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Void growth in tungsten wire
0001-6160~88 $3.00 + 0.00 Copyright @ 1988 Pergamon Press plc
Acta merall. Voi. 36, No. 9. pp. 2503-2514, 1988 Printed in Great Britain. All rights reserved
VOID GROWTH C L. RRIANT
IN TUNGSTEN
WIRE
and J. L. WALTER
General Electric Research and Development Center, P.O. Box 8, Schenectady, NY 12301, U.S.A. (Received 20 June 1987; in revised form 18 December
1987)
Abstract-This paper presents a study of void growth in tungsten wire. ‘&is wire contains potassium bubbles which are used to control grain growth and morpholop;y at elevated temperatures. Voids grow on the grain boundaries of this wire when it is operated in a lamp or heated in an uncoiied configuration. It was found that void growth was accelerated as the temperature of the wire, the stress on the wire, and the amount of oxygen in the environment were increased. It was also found that in the lamps operated without an externally applied stress (i.e. other than gravitational forces), voids did not grow unless there was a population of potassium bubbles with diameters greater than 0.08 pm. Based on these observations, it is proposed that the voids grow by creep. The larger pre-existing potassium bubbles serve as nuclei for these creep voids, since the low stresses and high temperatures preclude mechanically assisted nucleation at the high temperatures of lamp operation. It is also proposed that grain boundary sliding is an important source of stress in causing the voids to grow. Model calcuiations are used to support these conclusions. R&um&-Cet article ptisente une &ude de la croissance des caviti% dans un fif de tungstine. Ce fil contient des bulles de potassium que l’on utilise pour contrbler la croissance et la morphologie du grain aux temp6ratures Clevkes. Les cavit6s croissent sur les joints de grains de ce fil quand il fonctionne dans une lampe ou quand il est chaufl% dans une configuration d&o&e. La croissance des cavitts est a&l&e quand on augmente la temgrature du til, la contrainte sur Ie 6l et la quantiti d’oxy&ne dans l’environnement. On trouve en outre que dans les lampes qui fonctionnent sans l’application d’une contrainte exterieure (c’est-&dire), une contrainte autre que les forces de gravitation), les cavitis ne croissent pas $ qu’il n’y ait une population de bulles de potassium de diam&re sup6rieur li 0,08 pm. En nous basant sur ces observations, nous suggkrons que les cavids croissent par fluage. Les plus grosses bulles prt5existantes de potassium servent de germes pour ces cavit6s de &age, car Ies faibles contraines et les temp&atures &&es excluent une germination mhniquement assist&e pour les hautes temp6rature de fonctionnement de la lampe. Nous proposons&alement que le glissement intergranulaire est une source importante de contrainte qui peut faire croitre les cavitts. Nous utilisons des calculs de modblisation pour confirmer ces conclusions. Zusammenfassung-In dieser Arbeit wird das Ho~raumwa~hstum in wolframdraht untersucht. Dieser Draht enthalt Kaliumblasen, mit denen Komwachstum und -form bei erhijhten Temperaturen kontrolliert werden. Hohlrgume wachsen an den Komgrenzen dieses Drahtes, wenn er in einer Gliihlampc betrieben oder ungewendelt erhitzt wird. Das Hohlraumwachstum wurde stsrker, wenn die Drahttempertur, die elastische Spannung auf den Draht oder der Saurstoffant~l in der Umgebung erhhht wurden. AuBerdem ergab sich, daB in den Gliihlampen ohne eine lu&zre Spannung auf den Draht (d.h. keiner anderen als der Erdanziehung) die Hohlrlume nicht wuchsen, aul3er es lag eine Ansammlung von Kaliumblasen mit Durchmesser grader also 0.08pm vor. Ausgehend von diesen Beobachtungen wird dargelegt, daO die Hohllume durch Kriechen wachsen. Die gr65eren, vorher vorhandenen Kaliumblasen dienen also Orte, an denen die Hohlr%ume nukleieren; die niedrigen Spann~gen und die hohen Temperaturen schlieDen einen mechanisch unterstittzte Nukleation bei den hohen ~t~ebstem~raturen der Gliihlampe aus. AuBerdem wird gefolgert, dal3 die Korngrenzgleitung eine wichtige Spannungsquelle darstellt und so das Wachstum der Hohlriiume verursacht. Diese Folgerungen werden mit Modellrechnungen unterstiitzt.
1. INTRODUCTION Tungsten wire that is used for filaments in incandescent lamps has a unique microstructure. The wire is produced from a sintered powder-metallurgy ingot that is doped with potassium, aluminum, and silicon. During the sintering operation aluminum and silicon are almost completely evaporated from the ingot leaving less than about 100 wppm of potassium which is insoluble in tungsten and remains trapped in pores of the sintered ingot. As the ingot is unidirectionally deformed and lengthened through successive stages of rolling, swaging and w-ire drawing, these potassium-containing pores develop into ellip
soids that are less than 0.1 pm in width but greater than 1 pm in length [l]. When a filament made from this wire is heated in a lamp, the e&p&is break up into strings of bubbles, as shown in Fig. l(a). At the same time that bubble formation occurs, the wire is recrystallized to the point that a single grain may occupy the cross section of the wire. The boundaries between these grains have an undulating structure that is comprised of segments parallel to and perpendicular to the wire axis, as shown in Fig. l(b). Were it not for the presence of the potassium bubbles, the boundaries would be flat and the wire would have the bamboo structure of recrystallized pure tungsten wire. The flat boundaries could easily slide apart at
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BRIANT and WALTER:
VOID GROWTH
IN TUNGSTEN
WIRE
Fig. 2. Scanning electron micrograph of grain boundary fracture of wire A operated for 200 h and showing the large voids formed on the boundary.
operation [3,4]. These voids can greatly limit the life of the filament. To date, there have been only a few studies [3-6] that have characterized the growth of these voids and attempted to determine their origin. We present a further consideration of the formation and growth of voids in tungsten wire in this study. It will be shown that the growth of the voids is greatly increased by increasing the operating temperature, the stress on the filament, and the oxygen in the lamp environment. It is proposed that the voids grow by creep from the largest potassium bubbles already present on the grain boundaries, and that the stresses required for void growth are provided by grain boundary sliding. 2. EXPERIMENTAL
Fig. 1. (a) Transmission electron micrograph of doped tungsten wire heated 4 min at 2313 K. (b) Scanning electron micrograph of filament of wire B burned for 4.33 h. One can observe in this micrograph that the filament has a primary coil. It can be seen that this primary coil is also coiled; note
the out-of-focus turn underneath the focused one. Consequently, these are called “coiled-coil” filaments. the operating temperature, which, depending on the lamp design, ranges between 2700 and 3000 K, and cause failure of the filament. Because the bubbles are present, the boundaries are pinned at certain points as they migrate along the length of the wire. Consequently, the grains develop this interlocking morphology. This morphology is more resistant to grain boundary sliding and leads to longer filament life [l, 21. It has been demonstrated in the past that, under certain conditions, large voids as shown in Fig. 2 form on the grain boundaries of the wire during
PROCEDURE
The lamps used in this study were prepared by the General Electric Company Lighting Business Group. The lamp consists of a coiled-coil filament (see Fig. l(b) and its caption) encased in a small glass envelope containing a high pressure gas mixture of proprietary composition. During the course of this work we also used lamps that contained gas mixtures doped with specific amounts of oxygen. Most of the work reported in this study will compare filaments made from two wires, designated as A and B, manufactured by different processes. Filaments of wire A readily formed voids when operated at 2973 K, the standard temperature of these lamps, while filaments of wire B did not. Both wires were 63.5 pm in diameter. Three experimental techniques for growing voids were used in this study, two of which employed lamps with filaments made from wires A and B. In the first case, lamps were operated at 120 V, 60 Hz for specific periods of time. The second technique subjected the lamps to a stress by placing them on a spinning table, as shown in Fig. 3, while they were operated at the
BRIANT and WALTER:
2x)5
VOID GROWTH IN TUNGSTEN WIRE
Fig. 3. Spinning lamp tester used to apply stress to burning lamps.
same conditions stated above. Different stresses were applied to the filament encased in the envelope containing a standard lamp atmosphere by varying the rotational speed of the turntable. The stresses are reported as multiples of the gravitational stress, g. It should he noted that all of the lamps used in these first two types of tests had been subjected to a series of pulses of increasing voltage, referred to as flashing of the filament, to cause initial recrystallization of the filaments prior to the experiments described herein. Thus, all of the experiments with lamps employed fully recrystallized microstructures such as that shown in Fig. l(b). In addition to the study of void growth in filaments, we also examined void growth in uncoiled wire that was 178 pm in diameter. These experiments were performed with the sample in a stainless steel chamber that could be evacuated to 1 x lo-* torr. The samples were first recrystallized in vacuum by resistively heating them at 2673 K for 1 h. They were then cooled, a stress was applied, and oxygen was admitted into the system to the desired pressure. They were then resistively heated again to allow creep to occur. The results of all of the experiments were evaluated by high-resolution scanning electron microscopy. 3. EXPERIMENTAL 1. Lamp
? 5
80 -
3
60-
Wire A unburned
% tz ;
40-
1
7 20-
60
c
Win
A burned 1 h
.hl,_,,_,* 0.10
0.30
RIBULTS
0.50
Wlrr
0.90
0.70
A burned
tOOh
tests
We will first consider the results obtained on lamp filaments. We will be concerned with the effects of initial bubble size distribution, oxygen pressure in the till gas, stress and temperature on void growth. Figures 4 and 5 show histograms of the void diameters measured from scanning electron micrographs for filaments made from wires A and B, respectively. These histograms represent the void distribution on the grain boundaries in the initial&U.3bieH
100 - _
0
0.1
0.2
0.3
0.4
Bubble
0.S
0.6
diometcr
O.?
