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Self-diffusion of lead in oriented grain-boundaries
300
ACTA
METALLURGICA,
lated role of colloidal particles in the genesis of interior screw dislocations is apparent. If perfect platelets are grown in the absence of colloidal particles, then screw dislocations with a Burgers vector normal to the large face should only form peripherally by impingement of two platelets, never in the interior of a face. Such an experiment has already been reported. Kowarski4 in 1933 described the growth of perfect platelets of paratoluidine from the vapor. Translated into current terminology Kowarski reported that very thin platelets of para-toluidine grew at constant thickness on the surface of condensation from the vapor. The interference color of a given platelet was constant over long periods of time. Occasionally screw dislocations were generated on the extended faces and thickening occurred. He observed that the screw dislocations were always generated at the periphery of the platelets, and only at the point of intersection when neighboring platelets came into contact with one another during growth. He stated that the neighboring platelets were tilted slightly with respect to one another, which agrees exactly with observations of Newkirk on CdIz3 and the theoretical postulate.’ Since the growth occurred from the vapor phase, there can be no argument as to the absence of colloidal particles. The objection may be raised that there is some fundamental difference between growth from solution and from vapor. The accumulated evidence at this Laboratory appears to definitely eliminate the possibility. However, work is in progress to observe the growth of CdIz platelets from a solution free of colloidal particles. G. W. SEARS General Electric Research Laboratory Schenectady, New York References 1. F. C. Frank, Disc. Far. Sot. 5, 48 (1949). 2. J. C. Fisher, R. L. Fullman, and G. W. Sears, Acta Met. 2, 344 (1954). 3. J. B. Newkirk, to be published. 4. L. Kowarski, J. Chem. Phys. 32, 303 (1935). * Received December 9, 1954.
Self-Diffusion of Lead in Oriented Grain-Boundaries* It is now generally accepted that there are three paths for diffusion in polycrystalline metals, namely, through the lattice (lattice diffusion), along grain-boundaries (grain-boundary diffusion) and over the surface (surface diffusion). Diffusion experiments are of interest as they can give information about the properties of metals and because of the fact that diffusion plays a prominent role in many processes which can take place in metals (e.g., precipitation).
VOL.
,.p
3,
1955
99
.’
‘0--60----_r: I’
FIG. 1.
In order to avoid mathematical and experimental difliculties in investigating the complete diffusion behavior of a metal, it is advisable to study if possible, the different diffusion phenomena separately. Though it is easy to eliminate the complications due to surface diffusion, experiments on grain-boundary diffusion are practically always interfered with by lattice diffusion. This might give difficulties in analysing the experimental results obtained. For the case, however, that grainboundary diffusion is relatively important and provided that the number of atoms diffused along the boundaries is much larger than the number of atoms diffused through the lattice, Fisher’ has worked out a method which enables the analysis of the penetration curves, deviating from those obtained by homogeneous diffusion. With the aid of this method Hoffman and TurnbulP and Slifkin, Lazarus and Tomizuka3 calculated the grain-boundary diffusion constant for silver. For zinc this was done by Wajda4 and for lead by Okkerse.5 Smoluchowski et aPg have shown by means of an etching method that the penetration of silver and zinc in copper is a function of the orientation difference between neighboring grains. Similar observations, but explained in a different way, were made by Turnbull and Hoffman.rO One of the corollaries of the dislocation model is that the diffusion coefficient may be expected to be different in different directions, relative to the position of the dislocation lines in the grain-boundary. As a matter of fact such an anisotropy of the grainboundary diffusion was demonstrated by Couling and SmoluchowskP by means of autoradiographs. Because of the fact that in our laboratory grainboundary and lattice diffusion in lead were investigated with the aid of the sectioning technique and as it turned out to be possible to distinguish between these two phenomena, it seemed worth while to try to determine the influence of the structure of the boundary on the self-diffusion of lead in oriented grain-boundaries. In what follows a description of the technique applied and of the results obtained will be given. In order to have the disposal of grain boundaries with a well-defined structure, lead bicrystals were prepared after the method of Tiedema and Kooy.12 The dimensions of these bicrystals were: length 90 mm, width 30
LETTERS
TO
mm and thickness 6 mm. From the so-obtained bicrystalline “bars” cylinders with a diameter of 23 mm and a thickness of 5 mm were prepared by carefully sawing and machining (see Fig. 1). The specimens were only slightly deformed as appeared from Laue backreflection diagrams. After an anneal at 310°C in vacuum for 10 days, the deformed layer disappeared in most cases as was proved by Laue back-reflection photographs. This means a complete recovery of the deformed bicrystals. The method by which the penetration curves were obtained was the same as that used for single crystalline and polycrystalline specimens by Okkerse.6 The specimens were activated in the normal way with thorium B. Then they were annealed in vacuum at a temperature of 22O“C for about 70 hours. After this diffusion-anneal, sections with a thickness of approximately 10~ were turned off on a lathe. The radioactivity of each section was determined by means of a bell-shaped Geiger counter. In each case, 2.5 sections were examined. After the appropriate corrections, the penetration curve could be obtained from the weight and the activity of each section. The bicrystals were grown with orientations as given in Fig. 2. Figure 2a represents the orientations of bi-
THE
301
EDITOR
0
40
IN)
0’
200 d,s,o”cr
160
200
160 -
200 dmtonce
160
210 ,” Y
2.w
1. 00
0 0
40
cll) a
_
.
