- Email: [email protected]
Focus on hard and refractory materials advances
Focus on hard and refractory materials advances reakthroughs in nanomaterials, new wear-resistant cermets and the development of injection moulding techniques for rhenium were among the refractory metal, hardmetal and ceramics highlights at pM2TEC 2001. This large and well-attended annual 'banquet' for powder metallurgy (PM) and particulate materials advances, jointly organized by the Metal Powder Industries Federation and APMI International, was held in New Orleans, LA, USA, 13-17 May. With as many as seven simultaneous technical sessions, as well as the concurrent exhibition and a wide range of poster presentations, the following account is necessarily a selection from those sessions personally attended by the writer.
B
NanocrystaUine The project introduced by James Colaizzi of NanoPac Technologies (New Brunswick, NJ) by means of a paper with the mundane title 'Reduced pressure requirements for production of nanoceramic composites', combined two recent breakthroughs in nanomaterials research. These are the rapid solidification processing of metastable nanoceramic solid solutions and low-pressure transformation
A
C
I
I I AI203
Liquid
1900[
A!aOa + A!a0a:Ti0a
. . . . I + I3"AI203"TiO2
' +liq3d
1700I-
~~
+ liquid
AI203 + AI203.TiO 2
Alumina
I
I
I
I
I
wt% titania
I
I
I
I
Titania
Figure 1: Pseudobinary phase diagram of the Al203/TiO 2 system (after Coliazzi).
~4
MPR September
2001
assisted consolidation (TAC) of the resulting powders. A new generation of dense nanocomposites comprises a homogeneous network of two ultrafine ceramic phases, each of which retards the grain growth of the other. Normal TAC employs metastable starting materials, typically singlephase, and requires very high sintering pressures of up to 8 GPa. Because of the high pressures, scale-up to commercial components and full-size mechanical test specimens was expected to be very difficult but, surprisingly, muhiphase products gave similar ultrafine structures after processing at or near atmospheric pressure. Extremely complex microstructures can be developed by the various techniques described in the paper, for example up to five coexisting phases for 60A12OJ40TiO 2 in the relatively simple AI2OJTiO 2 systerd (Figure 1). One possible series of reactions in a more complex system is shown in
100 Precipit!ted
C°arseningFE
¢,1
20
B y "//Clustering
A Melt
,
Time (non-linear) IA
(homogeneousliquid)
I
Figure 2: Schematic of metamorphosis of metastable tetragonal parent structure YSZA (yttrium-stabilized zirconia-alumina) to alumina + monoclinic YSZ + stable tetragonal YSZ 'empty nest' parent state. A: Homogeneous liquid. B: Micron-sized crystals form during quenching. Very small regions with slightly higher alumina concentrations exist (represented as circles). C: Alumina-rich clusters form that are crystaUographically coherent and undetectable by X-ray diffraction, but stress the lattice enough to define coherent diffracting domains of 80 ~,. D: The fully solidified material begins the sintering process. The clusters precipitate into distinct alumina crystal lattices and grow to a size that can be detected by X-ray diffraction. The change of line spacing of the matrix represents the change in its c/a ratio as alumina is expelled. E: The alumina particles grow and consume most of the remaining alumina from the parent material. Also, some of the YSZ transforms from the tetragonal to the monoclinic structure. (After Coliazzi).
oo26-o657/m/$ - see front matter© 2ool ElsevierScienceLtd. All rights reserved.
The surface of the forged sample is smooth, without such defects as cracks and voids, indicating good bulk flow through the high-speed superplasticity of nanocrystalline PSZ-20mol%A1203. Answering an enquiry from the audience, the author said that no grain growth had been observed.
