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Ion-implanted potassium in tungsten
Volume 9, number
MATERIALS
9
ION-IMPLANTED
POTASSIUM
LETTERS
May 1990
IN TUNGSTEN
K.T. KIM and G. WELSCH ofMaterials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, LISA
Department
I March 1990
Received
Potassium was ion-implanted at 180 keV into recrystallized lamp-grade tungsten with dose of 5 X 1O’“/cm*. In the as-implanted condition potassium atoms are dispersed in a microcrystalline tungsten lattice. Upon annealing, very small and finely distributed potassium bubbles develop. The bubbles geometries are (0 11 }-faceted dodecahedra with truncation on {00 I } planes. The crystal orientation of condensed potassium precipitates inside the bubbles is identical with the orientation of the tungsten matrix.
1. Introduction Tungsten and potassium are bee metals with lattice parameters of 3.16 and 5.3 1 nm, respectively, at room temperature. Potassium is essentially insoluble in tungsten, but once trapped in the tungsten lattice, it clusters and is rather immobile even at high temperature [ l-51. Potassium’s large atomic diameter relative to that of tungsten ( 1.67 times larger) is believed a main reason for its insolubility and immobility. Arrays of fine potassium bubbles in lamp tungsten wire are known to have a profound effect on the recovery and recrystallization of lamp filaments [ 6- 141 in which longitudinal grain boundaries are pinned by stringers of potassium bubbles. This special dispersion of potassium is obtained by extensive wire-drawing of potassium-doped powder compacts. The effectiveness of potassium bubbles as high temperature barriers increases with the fineness of dispersion and with concentration. Bubbles smaller than 50 nm are useful for creep-resistance of lamp filament wire [ lo-121 which contains a total potassium dopant concentration of typically less than 100 ppm [ 131. Higher concentrations are difficult to achieve with the powder-metallurgical (P/M) process used for lamp wire fabrication [ 14 1. Ion implantation can achieve much higher potassium concentrations, although only in a relatively shallow surface layer. Implanted layers are suitable for research on the behavior of high potassium concen0 167-577x/90/$
03.50 0 Elsevier Science Publishers
B.V. (North-Holland)
trations. An additional advantage is that a much finer dispersion is obtained by ion implantation than is possible by the P/M method. Further, a desired concentration is easily obtained by choice of ion dose and energy. The objective of this work was to produce an ultrafine dispersion of potassium in tungsten and to investigate how the potassium develops into bubbles upon annealing [ 15 1.
2. Experimental A bulk sample of commercial doped tungsten (General Electric type 2 18), measuring 8.4 mm in diameter, was used for this experiment. This material is quite pure, except for about 0.03 at% potassium which is trapped in relatively large and easily distinguishable bubbles (fig. la). It was first recrystallization annealed at 2100°C in hydrogen atmosphere to a grain size of 0.1 to 0.5 mm. After preparation of a polished surface to mirror finish, 180 keV ion implantation with potassium was performed at room temperature to a dose of 5 x 1016/cm2, resulting in an implanted surface layer of about 150 nm thickness and a peak concentration of about 9 at% potassium. After implantation, annealing treatments were given at several temperatures from 800 to 2200°C for 1 h each in hydrogen atmosphere to study the evolution of potassium precipitates and bubbles in 295
Volume 9, number
MATERIALS
9
the implanted layer. The microstructural analysis was performed by transmission electron microscopy (Philips EM 400T microscope at 120 kV operating voltage). For this purpose thin foils containing the implanted layer were prepared by the following procedure. First, discs of 3 mm diameter and 0.1 mm thickness were mechanically prepared. The implanted surface was masked off with lacquer, and using single-jet-electropolishing the discs were thinned electrolytically from the unimplanted side.
