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Effects of powder mixing technique and tungsten powder size on the properties of tungsten heavy alloys
Journal of Materials Processing Technology 103 (2000) 288±292
Effects of powder mixing technique and tungsten powder size on the properties of tungsten heavy alloys S. Eroglu*, T. Baykara TUBITAK-MRC, Materials Research Department, PO Box 21, Gebze-Kocaeli 41470, Turkey Accepted 15 December 1999
Abstract Properties of tungsten heavy alloys are known to vary with process parameters such as sintering temperature, time and atmosphere. In this study, tensile properties of the 92.5W±5.25Ni±2.25Fe and 90W±7Ni±3Fe heavy alloys were found to be in¯uenced by powder mixing technique and tungsten powder size. For a given tungsten powder grade (mean particle size 10.5 or 3.4 mm), attritor milled alloys exhibited better tensile properties than those of the turbula-mixed alloys. Compared to ®ner grade, coarser tungsten powder generally resulted in alloys with higher ductility when turbula mixer was used. Overall, the alloys prepared from turbula mixed powders containing ®ner tungsten grade showed the lowest tensile properties. Metallographic analyses revealed that these alloys failed in more brittle fashion as a result of a high degree of contiguity of tungsten grains, which was inherited from powder mixing step. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Liquid-phase sintering; Tungsten heavy alloys; Powder mixing; Powder size
1. Introduction
2. Experimental procedures
Tungsten heavy alloys possess a unique combination of high density, ductility, strength and toughness. They are used extensively as kinetic energy penetrators, counterbalance weights and radiation shields. The alloys are produced from W, Ni, Fe and Cu powders by liquid-phase sintering in vacuum and/or in hydrogen atmosphere. Sintering in the presence of a liquid phase at temperatures about 15008C leads to fully dense alloys. However, full densi®cation does not ensure good mechanical properties, which are known to be very sensitive to processing variables. The variables studied [1±12] include sintering temperature, sintering time, sintering atmosphere, powder characteristics, heat-treatment temperature. Scant attention has been paid to powder mixing and powder size. The work reported in the present paper was carried out in order to investigate effects of powder mixing technique and tungsten powder size on the tensile properties and microstructures of 90W±7Ni±3Fe and 92.5W±5.25Ni± 2.25Fe heavy alloys.
2.1. Sample preparation
*
Corresponding author.
Elemental powders of tungsten (H.C. Starck), carbonyl nickel (Poudmet) and iron (Poudmet) were used to produce the alloys. Ni and Fe powders had Fisher particle sizes of 3 and 4.5 mm, respectively. Two grades of tungsten powders (HC150 and HC500) were employed. Mean particle sizes of HC500 and HC150 grades as measured with a sedigraph were 10.5 and 3.4 mm, respectively. Powders were weighed to give the compositions of 90%W±7%Ni±3%Fe and 92.5%W±5.25%Ni±2.25%Fe. Powder mixtures of 450 g were prepared for each tungsten grade using turbula mixer or attritor mill (Table 1). Dryturbula mixing was carried out at a speed of 85 rpm in a glass container (700 cm3) for 3 h. Zirconia balls with a diameter of 9 mm were charged with powders to reduce agglomeration and caking of the powders. The ball to powder weight ratio was kept at 1:10. Wet-attritor milling was carried out in ethyl alcohol (200 cm3) in a stainless steel tank (1300 cm3) with a WC arm at a speed of 400 rpm for 3 h. WC balls (diameter 4.76 mm) weighing 2750 g were used as milling media. Mean particle sizes of the
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 4 9 9 - 4
S. Eroglu, T. Baykara / Journal of Materials Processing Technology 103 (2000) 288±292
289
Table 1 Powder mixtures and microstructural characteristics of the alloys Composition
Tungsten powder grade
Powder mixing technique
Alloy designation
Tungsten mean grain size
