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Selective bubble deformation during thermomechanical processing in KSiAl-doped tungsten
REFRACTORY MEm.s
&HARDMATERIAls ELSEVIER
International Journal of Refractory Metals & Hard Materials 16 (1998) 51-57
Selective bubble deformation during thermomechanical in KSiAl-doped tungsten
processing
0. Horacseka*, Cs. L. Tbth”, A. Nagyb “Research Institute for Technical Physics of HAS, H-1325 PO. Box 76, Budapest, Hungary ‘GE-Tungsram Co. Ltd, H-1340 V&i tit 77, Budapest, Hungary
Received 31 October 1997; accepted 9 February 1998
Abstract The potassium-containing pores in KSiAl-doped sintered tungsten ingot are the precursors of the bubbles in the recrystallized wire. In this study the evolution of the inter- and intragranular bubble structures has been examined. Quantitative analysis of the bubble dispersion in the early stage of processing indicated that most of the potassium pores experienced less deformation than the predicted amount of the bubble formation theory. The discrepancy between the experimental facts and theory is explained by the size-dependent deformability of the potassium pores supporting the view that at the process temperatures the small pores (bubbles) are less deformable than the larger ones. The geometry of the lenticularly shaped intergranular bubbles was analyzed by measuring the contact (dihedral) angles at the intersections of the bubbles with the grain boundaries. The results indicated that the tungsten/potassium interface must have a relatively low interfacial tension suggesting that adsorbed impurities on the bubble surface lowered the surface energy of clean tungsten from about 2.3 to approx. 1.2 J/m*. The consequences of the sizedependent bubble deformation and low bubble surface energy in the evolution of the bubble structure and in the formation of extraordinary large bubbles are discussed. 0 1998 Elsevier Science Ltd. All rights reserved. Keywords:
Doped tungsten; Deformability of bubbles; Surface energy of K-W interface
1. Introduction
Tungsten, as a filament material in incandescent lamps, requires a high resistance to creep at the operating temperatures. To improve the mechanical properties of tungsten, a strengthening method is needed which suppresses power-law creep without permitting significant diffusion creep. The solution of this metallurgical problem is to impede the motion of dislocations and to increase the grain size in the recrystallized material. KSiAl-doping successfully satisfies these requirements: the fine potassium-filled bubbles in doped tungsten wires act as effective barriers against dislocation motion inhibiting not only the glide but also the non-conservative motion of the dislocations, while in the highly elongated large grains, strain by the Nabarro-Herring and/or Coble creep has little significance because for these processes the diffusion distance is of the order of the dimensions of the large grains. However, a more important component of the *Corresponding author.
creep strain may arise in doped tungsten from void growth [l] because in this case the diffusion distance is only half the void (growing bubble) spacing which is usually much shorter than the dimensions of the grains. This type of diffusion creep is a result of the removal of tungsten atoms from the surface of the growing voids and their subsequent deposition along those grain boundaries that are nearly perpendicular to the stress axis [2]. Consequently, void growth is always accompanied by creep because the progressive plating out of the removed tungsten atoms along the grain boundaries produces strain in the direction of the applied stress. Since the potassium bubbles on the grain boundaries above a critical size can act as void nuclei [3], the high temperature creep strength and life of the filament depend largely on the size distribution of the potassium bubbles in the wire. The present study is concerned with the evolution of the bubble dispersion in doped tungsten wires. The results suggested that at the process temperatures, the smaller pores (bubbles) are less deformable than the larger ones. The geometry of the lenticularly shaped
0263-4368/98/$ - see front matter 0 1998 Elsevier Science Ltd. All rights reserved. PII: SO263-4368(98)00010-9
52
0. Horucsek et al.llntemational Journal
qf Refractory Metals & Hard Materials 16 (1998) 51-57
grain boundary bubbles was analyzed by measuring the dihedral angles at the intersections of the bubbles with the grain boundaries. For the grain boundary energy/ bubble surface energy ratio, i.‘piyh= 0.92 was obtained indicating that surface-active impurities adsorbed on the bubble surface lowered the surface energy of clean tungsten near to the grain boundary energy (from 2.3 to approx.l.2 J/m’). To explain the appearance of very large bubbles in the recrystallized wire, some possible mechanisms are presented.
The geometry of the lenticularly shaped intergranular bubbles was quantitatively analyzed by measuring the contact (dihedral) angles at the intersections of the bubbles with the grain boundaries. Assuming that the equilibrium between bubble surface tension and grain boundary tension has been reached in the annealed samples and quenching from the high temperature had no effect on the equilibrium configuration at the junction of the bubble and grain boundary, the dihedral angles were measured on a number of enlarged TEM micrographs.
2. Experimental 3. Results and discussion The samples used in this study were prepared from a commercially doped sintered ingot which was processed by swaging and drawing to 0.39 mm wire. The residual pores of the ingot and their morphological changes during thermomechanical processing were examined by scanning electron microscopy (SEM). The bubble structures in swaged rods and in wires drawn to 2 mm diameter was analyzed on the longitudinally fractured surfaces. In these ‘thick’ samples that were prepared at the early stages of the thermomechanical processing, the potassium bubbles were sufficiently large to observe them on the SEM fractographs. Since in the case of 0.39 mm wire samples only the largest bubbles were detectable by SEM, to obtain more information about the entire bubble dispersion, transmission electron-microscopy (TEM) was used: thin foil technique for the examination of the intragranular bubble structure, and fracture replication TEM to observe the bubbles on the grain boundaries. The size distribution of the pores in the sintered ingot was characterized by a histogram that was obtained by measuring the diameters of the pores on the SEM fractographs and counting their number for the corresponding size ranges. Taking into consideration that the bubbles in the wire are formed from the potassium containing pores of the ingot having diameters of the order of 0.1 pm [4], the size distribution of the residual pores in the ingot was determined for the diameter range of O-1 pm. The size distributions of the inter- and intraganular bubbles in 0.39 mm diameter wire samples were determined from a number of TEM micrographs that were obtained by thin foil and fracture replication techniques. Since previous studies showed that the microstructural responses to different annealing conditions are different [5,6], two heating rates were applied: the heat up of the rapidly heated samples occurred within 10 s to 18Oo”C, while the temperature of the slowly heated ones were increased to the same temperature during 30 min and then the samples were annealed for 5 min in both cases.
