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Microstructure and mechanical properties of spark plasma sintered tungsten heavy alloys
Materials Science & Engineering A 710 (2018) 66–73
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Microstructure and mechanical properties of spark plasma sintered tungsten heavy alloys
MARK
⁎
N. Senthilnathana, , A. Raja Annamalaia,b, G. Venkatachalama a b
School of Mechanical Engineering, VIT University, Vellore 632014, India Centre for Innovative Manufacturing and Research, VIT University, Vellore 632014, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Tungsten heavy alloy Spark plasma sintering Contiguity Tensile strength Fractographs
The effect of cobalt (0.5 wt%, 1.0 wt%, 1.5 wt% and 2.0 wt%), as an alloying element, on the microstructure and mechanical properties of W-Ni-Fe tungsten heavy alloy (WNF) prepared through spark plasma sintering (SPS) process is investigated in this work. The sintering is performed at 1400 °C with a heating rate of 100 °C/min and holding time for a period of 2 min. The properties of the cobalt added heavy alloys (WNFC) are found to be superior to that of the tungsten heavy alloy without cobalt addition. The 1.0% cobalt alloy is observed to give higher yield and tensile strengths compared to other alloys. As the mechanical properties of the alloys depend on the microstructural features, a detailed study on the influence of the microstructural parameters such as average grain size, contiguity and matrix volume fraction on the properties of the alloys is carried out. The average tungsten grain size of WNF alloy is 12.3 µm and that of WNFC alloys is from 9.5µm to 11.5 µm. The control of grain size is significantly evident in the case of spark plasma sintered alloys. The yield strength is found to be influenced by the W-grain size of the microstructure. The contiguity of the WNFC alloys is observed to decrease with increase in percentage of cobalt addition. The fractograph analysis of the tensile tested specimen helps in understanding the tensile behaviour of the alloys. The WNFC alloys show predominantly W-grain cleavage fracture compared to the WNF alloy, possibly due to the good cohesive strength of the matrix phase and W/ matrix interface, because of cobalt addition and also due to the high heating rate followed in the SPS process.
1. Introduction The tungsten heavy alloys (WHA's) are used in applications where higher density, good mechanical properties and workability are required. A typical WHA consists of 88–98 wt% of tungsten, with low melting alloying elements of nickel, iron or copper constituting the remaining proportion [1]. The heavy alloys are fabricated as a bcc structured W phase dispersed in a fcc binder matrix phase of Ni-Fe-W or Ni-Cu-W. Due to its uniqueness of possessing high strength as well as ductility, the WHAs are used in variety of applications such as gyroscope rotors, counter balance weights, kinetic energy penetrators, radiation shields, vibration dampers, rocket nozzles, machining tools and electrical contacts [2]. To attain higher strength and hardness, these alloys are subjected to post sintering treatments such as swaging and aging [3–5]. There is a continuous research in improving the mechanical properties of the base alloys without any post sintering processes i.e. to obtain near net-shaped samples by including other alloying elements like cobalt, rhenium, and molybdenum in the binder matrix and by using novel sintering techniques and improving the processing
⁎
conditions. Therefore, the understanding of the effects of alloying elements and the best process conditions are necessary to obtain a good performance alloy material. The small additions of rhenium and molybdenum are found to control the tungsten grain growth and refine the grains during liquid phase sintering of the alloy, thereby, improving its strength and hardness [6–8]. The molybdenum addition decreases the melting point of the heavy alloy and produces fine W grains. Though the rhenium alloying gives a better performance than molybdenum, its use is limited due to the high cost involved in processing the element. The use of cobalt as alloying element increases strength and ductility of the alloy. The impact strength of the alloy is also increased [6,9]. The presence of cobalt offers solid solution strengthening of the binder matrix as well as strengthens the tungsten-matrix interface. The WHAs are generally processed through powder metallurgy technique. The sintering through conventional technique requires high dwell time of several hours to obtain a dense alloy and also leads to a coarser microstructure and degradation of mechanical properties, if the process conditions are not ideally controlled. During the last two
Corresponding author. E-mail address: [email protected] (N. Senthilnathan).
http://dx.doi.org/10.1016/j.msea.2017.10.080 Received 16 May 2017; Received in revised form 23 October 2017; Accepted 24 October 2017 Available online 25 October 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved.
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SEM micrographs taken from the prepared surfaces of the samples [23]. The contiguity (CWW) is calculated by measuring the number of tungsten-tungsten grain contacts (NWW) and tungsten-matrix interfaces (NWM) using the line-intercept method [24] and applying the same in the Eq. (1),
decades, different sintering techniques have evolved and used by researchers like microwave sintering [10–13], two stage sintering [14,15] and field assisted sintering technique (FAST) or spark plasma sintering (SPS) [16,17]. The objective of using these new techniques is to process the alloys at faster heating rate and thereby controlling the grain growth. The research focus is also on obtaining the required mechanical properties at a lower sintering temperature by using nano sized powder particles, high energy ball milling process [18–20] and solid state processing of the alloy, combining with novel sintering techniques. Spark plasma sintering (SPS) is a novel technique used for the past few years in consolidating metal powders and their alloys [16]. A pulsed direct current is supplied to the metal powders that are kept inside a die with simultaneous application of uniaxial pressure (< 100 MPa). Prior compaction of the powders is not required. The sintering cycle is very short due to the high heating rate (up to 300 °C/ min) which is possible in the SPS process. The overall process of densification is completed within few minutes. The grain growth during the sintering process is restricted due to the rapid rate of heating and thereby better mechanical properties are achieved. The SPS process is also used to consolidate tungsten heavy alloys [21,22]. The present work investigates the effect of different proportions of cobalt addition to W-Ni-Fe system processed through spark plasma sintering technique.
CWW =
(1)
The matrix volume fraction is measured using the SEM micrographs of the prepared samples by point counting method (ASTM E562-99e1). The microhardness of the samples is measured using digital Vicker's hardness tester (Economet VH-1D, Chennai Metco Pvt Ltd.) by applying 50 g of load for 10 s. The indentations are created on ten random locations of the sample surface and the mean of the ten readings is taken as the test result. The tensile specimens are prepared from the sintered samples following ASTM E-8 standards and experimented in a universal tensile testing machine (Instron 8801). The fractured surfaces of the tensile tested samples are inspected using scanning electron microscope (SEM). 3. Results and discussions The compositions of the alloys WNF, WNFC1, WNFC2, WNFC3 and WNFC4 are shown in Table 2. The alloys are sintered using spark plasma sintering technique at a temperature of 1400 °C with a heating rate of 100 °C/min.
