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Secondary recrystallization of pure molybdenum wires
Vol. 34, No. Printed in Great Britain
AC@ metaN.
7, pp. 1329-t334,
SECONDARY
1986
Sol-6140/86 $3.00+ 0.00 Pergamon Journals Ltd
RECRYSTALLIZATION MOLYBDENUM WIRES
OF PURE
Y. OHBA National Research Institute for Metals, Tokyo 153, Japan (Received 30 May 1985; in reoised,fhwt 6 October 1985)
Abstract-The secondary recrystallization ofdrawn pure MO wires has been studied; a particular attention has been paid on the effect of temperature gradients in the annealing process. The secondary recrystallized grains being very elongated along the wire axis have been obtained by annealing in a moderate temperature gradient furnace at about 2000°C. Orientations of the wire axis of the secondary recrystallized grains are mainly (023) or (135). while (Of l} oriented grains are less frequently observed. In contrast, the primary recrystallized texture consists of the (01 I) main component and (023)-<012) sub-component; many (011) oriented grains have large grain sizes. The observed discrepancy in orientation between the primary and secondary recrystallized grains is explained in terms of the difference in grain boundary mobilities which depend on the character of grain boundaries. R&sum&-Nous avons itudik la recristallisation secondaire de fits de MO pur; nous avons particulitrement Porte attention a l’effet des gradients de temperature dans le phtnomtne de recuit. Nous avons obtenu des grains de recristallisation secondaire tres allot@ le long de I’axe du fil par un recuit a environ 2000.C dans un four a gradient de temperature modert. L’orientation de l’axe du fil des grains de recristalhsation secondaire est surtout (023) ou <135), alors que Ton observe mains fr~quemment des grains d’orientation (01 I). Par contre, la texture de recristallisation primaire est constitute par la composante principale (011) et les composantes secondaires (0.23)-(012). Nous expliquons la difference d’orientation observte entre les grains de recristalhsations primaire et secondaire par une difierence de mobilitt des joints de grains dependant de leur cam&m. Zusammenfassung-Die sekundlre Rekristallisation reiner gezogener Mo-Dmhte wurde untersucht. Der EinfluB von Temperaturgradienten wlhrend des Ausheilprozesses wurde besonders beachtet. Nach Ausheilen in einem Ofen bei etwa 2000°C bei maI3igem Temperaturgradienten wurden sekundlr rekristallisierte Kiirner erhalten, die entlang der Drahtachse sehr ausgedehnt waren. Die Orientierung dieser Kijrner in Richtung der Drahtachse war haupts~chlich (023) oder (135); (01 I)-orientierte Kiimer waren weniger ha&g. Dagegen besitzt die primlr rekristallisierte Textur eine (01 I )-Hauptkomponente und (023)-(012)-Unterkomponenten; viele (01 I)-orientierte KGrner sind gro8. Die beobachtete Diskrepanz in der Orientierung von prim& und sekundir rekristallisierten Kornern wird mit einem Unterschied in der Beweglichkeit der Korngrenzen erkllrt, die von der Art der Korngrenze abhfingt.
1. INTRODUCTION
Fine filamentary wires of MO and W have been used as materials for electron-tube, emitters of thermoelectron, incandescent lamps and so on. A problem associated with these wires is fracture occurring along the grain boundaries. A well-known solution for this problem, however, is known to add some dopants before the working process. The effect of dopants is to produce bundles of elongated but interwoven grains at high temperature [I, 21. The best solution for this problem, however, is to use single crystal wires instead; here obviously no problem arises from grain boundaries. When commercially pure MO wires drawn to a proper degree are annealed at 2OOOC, very coarse secondary recrystallized grains are formed. Therefore, we shall examine in this study whether or not growth of a long pure MO single crystal wire is possible by exploiting this phenomenon. Since the phenomenon is closely related to the
secondary recrystallization, it is preferable to understand the detailed mechanism of the secondary recrystallization. Recently, powerful modern experimental techniques such as Kossel diffraction, electron channeling pattern (E.C.P. method) and threedimensional analysis of the texture have been used for this purpose [3-51. In order to understand the mechanism of the secondary recrystallization of pure MO wires, some experiments using the E.C.P.-method are performed and the results will be reported.
2. EXPERIMENTAL Materials drawn pure 99.95%; they K.K., using
used in this study were commercially MO (nondoped) wires of total purity were manufactured by Tokyo Tungsten the powder metallurgical process. The
main impurities are shown in Table 1. A sequence of the working processes given to the wires is shown in Table 2. Uniform temperature annealing (i.e.