l
EicrystalI
in,,
240
FIG. 3.
1
FIG. 2.
It103
crystal I. As can be seen from the drawings, both crystals have a common [llO]-direction which coincides with the longitudinal direction of the specimen, whereas each crystal has a (Ill)-plane which makes an angle of 81 degrees with the surface of the specimen. In bicrystal II (Fig. 2b) both crystals have a common [211]-direction coinciding with the longitudinal direction of the specimen. Again a (Ill)-plane of each crystal makes an angle of 81 degrees with the surface. Finally in bicrystal III (Fig. 2c) the two crystals have a (ill)-plane perpendicular to the surface of the specimen making angles of 9 degrees with the longitudinal direction. They have a common [l lo]-direction perpendicular to the surface. From bicrystal I, two specimens were examined whereas from the bicrystals II and III three specimens were analysed. The penetration curves of different
302
ACTA
~~TALL~RGI~A,
specimens from the same bicrystal were very similar. Typical curves for each bicrystal are shown in Fig. 3. From the results obtained it seems to us that the following conclusions can be drawn : a. In bicrystal I no detectable grain-boundary diffusion occurs. If the penetration curve is analysed in the normal way a lattice diffusion constant of .5.2OX1O-12 cm2/sec can be calculated. This value is 17 percent higher than the value calculated from the equation LI= 1.17 exp( - 2j7OO/~T) cm2/sec as given by Okkerse.j This deviation is within the accuracy of the experiments. In the case of bicrystal I the dislocation lines are expected to be perpendicular to the diffusion direction. b. The deviating penetration curves obtained for the bicrystals II and III can neither be analysed in the normal way, nor by the method of Fisher.’ As far as we can see this must be ascribed to the fact that the conditions required for the availability of Fisher’s method are not satisfied. Still, there is a marked difference between the curves II and III. The penetration is much larger in the last case. Now in bicrystal III the dislocation lines are expected to be parallel to the diffusion direction. For bicrystal II it is not so easy to say what the position of the dislocation lines is with respect to the diffusion direction. It is, however, clear that diffusion in the boundary is much faster than for bicrystal X but slower than for bicrystal III. G. From these experiments it follows that the diffusion in grain-boundaries is influenced strongly by the position of the dislocation lines with respect to the diffusion direction. For the case that the lines are parallel to the diffusion direction (bicrystal III) diffusion goes much faster than for the case that these fines are per~ndicular to the diffusion direction.
Laboratory for Physical Technical University, Delft, Netherlands
Chemistry,
References
VOL.