3. Results and discussion 3. I. As-implanted microstructure The microstructure
of as-received
recrystallized
LETTERS
May 1990
tungsten is shown in fig. 1. It shows the interior of a single grain, which contains dislocation arrays and networks as subboundaries. Most regions are dislocation-free, except where some dislocations have been trapped by potassium bubbles. These bubbles are the result of factory-doping and have sizes of 20 to 300 nm. They are significantly larger and more widely spaced than those produced by ion-implantationdoping and are easily discerned from the latter. A change in structure to microcrystalline was found in the as-implanted surface layer (fig. 2). Electron diffraction pattern taken near the edge of the hole in the TEM foil, area A in fig. 2a, no longer shows individual diffraction spots, but shows diffraction rings instead (fig. 2b). No diffraction spots or rings of potassium were found. This suggests that the as-implanted potassium is distributed in the surface layer in the form of a pseudo-solid solution. A thicker section of the TEM foil, area B in fig. 2a, contains both implanted surface layer and unimplanted substrate. The diffraction patterns from such area contain diffraction rings from the implanted surface layer and discrete diffraction spots from unimplanted substrate (fig. 2~). If a random dispersion of potassium atoms in the tungsten lattice is assumed, the average spacing of potassium atoms at the concentration peak is approximately 2.2 times the interatomic distance of tungsten. Because of their relatively large atomic size, potassium atoms must cause severe lattice distortions in the tungsten to accommodate the misfit. The diffraction spot spacings provide a calibration for the ring diameters. The radii of the diffraction rings are slightly smaller, by about 1.4%, than the corresponding reciprocal lattice spacings of unimplanted tungsten crystal. This indicates a lattice expansion of the implanted tungsten. The crystallite size is so fine (grains are less than 5 nm in diameter) that no dislocations can be observed. 3.2. Implanted and annealed microstructure
Fig. 1 (a) Microstructure in a grain of as-received recrystallized lamp-grade tungsten (2 1I-type) and (b) corresponding selected area diffraction pattern.
296
Upon heating, recovery and recrystallization take place with recrystallization typically beginning above 900’ C [ 11,16 1. Because of the insolubility of potassium in tungsten this results in the formation of potassium precipitates and the vapor pressure of potassium leads to the formation of bubbles. Vacancies, which are part of the radiation damage pro-
Volume 9. number
9
MATERIALS
LETTERS
May 1990
Fig. 2. (a) TEM micrograph of 5 x 10’” K atoms/cm* implanted tungsten. (b) A diffraction pattern taken from area A (war the edge of the hole in the TEM foil) shows sharp diffraction rings from a microcrystalline grain structure. (c) A diffractton pattern taken from area B (thicker section of the TEM foil. which contains both implanted layer and unimplanted substrate) shows both diffractton rungs from the implanted surface layer and discrete diffraction spots from unimplanted substrate.
duced during ion implantation, agglomerate during annealing and either form voids [ 17,18 ] or contribute to the bubble volume. The temperature dependence of potassium bubble growth for isochronal ( 1 h) annealing treatments is shown in fig. 3. After annealing at 8OO”C, a very high density of extremely fine bubbles was obtained. The measured mean bubble diameter was about 7.5 nm. With increasing annealing temperature, the bubble size increased to sizes
between IO and 200 nm at 22OO’C. After cooling to room temperature, the large bubbles are partially filled with precipitate phase of elemental potassium. This is analyzed in more detail below. The recrystallization during annealing caused the diffraction rings to disappear. Discrete diffraction spots of large tungsten grains developed again. but also extra diffraction spots appeared. The extra spots after annealing at 1200°C (fig. 4) were identified as 297
MATERIALS LETTERS
Volume 9, number 9
Fig. 3. Temperature dependence of potassium bubble growth in I h annealingireatment lOOO”C, (c) 12OO”C,(d) 22OO”C*, *additional annealing for 30 min at 2300°C.
diffraction and double diffraction spots of elemental potassium precipitates. Although a selected area for diffraction contains many bubbles, a single crystal orientation relationship of potassium with the tungsten matrix is observed. The potassium precipitates in the bubbles are oriented in congruence with the tungsten matrix, i.e. LOO1lwll[OOlIK and
[OIO]w]] [OIOIK.
This orientation relation has previously been found by Snow [ 191 in recrystallized lamp tungsten wire.
298
May 1990
- dose: 5 X lOI K atoms/cm’.