Tungsten (vol.%)
Contiguity
90W±7Ni±3Fe
Coarser (10.5 mm)
Attritor Turbula Attritor Turbula
90W-C-A 90W-C-T 90W-F-A 90W-F-T
18.720.26 19.480.63 17.740.77 18.561.16
76.892.43 82.041.40 77.712.32 80.823.03
0.280.06 0.290.08 0.300.03 0.350.08
Attritor Turbula Attritor Turbula
92.5W-C-A 92.5W-C-T 92.5W-F-A 92.5W-F-T
18.300.45 19.320.60 20.030.68 19.171.35
82.551.27 84.983.41 84.504.20 84.042.42
0.320.07 0.370.06 0.410.06 0.440.05
Finer (3.4 mm) 92.5W±5.25Ni±2.25Fe
Coarser (10.5 mm) Finer (3.4 mm)
powder mixtures were measured with a laser particle size analyser. For the turbula mixtures containing coarser tungsten powder, it was measured to be about 12.3 mm. With attritor milling, the mean particle size was reduced to 7.5 mm. When the powder mixtures of ®ner tungsten was attritor milled, the mean particle size decreased from 5.7 to 3.6 mm. Cylindrical bars (about 23 mm diameter100 mm length) were cold isostatically pressed at 230 MPa and sintered in hydrogen atmosphere at 14808C for 0.5 h. All samples achieved densities higher than 99.6% of theoretical density. 2.2. Characterization Tensile test specimens with a gauge length of 16 mm and diameter of 4 mm were machined from the sintered bars. Prior to tensile testing, the specimens were heat-treated in vacuum at a base pressure of 30 mTorr at 12508C for 3 h to remove hydrogen from the samples, which causes hydrogen embrittlement. A minimum of three tensile tests for each condition was carried out at a cross-head speed of 0.5 mm/ min using an Instron machine. Elongation was determined by ®tting the broken pieces back together and measuring the length of the gauge marks on the sample. Fracture surfaces were examined by a scanning electron microscope (SEM). For quantitative microstructural analysis, samples were prepared using standard metallographic techniques and examined with an image analyzer equipped with an optical microscope, a CCD camera, a frame grabber and a computer. Grey level images of the alloys were thresholded to produce binary presentations of the microstructures. Volume fraction, mean size, size distribution and contiguity of tungsten grains were measured from the binary images. Contiguity de®ned as the relative fraction of tungsten±tungsten interfacial area, was determined by placing grid lines over the binary image and counting the number of tungsten±tungsten and tungsten-matrix intercepts. It was calculated using the equation CW2NWW/(2NWWNWM), where NWW and NWM are the number of tungsten±tungsten grain boundaries and number of tungsten-matrix interfaces intercepted, respectively.
3. Results 3.1. Tensile properties Process variables were powder mixing technique and tungsten powder size for each composition. In order to see the effect of one variable on the properties, the other was kept constant: same powder grade was used while mixing technique was changed or vice versa. With this view in mind, the resultant properties were compared. Fig. 1 shows effects of mixing technique and tungsten powder size on the tensile properties of the 90W±7Ni± 3Fe alloys. For the alloys prepared from coarser tungsten grade, turbula mixed powders (90W-C-T) yielded a slightly higher tensile strength and lower ductility than those of attritor milled powders (90W-C-A). When ®ner grade was used, attritor milled powders (90W-F-A) resulted in higher ductility and strength values than those obtained from turbula mixtures (90W-F-T). Amongst the turbula-mixed alloys, the alloys of coarser grade (90W-C-T) had higher tensile properties compared to ®ner grade alloys (90W-F-T). As for the attritor milled alloys, no signi®cant changes in the tensile strength were observed with respect to powder size, but ductility was slightly higher when ®ner tungsten powder was used.
Fig. 1. Variations of (a) tensile strength and (b) elongation of the 90W± 7Ni±3Fe heavy alloys with powder mixing technique and tungsten powder particle size (Aattritor milled, Tturbula mixed, Ccoarser tungsten grade, F®ner tungsten grade).