(pm
0.6
0.9
f
Fig. 4. Histograms of bubbk sires for wire A in the initial condition and after operating for 1 and 100h. Note that a different scale for bubbk diameter is used for the data taken from samples burned 1 and 100h.
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BRIANT and WALTER:
100
VOID GROWTH
IN TUNGSTEN
Table 1. Number of voids per unit area (PI,) in grain boundaries for different burning times
I--
Wire Wire
6 unburned
Wire
B burned
1h
60
f---I
0.04
I
0.08
t
0.12
id
0.16
2 60
3 * b
60
f;
40
s
20
0.00
Wlrr
0.04
Bubble
6 burnrd
100
0.06
diameter
h
0.16
0.12
(pm
f
Fig. 5. Histograms of bubble sizes for wire B in the initial condition and after operating for 1 and 100 h. Note that, unlike the results in Fig. 4, the same scale is used for all figures.
recrystallized condition and after operating the lamps for 1 and 100 h. In the initial condition, the grain boundaries contain many very small pota~ium-filied bubbles as a result of processing. After operating for a given time, the bubbles have grown and become faceted voids as is shown in Fig. 2. There are several points that should be noted about the results shown in Figs 4 and 5. The first is that void growth occurs much more extensively in wire A than in wire B. (Note the different scales in the figures.) The second point is that in the initial condition, grain boundaries in wire A contain a finite number of voids (or bubbles) with diameters greater than 0.08 Frn. None of the bubbles on the grain boundaries in wire B have diameters greater than 0.08 ,um. The third point to note is that although some bubbles grow in wire A to
Burning time (h)
A A A A
2s 100
B
0
B” B
$
WIRE
0
I
2: 100
(cl%) 3.47 x 0.52 x 0.35 x 0.32 x
lop 109 109 IO9
4.1 2.6 3.6 2.2
109 lo9 IO9 109
x x x x
quite large sixes, there is always a population of voids below 0.08nm in diameter as well. Table 1 gives the average number of bubbles per unit area, N,, for each wire after different times of ovation. These numbers were based on measurements taken from six to ten boundaries and the variability among boundaries is approximately f 10%. The counts indicate a reduction in N. with increasing time for wire A, suggesting that some bubble or void growth is occurring and absorbing some of the small bubbles. There is no statistically meaningful change in N, for wire B. The fact that there is a reduction in N. for wire A and no change in N, for wire B suggests that no new bubbles are being formed on the grain boundaries during operation. The large voids in wire A had the faceted appearance shown in Fig. 2. There was also some localization of the voids on various parts of the fracture surface. This localization could be seen most clearly on boundary areas with large undulations. For example, in Fig. 6 it is seen that the larger voids have formed at the peaks and valleys of the undulations and that there has been little void formation on the sides. Also, sub-boundary often served as preferred sites of void growth as shown in Fig. 6(C). Voids grew less in wire B, but the growth that was observed was always associated with asperities on the grain boundary surface such as low angle boundaries, sharp undulations, or small ridges that apparently occurred on the boundary as a result of grain boundary migration [7]. An example is shown in Fig. 7. Table 2 shows the effect of operating temperature on void formation for samples made from wire A that were operated for 24 h. (Note that the last two columns in the table present calculated values that will be considered in the Discussion.) No voids were observed on the grain boundaries of samples operated at 2023 and 2123 IL After operating at 2473 K, one void with a diameter greater than 0.2pm was found after examining eight grain boundaries. Grain boundaries of filaments operated at 2623 and 2973 K had many large voids on each grain boundary. Clearly, increasing the temperature increases the rate of void growth. Figure 8 shows the effect of increasing the applied stress on the filaments in lamps made from wires A
BRIANT and WALTER:
VOID GROWTH
IN TUNGSTEN
WIRE
Fig. 6. (A) SEM of the grain boundary fracture surface of wire A operated for 20.5 h. (B) SEM of the grain boundary fracture surface of wire A operated for 20.5 h. (C) SEM of the grain boundary fracture surface of wire A operated for 144 h showing the formation of voids along the low-angle grain boundary.
and B. Increasing
the stress causes an increase in the average size of the voids formed in both wires during the 5-h period of the test. However, the effect of stress on void growth is much greater for wire A than for wire B. In these stressed lamps, there was clear evidence of grain boundary sliding as shown in Fig. 9. This figure shows a grain boundary in wire A that is nearly perpendicular to the wire axis and which underwent sliding during a test at 6g for 5 h. Figure 10 shows the effect of oxygen on void growth. Here, the average diameter of the ten largest voids found from examining six to ten grain boundaries is plotted as a function of the initial oxygen content in lamps made from wire A and operated for 24 h. In these lamps, the fill gas was krypton plus oxygen. It is clear that an increase in the initial oxygen content caused an increase in the void size for a fixed operating time. Figure 11 shows a similar plot
for data obtained on filaments made from both wires A and B in which oxygen was added to the standard fill gas. It is to be noted that the effect of oxygen is not as great when lamps contain the proprietary fill gas since the more complex chemistry of this gas mixture lowers the activity of oxygen on the wire surface. It is also noted that the filaments of wire B are much less susceptible to void formation in an oxygen-containing environment. In lamps made from either wire, the combination of increased oxygen and stress causes a significant increase in void growth as is seen in Fig. 12. It should be noted that when both the oxygen content and stress are increased, large voids also form in filaments made from wire B. A scanning electron micrograph of these voids is shown in Fig. 13. In addition to lamps made from wires A and B, lamps were also made from wires produced by
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BRIANT and WALTER:
VOID GROWTH IN TUNGSTEN WIRE
Fig. 9. SEM of a filament of wire A operated for 5 h at 6g showing offset grain houndary.
Fig. 7. SEM of the grain boundary fracture surface of wire B with 0.1% oxygen operated for 5 h at 56. This micrograph shows that voids have preferentially formed at the peaks and valleys of large undulations.
3.0 r
2.5 -
different thermomechanical schedules. Table 3 lists the average diameter of the ten largest bubbles in filaments in the initial recrystallized condition and
whether or not voids formed after operating the lamp for 24 h. Again, it is clear that if the initial bubble diameter never exceeds aproximately 0.08 ,um, littie void growth occurs.
z a
2.0
.
h
1.5 -
z v
t.o-
0.5
-
Table 2. Operating temperature (K)
Voids
Caktiated time to form voids
Normalized time to form
DreScnt
all
VOids
2023 2123 2413 2623 2973
No No 1 void Y&S Yes
30 11 0.51 0.2 0.03
1000 366 19 6.6
t
O.oOOl
CWOl
02
0.01
0.1
in kr (%.I
Fig. 10.Plot of the average value of the diameters of the ten largest voids in wire A as a function of oxygen content in lamps filled with krypton. Error bars represent one standard deviation.
Fig. 8. Plot of the maximum void diameter as a function of stress in lamps operated for 5 h. Error bar represents the standard deviation of values from different filaments.
BRIAN?- and WALTER:
o1
0.001
2509
VOID (SROWTH IN TUNGSTEN WIRE
0.1
0.01
Oxygenf%) Fig. 11.Plot of the average value of the diameters of the ten largest voids in wires A and B as a function of oxygen
content in lamps with standard atmospheres. Error bars represent one standard deviation. Fig. 13. SEM of a grain boundary fracture of wire B b&n 5 h at 5g showing large voids.
2. Straight wire tests This section considers the results of tests run on 178pm straight wire under conditions of varying stress and oxygen contents. Figure 14 shows the average diameter of the ten largest voids on grain boundaries plotted as a function of test time for straight wires dead-loaded to three levels of tensile stress. The plot shows that the voids grow more rapidly with increasing applied stress. There is also an initial period of rapid growth which is followed by much slower growth. The final data point on each curve represents a sample that failed during the test. The dashed lines drawn on the figure are calculated from a model and will be considered in the Discussion.
10 t
l
Wire A
o
WweAtO.l%02
0
8 ‘;
0
t
i
Tabk 3. Void charac&stic~for iampsmade from wins prosxad diffcrentlY
Ave. diemeter
of ten largestbubbks in
unburned filament (pm)
burning 24 h
A B
0.1 0.055 0.206 0.087
YeS No YCS No
The effect of oxygen was investigated in two ways. In the first set of experiments, wires were tested for 4.5 h at 2673 K under different applied loads and at different pressures of oxygen as measured at room temperature. The results in Table 4 show the minimum oxygen concentration required to obtain void growth at each stress. It is clear that as the stress
l.a
I-
.
