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Note on the powder metallurgy of rhenium
JOURNAL OF THE LESS-COMMON
NOTE
ON THE
POWDER
METALS
METALLURGY
OF
RHENIUM
A. R. POSTER Sylvania Electric
Products
Inc., Chemical and Metallurgical (Received
Division,
Towanda,
Pa. (U.S.A.)
April rrth, 1961)
SUMMARY The standard powder metallurgy practice of pressing and sintering was used to produce high density samples from a rhenium metal powder. Rhenium powder, of the type described, was found to be readily moldable into green compacts and these compacts sintered in rather short times to high density. It was found that sintered density depends to a greater extent upon sintering time than upon green density.
INTRODUCTION
Rhenium metal has been prepared by a number of processes, among which are (I) vapor deposition by dissociation of the chloride, (2) melting and (3) hydrogen reduction of either potassium perrhenate or ammonium perrhenate. Because of its high melting temperature (317o”C), melting of rhenium is rather difficult, although laboratory ingots have been made this way. The availability of high purity metal powder which is made by the reduction of potassium and ammonium perrhenate has lead to the use of powder metallurgy as the predominate method of manufacture. Rhenium metal is interesting from a number of standpoints. It is only second to tungsten, among the metallic elements, in melting pointi. Its density of 21.0 g/ml is higher than that of tungsten 2. Annealed material has exhibited tensile strengths of greater than 120,000 p.s.i. with 25% ductility at room temperature, and it is somewhat harder and more resistant to abrasion than tungsteni. Other properties, such as its corrosion resistance and electrical properties, make it promising for such applications as incandescent lamp filaments and electrical contacts%4. In this investigation the powder metallurgy process was used to form small compacts of a rhenium powder in order to investigate, in a preliminary way, some of the parameters which are involved in its compacting and sintering. PROPERTIES
OF THE POWDER
Physical and chemical properties of the powder are presented in Figs. I and 2 and in Table I. The particle size distribution was measured by use of the Cenco Photelometer and is shown in Fig. I. This figure shows that the largest particles were 16 ,x, and that approximately 75% of the particles were below g p in size. The average particle size is small, and also it can be seen that the particle size distribution is uniform. J. Less-Common
Metals,
4 (1962)
2-8
POWDER
METALLURGY
OF RHENIUM
Figure 2 is a photograph of this powder at IOOO magnification. The uniform distribution of particle sizes can also be seen in this photograph as well as the extreme
Fig. I. Particle size distribution.
Fig. z. Particles of rhenium powder.
(X
1000).
3
A. R. POSTER
4
TABLE PHYSICAL
AND
CHEMICAL
I
PROPERTIES
OF RHENIUM
Screen size
IOOo/o
Potassium by flame photometry (sensitivity o.oooz”~)
<0.0002y4
Fe
POWDER
mesh
-325
o.oo12°/~
Al
Ca
Mg
<
p.p.m. p.p.m.
I p.p.m.
irregularity of particle shape. In some instances, large agglomerates appear to be made up of many small particles. SAMPLE PREPARATION
Green compacts were formed by mechanical pressure application using a hardened steel die which had a cylindrical cavity of 0.176 in. diameter. Because of the high cost of the powder, only 0.1 g was used for each compact. Pressure was applied double action, and the green compacts were formed without the aid of either die wall or interparticle lubrication. Sintering of the compacts was accomplished in a molybdenum wound muffle furnace at 1750°C f 25’C under a cover gas of purified hydrogen of -20 dew point. The compacts were heated from room temperature to 1750°C over a period of 0.5 h by slow stoking into the furnace hot zone. This was done in order to avoid cracking of the compacts due to thermal shock and/or release of adhered gases. Compacts were formed at various pressures between 7.5 and 50 t.s.i. and sintered for 16 h. Compacts were also formed at 20 t.s.i. and sintered for various times from 0.5 to 16 h. Densities of the green and sintered compacts were taken by air weightmeasured volume method. RESULTS
Tables II and III give the density data for the various samples. This data is also graphed in Figs. 3 and 4. TABLE GREEN
AND
SINTERED
compacting presswe (t.s.i.)
7.5
DENSITY
II VS. COMPACTING
Green density I.%!~)
7.48
PRESSURE
Sirtned
density
f&W)
16.15
20
8.85
x9.08
30
9.80
19.60
40
II.22
19.30
50
II.55
r9.32
The green density curve, in Fig. 3, increases quite regularly with increasing compacting pressure, and appears to become constant at approximately 11.5-1~0 g/ml J. Less-Common Melds,
4 (1962) 2-S
POWDER METALLURGY OF RHENIUM
5
density for pressures greater than 50 t.s.i. This is quite low when compared to other refractory metal powders. For example, tungsten powder, of equal particle size, when compacted at only IO t.s.i. gives a green density of 10.5g/m15. Although the theore-
>.