According to previous studies [7,8], the SEM fractographs showed that the sintered ingot had a mixed residual porosity consisting of numerous potassiumcontaining tiny pores and fewer relatively large irregularly shaped metallurgical voids. The frequency distribution of the pore diameters for the size range of O-l pm is shown in Fig. 1. Although SEM did not allow accurate measurement of pore diameters below 0.1 pm, a more detailed examination of the smallest pore population by replication TEM technique revealed that the first column in Fig. 1 implies many pores with diameters less than 0.05 pm. In the case of 0.39 mm diameter wire samples, the size distribution of the bubbles within the grains and that on the grain boundaries for the two differently heated wires are given in Figs 2 and 3. Table 1 shows the average diameters for the examined bubble populations. The histograms in Fig. 2 and Fig. 3 show that the annealed (primary recrystallized) wires also contained relatively large bubbles with diameters larger than 0.05 Lirn. Comparing these histograms with the size
0,l
0,3
0,5
0,7
0,9
I,1
DIAMETER [pm] Fig. 1. Size distribution of the potassium pores in the sintered ingot. (The first column contains a lot of pores with diameters 0.05 ,um or less).
53
0. Horacsek et al.lIntemational Journal of Refractory Metals & Hard Materials I6 (1998) 51-57
distribution of the potassium pores of the sintered ingot (Fig. l), it can be seen that the largest bubbles in the wire had larger diameters than the smallest pores of the sintered ingot, i.e. the size distributions for the bubbles and pores were overlapping. It was often observed [9] that the largest bubbles in the annealed wires are present as isolated ones or they are arranged in short rows containing only a few bubbles as shown in Fig. 4. In order to analyze the problem of the existence of these large bubbles in the wire, we will consider the predictions of the theory and compare them with our experimental results.
identical to that of the bulk material. The Iengthto-width ratio of an elongated pore (frequently called potassium ellipsoid), L/W, can be related to the rod (or wire) diameter before, Db and after deformation, D,, by the expression L/W = 1.2(DJD,)3
50 intragranular bubbles slow heating 40 -T & ;-’ 0
3.1. Comparison of bubble formation theory with experimental results
(1)
average diameter d = 30,4 nm 30 --
5
According to the bubble formation model of Moon and Koo [lo], as the sintered ingot is processed to wire, the residual pores undergo a change in shape
2
20.-
intragranularbubbles
10
30
50
70
90
110
130
150
170
190
rapid heating
BUBBLE DIAMETER [nm] average diameter d = 27.0 nm
T
intergranular bubbles slow heating average diameter d = 47.3
10
30
50
70
90 110
130
BUBBLE DIAMETER
(a)
150
170
190
[nm]
intergranular bubbles rapid heating average diameter dz42.6 nm
0
II
IC) (b)
3a
10
@I
50
70
90
110
130
BUBBLE DIAMETER
150
170
190
[nm]
Fig. 3. (a) Size distribution of the bubbles within the grains in 0.39 mm diameter wire after slow heating; (b) size distribution of the bubbles on the grain boundaries in 0.39 mm diameter wire after slow heating.
Table 1 Average diameters of intraslowly heated wires Annealing condition
BUBBLE DIAMETER [nm]
Fig. 2. (a) Size distribution of the bubbles within the grains in 0.39 mm diameter wire after rapid heating; (b) size distribution of the bubbles on the grain boundaries in 0.39 mm diameter wire after rapid heating.
3a I
Rapid Slow
and intergranular
Average -__
bubble
bubbles
diameter
in rapidly
and
(nm)
lntragranular bubbles
Intergranular bubbles
27.0 30.4
42.6 41.3
54
0. Horacsek et al.llnternational Journal of Hefractoly Metals di Hard Materials 16 (1998) 51-57
50 ,
2 Fig. 4. Thin-foil TEM micrograph 0.39 mm diameter wire.
showing
large
isolated
bubbles
in
If L/W exceeds a critical value of approx. 10 (more exactly: 8.89) the ellipsoid becomes unstable and at sufficiently high annealing temperature breaks up into a row of bubbles, while below this critical value the ellipsoid contracts and spheroidizes. For example, if the reduction in cross section is just the critical (76.7%) each pore in the deformed material is elongated to a L/W ratio of approx. 10, and during annealing each ellipsoid will break up into two bubbles. Since the sintered ingot has a wide variety of pore sizes, relation (1) predicts that at a given stage of processing, the length of the elliptically elongated pores in a deformed rod or wire might be very different, but LIW must be identical for all ellipsoids. This means that after breakup, the size of the bubbles and the length of the rows might be very different, but each row must contain bubbles in the same number To compare the theoretical predictions with the experimental facts, we examined the bubble structure of a 2 mm diameter wire which was processed from a heat-treated 5.85 mm diameter rod. As a result of the applied thermomechanical treatment, in this rod the deformed pores were only slightly elongated (L/W - 4) at the as-swaged condition, and therefore, during annealing, the ellipsoids went back to their original spherical shape (with the exception of a few large metallurgical voids, which could not perfectly spheroidize). Considering that the heat-treated Dh = 5.85 mm diameter rod was worked down to D, = 2 mm wire, one obtains from relation (1) for the L/W ratio of the potassium ellipsoids: L/W = 30, predicting that on annealing, each ellipsoid will break up into a row of bubbles and each row will contain bubbles in the same (six) number. The calculated results deviated from the experimental facts. In the heat-treated 2 mm diameter wire no ellipsoids were observed, indicating that each potassium ellipsoid had broken up into a row of bubbles or
3
4
5
6
7
8
9
10
NUMBER OF BUBBLES IN A ROW Fig. 5. Frequency of bubble rows consisting bubbles in annealed 2 mm diameter wire.