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Table 1. Main impurities of pure MO (mass ppm) C
N
0
Fe
si
Al
K
Ca
IO
15
13
37
29
33
<16
<2
Table 2. Working processes of pure MO wires Ingot 20 mm sq. x 790 mm I Swaging at lOOi&1200°C Drawing from 5.2mm to l&mm at 70%8oo”C Intermediate annealing at 8~l~~C for 2-4 h Drawing to 1.0-0.6 mm at 70@-750°C
anneahng in a furnace whose temperature is kept constant in a certain range) was carried out in a furnace evacuated with an ion pump, while annealing by local heating was done in an electron-beam zone refining furnace. To reveal the metallographic structures of the specimens, they were etched by Murakami’s reagent. Orientations of the recrystallized grains having sizes larger than 100pm were measured by employing the X-ray micro Laue method, and those less than 1OO~m by the E.C.P. method, respectively. The E.C.P. was taken with a scanning electron microscope (JEOL JSM-35CF) operated at 30 kV. The fiber structure of the drawn wire was examined by taking the micro Laue back reflection over the vertical section.
3. RESULTS
3.1. Uniform temperature annealing Specimens were selected from as-received drawn wires on the basis of VHN; the VHN measured on the cross sections of the selected wires were in a range from 270 to 310. Their diameters were usually in the range of 2-3 mm. When these specimens were annealed at 1700-2000°C with a heating rate of 2000”C/40min in a uniform temperature furnace, coarse recrystallized grains of a few cm in length were formed [6], although the grain sizes were fairly scattered from specimen to specimen. When specimens
Photograph 1. Showing the primary recrystallized structure of wires. Upper and lower are wiresof diameter 2 and 3 mm, respectively. The upper specimen has higher VHN (345). Arrow shows the secondary recrystallized grain.
011
001
(0)
Fig. 1. Orientations of the wire axis for the secondary recrystallized grains formed by temperature uniform annealing. (a) Specimens are annealed at 2000°C. (b) Specimens are etched down to half diameter and annealed at 2000°C. 0, Grains of above IOmm in length along the wire axis, 0, grains of - 1 mm diameter; 0, grains of 150-500 p diameter.
with VHN less than the range mentioned above were annealed, the primary recrystallized grains grew normally and no coarse grain was observed. When specimens with higher VHN were annealed, a high density of coarse grains were formed, but their sizes became small. The coarse grains were more frequently observed near the circumferential part of the wire than in the central part. They grew approximately in the spherical shape at an early stage of annealing. This phenomenon is different from the cases of doped W and MO wires [l, 21. Photograph 1 shows a metallographic structure over the cross section of the specimen with diameter 3 mm, annealed at 1500°C for 5 min. We observe no secondary recrystallized grains but primary recrystallized grains having an average grain size of 2&30pm. Therefore, the coarse grains mentioned above must be the secondary recrystallized grains formed above 1500°C. It can be seen from Photograph 1 that the primary recrystallized grains had roughly spherical shape in most cases, although they were a little elongated along the wire axis; the average grain size was slightly larger in the circumferential part than in the central part of the specimen. It is also noted that the uniformity of the grain size distribution was slightly worse in the former than in the latter. Figure l(a) shows orientations of the specimen axis of the coarse grains formed by the uniform temperature annealing at 2000°C. These orientations were distributed from (011) to (113) and tended to concentrate around (023) and (135). These results are similar to those of wires of doped W [l, 21. Figure l(b) shows the orientations of coarse grams in the specimens whose surface layers were initially etched down to the half diameter and then annealed in the same condition. The observed orientations were closer to (023), as compared with those of the unetched specimens. For the both etched and unetched specimens, the secondary recrystallized coarse grains with orientation close to (011) were less frequently observed than in the case of (023) or (135) oriented grains.
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Fig. 2. Confieurations and sizes of the secondarv recrystallized grains in annealing by local heating. (a), ‘ich(4 Cases annealed below 2OOO’C (b). (d) Cases annealed above 2000°C.
3.2. Annealing by local heating Specimens were locally heated using an electronbeam and we examined the changes in the state of the secondary recrystallization such as the orientations and sizes of the grains. When the hottest zone of the specimen was at 2OOO”C, the temperature changed to about 500°C at 10mm away from the hottest zone. A standard heating rate was 2OOO”C/20 min and the heating times were 15-80min. The secondary recrystallization was usually observed when the heating temperature was about 2000°C. The orientation and the size of the secondary recrystallized grains were observed to depend on the positions of the heated zone and on the heating conditions. Typical configurations of the secondary recrystallized grains are shown in Fig. 2. The pattern (a), (c) and (e) were usually formed when the specimens were heated below 2OOO”C, while (b) and (d) were observed when the specimens were heated above 2000°C. Figure 3
shows the orientations of the secondary recrystallized grains. The orientation relationships were approximately the same as those obtained by uniform temperature annealing. The pattern (a) of Fig. 2 shows grains growing to the low temperature sides. This pattern appeared rarely in a specimen annealed by local heating.
