Principles and Applications to Materials Problems (Materials Research Society, Pittsburgh, PA, 1989). PI M.K. Miller and K.F. Russell, Surf. Sci. 246 (1991) 299. [91 M.K. Miller, P.A. Beaven and G.D.W. Smith, Metall. Trans. 12 A (1981) 1197. PO1 M.K. Miller, P.A. Beaven, S.S. Brenner and G.D.W. Smith, Metall. Trans. 14 A (1983) 1021. WI K.A. Taylor, L. Chang, E.D.W. Smith, M. Cohen and J.B. Vander Sande, Metall. Trans. A20 (1989) 2717.