290
S. Eroglu, T. Baykara / Journal of Materials Processing Technology 103 (2000) 288±292
powders. Turbula mixtures containing coarser grade tungsten powder (92.5W-C-T) led to the alloys with better tensile properties while turbula mixed powders with ®ner grade tungsten powders (92.5W-F-T) yielded less ductile and strong alloys. Among the attritor milled alloys, the alloy of coarser tungsten grade exhibited higher ductility with no signi®cant change in tensile strength. 3.2. Metallographic examinations
Fig. 2. Variations of (a) tensile strength and (b) elongation of the 92.5W± 5.25Ni±2.25Fe heavy alloys with powder mixing technique and tungsten powder particle size.
The tensile properties of the 92.5%W±5.25%Ni±2.25%Fe alloys are shown in Fig. 2. For both tungsten grades, attritor milled powders always resulted in heavy alloys with higher tensile strength and ductility compared to turbula mixed
Microstructural characteristics of the alloys were determined in order to explain property variations. Table 1 shows mean grain size, volume fraction and contiguity of the tungsten phase for all the alloys. There appears to be no signi®cant difference in mean grain size with values around 20 mm. Tungsten contents are measured to be 76.89±82.04 and 82.55±84.98 vol.% for the 90W±7Ni±3Fe and 92.5W± 5.25Ni±2.25Fe alloys, respectively. There is a systematic variation in contiguity values. For the 90W±7Ni±3Fe heavy alloys, attritor milling and coarser tungsten grade led to
Fig. 3. Binary images of (a) the 90W-C-A, (b) 90W-F-T, (c) 92.5W-C-A, and (d) 92.5W-F-T alloys.
S. Eroglu, T. Baykara / Journal of Materials Processing Technology 103 (2000) 288±292
lower contiguity values. Whereas, turbula mixing and ®ner tungsten powder resulted in alloys with high contiguities. The same trend was observed for the 92.5W±5.25Ni±2.25Fe heavy alloys. Generally, higher contiguity values were obtained from the 92.5W±5.25Ni±2.25Fe heavy alloys compared to the 90W±7Ni±3Fe alloys. This is attributed to higher W volume content. The binary images of higher and lower contiguity alloys for both compositions at a low magni®cation are displayed in Fig. 3. As seen from the micrographs, the low contiguity alloys have a matrix phase homogeneously distributed throughout the microstructure. Whereas, higher contiguity alloys exhibit matrix phase pools, indicating that the microstructure is less homogeneous. 4. Discussion Contiguity is known to be the most important microstructural property that affects mechanical properties of liquidphase sintered materials such as heavy alloys, WC/Co hard metals. The higher the contiguity, poorer are the mechanical properties. It increases with the solid phase, but decreases with the increasing of sintering temperature [1]. Noh et al. reported [13] that for a given composition, contiguity decreases with increasing heat treatment cycle. Experimental ®ndings in the present study, clearly shows that conguity also depends on powder mixing technique and on powder size. Among the alloys investigated, the turbula mixed heavy alloys of ®ner tungsten grade exhibited the lowest tensile
291
properties and had the highest contiguity. It is known that ®ne powders have a greater tendency to form locally densi®ed agglomerates and even lumps. During turbula mixing of the powders containing ®ner tungsten grade, agglomerates and lumps were observed. In the turbula mixer, such agglomerations naturally impeded through mixing of the W, Ni and Fe powders. Consequently, liquid matrix pools or tungsten aggregates formed during sintering, increase tungsten±tungsten grain contact areas or contiguity. Since tungsten±tungsten grain boundaries are inherently weak, increased contiguity between tungsten grains makes the alloy to fail at lower strains by providing a lower energy path for cracks to propagate. Therefore, higher contiguity alloys exhibit more tungsten±tungsten grain boundary failures signalled by ¯at facets seen in Fig. 4a and b. The alloys with higher ductility have predominantly transgranular fracture evidenced by cleavage of tungsten grains with river markings (Fig. 4c and d). Attritor milling or turbula mixing of coarser tungsten led to powder mixtures that yielded sintered microstructures with more homogeneously distributed matrix phase and lower contiguity values. Lower contiguity suggests that more cracks are blunted by the ductile matrix and signi®cant amount of deformation in tungsten grains (Fig. 5) takes place prior to the failure of the alloy during tensile test, thus increasing ductility and strength. It should be noted that similar observations have been reported [14,15] for the tungsten carbide/cobalt system. Finer tungsten carbide starting powder size gave rise to higher contiguities which are inherited from earlier stages of powder preparation [14]. Gurland [15] showed that ball milling in acetone increased impact strength of the tungsten
Fig. 4. Fracture surfaces of higher (a, b) and lower (c, d) contiguity alloys: (a) 90W-F-T, (b) 92.5W-F-T, (c) 90W-C-A and (d) 92.5W-C-A.
