Sintering atmosphere effects on tensile properties of ... - Springer Link

Sintering Atmosphere Effects on Tensile Properties of Heavy Alloys A. BOSE and R. M. GERMAN The sintering atmosphere has a direct bearing on the residual porosity which in turn has a strong negative influence on the tensile properties of W-Ni-Fe heavy alloys. The present investigation uses various sintering atmospheres to understand pore formation, densification, microstructure, and tensile properties of heavy alloys with tungsten contents ranging from 88 to 97 wt pct. Pore formation when sintering in a dry hydrogen atmosphere is linked to water vapor generation and its entrapment in pores. A hydrogen dew point effect is associated with solution-reprecipitation of tungsten during liquid phase sintering. The beneficial effect of vacuum sintering has been analyzed in terms of removal of the gases before pore closure. Property degradation during long time vacuum sintering is attributed to preferential matrix vaporization. The negative effect of long sintering times in dry hydrogen is attributed to pore coarsening, which is removed by a three-stage sintering atmosphere treatment.

I.

INTRODUCTION

TUNGSTEN heavy alloys are a class of composites characterized by a high density. They are processed by classic liquid phase sintering from W-Ni-Fe or W-Ni-Cu elemental powder mixes, where the tungsten content ranges from 80 to 98 wt pct. The final sintered alloy consists of nearly pure tungsten grains dispersed in a ductile matrix which has an approximate composition of 53Ni-23Fe-24W. [~I The typical mean tungsten grain size varies from 20 to 60 micrometers, depending on the initial particle size, volume fraction of tungsten, sintering temperature, and sintering time. The combination of density, ductility, strength, conductivity, machinability, and corrosion resistance makes the heavy alloys unique in the field of materials. Due to this property combination, these alloys are used in many areas, [2'3'41 such as radiation shields, counter weights, kinetic energy penetrators, vibration damping devices, and heavy duty electrical contacts. Though the earliest work on heavy alloys dates back to the 1930's, tsj the effect of all the processing parameters on the properties of heavy alloys is not yet well understood. Considerable property variations have been reported for the same alloy composition apparently sintered under similar conditions. This is definitely due to subtle processing differences. Apart from the variation of properties brought about by a change in composition, many other factors are thought to affect the final properties of the heavy alloys. [6-36]Porosity greater than 0.5 pct can drastically reduce the mechanical properties, especially the ductility. EglIntermetallic precipitation is to be avoided; this is typically done by eliminating copper, using a 7Ni:3Fe ratio, and low volume fraction of matrix. [4Ar-t8,3~ Impurity segregation and embrittlement of interfaces can be controlled by using pure powders and post-sintering heat treatments such as quenching from high temperatures. 113'17'2~ Hydrogen embrittlement can be controlled by changing gases within the sintering cycle, tgl using vacuum anneal as a post-sintering heat treatment, I1~ or using vacuum as the sintering atmosphere.[36] A. BOSE, Postdoctoral Associate, and R.M. GERMAN, Professor, with the Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. Manuscript submitted April 10, 1987.

In spite of all these factors, incomplete densification is still the primary concern. Incomplete densification may result from too short a sintering time, solidification shrinkage due to fast cooling through the solidus, trapped gases within the pores, or pores formed by a reaction product during sintering. The sintering atmosphere plays a vital role in controlling the residual porosity which in turn controls the final sintered properties. This paper addresses the roles of sintering atmospheres and alloy composition on pore elimination during sintering. The importance of hydrogen embrittlement and pore coarsening are discussed in light of the observations. Based on the theoretical concepts, processes have been developed which can eliminate pore coarsening phenomena associated with long sintering times.

II.

EXPERIMENTAL APPROACH

Nickel and iron elemental powders were mixed in the ratio of 7 to 3 by weight. To this premix, the desired quantity of tungsten powder was added. The tungsten contents used for this investigation were 88, 90, 93, 95, and 97 wt pct. The total mixing time for the alloys was 90 minutes. The powder characteristics are given in Table I. Flat, tensile specimens were compacted in a double action floating die using zinc stearate as a die wall lubricant. Table I.

