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Defects in doped tungsten seen with the field-ion microscope
SHORTCOMMUNICATIONS
242
earth alloys derived from the metallographic observation of alloys which have been mechanically polished, could be different from that obtained from X-ray observations of the annealed powders, as mechanical polishing could produce a phase transformation in alloys close to the c.p.h./Sm-type and the Sm-type/d.h. phase boundaries. As an extension of this work, a detailed study of the faulting probabilities of the c.p.h. rare-earth metals and alloys using WILSON’Sanalysisso6, is currently being carried out in the authors’ laboratorylo.
I I. R. HARRIS AND G. V. RAYNOR, J. Less-Common Metals, II lrg66) 286. 2 M. NORMAN, I. R. HARRIS AND G. V. RAYNOR, J. Less-Common, Metals, II (1966) 436. 3 A. JAYARAMAN, R. C. SHERWOOD, H. J. WILLIAMS AND E. CORENZWIT, Phys. Rev., 148 502. 4 I. R. HARRIS, C. C. KOCH AND G. V. RAYNOR, J. Less-Common Metals, II 11966) 395. 5 A. J. C. WILSON, X-ray Optics, Methuen, London, 1949. 6 A. J. C. WILSON, Proc. Roy. Sot. (London), A 180 (1042) 277. 7 0. S. EDWARDS AND H. LIPSON, PYOC.Roy. Sot. (London), A 180 (1942) 268. 8 V. G. RIVLIN, W. HUME-ROTHERY AND B. RYDER, Acta. Met., IO (1962) 1143. g R. E. SMALLMAN AND K. H. WESTMACOTT, Phil. Mag., 2 (1957) 669. IO I. R. HARRIS, unpublished work. II J. W. CHRISTIAN, ActaCryst., 7 (1954) 415.
(Ig66)
Received October z7th, 1966 * Present address: Oak Ridge National Laboratory,
J. Less-Common
Tennessee,
U.S.A.
Metals, 12 (1967) 239-242
Defects in doped tungsten seen with the field-ion microscope The observations reported here were made during an examination, by field-ion microscopy, of tungsten wires which contained small quantities of various oxides. The fine scale structure of these wires, partic~~ly as regards the form and distribution of the impurities, is of interest because unlike pure tungsten they develop stable grain structures when used as lamp filaments 1~2.Some details of the process of recrystallisation have led to speculations in which the impurities are envisaged to be in the form of strings and concentric tubes parallel to the wire axis3+4+5. It is well known that thoriated tungsten contains discrete particles of thoria which would be expected to inhibit boundary migration and lead to a stable fine-grained structure but there appears to be no evidence concerning the form of the traces of alumina and silica present in wires which achieve stability by exaggerated grain growth and the formation of spliced grain boundaries. Specimens were prepared from vacuum-annealed wires of (a) pure tungsten, (b) tungsten which contained traces of alumina, silica and alkali, and (c) tungsten which contained 0.6% of thoria. They were examined in a field-ion microscope in which helium was used as an imaging gas and liquid nitrogen as a specimen coolant. J. Less-Common
Metals, 12 (1967) 242-246
SHORT COMMUNICATIONS
243
In the present work the samples of pure tungsten invariably gave the familiar image of a perfect crystal, as did many of the impure specimens. But, quite frequently, field evaporation of the latter revealed irregularities, probably connected with the oxide additions and the special recrystallisation behaviour. Occasionally in the samples of tungsten with traces of alumina and silica a small area of the image, which was previously regular, appeared disordered and showed unusual brightness effects. An example is shown in Fig. I where a region of enhanced brightness is indicated by the arrow. As successive atom layers were stripped from the surface the appearance of the irregular area varied; sometimes parts of it became darker than the surroundings and occasionally it became perfect for a few layers. These types of irregularities persisted during the evaporation of several hundreds of atom layers, after which no further signs of them were seen, at least in the same part of the image. It seems reasonable to conclude that the irregularities consisted
Fig. I. The arrow indicates an irregular region due to the intersection phase with the surface. Imaged at 7.5 kV in 2.9 ,u of helium.
