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Development of the bubble structure from selectively deforming potassium-pores in doped tungsten wires
International Journal of Refractory Metals & Hard Materials 22 (2004) 9–15 www.elsevier.com/locate/ijrmhm
Development of the bubble structure from selectively deforming potassium-pores in doped tungsten wires O. Horacsek, L. Bartha
*
Research Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences, P.O. Box 49. 1525 Budapest, Hungary Received 6 August 2003; accepted 26 September 2003
Abstract As the AKS-doped sintered ingot is processed into wire, the smaller potassium-containing pores appear to be less deformable than the larger ones. In the present work the consequences of this selective pore deformation were investigated on the characteristics of the developing bubble dispersion in different heat-treated samples. The results indicated that the refinement of the potassium distribution during thermomechanical processing occurs more effectively if the potassium in the sintered ingot is enclosed in relatively large pores. It was concluded from a simple estimation that the size-dependent deformation of the potassium-pores exerts an equalizing effect on the potassium distribution manifesting itself in the fact that an initial broad size distribution of potassium-pores in the sintered ingot will be moderated during thermomechanical processing to a relatively narrow size distribution of the bubbles in the wire. In light of the size-dependent pore deformation, the occurrence of the isolated single bubbles in AKS-doped tungsten wires was also briefly discussed. 2003 Elsevier Ltd. All rights reserved. Keywords: Doped tungsten; Size-dependent pore deformation; Bubble dispersion; Bubble strengthening
1. Introduction Tungsten lamp filaments must operate at high temperatures for long times without significant distortion of the initial coil geometry. To meet these requirements, a bubble strengthening method is applied by a special alloying procedure that is called AKS-doping referring to the three dope elements of Al, K, and Si. As a result of doping, sintered tungsten ingots contain 60–80 lg/g potassium in metallic form incorporated in closed submicron sized pores. When the ingot is processed by swaging and wire drawing, these potassium-filled pores (bubbles) are elliptically elongated into long tubes in the working direction. As the diameter of the rod or wire is reduced, the potassium-containing tubes, that are referred mostly to as potassium-ellipsoids, become narrower and longer. The morphology of the ellipsoids is usually characterised by their length-to-width ratio, l=w [1]. If the l=w ratio exceeds a critical value, the elongated pore becomes thermodynamically unstable and breaks up into smaller spherical bubbles at sufficiently high
*
Corresponding author. Tel.: +36-1-392-2222; fax: +36-1-392-2226. E-mail address: [email protected] (L. Bartha).
0263-4368/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2003.10.002
annealing temperature. In this way, as a result of the thermomechanical treatment, characteristic bubble rows are formed in doped tungsten accomplishing thereby the required high temperature strength of the wire. Because of the importance of the bubble formation in determining the quality of tungsten lamp wires, much attention has been devoted to study the effect of the thermomechanical processing on the developing bubble dispersion [2–10]. The mechanism and kinetics of bubble formation in doped tungsten were first studied by Moon and Koo [1]. The presented bubble formation model proved to be a considerable progress in the understanding of the effect of AKS-doping on the enhancement of the high temperature creep strength of tungsten lamp wires. This work was based on the assumption that during swaging and wire drawing the volume of the potassiumcontaining pore is conserved and the morphological change of the pore is proportional to that of the deforming rod or wire. Recent studies showed, however, that some discrepancies exist between the theoretically predicted and experimentally observed behaviour of the deforming pores as the sintered ingot is processed into wire. It was found that at the working temperatures, the deformation of the potassium-containing pores
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O. Horacsek, L. Bartha / International Journal of Refractory Metals & Hard Materials 22 (2004) 9–15
(bubbles) occurs selectively depending on their size [8]. It was also found that the bubbles in thin wires are much larger than their calculated size derived from the theoretical model [7]. The aim of the present work was to investigate the consequences of the size-dependent deformation of the potassium-pores on the features of the bubble dispersion in the thermomechanically processed materials.
2. Experimental results
in the 2 mm diameter wire experienced less deformation than the larger ones, while some small pores retained their original spherical shape. The SEM micrograph in Fig. 2 obtained from a swaged and (at 1800 C for 5 min) heat treated 3 mm diameter rod shows that within the given part of the sample only one large pore has achieved sufficient elongation for ovulation and breakup into three smaller bubbles. Fig. 3, taken from another part of the same sample shows differently elongated ellipsoids undergoing fluctuations along their lengths and indications that the ends of the ellipsoids are preferred sites for the ovulation process. As a result of the size-dependent (selective) pore deformation, in the heat-treated samples the number of the brokenup bubbles in the bubble rows varied from two to higher values and a number of randomly distributed bubbles appeared as isolated single ones. Although with increasing reduction in the cross-section the frequency of these isolated bubbles decreased, several isolated bubbles were found in 1 mm
For the present work AKS-doped tungsten samples taken from sintered ingots, swaged rods and drawn wires were examined to follow up the changes in the morphology of the potassium-containing pores during processing. The morphological changes were analysed at the early stages of the working process where the brokenup bubbles were sufficiently large to observe them by scanning electron microscopy (SEM) getting information about the evolution of the bubble dispersion in the processed products. The fractured surface of the samples was examined both in as-worked and heat treated conditions. According to the bubble formation model [1], as the amount of the deformation increased, the average l=w ratio of the elongated potassium-pores increased. However, it was found that even after significant reduction in the cross-section (>88.3%), a number of the small potassium-pores retained its original spherical (l=w ¼ 1) shape. Although exact determination of the l=w ratios presented difficulties with the measurement of the dimensions of the long, thin ellipsoids, the micrographs provided a qualitative picture about the behaviour of the potassium-pores in the deforming material. In support of our previous interpretation [8], Fig. 1 presents additional direct evidence for the size-dependent pore deformation showing that during the mechanical working the small potassium-pores
Fig. 2. SEM micrograph of a heat-treated swaged rod demonstrating the effect of selective pore deformation. While the large pore achieved sufficient deformation for breakup into three bubbles, the smaller pores restored their original equilibrium size and shape.