21.7 MN/m2
l
q4.4 MN/m2
0.S 1 - .m
E
e
I-
O Wire
Voids formed on
Wire
.
7.1 MN/m2
B + 0.1 *da02 0
8
0 0
0.6
r3 a8
0.4
2 0.2 0
12345678
stress (g’* 1 Plot of the maximum void diameter as a
Fig. 12. (a) function of stress for Iamps of wire A with and without
oxygen in the lamp atmosphere. Lamps were operated for 5 h. (h) Plot of the maximum void diameter as a function of stress for lamps of wire B with and without oxygen in the lamp atmosphere. Lamps were operated for 5 h.
1 I
0
10
I 20
Time(h)
Fig. 14. Plot of the average value of the diameters of the ten largest voids as a function of time for the 178pm win dead-loaded to three levels of tensile stress.
2510
BRIANT and WALTER:
VOID GROWTH
Table 4. Minimum oxygen pressure required to cause void growth in straight wires tested at 2473K for 4.5 h Applied stnSs (MN/m’)
PO2 5 x lo-’
21.7 14.4 7.1 1.6
1x 10-J 1x IO--’ 2 x 10-z
decreases a higher partial pressure of oxygen is required to cause void growth. In the other set of experiments, a constant load of 14.4 MN/m* and a temperature of 2673 K were used and the times to failure and void diameters were measured. Figure 15 shows the times to failure plotted as a function of oxygen partial pressure; as the oxygen content increases, the life becomes shorter. In each wire the void diameters at failure were similar and the largest ones were of the order of 1.Opm. Scanning electron microscopy showed that the voids formed in the straight wires were similar to those found in the filaments from the lamps. The voids were faceted and the largest ones were associated with asperities on the grain boundaries. Scanning electron microscopy also provided very clear evidence for grain boundary sliding in the straight wire samples. Figure 16(A) shows the surface of one of the straight wire samples where it appears that the grams are sliding apart. Figure 16(B) shows a void on an internal boundary. The offset in the void across the boundary is typical of voids formed in sliding boundaries [8,9]. This sliding is similar to that shown in Fig. 9 for the filament. 4. DISCUSSION
The discussion begins by stating an interpretation of the results. The experimental results presented
30 r x 25 c ?! 3 5
x
above are then combined with calculations based on a simple model to substantiate the interpretation. It is proposed that the voids on the grain boundaries grow from the potassium bubbles initially present there. Growth of the voids occurs as a result of creep. Of the many bubbles present on the boundary after the initial recrystallization, only those with diameters greater than about 0.08 pm will grow; all others are below the critical diameter for growth. The gravitational stress on the filament is too low to cause growth of bubbles of any size on the grain boundary even if oxygen is present to lower the void surface tension. However, grain boundary sliding can occur at these temperatures and can intensify the stress at asperities on the grain boundaries. At these points, stress intensification can cause growth of the larger bubbles found in wire A. This effect will also give rise to the spatial distribution of the voids observed on the grain boundaries. If an additional stress is applied to the filament, voids will grow in filaments of wire B because the critical radius for growth is lowered. These points are considered in greater detail below. I. Void nucleation There are two stages to the formation of voidsnucleation and growth. In the as-drawn doped tungsten wire, potassium is present as very thin ellipsoids. When the wire is recrystallized during the flashing sequence, these ellipsoids break up into rows of small bubbles, and the internal pressure produced
x
15-
lo-
F S-
0'
WIRE
p ccexp[ -%I
20-
2 g
IN TUNGSTEN
x I 10-3
10-z
I 10-l
PO2 (torr) Fig. IS. Plot of the time to failure of 178 pm wire loaded to 14.4 MN/m* as a function of oxygen partial pressure. The test temperature was 2673 K.
where y is the void surface energy, a is the angle between the grain boundary and the void surface, and a, is the normal stress on the boundary. If one uses parameters appropriate to those for an operating filament (see Table S), the exponential is essentially zero and nucleation should not occur. The value of the exponential is also very near zero for all stresses used in the straight wire tests and in the spinning lamp tests. Therefore, it is assumed that nucleation does not occur at the temperatures used in this study, a result that is consistent with the data in Table 1. Also, at these high temperatures, nucleation rarely occurs
BRIANT and WALTER:
VOID GROWTH IN TUNGSTEN WIRE
2511
Table 5. Values used for calculations Parameter
Value
Comment or Ref.
1000 erg/cm’( I J/m’) 4.18 3.14 2.24 x lb N/m’ 2913 1 x 10-flm’ Wpm 3.3 x lo-‘ exp( -92,OOO/RT) m’/s
Y
F,(a) F,(a) =. T n 6 4
because entropy will constantly break up a subcritical nucleus as it forms. 2. Void growth The size of the critical radius for void growth, r,, is given by the expression [IO]
2Y
rc=6
where y is the surface energy of the void and u is the applied stress. The values for r, calculated using the parameters given in Table 5 for the fiIament and the straight wires are listed in Table 6. These values of r, for the filaments are clearly much larger than the radii of any of the bubbles measured in the filaments in their initial recrystallized condition. This calculation would suggest that void growth should never occur in the filaments. Since the voids do grow, they must do so as a result of increased stress or lowered void surface tension, both of which could lower the value of r,. The value chosen for y, l~erg~~2, is typical of grain boundary energies f 1I]. The effect of oxygen on void growth may result from a lowering of y. However, even if we reduce y to 500 erg/cm2, a value typical of an impurity covered grain boundary ill], we still obtain values of r, that are too large. Therefore, we must consider whether the stresses could be increased to lower r,. Significant increases in stress could be caused by grain boundary sliding. In Figs 9 and 16 there was evidence that in both the filaments and in the straight wire tests grain boundary sliding did occur. Sliding causes stresses to be built up at the asperities along the grain boundaries. The question is whether these stresses are sufficiently high to cause a decrease in r,. In order to estimate the value of such a stress increase, we assume that the undulations of the grain boundary can be expressed as a sine wave. Then, the stress build-up on the grain boundary is given by [ 141
Ref. [I I] Value for Value for Value for Value for
spherical void spherical void filament-Ref. [12) operating dlament
Ref. 113)
where 7a is the applied shear stress. i. is the wave length of the undulations, and h is the amplitude of the undulations. Therefore, as A increases and h decreases, we would expect more sliding to occur and, consequently, the stress resulting from sliding to increase. Examination of the filaments showed that there were a variety of undulations on the grain boundaries. First, there were the larger variations such as those shown in Figs l(b) and 6(A). Measurements of these gave an average value for i/h of 13 and a maximum value of 30. There were also fine undulations such as those shown in Figs 6(B) and (C) for which reliable measurements of A/h could not be obtained. Using the values of I/h obtained for the
(3)
Tabk 6. Values of the critical radius bm) Condition
Lamp filament Straight wire (o 21.7) Strainht wire (a 7.1)
~=lOOO 0.8928 0.0920 0.2816
7=sooc@m~
Fig. 16. (A) SEM of a surface of 178 pm wire showing grain boundaries sliding. (B) SEM of a grain boundary of 178 brn wire showing offsets on the boundary.