3.3. Growth of long single crystals With a purpose of obtaining the pattern (a) more easily, the uniform temperature furnace was modified to set up a moderate temperature gradient (30_40”C/cm). The specimens were heated up in this furnace until the temperature was 2000°C at one end of the specimen and they were kept for 1 h. Secondary recrystallized grains formed near or in the hottest zone tended to rapidly grow to the low temperature side. Photograph 2 shows two such examples; the crystal grew as far as a position of about 1450°C. Their orientations of the specimen axis were (135) or (023) in most cases. The same process can be possibly applied to produce long pure MO single crystal sheet, since a similar phenomenon has been observed on pure MO sheet (7).
3.4. Primary recrystallized texture (0)
(b)
Fig. 3. Orientations of the wire axis for the secondary recrystallized grains formed by annealing by local heating. (a) Specimens are annealed below 2000°C. (b) Specimens are annealed above 2000°C. 0, grains of above 3 mm in length
along
the wire axis; 0, grains
of - 1 mm diameter;
l , grains of 150-500 pm diameter.
The fiber structure of drawn wires was examined; a fairly sharp (011) fiber structure was formed at a central part of the specimen, but the sharpness of the structure was lost to a considerable degree at the circumferential part. Since the primary recrystallized texture is the direct source of the secondary recrystallization, it is important to know the detailed
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111
i
I
10cm Photograph
2. Single crystals grown in moderate tem-
perature gradient.
A A /cl A
001
structure, Both the orientation and size of each primary recrystallized grain were measured on the’ cross section of the specimen, using the E.C.P. method. The specimen annealed at 1500°C for 5 min was used. Figure 4 shows the regions where the orientation of grains was measured; about 40 grains were measured at each region. In addition, some grains having a large grain size were selected to measure the orientations. It is noted in Fig. 5 that the (011) component was dominant, although the subcomponent of (023)-(012) was significantly strong. This observation agreed with the result of the neutron diffraction by Swalin and Geisler [g]. The same results are shown in the form of (001) pole figures in Fig. 6; the anisotropy of (001) pole density is commonly seen about the specimen axis. These may correspond to a cylindrical or a partial cylindrical texture which has been reported in wires of the other metals [9, lo]. No attempt, however, was made to understand the details of the anisotropy in the present study.
4. DISCUSSION 4. I. Growth of secondary recrystallized grains Let us consider a reason why a long single crystal can grow only when a pure MO wire is annealed in a moderate tem~rature gradient ~Photograph 2). When a pure MO wire having the primary re-
a
OH
001
III
011
CKn
C
011
b
111
001
d
Oil
Fig. 5. Orientations of the wire axis for the primary recrystallized grains, (a), (b) Results near circumferential parts of the specimen. (c), (d) Results in central parts of the specimens.
crystallized texture shown in Photograph 1 is annealed in a uniform temperature furnace, secondary recrystallization proceeds only with difficulties owing to the slightly large average grain size and to the relatively simple primary recrystallized texture. Assuming that the density of preferential nucleation sites for the secondary recrystallization is low in the present wires, secondary recrystallized coarse grains will be formed in the uniform temperature anneafing. When the temperature gradient is too large, however, such preferential sites would be unlikely to be found in the heated zone. In conclusion, only when the s~~imens are anneaied in a moderate temperature gradient, a secondary recrystallization would occur at a single preferential site in or near the hottest zone and a single grain will grow before the other secondary recrystallized grains will. 4.2. Mechanism of secondary recrystallization
Fig, 4. Showing regions where orientatjons of the primary recrystallized grains were measured by the E.C.P. method. The large circle shows a cross section of the wire normal to the wire axis. An arrow shows a reference direction.