3,
1955
Formation of the Intermetallic Compound PtZn at Room Temperature*
After zinc was electrodeposited on platinum, early chemists concerned with electroanalysis of brasses and similar alloys observed that when the electrodeposit was removed in acid, a black residue remained firmly attached to the platinum. This deposit was unattacked by nitric or hydrochloric acids, and was stated to be platinum black.1*2J A similar residue was not observed when copper was electrodeposited instead, accounting for the recommendation that a layer of copper on platinum precede electrodeposition of zinc. With zinc, the residual film, although strongly resembling platinum black, was not analyzed by those who stated its composition, the identification being based solely on appearance and lack of attack by acids. If the compound is platinum black, as claimed, the mechanics of its formation at such low temperatures (25560°C) and within short times (one to several hours) is of some interest and, hence, appeared to deserve further investigation. A commercial zinc plating bath was prepared based on zinc sulfate4 employing a current density of 1.5 amp/sq cm. Platinum cathodes were cleaned in nitric acid, washed in water, and then heated to a bright red temperature in an oxygen-gas flame. Immediately after cleaning, the platinum was plated with zinc for a period of 1.5 minutes at room temperature. This provided a deposit approximately 7X1F4 cm thick. The zinc-plated platinum was then either aged at room temperature for a definite period of time, or heated somewhat above room temperature under specified conditions. Following this period, the zinc was removed by dissolution in 15 voI.o& nitric acid at room temperature. In ali cases, a black or gray residue remained on the platinum, insoluble in hot concentrated nitric or hot concentrated hydrochtoric acids. The B. OKKERSE residual film, however, was attacked by aqua regia. T. J. TIEDEMA It was apparent visually that the longer the time of W. (2. BURGERS aging, or the higher the temperature of heating, the darker was the film. But even when the film was stripped immediately after plating, evidence of a superficial residue was faintly apparent. This meant that aging at room temperature for 1.5 minutes sufficed to produce
1. J. C. Fisher, J. Appl. Phys. 22, 74 (1951). 2. R. E. Hoffman and D. Turnbull, J. Appl. P hys. 22,634,984 (1951). 3. L. Slifkin, D. Lazarus, and T. Tomizuka, J. Appl. Phys. 23, 1032 (1952). 4. E. S. Wajda, Acta Met. 2, 184 (19.54). 5. B. Okkerse, Acts Met. 2, 551 (1954). 6. R. Smoluchowski et al, Phys. Rev. 76,470 (1949). 7. R. Smoluchowski et al, Phys. Rev. 83, 163 (1951). 8. R. Smoluchowski et al, J. Appl. Phys. 22, 1260 (1951). 9. R. Smoluchowski et al, J. Appt. Phys. 23, 785 (1952). 10. D. Turnbull and R. E. Hoffman, Acta Met. 2, 419 (1954). 11. L. Couling and R. Smoluchowski, Phys. Rev. 91, 246 (1953). 12. T. J. Tiedema and C. L. Kooy, Ned. T. Nat. 2,419 (1954). * Received
December
27, 19.54.
TABLE I.Effect of aging on weight after immersion
Aging temperature
Room Room 56°C
* Time in plating bath.
gain of &-plated in HNO,.
Pt cathodes
Aging time
0 24 1 1 1
(15 min)* hrs hr hr hr
1.3 2.8
ACTA
METALLURGICA,
lated role of colloidal particles in the genesis of interior screw dislocations is apparent. If perfect platelets are grown in the absence of colloidal particles, then screw dislocations with a Burgers vector normal to the large face should only form peripherally by impingement of two platelets, never in the interior of a face. Such an experiment has already been reported. Kowarski4 in 1933 described the growth of perfect platelets of paratoluidine from the vapor. Translated into current terminology Kowarski reported that very thin platelets of para-toluidine grew at constant thickness on the surface of condensation from the vapor. The interference color of a given platelet was constant over long periods of time. Occasionally screw dislocations were generated on the extended faces and thickening occurred. He observed that the screw dislocations were always generated at the periphery of the platelets, and only at the point of intersection when neighboring platelets came into contact with one another during growth. He stated that the neighboring platelets were tilted slightly with respect to one another, which agrees exactly with observations of Newkirk on CdIz3 and the theoretical postulate.’ Since the growth occurred from the vapor phase, there can be no argument as to the absence of colloidal particles. The objection may be raised that there is some fundamental difference between growth from solution and from vapor. The accumulated evidence at this Laboratory appears to definitely eliminate the possibility. However, work is in progress to observe the growth of CdIz platelets from a solution free of colloidal particles. G. W. SEARS General Electric Research Laboratory Schenectady, New York References 1. F. C. Frank, Disc. Far. Sot. 5, 48 (1949). 2. J. C. Fisher, R. L. Fullman, and G. W. Sears, Acta Met. 2, 344 (1954). 3. J. B. Newkirk, to be published. 4. L. Kowarski, J. Chem. Phys. 32, 303 (1935). * Received December 9, 1954.
Self-Diffusion of Lead in Oriented Grain-Boundaries* It is now generally accepted that there are three paths for diffusion in polycrystalline metals, namely, through the lattice (lattice diffusion), along grain-boundaries (grain-boundary diffusion) and over the surface (surface diffusion). Diffusion experiments are of interest as they can give information about the properties of metals and because of the fact that diffusion plays a prominent role in many processes which can take place in metals (e.g., precipitation).
VOL.
,.p
3,
1955
99
.’
‘0--60----_r: I’
FIG. 1.