BRIANT and WALTER:
2512
VOID GROWTH IN TUNGSTEN WIRE
larger undulations, we calculate, for the average value of I/h, r, of 0.0546 pm and, for the maximum value, r, of 0.0235 pm (critical diameters of 0.109 and 0.047 pm, respectively). The average value is clearly near the upper range of bubble diameters in filaments of wire A in the initial condition (see Fig. 4). However, the calculated average value is still well outside that measured for wire B (see Fig. 5). If the applied stress on the filament is increased to, say, six times the gravitational stress through the spinning test, r, drops to 0.0108 pm, a bubble size found in both wires. Because of the difficulty in obtaining accurate values of y, I/h, and, in the case of the filament, a, our calculated values for r, primarily demonstrate a qualitative consistency between the model and the measured values of the bubble diameter. A stronger test of the model is based on the measurement of void sizes. It was proposed that voids do not form in wire B because there are no bubbles initially present with diameters greater than the critical value, although voids with diameters larger than this value are present in wire A. Examination of the histograms shown in Figs 4 and 5 shows that the critical diameter must be about 0.08 pm. Therefore, if this hypothesis is correct, the population of large voids that form on the grain boundaries during burning must be similar in number to that population of bubbles with diameters greater than 0.08 pm that were present prior to operation. It is seen in Table 1 that the average number of bubbles on the grain boundaries of a filament of wire A in the initial condition is 3.47 x 109cme2. Therefore, in a given grain boundary that comprises the entire cross section of the wire there will be approximately 110,000 bubbles. Using the data in the histogram in Fig. 4(a), it can be concluded that, of that number of bubbles, approximately 6500 will have diameters above 0.08 pm. Metallographic measurements of the number of voids in the grain boundaries of filaments after operation that also have diameters greater than 0.08 pm is given in Table 7. The apporoximate uniformity of the number of bubbles with diameters greater than 0.08 pm, regardless of time, may be seen. Therefore, the number of bubbles with diameters greater than 0.08 pm that were initially present in the unburned filament can account for all of the large voids that form during operation. To estimate the effect of temperature on void growth we use the following expression developed by Raj and Ashby [IO] for the time required for the voids to grow to the point that they occupy 50% of the grain boundary area, 3J;; lM =%-
kT ?&8
1 F”(a) b,p3/2 F22(a)
where f2 is the atomic volume, D,S is the grain boundary diffusion coefficient times the boundary thickness, and p is the number of voids per unit area of the boundary. In this expression the temperature dependence enters through kT in the numerator and
Table 7. The number of voids with diameter greater than 0.08 pm in single grain boundary Operating time fhl 0
Number of voids 6500 6600 7000 4500
2: 100
D,, in the denominator. Values used for various parameters in this equation are also given in Table 5. The calculated values obtained for 1, are too small by one to two orders of magnitude, as can be seen in Table 2, which is probably caused by a precise lack of knowledge of the value of D,S for the system. However, if we consider that, in 1 h, a major fraction of void growth has occurred at 2973 K and use this information to normalize the other results, we obtain the values given in the last column of Table 2. These results suggest that between 2473 and 2123 K we should expect to see a significant decrease in void growth; experimental results are in agreement with this prediction. The tIna1 point to consider is the kinetics of void growth. This factor is best described by considering the results of the straight wire test shown in Fig. 14. An expression for the change in the volume of the voids as a function of time is given by [lo]
x
(c_ -2;)(1-$)
[
logJ;)-;+$(l
-$)
I
(5)
where rB is the radius of the bubble and 1 is the spacing between the bubbles. In Fig. 14, the dashed lines show the prediction of the model. These calculated results are in reasonable agreement with the measured values. Again our value for DbS is probably too high, but since the diameter plotted in Fig. 14 is proportional to the cube root of this quantity, as shown by equation (5), the calculated values are less sensitive to it. All of the analysis of void growth above assumes that the underlying mechanism is one of creep stimulated by grain boundary sliding. Another mechanism that might be considered is that of growth by internal pressure. As mentioned above, the potassium in the bubbles is certainly in the vapor state at these temperatures and does cause the bubbles to expand during the initial recrystallization. However, there are several reasons to discount this mechanism as the cause of growth to large sizes. First, the studies of potassium bubble growth in unstressed material [l, 151 show that bubbles may grow up to approximately 1000 8, in diameter. This size is considerably smaller than the large cavities observed in wire A in
BRIANT and WALTER
VOID GROWTH IN TUNGSTEN
this study. Secondly, experiments in our laboratory suggest that at these elevated temperatures the pressure induced bubble growth is complete in less than 1 min. This fact would be inconsistent with the results here where bubbles continued to grow for at least the first 24 h. Finally, it seems unlikely that a model based on internal pressure could account for the difference between wires A and B and the spatial distribution of the large bubbles across the grain boundary surface. Another point that should be discussed is the following. If one describes the process of nucleation and void growth as a classical nucleation phenomenon, then all voids below the critical size should shrink to zero radius. In this particular application the voids do not shrink to this extreme because the potassium vapor within them will keep them open. Consequently, a more exact analogy for this problem would be that of water nucleation on ions [16,17]. In that situation the water molecules collect around the ions to form a very small cluster, which is often referred to as a prenucleation embryo. However, for this cluster to grow to droplet size, it still must overcome a surface energy barrier and to do so requires a supersaturation of the vapor. The free energy curve in this case has the shape given in Fig. 17. Similarly, in the case reported in this paper there will first be a decrease in free energy caused by expansion of the bubbles as a result of internal vapor pressure. However, for these bubbles to continue to grow to a large size a stress must be applied to overcome the surface energy. 3. Comparison with other studies We now wish to compare our results to other studies. Horacsek [3], in agreement with our results, also showed that voids appear to grow from the potassium bubbles which served as pre-existing nuclei. He showed that void growth occurred at very low stresses in doped tungsten but not in undoped tungsten where these nuclei would not be available. He further pointed out that at the high temperatures used in his experiments void nucleation would be very difficult. Also, in agreement with this study, the work of Pugh (181 has emphasized the importance of
AG
Fig. 17. A plot of the change in free energy vs embryo radius for a situation in which the energy first decreases, then increases, and then decreases again with increasing size.
WIRE
2513
gr&ilYbsurtdary sliding in tungsten at high temperatures and low stresses. He presents clear pictures of samples of K-doped tungsten wire pulling completely apart along grain boundaries. Also, a number of studies [19-231 have shown that the test environment can affect cavity growth, as the oxygen did in these experiments. Other studies [4-6] have suggested that migration of bubbles through stress or temperature gradients could be responsible for cavity growth. However, we saw no evidence of this mechanism in our study. 5. SUMMARY AND CONCLUSIONS Growth of voids on grain boundaries of doped tungsten wire lamp filaments was examined. Filaments of wire A readily formed voids during operation at 2973 K while filaments of wire B did not. None of the potassium bubbles initially present on the grain boundaries in wire B had diameters larger than 0.08 pm while wire A contained some bubbles with diameters greater than 0.08 pm. Measurements of void growth were made as a function of time, operating temperature, the presence of oxygen in the lamp atmosphere, and stress applied to the filament. It was seen that void growth increased as the temperature, the oxygen content and the stress on the filament increased. While void growth occurred in both wires under conditions of high applied stress and high oxygen concentration in the lamp atmosphere, growth was much less for wire B than for wire A. The voids were generally faceted and the larger voids always formed at the peaks and valleys of undulations on the grain boundaries. Tests on straight wires 178 pm in diameter gave the same results as were obtained with the 63.5 pm diameter wire filaments. Calculations of the critical void diameter for growth by creep, the void nucleation and growth rates, the effect of temperature on growth rate and the effect of the presence of undulations on the grain boundaries on the growth rate were made and were compared with the experimental results. It was concluded that these voids grew by creep. Nucleation of voids at the high temperatures and low stresses impressed upon the filament is virtually impossible. Therefore, voids grow from the largest potassium bubbles initially present on the grain boundaries. The void surface energy is decreased by the presence of oxygen and the local stress normal to the grain boundary is increased by grain boundary sliding. Under these conditions, the critical radius for void growth drops to a value close to that of the largest bubbles found in wire A. Since wire B did not contain bubbles as large as the critical radius, voids did not grow unless an external stress was applied. Acknowledgemenfs-The authors extend their thanks to Robert Arena and Richard Holcomb of the Glass and Metallurgical Products Department and the Lighting Business Group, respectively, of the General Electric Company
2514
BRIANT and WALTER:
VOID GROWTH
for supplying many of the samples used in this study. They also extend their appreciation to Craig Robertson of this laboratory for the scanning electron microscopy. They would like to thank Drs D. Bly, J. Pugh and M. Vuckcevish and Mr Robert Arena of the Lighting Business Group for many helpful discussions. RF‘FJZRENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
D. Snow, MetatY Trans. A. 7A, 783 (1976). J. L. Walter, Trans. AIME 239, 272 (1967). 0. Horacsek, Z. Metallk. 65, 318 (1974). S. Garbe and S. Hanlon, Phi&s J. Res. 38,248 (1983). C. W. Dawson, Metall. Trans. 3, 3103 (1972). J. Brett and S. Friedman, Metall. Truns. 3, 769 (1972). J. L. Walter and H. E. Chne, Ser. meta@. 20, 1257 (1986). T. Watanabe, Metail. Trans. 14A, 531 (1983). S. H. Goods and W. D. Nix, Acta metail. 26,739 (1987). R. Raj and M. F. Ashby, Acta metall. 23, 653 (1975). M. C. Inman, D. McLean and H. R. Tipler, Proc. R. Sot. A 273, 538 (I 963).
IN TUNGSTEN
WIRE
12. M. R. Vuckcevich, General Electric Report. Lighting Research and Technology Operation, Cleveland, Ohio (1983). 13. K. G. Kreider and G. Bruggemann, Trans. TMS-AIME 23!?, 1222 (1967). 14. R. Raj and M. F. Ashby, ~eta~i. Trans. 2, 1113(1971). 15. J. L. Walter, P. Rao and R. R. Russell, Med. Trans. A 6A, 1777 (1975). 16. B. J. Mason, The Physics ofClou&. Oxford University Press (1971). 17. C. L. Briant and J. J. Burton, J. atmospheric Sci. 33, 1357 (1976). 18. J. Pugh, Metal!. Trans. 4, 533 (1973). 19. A. S. Argon, Ser. Metall. 17, 5 (1983). 20. 0. Horascsek, 8th Congress on Material Testing, Vol. 1. Budapest (1982). 21. P. Lagarde and M. Biscondi, Grain 3oun~aries in Engineering Materials (edited by Walter, Westbrook and Woodford), p. 367. Claitor’s, Baton Rouge (1975). 22. I.-Wei Chen, Metall. Trans. 14A, 2289 (1983). 23. R. I-I. Bricknell and D. A. Woodford, Acta mecaN. 30, 257 (1982).