(a) 8OO”C, (b)
Shape, size, and distribution of potassium bubbles in an implanted layer after annealing at 1200°C is shown in fig. 5. Best bubble imaging was obtained by under-focusing the objective lens of the electron microscope. This, however, has the consequence, that dislocations which are also present, are not clearly imaged. The bubble surfaces have a strong tendency to develop {0 1 1 } facets of tungsten. The bubbles are {0 11 }-faceted dodecahedra with truncation on (00 1} planes and with rounded corners. They are identical to those observed in doped tungsten lamp wire [4,201.
Volume 9, number
9
MATERIALS
May 1990
LETTERS
. d ‘5
n
.
0
lo
d *
‘3
l
0 1
‘3
.
0 b ‘
.
h
“01 l*o
.
3
*_
0
0
*
.
d
l
0 *
.
0
.
I?
0
d
0
Fig. 4. Diffraction patterns from tungsten and potassium which have epitaxial relationship. (a) [ 0011 zone, (b) [ 0 I I ] zone, (c) [ 11 I ] - and A, first- and second-order K double diffraction zone. 0, W matrix spot; l and o, first- and second-order K spots, respectively; spots, respectively; q , higher than second-order K double diffraction spots.
299
Volume 9, number 9
MATERIALS LETTERS
May 1990
TEM foils of up to 300 nm can be readily imaged at an acceleration voltage of 120 kV. Most likely, such foils contain the entire implanted layer depth, unless a portion has been removed during the foil-thinning procedure. If all bubbles are imaged, the total bubble volume per unit area of implantation layer can be estimated. This volume can be compared with the total amount of implanted potassium, assuming that none has been lost during annealing or foil preparation. Bubbles were measured and counted at a magnification of 80000, and the total bubble volume was determined by use of a Zeiss Videoplan particle size analyzer. Because of the extremely small size (less than 10 nm) of many of the bubbles it is likely that the total bubble volume is underestimated. Nevertheless, by this approach we obtain a total bubble volume per unit implantation area of 1.6X lOi nm3/cm2 after annealing at 1200’ C. The number of potassium atoms per bubble volume is the implanted dose divided by the total bubble volume per unit area of implantation layer. The measured values yield a number of potassium atoms per bubble volume of 3 1.4 atoms/nm3, which is much higher than the theoretically possible 13.4 atoms/nm3 of crystalline potassium. The total bubble volume should be much larger than the observed value to accommodate all the implanted potassium atoms in the bubbles. Besides underestimation of the bubble volume, it is likely that potassium atoms may have escaped during annealing along short circuit paths from the implanted layer. It is also possible that a certain fraction of potassium atoms in the implanted volume is still atomically dispersed, and therefore invisible.
4. Conclusions After potassium implantation, initially largegrained, recrystallized tungsten is turned into microcrystalline material of less than 5 nm grain size. Asimplanted potassium is dispersed in the form of a pseudo-solution and causes lattice expansion of the microcrystalline tungsten. Upon annealing, recovery and recrystallization take place, and bubbles are formed in a recrystallized tungsten lattice. The bubbles are {0 11 }-faceted dodecahedra with truncation on {00 1 } planes and with rounded corners. They are
Volume 9, number
9
MATERIALS
partially filled with precipitates of elemental potassium. The potassium precipitates in the bubbles are oriented in congruence with the tungsten matrix, i.e. 1001
lw IILOO1 IK and
[OlO]wll [OlO]k.
The bubble morphology and the crystal orientation of the potassium precipitates are identical to those observed in conventionally doped tungsten lamp wire. However, by implantation and subsequent annealing, it is possible to obtain very small and more finely distributed potassium bubbles with a number density that cannot be achieved by conventional doping.
Acknowledgement Dr. Don Bly supplied lamp-grade tungsten for the experiments and provided recrystallization-annealing at high temperature. We thank Dr. D.L. Bly, Dr. M.R. Vukcevich, Dr. J.L. Walter and Dr. W.K. Brinn of General Electric Co. and Dr. F. Ernst of MPI Stuttgart for discussions.
References [ 1] R.C. Koo, TMS-AIME 239 ( 1967) 1996.