discharge
Electric source
Tungsten and r h e n i u m The effect of nickel content on the activated sintering of submicron tungsten powder contacts was investigated and reported by Dong Mook Won of Rapidus Inc (Seoul, Korea). The research was rather more restrictive than implied by the paper's title, since the range of nickel additions was 0.02 to 0.4 wt%, and the main effects considered (largely by dilatometer measurements) were densification and grain growth. The objective was to improve the workability and sinterability of nickel-doped tungsten. Supplied by Korea Tungsten, the tungsten powder had an average particle size of 0.57 pm, polyhedral shape and minimum purity of 99.9%. Nickel was added as a salt solution, then dried and reduced to metal. Powders were initially pressed in a cylindrical die, then cold isostatically repressed in a rubber mould to remove anisotropy. Linear shrinkage versus temperature curves obtained from the dilatometer during heating to 1400 °C showed that sintering was very sensitive to both heating rate and to the proportion of nickel dopant (Figure 5, a and b). Microstructure was also highly
7 7 7Z
Figure 3: Integrated high-speed nanoprocessing for the intelligent sintering and near-net-shape forming of bulk nanocrystalline ceramics. (After Hiroshi Kimura.) t.oI Fully dense
mol%A[203 o.8[PSZ-20 1466 K 115 MPa o.6F
E g #
0.4[
0"21 t~oo
t2'oo 14'oo Surfacetemperature, T(K)
Figure 4: Compressive true strain for the full-density PSZ-2omol% AI203 specimen with lo mm height and lo mm diameter, as synthesized at 1466 K under the initial applied pressure of 115 MPa. (After Hiroshi Kimura.) more detail in Figure 2, but the interested reader is strongly advised to read the full paper when published. When developed, the new process could become a highly versatile production method for advanced ultrafine ceramics. Yezdi B Pithawalla of Virginia Commonwealth University described a method for the preparation and characterization of ultrafine powders of intermetallic nanoparticles. The paper, whilst emphasizing the improved toughness and strength of ultrafine microstructures, had particular relevance to iron aluminide. Surprisingly, nanosized
www.metal-powder.net
FeA1 with <30 nm grain size was found to be magnetic, though more conventionally sized powders and sintered products are non-magnetic. Hiroshi Kimura of Japan's National Defence Academy dealt with the high-speed superplastic forming of consolidated nanocrystalline PSZ-AlzO 3. Pulse-current heating was used to consolidate amorphous PSZ (partially stabilized zirconia)-20mol%alumina powder in an electric field, to synthesize the bulk nanocrystalline product (Figure 3), and then to conduct the superplastic near-netshape forming of a disc. For a 10 mm high sample compressed at 115 MPa, a temperature of 1466 K is needed to achieve full density; as pressure is increased to 150 MPa and height decreased to 5 mm, the temperature needed for full density decreases to 1390 K (Figure 4).
g c" -~ F-
18 (a) .........1°C.min ..... -, 14 ~ 3"C.min" 12 - - 5°Crnin-1 I n ~ lO°C-min"1 8 20"c'min''
...... (dff
16
i lll/
IIII i IIII
////
o~"18
(b)
._.16 8,14 12 t- 10 6 ~ •-
0400 600
800
1000 1200 1400
Temperature (°C)
4 2 0400
600
800
1000 1200 1400
Temperature (°C)
Figure 5: Sintering shrinkage curves obtained by dilatometry during the heating-up stage with different heating rates for (a) W-o.6wt%Ni and (b) W-o.4wt%Ni. (After Dong Mook Won.)
M P R S e p t e m b e r 2001
15
dependent on these parameters, with substantial grain growth at the lowest heating rates of 1 to 3 °C.min'l, especially for the specimens with 0.06%Ni. With 0.4%Ni, the tests showed neither the excessive grain growth for low heating rates nor the retention of fine microstructure with the higher rates. C.M. Wang of Concurrent Technologies Corp (Johnstown, PA) investigated powder injection moulding (PIM) to fabricate tungsten and rhenium components. In this research project, the researcher used tungsten - which has generally similar properties as a comparatively inexpensive 'stand-in' for rhenium. Specific gravity values and melting points are similar, 19.3 and 3410 °C for tungsten, 21.1 and 3180 °C for rhenium. With its near-net-shape characteristics, PIM is a promising route for the fabrication of small and even medium-sized components in high-priced metals like rhenium, and moreover offers good and consistent properties. Contrasting with typical commercial practice, which favours large batches, a technique was developed for mixing and moulding relatively small amounts of rhenium feedstock. This was tested with commercially pure tungsten (99.99% purity, 1.5 pm grain size) before trials with the high-priced spherical rhenium powder. Binder was a wax-polymer composite, combining paraffin wax, polyolefin polymer, carnauba wax and stearic acid. In debinding, the paraffin wax is removed first, then the carnauba wax and finally the polyolefin polymer.
± Plunger and barrel
Figure 6: Schematic of capillary rheometry (after C.M. Wang).