Acta merall. Voi. 36, No. 9. pp. 2503-2514, 1988 Printed in Great Britain. All rights reserved
VOID GROWTH C L. RRIANT
IN TUNGSTEN
WIRE
and J. L. WALTER
General Electric Research and Development Center, P.O. Box 8, Schenectady, NY 12301, U.S.A. (Received 20 June 1987; in revised form 18 December
1987)
Abstract-This paper presents a study of void growth in tungsten wire. ‘&is wire contains potassium bubbles which are used to control grain growth and morpholop;y at elevated temperatures. Voids grow on the grain boundaries of this wire when it is operated in a lamp or heated in an uncoiied configuration. It was found that void growth was accelerated as the temperature of the wire, the stress on the wire, and the amount of oxygen in the environment were increased. It was also found that in the lamps operated without an externally applied stress (i.e. other than gravitational forces), voids did not grow unless there was a population of potassium bubbles with diameters greater than 0.08 pm. Based on these observations, it is proposed that the voids grow by creep. The larger pre-existing potassium bubbles serve as nuclei for these creep voids, since the low stresses and high temperatures preclude mechanically assisted nucleation at the high temperatures of lamp operation. It is also proposed that grain boundary sliding is an important source of stress in causing the voids to grow. Model calcuiations are used to support these conclusions. R&um&-Cet article ptisente une &ude de la croissance des caviti% dans un fif de tungstine. Ce fil contient des bulles de potassium que l’on utilise pour contrbler la croissance et la morphologie du grain aux temp6ratures Clevkes. Les cavit6s croissent sur les joints de grains de ce fil quand il fonctionne dans une lampe ou quand il est chaufl% dans une configuration d&o&e. La croissance des cavitts est a&l&e quand on augmente la temgrature du til, la contrainte sur Ie 6l et la quantiti d’oxy&ne dans l’environnement. On trouve en outre que dans les lampes qui fonctionnent sans l’application d’une contrainte exterieure (c’est-&dire), une contrainte autre que les forces de gravitation), les cavitis ne croissent pas $ qu’il n’y ait une population de bulles de potassium de diam&re sup6rieur li 0,08 pm. En nous basant sur ces observations, nous suggkrons que les cavids croissent par fluage. Les plus grosses bulles prt5existantes de potassium servent de germes pour ces cavit6s de &age, car Ies faibles contraines et les temp&atures &&es excluent une germination mhniquement assist&e pour les hautes temp6rature de fonctionnement de la lampe. Nous proposons&alement que le glissement intergranulaire est une source importante de contrainte qui peut faire croitre les cavitts. Nous utilisons des calculs de modblisation pour confirmer ces conclusions. Zusammenfassung-In dieser Arbeit wird das Ho~raumwa~hstum in wolframdraht untersucht. Dieser Draht enthalt Kaliumblasen, mit denen Komwachstum und -form bei erhijhten Temperaturen kontrolliert werden. Hohlrgume wachsen an den Komgrenzen dieses Drahtes, wenn er in einer Gliihlampc betrieben oder ungewendelt erhitzt wird. Das Hohlraumwachstum wurde stsrker, wenn die Drahttempertur, die elastische Spannung auf den Draht oder der Saurstoffant~l in der Umgebung erhhht wurden. AuBerdem ergab sich, daB in den Gliihlampen ohne eine lu&zre Spannung auf den Draht (d.h. keiner anderen als der Erdanziehung) die Hohlrlume nicht wuchsen, aul3er es lag eine Ansammlung von Kaliumblasen mit Durchmesser grader also 0.08pm vor. Ausgehend von diesen Beobachtungen wird dargelegt, daO die Hohllume durch Kriechen wachsen. Die gr65eren, vorher vorhandenen Kaliumblasen dienen also Orte, an denen die Hohlr%ume nukleieren; die niedrigen Spann~gen und die hohen Temperaturen schlieDen einen mechanisch unterstittzte Nukleation bei den hohen ~t~ebstem~raturen der Gliihlampe aus. AuBerdem wird gefolgert, dal3 die Korngrenzgleitung eine wichtige Spannungsquelle darstellt und so das Wachstum der Hohlriiume verursacht. Diese Folgerungen werden mit Modellrechnungen unterstiitzt.
1. INTRODUCTION Tungsten wire that is used for filaments in incandescent lamps has a unique microstructure. The wire is produced from a sintered powder-metallurgy ingot that is doped with potassium, aluminum, and silicon. During the sintering operation aluminum and silicon are almost completely evaporated from the ingot leaving less than about 100 wppm of potassium which is insoluble in tungsten and remains trapped in pores of the sintered ingot. As the ingot is unidirectionally deformed and lengthened through successive stages of rolling, swaging and w-ire drawing, these potassium-containing pores develop into ellip
soids that are less than 0.1 pm in width but greater than 1 pm in length [l]. When a filament made from this wire is heated in a lamp, the e&p&is break up into strings of bubbles, as shown in Fig. l(a). At the same time that bubble formation occurs, the wire is recrystallized to the point that a single grain may occupy the cross section of the wire. The boundaries between these grains have an undulating structure that is comprised of segments parallel to and perpendicular to the wire axis, as shown in Fig. l(b). Were it not for the presence of the potassium bubbles, the boundaries would be flat and the wire would have the bamboo structure of recrystallized pure tungsten wire. The flat boundaries could easily slide apart at
2503
2504
BRIANT and WALTER:
VOID GROWTH
IN TUNGSTEN
WIRE
Fig. 2. Scanning electron micrograph of grain boundary fracture of wire A operated for 200 h and showing the large voids formed on the boundary.
operation [3,4]. These voids can greatly limit the life of the filament. To date, there have been only a few studies [3-6] that have characterized the growth of these voids and attempted to determine their origin. We present a further consideration of the formation and growth of voids in tungsten wire in this study. It will be shown that the growth of the voids is greatly increased by increasing the operating temperature, the stress on the filament, and the oxygen in the lamp environment. It is proposed that the voids grow by creep from the largest potassium bubbles already present on the grain boundaries, and that the stresses required for void growth are provided by grain boundary sliding. 2. EXPERIMENTAL
Fig. 1. (a) Transmission electron micrograph of doped tungsten wire heated 4 min at 2313 K. (b) Scanning electron micrograph of filament of wire B burned for 4.33 h. One can observe in this micrograph that the filament has a primary coil. It can be seen that this primary coil is also coiled; note
the out-of-focus turn underneath the focused one. Consequently, these are called “coiled-coil” filaments. the operating temperature, which, depending on the lamp design, ranges between 2700 and 3000 K, and cause failure of the filament. Because the bubbles are present, the boundaries are pinned at certain points as they migrate along the length of the wire. Consequently, the grains develop this interlocking morphology. This morphology is more resistant to grain boundary sliding and leads to longer filament life [l, 21. It has been demonstrated in the past that, under certain conditions, large voids as shown in Fig. 2 form on the grain boundaries of the wire during
PROCEDURE
The lamps used in this study were prepared by the General Electric Company Lighting Business Group. The lamp consists of a coiled-coil filament (see Fig. l(b) and its caption) encased in a small glass envelope containing a high pressure gas mixture of proprietary composition. During the course of this work we also used lamps that contained gas mixtures doped with specific amounts of oxygen. Most of the work reported in this study will compare filaments made from two wires, designated as A and B, manufactured by different processes. Filaments of wire A readily formed voids when operated at 2973 K, the standard temperature of these lamps, while filaments of wire B did not. Both wires were 63.5 pm in diameter. Three experimental techniques for growing voids were used in this study, two of which employed lamps with filaments made from wires A and B. In the first case, lamps were operated at 120 V, 60 Hz for specific periods of time. The second technique subjected the lamps to a stress by placing them on a spinning table, as shown in Fig. 3, while they were operated at the
BRIANT and WALTER:
2x)5
VOID GROWTH IN TUNGSTEN WIRE
Fig. 3. Spinning lamp tester used to apply stress to burning lamps.
same conditions stated above. Different stresses were applied to the filament encased in the envelope containing a standard lamp atmosphere by varying the rotational speed of the turntable. The stresses are reported as multiples of the gravitational stress, g. It should he noted that all of the lamps used in these first two types of tests had been subjected to a series of pulses of increasing voltage, referred to as flashing of the filament, to cause initial recrystallization of the filaments prior to the experiments described herein. Thus, all of the experiments with lamps employed fully recrystallized microstructures such as that shown in Fig. l(b). In addition to the study of void growth in filaments, we also examined void growth in uncoiled wire that was 178 pm in diameter. These experiments were performed with the sample in a stainless steel chamber that could be evacuated to 1 x lo-* torr. The samples were first recrystallized in vacuum by resistively heating them at 2673 K for 1 h. They were then cooled, a stress was applied, and oxygen was admitted into the system to the desired pressure. They were then resistively heated again to allow creep to occur. The results of all of the experiments were evaluated by high-resolution scanning electron microscopy. 3. EXPERIMENTAL 1. Lamp
? 5
80 -
3
60-
Wire A unburned
% tz ;
40-
1
7 20-
60
c
Win
A burned 1 h
.hl,_,,_,* 0.10
0.30
RIBULTS
0.50
Wlrr
0.90
0.70
A burned
tOOh
tests
We will first consider the results obtained on lamp filaments. We will be concerned with the effects of initial bubble size distribution, oxygen pressure in the till gas, stress and temperature on void growth. Figures 4 and 5 show histograms of the void diameters measured from scanning electron micrographs for filaments made from wires A and B, respectively. These histograms represent the void distribution on the grain boundaries in the initial&U.3bieH
100 - _
0
0.1
0.2
0.3
0.4
Bubble
0.S
0.6
diometcr
O.?