2. Experimental procedure The as-received powders of tungsten, cobalt, nickel and iron are blended in the required proportions to design five grades of alloy, namely, WNF (92W-5.6Ni-2.4Fe), WNFC1 (91.5W-5.6Ni-2.4Fe-0.5Co), WNFC2 (91W-5.6Ni-2.4Fe-1.0Co), WNFC3 (90.5W-5.6Ni-2.4Fe-1.5Co) and WNFC4 (90W-5.6Ni-2.4Fe-2.0Co). The characteristics of the powders are presented in Table 1. The blending of powders is done in a VMixer for one hour. The compositions are sintered in a spark plasma sintering furnace (Dr. Sinter, Fuji Electronic Industrial Co. Ltd.), at a temperature of 1400 °C with heating rate of 100 °C/min. The blended powder is placed in a graphite die. The powders are separated from the punch and die by a thin graphite foil for easy removal of the sintered part. The sintering is carried out at the required temperature in vacuum with simultaneous application of 30 MPa pressure on the punch and die system. The samples of 30 mm diameter with average height of 7 mm are obtained. The sintered density is measured using Archimedes density measurement principle. All the samples are initially ground using silicon carbide abrasive papers of 240, 320, 400, 600, 800, 1000 and 1200 grit. The grinding is followed by cloth polishing using a disc polisher (Make: Bainpol-VT, Chennai Metco Pvt Ltd.) with aluminium oxide powder abrasive suspended in water solution. The polished samples are etched using murakami agent (100 ml Distilled water, 10 g Potassium ferricyanide and 10 g Sodium hydroxide) to highlight the microstructural features. The photo micrographs of the sintered samples are obtained through an optical microscope (Zeiss-Axio, Chennai Metco Pvt Ltd.) with digital image acquisition capability. The micrographs with higher magnification are obtained through scanning electron microscope (SEM, ZEISS EVO 18) provided with an energy dispersive spectrometer (EDS) system. The chemical composition of the alloys are analysed using EDS. The average grain size is measured using line-intercept method from
3.1. Densification The supply of high current and high heating rate, followed in the SPS process, results in good physical activation of the powder particles, thereby, cleaning the particle surfaces [25]. The sintering necks start to form between the particles and local diffusion takes place. At faster heating rates the powder particles are subjected to good activation and the sintering mechanism of neck formation and diffusion between the particles occur at a lower temperature [25–27]. An ideal heating rate of 100 °C/min is followed in this experiment, as the compact can be exposed to higher temperatures for a longer period of time, so that, the mass transfer is more efficient. The reduction of surface free energy of the particles is required for the solid-state sintering mechanism to progress. This is achieved through atomic diffusion and mass transfer between the particles [25]. The densification process continues with increase of temperature. A solid solution of mutually soluble elements is formed and they spread over the W particle surfaces through surface diffusion or viscous flow [28]. The pores in the structure get filled simultaneously, resulting in enhancement in values. The possible transfer mechanisms include coalescence of W-W particle contacts, dissolution and precipitation through the binder solution and surface diffusion at W-matrix boundary [29,30]. The Fig. 1 shows the variation of sintered relative density of the tungsten heavy alloys through the SPS process. The cobalt is added to the W-Ni-Fe system to enhance the sintering behaviour of the alloy [6,9]. The cobalt addition provides solid solution strengthening of the matrix phase and increases the tungsten dissolution in the phase [31]. Table 2 Composition of tungsten heavy alloys.
Table 1 Powder characteristics.
Alloy Powder
W
Ni
Co
Fe
Particle size (µm) Purity % Supplier
10
3
4
5
99.9 SigmaAldrich 19.3
99.9 SigmaAldrich 8.908
99.9 SigmaAldrich 8.9
99.9 SigmaAldrich 7.87
Density (g/cm3)
2NWW NWM + 2NWW
WNF WNFC1 WNFC2 WNFC3 WNFC4
67
Composition, wt% W
Ni
Fe
Co
92 91.5 91 90.5 90
5.6 5.6 5.6 5.6 5.6
2.4 2.4 2.4 2.4 2.4
– 0.5 1.0 1.5 2.0
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[9,34,35]. By varying the heating rate and intensity of the current, a higher density could be possibly achieved. 3.2. Microstructure analysis The optical and SEM micrographs of the heavy alloys are shown in Fig. 3. The microstructure reveals polyhedral tungsten grains surrounded by a ductile matrix phase of Ni-Fe-Co-W. The variation in microstructure is attributed to the effect of cobalt on diffusion of tungsten in the matrix phase [13,33]. As the percentage of Co addition is increased, there is a greater degree of tungsten spheroidization observed in the microstructure. The mechanical properties of the tungsten heavy alloys depend on the microstructural parameters such as W-grain size, contiguity, matrix volume fraction and tungsten solubility in the binder phase. The measured parameters are presented in Table 3. The addition of cobalt to the tungsten heavy alloy is found to refine the grain size of tungsten. The W grain size of WNFC alloys is lesser than that of WNF alloy. A decrease in grain size is found to improve the strength of the alloy. The grain size of the WNFC alloys decreases with increase in cobalt content. It is found to be least for WNFC2 alloy containing 1 wt% Co. The addition of cobalt promotes changes in the structure of the tungsten particles by dissolving more tungsten in the matrix phase and thereby strengthening the phase. When the cobalt content exceeds 1.5 wt%, the grain size is observed to increase. This is due to the changes in the matrix volume fraction and decrease in the amount of tungsten dissolution in the matrix phase [36,37]. This change is attributed to the presence of other compound phases in the structure for WNFC3 and WNFC4 alloys, as observed in the X-ray diffraction patterns. Though only Ni-W intermetallic phase is visible, there is a possibility of the presence of other compounds that are not visible in the XRD results due to its small quantity. Hence further additions of cobalt may not improve the performance of the heavy alloys. In addition to the cobalt effect, the finer W-grain size is also due to the higher heating rate followed in the SPS process which decreases the total diffusion time and thereby resulting in reduced coarsening of the grain. The re-precipitation of superfluous tungsten on larger grains is limited due to lower sintering time and temperature (1400 °C) followed in the process. This restricts grain growth of tungsten. The grain sizes are significantly reduced even with a higher initial W particle size compared to the grain sizes of liquid phase sintered tungsten heavy alloys processed through other sintering techniques obtained in the literatures [13,31,38]. The SPS sintered tungsten heavy alloys, with the sintering temperature between 1300 °C and 1400 °C, show solid state sintered microstructure with high contiguity [21,35]. The microstructure with small contiguity provides good strength and ductility. High contiguity structures lead to brittleness of the phases. Contiguity has a significant influence on the tensile behaviour of the alloy. The W-W contiguity is found to be 0.64 for WNF alloy without cobalt addition. The contiguity varies with the cobalt content. It varies from 0.43 to 0.54 for cobalt added alloys. It is observed to decrease with increase of cobalt content in the alloy. The variation in the trend with higher cobalt content, as visible in WNFC4 alloy, is attributed to the change in the amount of dissolution of tungsten in the matrix phase and the volume fraction of the matrix [39]. The contiguity is observed to decrease with the decrease of tungsten grain size. This trend is in consistent with the results observed by Yuanyuan et al. [35]. The decrease in contiguity leads to improvement in mechanical properties of heavy alloys [13]. The volume fraction of the matrix phase for WNFC alloys varies from 7 to 15 vol%. The matrix volume fraction is found to increase with the increase in cobalt addition [40]. The increase in volume fraction of the matrix phase is due to the dissolution of the sharp edges of the tungsten grains, which are considered to be weak contacts, and the ability of cobalt to increase the level of tungsten solubility in the matrix. The amount of tungsten dissolved in the matrix phase is measured from EDS analysis. The addition of cobalt is implied to increase the solubility of
Fig. 1. Variation of density in WNF alloys with increasing % cobalt addition.