LETTERS
May 1990
[ 21 G. Das and S.V. Radcliffe, TMS-AIME 242 ( 1968) 2 191. [ 3 ] D.M. Moon and R.C. Koo, Met. Trans. 2 ( 197 I ) 2 I 15. [4] D.B. Snow, Met. Trans. 3 (1972)
2553.
[ 51H.G. Sell, D.F. Stein, R. Stickler, A. Joshi and E. Berkey, J. Inst. Met. 100 (1972) 275. [6] R.A. Swalin and A.H. Geisler, J. Inst. Met. 86 (1957/58) 129. [ 71 G.D. Rieck, Acta Met. 9 ( 196 I ) 825. [8] J.L. Walter, TMS-AIME 239 (1967) 272. [9] J.L. Meijering and G.D. Rieck, Phillips Techn. Rev. 19 (1957/58) 109. IO] H. Warlimont, G. Necker and H. Schultz, Z. Metallkd. 66 (1975) 279. 111 G. Welsch, B.J. Young and R.F. Hehemann, in: Proceedings of the 5th International Conference on the Strength of Metals and Alloys ( 1979) eds. P. Haasen, V. Gerold and G. Kostorz (Pergamon Press, Oxford, 1980) p. 1693. 121 G. Welsch and J.L. Walter, Encycl. Mater. Sci. Eng. ( 1989), submitted for publication. 131 H.G. Sell and G.W. King, Research/development (July, 1972) p. 18. 141 C.J. Smithells, Tungsten - a treatise of its metallurgy, properties, and application (Chapman and Hall, London, 1952) p. 135. 151 K.T. Kim, Ph.D. Thesis, Case Western Reserve University (1989). [ 161 H. Schultz, Acta Met. 12 (1964) 649. [ 171 K.D. Rasch, R.W. Siegel and H. Schultz, Phil. Mag. A 41 (1980) 91. [ 181 H. Schultz, Z. Metallkd. 78 ( 1987) 469. [ 191 D.B. Snow, Met. Trans. 5 (1974) 2375. [ 201 J.L. Walter, P. Rao and R.R. Russell, Met. Trans. 6A ( 1975) 1775.
301
MATERIALS
9
ION-IMPLANTED
POTASSIUM
LETTERS
May 1990
IN TUNGSTEN
K.T. KIM and G. WELSCH ofMaterials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, LISA
Department
I March 1990
Received
Potassium was ion-implanted at 180 keV into recrystallized lamp-grade tungsten with dose of 5 X 1O’“/cm*. In the as-implanted condition potassium atoms are dispersed in a microcrystalline tungsten lattice. Upon annealing, very small and finely distributed potassium bubbles develop. The bubbles geometries are (0 11 }-faceted dodecahedra with truncation on {00 I } planes. The crystal orientation of condensed potassium precipitates inside the bubbles is identical with the orientation of the tungsten matrix.
1. Introduction Tungsten and potassium are bee metals with lattice parameters of 3.16 and 5.3 1 nm, respectively, at room temperature. Potassium is essentially insoluble in tungsten, but once trapped in the tungsten lattice, it clusters and is rather immobile even at high temperature [ l-51. Potassium’s large atomic diameter relative to that of tungsten ( 1.67 times larger) is believed a main reason for its insolubility and immobility. Arrays of fine potassium bubbles in lamp tungsten wire are known to have a profound effect on the recovery and recrystallization of lamp filaments [ 6- 141 in which longitudinal grain boundaries are pinned by stringers of potassium bubbles. This special dispersion of potassium is obtained by extensive wire-drawing of potassium-doped powder compacts. The effectiveness of potassium bubbles as high temperature barriers increases with the fineness of dispersion and with concentration. Bubbles smaller than 50 nm are useful for creep-resistance of lamp filament wire [ lo-121 which contains a total potassium dopant concentration of typically less than 100 ppm [ 131. Higher concentrations are difficult to achieve with the powder-metallurgical (P/M) process used for lamp wire fabrication [ 14 1. Ion implantation can achieve much higher potassium concentrations, although only in a relatively shallow surface layer. Implanted layers are suitable for research on the behavior of high potassium concen0 167-577x/90/$
03.50 0 Elsevier Science Publishers
B.V. (North-Holland)
trations. An additional advantage is that a much finer dispersion is obtained by ion implantation than is possible by the P/M method. Further, a desired concentration is easily obtained by choice of ion dose and energy. The objective of this work was to produce an ultrafine dispersion of potassium in tungsten and to investigate how the potassium develops into bubbles upon annealing [ 15 1.