of a fine string of foreign
J. Less-Common Metals,
12 (1967) 242-246
SHORT COMMUNICATIONS
244
of strings of tiny particles of foreign phase, several hundreds of atoms long and a few atoms across, lying along the direction of the wire. Irregularities, believed to be due to thoria, were observed in the thoriated tungsten, but fine, axially-aligned strings were not seen. Two samples of thoriated wire behaved particularly strangely. Their completely irregular image first appeared at 3 kV, filled the screen at 5 kV, and was observed until the specimen failed at 18.5 kV. As the voltage was raised the specimen apparently evaporated randomly while the image remained in focus. This behaviour is completely unlike that of tungsten, even that for badly-prepared samples, and suggests that the tip of the specimen may have been comprised of thoria. In the impure materials, but never in the pure tungsten, a dark region, presumably a hole, was often observed. The holes appeared during field evaporation of a perfect surface and, provided the tip did not break, further evaporation eventually re-established the perfect image. They varied in both shape and size, ranging from thirty to several hundreds of Angstroms across.
Fig. 2. A hole and associated surface damage imaged at J. Less-Common
Metab,
12
(1967) 242-246
12
kV
in
2.2
p
of helium.
245
SHORT CO~~U~ICATIO~S
The holes were usually approximately spherical and surrounded by a distorted surface as in Fig. 2 where the image around the hole is particularly irregular and severely streaked. When the holes appeared the image vibrated briefly; a fact, which taken in conjunction with the surface damage, suggests that they were revealed or formed by localised fracture of the tip under the very large stress imposed by the electric field. An unusual hole is illustrated in Fig. 3 where the dark area almost encircles the centre of the image and is surrounded by an essentially regular surface. The holes may be small cavities left in the material after sintering during manufacture or be due to included particles which were pulled out bij the electric field. In either case when, by field evaporation, the covering layer of tungsten becomes very thin it may fail mechanically. Indeed, a hole in a thoriated sample appeared suddenly at a voltage below that required for evaporation while the relevant area was in focus. Although cavities have been observed in pure tungsten sheet by electron microscopy7,
Fig. 3. A crescent-shaped hole imaged at 6. I kV in 3 y of helium. With further field evaporation hole closed up from its ends and was seventeen atomic layers deep. J. Less-Common
Met&,
the
12 (1967) 242-246
246
SHORT COMMUNICATIONS
and occasionally in wire by field-ion microscopy*, none was seen in the pure samples studied in the present work, probably because the quantities of material examined were extremely small. However, the frequent occurrence of holes in the impure tungsten suggests that either some were associated with inclusions or that cavities are more numerous than in pure tungsten, The observations reported here are in accordance with the presence of discrete particles of thoria in thoriated tungsten wire, such as have been seen by optical and electron microscopy. A random dispersion of thoria particles is held responsible for the fine-grained, equiaxed structure of thoriated wire. On the other hand, the reasons why exaggerated grain growth and overlapping grain boundaries occur in tungsten doped with traces of alumina and silica, are not clear. It is unlikely that the dope is present as r~domly-dis~rsed particles as this would lead to a uniform grain size. In fact, an estimate using the Zener criterion indicates that 70 p.p.m. of additive in the form of spheres of radius 10-8 to 10-5 cm radius is sufficient to yield a maximum grain size of 10-s to 10-2 cm. A possible solution suggested by the field-ion-microscope observations is that the dope is present as long, thin strings parallel to the axis. Such an arrangement would offer a higher resistance to boundary migration normal to the axis than to that parallel to the axis, and might lead to an elongated grain structure. This work was undertaken as part of the general research programme of the National Physical Laboratory and is published by permission of the Director of the Laboratory. The wires were kindly supplied by G. E. C., Wembley. The author is grateful to D. McLean for encouragement and helpful discussion and to A. Head for assistance with the experimental procedures. Metallurgical Division, National Physical Laboratory, Teddington, Middlesex (Gt. Britain) I z 3 4 5 6 7
C. D. G. G. G. A. H.
J. SMITHELLS, Tungsten, Chapman and Hall, London, 1952. J. JONES AND A. LEACH, Metaltwgia, 60 (357) (1959) 7. D. RIECKE, Acta. Met., 4 (1956) 47. D. RIECKL, Acta. Met., 6 (1958) 360. D. RIECKE, A&z. Met., 9 (1961) 8~5. WRONSKI AND A. FOURDEUX, J. Less-Gommo% Metals, 8 (1965) qg. I?. RYAN AND J. SUMTER,J. Less-Commo~t Ikfetds, g (1965) 307.
Received August xrst, 1966 J. Less-Common
Metals,
IZ (1967) 242-246
G.
