Combination of mechanical alloying and two-stage sintering of a 93W/5.6Ni/1.4Fe tungsten heavy alloy

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1 Materials Science and Engineering A344 (2003) 253/260 Combination of mechanical alloying and two-stage sintering of a 93W/5.6Ni/1.4Fe tungsten heavy alloy Soon H. Hong a, Ho J. Ryu b, a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Guseong-dong, Yuseong-gu, Daejeon , South Korea b DUPIC, Korea Atomic Energy Research Institute, 150 Deokjin-dong, Yuseong-gu, Daejeon , South Korea Received 7 March 2002; received in revised form 4 June 2002 Abstract The microstructural evolution and mechanical properties of a mechanically alloyed and two-stage sintered tungsten heavy alloy were investigated. Elemental powders of tungsten, nickel and iron of a composition corresponding to 93W /5.6Ni /1.4Fe were mechanically alloyed in a tumbler ball mill for 72 h. Mechanically alloyed powders were solid-state sintered at C for 1 hr in a hydrogen atmosphere followed by secondary sintering at 1445/1485 8C for a sintering time ranging from 4 to 90 min. Solid-state sintered tungsten heavy alloys exhibited full densification (above 99% in relative density) due to the enhanced sintering resulting from mechanical alloying. Secondary sintering with a rapid heating rate changed the microstructures of the solid-state sintered alloy with contiguous tungsten phases into a dispersion alloy with spherical tungsten particles embedded in the W /Ni /Fe matrix, maintaining fine tungsten particle due to the combination of a mechanical alloying and a short sintering time. The two-stage sintered tungsten heavy alloy from mechanically alloyed powders showed finer tungsten particle (about 6 mm in diameter) than in conventional liquid-phase sintered tungsten heavy alloys. The mechanical properties of a tungsten heavy alloy were found to be dependent on the microstructural parameters such as tungsten particle size, matrix volume fraction and tungsten/tungsten contiguity which are controllable through the two-stage sintering process. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Tungsten heavy alloy; Mechanical alloying; Two-stage sintering; Microstructural parameter; Mechanical properties 1. Introduction Corresponding author. Tel.: / ; fax:/ address: hjryu@kaeri.re.kr (H.J. Ryu). Tungsten heavy alloys (WHAs) are high density structural alloys used for kinetic energy penetrators, counter weights, radiation shields and electrical contacts [1]. Approximately 5/10 wt.% of nickel and iron are commonly added to tungsten to form a ductile solid solution matrix of W /Ni /Fe in WHAs. Liquid-phase sintering of blended elemental powders of tungsten, nickel and iron at a temperature above C is a conventional fabrication process for high density WHAs [2]. Liquid-phase sintered WHAs show a typical microstructure where spherical bcc tungsten particles are dispersed in a fcc solid solution matrix [3]. Recently, several studies have been carried out to enhance the mechanical properties of WHAs in order to improve the penetration capabilities for kinetic energy penetrator applications [4 /6]. The major disadvantage of WHAs is that they have a lesser penetration capability than depleted uranium alloys, which are another penetrator material [7]. Depleted uranium alloys exhibit superior penetration performance due to the so-called self-sharpening behavior, in which a penetrator shape maintains the penetrator profile during penetration against armor targets [8]. WHAs, however, develop mushroom-like heads during penetration, which result in a lower penetration depth compared with depleted uranium alloys. Therefore it is needed to develop advanced WHAs with high penetration cap /02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S ( 0 2 )