Fig. 2. XRD patterns of the sintered tungsten heavy alloys.
The adhesion between the tungsten phase and the matrix phase is improved which leads to provide good mechanical properties for the alloy. The lower density for WNF alloy compared to WNFC alloy is attributed to the presence of residual pores and relatively weaker interface strength between the tungsten and matrix phase [32]. In WNFC alloys the density decreases with increasing cobalt content. The maximum density of 93.36% of theoretical density is obtained for WNFC1 alloy with 0.5 wt% cobalt addition. As the cobalt content is increased further, with corresponding reduction in wt% of W, the density of the alloys are found to decrease. This is attributed to the density difference between the tungsten phase and the binder phase which decreases the compactness of the sample. The enhancement in the sintering behaviour of the tungsten heavy alloys depends on the percentage of the alloying element [33]. The reduction in density for the alloys with 1 wt% of Co addition and above may be due to the formation of intermetallic compounds that retards the diffusion mechanism during the sintering process. The X-ray diffraction patterns of the alloys are shown in Fig. 2. The appearance of bcc tungsten phase is seen as major peaks (JCPDS 89-4900) in all the alloys, with relatively weak peaks of Ni-Fe solid solution (JCPDS 47-1417) in WNF and WNFC4 alloys. The presence of small amount of intermetallic phase of Ni-W (JCPDS 47-1172) in WNFC4 alloy may be the possible reason for reduced density for this alloy. With increase in addition of cobalt, the diffraction peaks of the tungsten phase is found to decrease. The maximum density, obtained in the literatures for different compositions of WNF alloys using nano powders and sintered through spark plasma sintering and vacuum sintering at identical temperature conditions, ranges from 93% to 95% 68
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Fig. 3. Optical and SEM images of WNF(a, b), WNFC1(c, d), WNFC2(e, f), WNFC3 (g, h) and WNFC4(i, j) respectively.
and matrix strengthening effect [33,41]. The strength of the WHA's depends on the sintering behaviour and microstructural factors such as tungsten grain size, cohesion between tungsten and matrix phase, contiguity and other microstructural factors [35,38]. The yield and tensile strengths of WNFC alloys are higher than that of WNF alloy. The yield strength increases from 686 MPa for WNF alloy to 770 MPa for WNFC1 alloy. The alloy is strengthened through solid solution hardening which is the major advantage in cobalt added heavy alloys. The binder Ni-Fe-Co provides a ductile bonding matrix for the heavy alloy and also strengthens the interfacial strength between the tungsten and the matrix phase. The grain size also influences the yield strength of the alloy. When the grain size decreases, there is a lower possibility of finding energetic atom clusters to move the dislocations formed at the grain boundaries to initiate plasticity [42]. This leads to Hall-Petch strengthening, resulting in higher yield stress for the alloy. According to Hall-Petch relation, the grain size is found to influence the yield strength of the alloy by inverse correlation [43]. The alloy with 1%
tungsten in the matrix phase [31]. The dissolved tungsten in WNFC alloys is found to vary in the range of 33–44%.
3.3. Mechanical properties The strength properties and the ductility of tungsten heavy alloys are found to be optimal with Ni/Fe ratios ranging from 2 to 4 [7]. The ratio of 7:3 is suggested to provide good properties of the alloy by avoiding the formation of intermetallic compounds [13]. The influence of cobalt, as an additive to the tungsten heavy alloy of W-Ni-Fe, on the microstructure and mechanical properties of the alloy are analysed while keeping the ratio of Ni:Fe constant. The experimental results of the mechanical properties of the alloys are listed in Table 4. The hardness is noted to increase with increase in cobalt addition. Further, the hardness of WNFC alloys is greater than the WNF alloy. The alloy with 2 wt% cobalt addition gives the highest hardness of 499Hv50. The increase in hardness is attributed to the cobalt related surface activation 69
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Fig. 3. (continued)
depends on the diffusion of atoms which is a function of sintering temperature and time. The other possible reason for lower ductility in W-Ni-Fe system is the presence of residual porosity in the sintered alloy [32]. The W-W interfaces are the low energy preferred path for crack nucleation. A lesser contiguity value implies that such crack nucleation sites are less in WNFC alloys thereby resulting in greater ductility. The cobalt addition leads to increase in tungsten-matrix and tungstentungsten interface cohesive strength thereby enhancing ductility [13]. Generally, in tungsten heavy alloys, larger grain size with lower contiguity contributes to higher ductility, but with some loss in strength of the alloy [33]. However, the SPS processed alloys show controlled grain growth, giving a finer grain size and higher contiguity compared to the liquid phase sintered (LPS) W-Ni-Fe and W-Ni-Fe-Co alloys by different sintering techniques, but, provides good ductility and strength for the alloys. Table 5 gives an overview of the mechanical properties of tungsten heavy alloys obtained through various sintering processes from reference literatures for comparison with the results obtained in the present work. As sintering in SPS is done in vacuum, there is no problem of hydrogen embrittlement and also impurity segregation in the matrix and interface is avoided due to high heating rate.