Fig. 1. SEM micrograph showing differently elongated potassiumpores in 2 mm diameter AKS-doped tungsten wire.
Fig. 3. SEM micrograph showing potassium ellipsoids and spheroidized isolated single bubbles in swaged and annealed rod.
O. Horacsek, L. Bartha / International Journal of Refractory Metals & Hard Materials 22 (2004) 9–15
11
80
INGOT U
FREQUENCY [%]
70 60 50 40 30 20 10 0 0
Fig. 4. SEM micrograph of heat-treated 1 mm diameter wire showing a variety of the number of bubbles in the rows.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
DIAMETER [µm]
(a) 80
INGOT W
FREQUENCY [%]
70
diameter wire as shown in Fig. 4. In these samples (after annealing at 1800 C for 10 min) the number of the bubbles in the individual rows varied from 2 to 16, but on average, it amounted only to approximately 6. To study the effect of the size-dependent deformation of the potassium-pores on the evolution of the bubble dispersion, the recrystallization behaviour of two 0.39 mm diameter wires with different process history was comparatively investigated. The wires were processed from two ingots in which the size distribution of the potassium-pores was different. The use of these starting materials was based on the results of previous studies that have shown that H2 O-washing of the doped tungsten blue oxide resulted in a marked decrease in the frequency of the large (>0.5 lm) potassium-pores in the sintered ingot [11]. Thus, the ingots were sintered from two different batches of hydrogen reduced AKS-doped tungsten powders. While the first powder batch was produced by the standard AKS-doping technology, the second batch was obtained by normally doped and subsequently H2 O-washed tungsten blue oxide. The size distribution of the potassium-pores in the sintered ingots was determined by measuring the pore diameters on the fractured surfaces and counting their relative frequency for the corresponding size ranges. Since the bubble rows in doped tungsten wires are formed from the submicron sized pore population of the sintered ingot [12], the histograms were made for the diameter range of 0–1 lm. The pore size distributions for the sintered ingots obtained from unwashed and washed blue oxides, designated as ingot U and ingot W, are given in Fig. 5. Although the washing treatment was accompanied by some loss of potassium (10%), the difference in the size distribution of the potassium pores in the two ingots allowed to investigate the effect of the size-dependent pore deformation on the developing bubble dispersions in the wires. The characterization of the bubble dispersion was carried out indirectly by monitoring the re-
60 50 40 30 20 10 0 0
(b)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
DIAMETER [µm]
Fig. 5. (a) Frequency distribution of the pore diameters in ingot U, (b) Frequency distribution of the pore diameters in ingot W.
crystallization processes occurring in 0.39 mm diameter wires. Since the movements of the grain boundaries in doped tungsten wires are controlled by the bubble rows [13–15] and the effect of these growth-controlling bubble barriers can be observed during recrystallization by emission electron microscopy (EEM), this technique seemed to be a suitable indirect method to obtain a comprehensive picture about the bubble dispersion that exists (and control the movements of the grain boundaries) in the wire [11]. The examinations showed that the grain growth process started at a lower temperature and the recrystallized grains exhibited lower aspect ratios if the wire was processed from ingot W in which pores above 0.5 lm were detected only in a limited number onset of grain growth: Tu ¼ 2150 C, Tw ¼ 1950 C; grain aspect ratio: ½garu ¼ 16 ½garw ¼ 10). A further consequence of the lack of the larger potassium-pores in ingot W manifested itself during wire recrystallization in a change in the character of the grain boundary motion. While in wire U (processed from ingot U), the motion of the grain boundaries perpendicular to the wire axis was restricted mostly by long barriers, the lateral grain growth in wire W appeared to be impeded by shorter obstacles, as shown in Fig. 6.
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Fig. 6. (a) EEM micrograph showing grain growth process in wire U reflecting that grain boundary movements were impeded by long bubble barriers perpendicular to the wire axis, (b) EEM micrograph showing grain growth process in wire W reflecting that the laterally moving grain boundaries experienced shorter bubble barriers compared with wire U.
3. Discussion Technological studies on doped tungsten showed that the high temperature properties of the wires are affected during the different processing steps by several factors [16,17]. The amount, chemical compositions and sizes of the incorporated potassium-containing inclusions within the reduced tungsten powder may well influence the properties of the sintered ingot and thereby the behaviour of the wire [18]. In the case of a given starting material, the final properties of the bubble dispersion are determined by the last step of the production process, when the sintered ingot is converted into fine wire. According to the bubble formation model of Moon and Koo (1), the length-to-width ratio of an elongated pore can be related to the rod (or wire) diameter before, Di , and after working, Df , by 3
l=w ¼ 1:2ðDi =Df Þ ;
ð1Þ
while the number of the bubbles formed from each ellipsoid, N , is related to the amount of deformation by N ¼ 0:28ðDi =Df Þ3 :
the deformed material might be very different, but the length-to-width ratio, l=w, must be identical for all ellipsoids independently of the original (undeformed) size of the potassium pore. Consequently, if the l=w ratio exceeds a critical value (@10), and the annealing temperature is sufficiently high, then the number of the brokenup bubbles in each bubble row must be the same, according to Eq. (2). The experimental results showed, however, that the elongated potassium-pores in the deformed samples exhibited different l=w ratios, while the smallest pores kept their original spherical shape indicating that they were practically undeformable during working (Figs. 1– 3). Consequently, the number of the brokenup bubbles in the rows of the heat-treated samples deviated from the theoretically derived value of Eq. (2). Considering that the 1 mm diameter wire was processed from a heat-treated Di ¼ 5:85 mm diameter swaged rod, we obtain from relation (1) for the length-to-width ratio of the elongated pores l=w ffi 240 and from relation (2) for the number of the bubbles in each row: N ffi 56. In contrast to relation (2), the experimental results showed that the number of the bubbles in the rows varied in a broad range from 2 to 16. Obviously, this variety in the number of the bubbles within the rows must stem from the differently elongated potassium ellipsoids whose length-to-width ratios must be ranged from l=w ffi 10 to l=w ffi 80. The difference between the theoretically calculated and experimentally found values indicates that both the l=w ratio of the deformed pores and the number of the bubbles in the rows, N , are considerably overestimated by Eqs. (1) and (2). The wide variety of the number of the bubbles in the rows and the presence of the isolated single bubbles in the heat-treated 1 mm diameter wire indicates that the size-dependent pore deformation has a marked effect on the developing bubble dispersion. Assuming that at a given stage of mechanical working a large, and hence, relatively easily deforming potassium pore with radius R1 breaks up into n1 smaller bubbles with (uniform) radii r1 , while a less deformable smaller one with radius R2 breaks up into only n2 bubbles with radii r2 ðn2 < n1 Þ and the potassium behaves as a perfect gas, than from the ideal gas law: 8pcR21 8pcr12 ¼ n1 3 3
and
8pcR22 8pcr22 ¼ n2 ; 3 3
ð3Þ