2 254 S.H. Hong, H.J. Ryu / Materials Science and Engineering A344 (2003) 253/260 Fig. 1. Schematic illustration of secondary sintering equipment using a tube furnace with a push rod under a flowing hydrogen atmosphere. abilities to replace depleted uranium alloys of which use is a cause for environmental concern. The self-sharpening behavior of kinetic energy penetrator material is known to be associated with localized shear deformation, which occurs when a material is deformed at a high strain-rate [9]. Many investigations on enhancement of localized shear deformation during penetration by controlling microstructural factors in WHAs have been reported [10 /12]. Microstructural factors, such as tungsten particle size, matrix volume fraction and tungsten-tungsten contiguity, affect the mechanical properties and fracture behavior of WHAs. Tungsten /tungsten contiguity is defined as a fraction of contiguous tungsten /tungsten interfacial area to total interfacial area [13]. It has been reported that the localized shear deformation could be enhanced by refinement of the microstructure of WHA [14,15]. Many researchers have investigated refinement of WHAs by alloying with refractory elements such as Mo and Re [16], cold working followed by recrystallization [17], solid-state sintering [18,19], mechanical alloying (MA) [20,21] and oxide dispersion [22]. However, the attempts to refine the microstructures of WHAs resulted in less than optimum mechanical properties due to incomplete densification or brittleness of sintered WHAs. In this study, a two-stage sintering process of a mechanically alloyed WHA was suggested in order to refine the microstructure and control the mechanical properties of a WHA. Solid-state sintering was carried out to achieve full densification of mechanically alloyed WHA, and secondary sintering with a rapid heating rate and a short holding time was subsequently carried out to control the microstructural features and obtain the minimum coarsening of tungsten particles. The correlation between microstructure controlled by two-stage sintering process and mechanical properties of a 93W / 5.6Ni/1.4Fe WHA was investigated. mill was 255 mm, and tool steel balls of 8 mm in diameter were used as the milling media. The milling speed was 75 rpm, milling time was varied up to 72 h, ball-to-powder ratio was 20:1 by weight, and the ball filling ratio was 15% by volume. A chemical analysis of the milled powders was carried out employing atomic absorption spectroscopy. The mechanically alloyed powders were consolidated into a green compact by die compaction under a pressure of 100 MPa. The green compacts were solid-state sintered at a temperature of C for 1 h in a hydrogen atmosphere. Solid-state sintered WHA was subsequently sintered at temperatures ranging from 1445 to C for a sintering time ranging from 4 to 90 min in a hydrogen atmosphere using rapid sintering achieved by rapidly inserting the sample into the furnace hot zone (Fig. 1). Fig. 2 shows a two-stage sintering cycle when the secondary sintering temperature is C and the secondary sintering time is 4 min. The two-stage sintered specimens were annealed at C for 1 h in a nitrogen atmosphere and then water-quenched to prevent hydrogen embrittlement and impurity segregation during the cooling stage [23]. Densities of the sintered specimens were measured by the water immersion method. The size of tungsten particles, volume fraction of matrix phase and tungsten /tungsten contiguity of sintered WHAs were characterized using scanning electron microscopy. Room temperature tensile tests were performed at a strain rate of 1.33/10 3 s 1. Elongation-to-failure 2. Experimental procedures Tungsten powders of 2.5 mm, nickel powders of 2.5 mm and iron powders of 3.5 mm in average diameter were mechanically alloyed in a tumbler ball mill to fabricate the refined WHA powders with a composition of 93W/5.6Ni/1.4Fe by weight. The diameter of the Fig. 2. The sintering temperature, sintering time and heating rate for a two-stage sintering of a tungsten heavy alloy when the secondary sintering is carried out at C for 4 min.