.?1
= 14. : _ 12.
Compacting
pressure (t.s.i.)
Fig. 3. Green and sintered density as function of compacting pressure.
Fig. 5. Structure
80.
Sintering
time
(h)
Fig. 4. Sintered density vs. time at 1750°C temperature for rhenium compacts.
of sintered rhenium after 0.5 h at 175o’C. grains per mma. (X 1000) J.
Grain count
Less-Common
=
Metals,
~1,600
4 (1962) 2-8
A. R. POSTER
6
tical density for rhenium is slightly higher than for tungsten (19.3 g/ml for tungsten and 21.0 g/ml for rhenium), the difference in density at comparable pressures is significant. Although strength measurements were not performed on the green compacts, it was noted that the compacts were of sufficient strength, even at the lower pressures, to be readily handled. The sintered density curve, which is given on Fig. 3, shows that the final density depends only to a certain extent upon the compacting pressure and to a greater extent upon sintering time. Sintered density increases quite rapidly for the lower
Fig.
6. Structure
of sintered
rhenium after 2.0 h at 1750°C. Grain grains per mma. (X 1000)
TABLE
count
=
15,400
III
SINTERED DENSITY VS. TIME AT 1750°C Sinfering
time(h)
Sintered
dmity (g/ml)
0.3 0.5 2.0
8.85 13.80 15.10 17.02
4.0 16.0
17.35 19.08
0.0 (green)
J. Less-Comnzon Metals, 4 (1962) 2-8
POWDER METALLURGY
OF RHENIUM
7
compacting pressures and appears to reach a maximum at 25-30 t.s.i., while maximum green density is not obtained until approximately 50 t.s.i. pressure. A slight drop in the sintered density is noted for compacting pressures greater than 30 t.s.i. It is not known if this slight density decrease is real or an error in measurement. Since this decrease in sintered density occurs directly after attainment of maximum density, it is likely that there is some connection between these occurrences. It is also possible that entrapment of gases in the green compact is responsible for this decrease. This, however, was not further investigated.
Fig. 7. Structure of sintered rhenium after 16 h at 1750%. Grain count = 10,800 grains per mma. (X 1000)
Sinterecl density as a function of sintering time at 1750°C f 25’C, is shown in Fig. 4 for samples which were compacted at 20 t.s.i. An extremely rapid increase in density is noted for even the first increments of sintering time. This increase is from 8.8 g/ml, for the green compact, to approximately 13.8 g/ml during the first 20 min and to approximately 17.0 g/ml after only 2 h. Density continues to slowly increase for times greater than 2 h and, in these experiments, increased to 19.0 g/ml in 16 h. Figures 5, 6 and 7 are photographs, at IOOOmagnifications, of the metal structure after compaction at 20 t.s.i. and 0.5, 2 and 16 h sintering at 1750°C. Fig. 5 shows that new grains, which are significantly larger than the original powder particles, J. Less-Common Metals,
4 (1962) 2-8
8
A. R.
POSTER
are formed already at 0.5 h. The porosity, which is the dark area, is still associated strongly with the grain boundaries in this photograph. In Fig. 6 the compact has received two hours heating and is approximately 17.8 g/ml dense. The remaining pores are longer, fewer in number and many pores are no longer associated with grain boundaries. Further heating results in increased grain size as shown in Fig. 7. The large number of pores, which are shown in this last figure, indicate that further densification is possible. CONCLUSIONS
Rhenium powder, of the type described, is readily fabricated into green compacts of greater than 60 o/oof theoretical density. Sintering progresses quite rapidly with time, and densification depends to a greater extent on time than on green density. It is unfortunate that only very small compacts could be formed, however, little trouble is foreseen in forming larger parts such as bars for subsequent processing. REFERENCES 1 L. W.
KATES, Materials G Methods, 3g (1954) 88. 2 C. T. SIMS, C. M. CRAIGHEAD AND R. I. JAFFEE, J. Metals, 7, AIME Trans., 203 (1955) 168. 8 G. B. GAINES, C. T. SIMS AND R. I. JAFFBE, J. Electvochem. Sot., 106 (1959) 881. 4 I. E. CAMPBELL. D. M. ROSENBAUM AND B. W. GONSER, .I. Less-Common Metals, I (1959) 185. 6 A. R. POSTER, Factors which affect the compaction of- tungsten powders, Sylvania Elccfvic Products Inc. Rep., Towanda, Pa.