Table 3 Microstructural parameters of the tungsten heavy alloys. Alloy
Grain size µm
Contiguity
Matrix volume fraction (vol%)
Dissolved tungsten in matrix (wt%)
WNF WNFC1 WNFC2 WNFC3 WNFC4
12.3 ± 5.2 11.56 ± 3.5 9.48 ± 3.2 9.68 ± 4.1 11.1 ± 6.3
0.64 ± 0.04 0.54 ± 0.02 0.47 ± 0.03 0.43 ± 0.03 0.50 ± 0.04
18.7 ± 2.6 7.9 ± 2.7 8.9 ± 1.3 15.4 ± 3.1 12.3 ± 2.3
28.5 ± 3.7 33.6 ± 2.8 35.2 ± 1.6 43.7 ± 3.2 38.4 ± 3.5
Table 4 Observed mechanical properties of the tungsten heavy alloys. Alloy
Yield strength MPa
Tensile strength MPa
% Elongation
Microhardness Hv50
WNF WNFC1 WNFC2 WNFC3 WNFC4
686 770 1300 1080 1000
975 961 1508 1330 1256
12 16 20 21 18
385 ± 14 455 ± 11 467 ± 08 471 ± 16 499 ± 11
3.4. Fractography
cobalt addition shows yield strength of 1300 MPa with a fine tungsten grain size of 9.48 µm. Further, the obtained yield strength results for WNFC alloys are greater than the values found in other research works [31,41,44] processed through different sintering techniques. The lower tensile strength in the WNF alloy may be due to the high contiguity and relatively lesser cohesive strength between the tungsten grains. The cobalt containing alloys show higher elongation than WNF alloy. This is attributed to several reasons such as higher solubility of tungsten in the matrix phase, strength of the tungsten-tungsten and tungsten-matrix interfaces, lower contiguity and connectivity [6]. Ductility depends on bonding between the particles and phases. The degree of bonding
The addition of cobalt and the sintering through SPS process have a significant effect on the fracture modes of the alloys. The SEM micrographs of the fractured surfaces of the tensile tested specimens are shown in Fig. 4. The WNF fractured structure is observed to exhibit predominantly local matrix and interface failure. The 0.5 wt% cobalt alloy (WNFC1) shows more of matrix interface fracture mode and local matrix ductile tearing with few regions of W-grain cleavage features. Therefore, these two alloys confirm the lower tensile behaviour nature [38,45]. The tungsten heavy alloy with 1.0 wt% cobalt (WNFC2) indicates the presence of more regions of transgranular tungsten cleavage 70
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Table 5 Mechanical properties of sintered tungsten heavy alloys obtained in reference literatures. Composition
Processing method
Yield Strength MPa
Tensile strength MPa
% Elongation
Reference
92.5W–6.4Ni–1.1Fe
– –
[10]
90W–5Ni–2Fe–3Co
Sintering at 1480 °C in H2 for 120 min
90.5W–7.1Ni–1.65Fe–0.50Co–0.25Mo
Sintering at 1460 °C in H2 atmosphere for 90 min
93W–3.0Ni–2.0Fe–2.0Co
Sintering under vacuum condition at 1400 °C for 60 min
805 ± 14 642 ± 23 750 (At 1400 °C) 1185 (At 1500 °C) 1200 989 1050 675 993 608 976 608 920 (Heat treated) 980 ± 40
11.2 ± 1.1 3.5 ± 0.8 4.2 16.4
90W–7Ni–3Fe 90W–6Ni–2Fe–2Co 90W–7.2-Ni–1.8Fe–1Co 90W–6Ni–2Fe–2Co
Microwave sintering at 1500 °C for 20 min in H2 atmosphere. Conventional sintering at 1500 °C for 20 min in H2 atmosphere. Microwave sintering at 1400 °C and 1500 °C for 5 min in reducing atmosphere Sintering under vacuum condition at 1350 °C for 60 min Microwave sintering at 1470 °C for 60 min in H2 atmosphere Sintering at 1470 °C in H2 atmosphere for 90 min Sintering at 1480 °C in H2 for 120 min
93W–4.9Ni–2.1Fe
673 667 (Heat treated) 603 658 (Heat treated) 542 642 (Heat treated)
16 4 27 1 24 5
[15] [30] [13] [19] [31] [31] [41] [44]
Fig. 4. Fracture surfaces of the tungsten heavy alloys. (a) WNF (b) WNFC1 (c) WNFC2 (d) WNFC3 and (e) WNFC4. W-tungsten grain cleavage, W-W – tungsten decohesion, M-Matrix rupture, MC-Microcracks and P-pore.
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Acknowledgements
fracture and few features of matrix interface failure. This is in consistent with the higher tensile strength obtained for the alloy. The higher amount of intergranular matrix phase in WNFC alloys compared to the WNF alloy contributed to better densification of the cobalt added alloys [38]. In SPS technique, due to the intense heat and high heating rate, the impurities get homogeneously distributed instead of segregating at the matrix interface. Moreover, the cobalt addition increases the cohesive strength between the tungsten-matrix and W-W interface, thereby enhancing ductility and strength [13]. The liquid phase sintered W-Ni-Fe-Co alloy in the work by Jiten das et al. and ravi kiran et al. [31,38] through conventional sintering process showed primarily matrix or interface failure mode, that is related to poor tensile properties which transforms to transgranular W-cleavage fracture only after post-sintering heat treatment or swaging operations. The requirement of post sintering operations can be negated by obtaining near-net shape samples using SPS technique through proper control of the alloy composition and SPS process parameters. The alloys, with 1.5 wt% and 2 wt% cobalt (WNFC3 and WNFC4), show more features of tungsten-tungsten and tungsten-matrix debonding compared to 1.0 wt% cobalt alloy in addition to the W-cleavage fracture modes. This could be the reason for lower tensile properties for these alloys in comparison with WNFC2. The micro-cracks are not predominant in the fracture modes of WNFC alloys, while large number of micro-cracks are visible in WNF alloy as realized in this work and also in consistent with other works [21]. This can be attributed to the improved tungsten-tungsten cohesive strength of the WNFC alloys due to cobalt addition. The presence of micro-cracks in the tungsten grain boundaries is due to the morphology of the alloy containing polygon W-grains with high contiguity that may result in stress concentration at the triple tungsten boundary junctions. But, the addition of cobalt minimizes this effect.