With the aid of a computer program and flow-network model, results from capillary rheometry (Figure 6) were successfully correlated with moulding pressure in the scale-up manufacturing process. One of the first parts to be made with the W feedstock was a typically shaped tensile specimen 68 mm long, 19 mm wide and 3 mm thick. Moulding pressure, gate size and moulding temperature could all be defined by reference to the viscosity data obtained from the rheoraetry. Follow-up sintering tests were said to be in progress. Further discussion of advances in rhenium powder metallurgy (PM) will be covered in a separate article on the work of Rhenium Alloys Inc in a later issue. Cemented carbides
Some years ago, US carbide manufacturer Newcomer invented, patented and commercialized a hardmetal in which spherical agglomerates of a hard, finegrained, wear-resistant WC/Co composition were dispersed in a much tougher matrix of coarse WC/Co. That was indeed a novel concept and was described and illustrated in some detail by the writer in Metal Powder Report (April 1995, pp. 14-17).
wt% 6
~o
3
.659
lo.9
~1
~8
3
"~34o
I3.4
16
25
3
1~39
~6.7
AfterXin Deng
~L6
MPR September 2ool
Die
A team of researchers from the University of Alabama, Smith International and Rogers Tool Works (part of the Kennametal organization) have now developed - and patented - a hardmetal in which agglomerates of a hard, fine-grained, wear-resistant W C / C o composition are dispersed in a much tougher matrix of metallic cobalt. This was presented by Xin Deng of the University of Alabama as 'Double cemented carbide composite - a novel composite concept'. The earlier work mentioned above is not included in this paper's list of references and the author claimed to have been unaware of the Newcomer developments. Let's not be churlish, however. The paper records some excellent research activity and includes useful property data and microstructural photographs. Presumably - though not necessarily - the metal-matrix material will be more shock-resistant and resistant to cracking than Newcomer's, though with less resistance to abrasive wear. It would be interesting to see both types of materials tested and evaluated under near-identical conditions. Microstructures of the two classes of sintered hardmetal are quite distinctive. Table 1 gives properties of the three kinds of granules tested in the present work. They were sorted by sieving into five size classes, then hot-pressed with 30 vol% additional cobalt binder. Abrasive wear was measured according to ASTM B611 and ASTM G65 for high and low stresses respectively. Figure 7 shows that 'DC carbide' had higher fracture toughness than
www.metal-powder.net
Ceramics and cermets 50
Granule Co vol%:
45 E 13_ o
,o10
4O
18
35
Larger MFP ~ .,,,,,~,~,,~
30 25
U~
=~ 20 ¢-
o~ 15 ~.
to
/
Conventional c e m e n t e d carbide
-
5,
I
I
I
I
I
10
20
30
40
50
60
Total Co content (vol%) Figure 7: Effect of total cobalt content on the toughness of DC carbide
(after Xin Deng).
45 40 35 u) .~ ~ 30 o~E 25 O
Granule size increases
~'
20
• ~ "5 m
15 10 5
LL
o
o
•
•
Granule co vol%:
•
* 10
• 18
25 • 18 - for granule size effect
Conventional cemented carbide 1'0 15 High stress wear resistance (krev.cm -3)
20
Figure 8: Relationship between fracture toughness and high-stress wear resistance of DC carbide (after Xin Deng).
lO • > O o
8
E-
7
E E
o
6 5 4
E '-s
3 2
O
9
o
Granule Co vol%:
4, 10 18 •
Conventional cemented carbide
10
25
~
~ ,...-----
20 30 Total Co content (vol%)
40
50
Figure 9: Effect of total Co content on low-stress wear resistance of DC carbide
(after Xin Deng).
conventional carbide with the same total cobalt binder content. Figure 8 gives an indication of the relationship between fracture toughness and high-stress wear resistance, whilst Figure 9 describes the effect of total cobalt on low-stress wear. The presenter pointed out that, by altering any of a number of parameters to emphasize
www.metal-powder.net
specific mechanical properties, the material could be optimized for particular applications. In the ensuing discussion, Xin Deng also said that future work could include a heat-treatable steel matrix. It would be interesting to see whether the precipitated carbides within such a steel matrix would bring it within the ambit of Newcomer's patents.
The user's choice of wear-resistant cermets has been further expanded, as illustrated by an investigation into the abrasive wear properties of Cr-Cr3Si and Cr-Cr3Si-X composites, reported by Joseph W Newkirk of the University of Missouri-Rolla. Composites in this system of metal-bonded refractory metal silicides are characterized by hardness, high stiffness, good thermal conductivity, chemical resistance and resistance to hightemperature creep and oxidation. Expansivity is similar to that of chromium and the sintered material is machinable by EDM. It was originally intended for jet-engine components, but properties were not sufficiently in advance of currently used materials. The target application is now abrasionresistant parts. Production is by mechanical alloying, then hot pressing in an inert argon atmosphere to 92-98% of theoretical density. The liquid phase forms rapidly, necessitating tight control of sintering temperature. Other alloying elements, such as titanium and erbium (as up to 10% erbia, Er203) have also been added to improve selected properties. Up to 10% by volume of such carbides as TiC, NbC and SiC (especially NbC) made a significant improvement to Vickers hardness, with little effect on Kk fracture toughness. Pin-on-drum abrasion tests were carried out against comparatively soft 150 grit garnet and the much harder 400 grit silicon carbide. The basic 75Cr3Si/25Cr composition has been found to give good resistance to wear from both abrasives. Carbide additions improved performance against the softer garnet, but not against SiC.