MEYRICK
242
earth alloys derived from the metallographic observation of alloys which have been mechanically polished, could be different from that obtained from X-ray observations of the annealed powders, as mechanical polishing could produce a phase transformation in alloys close to the c.p.h./Sm-type and the Sm-type/d.h. phase boundaries. As an extension of this work, a detailed study of the faulting probabilities of the c.p.h. rare-earth metals and alloys using WILSON’Sanalysisso6, is currently being carried out in the authors’ laboratorylo.
I I. R. HARRIS AND G. V. RAYNOR, J. Less-Common Metals, II lrg66) 286. 2 M. NORMAN, I. R. HARRIS AND G. V. RAYNOR, J. Less-Common, Metals, II (1966) 436. 3 A. JAYARAMAN, R. C. SHERWOOD, H. J. WILLIAMS AND E. CORENZWIT, Phys. Rev., 148 502. 4 I. R. HARRIS, C. C. KOCH AND G. V. RAYNOR, J. Less-Common Metals, II 11966) 395. 5 A. J. C. WILSON, X-ray Optics, Methuen, London, 1949. 6 A. J. C. WILSON, Proc. Roy. Sot. (London), A 180 (1042) 277. 7 0. S. EDWARDS AND H. LIPSON, PYOC.Roy. Sot. (London), A 180 (1942) 268. 8 V. G. RIVLIN, W. HUME-ROTHERY AND B. RYDER, Acta. Met., IO (1962) 1143. g R. E. SMALLMAN AND K. H. WESTMACOTT, Phil. Mag., 2 (1957) 669. IO I. R. HARRIS, unpublished work. II J. W. CHRISTIAN, ActaCryst., 7 (1954) 415.
(Ig66)
Received October z7th, 1966 * Present address: Oak Ridge National Laboratory,
J. Less-Common
Tennessee,
U.S.A.
Metals, 12 (1967) 239-242
Defects in doped tungsten seen with the field-ion microscope The observations reported here were made during an examination, by field-ion microscopy, of tungsten wires which contained small quantities of various oxides. The fine scale structure of these wires, partic~~ly as regards the form and distribution of the impurities, is of interest because unlike pure tungsten they develop stable grain structures when used as lamp filaments 1~2.Some details of the process of recrystallisation have led to speculations in which the impurities are envisaged to be in the form of strings and concentric tubes parallel to the wire axis3+4+5. It is well known that thoriated tungsten contains discrete particles of thoria which would be expected to inhibit boundary migration and lead to a stable fine-grained structure but there appears to be no evidence concerning the form of the traces of alumina and silica present in wires which achieve stability by exaggerated grain growth and the formation of spliced grain boundaries. Specimens were prepared from vacuum-annealed wires of (a) pure tungsten, (b) tungsten which contained traces of alumina, silica and alkali, and (c) tungsten which contained 0.6% of thoria. They were examined in a field-ion microscope in which helium was used as an imaging gas and liquid nitrogen as a specimen coolant. J. Less-Common
Metals, 12 (1967) 242-246
SHORT COMMUNICATIONS
243
In the present work the samples of pure tungsten invariably gave the familiar image of a perfect crystal, as did many of the impure specimens. But, quite frequently, field evaporation of the latter revealed irregularities, probably connected with the oxide additions and the special recrystallisation behaviour. Occasionally in the samples of tungsten with traces of alumina and silica a small area of the image, which was previously regular, appeared disordered and showed unusual brightness effects. An example is shown in Fig. I where a region of enhanced brightness is indicated by the arrow. As successive atom layers were stripped from the surface the appearance of the irregular area varied; sometimes parts of it became darker than the surroundings and occasionally it became perfect for a few layers. These types of irregularities persisted during the evaporation of several hundreds of atom layers, after which no further signs of them were seen, at least in the same part of the image. It seems reasonable to conclude that the irregularities consisted
Fig. I. The arrow indicates an irregular region due to the intersection phase with the surface. Imaged at 7.5 kV in 2.9 ,u of helium.