The authors gratefully acknowledge VIT University, Vellore, India for the support through Seed Grant for Research. References [1] Nicolas. G, Voltz. M, Tungsten heavy metal alloys – relations between the crystallographic texture and the internal stress distribution, in: Proceedings of the 15th International Plansee Seminar, 1, 2001, pp. 780–781. [2] Y. Sahin, Recent progress in processing of tungsten heavy alloys, J. Powder Technol. (2014) 1–22. [3] Don-Kuk Kim, Sunghak Lee, Joon Woong Noh, Dynamic and quasi-static torsional behavior of tungsten heavy alloy specimens fabricated through sintering, heattreatment, swaging and aging, Mater. Sci. Eng. A 247 (1998) 285–294. [4] Yang Yu, Wencong Zhang, Yu Chen, Erde Wang, Effect of swaging on microstructure and mechanical properties of liquid phase sintered 93W-4.9(Ni,Co)-2.1Fe alloy, Int. J. Refract. Met. Hard Mater. 44 (2014) 103–108. [5] Nuri durlu, Kaan Caliskan, Sakir Bor, Effect of swaging on microstructure and tensile properties of W-Ni-Fe alloy, Int. J. Refract. Met. Hard Mater. 42 (2014) 126–131. [6] U. Ravi kiran, A. Panchal, M. Sankaranarayana, G.V.S. Nageswararao, T.K. Nandy, Effect of alloying addition and microstructural parameters on mechanical properties of 93% tungsten heavy alloys, Mater. Sci. Eng. A 640 (2015) 82–90. [7] R.M. German, Microstructure and impurity effects on tungsten heavy alloys, final report, U.S. Army Research Office, 1990, pp. 1–19. [8] D.P. Xiang, L. Ding, Y.Y. Li, X.Y. Chen, T.M. Zhang, Fabricating fine-grained tungsten heavy alloy by spark plasma sintering of low-energy ball-milled W–2Mo–7Ni–3Fe powders, Mater. Sci. Eng. A 578 (2013) 18–23. [9] Zeinab Abdel Hamid, Sayed Farag Moustafa, Walid Mohamed Daoush, Fatema Abdel Mouez, Moha Hassan, Fabrication and characterization of tungsten heavy alloys using chemical reduction and mechanical alloying methods, Open J. Appl. Sci. 3 (2013) 15–27. [10] A. Upadhyaya, S.K. Tiwari, P. Mishra, Microwave sintering of W–Ni–Fe alloy, Scr. Mater. 56 (2007) 5–8. [11] Avijit Mondal, Anish Upadhyaya, Dinesh Agrawal, Microwave sintering of tungsten based alloys, in: Proceedings of the International Conference on tungsten, refractory and hard materials VII, 3, 2008, pp. 122–133. [12] Avijit Mondal, Anish Upadhyaya, Dinesh Agrawal, Microwave and conventional sintering of 90W-7Ni-3Cu alloys with premixed and prealloyed binder phase, Mater. Sci. Eng. A 527 (2010) 6870–6878. [13] G. Prabhu, N. Arvind kumar, M. Sankaranarayana, T.K. Nandy, Tensile and impact properties of microwave sintered tungsten heavy alloys, Mater. Sci. Eng. A 607 (2014) 63–70. [14] K.H. Lee, S.I. Cha, H.J. Ryu, S.H. Hong, Effect of two-stage sintering process on microstructure and mechanical properties of ODS tungsten heavy alloy, Mater. Sci. Eng. A 458 (1) (2007) 323–329. [15] Jiajia Zhang, Wensheng Liu, Yunzhu Ma, Xiaoshan Ye, Yayu Wu, An improved twostage sintering method for tungsten heavy alloys: conventional solid-phase sintering followed by microwave heating, Int. J. Mater. Res. 106 (1) (2015) 80–82. [16] M. Saiprasad, R. Atchayakumar, K. Thiruppathi, S. Raghuraman, Consolidation of copper and aluminium powders by spark plasma sintering, Mater. Sci. Eng. 149 (2016) 012057. [17] K. Hu, X. Li, S. Qu, Y. Li, Spark plasma sintering of W-5.6Ni-1.2Fe heavy alloys: densification and grain growth, Metall. Mater. Trans. A 44A (2013) 923–933. [18] S.H. Islam, M.A. Al-Eshaikh, Z. Huda, Synthesis and characterization of high energy ball milled tungsten heavy alloyed powders, Arab. J. Sci. Eng. 38 (9) (2013) 2503–2507. [19] K.B. Povarova, M.I. Alymov, O.S. Gavrilin, A.A. Drozdov, A.I. Kachnov, N.L. Korenovskii, I.O. Bannykh, Structure and properties of W-Ni-Fe-Co heavy alloys compacted from nano powders, Russ. Metall. (2008) 52–55. [20] H. Elshimy, Z. Heiba, K. El-Sayed, Structural, microstructural, mechanical and magnetic characterization of ball milled tungsten heavy alloys, Adv. Mater. Phys. Chem. 4 (2014) 237–257. [21] X. Li, K. Hu, S. Qu, Li. Li, C. Yang, 93W–5.6Ni–1.4Fe heavy alloys with enhanced performance prepared by cyclic spark plasma sintering, Mater. Sci. Eng. A 599 (2014) 233–241. [22] Mxolisi Brendon Shongwe, Saliou Diouf, Mondiu Olayinka Durowoju, Peter Apata Olubambi, Munyadziwa Mercy Ramakokovhu, Babatunde Abiodun Obadele, A comparative study of spark plasma sintering and hybrid spark plasma sintering of 93W–4.9Ni–2.1Fe heavy alloy, Int. J. Refract. Met. Hard Mater. 55 (2016) 16–23. [23] ASTM E112-96, Standard Test Methods for Determining Average Grain Size, 16 ASTM, Philadelphia, PA, 1996. [24] J. Gurland, The measurement of grain contiguity in two-phase alloys, Trans. Met. Soc AIME 212 (1958) 452–455. [25] Ke Hu, Xiao-qiang Li, Chao Yang, Yuan-yuan Li, Densification and microstructure evolution during SPS consolidation process in W-Ni-Fe System, Trans. Nonferrous Met. Soc. China 21 (2011) 493–501. [26] M. Lungu, V. Tsakiris, E. Enescu, D. Patroi, V. Marinescu, D. Talpeanu, Development of W-Cu-Ni electrical contact materials with enhanced mechanical properties by spark plasma sintering process, Acta Phys. Pol. A 125 (2014) 327–330. [27] N.S. Ermakova, M.S. Yurlova, Grigoryev, Properties of tungsten heavy alloys prepared by spark-plasma sintering, IOP Conf. Ser. Mater. Sci. Eng. 130 (2016) 1–6.