of different
number
of
had gone back to its spherical shape during annealing. Although the theory predicts that, at the given stage of deformation, all bubbles in the wire will arranged in rows, we found that most of the bubbles in the annealed wire were randomly distributed and only 35% of the bubbles were ordered into rows that contained bubbles in different numbers (from 2 to 9). The result of our measurements is given in Fig. 5 showing the frequency of the rows with different numbers of bubbles. Since this variety in the number must stem from the different L/W ratios of the ellipsoids, Fig. 5 shows that the potassium bubbles were not equally deformable at the working temperature. The fact that only 35% of the bubbles were ordered into rows and most of the rows contained less bubbles than the predicted number of six, indicates that most of the potassium ellipsoids experienced less deformation than the theoretical amount calculated from relation (1). (The detected few rows that contained more than six bubbles, can be derived from some large metallurgical voids whose L/W ratio was greater than 30 as a result of their cumulative elongation.) The discrepancy between theory and experimental facts can bc explained by the effect of two competing processes [ 1I, 121 that take place at the reIatively high working temperatures: the elongation of the bubble and simultaneous striving to restore its original spherical shape can result in a smaller L/W ratio of the ellipsoid than the calculated value from eqn (1). Since the time required to restore the original equilibrium shape of the deformed pore (bubble) is proportional to the fourth power of its diameter [13], it is to be expected, that the deformability of the potassium pores in doped tungsten decreases sharply with increasing diameter. As a result of this size-dependent deformability of the bubbles, bubble rows containing only a few bubbles will form from the smallest potassium
0. Horacsek et al.lIntemational Journal of Refractory Metals & Hard Materials 16 (1998) 51-57
pores of the sintered ingot or they will be introduced into the wire even without sufficient elongation for breakup. In this way more or less small potassium pores of the sintered ingot may appear in the recrystallized wire even in their original size as isolated bubbles. The introduction of the smallest, and hence, practically undeformable pores into the wire seems to be a possible mechanism which may account (partly) for the existence of the large bubbles that have similar diameters to the smallest pores of the sintered ingot (a potassium pore which is ‘small’ in the sintered ingot will be ‘large’ as a bubble in the wire). The size-dependent deformability of the potassium bubbles seems to be supported directly by the SEM micrograph in Fig. 6, showing that the small bubbles experienced less deformation than the larger ones. The micrograph, taken from the 2 mm diameter wire at the as-worked condition demonstrates, that besides some relatively long ellipsoids, a number of spherical or only slightly elongated small bubbles are also present in the deformed wire that are apparently unable to form bubble rows by breakup. 3.2. Formation of large bubbles during annealing of the wire
The difference in the size distributions of the bubbles between the rapidly and slowly heated 0.39 mm diameter wires (Fig. 2, Fig. 3, Table 1) indicates that, during annealing, bubble coarsening occurred in the wire, which manifested itself both within the grains [Fig. 3(a)] and on the grain boundaries [Fig. 3(b)]. Since potassium has no solubility in tungsten, and hence, bulk diffusion of potassium is highly improbable, the appearance of the large intragranular bubbles in the slowly heated wire can be
Fig. 6. SEM micrograph 2 mm diameter as-worked
showing wire.
differently
deformed
bubbles
in
55
explained only by a coalescence mechanism which may occur in two ways: (i) coalescence by migrating bubbles (when the bubbles are dragged with moving grain boundaries) and (ii) in situ coalescence (when the bubbles can not expand or change their shape in situ without touching of their neighbors). A recent study [14] has shown that coalescence by migrating bubbles is the most probable mechanism by which large bubbles could form in the wire during annealing. On rapid heating, the migration of the fiber boundaries is driven by a sudden release of a large stored energy and detachment may occur readily from the bubbles. Therefore, the initial bubble distribution in the wire is not considerably modified by boundary movement. On slow heating, however, the bubbles could remain attached to the boundaries of the widening fibers which could move slowly enough to drag along the bubbles. In this case the velocity of the boundary migration is controlled by the movement of the bubbles that are attached to the boundary [U-17]. As the fibers broaden, partial or even entire bubble rows could be dragged by the boundary segments moving in the transverse direction and, as a result of row-row collisions and bubble coalescence, large bubbles could form in the wire. As a result of the wide variety in the spatial distribution of the potassium pores in the sintered ingot, the distance by which the bubble rows are separated from each other in the wire may be extremely small. At such places collision and coalescence of bubbles of adjacent rows requires only a slight displacement of the laterally moving fiber boundaries by which the bubbles are dragged to the neighbouring bubble row. Although the result of row-row collision is, in fact, a new bubble row, after collision, the new row is less ‘regular’ than an intact original row for which the diameter of each bubble must be the same and the spacing between them must be identical [lo]. In contrast, owing to the coalesced bubbles which form at the contact part of the original rows, the bubble size and inter-bubble spacing in the new row will be different, as shown in Fig. 7. Repetition of row-row collision and further bubble coalescence during recrystallization can lead to the development of extraordinary ‘bubble rows’ consisting of a few large coalesced bubbles with variable sizes and spacings (Fig. 8). The potassium bubbles in the recrystallized wires exhibited different shapes depending on whether they were located within the grains or on the grain boundaries. While the intragranular bubbles had always an equiaxed (practically spherical) shape, the intergranular bubbles appeared like lenses (Fig. 9). This means, that when a potassium bubble gets attached to a grain boundary, its diameter in the boundary will be larger than its diameter was within the grain. Since the shape of a bubble in the grain boundary is determined
56
0. Horacsek et al./lnternutional Journal of R
Fig. 10. Equilibrium grain boundary. Fig. 7. Fracture replication TEM micrograph showing row with variable sizes and spacings (0.39 mm diameter
a long bubble wire).