As shown in Figs 1 and 3, a iarge number of orientations of the secondary recrystallized grains deviated more than several degrees from (011) in the both cases of uniform temperature annealing and of annealing by local heating. As mentioned in Section 3.4, the main component of the primary recrystallized texture was (011) and the sub-component (023)(012). Many (011) oriented grains having a larger grain size than the average one were observed. When several primary recrystallized grains of an especially large size were selected and their orientations were measured, again about half of them were found to be ciose to (011) orientation. If the size factor of the grains in the primary recrystallized texture is a key factor to control the secondary recrystallization, then it would be hard to understand why the secondary
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Circumferential part
Central
OF MO WIRES
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b
part
Fig. 6. (001) pole figures of orientations of the primary recrystallized grains. (a), (b) Results near circumferential parts of the specimen. (c), (d) Results in central parts of the specimen. The arrow indicates the reference direction marked in Fig. 4.
recrystallized grains with orientation close to (011) are less frequently observed (Figs 1 and 3). Let us consider a reason why the secondary recrystallized grains with orientation close to (01 I) are so less frequently observed. Some models on the geometrical relationships among the primary recrystallized grains are showing in Fig. 7. Suppose that a specific grain grows at the expense of the small neighboring grains, as shown in model (a). In this case, the boundary migration occurring during the grain growth is difficult in the direction along the specimen axis, because the boundaries in model (a) is of pure twist type. It is likely that the mobility of a pure twist boundary is small as compared to those of the tilt and mixed type boundaries. Indeed, difference in the mobilities of the twist and tilt boundaries has been experimentally confirmed in Al and Cu [I I-I 51; these investigations have been performed by using lightly strained and polygonized matrix or capillary force. Model (b) shows a case where the growth in the
direction along the specimen axis is possible, but that in the lateral direction is difficult. This is because a central specific grain is bounded by small angle boundaries in the lateral direction. In this case the nucleation of the secondary recrystallization would also be difficult, because the nucleus prefers to grow spherically in the case of pure MO. In model (c) (023) or < 135) oriented secondary recrystallized grains would be formed. According to Hillert’s theory, the local orientation relationships around the nucleus would no longer be an important limiting factor for the growth, once the nucleation is completed [16]. This is a reason why a single crystal wire with its axis parallel to (023) or (135) is frequently observed. A supplementary consideration may be made on the origin of ( 135) oriented secondary recrystallized grains. Comparing the experimental results of the unetched and etched specimens [Fig. I (a) and (b)]. we conjecture that this oriented secondary recrystallized grains are preferentially formed close to the circum-
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the primary recrystallized texture (Fig. 5). This observation implies that (135) oriented primary recrystallized grains have by far the favorable environments for the secondary recrystallization. Lastly, Fig. 8 shows the results of the etched specimen locally annealed above 2300°C; the secondary recrystallized grains with orientation close to (011) appear to grow easily. This observation may be attributed to the fact that the boundary mobilities become independent of the boundary types at such high temperatures. Similar results have been reported in doped W wires and in the experiments of the boundary migration of pure Pb [2, 171.
b
C
Fig. 7. Models showing the early stages of the secondary recrystallization to explain the effects of geometrica! factors. Central grains show specific grains to become nuclei. 1, (011) Orientation (almost parallel to wire axis; U, rotation.
part of the specimens. Although (135) oriented secondary recrystallized grains are one of the main components of the secondary recrystallization, grains with this orientation are less frequently observed than those with (023)-(012) orientation in ferential
111
Acknowledgements-The author expresses a deep appreciation to Dr K. Onawa for his critical reading of the manuscript and for his helpful comments. The author is very grateful to Dr A. Matushita and to Dr E. Furubayashi for their helpful discussions.
REFERENCES 1. G. D. Rieck, Acra metall. 6, 360 (1958). 2. H. Yamamoto, J. Japan Inst. Metals 37, 1 (1973). 3. N. C. Pease. D. W. Jones. M. H. L. Wise and W. B. Hutchinson,‘Meral Sci. 15; 203 (1981). 4. Y. Inokuchi, C. Maeda, Y. Ito and H. Shimanaka, Trans. Iron Steel Inst. Japan 23, 441 (1983). 5. S. Mishra, C. Darmann and K. Liicke, Acta metall. 32, 2185 (1984).
6. N. K. Chen, R. Maddin and R. B. Pond, J. Metals 46, 461 (1951). 7. Y. Ohba, unpublished data. 8. R. A. Swalin and A. H. Geisler, J. Inst. Metals 85, 129 (1957). 9. S. Leber, Trans. Am. Sot. Metals 53, 697 (1961). 10. G. Linssen, H. D. Mengelberg and H. P. Stiiwe, Z. Metal. 55, 600 (1964). 11. H. Yoshida, B. Liebmann and K. Liicke, Acta metall. 7, 51 (1959). 12. B. B. Rath’and Hsun Hu, Trans. Am. Inst. Min. Engrs 245, 1243 (1969).