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S. Eroglu, T. Baykara / Journal of Materials Processing Technology 103 (2000) 288±292
References
Fig. 5. Binary image of the 92.5W-C-A alloy taken near the fracture surface parallel to the loading direction (from top to bottom in the page). Tungsten grains (white areas) are elongated in the direction of applied load during tensile testing.
carbide±cobalt alloys compared to those of powders mixed with other techniques such as dry or wet blending. 5. Conclusions This study showed that powder mixing technique and tungsten powder size greatly in¯uence the properties of heavy alloys. For a given tungsten powder size, wet attritor milled mixtures resulted in heavy alloys with higher ductility compared to turbula mixed powders. Coarser tungsten powders led to better ductility in the turbula mixed alloys compared to ®ner tungsten powders. Similar trends were generally observed for the tensile strengths of the alloys. Amongst the tungsten heavy alloys studied, turbula mixed alloys prepared from ®ner tungsten powders showed the lowest tensile properties. This behavior was attributed to higher tungsten±tungsten contiguities caused by a less ef®cient mixing of powders in the turbula mixer. Acknowledgements Financial supports of NATO-SFS programme and TUBITAK-MRC are greatly acknowledged. Thanks are due to F. Balli, Z. Oktem, O. Bulut and H. Yigiter for their help during various stages of this study.
[1] R.M. German, L.L. Bourguignon, B.H. Rabin, Microstructure limitations of high tungsten content heavy alloys, J. Metal. 37 (1985) 36±39. [2] R.M. German, Critical developments in tungsten heavy alloys, in: A. Bose, R.J. Dowding (Eds.), Tungsten and Tungsten Alloys, MPIF, Princeton, NJ, 1992, pp. 3±13. [3] S. Farooq, P.B. Kemp, R.M. German, A. Bose, Effect of initial oxygen content and sintering atmosphere dew point on the properties of tungsten based heavy alloys, Int. J. Refract. Met. Hard Mater. 8 (1989) 236±243. [4] A. Bose, R.M. German, Sintering atmosphere effects on tensile properties of heavy alloys, Met. Trans. A 19 (1988) 2467±2476. [5] T. Kishi, R.M. German, Processing effects on the mechanical properties of tungsten heavy alloys, Int. J. Refract. Met. Hard Mater 9 (1990) 40±45. [6] H.K. Yoon, S.H. Lee, S.-J.L. Kang, D.N. Yoon, Effect of vacuum treatment on mechanical properties of W±Ni±Fe heavy alloy, J. Mater. Sci. 18 (1983) 1374±1380. [7] L.L. Bourguignon, R.M. German, Sintering temperature effects on a tungsten heavy alloy, Int. J. Powder Metall. 24 (1988) 115±121. [8] B.H. Rabin, R.M. German, Microstructure effects on tensile properties of tungsten±nickel±iron composites, Met. Trans. A 19 (1988) 1523±1532. [9] W.E. Gurwell, Solid state sintering of tungsten heavy alloys, in: A. Bose, R. J. Dowding (Eds.), Tungsten and Refractory Metals 2, MPIF, Princeton, NJ, 1994, pp. 65±75. [10] J.S. Lee, T.H. Kim, T.G. Kang, S. Lee, M.H. Hong, J.W. Noh, The effect of W powder size on the microstructure and mechanical properties of W±25 wt.