4. Conclusions 1. The maximum relative density of 93.36% is observed for the tungsten heavy alloy with 0.5 wt% cobalt addition. The relative density is found to decrease with increase in the percentage of cobalt addition. 2. The spark plasma sintered WNF and WNFC alloys show finer Wgrain size with respect to the initial particle size. The W-grain size of WNF alloy is 12.3 µm. The alloy with 1 wt% cobalt gives the least average grain size of 9.48 µm. The contiguity of cobalt added alloys are lower compared to the WNF alloy, which implies enhanced mechanical properties for WNFC alloys. The W-W contiguity is found to decrease with increase of cobalt percentage. 3. The addition of cobalt improves the dissolution of tungsten in the matrix phase. This is validated with increasing matrix volume fraction and decreasing contiguity with increase in the percentage addition of cobalt. 4. The tensile properties of the WNFC alloys are superior to the WNF alloy. The yield strength of the alloys is found to depend on the tungsten grain size. The alloy with 1 wt% cobalt gives the highest yield strength of 1300 MPa and tensile strength of 1508 MPa. 5. The mode of fracture in WNF alloy is intergranular with tungstenmatrix debonding dominating the fractograph. In contrast, the WNFC alloys, especially the alloys with 1 wt% and more, fail predominantly by transgranular failure mode with more of W-grain cleavage. This may be the reason for superior tensile properties for the cobalt added alloys. 6. The present investigations reveal the effectiveness of SPS as a sintering technique and cobalt as an effective alloying element to obtain higher strength alloys with good ductility. By proper control of the process parameters, it is possible to obtain tungsten heavy alloys with enhanced mechanical properties in as-sintered stage itself, thereby avoiding post sintering processes.
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Powder Metall. Metal. Ceram. 46 (2007) 517–524. [37] Yu.V. Milman, The effect of structural state and temperature on mechanical properties and deformation mechanisms of WC-Co hard alloy, J. Superhard Mater. 36 (2014) 65–81. [38] G. Jiten das, Appa Rao, S.K. Pabi, Microstructure and mechanical properties of tungsten heavy alloys, Mater. Sci. Eng. A 527 (2010) 7841–7847. [39] H.E. Exner, J. Gurland, A review of parameters influencing some mechanical properties of tungsten carbide-cobalt alloys, Powder Metall. 13 (1970) 13–31. [40] B. Katavic, M. Nikacevic, Z. Odanovic, Effect of cold swaging and heat treatment on properties of the P/M 91W-6Ni-3Co heavy alloy, Sci. Sinter. 40 (2008) 319–331. [41] G. Jiten das, Appa Rao, S.K. Pabi, M. Sankaranarayanan, T.K. Nandy, Thermomechanical processing, microstructure and tensile properties of a tungsten heavy alloy, Mater. Sci. Eng. A 613 (2014) 48–49. [42] Ju-Young Kim, Dongchan Jang, Julia R. Greer, Tensile and compressive behaviour of tungsten, molybdenum, tantalum and niobium at the nanoscale, Acta Mater. 58 (2010) 2355–2363. [43] Hansen Niels, Hall-Petch relation and boundary strengthening, Scr. Mater. 51 (2004) 801–806. [44] C.C. Fu, L.J. Chang, Y.C. Huang, P.W. Wong, J.S.C. Jang, Microstructure and mechanical properties of solid phase sintered heavy tungsten alloy, Adv. Mater. Res. 15–17 (2007) 575–580. [45] M. Kaczorowski, P. Skoczylas, A. Krzynska, The influence of Fe content on spreading ability of tungsten heavy alloys matrix on tungsten surface, Arch. Foundry Eng. 11 (2011) 103–106.
[28] H.R. De Macedo, A.G.P. da Silva, D.M.A. de Melo, The spreading of cobalt, nickel and iron on tungsten carbide and the first stage of hard metal sintering, Mater. Lett. 57 (2003) 3924–3932. [29] James R. Spencer, James A. Mullendore, Relationship between composition, structure, properties, thermo-mechanical processing and ballistic performance of tungsten heavy alloys, Rep. U.S. Army Mater. Technol. Lab. (1991). [30] Akhtar Farid, An investigation on the solid state sintering of mechanically alloyed nano-structured 90W-Ni-Fe tungsten heavy alloy, Int. J. Refract. Met. Hard Mater. 26 (2008) 145–151. [31] U. Ravi kiran, M. Sankaranarayana, G.V.S. Nageswararao, T.K. Nandy, Effect of cobalt addition on microstructure and mechanical properties of tungsten heavy alloys, Trans. Indian Inst. Met. 70 (2017) 615–622. [32] R.M. German, K.S. Churn, Sintering atmosphere effects on the ductility of W-Ni-Fe heavy metals, Metall. Trans. A 15 (4) (1984) 747–754. [33] J. Wittenauer, T.G Nieh, Development of fine-grained ductile tungsten alloys for anti-armor applications, A Report for U.S. Army Research Office, by Lockheed Missiles and Space Company, Inc, 1991. [34] Xiao-qiang Li, Hong-wei Xin, Ke Hu, Yuan-yuan Li, Microstructure and properties of ultra-fine tungsten heavy alloys prepared by mechanical alloying and electric current activated sintering, Trans. Nonferrous Met. Soc. China 20 (2010) 443–449. [35] Yuanyuan Li, Ke Hu, Xiaoqiang Li, Xuan Ai, Shengguan Qu, Fine grained 93W5.6Ni-1.4Fe heavy alloys with enhanced performance prepared by spark plasma sintering, Mater. Sci. Eng. A 573 (2013) 245–252. [36] A.V. Laptev, Structure and properties of WC-Co alloys in solid phase sintering,
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Microstructure and mechanical properties of spark plasma sintered tungsten heavy alloys
MARK
⁎
N. Senthilnathana, , A. Raja Annamalaia,b, G. Venkatachalama a b
School of Mechanical Engineering, VIT University, Vellore 632014, India Centre for Innovative Manufacturing and Research, VIT University, Vellore 632014, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Tungsten heavy alloy Spark plasma sintering Contiguity Tensile strength Fractographs
The effect of cobalt (0.5 wt%, 1.0 wt%, 1.5 wt% and 2.0 wt%), as an alloying element, on the microstructure and mechanical properties of W-Ni-Fe tungsten heavy alloy (WNF) prepared through spark plasma sintering (SPS) process is investigated in this work. The sintering is performed at 1400 °C with a heating rate of 100 °C/min and holding time for a period of 2 min. The properties of the cobalt added heavy alloys (WNFC) are found to be superior to that of the tungsten heavy alloy without cobalt addition. The 1.0% cobalt alloy is observed to give higher yield and tensile strengths compared to other alloys. As the mechanical properties of the alloys depend on the microstructural features, a detailed study on the influence of the microstructural parameters such as average grain size, contiguity and matrix volume fraction on the properties of the alloys is carried out. The average tungsten grain size of WNF alloy is 12.3 µm and that of WNFC alloys is from 9.5µm to 11.5 µm. The control of grain size is significantly evident in the case of spark plasma sintered alloys. The yield strength is found to be influenced by the W-grain size of the microstructure. The contiguity of the WNFC alloys is observed to decrease with increase in percentage of cobalt addition. The fractograph analysis of the tensile tested specimen helps in understanding the tensile behaviour of the alloys. The WNFC alloys show predominantly W-grain cleavage fracture compared to the WNF alloy, possibly due to the good cohesive strength of the matrix phase and W/ matrix interface, because of cobalt addition and also due to the high heating rate followed in the SPS process.