Fig. 9. TEM micrograph showing spherical and lentiform bubbles in the grain boundary.
bubbles
within
the grain
at the junction
of a bubble
and a
by the vectorial balance between the grain boundary tension and bubble surface tension (Fig. lo), the equilibrium shape can be characterized by the enlarged diameter of the grain boundary bubble, D, and by the contact (dihedral) angle 0 formed at the intersection of the grain boundary and bubble: 2 cosoi2
Fig. 8. SEM micrograph showing irregular bubble rows in 0.39 mm diameter wire. (The bar on the micrograph represents 1 pm).
configuration
= &ll’h
(2)
where yh is the bubble surface energy and 1~~is the grain boundary energy. The diameter increase in the boundary will be significant and comparable with the original (spherical) bubble diameter if 0 = 120”. In this case the extension of the bubbles in the boundary can lead to in situ coalescence in a closely spaced bubble population. Since the shape change of the potassium bubbles may have a consequence on the evolution of the intergranular bubble population, we attempted to measure the dihedral angles at the junctions of the bubbles and grain boundaries. The measurements were made on enlarged TEM micrographs obtained from thin foils which contained grain boundaries to which bubbles were attached. Based on a number of measurements, we obtained for the dihedral angle approx. 125”. From relation (2) the measurement gives: ys/y,, = 0.92 indicating that the bubble surface (tungsten/potassium interface) energy practically equals the grain boundary energy. Considering the two generally accepted values for tungsten: 2.3 J/m* for the surface energy and 1.08 J/m* for the grain boundary energy, the ratio ys/yt, = 0.92 suggests that adsorbed impurities on the bubble surface (e.g. cosegregation of oxygen onto the W/K interface [18]) reduced the surface energy of clean tungsten from 2.3 to approx. 1.2 J/m*. The measured small dihedral angle suggests that the potassium bubbles suffer a fairly large extension when they get to the grain boundaries during recrystallization. While the spacing between the bubbles in a given row is, in principle, determined by the diameter of these bubbles [lo], there is no theoretical limitation for
0. Horacsek et al.lIntemational Journal of Refractory Metals & Hard Materials 16 (1998) 51-57
the closeness of the adjacent rows. Therefore, bubble coalescence between adjacent bubble rows may occur in situ even without any displacement of the bubbles when they become attached to the grain boundary (and the result is similar to Fig. 8). This in situ bubble coalescence could also contribute to the formation of the observed large (110-170 nm) bubbles on the grain boundaries, as shown in Fig. 2(b) and Fig. 3(b).
4. Conclusions Experimental results supported the view [ll, 121 that the deformability of the potassium pores of the sintered ingot depends on their size: at the working temperatures the small pores (bubbles) are less deformable than the larger ones. As a result of selective pore deformation, relatively large bubbles can form in the wire from the smallest potassium-pore population of the sintered ingot. The measured yg/yb= 0.92 ratio indicated that the tungsten/potassium interface had a relatively low surface energy due to adsorbed impurities which lowered the surface energy of clean tungsten from 2.3 to approx. 1.2 J/m2 on the bubble surface. The low bubble surface energy promotes in situ coalescence where the interbubble spacing is comparable with the diameter of the bubbles.
Acknowledgements
The authors would like to thank Drs C.L. Briant and B.P. Bewlay for their very helpful discussions on these topics. They would also like to thank Mrs Olga Geszti for performing the TEM. The authors are grateful to the Hungarian National Science Foundation (OTKA) project No T 016597 for supporting the present work.
References [l] Horacsek 0. High-temperature fracture of non-sag wires. In: Pink E, Bartha L, editors. The metallurgy of doped/non-sag tungsten, London: Elsevier, 1989: 251-65.
[21 Harris
57
JE, Tucker MO, Greenwood GW. The diffusional creep strain due to the growth of intergranular voids. Metal Science 1974;8:311-314. wire. Acta [31 Briant CL, Walter JL. Void growth in tungsten Metallica 1988;36:2503-2514. [41 Horacsek 0. Features and formation of bubbles. In: Pink E, Bartha L, editors. The metallurgy of doped/non-sag tungsten. London: Elsevier, 1989:175-88. [51 Tanoue K, Masaoka E, Matsuda H. Secondary recrystallization and high temperature strength in commercial tungsten wires. In Proceedings of the 1st International Conference on the Metallurgy and Materials Science of Tungsten, Titanium, Rare Earths and Antimony, vol. 2. 1988: 730-35. PI Briant CL, Horacsek 0, Horacsek K. The effect of wire history on the coarsened structure and secondary recrystallization of doped tungsten. Metallurgy Transactions A 1993;24:843-851. of [71 Walter JL, Lou KA, Vukcevich MR. On the measurement potassium and other impurities in voids in sintered ingots of doped tungsten. In: Bildstein H, Ortner RM, editors. Proceedings of the 12th Plansee Seminar, vol. 1. Reutte, 1989:493-512. PI Horacsek 0, Tbth Cs, GasI I, Horacsek K. Relation between pore size distribution in sintered tungsten ingots and the creep strength of the wires. In: Bildstein H, Ortner RM, editors. Proceedings of the 12th Plansee Seminar, vol. 1. Reutte, 1989: 513-21. on the forma191 Horacsek 0, Menyhard M, Lgbar J. Observations tion of bubble dispersion in doped tungsten. High Temperature Materials and Processes 1995;14:207-213. [lOI Moon DM, Koo RC. Mechanism and kinetics of bubble formaMetallurgy Transactions tion in doped tungsten. 1971;2:2115-2122. MR. Spheroidization of potassium bubbles in [ill Vukcevich tungsten. Proceedings of the 5th International Tungsten Symposium. Shrewsbury, UK: MPR Publishing, 1992: 157-166. WI Briant CL. Warm rolling and swaging of tungsten. Proceedings of the 5th International Tungsten Symposium. Shrewsbury, UK: MPR Publishing, 1992:169-182. and volumeu31 Nichols FA, Mullins WW. Surface (interface-) diffusion contributions to morphological changes driven by capillarity. Transactions of TMS-AIME 1965;233:1840-1888. [I41 Horacsek 0, Briant CL, Horacsek K. Effect of heating rate on the recrystallization behaviour of doped tungsten. High Temperature Materials and Processes 1997;16:15-27. u51 Speight MV, Greenwood GW. Grain boundary mobility and its effects in materials containing inert gases. Phil. Mag. 1964;9:683-689. 1161 Nichols FA. Further comments on the theory of grain growth in porous compacts. Journal of the American Ceramic Society 1968;51: 468-469. and grain growth. [I71 Brook RJ. Pore-grain boundary interactions Journal of the American Ceramic Society 1969;52:56-57. of bubbles. In: Pink E, Bartha L, WI Gail I. Thermochemistry editors. The metallurgy of doped/non-sag tungsten. London: Elsevier, 1989:141-74.