%Cu alloy, in: A. Bose, R.J. Dowding (Eds.), Tungsten and Refractory Metals 2, MPIF, Princeton, NJ, 1994, pp. 259±266. [11] H. Danninger, The in¯uence of the W starting powder size on sintering of tungsten heavy alloys, in: H. Bildstein, R. Eck. (Eds.),13th Plansee Seminar, Reutte, Metalwerk Plansee, 1993, pp. 405±419. [12] S. Eroglu, T. Baykara, Effect of processing variables on densi®cation of the 92.5%W±5.25%Ni±2.25%Fe heavy metals, in: Powder Metallurgy World Congress, Paris, 1994, pp. 2013±2016. [13] J.-W. Noh, E.-P. Kim, H.-S. Song, W.-H. Baek, K.-S. Churn, S.-K.L. Kang, Matrix penetration of W/W grain boundaries, Met. Trans. A 24 (1993) 2411±2416. [14] J.W. Lee, D. Jaffrey, J.D. Browne, In¯uence of process variables on sintering of WC±25 wt.%Co, Powder Metall. 2 (1980) 57±64. [15] J. Gurland, The in¯uence of basic design and processing variables on the properties of sintered WC±Co alloys, in: W. Leszynski (Ed.), Powder Metallurgy, Interscience, New York, 1961, pp. 661±682.
Effects of powder mixing technique and tungsten powder size on the properties of tungsten heavy alloys S. Eroglu*, T. Baykara TUBITAK-MRC, Materials Research Department, PO Box 21, Gebze-Kocaeli 41470, Turkey Accepted 15 December 1999
Abstract Properties of tungsten heavy alloys are known to vary with process parameters such as sintering temperature, time and atmosphere. In this study, tensile properties of the 92.5W±5.25Ni±2.25Fe and 90W±7Ni±3Fe heavy alloys were found to be in¯uenced by powder mixing technique and tungsten powder size. For a given tungsten powder grade (mean particle size 10.5 or 3.4 mm), attritor milled alloys exhibited better tensile properties than those of the turbula-mixed alloys. Compared to ®ner grade, coarser tungsten powder generally resulted in alloys with higher ductility when turbula mixer was used. Overall, the alloys prepared from turbula mixed powders containing ®ner tungsten grade showed the lowest tensile properties. Metallographic analyses revealed that these alloys failed in more brittle fashion as a result of a high degree of contiguity of tungsten grains, which was inherited from powder mixing step. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Liquid-phase sintering; Tungsten heavy alloys; Powder mixing; Powder size
1. Introduction
2. Experimental procedures
Tungsten heavy alloys possess a unique combination of high density, ductility, strength and toughness. They are used extensively as kinetic energy penetrators, counterbalance weights and radiation shields. The alloys are produced from W, Ni, Fe and Cu powders by liquid-phase sintering in vacuum and/or in hydrogen atmosphere. Sintering in the presence of a liquid phase at temperatures about 15008C leads to fully dense alloys. However, full densi®cation does not ensure good mechanical properties, which are known to be very sensitive to processing variables. The variables studied [1±12] include sintering temperature, sintering time, sintering atmosphere, powder characteristics, heat-treatment temperature. Scant attention has been paid to powder mixing and powder size. The work reported in the present paper was carried out in order to investigate effects of powder mixing technique and tungsten powder size on the tensile properties and microstructures of 90W±7Ni±3Fe and 92.5W±5.25Ni± 2.25Fe heavy alloys.