1. Introduction The tungsten heavy alloys (WHA's) are used in applications where higher density, good mechanical properties and workability are required. A typical WHA consists of 88–98 wt% of tungsten, with low melting alloying elements of nickel, iron or copper constituting the remaining proportion [1]. The heavy alloys are fabricated as a bcc structured W phase dispersed in a fcc binder matrix phase of Ni-Fe-W or Ni-Cu-W. Due to its uniqueness of possessing high strength as well as ductility, the WHAs are used in variety of applications such as gyroscope rotors, counter balance weights, kinetic energy penetrators, radiation shields, vibration dampers, rocket nozzles, machining tools and electrical contacts [2]. To attain higher strength and hardness, these alloys are subjected to post sintering treatments such as swaging and aging [3–5]. There is a continuous research in improving the mechanical properties of the base alloys without any post sintering processes i.e. to obtain near net-shaped samples by including other alloying elements like cobalt, rhenium, and molybdenum in the binder matrix and by using novel sintering techniques and improving the processing
⁎
conditions. Therefore, the understanding of the effects of alloying elements and the best process conditions are necessary to obtain a good performance alloy material. The small additions of rhenium and molybdenum are found to control the tungsten grain growth and refine the grains during liquid phase sintering of the alloy, thereby, improving its strength and hardness [6–8]. The molybdenum addition decreases the melting point of the heavy alloy and produces fine W grains. Though the rhenium alloying gives a better performance than molybdenum, its use is limited due to the high cost involved in processing the element. The use of cobalt as alloying element increases strength and ductility of the alloy. The impact strength of the alloy is also increased [6,9]. The presence of cobalt offers solid solution strengthening of the binder matrix as well as strengthens the tungsten-matrix interface. The WHAs are generally processed through powder metallurgy technique. The sintering through conventional technique requires high dwell time of several hours to obtain a dense alloy and also leads to a coarser microstructure and degradation of mechanical properties, if the process conditions are not ideally controlled. During the last two
Corresponding author. E-mail address: [email protected] (N. Senthilnathan).
http://dx.doi.org/10.1016/j.msea.2017.10.080 Received 16 May 2017; Received in revised form 23 October 2017; Accepted 24 October 2017 Available online 25 October 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved.
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SEM micrographs taken from the prepared surfaces of the samples [23]. The contiguity (CWW) is calculated by measuring the number of tungsten-tungsten grain contacts (NWW) and tungsten-matrix interfaces (NWM) using the line-intercept method [24] and applying the same in the Eq. (1),
decades, different sintering techniques have evolved and used by researchers like microwave sintering [10–13], two stage sintering [14,15] and field assisted sintering technique (FAST) or spark plasma sintering (SPS) [16,17]. The objective of using these new techniques is to process the alloys at faster heating rate and thereby controlling the grain growth. The research focus is also on obtaining the required mechanical properties at a lower sintering temperature by using nano sized powder particles, high energy ball milling process [18–20] and solid state processing of the alloy, combining with novel sintering techniques. Spark plasma sintering (SPS) is a novel technique used for the past few years in consolidating metal powders and their alloys [16]. A pulsed direct current is supplied to the metal powders that are kept inside a die with simultaneous application of uniaxial pressure (< 100 MPa). Prior compaction of the powders is not required. The sintering cycle is very short due to the high heating rate (up to 300 °C/ min) which is possible in the SPS process. The overall process of densification is completed within few minutes. The grain growth during the sintering process is restricted due to the rapid rate of heating and thereby better mechanical properties are achieved. The SPS process is also used to consolidate tungsten heavy alloys [21,22]. The present work investigates the effect of different proportions of cobalt addition to W-Ni-Fe system processed through spark plasma sintering technique.
CWW =
(1)
The matrix volume fraction is measured using the SEM micrographs of the prepared samples by point counting method (ASTM E562-99e1). The microhardness of the samples is measured using digital Vicker's hardness tester (Economet VH-1D, Chennai Metco Pvt Ltd.) by applying 50 g of load for 10 s. The indentations are created on ten random locations of the sample surface and the mean of the ten readings is taken as the test result. The tensile specimens are prepared from the sintered samples following ASTM E-8 standards and experimented in a universal tensile testing machine (Instron 8801). The fractured surfaces of the tensile tested samples are inspected using scanning electron microscope (SEM). 3. Results and discussions The compositions of the alloys WNF, WNFC1, WNFC2, WNFC3 and WNFC4 are shown in Table 2. The alloys are sintered using spark plasma sintering technique at a temperature of 1400 °C with a heating rate of 100 °C/min.
2. Experimental procedure The as-received powders of tungsten, cobalt, nickel and iron are blended in the required proportions to design five grades of alloy, namely, WNF (92W-5.6Ni-2.4Fe), WNFC1 (91.5W-5.6Ni-2.4Fe-0.5Co), WNFC2 (91W-5.6Ni-2.4Fe-1.0Co), WNFC3 (90.5W-5.6Ni-2.4Fe-1.5Co) and WNFC4 (90W-5.6Ni-2.4Fe-2.0Co). The characteristics of the powders are presented in Table 1. The blending of powders is done in a VMixer for one hour. The compositions are sintered in a spark plasma sintering furnace (Dr. Sinter, Fuji Electronic Industrial Co. Ltd.), at a temperature of 1400 °C with heating rate of 100 °C/min. The blended powder is placed in a graphite die. The powders are separated from the punch and die by a thin graphite foil for easy removal of the sintered part. The sintering is carried out at the required temperature in vacuum with simultaneous application of 30 MPa pressure on the punch and die system. The samples of 30 mm diameter with average height of 7 mm are obtained. The sintered density is measured using Archimedes density measurement principle. All the samples are initially ground using silicon carbide abrasive papers of 240, 320, 400, 600, 800, 1000 and 1200 grit. The grinding is followed by cloth polishing using a disc polisher (Make: Bainpol-VT, Chennai Metco Pvt Ltd.) with aluminium oxide powder abrasive suspended in water solution. The polished samples are etched using murakami agent (100 ml Distilled water, 10 g Potassium ferricyanide and 10 g Sodium hydroxide) to highlight the microstructural features. The photo micrographs of the sintered samples are obtained through an optical microscope (Zeiss-Axio, Chennai Metco Pvt Ltd.) with digital image acquisition capability. The micrographs with higher magnification are obtained through scanning electron microscope (SEM, ZEISS EVO 18) provided with an energy dispersive spectrometer (EDS) system. The chemical composition of the alloys are analysed using EDS. The average grain size is measured using line-intercept method from
3.1. Densification The supply of high current and high heating rate, followed in the SPS process, results in good physical activation of the powder particles, thereby, cleaning the particle surfaces [25]. The sintering necks start to form between the particles and local diffusion takes place. At faster heating rates the powder particles are subjected to good activation and the sintering mechanism of neck formation and diffusion between the particles occur at a lower temperature [25–27]. An ideal heating rate of 100 °C/min is followed in this experiment, as the compact can be exposed to higher temperatures for a longer period of time, so that, the mass transfer is more efficient. The reduction of surface free energy of the particles is required for the solid-state sintering mechanism to progress. This is achieved through atomic diffusion and mass transfer between the particles [25]. The densification process continues with increase of temperature. A solid solution of mutually soluble elements is formed and they spread over the W particle surfaces through surface diffusion or viscous flow [28]. The pores in the structure get filled simultaneously, resulting in enhancement in values. The possible transfer mechanisms include coalescence of W-W particle contacts, dissolution and precipitation through the binder solution and surface diffusion at W-matrix boundary [29,30]. The Fig. 1 shows the variation of sintered relative density of the tungsten heavy alloys through the SPS process. The cobalt is added to the W-Ni-Fe system to enhance the sintering behaviour of the alloy [6,9]. The cobalt addition provides solid solution strengthening of the matrix phase and increases the tungsten dissolution in the phase [31]. Table 2 Composition of tungsten heavy alloys.