&HARDMATERIAls ELSEVIER
International Journal of Refractory Metals & Hard Materials 16 (1998) 51-57
Selective bubble deformation during thermomechanical in KSiAl-doped tungsten
processing
0. Horacseka*, Cs. L. Tbth”, A. Nagyb “Research Institute for Technical Physics of HAS, H-1325 PO. Box 76, Budapest, Hungary ‘GE-Tungsram Co. Ltd, H-1340 V&i tit 77, Budapest, Hungary
Received 31 October 1997; accepted 9 February 1998
Abstract The potassium-containing pores in KSiAl-doped sintered tungsten ingot are the precursors of the bubbles in the recrystallized wire. In this study the evolution of the inter- and intragranular bubble structures has been examined. Quantitative analysis of the bubble dispersion in the early stage of processing indicated that most of the potassium pores experienced less deformation than the predicted amount of the bubble formation theory. The discrepancy between the experimental facts and theory is explained by the size-dependent deformability of the potassium pores supporting the view that at the process temperatures the small pores (bubbles) are less deformable than the larger ones. The geometry of the lenticularly shaped intergranular bubbles was analyzed by measuring the contact (dihedral) angles at the intersections of the bubbles with the grain boundaries. The results indicated that the tungsten/potassium interface must have a relatively low interfacial tension suggesting that adsorbed impurities on the bubble surface lowered the surface energy of clean tungsten from about 2.3 to approx. 1.2 J/m*. The consequences of the sizedependent bubble deformation and low bubble surface energy in the evolution of the bubble structure and in the formation of extraordinary large bubbles are discussed. 0 1998 Elsevier Science Ltd. All rights reserved. Keywords:
Doped tungsten; Deformability of bubbles; Surface energy of K-W interface
1. Introduction
Tungsten, as a filament material in incandescent lamps, requires a high resistance to creep at the operating temperatures. To improve the mechanical properties of tungsten, a strengthening method is needed which suppresses power-law creep without permitting significant diffusion creep. The solution of this metallurgical problem is to impede the motion of dislocations and to increase the grain size in the recrystallized material. KSiAl-doping successfully satisfies these requirements: the fine potassium-filled bubbles in doped tungsten wires act as effective barriers against dislocation motion inhibiting not only the glide but also the non-conservative motion of the dislocations, while in the highly elongated large grains, strain by the Nabarro-Herring and/or Coble creep has little significance because for these processes the diffusion distance is of the order of the dimensions of the large grains. However, a more important component of the *Corresponding author.
creep strain may arise in doped tungsten from void growth [l] because in this case the diffusion distance is only half the void (growing bubble) spacing which is usually much shorter than the dimensions of the grains. This type of diffusion creep is a result of the removal of tungsten atoms from the surface of the growing voids and their subsequent deposition along those grain boundaries that are nearly perpendicular to the stress axis [2]. Consequently, void growth is always accompanied by creep because the progressive plating out of the removed tungsten atoms along the grain boundaries produces strain in the direction of the applied stress. Since the potassium bubbles on the grain boundaries above a critical size can act as void nuclei [3], the high temperature creep strength and life of the filament depend largely on the size distribution of the potassium bubbles in the wire. The present study is concerned with the evolution of the bubble dispersion in doped tungsten wires. The results suggested that at the process temperatures, the smaller pores (bubbles) are less deformable than the larger ones. The geometry of the lenticularly shaped
0263-4368/98/$ - see front matter 0 1998 Elsevier Science Ltd. All rights reserved. PII: SO263-4368(98)00010-9
52
0. Horucsek et al.llntemational Journal
qf Refractory Metals & Hard Materials 16 (1998) 51-57
grain boundary bubbles was analyzed by measuring the dihedral angles at the intersections of the bubbles with the grain boundaries. For the grain boundary energy/ bubble surface energy ratio, i.‘piyh= 0.92 was obtained indicating that surface-active impurities adsorbed on the bubble surface lowered the surface energy of clean tungsten near to the grain boundary energy (from 2.3 to approx.l.2 J/m’). To explain the appearance of very large bubbles in the recrystallized wire, some possible mechanisms are presented.
The geometry of the lenticularly shaped intergranular bubbles was quantitatively analyzed by measuring the contact (dihedral) angles at the intersections of the bubbles with the grain boundaries. Assuming that the equilibrium between bubble surface tension and grain boundary tension has been reached in the annealed samples and quenching from the high temperature had no effect on the equilibrium configuration at the junction of the bubble and grain boundary, the dihedral angles were measured on a number of enlarged TEM micrographs.
2. Experimental 3. Results and discussion The samples used in this study were prepared from a commercially doped sintered ingot which was processed by swaging and drawing to 0.39 mm wire. The residual pores of the ingot and their morphological changes during thermomechanical processing were examined by scanning electron microscopy (SEM). The bubble structures in swaged rods and in wires drawn to 2 mm diameter was analyzed on the longitudinally fractured surfaces. In these ‘thick’ samples that were prepared at the early stages of the thermomechanical processing, the potassium bubbles were sufficiently large to observe them on the SEM fractographs. Since in the case of 0.39 mm wire samples only the largest bubbles were detectable by SEM, to obtain more information about the entire bubble dispersion, transmission electron-microscopy (TEM) was used: thin foil technique for the examination of the intragranular bubble structure, and fracture replication TEM to observe the bubbles on the grain boundaries. The size distribution of the pores in the sintered ingot was characterized by a histogram that was obtained by measuring the diameters of the pores on the SEM fractographs and counting their number for the corresponding size ranges. Taking into consideration that the bubbles in the wire are formed from the potassium containing pores of the ingot having diameters of the order of 0.1 pm [4], the size distribution of the residual pores in the ingot was determined for the diameter range of O-1 pm. The size distributions of the inter- and intraganular bubbles in 0.39 mm diameter wire samples were determined from a number of TEM micrographs that were obtained by thin foil and fracture replication techniques. Since previous studies showed that the microstructural responses to different annealing conditions are different [5,6], two heating rates were applied: the heat up of the rapidly heated samples occurred within 10 s to 18Oo”C, while the temperature of the slowly heated ones were increased to the same temperature during 30 min and then the samples were annealed for 5 min in both cases.