3 S.H. Hong, H.J. Ryu / Materials Science and Engineering A344 (2003) 253/ was determined from the change in length of specimen after tensile test. 3. Results and discussion The result of chemical analysis of the milled powders shows that Fe content was increased with milling time due to the wear of the grinding media as shown in Fig. 3. The powder content of W, Ni, and Fe was adjusted considering the rate of the increase in Fe content which was wt.% h 1 in this study. Previous results on a mechanically alloyed 93W / 5.6Ni/1.4Fe WHA showed that average lamellar spacing in the alloyed powder was reduced below 0.2 mm and the crystallite size was about 16 nm after milling of 72 h with a rotation speed for 75 rpm, a ball-to-powder ratio of 20:1 by weight and a ball filling ratio of 15% by volume [21]. When a mechanically alloyed 93W /5.6Ni/ 1.4Fe WHA was solid-state sintered at C for 1 h, the microstructure showed that the tungsten grains were interconnected each other as shown in Fig. 4a. The solid-state sintered WHA from mechanically alloyed powders showed high relative density above 99% which is higher than a WHA without MA (93% at C) [19,21]. The stored energy and defects by severe cold working and fine and homogeneous distribution of matrix phase by MA process enhances the sintering process and results in a higher density. The average size of tungsten particles in the solid-state sintered alloy was about 3 mm. These results indicate that the MA followed Fig. 4. Scanning electron micrographs showing (a) the typical microstructure of a 93W/5.6Ni/1.4Fe tungsten heavy alloy solid-state sintered at C and (b) a microcrack nucleated at a tungsten/ tungsten interface boundary during a tensile test. Fig. 3. Variation of Ni and Fe content (wt.%) of tungsten heavy alloy powders as a function of milling time when the initial composition is 93W/5.6Ni/1.4Fe. by solid-state sintering is very effective in refining the microstructure of a WHA while simultaneously achieving full densification. The volume fraction of matrix phase and the area fraction of contiguous tungsten/ tungsten grain boundary, so called tungsten/tungsten contiguity, of solid-state sintered WHA were measured as 11% and 0.74, respectively. Compared with a conventional liquid-phase sintered WHA with the same composition [24], matrix volume fraction is reduced and tungsten/tungsten contiguity is increased by solid-state sintering of a WHA. The volume fraction of matrix phase was reduced due to the decrease in the solubility of tungsten into the matrix at a lower sintering temperature and the increase in tungsten /tungsten contiguity is due to the decrease in matrix volume fraction and increase in dihedral angle between tungsten

4 256 S.H. Hong, H.J. Ryu / Materials Science and Engineering A344 (2003) 253/260 particles in the matrix phase at lower sintering temperature [25]. Compared with a liquid-phase sintered WHA with the same composition [24], the tensile strength increased from 950 to 1100 MPa, and the elongation decreased from 30 to 0.5% by solid-state sintering of a WHA. Whereas solid-state sintered mechanically alloyed WHAs showed high level of tensile strength (about 1100 MPa), the elongation is considered to be unsuitable for many possible applications. The reduced elongation was attributed to the brittle character of tungsten / tungsten interfaces which is the weakest interfaces in WHAs [26]. Fig. 4b shows a microcrack at a tungsten/ tungsten interface of a solid-stated sintered 93W / 5.6Ni/1.4Fe tungsten heavy alloy in a tensile-tested sample. Fig. 5 shows scanning electron micrographs of twostage sintered 93W/5.6Ni /1.4Fe WHAs rapidly sintered again at temperatures ranging from 1445 to C for 4 min after solid-state sintering at C for 1 h. When sintered at a temperature below C, tungsten particles remained a highly contiguous shape as in a solid-state sintered microstructure of Fig. 6. The variation of the matrix volume fraction and W/W contiguity of two-stage sintered 93W/5.6Ni/1.4Fe tungsten heavy alloys with secondary sintering temperature ranging from 1445 to C. a WHA. However, tungsten particles are spherical in shape*/as in a WHA liquid phase sintered microstructure*/when sintered at a temperature above Fig. 5. Scanning electron micrographs of two-stage sintered 93W/5.6Ni/1.4Fe tungsten heavy alloys liquid-phase sintered at (a) C, (b) C, (c) C and (d) C for 4 min after solid-state sintering at C for 1 h.