2.1. Sample preparation
*
Corresponding author.
Elemental powders of tungsten (H.C. Starck), carbonyl nickel (Poudmet) and iron (Poudmet) were used to produce the alloys. Ni and Fe powders had Fisher particle sizes of 3 and 4.5 mm, respectively. Two grades of tungsten powders (HC150 and HC500) were employed. Mean particle sizes of HC500 and HC150 grades as measured with a sedigraph were 10.5 and 3.4 mm, respectively. Powders were weighed to give the compositions of 90%W±7%Ni±3%Fe and 92.5%W±5.25%Ni±2.25%Fe. Powder mixtures of 450 g were prepared for each tungsten grade using turbula mixer or attritor mill (Table 1). Dryturbula mixing was carried out at a speed of 85 rpm in a glass container (700 cm3) for 3 h. Zirconia balls with a diameter of 9 mm were charged with powders to reduce agglomeration and caking of the powders. The ball to powder weight ratio was kept at 1:10. Wet-attritor milling was carried out in ethyl alcohol (200 cm3) in a stainless steel tank (1300 cm3) with a WC arm at a speed of 400 rpm for 3 h. WC balls (diameter 4.76 mm) weighing 2750 g were used as milling media. Mean particle sizes of the
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 4 9 9 - 4
S. Eroglu, T. Baykara / Journal of Materials Processing Technology 103 (2000) 288±292
289
Table 1 Powder mixtures and microstructural characteristics of the alloys Composition
Tungsten powder grade
Powder mixing technique
Alloy designation
Tungsten mean grain size
Tungsten (vol.%)
Contiguity
90W±7Ni±3Fe
Coarser (10.5 mm)
Attritor Turbula Attritor Turbula
90W-C-A 90W-C-T 90W-F-A 90W-F-T
18.720.26 19.480.63 17.740.77 18.561.16
76.892.43 82.041.40 77.712.32 80.823.03
0.280.06 0.290.08 0.300.03 0.350.08
Attritor Turbula Attritor Turbula
92.5W-C-A 92.5W-C-T 92.5W-F-A 92.5W-F-T
18.300.45 19.320.60 20.030.68 19.171.35
82.551.27 84.983.41 84.504.20 84.042.42
0.320.07 0.370.06 0.410.06 0.440.05
Finer (3.4 mm) 92.5W±5.25Ni±2.25Fe
Coarser (10.5 mm) Finer (3.4 mm)
powder mixtures were measured with a laser particle size analyser. For the turbula mixtures containing coarser tungsten powder, it was measured to be about 12.3 mm. With attritor milling, the mean particle size was reduced to 7.5 mm. When the powder mixtures of ®ner tungsten was attritor milled, the mean particle size decreased from 5.7 to 3.6 mm. Cylindrical bars (about 23 mm diameter100 mm length) were cold isostatically pressed at 230 MPa and sintered in hydrogen atmosphere at 14808C for 0.5 h. All samples achieved densities higher than 99.6% of theoretical density. 2.2. Characterization Tensile test specimens with a gauge length of 16 mm and diameter of 4 mm were machined from the sintered bars. Prior to tensile testing, the specimens were heat-treated in vacuum at a base pressure of 30 mTorr at 12508C for 3 h to remove hydrogen from the samples, which causes hydrogen embrittlement. A minimum of three tensile tests for each condition was carried out at a cross-head speed of 0.5 mm/ min using an Instron machine. Elongation was determined by ®tting the broken pieces back together and measuring the length of the gauge marks on the sample. Fracture surfaces were examined by a scanning electron microscope (SEM). For quantitative microstructural analysis, samples were prepared using standard metallographic techniques and examined with an image analyzer equipped with an optical microscope, a CCD camera, a frame grabber and a computer. Grey level images of the alloys were thresholded to produce binary presentations of the microstructures. Volume fraction, mean size, size distribution and contiguity of tungsten grains were measured from the binary images. Contiguity de®ned as the relative fraction of tungsten±tungsten interfacial area, was determined by placing grid lines over the binary image and counting the number of tungsten±tungsten and tungsten-matrix intercepts. It was calculated using the equation CW2NWW/(2NWWNWM), where NWW and NWM are the number of tungsten±tungsten grain boundaries and number of tungsten-matrix interfaces intercepted, respectively.