According to previous studies [7,8], the SEM fractographs showed that the sintered ingot had a mixed residual porosity consisting of numerous potassiumcontaining tiny pores and fewer relatively large irregularly shaped metallurgical voids. The frequency distribution of the pore diameters for the size range of O-l pm is shown in Fig. 1. Although SEM did not allow accurate measurement of pore diameters below 0.1 pm, a more detailed examination of the smallest pore population by replication TEM technique revealed that the first column in Fig. 1 implies many pores with diameters less than 0.05 pm. In the case of 0.39 mm diameter wire samples, the size distribution of the bubbles within the grains and that on the grain boundaries for the two differently heated wires are given in Figs 2 and 3. Table 1 shows the average diameters for the examined bubble populations. The histograms in Fig. 2 and Fig. 3 show that the annealed (primary recrystallized) wires also contained relatively large bubbles with diameters larger than 0.05 Lirn. Comparing these histograms with the size
0,l
0,3
0,5
0,7
0,9
I,1
DIAMETER [pm] Fig. 1. Size distribution of the potassium pores in the sintered ingot. (The first column contains a lot of pores with diameters 0.05 ,um or less).
53
0. Horacsek et al.lIntemational Journal of Refractory Metals & Hard Materials I6 (1998) 51-57
distribution of the potassium pores of the sintered ingot (Fig. l), it can be seen that the largest bubbles in the wire had larger diameters than the smallest pores of the sintered ingot, i.e. the size distributions for the bubbles and pores were overlapping. It was often observed [9] that the largest bubbles in the annealed wires are present as isolated ones or they are arranged in short rows containing only a few bubbles as shown in Fig. 4. In order to analyze the problem of the existence of these large bubbles in the wire, we will consider the predictions of the theory and compare them with our experimental results.
identical to that of the bulk material. The Iengthto-width ratio of an elongated pore (frequently called potassium ellipsoid), L/W, can be related to the rod (or wire) diameter before, Db and after deformation, D,, by the expression L/W = 1.2(DJD,)3
50 intragranular bubbles slow heating 40 -T & ;-’ 0
3.1. Comparison of bubble formation theory with experimental results
(1)
average diameter d = 30,4 nm 30 --
5
According to the bubble formation model of Moon and Koo [lo], as the sintered ingot is processed to wire, the residual pores undergo a change in shape
2
20.-
intragranularbubbles
10
30
50
70
90
110
130
150
170
190
rapid heating
BUBBLE DIAMETER [nm] average diameter d = 27.0 nm
T
intergranular bubbles slow heating average diameter d = 47.3
10
30
50
70
90 110
130
BUBBLE DIAMETER
(a)
150
170
190
[nm]
intergranular bubbles rapid heating average diameter dz42.6 nm
0
II
IC) (b)
3a
10
@I
50
70
90
110
130
BUBBLE DIAMETER
150
170
190
[nm]
Fig. 3. (a) Size distribution of the bubbles within the grains in 0.39 mm diameter wire after slow heating; (b) size distribution of the bubbles on the grain boundaries in 0.39 mm diameter wire after slow heating.
Table 1 Average diameters of intraslowly heated wires Annealing condition
BUBBLE DIAMETER [nm]
Fig. 2. (a) Size distribution of the bubbles within the grains in 0.39 mm diameter wire after rapid heating; (b) size distribution of the bubbles on the grain boundaries in 0.39 mm diameter wire after rapid heating.
3a I
Rapid Slow
and intergranular
Average -__
bubble
bubbles
diameter
in rapidly
and
(nm)
lntragranular bubbles
Intergranular bubbles
27.0 30.4
42.6 41.3
54
0. Horacsek et al.llnternational Journal of Hefractoly Metals di Hard Materials 16 (1998) 51-57
50 ,
2 Fig. 4. Thin-foil TEM micrograph 0.39 mm diameter wire.
showing
large
isolated
bubbles
in
If L/W exceeds a critical value of approx. 10 (more exactly: 8.89) the ellipsoid becomes unstable and at sufficiently high annealing temperature breaks up into a row of bubbles, while below this critical value the ellipsoid contracts and spheroidizes. For example, if the reduction in cross section is just the critical (76.7%) each pore in the deformed material is elongated to a L/W ratio of approx. 10, and during annealing each ellipsoid will break up into two bubbles. Since the sintered ingot has a wide variety of pore sizes, relation (1) predicts that at a given stage of processing, the length of the elliptically elongated pores in a deformed rod or wire might be very different, but LIW must be identical for all ellipsoids. This means that after breakup, the size of the bubbles and the length of the rows might be very different, but each row must contain bubbles in the same number To compare the theoretical predictions with the experimental facts, we examined the bubble structure of a 2 mm diameter wire which was processed from a heat-treated 5.85 mm diameter rod. As a result of the applied thermomechanical treatment, in this rod the deformed pores were only slightly elongated (L/W - 4) at the as-swaged condition, and therefore, during annealing, the ellipsoids went back to their original spherical shape (with the exception of a few large metallurgical voids, which could not perfectly spheroidize). Considering that the heat-treated Dh = 5.85 mm diameter rod was worked down to D, = 2 mm wire, one obtains from relation (1) for the L/W ratio of the potassium ellipsoids: L/W = 30, predicting that on annealing, each ellipsoid will break up into a row of bubbles and each row will contain bubbles in the same (six) number. The calculated results deviated from the experimental facts. In the heat-treated 2 mm diameter wire no ellipsoids were observed, indicating that each potassium ellipsoid had broken up into a row of bubbles or
3
4
5
6
7
8
9
10
NUMBER OF BUBBLES IN A ROW Fig. 5. Frequency of bubble rows consisting bubbles in annealed 2 mm diameter wire.