2. Experimental A bulk sample of commercial doped tungsten (General Electric type 2 18), measuring 8.4 mm in diameter, was used for this experiment. This material is quite pure, except for about 0.03 at% potassium which is trapped in relatively large and easily distinguishable bubbles (fig. la). It was first recrystallization annealed at 2100°C in hydrogen atmosphere to a grain size of 0.1 to 0.5 mm. After preparation of a polished surface to mirror finish, 180 keV ion implantation with potassium was performed at room temperature to a dose of 5 x 1016/cm2, resulting in an implanted surface layer of about 150 nm thickness and a peak concentration of about 9 at% potassium. After implantation, annealing treatments were given at several temperatures from 800 to 2200°C for 1 h each in hydrogen atmosphere to study the evolution of potassium precipitates and bubbles in 295
Volume 9, number
MATERIALS
9
the implanted layer. The microstructural analysis was performed by transmission electron microscopy (Philips EM 400T microscope at 120 kV operating voltage). For this purpose thin foils containing the implanted layer were prepared by the following procedure. First, discs of 3 mm diameter and 0.1 mm thickness were mechanically prepared. The implanted surface was masked off with lacquer, and using single-jet-electropolishing the discs were thinned electrolytically from the unimplanted side.
3. Results and discussion 3. I. As-implanted microstructure The microstructure
of as-received
recrystallized
LETTERS
May 1990
tungsten is shown in fig. 1. It shows the interior of a single grain, which contains dislocation arrays and networks as subboundaries. Most regions are dislocation-free, except where some dislocations have been trapped by potassium bubbles. These bubbles are the result of factory-doping and have sizes of 20 to 300 nm. They are significantly larger and more widely spaced than those produced by ion-implantationdoping and are easily discerned from the latter. A change in structure to microcrystalline was found in the as-implanted surface layer (fig. 2). Electron diffraction pattern taken near the edge of the hole in the TEM foil, area A in fig. 2a, no longer shows individual diffraction spots, but shows diffraction rings instead (fig. 2b). No diffraction spots or rings of potassium were found. This suggests that the as-implanted potassium is distributed in the surface layer in the form of a pseudo-solid solution. A thicker section of the TEM foil, area B in fig. 2a, contains both implanted surface layer and unimplanted substrate. The diffraction patterns from such area contain diffraction rings from the implanted surface layer and discrete diffraction spots from unimplanted substrate (fig. 2~). If a random dispersion of potassium atoms in the tungsten lattice is assumed, the average spacing of potassium atoms at the concentration peak is approximately 2.2 times the interatomic distance of tungsten. Because of their relatively large atomic size, potassium atoms must cause severe lattice distortions in the tungsten to accommodate the misfit. The diffraction spot spacings provide a calibration for the ring diameters. The radii of the diffraction rings are slightly smaller, by about 1.4%, than the corresponding reciprocal lattice spacings of unimplanted tungsten crystal. This indicates a lattice expansion of the implanted tungsten. The crystallite size is so fine (grains are less than 5 nm in diameter) that no dislocations can be observed. 3.2. Implanted and annealed microstructure
Fig. 1 (a) Microstructure in a grain of as-received recrystallized lamp-grade tungsten (2 1I-type) and (b) corresponding selected area diffraction pattern.