J. Less-Common
Metals,
4 (1962) 2-8
NOTE
ON THE
POWDER
METALS
METALLURGY
OF
RHENIUM
A. R. POSTER Sylvania Electric
Products
Inc., Chemical and Metallurgical (Received
Division,
Towanda,
Pa. (U.S.A.)
April rrth, 1961)
SUMMARY The standard powder metallurgy practice of pressing and sintering was used to produce high density samples from a rhenium metal powder. Rhenium powder, of the type described, was found to be readily moldable into green compacts and these compacts sintered in rather short times to high density. It was found that sintered density depends to a greater extent upon sintering time than upon green density.
INTRODUCTION
Rhenium metal has been prepared by a number of processes, among which are (I) vapor deposition by dissociation of the chloride, (2) melting and (3) hydrogen reduction of either potassium perrhenate or ammonium perrhenate. Because of its high melting temperature (317o”C), melting of rhenium is rather difficult, although laboratory ingots have been made this way. The availability of high purity metal powder which is made by the reduction of potassium and ammonium perrhenate has lead to the use of powder metallurgy as the predominate method of manufacture. Rhenium metal is interesting from a number of standpoints. It is only second to tungsten, among the metallic elements, in melting pointi. Its density of 21.0 g/ml is higher than that of tungsten 2. Annealed material has exhibited tensile strengths of greater than 120,000 p.s.i. with 25% ductility at room temperature, and it is somewhat harder and more resistant to abrasion than tungsteni. Other properties, such as its corrosion resistance and electrical properties, make it promising for such applications as incandescent lamp filaments and electrical contacts%4. In this investigation the powder metallurgy process was used to form small compacts of a rhenium powder in order to investigate, in a preliminary way, some of the parameters which are involved in its compacting and sintering. PROPERTIES
OF THE POWDER
Physical and chemical properties of the powder are presented in Figs. I and 2 and in Table I. The particle size distribution was measured by use of the Cenco Photelometer and is shown in Fig. I. This figure shows that the largest particles were 16 ,x, and that approximately 75% of the particles were below g p in size. The average particle size is small, and also it can be seen that the particle size distribution is uniform. J. Less-Common
Metals,
4 (1962)
2-8
POWDER
METALLURGY
OF RHENIUM
Figure 2 is a photograph of this powder at IOOO magnification. The uniform distribution of particle sizes can also be seen in this photograph as well as the extreme
Fig. I. Particle size distribution.
Fig. z. Particles of rhenium powder.
(X
1000).
3
A. R. POSTER
4
TABLE PHYSICAL
AND
CHEMICAL
I
PROPERTIES
OF RHENIUM
Screen size
IOOo/o
Potassium by flame photometry (sensitivity o.oooz”~)
<0.0002y4
Fe
POWDER
mesh
-325
o.oo12°/~
Al
Ca
Mg
<
p.p.m. p.p.m.
I p.p.m.
irregularity of particle shape. In some instances, large agglomerates appear to be made up of many small particles. SAMPLE PREPARATION
Green compacts were formed by mechanical pressure application using a hardened steel die which had a cylindrical cavity of 0.176 in. diameter. Because of the high cost of the powder, only 0.1 g was used for each compact. Pressure was applied double action, and the green compacts were formed without the aid of either die wall or interparticle lubrication. Sintering of the compacts was accomplished in a molybdenum wound muffle furnace at 1750°C f 25’C under a cover gas of purified hydrogen of -20 dew point. The compacts were heated from room temperature to 1750°C over a period of 0.5 h by slow stoking into the furnace hot zone. This was done in order to avoid cracking of the compacts due to thermal shock and/or release of adhered gases. Compacts were formed at various pressures between 7.5 and 50 t.s.i. and sintered for 16 h. Compacts were also formed at 20 t.s.i. and sintered for various times from 0.5 to 16 h. Densities of the green and sintered compacts were taken by air weightmeasured volume method. RESULTS
Tables II and III give the density data for the various samples. This data is also graphed in Figs. 3 and 4. TABLE GREEN
AND
SINTERED
compacting presswe (t.s.i.)
7.5
DENSITY
II VS. COMPACTING
Green density I.%!~)
7.48
PRESSURE
Sirtned
density
f&W)
16.15
20
8.85
x9.08
30
9.80
19.60
40
II.22
19.30
50
II.55
r9.32
The green density curve, in Fig. 3, increases quite regularly with increasing compacting pressure, and appears to become constant at approximately 11.5-1~0 g/ml J. Less-Common Melds,
4 (1962) 2-S
POWDER METALLURGY OF RHENIUM
5
density for pressures greater than 50 t.s.i. This is quite low when compared to other refractory metal powders. For example, tungsten powder, of equal particle size, when compacted at only IO t.s.i. gives a green density of 10.5g/m15. Although the theore-
>.