where c is the specific surface energy of the tungsten/ potassium interface. From these relations one obtains:
ð2Þ
Relation (1) predicts that at a given stage of mechanical working, the lengths of the elliptically elongated pores in
R1 r1 ¼ pffiffiffiffiffi n1
and
R2 r2 ¼ pffiffiffiffiffi : n2
ð4Þ
O. Horacsek, L. Bartha / International Journal of Refractory Metals & Hard Materials 22 (2004) 9–15
Combining these formulae leads to the expression: rffiffiffiffiffi r 1 R1 n 2 : ð5Þ ¼ r 2 R2 n 1 Since n2 < n1 , and therefore rffiffiffiffiffi n2 < 1; n1
ð6Þ
from relations (5) and (6), we obtain: r 1 R1 < : r 2 R2
ð7Þ
Obviously, if according to the bubble formation model [1], all potassium pores were equally deformable and therefore n1 ¼ n2 , then from relation (5) it follows that R1 =R2 ¼ r1 =r2 i.e. the ratio of the pore radii in the ingot would be equal to the ratio of the bubble radii in the corresponding rows of the worked and heat-treated material. Because of the size-dependent pore deformation, however, relation (7) indicates that the ratio of the radii of a large and a small potassium pore, R1 =R2 , is higher than the ratio of the radii of the brokenup bubbles, r1 =r2 , in the corresponding bubble rows that are formed from the elongated parent pores. For example, if we assume that a large potassium pore with radius R1 ¼ 0:3 lm breaks up into n1 ¼ 16 smaller bubbles with radii r1 , while at the same time a smaller and, hence, less deforming pore with radius R2 ¼ 0:05 lm produces only n2 ¼ 4 bubbles with radii r2 , then according to relation (5), we obtain r1 =r2 ¼ 3. In this case the original ratio of the radii of the parent pores, R1 =R2 ¼ 6 decreases to r1 =r2 ¼ 3 for the ratio of the radii of the brokenup bubbles in the corresponding rows. This means that as the sintered ingot is processed to fine wire, the size-dependent pore deformation exerts an ‘‘equalization’’ effect on the potassium distribution. Therefore, the difference in the degree of the potassiumdispersion between two sintered ingots decreases during their thermomechanical processing. This conclusion is supported by previous experimental results which showed that the initial difference in the potassium distribution between two sintered ingots decreases during processing, contributing to the development of similar bubble structures in the wires [14]. Since a highly elongated large potassium-pore breaks up into a lot of small bubbles, while a poorly deforming small pore creates only fewer and therefore relatively large bubbles (compared with the size of the ‘‘parent’’ pore), the refinement of the potassium distribution during thermomechanical processing occurs more effectively if the given amount of potassium (60–80 lg/g) is enclosed in relatively large pores in the ingot. In this case long bubble rows containing numerous small bubbles will form in the wire contributing to the required bubble strengthening of the matrix and to the development of a highly elongated interlocking recrystallized grain structure.
13
It should be noted that potassium in the bubbles presumably does not exactly obey the perfect gas law, and hence, the above calculation must be considered only approximate. However, if we repeat the estimation under the assumption that potassium in the bubbles behaves like a liquid, i.e., the volume of the bubbles is conserved during the breakup process, then the results do not differ essentially from the data of the former calculation. In this case relation (5) changes to rffiffiffiffiffi r 1 R 1 3 n2 ¼ ; ð8Þ r 2 R 2 n1 i.e. the square-root in Eq. (5) changes to cube-root in Eq. (8), from which considering that n2 < n1 , one obtains again relation (7). The histograms in Fig. 5 show that larger (>0,4 lm) pores were present in a noticeable number only in ingot U. Considering that a large potassium pore can produce more bubbles aligned in longer rows than a less deformable smaller one, it is reasonable to expect that more long bubble rows will develop in wire U than in wire W. This expectation was supported by the EEM experiments. In agreement with previous observations, Fig. 6 shows that lateral grain growth in wire U, was impeded by longer barriers compared to wire W in which the moving boundaries experienced mostly shorter obstacles perpendicular to the wire axis. Obviously, the more the large potassium-pores in the ingot, the more long bubble barriers will restrict geain boundary motion. Therefore the great number of the large potassium-pores in ingot U resulted in the development of a highly elongated recrystallized grain structure in wire U. On the other hand, the smaller potassium-pores in ingot W produced shorter bubble rows in the wire that resulted in a lower recrystallization temperature and smaller aspect ratio of the recrystallized grains. Thus, the difference in the size distribution of the potassium-pores between the sintered ingots may account for the difference in the recrystallization behaviour, and hence, in the difference in the bubble dispersion between the two wires. This means that the recrystallization experiments supported the view that more bubbles can form from a large potassium pore than from a less deformable smaller one, i.e., the ‘‘bubble multiplication ability’’ of the large potassiumpores is greater than that of the smaller ones. The above experimental results showed that in doped tungsten, the l=w ratio of the deformed potassiumpores, and hence, the number of the brokenup bubbles within the rows may considerably deviate from the expected parameters predicted by Eqs. (1) and (2). The discrepancy between theory and experiments can be attributed to several factors. At the early stages of processing when the working temperature is high (e.g. swaging is carried out above 1400 C), two competing processes may occur during deformation [2,5]. The
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O. Horacsek, L. Bartha / International Journal of Refractory Metals & Hard Materials 22 (2004) 9–15
elongation of a deforming potassium-pore and the simultaneously acting tendency to restore its original shape can result in a smaller l=w ratio than the predicted value of Eq. (1). Since at the working temperature the time required to achieve the equilibrium shape of a small (<0.05 lm) potassium-pore may be very short (comparable with the time during which the deforming part of the rod/wire experiences the working temperature), the smallest potassium-pores cannot maintain their deformed non-equilibrium shape. In this case the number of the brokenup bubbles in the row will be less than the calculated number of Eq. (2), or in extreme cases some small potassium-pores of the sintered ingot may be introduced into the wire (without sufficient elongation for breakup) in their original size as isolated single bubbles. For the occurrence of the single bubbles a mechanism of deformation induced ‘‘in situ spheroidization’’ was proposed in a recent work by Schade [10]. As the processing proceeds, break up of the deforming potassium-pores into shorter segments may also result in fewer bubbles in the rows than the theoretically predicted number. A further explanation of the discrepancy between theory and experiment was proposed by Nagy (7) who suggested that if the potassium-bubble exists in a two-phase state, its deformation can lead to a lower l=w ratio compared to a bubble that is filled with only a dense fluid. In this case the breakup process would lead also to bubble rows that contain less bubbles than the calculated number of Eq. (2). In summary, while at the early stages of processing the high working temperature makes difficult to lengthen the potassiumpores concomitantly with the bulk material, as the processing proceeds, further mechanisms may also contribute to the formation of shorter bubble rows and greater bubble sizes than the predictions of the theory.