296
Upon heating, recovery and recrystallization take place with recrystallization typically beginning above 900’ C [ 11,16 1. Because of the insolubility of potassium in tungsten this results in the formation of potassium precipitates and the vapor pressure of potassium leads to the formation of bubbles. Vacancies, which are part of the radiation damage pro-
Volume 9. number
9
MATERIALS
LETTERS
May 1990
Fig. 2. (a) TEM micrograph of 5 x 10’” K atoms/cm* implanted tungsten. (b) A diffraction pattern taken from area A (war the edge of the hole in the TEM foil) shows sharp diffraction rings from a microcrystalline grain structure. (c) A diffractton pattern taken from area B (thicker section of the TEM foil. which contains both implanted layer and unimplanted substrate) shows both diffractton rungs from the implanted surface layer and discrete diffraction spots from unimplanted substrate.
duced during ion implantation, agglomerate during annealing and either form voids [ 17,18 ] or contribute to the bubble volume. The temperature dependence of potassium bubble growth for isochronal ( 1 h) annealing treatments is shown in fig. 3. After annealing at 8OO”C, a very high density of extremely fine bubbles was obtained. The measured mean bubble diameter was about 7.5 nm. With increasing annealing temperature, the bubble size increased to sizes
between IO and 200 nm at 22OO’C. After cooling to room temperature, the large bubbles are partially filled with precipitate phase of elemental potassium. This is analyzed in more detail below. The recrystallization during annealing caused the diffraction rings to disappear. Discrete diffraction spots of large tungsten grains developed again. but also extra diffraction spots appeared. The extra spots after annealing at 1200°C (fig. 4) were identified as 297
MATERIALS LETTERS
Volume 9, number 9
Fig. 3. Temperature dependence of potassium bubble growth in I h annealingireatment lOOO”C, (c) 12OO”C,(d) 22OO”C*, *additional annealing for 30 min at 2300°C.
diffraction and double diffraction spots of elemental potassium precipitates. Although a selected area for diffraction contains many bubbles, a single crystal orientation relationship of potassium with the tungsten matrix is observed. The potassium precipitates in the bubbles are oriented in congruence with the tungsten matrix, i.e. LOO1lwll[OOlIK and
[OIO]w]] [OIOIK.
This orientation relation has previously been found by Snow [ 191 in recrystallized lamp tungsten wire.
298
May 1990
- dose: 5 X lOI K atoms/cm’.
(a) 8OO”C, (b)
Shape, size, and distribution of potassium bubbles in an implanted layer after annealing at 1200°C is shown in fig. 5. Best bubble imaging was obtained by under-focusing the objective lens of the electron microscope. This, however, has the consequence, that dislocations which are also present, are not clearly imaged. The bubble surfaces have a strong tendency to develop {0 1 1 } facets of tungsten. The bubbles are {0 11 }-faceted dodecahedra with truncation on (00 1} planes and with rounded corners. They are identical to those observed in doped tungsten lamp wire [4,201.
Volume 9, number
9
MATERIALS
May 1990
LETTERS
. d ‘5
n
.
0
lo
d *
‘3
l
0 1
‘3
.
0 b ‘
.
h
“01 l*o
.
3
*_
0
0
*
.
d
l
0 *
.
0
.
I?
0
d
0
Fig. 4. Diffraction patterns from tungsten and potassium which have epitaxial relationship. (a) [ 0011 zone, (b) [ 0 I I ] zone, (c) [ 11 I ] - and A, first- and second-order K double diffraction zone. 0, W matrix spot; l and o, first- and second-order K spots, respectively; spots, respectively; q , higher than second-order K double diffraction spots.
299
Volume 9, number 9
MATERIALS LETTERS
May 1990
TEM foils of up to 300 nm can be readily imaged at an acceleration voltage of 120 kV. Most likely, such foils contain the entire implanted layer depth, unless a portion has been removed during the foil-thinning procedure. If all bubbles are imaged, the total bubble volume per unit area of implantation layer can be estimated. This volume can be compared with the total amount of implanted potassium, assuming that none has been lost during annealing or foil preparation. Bubbles were measured and counted at a magnification of 80000, and the total bubble volume was determined by use of a Zeiss Videoplan particle size analyzer. Because of the extremely small size (less than 10 nm) of many of the bubbles it is likely that the total bubble volume is underestimated. Nevertheless, by this approach we obtain a total bubble volume per unit implantation area of 1.6X lOi nm3/cm2 after annealing at 1200’ C. The number of potassium atoms per bubble volume is the implanted dose divided by the total bubble volume per unit area of implantation layer. The measured values yield a number of potassium atoms per bubble volume of 3 1.4 atoms/nm3, which is much higher than the theoretically possible 13.4 atoms/nm3 of crystalline potassium. The total bubble volume should be much larger than the observed value to accommodate all the implanted potassium atoms in the bubbles. Besides underestimation of the bubble volume, it is likely that potassium atoms may have escaped during annealing along short circuit paths from the implanted layer. It is also possible that a certain fraction of potassium atoms in the implanted volume is still atomically dispersed, and therefore invisible.