3. Results 3.1. Tensile properties Process variables were powder mixing technique and tungsten powder size for each composition. In order to see the effect of one variable on the properties, the other was kept constant: same powder grade was used while mixing technique was changed or vice versa. With this view in mind, the resultant properties were compared. Fig. 1 shows effects of mixing technique and tungsten powder size on the tensile properties of the 90W±7Ni± 3Fe alloys. For the alloys prepared from coarser tungsten grade, turbula mixed powders (90W-C-T) yielded a slightly higher tensile strength and lower ductility than those of attritor milled powders (90W-C-A). When ®ner grade was used, attritor milled powders (90W-F-A) resulted in higher ductility and strength values than those obtained from turbula mixtures (90W-F-T). Amongst the turbula-mixed alloys, the alloys of coarser grade (90W-C-T) had higher tensile properties compared to ®ner grade alloys (90W-F-T). As for the attritor milled alloys, no signi®cant changes in the tensile strength were observed with respect to powder size, but ductility was slightly higher when ®ner tungsten powder was used.
Fig. 1. Variations of (a) tensile strength and (b) elongation of the 90W± 7Ni±3Fe heavy alloys with powder mixing technique and tungsten powder particle size (Aattritor milled, Tturbula mixed, Ccoarser tungsten grade, F®ner tungsten grade).
290
S. Eroglu, T. Baykara / Journal of Materials Processing Technology 103 (2000) 288±292
powders. Turbula mixtures containing coarser grade tungsten powder (92.5W-C-T) led to the alloys with better tensile properties while turbula mixed powders with ®ner grade tungsten powders (92.5W-F-T) yielded less ductile and strong alloys. Among the attritor milled alloys, the alloy of coarser tungsten grade exhibited higher ductility with no signi®cant change in tensile strength. 3.2. Metallographic examinations
Fig. 2. Variations of (a) tensile strength and (b) elongation of the 92.5W± 5.25Ni±2.25Fe heavy alloys with powder mixing technique and tungsten powder particle size.
The tensile properties of the 92.5%W±5.25%Ni±2.25%Fe alloys are shown in Fig. 2. For both tungsten grades, attritor milled powders always resulted in heavy alloys with higher tensile strength and ductility compared to turbula mixed
Microstructural characteristics of the alloys were determined in order to explain property variations. Table 1 shows mean grain size, volume fraction and contiguity of the tungsten phase for all the alloys. There appears to be no signi®cant difference in mean grain size with values around 20 mm. Tungsten contents are measured to be 76.89±82.04 and 82.55±84.98 vol.% for the 90W±7Ni±3Fe and 92.5W± 5.25Ni±2.25Fe alloys, respectively. There is a systematic variation in contiguity values. For the 90W±7Ni±3Fe heavy alloys, attritor milling and coarser tungsten grade led to
Fig. 3. Binary images of (a) the 90W-C-A, (b) 90W-F-T, (c) 92.5W-C-A, and (d) 92.5W-F-T alloys.