4. Conclusions As a result of the selective deformation of the potassium-pores in doped tungsten, a considerable part of the bubble rows, preferentially those that are formed from the smallest pore population of the sintered ingot, contain fewer bubbles than the theoretically predicted number of the bubble formation theory based on the assumption that during warm working, each pore endure the same change in shape proportional to that of the bulk material. The selective deformation of the potassium-containing pores exerts an ‘‘equalizing’’ effect on the potassium distribution that manifests itself in the fact that an initial broad size distribution of the potassium-pores in the sintered ingot will be moderated during thermomechanical processing to a relatively narrow size distribution of the brokenup bubbles in the wire.
The refinement of the potassium distribution during thermomechanical processing may occur more effectively if the given amount of potassium is enclosed in relatively large pores in the sintered ingot. In this case long bubble rows can form in the wire contributing to the required dispersion strengthening of the matrix and to the development of a highly elongated recrystallized grain structure.
Acknowledgements The work was supported by the National Research Fund (OTKA) contract Nr: T 32730.
References [1] Moon DM, Koo RC. Mechanism kinetics of bubble formation in doped tungsten. Metall Trans 1971;2:2115–22. [2] Vukcevich MR. Spheroidization of potassium bubbles in tungsten. 5th Int. Tungsten Symp. Proceedings, 1991;p. 157–68. [3] Briant CL. Potassium bubbles in tungsten wire. Metall Trans 1993;24A:1073–84. [4] Liu X, Zuo, T. A study on the dispersion of K-filled bubbles and recrystallization of doped tungsten wires. 13th Plansee Seminar Proceedings RM 49, 1996;1:390–404. [5] Briant CL. Warm rolling and swaging of tungsten. 5th Int. Tungsten Symp. Proceedings, 1991; p. 169–82. [6] Bewlay BP, Briant CL. The formation and the role of potassium bubbles in NS-doped tungsten, in: Bartha L, Lassner E, Schubert WD, Lux B, editors. The Chemistry of Non-Sag Tungsten. Great Britain: Elsevier Applied Science; 1995. p. 137–59. [7] Nagy A. Van-der-Waals approximation for potassium bubbles in tungsten. Metall Mater Trans 1996;27B:987–92. [8] Horacsek O, T oth CsL, Nagy A. Selective bubble deformation during thermomechanical processing in KSiAl-doped tungsten. Int J Refr Metals & Hard Mater 1998;16:51–7. [9] Harmat P, Gr osz T, Rosta L, Bartha L. A novel method for shape analysis: deformation of bubbles during wire drawing in doped tungsten. 15th Plansee Seminar Proceedings RM 76, 2001;4:108–117. [10] Schade P. Bubble evolution and effects during tungsten processing. Int J Refr Metals & Hard Mater 2002;20:301–9. [11] Horacsek O, Bartha L. Influence of surface particles of AKSdoped TBO on the NS-structure of tungsten wires. Int J Refr Metals & Hard Mater 2002;20:271–6. [12] Horacsek O, T oth Cs, Gaal I, Horacsek K. Relation between pore size distribution in sintered tungsten ingots and creep strength of the wires. 12th Plansee Seminar Proceedings RM 49, 1989;1:513–521. [13] Yamazaki S, Ogura S, Fukazawa Y, Hatae N. The effect of bubble dispersion on high-temperature tensile behaviour of doped tungsten wire. High Temp High Presssures 1978;10:329–39. [14] Briant CL, Horacsek O, Horacsek K. The effect of wire history on the coarsened substructure and secondary recrystallization of doped tungsten. Metall Trans 1993;24A:843–51. [15] St€ uwe HP. A model for the formation of the non-sag structure of potassium-doped tungsten wire. Metall Trans 1986;17A:1455–9. [16] Mullendore JA. The technology of doped-tungsten wire manufacturing, in: Pink E, Bartha L, editors. The metallurgy of doped/ non-sag tungsten. London: Elsevier Science Publishers Ltd; 1989. p. 61–81.
O. Horacsek, L. Bartha / International Journal of Refractory Metals & Hard Materials 22 (2004) 9–15 [17] Schob O. General quality control and mechanical tests, in: Pink E, Bartha L, editors. The metallurgy of doped/non-sag tungsten. London: Elsevier Science Publishers Ltd; 1989. p. 83–112.