(pm
0.6
0.9
f
Fig. 4. Histograms of bubbk sires for wire A in the initial condition and after operating for 1 and 100h. Note that a different scale for bubbk diameter is used for the data taken from samples burned 1 and 100h.
2506
BRIANT and WALTER:
100
VOID GROWTH
IN TUNGSTEN
Table 1. Number of voids per unit area (PI,) in grain boundaries for different burning times
I--
Wire Wire
6 unburned
Wire
B burned
1h
60
f---I
0.04
I
0.08
t
0.12
id
0.16
2 60
3 * b
60
f;
40
s
20
0.00
Wlrr
0.04
Bubble
6 burnrd
100
0.06
diameter
h
0.16
0.12
(pm
f
Fig. 5. Histograms of bubble sizes for wire B in the initial condition and after operating for 1 and 100 h. Note that, unlike the results in Fig. 4, the same scale is used for all figures.
recrystallized condition and after operating the lamps for 1 and 100 h. In the initial condition, the grain boundaries contain many very small pota~ium-filied bubbles as a result of processing. After operating for a given time, the bubbles have grown and become faceted voids as is shown in Fig. 2. There are several points that should be noted about the results shown in Figs 4 and 5. The first is that void growth occurs much more extensively in wire A than in wire B. (Note the different scales in the figures.) The second point is that in the initial condition, grain boundaries in wire A contain a finite number of voids (or bubbles) with diameters greater than 0.08 Frn. None of the bubbles on the grain boundaries in wire B have diameters greater than 0.08 ,um. The third point to note is that although some bubbles grow in wire A to
Burning time (h)
A A A A
2s 100
B
0
B” B
$
WIRE
0
I
2: 100
(cl%) 3.47 x 0.52 x 0.35 x 0.32 x
lop 109 109 IO9
4.1 2.6 3.6 2.2
109 lo9 IO9 109
x x x x
quite large sixes, there is always a population of voids below 0.08nm in diameter as well. Table 1 gives the average number of bubbles per unit area, N,, for each wire after different times of ovation. These numbers were based on measurements taken from six to ten boundaries and the variability among boundaries is approximately f 10%. The counts indicate a reduction in N. with increasing time for wire A, suggesting that some bubble or void growth is occurring and absorbing some of the small bubbles. There is no statistically meaningful change in N, for wire B. The fact that there is a reduction in N. for wire A and no change in N, for wire B suggests that no new bubbles are being formed on the grain boundaries during operation. The large voids in wire A had the faceted appearance shown in Fig. 2. There was also some localization of the voids on various parts of the fracture surface. This localization could be seen most clearly on boundary areas with large undulations. For example, in Fig. 6 it is seen that the larger voids have formed at the peaks and valleys of the undulations and that there has been little void formation on the sides. Also, sub-boundary often served as preferred sites of void growth as shown in Fig. 6(C). Voids grew less in wire B, but the growth that was observed was always associated with asperities on the grain boundary surface such as low angle boundaries, sharp undulations, or small ridges that apparently occurred on the boundary as a result of grain boundary migration [7]. An example is shown in Fig. 7. Table 2 shows the effect of operating temperature on void formation for samples made from wire A that were operated for 24 h. (Note that the last two columns in the table present calculated values that will be considered in the Discussion.) No voids were observed on the grain boundaries of samples operated at 2023 and 2123 IL After operating at 2473 K, one void with a diameter greater than 0.2pm was found after examining eight grain boundaries. Grain boundaries of filaments operated at 2623 and 2973 K had many large voids on each grain boundary. Clearly, increasing the temperature increases the rate of void growth. Figure 8 shows the effect of increasing the applied stress on the filaments in lamps made from wires A
BRIANT and WALTER:
VOID GROWTH
IN TUNGSTEN
WIRE
Fig. 6. (A) SEM of the grain boundary fracture surface of wire A operated for 20.5 h. (B) SEM of the grain boundary fracture surface of wire A operated for 20.5 h. (C) SEM of the grain boundary fracture surface of wire A operated for 144 h showing the formation of voids along the low-angle grain boundary.
and B. Increasing
the stress causes an increase in the average size of the voids formed in both wires during the 5-h period of the test. However, the effect of stress on void growth is much greater for wire A than for wire B. In these stressed lamps, there was clear evidence of grain boundary sliding as shown in Fig. 9. This figure shows a grain boundary in wire A that is nearly perpendicular to the wire axis and which underwent sliding during a test at 6g for 5 h. Figure 10 shows the effect of oxygen on void growth. Here, the average diameter of the ten largest voids found from examining six to ten grain boundaries is plotted as a function of the initial oxygen content in lamps made from wire A and operated for 24 h. In these lamps, the fill gas was krypton plus oxygen. It is clear that an increase in the initial oxygen content caused an increase in the void size for a fixed operating time. Figure 11 shows a similar plot
for data obtained on filaments made from both wires A and B in which oxygen was added to the standard fill gas. It is to be noted that the effect of oxygen is not as great when lamps contain the proprietary fill gas since the more complex chemistry of this gas mixture lowers the activity of oxygen on the wire surface. It is also noted that the filaments of wire B are much less susceptible to void formation in an oxygen-containing environment. In lamps made from either wire, the combination of increased oxygen and stress causes a significant increase in void growth as is seen in Fig. 12. It should be noted that when both the oxygen content and stress are increased, large voids also form in filaments made from wire B. A scanning electron micrograph of these voids is shown in Fig. 13. In addition to lamps made from wires A and B, lamps were also made from wires produced by
2508
BRIANT and WALTER:
VOID GROWTH IN TUNGSTEN WIRE
Fig. 9. SEM of a filament of wire A operated for 5 h at 6g showing offset grain houndary.
Fig. 7. SEM of the grain boundary fracture surface of wire B with 0.1% oxygen operated for 5 h at 56. This micrograph shows that voids have preferentially formed at the peaks and valleys of large undulations.
3.0 r
2.5 -
different thermomechanical schedules. Table 3 lists the average diameter of the ten largest bubbles in filaments in the initial recrystallized condition and
whether or not voids formed after operating the lamp for 24 h. Again, it is clear that if the initial bubble diameter never exceeds aproximately 0.08 ,um, littie void growth occurs.
z a
2.0
.
h
1.5 -
z v
t.o-
0.5
-
Table 2. Operating temperature (K)
Voids
Caktiated time to form voids
Normalized time to form
DreScnt
all
VOids
2023 2123 2413 2623 2973
No No 1 void Y&S Yes
30 11 0.51 0.2 0.03
1000 366 19 6.6
t
O.oOOl
CWOl
02
0.01
0.1
in kr (%.I
Fig. 10.Plot of the average value of the diameters of the ten largest voids in wire A as a function of oxygen content in lamps filled with krypton. Error bars represent one standard deviation.
Fig. 8. Plot of the maximum void diameter as a function of stress in lamps operated for 5 h. Error bar represents the standard deviation of values from different filaments.
BRIAN?- and WALTER:
o1
0.001
2509
VOID (SROWTH IN TUNGSTEN WIRE
0.1
0.01
Oxygenf%) Fig. 11.Plot of the average value of the diameters of the ten largest voids in wires A and B as a function of oxygen
content in lamps with standard atmospheres. Error bars represent one standard deviation. Fig. 13. SEM of a grain boundary fracture of wire B b&n 5 h at 5g showing large voids.
2. Straight wire tests This section considers the results of tests run on 178pm straight wire under conditions of varying stress and oxygen contents. Figure 14 shows the average diameter of the ten largest voids on grain boundaries plotted as a function of test time for straight wires dead-loaded to three levels of tensile stress. The plot shows that the voids grow more rapidly with increasing applied stress. There is also an initial period of rapid growth which is followed by much slower growth. The final data point on each curve represents a sample that failed during the test. The dashed lines drawn on the figure are calculated from a model and will be considered in the Discussion.
10 t
l
Wire A
o
WweAtO.l%02
0
8 ‘;
0
t
i
Tabk 3. Void charac&stic~for iampsmade from wins prosxad diffcrentlY
Ave. diemeter
of ten largestbubbks in
unburned filament (pm)
burning 24 h
A B
0.1 0.055 0.206 0.087
YeS No YCS No
The effect of oxygen was investigated in two ways. In the first set of experiments, wires were tested for 4.5 h at 2673 K under different applied loads and at different pressures of oxygen as measured at room temperature. The results in Table 4 show the minimum oxygen concentration required to obtain void growth at each stress. It is clear that as the stress
l.a
I-
.