Mechanical alloying Isaac Chang of Birmingham University described the 'Effect of nickel additions to the formation and crystallization of mechanically alloyed amorphous FeZrB powders'. Employing a high-energy
MPR
September
20ol
~7
Fritsch P5 planetary ball-mill, the research programme began in the 1970s, initially for ODS alloys but later extended to other metals, semiconductors and ceramics. Either elemental or pre-alloyed powders can be used as input. Recently, mechanical alloying has been applied to amorphous alloy powders with large zones of supercooled liquid. These can be consolidated into bulk form without crystallization.
4
Starting materials were elemental powders of iron, nickel, boron and pre-alloyed Zr70/Ni30 (wt%), with particle sizes between 25 and 120 IJm. Nominal compositions of the alloys chosen for study are listed in Table 2. Milling time was varied between 30 minutes and 27 hours, with a 10 minute pause after every 10 minutes' milling, to avoid overheating. Examination of the samples was mainly by X-ray diffraction and transmission electron microscopy. It was found that partial replacement of Fe with Ni in FeZrB enhances grain refinement, accelerates the 'amorphization' process and improves the thermal stability of the amorphous structure. Melting points were measured by differential scanning calorimetry (DSC) and a metastable phase diagram was
established for the amorphous alloy powder (Figure 10). Crystalline phases found in samples after annealing at various temperatures within DSC are listed in Table 3. Sylva Turenne of Montreal's Ecole Polytechnique introduced a paper with a long name but considerable technical importance, on the 'Improvement of thermoelectric properties of bismuth telluride alloys by controlling the microstructure through mechanical alloying and extrusion'. Bismuth telluride, Bi2T%, with variable additions of bismuth selenide, Bi2S%, is a semiconductor used to convert heat into electricity, or to absorb heat by passing a current (the Peltier effect), providing refrigeration without the use of gases that could harm the environment or be inaccessible in space.
~e 745
3
558 783
s8
MPR September 2ool.
www.metal-powder.net
The conventional production method is by directional solidification of a crucible of molten alloy, employing the so-called Bridgman technique. This method produces a single crystal with the desired directional properties, but suffers from segregation, brittleness of the crystal and low productivity. PM, on the other hand, shows considerable promise as an alternative, following the sequence mechanical alloying, pressing, sintering, high-temperature extrusion and annealing. Mechanical alloying was carried out in a 10-litre Union Process 1-S attritor, 20 cm in diameter and running at (typically) 440 rev/min. Input was pellets of pure bismuth, tellurium and selenium totalling 2.7 kg, with a small amount (nearly 500 ppm) of SbI 3 to optimize the charge carrier concentration. The agitator arms were of stainless steel and the 6.35 mm diameter bails of chromium steel, from which contamination was insufficient
to affect thermoelectric properties. The initially brittle materials were rapidly reduced in grain size and microstructures were examined after 30, 60 and 180 minutes. Homogeneity was improved at the longer times, producing an alloy (confirmed by X-ray diffraction) of Bi2Te2.s5Se0j 5. The alloyed powders were extracted under inert gas, pressed by single action at 285 MPa and sintered in argon. After 16 hours at temperatures up to 400 °C, sintered density was about 80%. Recent experiments indicate that the pressing and sintering steps may not be necessary, since similar results are obtained when proceeding directly to high-temperature extrusion. Extrusion elongates the grains in the extrusion direction by plastic flow along slip planes. There was little variation in thermoelectric properties with extrusion temperatures between 440 and 490 °C. The greatest variation was found with orientation of test specimens
I
com~unds i~.._ I
Tm
t
I o~-Fe+ 7-FeNi + I
i i
i ~,-FeNi + -1- compounds
t--
t
. m
ct<
t
-Am+ ~--Fe
~ Am + ),-FeNi .........~ ....................!............................................. Tx
j 4 .....................-I-
~
J~-
s
Tg
phase (Am) L
2
3
4
Ni increases Figure 10: Schematic metastable phase diagram showing the . phase transformation path as a function of Ni alloying content. 1: Fe63/Ni7/Zr2o/B2o; 2: Fe56/Nila/Zrlo/B2o; 3: Fe49/Ni2JZrm/B2o; 4: Fe4JNi28/Zqo/B2o. Small horizontal lines represent the various annealing temperatures used in the study. (After Chang.)
with respect to extrusion direction. In general, thermoelectric properties from the PM route were close to those produced by the far less efficient process of directional solidification.