In order to avoid mathematical and experimental difliculties in investigating the complete diffusion behavior of a metal, it is advisable to study if possible, the different diffusion phenomena separately. Though it is easy to eliminate the complications due to surface diffusion, experiments on grain-boundary diffusion are practically always interfered with by lattice diffusion. This might give difficulties in analysing the experimental results obtained. For the case, however, that grainboundary diffusion is relatively important and provided that the number of atoms diffused along the boundaries is much larger than the number of atoms diffused through the lattice, Fisher’ has worked out a method which enables the analysis of the penetration curves, deviating from those obtained by homogeneous diffusion. With the aid of this method Hoffman and TurnbulP and Slifkin, Lazarus and Tomizuka3 calculated the grain-boundary diffusion constant for silver. For zinc this was done by Wajda4 and for lead by Okkerse.5 Smoluchowski et aPg have shown by means of an etching method that the penetration of silver and zinc in copper is a function of the orientation difference between neighboring grains. Similar observations, but explained in a different way, were made by Turnbull and Hoffman.rO One of the corollaries of the dislocation model is that the diffusion coefficient may be expected to be different in different directions, relative to the position of the dislocation lines in the grain-boundary. As a matter of fact such an anisotropy of the grainboundary diffusion was demonstrated by Couling and SmoluchowskP by means of autoradiographs. Because of the fact that in our laboratory grainboundary and lattice diffusion in lead were investigated with the aid of the sectioning technique and as it turned out to be possible to distinguish between these two phenomena, it seemed worth while to try to determine the influence of the structure of the boundary on the self-diffusion of lead in oriented grain-boundaries. In what follows a description of the technique applied and of the results obtained will be given. In order to have the disposal of grain boundaries with a well-defined structure, lead bicrystals were prepared after the method of Tiedema and Kooy.12 The dimensions of these bicrystals were: length 90 mm, width 30
LETTERS
TO
mm and thickness 6 mm. From the so-obtained bicrystalline “bars” cylinders with a diameter of 23 mm and a thickness of 5 mm were prepared by carefully sawing and machining (see Fig. 1). The specimens were only slightly deformed as appeared from Laue backreflection diagrams. After an anneal at 310°C in vacuum for 10 days, the deformed layer disappeared in most cases as was proved by Laue back-reflection photographs. This means a complete recovery of the deformed bicrystals. The method by which the penetration curves were obtained was the same as that used for single crystalline and polycrystalline specimens by Okkerse.6 The specimens were activated in the normal way with thorium B. Then they were annealed in vacuum at a temperature of 22O“C for about 70 hours. After this diffusion-anneal, sections with a thickness of approximately 10~ were turned off on a lathe. The radioactivity of each section was determined by means of a bell-shaped Geiger counter. In each case, 2.5 sections were examined. After the appropriate corrections, the penetration curve could be obtained from the weight and the activity of each section. The bicrystals were grown with orientations as given in Fig. 2. Figure 2a represents the orientations of bi-
THE
301
EDITOR
0
40
IN)
0’
200 d,s,o”cr
160
200
160 -
200 dmtonce
160
210 ,” Y
2.w
1. 00
0 0
40
cll) a
_
.
l
EicrystalI
in,,
240
FIG. 3.
1
FIG. 2.
It103
crystal I. As can be seen from the drawings, both crystals have a common [llO]-direction which coincides with the longitudinal direction of the specimen, whereas each crystal has a (Ill)-plane which makes an angle of 81 degrees with the surface of the specimen. In bicrystal II (Fig. 2b) both crystals have a common [211]-direction coinciding with the longitudinal direction of the specimen. Again a (Ill)-plane of each crystal makes an angle of 81 degrees with the surface. Finally in bicrystal III (Fig. 2c) the two crystals have a (ill)-plane perpendicular to the surface of the specimen making angles of 9 degrees with the longitudinal direction. They have a common [l lo]-direction perpendicular to the surface. From bicrystal I, two specimens were examined whereas from the bicrystals II and III three specimens were analysed. The penetration curves of different
302
ACTA
~~TALL~RGI~A,
specimens from the same bicrystal were very similar. Typical curves for each bicrystal are shown in Fig. 3. From the results obtained it seems to us that the following conclusions can be drawn : a. In bicrystal I no detectable grain-boundary diffusion occurs. If the penetration curve is analysed in the normal way a lattice diffusion constant of .5.2OX1O-12 cm2/sec can be calculated. This value is 17 percent higher than the value calculated from the equation LI= 1.17 exp( - 2j7OO/~T) cm2/sec as given by Okkerse.j This deviation is within the accuracy of the experiments. In the case of bicrystal I the dislocation lines are expected to be perpendicular to the diffusion direction. b. The deviating penetration curves obtained for the bicrystals II and III can neither be analysed in the normal way, nor by the method of Fisher.’ As far as we can see this must be ascribed to the fact that the conditions required for the availability of Fisher’s method are not satisfied. Still, there is a marked difference between the curves II and III. The penetration is much larger in the last case. Now in bicrystal III the dislocation lines are expected to be parallel to the diffusion direction. For bicrystal II it is not so easy to say what the position of the dislocation lines is with respect to the diffusion direction. It is, however, clear that diffusion in the boundary is much faster than for bicrystal X but slower than for bicrystal III. G. From these experiments it follows that the diffusion in grain-boundaries is influenced strongly by the position of the dislocation lines with respect to the diffusion direction. For the case that the lines are parallel to the diffusion direction (bicrystal III) diffusion goes much faster than for the case that these fines are per~ndicular to the diffusion direction.