5 S.H. Hong, H.J. Ryu / Materials Science and Engineering A344 (2003) 253/ C. The matrix volume fraction increased and tungsten/tungsten contiguity decreased with secondary sintering temperature as shown in Fig. 6. The secondary sintering time was increased up to 90 min at C to investigate the effect of secondary liquid-phase sintering time on the microstructural evolution of a two-stage sintered 93W /5.6Ni/1.4Fe WHA. The average size of tungsten particles increased from 6 to 27 mm with increasing secondary sintering time from 4 to 90 min as shown in Fig. 7. The relationship between tungsten particle size and secondary sintering time was found to satisfy the LSW theory [27,28] which describes the coarsening of particles through the diffusion of solute element as follows: r 3 t r3 0 k(tt 0 ) (1) where r t is average radius of particles at time, t, r 0 is average radius of particles at time, t 0, and k is a coarsening rate constant. At C, k for a mechanically alloyed 93W/5.6Ni /1.4Fe WHA was measured as 25.5 mm 3 min 1, as shown in Fig. 8. The tungsten particle size of a two-stage sintered WHA can be controlled by the secondary sintering time in the twostage sintering process. The correlation between microstructural factors and mechanical properties were obtained from tensile tests of a solid-state sintered 93W /5.6Ni/1.4Fe WHA, twostage sintered 93W /5.6Ni /1.4Fe WHAs and a liquid phase sintered 93W /5.6Ni/1.4Fe WHA at C for 1 h. The fractographies of tensile tested samples of the 93W /5.6Ni/1.4Fe WHAs show that the different fracture modes operate for the different sintering conditions as shown in Fig. 9. Tungsten heavy alloys also exhibit four fracture modes such as tungsten cleavage, matrix rupture, tungsten /tungsten interfacial debonding, and tungsten /matrix interfacial debonding. Most fractured interfaces are composed of tungsten /tungsten interface boundaries in solid-state sintered 93W /5.6Ni/ 1.4Fe WHA and two-stage sintered 93W /5.6Ni/1.4Fe WHA, whereas tungsten cleavage is mainly observed in liquid-phase sintered 93W/5.6Ni /1.4Fe WHA. The mechanical properties are explained in terms of microstructural factors such as tungsten grain size, matrix volume fraction and tungsten /tungsten contiguity. The high yield strength of a WHA is obtained by decreasing tungsten particle size and matrix volume fraction. Ryu et al. suggested using the Hall /Petch type equation of two-phase alloy that the yield strength of a tungsten heavy alloy is a function of tungsten particle size and matrix volume fraction as follows [19,29], 1 1=2 VM s y s 0 k 1 (2) DV M where s y is the yield strength, s 0 is the intrinsic strength, k 1 is a constant, V M is the matrix volume fraction and D Fig. 7. Scanning electron micrographs of two-stage sintered 93W/ 5.6Ni/1.4Fe tungsten heavy alloys secondarily sintered at C for (a) 4 min, (b) 15 min and (c) 90 min after solid-state sintering at C for 1 h.

6 258 S.H. Hong, H.J. Ryu / Materials Science and Engineering A344 (2003) 253/260 Fig. 8. (a) The variation of the tungsten particle size with secondary sintering time for two-stage sintered 93W/5.6Ni/1.4Fe tungsten heavy alloys. (b) Tungsten particle radius cubed vs. secondary sintering time. is the average diameter of tungsten particles. Fig. 10 shows that the yield strengths of WHAs are well represented as a function of (1/V M /DV M ) 1/2 as shown in Eq. (2). Based on the experimental results showing that decrease in matrix volume fraction and increase in tungsten/tungsten contiguity decreases the elongation to failure of tungsten heavy alloys [26,30], an empirical correlation between the elongation to failure and microstructures of 93W /5.6Ni/1.4Fe WHAs have been suggested as follows [31], o o 0 k 2 V M (1C w ) (3) where o is the elongation to failure, o 0 is a constant which has no explicit physical meaning (o /0 when k 2 V M (1/C w ) is less than o 0 ), k 2 is a constant and C w is the tungsten /tungsten contiguity. Elongations of WHAs sintered at various conditions linearly correlate with V M (1/C w ) as shown in Fig. 11. Although it is reasonable that higher tungsten /tungsten contiguity provides sites for microcrack nucleation and lower matrix volume fraction provide readier microcrack link up without blunting of crack [32,33], more understanding is required to elucidate the physical meaning of Eq. (3). Fig. 9. Scanning electron miocrographs showing tensile fracture surfaces of (a) solid-state sintered, (b) two-stage sintered, and (c) liquid-phase sintered 93W/5.6Ni/1.4Fe tungsten heavy alloys.