S. Eroglu, T. Baykara / Journal of Materials Processing Technology 103 (2000) 288±292
lower contiguity values. Whereas, turbula mixing and ®ner tungsten powder resulted in alloys with high contiguities. The same trend was observed for the 92.5W±5.25Ni±2.25Fe heavy alloys. Generally, higher contiguity values were obtained from the 92.5W±5.25Ni±2.25Fe heavy alloys compared to the 90W±7Ni±3Fe alloys. This is attributed to higher W volume content. The binary images of higher and lower contiguity alloys for both compositions at a low magni®cation are displayed in Fig. 3. As seen from the micrographs, the low contiguity alloys have a matrix phase homogeneously distributed throughout the microstructure. Whereas, higher contiguity alloys exhibit matrix phase pools, indicating that the microstructure is less homogeneous. 4. Discussion Contiguity is known to be the most important microstructural property that affects mechanical properties of liquidphase sintered materials such as heavy alloys, WC/Co hard metals. The higher the contiguity, poorer are the mechanical properties. It increases with the solid phase, but decreases with the increasing of sintering temperature [1]. Noh et al. reported [13] that for a given composition, contiguity decreases with increasing heat treatment cycle. Experimental ®ndings in the present study, clearly shows that conguity also depends on powder mixing technique and on powder size. Among the alloys investigated, the turbula mixed heavy alloys of ®ner tungsten grade exhibited the lowest tensile
291
properties and had the highest contiguity. It is known that ®ne powders have a greater tendency to form locally densi®ed agglomerates and even lumps. During turbula mixing of the powders containing ®ner tungsten grade, agglomerates and lumps were observed. In the turbula mixer, such agglomerations naturally impeded through mixing of the W, Ni and Fe powders. Consequently, liquid matrix pools or tungsten aggregates formed during sintering, increase tungsten±tungsten grain contact areas or contiguity. Since tungsten±tungsten grain boundaries are inherently weak, increased contiguity between tungsten grains makes the alloy to fail at lower strains by providing a lower energy path for cracks to propagate. Therefore, higher contiguity alloys exhibit more tungsten±tungsten grain boundary failures signalled by ¯at facets seen in Fig. 4a and b. The alloys with higher ductility have predominantly transgranular fracture evidenced by cleavage of tungsten grains with river markings (Fig. 4c and d). Attritor milling or turbula mixing of coarser tungsten led to powder mixtures that yielded sintered microstructures with more homogeneously distributed matrix phase and lower contiguity values. Lower contiguity suggests that more cracks are blunted by the ductile matrix and signi®cant amount of deformation in tungsten grains (Fig. 5) takes place prior to the failure of the alloy during tensile test, thus increasing ductility and strength. It should be noted that similar observations have been reported [14,15] for the tungsten carbide/cobalt system. Finer tungsten carbide starting powder size gave rise to higher contiguities which are inherited from earlier stages of powder preparation [14]. Gurland [15] showed that ball milling in acetone increased impact strength of the tungsten
Fig. 4. Fracture surfaces of higher (a, b) and lower (c, d) contiguity alloys: (a) 90W-F-T, (b) 92.5W-F-T, (c) 90W-C-A and (d) 92.5W-C-A.
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References
Fig. 5. Binary image of the 92.5W-C-A alloy taken near the fracture surface parallel to the loading direction (from top to bottom in the page). Tungsten grains (white areas) are elongated in the direction of applied load during tensile testing.
carbide±cobalt alloys compared to those of powders mixed with other techniques such as dry or wet blending. 5. Conclusions This study showed that powder mixing technique and tungsten powder size greatly in¯uence the properties of heavy alloys. For a given tungsten powder size, wet attritor milled mixtures resulted in heavy alloys with higher ductility compared to turbula mixed powders. Coarser tungsten powders led to better ductility in the turbula mixed alloys compared to ®ner tungsten powders. Similar trends were generally observed for the tensile strengths of the alloys. Amongst the tungsten heavy alloys studied, turbula mixed alloys prepared from ®ner tungsten powders showed the lowest tensile properties. This behavior was attributed to higher tungsten±tungsten contiguities caused by a less ef®cient mixing of powders in the turbula mixer. Acknowledgements Financial supports of NATO-SFS programme and TUBITAK-MRC are greatly acknowledged. Thanks are due to F. Balli, Z. Oktem, O. Bulut and H. Yigiter for their help during various stages of this study.
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