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[18] Schubert WD, Lux B, Zeiler B. Formation and incorporation of dopant phases during technical reduction of NS-doped tungsten blue oxide. Int J Refr Metals & Hard Mater 1995;13:119–35.
Development of the bubble structure from selectively deforming potassium-pores in doped tungsten wires O. Horacsek, L. Bartha
*
Research Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences, P.O. Box 49. 1525 Budapest, Hungary Received 6 August 2003; accepted 26 September 2003
Abstract As the AKS-doped sintered ingot is processed into wire, the smaller potassium-containing pores appear to be less deformable than the larger ones. In the present work the consequences of this selective pore deformation were investigated on the characteristics of the developing bubble dispersion in different heat-treated samples. The results indicated that the refinement of the potassium distribution during thermomechanical processing occurs more effectively if the potassium in the sintered ingot is enclosed in relatively large pores. It was concluded from a simple estimation that the size-dependent deformation of the potassium-pores exerts an equalizing effect on the potassium distribution manifesting itself in the fact that an initial broad size distribution of potassium-pores in the sintered ingot will be moderated during thermomechanical processing to a relatively narrow size distribution of the bubbles in the wire. In light of the size-dependent pore deformation, the occurrence of the isolated single bubbles in AKS-doped tungsten wires was also briefly discussed. 2003 Elsevier Ltd. All rights reserved. Keywords: Doped tungsten; Size-dependent pore deformation; Bubble dispersion; Bubble strengthening
1. Introduction Tungsten lamp filaments must operate at high temperatures for long times without significant distortion of the initial coil geometry. To meet these requirements, a bubble strengthening method is applied by a special alloying procedure that is called AKS-doping referring to the three dope elements of Al, K, and Si. As a result of doping, sintered tungsten ingots contain 60–80 lg/g potassium in metallic form incorporated in closed submicron sized pores. When the ingot is processed by swaging and wire drawing, these potassium-filled pores (bubbles) are elliptically elongated into long tubes in the working direction. As the diameter of the rod or wire is reduced, the potassium-containing tubes, that are referred mostly to as potassium-ellipsoids, become narrower and longer. The morphology of the ellipsoids is usually characterised by their length-to-width ratio, l=w [1]. If the l=w ratio exceeds a critical value, the elongated pore becomes thermodynamically unstable and breaks up into smaller spherical bubbles at sufficiently high
*
Corresponding author. Tel.: +36-1-392-2222; fax: +36-1-392-2226. E-mail address: [email protected] (L. Bartha).
0263-4368/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2003.10.002
annealing temperature. In this way, as a result of the thermomechanical treatment, characteristic bubble rows are formed in doped tungsten accomplishing thereby the required high temperature strength of the wire. Because of the importance of the bubble formation in determining the quality of tungsten lamp wires, much attention has been devoted to study the effect of the thermomechanical processing on the developing bubble dispersion [2–10]. The mechanism and kinetics of bubble formation in doped tungsten were first studied by Moon and Koo [1]. The presented bubble formation model proved to be a considerable progress in the understanding of the effect of AKS-doping on the enhancement of the high temperature creep strength of tungsten lamp wires. This work was based on the assumption that during swaging and wire drawing the volume of the potassiumcontaining pore is conserved and the morphological change of the pore is proportional to that of the deforming rod or wire. Recent studies showed, however, that some discrepancies exist between the theoretically predicted and experimentally observed behaviour of the deforming pores as the sintered ingot is processed into wire. It was found that at the working temperatures, the deformation of the potassium-containing pores
10
O. Horacsek, L. Bartha / International Journal of Refractory Metals & Hard Materials 22 (2004) 9–15
(bubbles) occurs selectively depending on their size [8]. It was also found that the bubbles in thin wires are much larger than their calculated size derived from the theoretical model [7]. The aim of the present work was to investigate the consequences of the size-dependent deformation of the potassium-pores on the features of the bubble dispersion in the thermomechanically processed materials.
2. Experimental results
in the 2 mm diameter wire experienced less deformation than the larger ones, while some small pores retained their original spherical shape. The SEM micrograph in Fig. 2 obtained from a swaged and (at 1800 C for 5 min) heat treated 3 mm diameter rod shows that within the given part of the sample only one large pore has achieved sufficient elongation for ovulation and breakup into three smaller bubbles. Fig. 3, taken from another part of the same sample shows differently elongated ellipsoids undergoing fluctuations along their lengths and indications that the ends of the ellipsoids are preferred sites for the ovulation process. As a result of the size-dependent (selective) pore deformation, in the heat-treated samples the number of the brokenup bubbles in the bubble rows varied from two to higher values and a number of randomly distributed bubbles appeared as isolated single ones. Although with increasing reduction in the cross-section the frequency of these isolated bubbles decreased, several isolated bubbles were found in 1 mm
For the present work AKS-doped tungsten samples taken from sintered ingots, swaged rods and drawn wires were examined to follow up the changes in the morphology of the potassium-containing pores during processing. The morphological changes were analysed at the early stages of the working process where the brokenup bubbles were sufficiently large to observe them by scanning electron microscopy (SEM) getting information about the evolution of the bubble dispersion in the processed products. The fractured surface of the samples was examined both in as-worked and heat treated conditions. According to the bubble formation model [1], as the amount of the deformation increased, the average l=w ratio of the elongated potassium-pores increased. However, it was found that even after significant reduction in the cross-section (>88.3%), a number of the small potassium-pores retained its original spherical (l=w ¼ 1) shape. Although exact determination of the l=w ratios presented difficulties with the measurement of the dimensions of the long, thin ellipsoids, the micrographs provided a qualitative picture about the behaviour of the potassium-pores in the deforming material. In support of our previous interpretation [8], Fig. 1 presents additional direct evidence for the size-dependent pore deformation showing that during the mechanical working the small potassium-pores
Fig. 2. SEM micrograph of a heat-treated swaged rod demonstrating the effect of selective pore deformation. While the large pore achieved sufficient deformation for breakup into three bubbles, the smaller pores restored their original equilibrium size and shape.