4. Conclusions After potassium implantation, initially largegrained, recrystallized tungsten is turned into microcrystalline material of less than 5 nm grain size. Asimplanted potassium is dispersed in the form of a pseudo-solution and causes lattice expansion of the microcrystalline tungsten. Upon annealing, recovery and recrystallization take place, and bubbles are formed in a recrystallized tungsten lattice. The bubbles are {0 11 }-faceted dodecahedra with truncation on {00 1 } planes and with rounded corners. They are
Volume 9, number
9
MATERIALS
partially filled with precipitates of elemental potassium. The potassium precipitates in the bubbles are oriented in congruence with the tungsten matrix, i.e. 1001
lw IILOO1 IK and
[OlO]wll [OlO]k.
The bubble morphology and the crystal orientation of the potassium precipitates are identical to those observed in conventionally doped tungsten lamp wire. However, by implantation and subsequent annealing, it is possible to obtain very small and more finely distributed potassium bubbles with a number density that cannot be achieved by conventional doping.
Acknowledgement Dr. Don Bly supplied lamp-grade tungsten for the experiments and provided recrystallization-annealing at high temperature. We thank Dr. D.L. Bly, Dr. M.R. Vukcevich, Dr. J.L. Walter and Dr. W.K. Brinn of General Electric Co. and Dr. F. Ernst of MPI Stuttgart for discussions.
References [ 1] R.C. Koo, TMS-AIME 239 ( 1967) 1996.
LETTERS
May 1990
[ 21 G. Das and S.V. Radcliffe, TMS-AIME 242 ( 1968) 2 191. [ 3 ] D.M. Moon and R.C. Koo, Met. Trans. 2 ( 197 I ) 2 I 15. [4] D.B. Snow, Met. Trans. 3 (1972)
2553.
[ 51H.G. Sell, D.F. Stein, R. Stickler, A. Joshi and E. Berkey, J. Inst. Met. 100 (1972) 275. [6] R.A. Swalin and A.H. Geisler, J. Inst. Met. 86 (1957/58) 129. [ 71 G.D. Rieck, Acta Met. 9 ( 196 I ) 825. [8] J.L. Walter, TMS-AIME 239 (1967) 272. [9] J.L. Meijering and G.D. Rieck, Phillips Techn. Rev. 19 (1957/58) 109. IO] H. Warlimont, G. Necker and H. Schultz, Z. Metallkd. 66 (1975) 279. 111 G. Welsch, B.J. Young and R.F. Hehemann, in: Proceedings of the 5th International Conference on the Strength of Metals and Alloys ( 1979) eds. P. Haasen, V. Gerold and G. Kostorz (Pergamon Press, Oxford, 1980) p. 1693. 121 G. Welsch and J.L. Walter, Encycl. Mater. Sci. Eng. ( 1989), submitted for publication. 131 H.G. Sell and G.W. King, Research/development (July, 1972) p. 18. 141 C.J. Smithells, Tungsten - a treatise of its metallurgy, properties, and application (Chapman and Hall, London, 1952) p. 135. 151 K.T. Kim, Ph.D. Thesis, Case Western Reserve University (1989). [ 161 H. Schultz, Acta Met. 12 (1964) 649. [ 171 K.D. Rasch, R.W. Siegel and H. Schultz, Phil. Mag. A 41 (1980) 91. [ 181 H. Schultz, Z. Metallkd. 78 ( 1987) 469. [ 191 D.B. Snow, Met. Trans. 5 (1974) 2375. [ 201 J.L. Walter, P. Rao and R.R. Russell, Met. Trans. 6A ( 1975) 1775.
301