.?1
= 14. : _ 12.
Compacting
pressure (t.s.i.)
Fig. 3. Green and sintered density as function of compacting pressure.
Fig. 5. Structure
80.
Sintering
time
(h)
Fig. 4. Sintered density vs. time at 1750°C temperature for rhenium compacts.
of sintered rhenium after 0.5 h at 175o’C. grains per mma. (X 1000) J.
Grain count
Less-Common
=
Metals,
~1,600
4 (1962) 2-8
A. R. POSTER
6
tical density for rhenium is slightly higher than for tungsten (19.3 g/ml for tungsten and 21.0 g/ml for rhenium), the difference in density at comparable pressures is significant. Although strength measurements were not performed on the green compacts, it was noted that the compacts were of sufficient strength, even at the lower pressures, to be readily handled. The sintered density curve, which is given on Fig. 3, shows that the final density depends only to a certain extent upon the compacting pressure and to a greater extent upon sintering time. Sintered density increases quite rapidly for the lower
Fig.
6. Structure
of sintered
rhenium after 2.0 h at 1750°C. Grain grains per mma. (X 1000)
TABLE
count
=
15,400
III
SINTERED DENSITY VS. TIME AT 1750°C Sinfering
time(h)
Sintered
dmity (g/ml)
0.3 0.5 2.0
8.85 13.80 15.10 17.02
4.0 16.0
17.35 19.08
0.0 (green)
J. Less-Comnzon Metals, 4 (1962) 2-8
POWDER METALLURGY
OF RHENIUM
7
compacting pressures and appears to reach a maximum at 25-30 t.s.i., while maximum green density is not obtained until approximately 50 t.s.i. pressure. A slight drop in the sintered density is noted for compacting pressures greater than 30 t.s.i. It is not known if this slight density decrease is real or an error in measurement. Since this decrease in sintered density occurs directly after attainment of maximum density, it is likely that there is some connection between these occurrences. It is also possible that entrapment of gases in the green compact is responsible for this decrease. This, however, was not further investigated.
Fig. 7. Structure of sintered rhenium after 16 h at 1750%. Grain count = 10,800 grains per mma. (X 1000)
Sinterecl density as a function of sintering time at 1750°C f 25’C, is shown in Fig. 4 for samples which were compacted at 20 t.s.i. An extremely rapid increase in density is noted for even the first increments of sintering time. This increase is from 8.8 g/ml, for the green compact, to approximately 13.8 g/ml during the first 20 min and to approximately 17.0 g/ml after only 2 h. Density continues to slowly increase for times greater than 2 h and, in these experiments, increased to 19.0 g/ml in 16 h. Figures 5, 6 and 7 are photographs, at IOOOmagnifications, of the metal structure after compaction at 20 t.s.i. and 0.5, 2 and 16 h sintering at 1750°C. Fig. 5 shows that new grains, which are significantly larger than the original powder particles, J. Less-Common Metals,
4 (1962) 2-8
8
A. R.
POSTER
are formed already at 0.5 h. The porosity, which is the dark area, is still associated strongly with the grain boundaries in this photograph. In Fig. 6 the compact has received two hours heating and is approximately 17.8 g/ml dense. The remaining pores are longer, fewer in number and many pores are no longer associated with grain boundaries. Further heating results in increased grain size as shown in Fig. 7. The large number of pores, which are shown in this last figure, indicate that further densification is possible. CONCLUSIONS
Rhenium powder, of the type described, is readily fabricated into green compacts of greater than 60 o/oof theoretical density. Sintering progresses quite rapidly with time, and densification depends to a greater extent on time than on green density. It is unfortunate that only very small compacts could be formed, however, little trouble is foreseen in forming larger parts such as bars for subsequent processing. REFERENCES 1 L. W.
KATES, Materials G Methods, 3g (1954) 88. 2 C. T. SIMS, C. M. CRAIGHEAD AND R. I. JAFFEE, J. Metals, 7, AIME Trans., 203 (1955) 168. 8 G. B. GAINES, C. T. SIMS AND R. I. JAFFBE, J. Electvochem. Sot., 106 (1959) 881. 4 I. E. CAMPBELL. D. M. ROSENBAUM AND B. W. GONSER, .I. Less-Common Metals, I (1959) 185. 6 A. R. POSTER, Factors which affect the compaction of- tungsten powders, Sylvania Elccfvic Products Inc. Rep., Towanda, Pa.
J. Less-Common
Metals,
4 (1962) 2-8