13. R. Viswanathan and C. L. Bauer. Acta metall. 21, 1099 (1973).
14. B. B. Rath and Hsun Hu, Trans. Am. Inst. Min. Engrs 245, 1577 (1969).
15. D. Eylon, A. Rosen and S. Niedzwiedz, Acta metall. 17, Fig. 8. Orientations of the secondary recrystallized grains in the specimen etched down to half diameter and locally heated above 2300°C. 0, Grains of - 500 pm diameter; 0, grains of 150-500 pm diameter.
1013 (1969).
16. M. Hillert, Acta metall. 13, 227 (1965). 17. K. J. Aust and J. W. Rutter, Trans. Am. Inst. Min. Engrs 224, 111 (1962).
AC@ metaN.
7, pp. 1329-t334,
SECONDARY
1986
Sol-6140/86 $3.00+ 0.00 Pergamon Journals Ltd
RECRYSTALLIZATION MOLYBDENUM WIRES
OF PURE
Y. OHBA National Research Institute for Metals, Tokyo 153, Japan (Received 30 May 1985; in reoised,fhwt 6 October 1985)
Abstract-The secondary recrystallization ofdrawn pure MO wires has been studied; a particular attention has been paid on the effect of temperature gradients in the annealing process. The secondary recrystallized grains being very elongated along the wire axis have been obtained by annealing in a moderate temperature gradient furnace at about 2000°C. Orientations of the wire axis of the secondary recrystallized grains are mainly (023) or (135). while (Of l} oriented grains are less frequently observed. In contrast, the primary recrystallized texture consists of the (01 I) main component and (023)-<012) sub-component; many (011) oriented grains have large grain sizes. The observed discrepancy in orientation between the primary and secondary recrystallized grains is explained in terms of the difference in grain boundary mobilities which depend on the character of grain boundaries. R&sum&-Nous avons itudik la recristallisation secondaire de fits de MO pur; nous avons particulitrement Porte attention a l’effet des gradients de temperature dans le phtnomtne de recuit. Nous avons obtenu des grains de recristallisation secondaire tres allot@ le long de I’axe du fil par un recuit a environ 2000.C dans un four a gradient de temperature modert. L’orientation de l’axe du fil des grains de recristalhsation secondaire est surtout (023) ou <135), alors que Ton observe mains fr~quemment des grains d’orientation (01 I). Par contre, la texture de recristallisation primaire est constitute par la composante principale (011) et les composantes secondaires (0.23)-(012). Nous expliquons la difference d’orientation observte entre les grains de recristalhsations primaire et secondaire par une difierence de mobilitt des joints de grains dependant de leur cam&m. Zusammenfassung-Die sekundlre Rekristallisation reiner gezogener Mo-Dmhte wurde untersucht. Der EinfluB von Temperaturgradienten wlhrend des Ausheilprozesses wurde besonders beachtet. Nach Ausheilen in einem Ofen bei etwa 2000°C bei maI3igem Temperaturgradienten wurden sekundlr rekristallisierte Kiirner erhalten, die entlang der Drahtachse sehr ausgedehnt waren. Die Orientierung dieser Kijrner in Richtung der Drahtachse war haupts~chlich (023) oder (135); (01 I)-orientierte Kiimer waren weniger ha&g. Dagegen besitzt die primlr rekristallisierte Textur eine (01 I )-Hauptkomponente und (023)-(012)-Unterkomponenten; viele (01 I)-orientierte KGrner sind gro8. Die beobachtete Diskrepanz in der Orientierung von prim& und sekundir rekristallisierten Kornern wird mit einem Unterschied in der Beweglichkeit der Korngrenzen erkllrt, die von der Art der Korngrenze abhfingt.
1. INTRODUCTION
Fine filamentary wires of MO and W have been used as materials for electron-tube, emitters of thermoelectron, incandescent lamps and so on. A problem associated with these wires is fracture occurring along the grain boundaries. A well-known solution for this problem, however, is known to add some dopants before the working process. The effect of dopants is to produce bundles of elongated but interwoven grains at high temperature [I, 21. The best solution for this problem, however, is to use single crystal wires instead; here obviously no problem arises from grain boundaries. When commercially pure MO wires drawn to a proper degree are annealed at 2OOOC, very coarse secondary recrystallized grains are formed. Therefore, we shall examine in this study whether or not growth of a long pure MO single crystal wire is possible by exploiting this phenomenon. Since the phenomenon is closely related to the
secondary recrystallization, it is preferable to understand the detailed mechanism of the secondary recrystallization. Recently, powerful modern experimental techniques such as Kossel diffraction, electron channeling pattern (E.C.P. method) and threedimensional analysis of the texture have been used for this purpose [3-51. In order to understand the mechanism of the secondary recrystallization of pure MO wires, some experiments using the E.C.P.-method are performed and the results will be reported.