Kenneth J A Brookes Consulting Editor
•
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MPR September 2ool
19
B
NanocrystaUine The project introduced by James Colaizzi of NanoPac Technologies (New Brunswick, NJ) by means of a paper with the mundane title 'Reduced pressure requirements for production of nanoceramic composites', combined two recent breakthroughs in nanomaterials research. These are the rapid solidification processing of metastable nanoceramic solid solutions and low-pressure transformation
A
C
I
I I AI203
Liquid
1900[
A!aOa + A!a0a:Ti0a
. . . . I + I3"AI203"TiO2
' +liq3d
1700I-
~~
+ liquid
AI203 + AI203.TiO 2
Alumina
I
I
I
I
I
wt% titania
I
I
I
I
Titania
Figure 1: Pseudobinary phase diagram of the Al203/TiO 2 system (after Coliazzi).
~4
MPR September
2001
assisted consolidation (TAC) of the resulting powders. A new generation of dense nanocomposites comprises a homogeneous network of two ultrafine ceramic phases, each of which retards the grain growth of the other. Normal TAC employs metastable starting materials, typically singlephase, and requires very high sintering pressures of up to 8 GPa. Because of the high pressures, scale-up to commercial components and full-size mechanical test specimens was expected to be very difficult but, surprisingly, muhiphase products gave similar ultrafine structures after processing at or near atmospheric pressure. Extremely complex microstructures can be developed by the various techniques described in the paper, for example up to five coexisting phases for 60A12OJ40TiO 2 in the relatively simple AI2OJTiO 2 systerd (Figure 1). One possible series of reactions in a more complex system is shown in
100 Precipit!ted
C°arseningFE
¢,1
20
B y "//Clustering
A Melt
,
Time (non-linear) IA
(homogeneousliquid)
I
Figure 2: Schematic of metamorphosis of metastable tetragonal parent structure YSZA (yttrium-stabilized zirconia-alumina) to alumina + monoclinic YSZ + stable tetragonal YSZ 'empty nest' parent state. A: Homogeneous liquid. B: Micron-sized crystals form during quenching. Very small regions with slightly higher alumina concentrations exist (represented as circles). C: Alumina-rich clusters form that are crystaUographically coherent and undetectable by X-ray diffraction, but stress the lattice enough to define coherent diffracting domains of 80 ~,. D: The fully solidified material begins the sintering process. The clusters precipitate into distinct alumina crystal lattices and grow to a size that can be detected by X-ray diffraction. The change of line spacing of the matrix represents the change in its c/a ratio as alumina is expelled. E: The alumina particles grow and consume most of the remaining alumina from the parent material. Also, some of the YSZ transforms from the tetragonal to the monoclinic structure. (After Coliazzi).
oo26-o657/m/$ - see front matter© 2ool ElsevierScienceLtd. All rights reserved.
The surface of the forged sample is smooth, without such defects as cracks and voids, indicating good bulk flow through the high-speed superplasticity of nanocrystalline PSZ-20mol%A1203. Answering an enquiry from the audience, the author said that no grain growth had been observed.