Laboratory for Physical Technical University, Delft, Netherlands
Chemistry,
References
VOL.
3,
1955
Formation of the Intermetallic Compound PtZn at Room Temperature*
After zinc was electrodeposited on platinum, early chemists concerned with electroanalysis of brasses and similar alloys observed that when the electrodeposit was removed in acid, a black residue remained firmly attached to the platinum. This deposit was unattacked by nitric or hydrochloric acids, and was stated to be platinum black.1*2J A similar residue was not observed when copper was electrodeposited instead, accounting for the recommendation that a layer of copper on platinum precede electrodeposition of zinc. With zinc, the residual film, although strongly resembling platinum black, was not analyzed by those who stated its composition, the identification being based solely on appearance and lack of attack by acids. If the compound is platinum black, as claimed, the mechanics of its formation at such low temperatures (25560°C) and within short times (one to several hours) is of some interest and, hence, appeared to deserve further investigation. A commercial zinc plating bath was prepared based on zinc sulfate4 employing a current density of 1.5 amp/sq cm. Platinum cathodes were cleaned in nitric acid, washed in water, and then heated to a bright red temperature in an oxygen-gas flame. Immediately after cleaning, the platinum was plated with zinc for a period of 1.5 minutes at room temperature. This provided a deposit approximately 7X1F4 cm thick. The zinc-plated platinum was then either aged at room temperature for a definite period of time, or heated somewhat above room temperature under specified conditions. Following this period, the zinc was removed by dissolution in 15 voI.o& nitric acid at room temperature. In ali cases, a black or gray residue remained on the platinum, insoluble in hot concentrated nitric or hot concentrated hydrochtoric acids. The B. OKKERSE residual film, however, was attacked by aqua regia. T. J. TIEDEMA It was apparent visually that the longer the time of W. (2. BURGERS aging, or the higher the temperature of heating, the darker was the film. But even when the film was stripped immediately after plating, evidence of a superficial residue was faintly apparent. This meant that aging at room temperature for 1.5 minutes sufficed to produce
1. J. C. Fisher, J. Appl. Phys. 22, 74 (1951). 2. R. E. Hoffman and D. Turnbull, J. Appl. P hys. 22,634,984 (1951). 3. L. Slifkin, D. Lazarus, and T. Tomizuka, J. Appl. Phys. 23, 1032 (1952). 4. E. S. Wajda, Acta Met. 2, 184 (19.54). 5. B. Okkerse, Acts Met. 2, 551 (1954). 6. R. Smoluchowski et al, Phys. Rev. 76,470 (1949). 7. R. Smoluchowski et al, Phys. Rev. 83, 163 (1951). 8. R. Smoluchowski et al, J. Appl. Phys. 22, 1260 (1951). 9. R. Smoluchowski et al, J. Appt. Phys. 23, 785 (1952). 10. D. Turnbull and R. E. Hoffman, Acta Met. 2, 419 (1954). 11. L. Couling and R. Smoluchowski, Phys. Rev. 91, 246 (1953). 12. T. J. Tiedema and C. L. Kooy, Ned. T. Nat. 2,419 (1954). * Received
December
27, 19.54.
TABLE I.Effect of aging on weight after immersion
Aging temperature
Room Room 56°C
* Time in plating bath.
gain of &-plated in HNO,.
Pt cathodes
Aging time
0 24 1 1 1
(15 min)* hrs hr hr hr
1.3 2.8