21.7 MN/m2
l
q4.4 MN/m2
0.S 1 - .m
E
e
I-
O Wire
Voids formed on
Wire
.
7.1 MN/m2
B + 0.1 *da02 0
8
0 0
0.6
r3 a8
0.4
2 0.2 0
12345678
stress (g’* 1 Plot of the maximum void diameter as a
Fig. 12. (a) function of stress for Iamps of wire A with and without
oxygen in the lamp atmosphere. Lamps were operated for 5 h. (h) Plot of the maximum void diameter as a function of stress for lamps of wire B with and without oxygen in the lamp atmosphere. Lamps were operated for 5 h.
1 I
0
10
I 20
Time(h)
Fig. 14. Plot of the average value of the diameters of the ten largest voids as a function of time for the 178pm win dead-loaded to three levels of tensile stress.
2510
BRIANT and WALTER:
VOID GROWTH
Table 4. Minimum oxygen pressure required to cause void growth in straight wires tested at 2473K for 4.5 h Applied stnSs (MN/m’)
PO2 5 x lo-’
21.7 14.4 7.1 1.6
1x 10-J 1x IO--’ 2 x 10-z
decreases a higher partial pressure of oxygen is required to cause void growth. In the other set of experiments, a constant load of 14.4 MN/m* and a temperature of 2673 K were used and the times to failure and void diameters were measured. Figure 15 shows the times to failure plotted as a function of oxygen partial pressure; as the oxygen content increases, the life becomes shorter. In each wire the void diameters at failure were similar and the largest ones were of the order of 1.Opm. Scanning electron microscopy showed that the voids formed in the straight wires were similar to those found in the filaments from the lamps. The voids were faceted and the largest ones were associated with asperities on the grain boundaries. Scanning electron microscopy also provided very clear evidence for grain boundary sliding in the straight wire samples. Figure 16(A) shows the surface of one of the straight wire samples where it appears that the grams are sliding apart. Figure 16(B) shows a void on an internal boundary. The offset in the void across the boundary is typical of voids formed in sliding boundaries [8,9]. This sliding is similar to that shown in Fig. 9 for the filament. 4. DISCUSSION
The discussion begins by stating an interpretation of the results. The experimental results presented
30 r x 25 c ?! 3 5
x
above are then combined with calculations based on a simple model to substantiate the interpretation. It is proposed that the voids on the grain boundaries grow from the potassium bubbles initially present there. Growth of the voids occurs as a result of creep. Of the many bubbles present on the boundary after the initial recrystallization, only those with diameters greater than about 0.08 pm will grow; all others are below the critical diameter for growth. The gravitational stress on the filament is too low to cause growth of bubbles of any size on the grain boundary even if oxygen is present to lower the void surface tension. However, grain boundary sliding can occur at these temperatures and can intensify the stress at asperities on the grain boundaries. At these points, stress intensification can cause growth of the larger bubbles found in wire A. This effect will also give rise to the spatial distribution of the voids observed on the grain boundaries. If an additional stress is applied to the filament, voids will grow in filaments of wire B because the critical radius for growth is lowered. These points are considered in greater detail below. I. Void nucleation There are two stages to the formation of voidsnucleation and growth. In the as-drawn doped tungsten wire, potassium is present as very thin ellipsoids. When the wire is recrystallized during the flashing sequence, these ellipsoids break up into rows of small bubbles, and the internal pressure produced
x
15-
lo-
F S-
0'
WIRE
p ccexp[ -%I
20-
2 g
IN TUNGSTEN
x I 10-3
10-z
I 10-l
PO2 (torr) Fig. IS. Plot of the time to failure of 178 pm wire loaded to 14.4 MN/m* as a function of oxygen partial pressure. The test temperature was 2673 K.
where y is the void surface energy, a is the angle between the grain boundary and the void surface, and a, is the normal stress on the boundary. If one uses parameters appropriate to those for an operating filament (see Table S), the exponential is essentially zero and nucleation should not occur. The value of the exponential is also very near zero for all stresses used in the straight wire tests and in the spinning lamp tests. Therefore, it is assumed that nucleation does not occur at the temperatures used in this study, a result that is consistent with the data in Table 1. Also, at these high temperatures, nucleation rarely occurs
BRIANT and WALTER:
VOID GROWTH IN TUNGSTEN WIRE
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Table 5. Values used for calculations Parameter
Value
Comment or Ref.
1000 erg/cm’( I J/m’) 4.18 3.14 2.24 x lb N/m’ 2913 1 x 10-flm’ Wpm 3.3 x lo-‘ exp( -92,OOO/RT) m’/s
Y
F,(a) F,(a) =. T n 6 4
because entropy will constantly break up a subcritical nucleus as it forms. 2. Void growth The size of the critical radius for void growth, r,, is given by the expression [IO]
2Y
rc=6
where y is the surface energy of the void and u is the applied stress. The values for r, calculated using the parameters given in Table 5 for the fiIament and the straight wires are listed in Table 6. These values of r, for the filaments are clearly much larger than the radii of any of the bubbles measured in the filaments in their initial recrystallized condition. This calculation would suggest that void growth should never occur in the filaments. Since the voids do grow, they must do so as a result of increased stress or lowered void surface tension, both of which could lower the value of r,. The value chosen for y, l~erg~~2, is typical of grain boundary energies f 1I]. The effect of oxygen on void growth may result from a lowering of y. However, even if we reduce y to 500 erg/cm2, a value typical of an impurity covered grain boundary ill], we still obtain values of r, that are too large. Therefore, we must consider whether the stresses could be increased to lower r,. Significant increases in stress could be caused by grain boundary sliding. In Figs 9 and 16 there was evidence that in both the filaments and in the straight wire tests grain boundary sliding did occur. Sliding causes stresses to be built up at the asperities along the grain boundaries. The question is whether these stresses are sufficiently high to cause a decrease in r,. In order to estimate the value of such a stress increase, we assume that the undulations of the grain boundary can be expressed as a sine wave. Then, the stress build-up on the grain boundary is given by [ 141
Ref. [I I] Value for Value for Value for Value for
spherical void spherical void filament-Ref. [12) operating dlament
Ref. 113)
where 7a is the applied shear stress. i. is the wave length of the undulations, and h is the amplitude of the undulations. Therefore, as A increases and h decreases, we would expect more sliding to occur and, consequently, the stress resulting from sliding to increase. Examination of the filaments showed that there were a variety of undulations on the grain boundaries. First, there were the larger variations such as those shown in Figs l(b) and 6(A). Measurements of these gave an average value for i/h of 13 and a maximum value of 30. There were also fine undulations such as those shown in Figs 6(B) and (C) for which reliable measurements of A/h could not be obtained. Using the values of I/h obtained for the
(3)
Tabk 6. Values of the critical radius bm) Condition
Lamp filament Straight wire (o 21.7) Strainht wire (a 7.1)
~=lOOO 0.8928 0.0920 0.2816
7=sooc@m~
Fig. 16. (A) SEM of a surface of 178 pm wire showing grain boundaries sliding. (B) SEM of a grain boundary of 178 brn wire showing offsets on the boundary.