Fig. 1. SEM micrograph showing differently elongated potassiumpores in 2 mm diameter AKS-doped tungsten wire.
Fig. 3. SEM micrograph showing potassium ellipsoids and spheroidized isolated single bubbles in swaged and annealed rod.
O. Horacsek, L. Bartha / International Journal of Refractory Metals & Hard Materials 22 (2004) 9–15
11
80
INGOT U
FREQUENCY [%]
70 60 50 40 30 20 10 0 0
Fig. 4. SEM micrograph of heat-treated 1 mm diameter wire showing a variety of the number of bubbles in the rows.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
DIAMETER [µm]
(a) 80
INGOT W
FREQUENCY [%]
70
diameter wire as shown in Fig. 4. In these samples (after annealing at 1800 C for 10 min) the number of the bubbles in the individual rows varied from 2 to 16, but on average, it amounted only to approximately 6. To study the effect of the size-dependent deformation of the potassium-pores on the evolution of the bubble dispersion, the recrystallization behaviour of two 0.39 mm diameter wires with different process history was comparatively investigated. The wires were processed from two ingots in which the size distribution of the potassium-pores was different. The use of these starting materials was based on the results of previous studies that have shown that H2 O-washing of the doped tungsten blue oxide resulted in a marked decrease in the frequency of the large (>0.5 lm) potassium-pores in the sintered ingot [11]. Thus, the ingots were sintered from two different batches of hydrogen reduced AKS-doped tungsten powders. While the first powder batch was produced by the standard AKS-doping technology, the second batch was obtained by normally doped and subsequently H2 O-washed tungsten blue oxide. The size distribution of the potassium-pores in the sintered ingots was determined by measuring the pore diameters on the fractured surfaces and counting their relative frequency for the corresponding size ranges. Since the bubble rows in doped tungsten wires are formed from the submicron sized pore population of the sintered ingot [12], the histograms were made for the diameter range of 0–1 lm. The pore size distributions for the sintered ingots obtained from unwashed and washed blue oxides, designated as ingot U and ingot W, are given in Fig. 5. Although the washing treatment was accompanied by some loss of potassium (10%), the difference in the size distribution of the potassium pores in the two ingots allowed to investigate the effect of the size-dependent pore deformation on the developing bubble dispersions in the wires. The characterization of the bubble dispersion was carried out indirectly by monitoring the re-
60 50 40 30 20 10 0 0
(b)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
DIAMETER [µm]
Fig. 5. (a) Frequency distribution of the pore diameters in ingot U, (b) Frequency distribution of the pore diameters in ingot W.
crystallization processes occurring in 0.39 mm diameter wires. Since the movements of the grain boundaries in doped tungsten wires are controlled by the bubble rows [13–15] and the effect of these growth-controlling bubble barriers can be observed during recrystallization by emission electron microscopy (EEM), this technique seemed to be a suitable indirect method to obtain a comprehensive picture about the bubble dispersion that exists (and control the movements of the grain boundaries) in the wire [11]. The examinations showed that the grain growth process started at a lower temperature and the recrystallized grains exhibited lower aspect ratios if the wire was processed from ingot W in which pores above 0.5 lm were detected only in a limited number onset of grain growth: Tu ¼ 2150 C, Tw ¼ 1950 C; grain aspect ratio: ½garu ¼ 16 ½garw ¼ 10). A further consequence of the lack of the larger potassium-pores in ingot W manifested itself during wire recrystallization in a change in the character of the grain boundary motion. While in wire U (processed from ingot U), the motion of the grain boundaries perpendicular to the wire axis was restricted mostly by long barriers, the lateral grain growth in wire W appeared to be impeded by shorter obstacles, as shown in Fig. 6.
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O. Horacsek, L. Bartha / International Journal of Refractory Metals & Hard Materials 22 (2004) 9–15
Fig. 6. (a) EEM micrograph showing grain growth process in wire U reflecting that grain boundary movements were impeded by long bubble barriers perpendicular to the wire axis, (b) EEM micrograph showing grain growth process in wire W reflecting that the laterally moving grain boundaries experienced shorter bubble barriers compared with wire U.
3. Discussion Technological studies on doped tungsten showed that the high temperature properties of the wires are affected during the different processing steps by several factors [16,17]. The amount, chemical compositions and sizes of the incorporated potassium-containing inclusions within the reduced tungsten powder may well influence the properties of the sintered ingot and thereby the behaviour of the wire [18]. In the case of a given starting material, the final properties of the bubble dispersion are determined by the last step of the production process, when the sintered ingot is converted into fine wire. According to the bubble formation model of Moon and Koo (1), the length-to-width ratio of an elongated pore can be related to the rod (or wire) diameter before, Di , and after working, Df , by 3
l=w ¼ 1:2ðDi =Df Þ ;
ð1Þ
while the number of the bubbles formed from each ellipsoid, N , is related to the amount of deformation by N ¼ 0:28ðDi =Df Þ3 :
the deformed material might be very different, but the length-to-width ratio, l=w, must be identical for all ellipsoids independently of the original (undeformed) size of the potassium pore. Consequently, if the l=w ratio exceeds a critical value (@10), and the annealing temperature is sufficiently high, then the number of the brokenup bubbles in each bubble row must be the same, according to Eq. (2). The experimental results showed, however, that the elongated potassium-pores in the deformed samples exhibited different l=w ratios, while the smallest pores kept their original spherical shape indicating that they were practically undeformable during working (Figs. 1– 3). Consequently, the number of the brokenup bubbles in the rows of the heat-treated samples deviated from the theoretically derived value of Eq. (2). Considering that the 1 mm diameter wire was processed from a heat-treated Di ¼ 5:85 mm diameter swaged rod, we obtain from relation (1) for the length-to-width ratio of the elongated pores l=w ffi 240 