of different
number
of
had gone back to its spherical shape during annealing. Although the theory predicts that, at the given stage of deformation, all bubbles in the wire will arranged in rows, we found that most of the bubbles in the annealed wire were randomly distributed and only 35% of the bubbles were ordered into rows that contained bubbles in different numbers (from 2 to 9). The result of our measurements is given in Fig. 5 showing the frequency of the rows with different numbers of bubbles. Since this variety in the number must stem from the different L/W ratios of the ellipsoids, Fig. 5 shows that the potassium bubbles were not equally deformable at the working temperature. The fact that only 35% of the bubbles were ordered into rows and most of the rows contained less bubbles than the predicted number of six, indicates that most of the potassium ellipsoids experienced less deformation than the theoretical amount calculated from relation (1). (The detected few rows that contained more than six bubbles, can be derived from some large metallurgical voids whose L/W ratio was greater than 30 as a result of their cumulative elongation.) The discrepancy between theory and experimental facts can bc explained by the effect of two competing processes [ 1I, 121 that take place at the reIatively high working temperatures: the elongation of the bubble and simultaneous striving to restore its original spherical shape can result in a smaller L/W ratio of the ellipsoid than the calculated value from eqn (1). Since the time required to restore the original equilibrium shape of the deformed pore (bubble) is proportional to the fourth power of its diameter [13], it is to be expected, that the deformability of the potassium pores in doped tungsten decreases sharply with increasing diameter. As a result of this size-dependent deformability of the bubbles, bubble rows containing only a few bubbles will form from the smallest potassium
0. Horacsek et al.lIntemational Journal of Refractory Metals & Hard Materials 16 (1998) 51-57
pores of the sintered ingot or they will be introduced into the wire even without sufficient elongation for breakup. In this way more or less small potassium pores of the sintered ingot may appear in the recrystallized wire even in their original size as isolated bubbles. The introduction of the smallest, and hence, practically undeformable pores into the wire seems to be a possible mechanism which may account (partly) for the existence of the large bubbles that have similar diameters to the smallest pores of the sintered ingot (a potassium pore which is ‘small’ in the sintered ingot will be ‘large’ as a bubble in the wire). The size-dependent deformability of the potassium bubbles seems to be supported directly by the SEM micrograph in Fig. 6, showing that the small bubbles experienced less deformation than the larger ones. The micrograph, taken from the 2 mm diameter wire at the as-worked condition demonstrates, that besides some relatively long ellipsoids, a number of spherical or only slightly elongated small bubbles are also present in the deformed wire that are apparently unable to form bubble rows by breakup. 3.2. Formation of large bubbles during annealing of the wire
The difference in the size distributions of the bubbles between the rapidly and slowly heated 0.39 mm diameter wires (Fig. 2, Fig. 3, Table 1) indicates that, during annealing, bubble coarsening occurred in the wire, which manifested itself both within the grains [Fig. 3(a)] and on the grain boundaries [Fig. 3(b)]. Since potassium has no solubility in tungsten, and hence, bulk diffusion of potassium is highly improbable, the appearance of the large intragranular bubbles in the slowly heated wire can be
Fig. 6. SEM micrograph 2 mm diameter as-worked
showing wire.
differently
deformed
bubbles
in
55
explained only by a coalescence mechanism which may occur in two ways: (i) coalescence by migrating bubbles (when the bubbles are dragged with moving grain boundaries) and (ii) in situ coalescence (when the bubbles can not expand or change their shape in situ without touching of their neighbors). A recent study [14] has shown that coalescence by migrating bubbles is the most probable mechanism by which large bubbles could form in the wire during annealing. On rapid heating, the migration of the fiber boundaries is driven by a sudden release of a large stored energy and detachment may occur readily from the bubbles. Therefore, the initial bubble distribution in the wire is not considerably modified by boundary movement. On slow heating, however, the bubbles could remain attached to the boundaries of the widening fibers which could move slowly enough to drag along the bubbles. In this case the velocity of the boundary migration is controlled by the movement of the bubbles that are attached to the boundary [U-17]. As the fibers broaden, partial or even entire bubble rows could be dragged by the boundary segments moving in the transverse direction and, as a result of row-row collisions and bubble coalescence, large bubbles could form in the wire. As a result of the wide variety in the spatial distribution of the potassium pores in the sintered ingot, the distance by which the bubble rows are separated from each other in the wire may be extremely small. At such places collision and coalescence of bubbles of adjacent rows requires only a slight displacement of the laterally moving fiber boundaries by which the bubbles are dragged to the neighbouring bubble row. Although the result of row-row collision is, in fact, a new bubble row, after collision, the new row is less ‘regular’ than an intact original row for which the diameter of each bubble must be the same and the spacing between them must be identical [lo]. In contrast, owing to the coalesced bubbles which form at the contact part of the original rows, the bubble size and inter-bubble spacing in the new row will be different, as shown in Fig. 7. Repetition of row-row collision and further bubble coalescence during recrystallization can lead to the development of extraordinary ‘bubble rows’ consisting of a few large coalesced bubbles with variable sizes and spacings (Fig. 8). The potassium bubbles in the recrystallized wires exhibited different shapes depending on whether they were located within the grains or on the grain boundaries. While the intragranular bubbles had always an equiaxed (practically spherical) shape, the intergranular bubbles appeared like lenses (Fig. 9). This means, that when a potassium bubble gets attached to a grain boundary, its diameter in the boundary will be larger than its diameter was within the grain. Since the shape of a bubble in the grain boundary is determined
56
0. Horacsek et al./lnternutional Journal of R
Fig. 10. Equilibrium grain boundary. Fig. 7. Fracture replication TEM micrograph showing row with variable sizes and spacings (0.39 mm diameter
a long bubble wire).