discharge
Electric source
Tungsten and r h e n i u m The effect of nickel content on the activated sintering of submicron tungsten powder contacts was investigated and reported by Dong Mook Won of Rapidus Inc (Seoul, Korea). The research was rather more restrictive than implied by the paper's title, since the range of nickel additions was 0.02 to 0.4 wt%, and the main effects considered (largely by dilatometer measurements) were densification and grain growth. The objective was to improve the workability and sinterability of nickel-doped tungsten. Supplied by Korea Tungsten, the tungsten powder had an average particle size of 0.57 pm, polyhedral shape and minimum purity of 99.9%. Nickel was added as a salt solution, then dried and reduced to metal. Powders were initially pressed in a cylindrical die, then cold isostatically repressed in a rubber mould to remove anisotropy. Linear shrinkage versus temperature curves obtained from the dilatometer during heating to 1400 °C showed that sintering was very sensitive to both heating rate and to the proportion of nickel dopant (Figure 5, a and b). Microstructure was also highly
7 7 7Z
Figure 3: Integrated high-speed nanoprocessing for the intelligent sintering and near-net-shape forming of bulk nanocrystalline ceramics. (After Hiroshi Kimura.) t.oI Fully dense
mol%A[203 o.8[PSZ-20 1466 K 115 MPa o.6F
E g #
0.4[
0"21 t~oo
t2'oo 14'oo Surfacetemperature, T(K)
Figure 4: Compressive true strain for the full-density PSZ-2omol% AI203 specimen with lo mm height and lo mm diameter, as synthesized at 1466 K under the initial applied pressure of 115 MPa. (After Hiroshi Kimura.) more detail in Figure 2, but the interested reader is strongly advised to read the full paper when published. When developed, the new process could become a highly versatile production method for advanced ultrafine ceramics. Yezdi B Pithawalla of Virginia Commonwealth University described a method for the preparation and characterization of ultrafine powders of intermetallic nanoparticles. The paper, whilst emphasizing the improved toughness and strength of ultrafine microstructures, had particular relevance to iron aluminide. Surprisingly, nanosized
www.metal-powder.net
FeA1 with <30 nm grain size was found to be magnetic, though more conventionally sized powders and sintered products are non-magnetic. Hiroshi Kimura of Japan's National Defence Academy dealt with the high-speed superplastic forming of consolidated nanocrystalline PSZ-AlzO 3. Pulse-current heating was used to consolidate amorphous PSZ (partially stabilized zirconia)-20mol%alumina powder in an electric field, to synthesize the bulk nanocrystalline product (Figure 3), and then to conduct the superplastic near-netshape forming of a disc. For a 10 mm high sample compressed at 115 MPa, a temperature of 1466 K is needed to achieve full density; as pressure is increased to 150 MPa and height decreased to 5 mm, the temperature needed for full density decreases to 1390 K (Figure 4).
g c" -~ F-
18 (a) .........1°C.min ..... -, 14 ~ 3"C.min" 12 - - 5°Crnin-1 I n ~ lO°C-min"1 8 20"c'min''
...... (dff
16
i lll/
IIII i IIII
////
o~"18
(b)
._.16 8,14 12 t- 10 6 ~ •-
0400 600
800
1000 1200 1400
Temperature (°C)
4 2 0400
600
800
1000 1200 1400
Temperature (°C)
Figure 5: Sintering shrinkage curves obtained by dilatometry during the heating-up stage with different heating rates for (a) W-o.6wt%Ni and (b) W-o.4wt%Ni. (After Dong Mook Won.)
M P R S e p t e m b e r 2001
15
dependent on these parameters, with substantial grain growth at the lowest heating rates of 1 to 3 °C.min'l, especially for the specimens with 0.06%Ni. With 0.4%Ni, the tests showed neither the excessive grain growth for low heating rates nor the retention of fine microstructure with the higher rates. C.M. Wang of Concurrent Technologies Corp (Johnstown, PA) investigated powder injection moulding (PIM) to fabricate tungsten and rhenium components. In this research project, the researcher used tungsten - which has generally similar properties as a comparatively inexpensive 'stand-in' for rhenium. Specific gravity values and melting points are similar, 19.3 and 3410 °C for tungsten, 21.1 and 3180 °C for rhenium. With its near-net-shape characteristics, PIM is a promising route for the fabrication of small and even medium-sized components in high-priced metals like rhenium, and moreover offers good and consistent properties. Contrasting with typical commercial practice, which favours large batches, a technique was developed for mixing and moulding relatively small amounts of rhenium feedstock. This was tested with commercially pure tungsten (99.99% purity, 1.5 pm grain size) before trials with the high-priced spherical rhenium powder. Binder was a wax-polymer composite, combining paraffin wax, polyolefin polymer, carnauba wax and stearic acid. In debinding, the paraffin wax is removed first, then the carnauba wax and finally the polyolefin polymer.
± Plunger and barrel
Figure 6: Schematic of capillary rheometry (after C.M. Wang).
With the aid of a computer program and flow-network model, results from capillary rheometry (Figure 6) were successfully correlated with moulding pressure in the scale-up manufacturing process. One of the first parts to be made with the W feedstock was a typically shaped tensile specimen 68 mm long, 19 mm wide and 3 mm thick. Moulding pressure, gate size and moulding temperature could all be defined by reference to the viscosity data obtained from the rheoraetry. Follow-up sintering tests were said to be in progress. Further discussion of advances in rhenium powder metallurgy (PM) will be covered in a separate article on the work of Rhenium Alloys Inc in a later issue. Cemented carbides
Some years ago, US carbide manufacturer Newcomer invented, patented and commercialized a hardmetal in which spherical agglomerates of a hard, finegrained, wear-resistant WC/Co composition were dispersed in a much tougher matrix of coarse WC/Co. That was indeed a novel concept and was described and illustrated in some detail by the writer in Metal Powder Report (April 1995, pp. 14-17).