Powder Characteristics

Property W Vendor GTE Designation M35 Purity, pct 99.98 Fisher subsieve size,/zm 2.5 Mean size,*/xm 2.6 BET specific surface area, m2/g 0.23 Apparent density, g/cm 3 2.57 Major impurities (ppm) K(ll) Na(15) C(19) 0(770)

are

METALLURGICALTRANSACTIONS A

Ni INCO 123 99.99 2.8 3.3 2.19 2.15 Ca(10) Fe(30) Si(40)

Fe GAF HP 99.50 3.0 10.8 0.88 2.20 Ca(600) Al(600) Si(600) 0(300) Mn(2000)

*forward laser light scattering VOLUME 19A, OCTOBER 1988--2467

The compacts were pressed at 275 MPa, giving green densities in the range of 50 to 55 pct of theoretical, and green dimensions of around 5 mm height and 6.45 cm z projected area (about 1 square inch). The green tensile bars were sintered in a microprocessor controlled horizontal tube furnace. The temperature was held to ---5 ~ of the set point. A typical sintering and heat treatment cycle is shown in Figure 1. It consists of the following steps: (a) heat up to 800 ~ at 10 ~ (b) hold at 800 ~ for one hour, (c) heat from 800 ~ to the sintering temperature (usually 1480 ~ to 1500 ~ at 10 ~ (d) hold at the sintering temperature for the desired length of time (usually 30 minutes), (e) from the sintering temperature to 1420 ~ cool at 3 ~ and (f) furnace cool from 1420 ~ Sintered samples are generally heat treated in argon atmosphere, using a vertical furnace with automatic quenching facilities with the following cycle: (a) heat to 1100 ~ at 10 ~ (b) hold at 1100 ~ for 1 hour, and (c) water quench from 1100 ~ This treatment suppresses impurity segregation to the interfaces. [24'25'26] Beyond this basic sintering and heat treatment cycle, various atmosphere cycles have been investigated, as discussed in the next section. In a few cases, the heat treatment step was incorporated into the sintering cycle. The sintered and heat treated tensile bars were lapped to a 240 grit finish on the two flat surfaces; however, the two side faces could not be lapped. Density was determined by the water immersion technique. A 20 mm gage length was marked out on one of the flat surfaces and the samples were pulled to failure in tension at a crosshead speed of 0.004 mm/s. The properties reported here are the average of three specimens. III. BASIS FOR SELECTION OF THE SINTERING CYCLES Generally the atmosphere used for sintering heavy alloys is hydrogen. Dry hydrogen often causes blistering and swelling during liquid phase sintering. Sintering in dry hydrogen for 120 minutes causes excessive swelling. This problem can be resolved by using wet hydrogen in the sintering cycle. [371 SINTERING AND HEAT TREATMENT 1800

l

1

T

-

-

T

-

-

1480"C //~20"C

1500 1200

/

Ld ,,y

I

\

HO0*C

t

900 nW OW

6oo 3oo 0

~

0

I00

_

2oo

0

TIME (M~N.)

Fig. 1--Typical sintering and heat treatment cycle. 2468--VOLUME 19A, OCTOBER 1988

t

I00

200

For the majority of this study, a switch to a secondary inert gas atmosphere (dry argon having a dew point of - 7 0 ~ has been made near the end of the sintering cycle. A switch to an inert gas atmosphere late in the sintering cycle effectively removes the hydrogen embrittlement problem.[9] The usual sintering cycle consists of heating up to a temperature of 800 ~ and holding there for one hour. This is carded out in dry hydrogen to reduce oxides. The sintering temperature is higher than the solidus temperature, m'39] ensuring complete liquid phase sintering. Generally a short sintering time like 30 minutes has been optimum, t6'9'35] On completion of sintering, a cooling rate of 3 ~ to 1420 ~ is used in all cases to minimize solidification shrinkage. An initial atmosphere cycle was dry hydrogen (dew point of - 5 5 ~ for two-thirds of the sintering time, followed by a change to dry argon (dew point - 7 0 ~ Poor tensile results were obtained with this sintering cycle due to a high final porosity. Thus, the dew point of the hydrogen was changed to 0 ~ This was used up to two-thirds of the sintering time, followed by dry argon. These two modified sintering cycles are shown in Figures 2A and 2B, respectively. Figures 3 and 4 show the ductility and strength of various tungsten heavy alloys using these two atmospheres. The lower properties resulted from sintering in the dry hydrogen (dew point - 5 5 ~ followed by dry argon (dew point - 7 0 ~ sintering cycle (Figure 2A). Shifting the hydrogen dew point from - 5 5 ~ to 0 ~ (Figure 2B) resulted in an increase in the tensile properties. These results prove that the dew points of the hydrogen sintering atmosphere have an important beating on the tensile properties. The heavy alloys sintered with a 0 ~ dew point showed significant property improvements over those sintered in dry hydrogen (dew point - 5 5 ~ Hence, a more detailed investigation into the dew point effect was undertaken. For this part of the study an alloy of 88W-8.4Ni-3.6Fe was chosen as this alloy showed marked property changes with dew point. A sintering temperature of 1500 ~ was used and the hydrogen dew point was varied from - 4 4 to +30 ~ In each case, a shift to dry argon was made after two-thirds of the sintering time. This sintering cycle is shown in Figure 2C. The variations in the retained porosity and elongation with hydrogen dew point for the 88W alloy at 1500 ~ are shown in Figures 5 and 6. By changing the hydrogen dew point from - 4 4 to +30 ~ the sintered density changed from 99.2 to 99.9 pct, and the elongation changed from 23 pct to 33 pct. The dew point experiments demonstrated that water vapor formed inside the compact during sintering causes swelling. The dissolution of tungsten into the matrix releases dissolved oxygen which reacts with the absorbed hydrogen to form water vapor. The low solubility in the matrix leads to pore formation with subsequent coarsening during prolonged sintering. Any conditions that favor tungsten dissolution or water vapor formation will enhance bubble formation. To test this theory, three sets of experiments were designed. For one experiment, the sintering temperature was raised to 1540 ~ In the other case, the sintering temperature was kept at 1500 ~ while the heating rate was reduced to 5 ~ Finally, keeping the sintering temperature at 1500 ~ and heating rate at 10 ~ the sintering time was increased to 120 minutes. In all three experiments dry hydrogen (dew point of - 4 4 ~ was used as the sintering atmosphere with METALLURGICAL TRANSACTIONS A