2. EXPERIMENTAL Materials drawn pure 99.95%; they K.K., using
used in this study were commercially MO (nondoped) wires of total purity were manufactured by Tokyo Tungsten the powder metallurgical process. The
main impurities are shown in Table 1. A sequence of the working processes given to the wires is shown in Table 2. Uniform temperature annealing (i.e.
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Table 1. Main impurities of pure MO (mass ppm) C
N
0
Fe
si
Al
K
Ca
IO
15
13
37
29
33
<16
<2
Table 2. Working processes of pure MO wires Ingot 20 mm sq. x 790 mm I Swaging at lOOi&1200°C Drawing from 5.2mm to l&mm at 70%8oo”C Intermediate annealing at 8~l~~C for 2-4 h Drawing to 1.0-0.6 mm at 70@-750°C
anneahng in a furnace whose temperature is kept constant in a certain range) was carried out in a furnace evacuated with an ion pump, while annealing by local heating was done in an electron-beam zone refining furnace. To reveal the metallographic structures of the specimens, they were etched by Murakami’s reagent. Orientations of the recrystallized grains having sizes larger than 100pm were measured by employing the X-ray micro Laue method, and those less than 1OO~m by the E.C.P. method, respectively. The E.C.P. was taken with a scanning electron microscope (JEOL JSM-35CF) operated at 30 kV. The fiber structure of the drawn wire was examined by taking the micro Laue back reflection over the vertical section.
3. RESULTS
3.1. Uniform temperature annealing Specimens were selected from as-received drawn wires on the basis of VHN; the VHN measured on the cross sections of the selected wires were in a range from 270 to 310. Their diameters were usually in the range of 2-3 mm. When these specimens were annealed at 1700-2000°C with a heating rate of 2000”C/40min in a uniform temperature furnace, coarse recrystallized grains of a few cm in length were formed [6], although the grain sizes were fairly scattered from specimen to specimen. When specimens
Photograph 1. Showing the primary recrystallized structure of wires. Upper and lower are wiresof diameter 2 and 3 mm, respectively. The upper specimen has higher VHN (345). Arrow shows the secondary recrystallized grain.
011
001
(0)
Fig. 1. Orientations of the wire axis for the secondary recrystallized grains formed by temperature uniform annealing. (a) Specimens are annealed at 2000°C. (b) Specimens are etched down to half diameter and annealed at 2000°C. 0, Grains of above IOmm in length along the wire axis, 0, grains of - 1 mm diameter; 0, grains of 150-500 p diameter.
with VHN less than the range mentioned above were annealed, the primary recrystallized grains grew normally and no coarse grain was observed. When specimens with higher VHN were annealed, a high density of coarse grains were formed, but their sizes became small. The coarse grains were more frequently observed near the circumferential part of the wire than in the central part. They grew approximately in the spherical shape at an early stage of annealing. This phenomenon is different from the cases of doped W and MO wires [l, 21. Photograph 1 shows a metallographic structure over the cross section of the specimen with diameter 3 mm, annealed at 1500°C for 5 min. We observe no secondary recrystallized grains but primary recrystallized grains having an average grain size of 2&30pm. Therefore, the coarse grains mentioned above must be the secondary recrystallized grains formed above 1500°C. It can be seen from Photograph 1 that the primary recrystallized grains had roughly spherical shape in most cases, although they were a little elongated along the wire axis; the average grain size was slightly larger in the circumferential part than in the central part of the specimen. It is also noted that the uniformity of the grain size distribution was slightly worse in the former than in the latter. Figure l(a) shows orientations of the specimen axis of the coarse grains formed by the uniform temperature annealing at 2000°C. These orientations were distributed from (011) to (113) and tended to concentrate around (023) and (135). These results are similar to those of wires of doped W [l, 21. Figure l(b) shows the orientations of coarse grams in the specimens whose surface layers were initially etched down to the half diameter and then annealed in the same condition. The observed orientations were closer to (023), as compared with those of the unetched specimens. For the both etched and unetched specimens, the secondary recrystallized coarse grains with orientation close to (011) were less frequently observed than in the case of (023) or (135) oriented grains.