wt% 6
~o
3
.659
lo.9
~1
~8
3
"~34o
I3.4
16
25
3
1~39
~6.7
AfterXin Deng
~L6
MPR September 2ool
Die
A team of researchers from the University of Alabama, Smith International and Rogers Tool Works (part of the Kennametal organization) have now developed - and patented - a hardmetal in which agglomerates of a hard, fine-grained, wear-resistant W C / C o composition are dispersed in a much tougher matrix of metallic cobalt. This was presented by Xin Deng of the University of Alabama as 'Double cemented carbide composite - a novel composite concept'. The earlier work mentioned above is not included in this paper's list of references and the author claimed to have been unaware of the Newcomer developments. Let's not be churlish, however. The paper records some excellent research activity and includes useful property data and microstructural photographs. Presumably - though not necessarily - the metal-matrix material will be more shock-resistant and resistant to cracking than Newcomer's, though with less resistance to abrasive wear. It would be interesting to see both types of materials tested and evaluated under near-identical conditions. Microstructures of the two classes of sintered hardmetal are quite distinctive. Table 1 gives properties of the three kinds of granules tested in the present work. They were sorted by sieving into five size classes, then hot-pressed with 30 vol% additional cobalt binder. Abrasive wear was measured according to ASTM B611 and ASTM G65 for high and low stresses respectively. Figure 7 shows that 'DC carbide' had higher fracture toughness than
www.metal-powder.net
Ceramics and cermets 50
Granule Co vol%:
45 E 13_ o
,o10
4O
18
35
Larger MFP ~ .,,,,,~,~,,~
30 25
U~
=~ 20 ¢-
o~ 15 ~.
to
/
Conventional c e m e n t e d carbide
-
5,
I
I
I
I
I
10
20
30
40
50
60
Total Co content (vol%) Figure 7: Effect of total cobalt content on the toughness of DC carbide
(after Xin Deng).
45 40 35 u) .~ ~ 30 o~E 25 O
Granule size increases
~'
20
• ~ "5 m
15 10 5
LL
o
o
•
•
Granule co vol%:
•
* 10
• 18
25 • 18 - for granule size effect
Conventional cemented carbide 1'0 15 High stress wear resistance (krev.cm -3)
20
Figure 8: Relationship between fracture toughness and high-stress wear resistance of DC carbide (after Xin Deng).
lO • > O o
8
E-
7
E E
o
6 5 4
E '-s
3 2
O
9
o
Granule Co vol%:
4, 10 18 •
Conventional cemented carbide
10
25
~
~ ,...-----
20 30 Total Co content (vol%)
40
50
Figure 9: Effect of total Co content on low-stress wear resistance of DC carbide
(after Xin Deng).
conventional carbide with the same total cobalt binder content. Figure 8 gives an indication of the relationship between fracture toughness and high-stress wear resistance, whilst Figure 9 describes the effect of total cobalt on low-stress wear. The presenter pointed out that, by altering any of a number of parameters to emphasize
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specific mechanical properties, the material could be optimized for particular applications. In the ensuing discussion, Xin Deng also said that future work could include a heat-treatable steel matrix. It would be interesting to see whether the precipitated carbides within such a steel matrix would bring it within the ambit of Newcomer's patents.
The user's choice of wear-resistant cermets has been further expanded, as illustrated by an investigation into the abrasive wear properties of Cr-Cr3Si and Cr-Cr3Si-X composites, reported by Joseph W Newkirk of the University of Missouri-Rolla. Composites in this system of metal-bonded refractory metal silicides are characterized by hardness, high stiffness, good thermal conductivity, chemical resistance and resistance to hightemperature creep and oxidation. Expansivity is similar to that of chromium and the sintered material is machinable by EDM. It was originally intended for jet-engine components, but properties were not sufficiently in advance of currently used materials. The target application is now abrasionresistant parts. Production is by mechanical alloying, then hot pressing in an inert argon atmosphere to 92-98% of theoretical density. The liquid phase forms rapidly, necessitating tight control of sintering temperature. Other alloying elements, such as titanium and erbium (as up to 10% erbia, Er203) have also been added to improve selected properties. Up to 10% by volume of such carbides as TiC, NbC and SiC (especially NbC) made a significant improvement to Vickers hardness, with little effect on Kk fracture toughness. Pin-on-drum abrasion tests were carried out against comparatively soft 150 grit garnet and the much harder 400 grit silicon carbide. The basic 75Cr3Si/25Cr composition has been found to give good resistance to wear from both abrasives. Carbide additions improved performance against the softer garnet, but not against SiC.