of a fine string of foreign
J. Less-Common Metals,
12 (1967) 242-246
SHORT COMMUNICATIONS
244
of strings of tiny particles of foreign phase, several hundreds of atoms long and a few atoms across, lying along the direction of the wire. Irregularities, believed to be due to thoria, were observed in the thoriated tungsten, but fine, axially-aligned strings were not seen. Two samples of thoriated wire behaved particularly strangely. Their completely irregular image first appeared at 3 kV, filled the screen at 5 kV, and was observed until the specimen failed at 18.5 kV. As the voltage was raised the specimen apparently evaporated randomly while the image remained in focus. This behaviour is completely unlike that of tungsten, even that for badly-prepared samples, and suggests that the tip of the specimen may have been comprised of thoria. In the impure materials, but never in the pure tungsten, a dark region, presumably a hole, was often observed. The holes appeared during field evaporation of a perfect surface and, provided the tip did not break, further evaporation eventually re-established the perfect image. They varied in both shape and size, ranging from thirty to several hundreds of Angstroms across.
Fig. 2. A hole and associated surface damage imaged at J. Less-Common
Metab,
12
(1967) 242-246
12
kV
in
2.2
p
of helium.
245
SHORT CO~~U~ICATIO~S
The holes were usually approximately spherical and surrounded by a distorted surface as in Fig. 2 where the image around the hole is particularly irregular and severely streaked. When the holes appeared the image vibrated briefly; a fact, which taken in conjunction with the surface damage, suggests that they were revealed or formed by localised fracture of the tip under the very large stress imposed by the electric field. An unusual hole is illustrated in Fig. 3 where the dark area almost encircles the centre of the image and is surrounded by an essentially regular surface. The holes may be small cavities left in the material after sintering during manufacture or be due to included particles which were pulled out bij the electric field. In either case when, by field evaporation, the covering layer of tungsten becomes very thin it may fail mechanically. Indeed, a hole in a thoriated sample appeared suddenly at a voltage below that required for evaporation while the relevant area was in focus. Although cavities have been observed in pure tungsten sheet by electron microscopy7,
Fig. 3. A crescent-shaped hole imaged at 6. I kV in 3 y of helium. With further field evaporation hole closed up from its ends and was seventeen atomic layers deep. J. Less-Common
Met&,
the
12 (1967) 242-246
246
SHORT COMMUNICATIONS
and occasionally in wire by field-ion microscopy*, none was seen in the pure samples studied in the present work, probably because the quantities of material examined were extremely small. However, the frequent occurrence of holes in the impure tungsten suggests that either some were associated with inclusions or that cavities are more numerous than in pure tungsten, The observations reported here are in accordance with the presence of discrete particles of thoria in thoriated tungsten wire, such as have been seen by optical and electron microscopy. A random dispersion of thoria particles is held responsible for the fine-grained, equiaxed structure of thoriated wire. On the other hand, the reasons why exaggerated grain growth and overlapping grain boundaries occur in tungsten doped with traces of alumina and silica, are not clear. It is unlikely that the dope is present as r~domly-dis~rsed particles as this would lead to a uniform grain size. In fact, an estimate using the Zener criterion indicates that 70 p.p.m. of additive in the form of spheres of radius 10-8 to 10-5 cm radius is sufficient to yield a maximum grain size of 10-s to 10-2 cm. A possible solution suggested by the field-ion-microscope observations is that the dope is present as long, thin strings parallel to the axis. Such an arrangement would offer a higher resistance to boundary migration normal to the axis than to that parallel to the axis, and might lead to an elongated grain structure. This work was undertaken as part of the general research programme of the National Physical Laboratory and is published by permission of the Director of the Laboratory. The wires were kindly supplied by G. E. C., Wembley. The author is grateful to D. McLean for encouragement and helpful discussion and to A. Head for assistance with the experimental procedures. Metallurgical Division, National Physical Laboratory, Teddington, Middlesex (Gt. Britain) I z 3 4 5 6 7
C. D. G. G. G. A. H.
J. SMITHELLS, Tungsten, Chapman and Hall, London, 1952. J. JONES AND A. LEACH, Metaltwgia, 60 (357) (1959) 7. D. RIECKE, Acta. Met., 4 (1956) 47. D. RIECKL, Acta. Met., 6 (1958) 360. D. RIECKE, A&z. Met., 9 (1961) 8~5. WRONSKI AND A. FOURDEUX, J. Less-Gommo% Metals, 8 (1965) qg. I?. RYAN AND J. SUMTER,J. Less-Commo~t Ikfetds, g (1965) 307.
Received August xrst, 1966 J. Less-Common
Metals,
IZ (1967) 242-246
G.
MEYRICK