elongation, %

temperature, ~ iooo ,I I

I

I

I

--i

1500

I

I

I

~I

I

I

I

I

,5

1540"C

_LJ

1480~

I

I

pO

\

~/

/

300

-70oC

-

Hit

Ar

-44"C

-600C

I

I

I

I Fig. 2A

--

Fig. 2 B \

-~5 '.0 --

o /~ 1

I

I

II

1500

I 15000C

1480~

1200

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900

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"\

600

H=

Ar

500

O*C

-?O~C

o

15 --

-~

+I

I

{

I

1500

"

I

t

J

/

L

H=

Ar

-44*C

-60aC

F_J t

t

t

5 --

~

1 90

o

I

86

lSOO*C

1500*C

I0 --

88

I 92

1 94

I 96

I 98

weight percent tungsten ,,

j

900

600

HI to

0

I 50

0

I I00

I 150

-44+C

"70"C

+50~

300

,

At

"44~

I 200

I 2~

-60~

I

I

l

l

I

300

50

I00

150

~00

time,

I

250

300

rain

Fig. 3 - - T h e variation of elongation with tungsten contents for the sintering cycles shown in Figs. 2A and 2B.

ultimate tensile strength, MPo IOOO

1

temperoture, oC 1800

I

1500 -

.ll

I

I

I

z..J

14800C

i

I

I

I

[

~5OO*C I I*Cfmm ~ - " - ~

950

1

I

-

9

Fig. 2A

9

Fig. 2 B

I

1200

x\ 'L.

Ar

900 60O 300

-

\\

900

70"C

o

[I

I

1500

I

I

i

i

.11

I

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't

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]

I

1500"C

850 -

600

Vac

H2

HI

Vac

Af

- 50%

500

o:~1

I

-i

t200 [-

I

i

t

I

I

1480~

I

i

900

0

/

~c

- 50-C

Ar

I00

VOr \

-

H2

150

200

I

I

I

94

96

98

H2

18"C

-70"(

i

50

I 92

\\

\

He

I 90

Fig. 4 - - T h e variation of ultimate tensile strength with tungsten contents for the sintering cycles shown in Figs. 2A and 2B.

\\

\

600 ~- /

I 88

weight percent tungsten

1480~

/

r

800 86

"70~

1500

300

vac

- 70%

I

I

250

300

0

l

l

50

I00

150

I

t

I

200

250

500

time, rain

Fig. 2 - - T h e various sintering atmospheres and cycles used.