Mechanical alloying Isaac Chang of Birmingham University described the 'Effect of nickel additions to the formation and crystallization of mechanically alloyed amorphous FeZrB powders'. Employing a high-energy
MPR
September
20ol
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Fritsch P5 planetary ball-mill, the research programme began in the 1970s, initially for ODS alloys but later extended to other metals, semiconductors and ceramics. Either elemental or pre-alloyed powders can be used as input. Recently, mechanical alloying has been applied to amorphous alloy powders with large zones of supercooled liquid. These can be consolidated into bulk form without crystallization.
4
Starting materials were elemental powders of iron, nickel, boron and pre-alloyed Zr70/Ni30 (wt%), with particle sizes between 25 and 120 IJm. Nominal compositions of the alloys chosen for study are listed in Table 2. Milling time was varied between 30 minutes and 27 hours, with a 10 minute pause after every 10 minutes' milling, to avoid overheating. Examination of the samples was mainly by X-ray diffraction and transmission electron microscopy. It was found that partial replacement of Fe with Ni in FeZrB enhances grain refinement, accelerates the 'amorphization' process and improves the thermal stability of the amorphous structure. Melting points were measured by differential scanning calorimetry (DSC) and a metastable phase diagram was
established for the amorphous alloy powder (Figure 10). Crystalline phases found in samples after annealing at various temperatures within DSC are listed in Table 3. Sylva Turenne of Montreal's Ecole Polytechnique introduced a paper with a long name but considerable technical importance, on the 'Improvement of thermoelectric properties of bismuth telluride alloys by controlling the microstructure through mechanical alloying and extrusion'. Bismuth telluride, Bi2T%, with variable additions of bismuth selenide, Bi2S%, is a semiconductor used to convert heat into electricity, or to absorb heat by passing a current (the Peltier effect), providing refrigeration without the use of gases that could harm the environment or be inaccessible in space.
~e 745
3
558 783
s8
MPR September 2ool.
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The conventional production method is by directional solidification of a crucible of molten alloy, employing the so-called Bridgman technique. This method produces a single crystal with the desired directional properties, but suffers from segregation, brittleness of the crystal and low productivity. PM, on the other hand, shows considerable promise as an alternative, following the sequence mechanical alloying, pressing, sintering, high-temperature extrusion and annealing. Mechanical alloying was carried out in a 10-litre Union Process 1-S attritor, 20 cm in diameter and running at (typically) 440 rev/min. Input was pellets of pure bismuth, tellurium and selenium totalling 2.7 kg, with a small amount (nearly 500 ppm) of SbI 3 to optimize the charge carrier concentration. The agitator arms were of stainless steel and the 6.35 mm diameter bails of chromium steel, from which contamination was insufficient
to affect thermoelectric properties. The initially brittle materials were rapidly reduced in grain size and microstructures were examined after 30, 60 and 180 minutes. Homogeneity was improved at the longer times, producing an alloy (confirmed by X-ray diffraction) of Bi2Te2.s5Se0j 5. The alloyed powders were extracted under inert gas, pressed by single action at 285 MPa and sintered in argon. After 16 hours at temperatures up to 400 °C, sintered density was about 80%. Recent experiments indicate that the pressing and sintering steps may not be necessary, since similar results are obtained when proceeding directly to high-temperature extrusion. Extrusion elongates the grains in the extrusion direction by plastic flow along slip planes. There was little variation in thermoelectric properties with extrusion temperatures between 440 and 490 °C. The greatest variation was found with orientation of test specimens
I
com~unds i~.._ I
Tm
t
I o~-Fe+ 7-FeNi + I
i i
i ~,-FeNi + -1- compounds
t--
t
. m
ct<
t
-Am+ ~--Fe
~ Am + ),-FeNi .........~ ....................!............................................. Tx
j 4 .....................-I-
~
J~-
s
Tg
phase (Am) L
2
3
4
Ni increases Figure 10: Schematic metastable phase diagram showing the . phase transformation path as a function of Ni alloying content. 1: Fe63/Ni7/Zr2o/B2o; 2: Fe56/Nila/Zrlo/B2o; 3: Fe49/Ni2JZrm/B2o; 4: Fe4JNi28/Zqo/B2o. Small horizontal lines represent the various annealing temperatures used in the study. (After Chang.)
with respect to extrusion direction. In general, thermoelectric properties from the PM route were close to those produced by the far less efficient process of directional solidification.
Kenneth J A Brookes Consulting Editor
•
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