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Fig. 2. Confieurations and sizes of the secondarv recrystallized grains in annealing by local heating. (a), ‘ich(4 Cases annealed below 2OOO’C (b). (d) Cases annealed above 2000°C.
3.2. Annealing by local heating Specimens were locally heated using an electronbeam and we examined the changes in the state of the secondary recrystallization such as the orientations and sizes of the grains. When the hottest zone of the specimen was at 2OOO”C, the temperature changed to about 500°C at 10mm away from the hottest zone. A standard heating rate was 2OOO”C/20 min and the heating times were 15-80min. The secondary recrystallization was usually observed when the heating temperature was about 2000°C. The orientation and the size of the secondary recrystallized grains were observed to depend on the positions of the heated zone and on the heating conditions. Typical configurations of the secondary recrystallized grains are shown in Fig. 2. The pattern (a), (c) and (e) were usually formed when the specimens were heated below 2OOO”C, while (b) and (d) were observed when the specimens were heated above 2000°C. Figure 3
shows the orientations of the secondary recrystallized grains. The orientation relationships were approximately the same as those obtained by uniform temperature annealing. The pattern (a) of Fig. 2 shows grains growing to the low temperature sides. This pattern appeared rarely in a specimen annealed by local heating.
3.3. Growth of long single crystals With a purpose of obtaining the pattern (a) more easily, the uniform temperature furnace was modified to set up a moderate temperature gradient (30_40”C/cm). The specimens were heated up in this furnace until the temperature was 2000°C at one end of the specimen and they were kept for 1 h. Secondary recrystallized grains formed near or in the hottest zone tended to rapidly grow to the low temperature side. Photograph 2 shows two such examples; the crystal grew as far as a position of about 1450°C. Their orientations of the specimen axis were (135) or (023) in most cases. The same process can be possibly applied to produce long pure MO single crystal sheet, since a similar phenomenon has been observed on pure MO sheet (7).
3.4. Primary recrystallized texture (0)
(b)
Fig. 3. Orientations of the wire axis for the secondary recrystallized grains formed by annealing by local heating. (a) Specimens are annealed below 2000°C. (b) Specimens are annealed above 2000°C. 0, grains of above 3 mm in length
along
the wire axis; 0, grains
of - 1 mm diameter;
l , grains of 150-500 pm diameter.
The fiber structure of drawn wires was examined; a fairly sharp (011) fiber structure was formed at a central part of the specimen, but the sharpness of the structure was lost to a considerable degree at the circumferential part. Since the primary recrystallized texture is the direct source of the secondary recrystallization, it is important to know the detailed
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111
i
I
10cm Photograph
2. Single crystals grown in moderate tem-
perature gradient.
A A /cl A
001
structure, Both the orientation and size of each primary recrystallized grain were measured on the’ cross section of the specimen, using the E.C.P. method. The specimen annealed at 1500°C for 5 min was used. Figure 4 shows the regions where the orientation of grains was measured; about 40 grains were measured at each region. In addition, some grains having a large grain size were selected to measure the orientations. It is noted in Fig. 5 that the (011) component was dominant, although the subcomponent of (023)-(012) was significantly strong. This observation agreed with the result of the neutron diffraction by Swalin and Geisler [g]. The same results are shown in the form of (001) pole figures in Fig. 6; the anisotropy of (001) pole density is commonly seen about the specimen axis. These may correspond to a cylindrical or a partial cylindrical texture which has been reported in wires of the other metals [9, lo]. No attempt, however, was made to understand the details of the anisotropy in the present study.
4. DISCUSSION 4. I. Growth of secondary recrystallized grains Let us consider a reason why a long single crystal can grow only when a pure MO wire is annealed in a moderate tem~rature gradient ~Photograph 2). When a pure MO wire having the primary re-
a
OH
001
III
011
CKn
C
011
b
111
001
d
Oil
Fig. 5. Orientations of the wire axis for the primary recrystallized grains, (a), (b) Results near circumferential parts of the specimen. (c), (d) Results in central parts of the specimens.
crystallized texture shown in Photograph 1 is annealed in a uniform temperature furnace, secondary recrystallization proceeds only with difficulties owing to the slightly large average grain size and to the relatively simple primary recrystallized texture. Assuming that the density of preferential nucleation sites for the secondary recrystallization is low in the present wires, secondary recrystallized coarse grains will be formed in the uniform temperature anneafing. When the temperature gradient is too large, however, such preferential sites would be unlikely to be found in the heated zone. In conclusion, only when the s~~imens are anneaied in a moderate temperature gradient, a secondary recrystallization would occur at a single preferential site in or near the hottest zone and a single grain will grow before the other secondary recrystallized grains will. 4.2. Mechanism of secondary recrystallization
Fig, 4. Showing regions where orientatjons of the primary recrystallized grains were measured by the E.C.P. method. The large circle shows a cross section of the wire normal to the wire axis. An arrow shows a reference direction.