7 S.H. Hong, H.J. Ryu / Materials Science and Engineering A344 (2003) 253/ while showed low elongation (below 1% due to the low volume fraction of matrix phase and a large tungsten/ tungsten contiguity). Two-stage sintered WHA using mechanically alloyed powders showed finer tungsten particles than conventional liquid-phase sintered WHAs. Mechanical properties of WHA were found to be related to microstructural parameters such as tungsten particle size, matrix volume fraction and tungsten/ tungsten contiguity, all being able to be controlled by the two-stage sintering process. References Fig. 10. Variation of yield strength as a function of microstructural parameters including the matrix volume fraction, V M, and tungsten particle size, D, of solid-state sintered, two-stage sintered, and liquid phase sintered 93W/5.6Ni/1.4Fe tungsten heavy alloys. Fig. 11. Variation of elongation as a function of microstructural parameters including the matrix volume fraction, V M, and tungsten/ tungsten contiguity, C w, of solid-state sintered, two-stage sintered, and liquid phase sintered 93W/5.6Ni/1.4Fe tungsten heavy alloys. 4. Conclusions Finer spherical tungsten particles (less than 6 mm in diameter) can be obtained by two-stage sintering of mechanically alloyed 93W/5.6Ni/1.4Fe WHAs. When solid-state sintered at C, a mechanically alloyed WHA showed ultra-fine tungsten particles of about 3 mm in diameter with high relative density (above 99%). Solid-state sintered WHA from mechanically alloyed powders exhibited high tensile strength (about 1100 MPa) due to fine grain structure and high W content [1] R.M. German, in: A. Bose, R.J. Dowding (Eds.), Proc. Inter. Conf. on Tungsten and Tungsten Alloys, MPIF, Princeton, NJ, 1992, p. 1. [2] R.M. German, Sintering Theory and Practice, Wiley, New York, NY, 1996, p [3] W.D. Cai, Y. Li, R.J. Dowding, F.A. Mohamed, E.J. Lavernia, Rev. Particulate Mater. 3 (1995) 71. [4] C. Zubillaga, F. Castro, J.J. Urcola, M. Fuentes, Z. Metallkd. 80 (1989) 577. [5] R.G. O Donnell, R.L. Woodward, Met. Trans. A 21 (1990) 744. [6] D.K. Kim, S. Lee, W.H. Baek, Mater. Sci. Eng. A249 (1998) 197. [7] L.S. Magness, T.G. Farrand, Proc Army Science Conf., Durham, NC, 1990, p [8] L.S. Magness, Jr, Mech. Mater. 17 (1994) 147. [9] K.-M. Cho, S. Lee, S.R. Nutt, J. Duffy, Acta Met. Mater. 41 (1993) 923. [10] K.T. Ramesh, R.S. Coates, Met. Trans. A 23 (1992) [11] D.K. Kim, S. Lee, H.J. Ryu, S.H. Hong, J.W. Noh, Met. Mater. Trans. A 31 (2000) [12] A. Bose, H.R.A. Couque, J. Lankford, Jr, Int. J. Powder Met. 28 (1992) 383. [13] J. Gurland, Trans. AIME 212 (1958) 452. [14] A. Bose, H. Couque, J. Lankford, Jr, in: A. Bose, R.J. Dowding (Eds.), Proc. Inter. Conf. on Tungsten and Tungsten Alloys, MPIF, Princeton, NJ, 1992, p [15] S.H. Hong, H.J. Ryu, W.H. Baek, Mater. Sci. Eng. A 333 (2002) 187. [16] A. Bose, R.M. German, Met. Trans. A 19 (1988) [17] G.R. Goren-Muginstein, A. Rosen, Mater. Sci. Eng. A238 (1997) 351. [18] W.E. Gurwell, in: A. Bose, R.J. Dowding (Eds.), Proc. 2nd Inter. Conf. on Tungsten and Refractory Metals, MPIF, Princeton, NJ, 1994, p. 65. [19] H.J. Ryu, S.H. Hong, W.H. Baek, Mater. Sci. Eng. A291 (2000) 91. [20] M.L. Ovecoglu, B. Ozkal, C. Suryanarayana, J. Mater. Res. 11 (1996) [21] H.J. Ryu, S.H. Hong, W.H. Baek, J. Mater. Process. Tech. 63 (1997) 292. [22] S.H. Park, D.K. Kim, S. Lee, H.J. Ryu, S.H. Hong, Met. Mater. Trans. A 32 (2001) [23] S.H. Hong, S.J.L. Kang, D.N. Yoon, W.H. Baek, Met. Trans. A 22 (1991) [24] J.W. Noh, E.P. Kim, H.S. Song, W.H. Baek, K.S. Churn, S.J.L. Kang, Met. Trans. A 24 (1993) [25] R.M. German, L.L. Bourguignon, B.H. Rabin, J. Met. 37 (1985) 36.

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