Fig. 9. TEM micrograph showing spherical and lentiform bubbles in the grain boundary.
bubbles
within
the grain
at the junction
of a bubble
and a
by the vectorial balance between the grain boundary tension and bubble surface tension (Fig. lo), the equilibrium shape can be characterized by the enlarged diameter of the grain boundary bubble, D, and by the contact (dihedral) angle 0 formed at the intersection of the grain boundary and bubble: 2 cosoi2
Fig. 8. SEM micrograph showing irregular bubble rows in 0.39 mm diameter wire. (The bar on the micrograph represents 1 pm).
configuration
= &ll’h
(2)
where yh is the bubble surface energy and 1~~is the grain boundary energy. The diameter increase in the boundary will be significant and comparable with the original (spherical) bubble diameter if 0 = 120”. In this case the extension of the bubbles in the boundary can lead to in situ coalescence in a closely spaced bubble population. Since the shape change of the potassium bubbles may have a consequence on the evolution of the intergranular bubble population, we attempted to measure the dihedral angles at the junctions of the bubbles and grain boundaries. The measurements were made on enlarged TEM micrographs obtained from thin foils which contained grain boundaries to which bubbles were attached. Based on a number of measurements, we obtained for the dihedral angle approx. 125”. From relation (2) the measurement gives: ys/y,, = 0.92 indicating that the bubble surface (tungsten/potassium interface) energy practically equals the grain boundary energy. Considering the two generally accepted values for tungsten: 2.3 J/m* for the surface energy and 1.08 J/m* for the grain boundary energy, the ratio ys/yt, = 0.92 suggests that adsorbed impurities on the bubble surface (e.g. cosegregation of oxygen onto the W/K interface [18]) reduced the surface energy of clean tungsten from 2.3 to approx. 1.2 J/m*. The measured small dihedral angle suggests that the potassium bubbles suffer a fairly large extension when they get to the grain boundaries during recrystallization. While the spacing between the bubbles in a given row is, in principle, determined by the diameter of these bubbles [lo], there is no theoretical limitation for
0. Horacsek et al.lIntemational Journal of Refractory Metals & Hard Materials 16 (1998) 51-57
the closeness of the adjacent rows. Therefore, bubble coalescence between adjacent bubble rows may occur in situ even without any displacement of the bubbles when they become attached to the grain boundary (and the result is similar to Fig. 8). This in situ bubble coalescence could also contribute to the formation of the observed large (110-170 nm) bubbles on the grain boundaries, as shown in Fig. 2(b) and Fig. 3(b).
4. Conclusions Experimental results supported the view [ll, 121 that the deformability of the potassium pores of the sintered ingot depends on their size: at the working temperatures the small pores (bubbles) are less deformable than the larger ones. As a result of selective pore deformation, relatively large bubbles can form in the wire from the smallest potassium-pore population of the sintered ingot. The measured yg/yb= 0.92 ratio indicated that the tungsten/potassium interface had a relatively low surface energy due to adsorbed impurities which lowered the surface energy of clean tungsten from 2.3 to approx. 1.2 J/m2 on the bubble surface. The low bubble surface energy promotes in situ coalescence where the interbubble spacing is comparable with the diameter of the bubbles.
Acknowledgements
The authors would like to thank Drs C.L. Briant and B.P. Bewlay for their very helpful discussions on these topics. They would also like to thank Mrs Olga Geszti for performing the TEM. The authors are grateful to the Hungarian National Science Foundation (OTKA) project No T 016597 for supporting the present work.
References [l] Horacsek 0. High-temperature fracture of non-sag wires. In: Pink E, Bartha L, editors. The metallurgy of doped/non-sag tungsten, London: Elsevier, 1989: 251-65.
[21 Harris
57
JE, Tucker MO, Greenwood GW. The diffusional creep strain due to the growth of intergranular voids. Metal Science 1974;8:311-314. wire. Acta [31 Briant CL, Walter JL. Void growth in tungsten Metallica 1988;36:2503-2514. [41 Horacsek 0. Features and formation of bubbles. In: Pink E, Bartha L, editors. The metallurgy of doped/non-sag tungsten. London: Elsevier, 1989:175-88. [51 Tanoue K, Masaoka E, Matsuda H. Secondary recrystallization and high temperature strength in commercial tungsten wires. In Proceedings of the 1st International Conference on the Metallurgy and Materials Science of Tungsten, Titanium, Rare Earths and Antimony, vol. 2. 1988: 730-35. PI Briant CL, Horacsek 0, Horacsek K. The effect of wire history on the coarsened structure and secondary recrystallization of doped tungsten. Metallurgy Transactions A 1993;24:843-851. of [71 Walter JL, Lou KA, Vukcevich MR. On the measurement potassium and other impurities in voids in sintered ingots of doped tungsten. In: Bildstein H, Ortner RM, editors. Proceedings of the 12th Plansee Seminar, vol. 1. Reutte, 1989:493-512. PI Horacsek 0, Tbth Cs, GasI I, Horacsek K. Relation between pore size distribution in sintered tungsten ingots and the creep strength of the wires. In: Bildstein H, Ortner RM, editors. Proceedings of the 12th Plansee Seminar, vol. 1. Reutte, 1989: 513-21. on the forma191 Horacsek 0, Menyhard M, Lgbar J. Observations tion of bubble dispersion in doped tungsten. High Temperature Materials and Processes 1995;14:207-213. [lOI Moon DM, Koo RC. Mechanism and kinetics of bubble formaMetallurgy Transactions tion in doped tungsten. 1971;2:2115-2122. MR. Spheroidization of potassium bubbles in [ill Vukcevich tungsten. Proceedings of the 5th International Tungsten Symposium. Shrewsbury, UK: MPR Publishing, 1992: 157-166. WI Briant CL. Warm rolling and swaging of tungsten. Proceedings of the 5th International Tungsten Symposium. Shrewsbury, UK: MPR Publishing, 1992:169-182. and volumeu31 Nichols FA, Mullins WW. Surface (interface-) diffusion contributions to morphological changes driven by capillarity. Transactions of TMS-AIME 1965;233:1840-1888. [I41 Horacsek 0, Briant CL, Horacsek K. Effect of heating rate on the recrystallization behaviour of doped tungsten. High Temperature Materials and Processes 1997;16:15-27. u51 Speight MV, Greenwood GW. Grain boundary mobility and its effects in materials containing inert gases. Phil. Mag. 1964;9:683-689. 1161 Nichols FA. Further comments on the theory of grain growth in porous compacts. Journal of the American Ceramic Society 1968;51: 468-469. and grain growth. [I71 Brook RJ. Pore-grain boundary interactions Journal of the American Ceramic Society 1969;52:56-57. of bubbles. In: Pink E, Bartha L, WI Gail I. Thermochemistry editors. The metallurgy of doped/non-sag tungsten. London: Elsevier, 1989:141-74.