a switch to dry argon during the last 10 minutes of the hold period. The atmosphere and sintering cycles are compared in Figures 2D, 2E, and 2F, respectively. The dissolved oxygen in the tungsten which is not reduced by the one-hour hold at 800 ~ is released into the liquid matrix when the tungsten is taken into solution. This oxygen reacts with the dissolved hydrogen in the liquid maMETALLURGICAL TRANSACTIONS A

trix to form water vapor which is insoluble in the matrix. Any condition that will enhance the solution of tungsten in the liquid matrix will decrease the sintered density and mechanical properties, especially the ductility. Table II compares some of the effects due to variations in sintering temperature, sintering time, alloy composition, and heating rate when dry hydrogen is used as the sintering atmosphere after the liquid has formed. Sintering in an inert gas atmosphere leads to incomplete densification and extremely poor properties. [9] To demonstrate this effect, one experiment was carried out in an argon atmosphere, as shown in Figure 2G. It is best to have no gas in the pores during sintering. This is possible if a vacuum exists when the pores close. Thus, another experiment was VOLUME 19A, OCTOBER 1988-- 2469

retoined porosity, % I.O I

I

I

I

0.8

0.6

w

0.4

0.2

0 -50

I

I

I

I

-30

-I0

I0

30

50

dew point (~ Fig. 5 - - T h e variation of retained porosity with hydrogen dew points for an 88W alloy sintered at 1500 ~

elongation, % 35

I

I

I

I

30 -

25

20

I

-5(

-3o

I

I

-io

Io

I

30

50

dew point (~ Fig. 6 - - T h e variation of elongation with hydrogen dew points for an 88W alloy sintered at 1500 ~

Table II.

Alloy Pct W 88 90 88 88 88 88 88 88 88 88

Sintering Temp., ~ 1480 1480 1500 1540 1500 1500 1500 1500 1500 1500

Some Parameters Affecting the Retained Porosity in Heavy Alloys

Sintering Time, min 30 30 30 30 30 120 30 30 30 30

2470--VOLUME 19A, OCTOBER 1988

designed using a 0.006 Pa vacuum sintering atmosphere. To have sufficient oxide reduction, dry hydrogen was used up to 1300 ~ which was followed by a switch to vacuum. The vacuum was maintained over the complete high temperature hold period after which the system was backfilled with argon to remove any remaining pores. This sintering cycle is shown in Figure 2H. The mechanical properties obtained by vacuum sintering were excellent with lower tungsten contents. With high tungsten contents the tensile properties, especially ductility, were degraded. At the sintering temperatures there is preferential matrix vaporization as evidenced by a smaller matrix in metallographic sections and a density increase. This matrix loss leads to a property decrease, especially in those alloys where the initial matrix content is low (for example, 97W). Thus, it is best to avoid long sintering times and high tungsten contents when sintering in vacuum. Since the pores are completely sealed off within a very short time after the liquid forms, the hold time in vacuum could easily be shortened to 15 minutes. Based on these concepts, the sintering cycle shown in Figure 21 was tried where the vacuum heat treatment cycle was incorporated in the sintering cycle. Specimens sintered by this cycle were not subjected to the usual post-sintering heat treatment. There was concern that residual oxygen in the alloy was detrimental to the sintered properties. Thermodynamic calculations revealed that even using hydrogen dew points as high as + 50 ~ would not reverse the water vapor formation reaction. However, the reaction kinetics are lower when high dew points are used. Thus, using high dew points might solve the problem of bubble formation within the system, but could possibly lower the sintered properties due to residual oxygen. Two experiments were conducted using the general procedure outlined in Figure 2I, with the major variation being the presintering temperature. In these, a presintering of 60 minutes at 1000 ~ was used, after which, in one experiment the heating rate to the sintering temperature was kept at 11 ~ whereas in the other it was reduced to 4 ~ In both experiments dry hydrogen was used up to 1300 ~ followed by a switch to vacuum. Also in these the heat treatment step was incorporated into the sintering cycle. The two sintering cycles are outlined in Figures 2J and 2K, respectively. Sintering cycle 2K gives more time in dry hydrogen at temperatures higher than 1000 ~ thereby allowing better oxide reduction.