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VOID GROWTH IN TUNGSTEN WIRE
larger undulations, we calculate, for the average value of I/h, r, of 0.0546 pm and, for the maximum value, r, of 0.0235 pm (critical diameters of 0.109 and 0.047 pm, respectively). The average value is clearly near the upper range of bubble diameters in filaments of wire A in the initial condition (see Fig. 4). However, the calculated average value is still well outside that measured for wire B (see Fig. 5). If the applied stress on the filament is increased to, say, six times the gravitational stress through the spinning test, r, drops to 0.0108 pm, a bubble size found in both wires. Because of the difficulty in obtaining accurate values of y, I/h, and, in the case of the filament, a, our calculated values for r, primarily demonstrate a qualitative consistency between the model and the measured values of the bubble diameter. A stronger test of the model is based on the measurement of void sizes. It was proposed that voids do not form in wire B because there are no bubbles initially present with diameters greater than the critical value, although voids with diameters larger than this value are present in wire A. Examination of the histograms shown in Figs 4 and 5 shows that the critical diameter must be about 0.08 pm. Therefore, if this hypothesis is correct, the population of large voids that form on the grain boundaries during burning must be similar in number to that population of bubbles with diameters greater than 0.08 pm that were present prior to operation. It is seen in Table 1 that the average number of bubbles on the grain boundaries of a filament of wire A in the initial condition is 3.47 x 109cme2. Therefore, in a given grain boundary that comprises the entire cross section of the wire there will be approximately 110,000 bubbles. Using the data in the histogram in Fig. 4(a), it can be concluded that, of that number of bubbles, approximately 6500 will have diameters above 0.08 pm. Metallographic measurements of the number of voids in the grain boundaries of filaments after operation that also have diameters greater than 0.08 pm is given in Table 7. The apporoximate uniformity of the number of bubbles with diameters greater than 0.08 pm, regardless of time, may be seen. Therefore, the number of bubbles with diameters greater than 0.08 pm that were initially present in the unburned filament can account for all of the large voids that form during operation. To estimate the effect of temperature on void growth we use the following expression developed by Raj and Ashby [IO] for the time required for the voids to grow to the point that they occupy 50% of the grain boundary area, 3J;; lM =%-
kT ?&8
1 F”(a) b,p3/2 F22(a)
where f2 is the atomic volume, D,S is the grain boundary diffusion coefficient times the boundary thickness, and p is the number of voids per unit area of the boundary. In this expression the temperature dependence enters through kT in the numerator and
Table 7. The number of voids with diameter greater than 0.08 pm in single grain boundary Operating time fhl 0
Number of voids 6500 6600 7000 4500
2: 100
D,, in the denominator. Values used for various parameters in this equation are also given in Table 5. The calculated values obtained for 1, are too small by one to two orders of magnitude, as can be seen in Table 2, which is probably caused by a precise lack of knowledge of the value of D,S for the system. However, if we consider that, in 1 h, a major fraction of void growth has occurred at 2973 K and use this information to normalize the other results, we obtain the values given in the last column of Table 2. These results suggest that between 2473 and 2123 K we should expect to see a significant decrease in void growth; experimental results are in agreement with this prediction. The tIna1 point to consider is the kinetics of void growth. This factor is best described by considering the results of the straight wire test shown in Fig. 14. An expression for the change in the volume of the voids as a function of time is given by [lo]
x
(c_ -2;)(1-$)
[
logJ;)-;+$(l
-$)
I
(5)
where rB is the radius of the bubble and 1 is the spacing between the bubbles. In Fig. 14, the dashed lines show the prediction of the model. These calculated results are in reasonable agreement with the measured values. Again our value for DbS is probably too high, but since the diameter plotted in Fig. 14 is proportional to the cube root of this quantity, as shown by equation (5), the calculated values are less sensitive to it. All of the analysis of void growth above assumes that the underlying mechanism is one of creep stimulated by grain boundary sliding. Another mechanism that might be considered is that of growth by internal pressure. As mentioned above, the potassium in the bubbles is certainly in the vapor state at these temperatures and does cause the bubbles to expand during the initial recrystallization. However, there are several reasons to discount this mechanism as the cause of growth to large sizes. First, the studies of potassium bubble growth in unstressed material [l, 151 show that bubbles may grow up to approximately 1000 8, in diameter. This size is considerably smaller than the large cavities observed in wire A in
BRIANT and WALTER
VOID GROWTH IN TUNGSTEN
this study. Secondly, experiments in our laboratory suggest that at these elevated temperatures the pressure induced bubble growth is complete in less than 1 min. This fact would be inconsistent with the results here where bubbles continued to grow for at least the first 24 h. Finally, it seems unlikely that a model based on internal pressure could account for the difference between wires A and B and the spatial distribution of the large bubbles across the grain boundary surface. Another point that should be discussed is the following. If one describes the process of nucleation and void growth as a classical nucleation phenomenon, then all voids below the critical size should shrink to zero radius. In this particular application the voids do not shrink to this extreme because the potassium vapor within them will keep them open. Consequently, a more exact analogy for this problem would be that of water nucleation on ions [16,17]. In that situation the water molecules collect around the ions to form a very small cluster, which is often referred to as a prenucleation embryo. However, for this cluster to grow to droplet size, it still must overcome a surface energy barrier and to do so requires a supersaturation of the vapor. The free energy curve in this case has the shape given in Fig. 17. Similarly, in the case reported in this paper there will first be a decrease in free energy caused by expansion of the bubbles as a result of internal vapor pressure. However, for these bubbles to continue to grow to a large size a stress must be applied to overcome the surface energy. 3. Comparison with other studies We now wish to compare our results to other studies. Horacsek [3], in agreement with our results, also showed that voids appear to grow from the potassium bubbles which served as pre-existing nuclei. He showed that void growth occurred at very low stresses in doped tungsten but not in undoped tungsten where these nuclei would not be available. He further pointed out that at the high temperatures used in his experiments void nucleation would be very difficult. Also, in agreement with this study, the work of Pugh (181 has emphasized the importance of
AG
Fig. 17. A plot of the change in free energy vs embryo radius for a situation in which the energy first decreases, then increases, and then decreases again with increasing size.
WIRE
2513
gr&ilYbsurtdary sliding in tungsten at high temperatures and low stresses. He presents clear pictures of samples of K-doped tungsten wire pulling completely apart along grain boundaries. Also, a number of studies [19-231 have shown that the test environment can affect cavity growth, as the oxygen did in these experiments. Other studies [4-6] have suggested that migration of bubbles through stress or temperature gradients could be responsible for cavity growth. However, we saw no evidence of this mechanism in our study. 5. SUMMARY AND CONCLUSIONS Growth of voids on grain boundaries of doped tungsten wire lamp filaments was examined. Filaments of wire A readily formed voids during operation at 2973 K while filaments of wire B did not. None of the potassium bubbles initially present on the grain boundaries in wire B had diameters larger than 0.08 pm while wire A contained some bubbles with diameters greater than 0.08 pm. Measurements of void growth were made as a function of time, operating temperature, the presence of oxygen in the lamp atmosphere, and stress applied to the filament. It was seen that void growth increased as the temperature, the oxygen content and the stress on the filament increased. While void growth occurred in both wires under conditions of high applied stress and high oxygen concentration in the lamp atmosphere, growth was much less for wire B than for wire A. The voids were generally faceted and the larger voids always formed at the peaks and valleys of undulations on the grain boundaries. Tests on straight wires 178 pm in diameter gave the same results as were obtained with the 63.5 pm diameter wire filaments. Calculations of the critical void diameter for growth by creep, the void nucleation and growth rates, the effect of temperature on growth rate and the effect of the presence of undulations on the grain boundaries on the growth rate were made and were compared with the experimental results. It was concluded that these voids grew by creep. Nucleation of voids at the high temperatures and low stresses impressed upon the filament is virtually impossible. Therefore, voids grow from the largest potassium bubbles initially present on the grain boundaries. The void surface energy is decreased by the presence of oxygen and the local stress normal to the grain boundary is increased by grain boundary sliding. Under these conditions, the critical radius for void growth drops to a value close to that of the largest bubbles found in wire A. Since wire B did not contain bubbles as large as the critical radius, voids did not grow unless an external stress was applied. Acknowledgemenfs-The authors extend their thanks to Robert Arena and Richard Holcomb of the Glass and Metallurgical Products Department and the Lighting Business Group, respectively, of the General Electric Company
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BRIANT and WALTER:
VOID GROWTH
for supplying many of the samples used in this study. They also extend their appreciation to Craig Robertson of this laboratory for the scanning electron microscopy. They would like to thank Drs D. Bly, J. Pugh and M. Vuckcevish and Mr Robert Arena of the Lighting Business Group for many helpful discussions. RF‘FJZRENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
D. Snow, MetatY Trans. A. 7A, 783 (1976). J. L. Walter, Trans. AIME 239, 272 (1967). 0. Horacsek, Z. Metallk. 65, 318 (1974). S. Garbe and S. Hanlon, Phi&s J. Res. 38,248 (1983). C. W. Dawson, Metall. Trans. 3, 3103 (1972). J. Brett and S. Friedman, Metall. Truns. 3, 769 (1972). J. L. Walter and H. E. Chne, Ser. meta@. 20, 1257 (1986). T. Watanabe, Metail. Trans. 14A, 531 (1983). S. H. Goods and W. D. Nix, Acta metail. 26,739 (1987). R. Raj and M. F. Ashby, Acta metall. 23, 653 (1975). M. C. Inman, D. McLean and H. R. Tipler, Proc. R. Sot. A 273, 538 (I 963).
IN TUNGSTEN
WIRE
12. M. R. Vuckcevich, General Electric Report. Lighting Research and Technology Operation, Cleveland, Ohio (1983). 13. K. G. Kreider and G. Bruggemann, Trans. TMS-AIME 23!?, 1222 (1967). 14. R. Raj and M. F. Ashby, ~eta~i. Trans. 2, 1113(1971). 15. J. L. Walter, P. Rao and R. R. Russell, Med. Trans. A 6A, 1777 (1975). 16. B. J. Mason, The Physics ofClou&. Oxford University Press (1971). 17. C. L. Briant and J. J. Burton, J. atmospheric Sci. 33, 1357 (1976). 18. J. Pugh, Metal!. Trans. 4, 533 (1973). 19. A. S. Argon, Ser. Metall. 17, 5 (1983). 20. 0. Horascsek, 8th Congress on Material Testing, Vol. 1. Budapest (1982). 21. P. Lagarde and M. Biscondi, Grain 3oun~aries in Engineering Materials (edited by Walter, Westbrook and Woodford), p. 367. Claitor’s, Baton Rouge (1975). 22. I.-Wei Chen, Metall. Trans. 14A, 2289 (1983). 23. R. I-I. Bricknell and D. A. Woodford, Acta mecaN. 30, 257 (1982).