and from relation (2) for the number of the bubbles in each row: N ffi 56. In contrast to relation (2), the experimental results showed that the number of the bubbles in the rows varied in a broad range from 2 to 16. Obviously, this variety in the number of the bubbles within the rows must stem from the differently elongated potassium ellipsoids whose length-to-width ratios must be ranged from l=w ffi 10 to l=w ffi 80. The difference between the theoretically calculated and experimentally found values indicates that both the l=w ratio of the deformed pores and the number of the bubbles in the rows, N , are considerably overestimated by Eqs. (1) and (2). The wide variety of the number of the bubbles in the rows and the presence of the isolated single bubbles in the heat-treated 1 mm diameter wire indicates that the size-dependent pore deformation has a marked effect on the developing bubble dispersion. Assuming that at a given stage of mechanical working a large, and hence, relatively easily deforming potassium pore with radius R1 breaks up into n1 smaller bubbles with (uniform) radii r1 , while a less deformable smaller one with radius R2 breaks up into only n2 bubbles with radii r2 ðn2 < n1 Þ and the potassium behaves as a perfect gas, than from the ideal gas law: 8pcR21 8pcr12 ¼ n1 3 3
and
8pcR22 8pcr22 ¼ n2 ; 3 3
ð3Þ
where c is the specific surface energy of the tungsten/ potassium interface. From these relations one obtains:
ð2Þ
Relation (1) predicts that at a given stage of mechanical working, the lengths of the elliptically elongated pores in
R1 r1 ¼ pffiffiffiffiffi n1
and
R2 r2 ¼ pffiffiffiffiffi : n2
ð4Þ
O. Horacsek, L. Bartha / International Journal of Refractory Metals & Hard Materials 22 (2004) 9–15
Combining these formulae leads to the expression: rffiffiffiffiffi r 1 R1 n 2 : ð5Þ ¼ r 2 R2 n 1 Since n2 < n1 , and therefore rffiffiffiffiffi n2 < 1; n1
ð6Þ
from relations (5) and (6), we obtain: r 1 R1 < : r 2 R2
ð7Þ
Obviously, if according to the bubble formation model [1], all potassium pores were equally deformable and therefore n1 ¼ n2 , then from relation (5) it follows that R1 =R2 ¼ r1 =r2 i.e. the ratio of the pore radii in the ingot would be equal to the ratio of the bubble radii in the corresponding rows of the worked and heat-treated material. Because of the size-dependent pore deformation, however, relation (7) indicates that the ratio of the radii of a large and a small potassium pore, R1 =R2 , is higher than the ratio of the radii of the brokenup bubbles, r1 =r2 , in the corresponding bubble rows that are formed from the elongated parent pores. For example, if we assume that a large potassium pore with radius R1 ¼ 0:3 lm breaks up into n1 ¼ 16 smaller bubbles with radii r1 , while at the same time a smaller and, hence, less deforming pore with radius R2 ¼ 0:05 lm produces only n2 ¼ 4 bubbles with radii r2 , then according to relation (5), we obtain r1 =r2 ¼ 3. In this case the original ratio of the radii of the parent pores, R1 =R2 ¼ 6 decreases to r1 =r2 ¼ 3 for the ratio of the radii of the brokenup bubbles in the corresponding rows. This means that as the sintered ingot is processed to fine wire, the size-dependent pore deformation exerts an ‘‘equalization’’ effect on the potassium distribution. Therefore, the difference in the degree of the potassiumdispersion between two sintered ingots decreases during their thermomechanical processing. This conclusion is supported by previous experimental results which showed that the initial difference in the potassium distribution between two sintered ingots decreases during processing, contributing to the development of similar bubble structures in the wires [14]. Since a highly elongated large potassium-pore breaks up into a lot of small bubbles, while a poorly deforming small pore creates only fewer and therefore relatively large bubbles (compared with the size of the ‘‘parent’’ pore), the refinement of the potassium distribution during thermomechanical processing occurs more effectively if the given amount of potassium (60–80 lg/g) is enclosed in relatively large pores in the ingot. In this case long bubble rows containing numerous small bubbles will form in the wire contributing to the required bubble strengthening of the matrix and to the development of a highly elongated interlocking recrystallized grain structure.
13
It should be noted that potassium in the bubbles presumably does not exactly obey the perfect gas law, and hence, the above calculation must be considered only approximate. However, if we repeat the estimation under the assumption that potassium in the bubbles behaves like a liquid, i.e., the volume of the bubbles is conserved during the breakup process, then the results do not differ essentially from the data of the former calculation. In this case relation (5) changes to rffiffiffiffiffi r 1 R 1 3 n2 ¼ ; ð8Þ r 2 R 2 n1 i.e. the square-root in Eq. (5) changes to cube-root in Eq. (8), from which considering that n2 < n1 , one obtains again relation (7). The histograms in Fig. 5 show that larger (>0,4 lm) pores were present in a noticeable number only in ingot U. Considering that a large potassium pore can produce more bubbles aligned in longer rows than a less deformable smaller one, it is reasonable to expect that more long bubble rows will develop in wire U than in wire W. This expectation was supported by the EEM experiments. In agreement with previous observations, Fig. 6 shows that lateral grain growth in wire U, was impeded by longer barriers compared to wire W in which the moving boundaries experienced mostly shorter obstacles perpendicular to the wire axis. Obviously, the more the large potassium-pores in the ingot, the more long bubble barriers will restrict geain boundary motion. Therefore the great number of the large potassium-pores in ingot U resulted in the development of a highly elongated recrystallized grain structure in wire U. On the other hand, the smaller potassium-pores in ingot W produced shorter bubble rows in the wire that resulted in a lower recrystallization temperature and smaller aspect ratio of the recrystallized grains. Thus, the difference in the size distribution of the potassium-pores between the sintered ingots may account for the difference in the recrystallization behaviour, and hence, in the difference in the bubble dispersion between the two wires. This means that the recrystallization experiments supported the view that more bubbles can form from a large potassium pore than from a less deformable smaller one, i.e., the ‘‘bubble multiplication ability’’ of the large potassiumpores is greater than that of the smaller ones. The above experimental results showed that in doped tungsten, the l=w ratio of the deformed potassiumpores, and hence, the number of the brokenup bubbles within the rows may considerably deviate from the expected parameters predicted by Eqs. (1) and (2). The discrepancy between theory and experiments can be attributed to several factors. At the early stages of processing when the working temperature is high (e.g. swaging is carried out above 1400 C), two competing processes may occur during deformation [2,5]. The