Heat-Up Rate ~ 10 10 10 10 10 10 10 5 10 10

Hydrogen Dew Point, ~ -55 -55 -44 -44 -44 -44 -44 -44 -44 +30

Retained Porosity Pct 0.9 0.7 0.8 1.4 0.8 5.2 0.8 1.1 0.8 0.1

Elongation Pct 12 16 23 2 23 nil 23 20 23 33

UTS, MPa 876 890 805 531 805 nil 805 783 807 881

METALLURGICAL TRANSACTIONS A

Based on the above, a sintering schedule was designed to enhance oxide reduction before the liquid forms, as well as to avoid any bubble formation within the system. In this cycle, the samples were heated to 1300 ~ (after a hold of one hour at 800 ~ in dry hydrogen (dew point - 4 4 ~ followed by a switch to wet hydrogen (dew point + 18 ~ which was maintained for two-thirds of the sintering time, followed by a switch to dry argon (dew point around - 6 0 ~ The sintered samples were heat treated by water quenching after one hour at 1100 ~ The sintering schedule is depicted in Figure 2L, which will be termed the dry-wet hydrogen cycle. The elongation and ultimate tensile strength changes with tungsten content, sintered with the dry-wet cycle, are shown in Figures 7 and 8, respectively. The elongation is comparable to that obtained by the dry hydrogen-vacuumdry argon cycle except for the 97W alloy, where the properties of the vacuum sintered material are degraded due to preferential binder loss in vacuum. The elongation of the dry hydrogen-dry argon sintered specimens was inferior. The ultimate tensile strength in both the dry-wet hydrogen and vacuum cycles showed a definite peak around 93W. This peak is attributed to the increase in contiguity with the increase in tungsten content, resulting in premature failure due to the high population of W-W contacts.t4~ elongation, % 36 Ik..

/

i

I

32 28

-

~

i 9

Fig, 2A

9

Fig. 2H

2016 12 8

-

4

-

0 86

I 88

I 90

I 92

I 94

I 96

98

weight percent tungsten

Fig. 7--The variationof elongation with tungsten contents for the sintering cycles shown in Figs. 2A, 2H, and 2L.

Table

Alloy Pct W 90 90 90 93 93 93 93

III.

Role of Vacuum

Heat

ultimate tensile strength, MPa I000 I

30 min, 30 min, 30 min, 30 min, 30 min, 30 min, treatment 30 min, treatment

dry dry dry dry dry dry

I

/

I

~

I

I

* Fig. 2A 9 Fig. 2 H

950

9 2L

900 --

850 --

8OO 86

l 88

I 90

1 92

] 94

l 96

i 98

weight percent tungsten

Fig. 8--The variationof ultimate tensile strength with tungsten contents for the sintering cycles shown in Figs. 2A, 2H, and 2L. Table III lists the properties of samples that illustrate the effects of vacuum heat treatment in the same sintering cycle, and prereduction in dry hydrogen with varying heating rates. Vacuum sintering yields properties comparable to the dry-wet hydrogen cycle. Incorporation of vacuum during cooling eliminates the extra heat treatment step and gives comparable properties. In cases where vacuum sintering was used, the sintered densities were greater than theoretical due to the preferential vaporization of the matrix. The last two results in Table III demonstrate the beneficial effects of longer prereduction in dry hydrogen. Prolonged sintering reduces the density and room temperature ductility, t6,9,3s] Sintering times over 90 minutes result in lower ductilities. This is attributed to pore coarsening. Until now, long time sintering of heavy alloys without a loss in the sintered density and ductility has not been possible. One 10-hour sintering run was carried out on a 90W sample in a vacuum cycle similar to that described in Figure 21. Here the hold time in vacuum was 590 minutes after which a switch was made to argon for the last 10 minutes of the hold period. The samples lost matrix due to evaporation with a concomitant increase in the W content. In another case, sintering times up to 10 hours were used on an 88W and 95W

Treatment

Sintering Cycle and Heat Treatment 1480 ~ 1480 ~ 1480 ~ 1480 ~ 1480 ~ 1500 ~ no heat 1500 ~ no heat

t

H2, vac, dry Ar, quench from I ~ C H2, vac, dry Ar, vac, no heat treatment H2, wet H/, dry Ar, quench from 1100 ~ H2, vac, dry Ar, vac, no heat treatment H2, wet H2, dry Ar, quench from 1100 ~ H2, vac, dry Ar, vac, heating rate 11 ~

and Pre-Reduction

Figure Number

Theoretical Density

Elongation Pct

UTS, MPa

2H 21 2L 2I 2L

100.4 100.1 99.9 100.3 99.7

31 32 31 24 23

903 913 908 962 986

2J

100.7

26

923

2K

100.9

29

922

dry H2, vac, dry Ar, vac, heating rate 4 ~

METALLURGICALTRANSACTIONSA

VOLUME19A,OCTOBER1988--2471

alloy based on the dry-wet hydrogen cycle. The hold was in wet hydrogen (dew point + 18 ~ followed by a switch to dry argon for the last ten minutes of sintering. The dry-wet hydrogen cycle resulted in samples which were fully dense even after 10-hour sintering. Long time sintering of heavy alloys without a decrease in the sintered density was achieved. Prolonged holds at the sintering temperature cause a drop in the sintered densities and properties, especially in the elongations. The drop in the ductility (20 pct) for a 90W long time (600 minutes) vacuum sintered sample is due to the excessive matrix loss which is reflected in the sintered density which is much greater than the theoretical.