As shown in Figs 1 and 3, a iarge number of orientations of the secondary recrystallized grains deviated more than several degrees from (011) in the both cases of uniform temperature annealing and of annealing by local heating. As mentioned in Section 3.4, the main component of the primary recrystallized texture was (011) and the sub-component (023)(012). Many (011) oriented grains having a larger grain size than the average one were observed. When several primary recrystallized grains of an especially large size were selected and their orientations were measured, again about half of them were found to be ciose to (011) orientation. If the size factor of the grains in the primary recrystallized texture is a key factor to control the secondary recrystallization, then it would be hard to understand why the secondary
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RECRYSTALLIZATION
Circumferential part
Central
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b
part
Fig. 6. (001) pole figures of orientations of the primary recrystallized grains. (a), (b) Results near circumferential parts of the specimen. (c), (d) Results in central parts of the specimen. The arrow indicates the reference direction marked in Fig. 4.
recrystallized grains with orientation close to (011) are less frequently observed (Figs 1 and 3). Let us consider a reason why the secondary recrystallized grains with orientation close to (01 I) are so less frequently observed. Some models on the geometrical relationships among the primary recrystallized grains are showing in Fig. 7. Suppose that a specific grain grows at the expense of the small neighboring grains, as shown in model (a). In this case, the boundary migration occurring during the grain growth is difficult in the direction along the specimen axis, because the boundaries in model (a) is of pure twist type. It is likely that the mobility of a pure twist boundary is small as compared to those of the tilt and mixed type boundaries. Indeed, difference in the mobilities of the twist and tilt boundaries has been experimentally confirmed in Al and Cu [I I-I 51; these investigations have been performed by using lightly strained and polygonized matrix or capillary force. Model (b) shows a case where the growth in the
direction along the specimen axis is possible, but that in the lateral direction is difficult. This is because a central specific grain is bounded by small angle boundaries in the lateral direction. In this case the nucleation of the secondary recrystallization would also be difficult, because the nucleus prefers to grow spherically in the case of pure MO. In model (c) (023) or < 135) oriented secondary recrystallized grains would be formed. According to Hillert’s theory, the local orientation relationships around the nucleus would no longer be an important limiting factor for the growth, once the nucleation is completed [16]. This is a reason why a single crystal wire with its axis parallel to (023) or (135) is frequently observed. A supplementary consideration may be made on the origin of ( 135) oriented secondary recrystallized grains. Comparing the experimental results of the unetched and etched specimens [Fig. I (a) and (b)]. we conjecture that this oriented secondary recrystallized grains are preferentially formed close to the circum-
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the primary recrystallized texture (Fig. 5). This observation implies that (135) oriented primary recrystallized grains have by far the favorable environments for the secondary recrystallization. Lastly, Fig. 8 shows the results of the etched specimen locally annealed above 2300°C; the secondary recrystallized grains with orientation close to (011) appear to grow easily. This observation may be attributed to the fact that the boundary mobilities become independent of the boundary types at such high temperatures. Similar results have been reported in doped W wires and in the experiments of the boundary migration of pure Pb [2, 171.
b
C
Fig. 7. Models showing the early stages of the secondary recrystallization to explain the effects of geometrica! factors. Central grains show specific grains to become nuclei. 1, (011) Orientation (almost parallel to wire axis; U, rotation.
part of the specimens. Although (135) oriented secondary recrystallized grains are one of the main components of the secondary recrystallization, grains with this orientation are less frequently observed than those with (023)-(012) orientation in ferential
111
Acknowledgements-The author expresses a deep appreciation to Dr K. Onawa for his critical reading of the manuscript and for his helpful comments. The author is very grateful to Dr A. Matushita and to Dr E. Furubayashi for their helpful discussions.
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15. D. Eylon, A. Rosen and S. Niedzwiedz, Acta metall. 17, Fig. 8. Orientations of the secondary recrystallized grains in the specimen etched down to half diameter and locally heated above 2300°C. 0, Grains of - 500 pm diameter; 0, grains of 150-500 pm diameter.
1013 (1969).
16. M. Hillert, Acta metall. 13, 227 (1965). 17. K. J. Aust and J. W. Rutter, Trans. Am. Inst. Min. Engrs 224, 111 (1962).