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O. Horacsek, L. Bartha / International Journal of Refractory Metals & Hard Materials 22 (2004) 9–15
elongation of a deforming potassium-pore and the simultaneously acting tendency to restore its original shape can result in a smaller l=w ratio than the predicted value of Eq. (1). Since at the working temperature the time required to achieve the equilibrium shape of a small (<0.05 lm) potassium-pore may be very short (comparable with the time during which the deforming part of the rod/wire experiences the working temperature), the smallest potassium-pores cannot maintain their deformed non-equilibrium shape. In this case the number of the brokenup bubbles in the row will be less than the calculated number of Eq. (2), or in extreme cases some small potassium-pores of the sintered ingot may be introduced into the wire (without sufficient elongation for breakup) in their original size as isolated single bubbles. For the occurrence of the single bubbles a mechanism of deformation induced ‘‘in situ spheroidization’’ was proposed in a recent work by Schade [10]. As the processing proceeds, break up of the deforming potassium-pores into shorter segments may also result in fewer bubbles in the rows than the theoretically predicted number. A further explanation of the discrepancy between theory and experiment was proposed by Nagy (7) who suggested that if the potassium-bubble exists in a two-phase state, its deformation can lead to a lower l=w ratio compared to a bubble that is filled with only a dense fluid. In this case the breakup process would lead also to bubble rows that contain less bubbles than the calculated number of Eq. (2). In summary, while at the early stages of processing the high working temperature makes difficult to lengthen the potassiumpores concomitantly with the bulk material, as the processing proceeds, further mechanisms may also contribute to the formation of shorter bubble rows and greater bubble sizes than the predictions of the theory.
4. Conclusions As a result of the selective deformation of the potassium-pores in doped tungsten, a considerable part of the bubble rows, preferentially those that are formed from the smallest pore population of the sintered ingot, contain fewer bubbles than the theoretically predicted number of the bubble formation theory based on the assumption that during warm working, each pore endure the same change in shape proportional to that of the bulk material. The selective deformation of the potassium-containing pores exerts an ‘‘equalizing’’ effect on the potassium distribution that manifests itself in the fact that an initial broad size distribution of the potassium-pores in the sintered ingot will be moderated during thermomechanical processing to a relatively narrow size distribution of the brokenup bubbles in the wire.
The refinement of the potassium distribution during thermomechanical processing may occur more effectively if the given amount of potassium is enclosed in relatively large pores in the sintered ingot. In this case long bubble rows can form in the wire contributing to the required dispersion strengthening of the matrix and to the development of a highly elongated recrystallized grain structure.
Acknowledgements The work was supported by the National Research Fund (OTKA) contract Nr: T 32730.
References [1] Moon DM, Koo RC. Mechanism kinetics of bubble formation in doped tungsten. Metall Trans 1971;2:2115–22. [2] Vukcevich MR. Spheroidization of potassium bubbles in tungsten. 5th Int. Tungsten Symp. Proceedings, 1991;p. 157–68. [3] Briant CL. Potassium bubbles in tungsten wire. Metall Trans 1993;24A:1073–84. [4] Liu X, Zuo, T. A study on the dispersion of K-filled bubbles and recrystallization of doped tungsten wires. 13th Plansee Seminar Proceedings RM 49, 1996;1:390–404. [5] Briant CL. Warm rolling and swaging of tungsten. 5th Int. Tungsten Symp. Proceedings, 1991; p. 169–82. [6] Bewlay BP, Briant CL. The formation and the role of potassium bubbles in NS-doped tungsten, in: Bartha L, Lassner E, Schubert WD, Lux B, editors. The Chemistry of Non-Sag Tungsten. Great Britain: Elsevier Applied Science; 1995. p. 137–59. [7] Nagy A. Van-der-Waals approximation for potassium bubbles in tungsten. Metall Mater Trans 1996;27B:987–92. [8] Horacsek O, T oth CsL, Nagy A. Selective bubble deformation during thermomechanical processing in KSiAl-doped tungsten. Int J Refr Metals & Hard Mater 1998;16:51–7. [9] Harmat P, Gr osz T, Rosta L, Bartha L. A novel method for shape analysis: deformation of bubbles during wire drawing in doped tungsten. 15th Plansee Seminar Proceedings RM 76, 2001;4:108–117. [10] Schade P. Bubble evolution and effects during tungsten processing. Int J Refr Metals & Hard Mater 2002;20:301–9. [11] Horacsek O, Bartha L. Influence of surface particles of AKSdoped TBO on the NS-structure of tungsten wires. Int J Refr Metals & Hard Mater 2002;20:271–6. [12] Horacsek O, T oth Cs, Gaal I, Horacsek K. Relation between pore size distribution in sintered tungsten ingots and creep strength of the wires. 12th Plansee Seminar Proceedings RM 49, 1989;1:513–521. [13] Yamazaki S, Ogura S, Fukazawa Y, Hatae N. The effect of bubble dispersion on high-temperature tensile behaviour of doped tungsten wire. High Temp High Presssures 1978;10:329–39. [14] Briant CL, Horacsek O, Horacsek K. The effect of wire history on the coarsened substructure and secondary recrystallization of doped tungsten. Metall Trans 1993;24A:843–51. [15] St€ uwe HP. A model for the formation of the non-sag structure of potassium-doped tungsten wire. Metall Trans 1986;17A:1455–9. [16] Mullendore JA. The technology of doped-tungsten wire manufacturing, in: Pink E, Bartha L, editors. The metallurgy of doped/ non-sag tungsten. London: Elsevier Science Publishers Ltd; 1989. p. 61–81.
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[18] Schubert WD, Lux B, Zeiler B. Formation and incorporation of dopant phases during technical reduction of NS-doped tungsten blue oxide. Int J Refr Metals & Hard Mater 1995;13:119–35.