IV.

DISCUSSION

At the onset of heavy alloy sintering, the pore structure is open. Initial densification takes place by activated sintering even before the liquid phase forms.t261 With the formation of a liquid, rapid densification takes place and the open pore network is replaced by closed pores which gradually spheroidize. This occurs when the total porosity in the system is reduced to about 8 pet. [41'421 In his model of final stage of sintering, Markworth t43[ shows that the densification rate depends on the inverse of the pore radius and any internal gas pressure, as shown in Eq. [1],

--~

=

kTG2

-

Pg

[1]

where p is the fractional density, t is the time, dp/dt is the densification rate, l~ is the atomic volume, D is the diffusivity, k is Boltzmann's constant, T is the absolute temperature, G is the grain size, T is the surface energy, and Pg is the gas pressure in the pores. The removal of the last 8 pet porosity is governed by the sintering atmosphere trapped inside the closed pores. Due to liquid flow into the pore, the pore size will shrink, thus increasing the pressure of the gas within the pore. When the gas pressure within the pores increases to balance the surface energy effect, no further densification will occur. This is true only for gases which have no solubility in the matrix. If the entrapped gas has some solubility in the matrix, then it will diffuse out gradually, allowing material to flow in and fill the pore. [9] Hydrogen has some solubility and diffusivity in the matrix. [441 If hydrogen is trapped within the pores, then it will gradually diffuse out as the pores shrink. Trapped hydrogen may decrease the densification rate to some extent, but will not prevent complete densification. However, if an inert gas like argon or a gas which has a low solubility in the matrix like water vapor, is trapped within the pores, then it will hinder densification. Indeed, swelling is often seen during prolonged sin~9~ring dueto poret~oalescence)45] German and C urn have ected poe depressurization rate to the gas diffusivity. Based on that model, estimates have been made for the final sintered density variation with sintering time, for different atmospheres, as shown in Figure 9. The fastest densification is expected with vacuum sintering. This is closely followed by hydrogen, where complete densification should be obtained within 20 minutes. However, this is not seen in practice due to the water vapor formation which prevents complete densification. Argon as the sintering atmosphere trapped in the pores will result in 2472--VOLUME 19A, OCTOBER 1988

incomplete densification. Alternatively, sintering in hydrogen (Figure 2L) and vacuum (Figures 2H and 21) produces heavy alloys which are fully dense with good properties. Sintering in argon (Figure 2G) shows poor properties and incomplete densification. The calculations predict that a density of at least 99 pet of theoretical should be obtained with an inert gas atmosphere, assuming no coarsening. The samples sintered in argon showed sintered densities around 96.3 pet. The pores seen in Figure 10(b) are larger than the initial particle size and have coarsened by coalescence, thereby reducing the sintered density, t9[ Also, sintering in argon may influence wetting due to unreduced oxides. The microstructures of a sample sintered by the dry-wet hydrogen cycle (Figure 2L) and in argon (Figure 2G) are compared in Figure 10. Sintering in argon leads to small pores and jagged tungsten grains, indicating poor or incomplete wetting and incomplete dissolution of tungsten. In contrast, the microstructure of a sample sintered with the dry-wet hydrogen cycle shows no porosity and large rounded tungsten grains, evidencing complete wetting of tungsten. The elongation of the argon sintered 88W heavy alloys is 1 pet compared to 35 pct elongation for a dry-wet hydrogen sintered alloy. With the hydrogen sintered specimens there is a wide variation in the sintered properties. With no post-sintering treatments to remove the hydrogen embrittlement effect, poor mechanical properties result even though the specimen may be fully dense. This is expected since the ductility of nickel-iron polycrystalline alloys is affected by hydrogen.[46] Another way of reducing hydrogen embrittlement is by switching to an inert gas atmosphere at the end of the sintering cycle, t91 Generally in all of our investigations, a shift has been made to argon in the last 10 minutes of the

density, % I00

ogen im

~argon 99

95w olloy 1480"C

98

I0

20

40

80

sintering time, min Fig. 9 - - T h e variation of sintered density with sintering times for vacuum, hydrogen, and argon atmospheres. METALLURGICAL TRANSACTIONS A

(a)

40~J

(b) Fig. lO--Optical mJcrographs showing an 88W heavy alloy sintered by (a) the dry-wet hydrogen cycle (Fig. 2L) and (b) the argon cycle (Fig. 2G).

sintering cycle with a post sintering heat treatment of a water quench from 1100 ~ after an hour hold at 1100 ~ (in argon), to remove hydrogen. In spite of this, sintering in hydrogen with different dew points resulted in samples with different tensile properties. A comparison of the sintered tensile properties obtained by sintering in dry hydrogen (dew point - 5 5 ~ Figure 2(a)) and wet hydrogen (dew point 0 ~ Figure 2(b)), shows the importance of the dew point of the hydrogen atmosphere. Such behavior can be attributed to the reaction between residual oxygen in the tungsten and hydrogen in solution in the liquid matrix. Commercial tungsten powder has oxygen as an impurity (in this powder it was about 770 ppm). This oxygen can exist as surface oxides, adsorbed oxygen, or dissolved oxygen. The adsorbed and the surface oxides on the tungsten particles should be removed during the one-hour reduction hold. However, a part of the dissolved oxygen within the tungsten will remain even when the liquid forms. Possibly this dissolved oxygen could be reduced prior to densification by using very long annealing times in dry hydrogen. In the absence of such treatments, once the liquid forms the process of solution-reprecipitation releases fresh oxygen into the liquid phase. Hydrogen is soluble in the liquid matrix; thus, it will be present in the liquid if a hydrogen sintering atmoMETALLURGICALTRANSACTIONSA

sphere is used. Depending on the dew point and residual oxygen content, the soluble hydrogen will react with the released oxygen to form water vapor. This water vapor is essentially insoluble in the matrix; hence it forms small water vapor filled pores. Initially, the water vapor filled pores are small and low in population. Such small gas bubbles can exit to the sample surface by buoyancy driven pore migration. A change in the dew point would alter the kinetics of water vapor formation. As the dew point decreases, water vapor formation will exceed the rate of pore removal, resulting in swelling. However, a wet hydrogen atmosphere slows the rate of water vapor formation, allowing elimination of the water vapor filled pores. As illustrated in Table II, any condition which enhances the release of oxygen into the liquid matrix when sintering in dry hydrogen will induce pore growth and deteriorate the sintered tensile properties. Thus, heavy alloy properties are degraded with the use of a dry hydrogen sintering atmosphere. Additionally, higher binder contents increase the oxygen available for pore generation. Thus, a 88W alloy can show lower ductility compared to a 90W alloy. Also a higher sintering temperature or longer sintering time will release more oxygen, giving lower strength and ductility. The 88W alloy exhibits lower properties when sintered in dry hydrogen and at higher temperatures or longer times. Pore coarsening is also enhanced by longer sintering times. A slower heat-up rate means longer holding times in the liquid phase, which will enhance pore formation. Thus an 88W alloy sintered at 1500 ~ for 30 minutes using a heatup rate of 5 ~ will show inferior properties compared to samples where the heat-up rate was 10 ~ However, too rapid a heat-up rate is detrimental since it will fail to give reduction of the surface oxides. These property variations can be seen by comparing the results in Table II. Residual oxygen influences the tensile properties of the heavy alloys slightly, as shown in Table III where the tensile properties of 93W samples heated to 1500 ~ at two different heating rates (Figures 2J and 2K) are compared. A slower heating rate has more time in dry hydrogen, resulting in better oxide reduction and slightly improved ductility. A basic precept is that a gas with some diffusivity through the matrix (hydrogen), will slow but not hinder full densification. However, if the gas has no solubility (water vapor or inert gas), it will be compressed in the pore as densification occurs. In this case there will be an outer core where the matrix has refilled a pore and a gas filled inner pore. The process is shown schematically in Figure 11. Samples sintered in dry hydrogen can exhibit a microstructure showing complete pore filling (of pores previously filled with hydrogen) and partial pore filling (of pores which had water vapor), as shown in Figure 12. Prior investigators[1~'35]have shown that as-sintered heavy alloys have hydrogen contents of around 800 to 900 ppb, whereas the estimated solubility of hydrogen in heavy alloys is only about 400 ppb by weight. I35'44] This apparent two-fold increase over the solubility limit of hydrogen is due to hydrogen entrapped in the pores during pore closure as either hydrogen or water vapor. The dry-wet hydrogen cycle (Figure 2L) derives the benefit of better oxide reduction by using dry hydrogen up to 1250 ~ and counteracts swelling by using wet hydrogen after 1250 ~ Hydrogen degassing is achieved by switching to argon during the last 10 minutes of the sintering cycle. VOLUME 19A, OCTOBER 1988--2473

The ductility shows a drop with increasing tungsten content (Figure 7). This is due to an increase in the contiguity of the sintered materials. The contiguity will approach unity as the volume fraction of liquid tends to zero. Figure 13 shows the increase of the contiguity with the volume fraction of tungsten. This is in agreement with the trends reported for heavy alloysts~ as well as different carbides. [471In all cases, the contiguity increases with the increase in the volume fraction of solid and dihedral angle.t481 As the contiguity goes up, the population of tungsten-tungsten grain contacts increases. This results in a continuous decrease in the ductility with increasing tungsten contents since the tungsten-tungsten contacts are the weak link in the microstructure. V.

(a) Fig. 1 1 entrapped the liquid the liquid

(b)

Schematic representation of the pore filling process when the gas in the pore has (a) high solubility and diffusivity through matrix and (b) practically no solubility or diffusivity through matrix.

The properties obtained by this sintering cycle are extremely good, as can be seen from Figures 7 and 8. A further advantage becomes apparent when long sintering times are used. This processing cycle does not lower the sintered density of heavy alloys when sintered for times more than 100 minutes. It is postulated that by using a wet hydrogen atmosphere, the bubble formation is very slow. This allows the gas-filled bubbles to exit the samples before they can coalesce and grow.

(a)

CONCLUSIONS

The sintering atmosphere present just before the interconnected pores close has a major influence on the final density and properties of tungsten heavy alloys. Vacuum gives the fastest densification rate as there is no gas trapped in the pores. Any gas sealed in the pores inhibits densification, but the gases with high diffusivity and solubility in the liquid matrix will not greatly inhibit densification. Gases that have practically no solubility through the liquid matrix (argon or water vapor) are trapped in the pores, and prevent complete densification. Indeed, the stabilized pores can grow, causing swelling with long sintering times. The heavy alloys are susceptible to hydrogen embrittlement; thus, they show property benefits if a switch is made to an inert gas or vacuum atmosphere at the end of the sintering cycle. The use of a dry hydrogen atmosphere after the liquid forms causes a property degradation by the inhibition of full densification. A property dependence of heavy alloys on the dew point of the hydrogen atmosphere has been demonstrated. The results point to the formation and entrapment of water vapor during solution-reprecipitation of tungsten particles, which releases the dissolved oxygen into the liquid matrix. Using a dry hydrogen atmosphere and conditions that increase tungsten dissolution into the liquid causes property degradation. Vacuum sintering and vacuum heat treatment produces heavy alloys with good tensile

6 0 ~

(b) I

Fig. 12--Microstructure of an 88W heavy alloy sintered at 1500 ~ in dry hydrogen showing (a) matrix filling of a pore that had entrapped hydrogen and (b) partial matrix filling of a pore that had entrapped water vapor. 2474--VOLUME 19A, OCTOBER 1988

METALLURGICAL TRANSACTIONS A

Contiguity 1.0

I

0.8

I

I

1

I 0.8

I 0.9

1480 ~ C 30 min.

0.6

0.4

0.2 0 0.5

I 0.6 Volume

I 0.7

1.0

Fraction Tungsten

Fig. 13--Variation of contiguity with volume fraction of tungsten.

properties when the tungsten content is below 95 wt pct. Above this tungsten level preferential vaporization of the binder phase becomes acute. Residual oxygen is detrimental, as demonstrated by the beneficial effects of slower heating rates in dry hydrogen. Based on these findings, a dry-wet hydrogen cycle was designed to combine the beneficial effects of oxygen reduction, prevention of swelling, and removal of the hydrogen embrittlement effect. The main attribute of this sintering cycle is that it allows long time sintering of heavy alloys without the associated density degradation.

ACKNOWLEDGMENTS

This investigation has been conducted under sponsorship of the United States Army Research Office and California Research and Technology, Inc. The support of Dr. Ronald Brown of CRT is greatly appreciated. The authors would like to thank Dr. Tai-Shing Wei for assistance with the analysis and Dr. Barry H. Rabin for useful discussions. Thanks are also due to Prarthana Bose and Deepak Madan for their valuable help in the preparation of the manuscript.

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METALLURGICALTRANSACTIONS A