Table 1 Powder characteristics.
Alloy Powder
W
Ni
Co
Fe
Particle size (µm) Purity % Supplier
10
3
4
5
99.9 SigmaAldrich 19.3
99.9 SigmaAldrich 8.908
99.9 SigmaAldrich 8.9
99.9 SigmaAldrich 7.87
Density (g/cm3)
2NWW NWM + 2NWW
WNF WNFC1 WNFC2 WNFC3 WNFC4
67
Composition, wt% W
Ni
Fe
Co
92 91.5 91 90.5 90
5.6 5.6 5.6 5.6 5.6
2.4 2.4 2.4 2.4 2.4
– 0.5 1.0 1.5 2.0
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[9,34,35]. By varying the heating rate and intensity of the current, a higher density could be possibly achieved. 3.2. Microstructure analysis The optical and SEM micrographs of the heavy alloys are shown in Fig. 3. The microstructure reveals polyhedral tungsten grains surrounded by a ductile matrix phase of Ni-Fe-Co-W. The variation in microstructure is attributed to the effect of cobalt on diffusion of tungsten in the matrix phase [13,33]. As the percentage of Co addition is increased, there is a greater degree of tungsten spheroidization observed in the microstructure. The mechanical properties of the tungsten heavy alloys depend on the microstructural parameters such as W-grain size, contiguity, matrix volume fraction and tungsten solubility in the binder phase. The measured parameters are presented in Table 3. The addition of cobalt to the tungsten heavy alloy is found to refine the grain size of tungsten. The W grain size of WNFC alloys is lesser than that of WNF alloy. A decrease in grain size is found to improve the strength of the alloy. The grain size of the WNFC alloys decreases with increase in cobalt content. It is found to be least for WNFC2 alloy containing 1 wt% Co. The addition of cobalt promotes changes in the structure of the tungsten particles by dissolving more tungsten in the matrix phase and thereby strengthening the phase. When the cobalt content exceeds 1.5 wt%, the grain size is observed to increase. This is due to the changes in the matrix volume fraction and decrease in the amount of tungsten dissolution in the matrix phase [36,37]. This change is attributed to the presence of other compound phases in the structure for WNFC3 and WNFC4 alloys, as observed in the X-ray diffraction patterns. Though only Ni-W intermetallic phase is visible, there is a possibility of the presence of other compounds that are not visible in the XRD results due to its small quantity. Hence further additions of cobalt may not improve the performance of the heavy alloys. In addition to the cobalt effect, the finer W-grain size is also due to the higher heating rate followed in the SPS process which decreases the total diffusion time and thereby resulting in reduced coarsening of the grain. The re-precipitation of superfluous tungsten on larger grains is limited due to lower sintering time and temperature (1400 °C) followed in the process. This restricts grain growth of tungsten. The grain sizes are significantly reduced even with a higher initial W particle size compared to the grain sizes of liquid phase sintered tungsten heavy alloys processed through other sintering techniques obtained in the literatures [13,31,38]. The SPS sintered tungsten heavy alloys, with the sintering temperature between 1300 °C and 1400 °C, show solid state sintered microstructure with high contiguity [21,35]. The microstructure with small contiguity provides good strength and ductility. High contiguity structures lead to brittleness of the phases. Contiguity has a significant influence on the tensile behaviour of the alloy. The W-W contiguity is found to be 0.64 for WNF alloy without cobalt addition. The contiguity varies with the cobalt content. It varies from 0.43 to 0.54 for cobalt added alloys. It is observed to decrease with increase of cobalt content in the alloy. The variation in the trend with higher cobalt content, as visible in WNFC4 alloy, is attributed to the change in the amount of dissolution of tungsten in the matrix phase and the volume fraction of the matrix [39]. The contiguity is observed to decrease with the decrease of tungsten grain size. This trend is in consistent with the results observed by Yuanyuan et al. [35]. The decrease in contiguity leads to improvement in mechanical properties of heavy alloys [13]. The volume fraction of the matrix phase for WNFC alloys varies from 7 to 15 vol%. The matrix volume fraction is found to increase with the increase in cobalt addition [40]. The increase in volume fraction of the matrix phase is due to the dissolution of the sharp edges of the tungsten grains, which are considered to be weak contacts, and the ability of cobalt to increase the level of tungsten solubility in the matrix. The amount of tungsten dissolved in the matrix phase is measured from EDS analysis. The addition of cobalt is implied to increase the solubility of
Fig. 1. Variation of density in WNF alloys with increasing % cobalt addition.
Fig. 2. XRD patterns of the sintered tungsten heavy alloys.
The adhesion between the tungsten phase and the matrix phase is improved which leads to provide good mechanical properties for the alloy. The lower density for WNF alloy compared to WNFC alloy is attributed to the presence of residual pores and relatively weaker interface strength between the tungsten and matrix phase [32]. In WNFC alloys the density decreases with increasing cobalt content. The maximum density of 93.36% of theoretical density is obtained for WNFC1 alloy with 0.5 wt% cobalt addition. As the cobalt content is increased further, with corresponding reduction in wt% of W, the density of the alloys are found to decrease. This is attributed to the density difference between the tungsten phase and the binder phase which decreases the compactness of the sample. The enhancement in the sintering behaviour of the tungsten heavy alloys depends on the percentage of the alloying element [33]. The reduction in density for the alloys with 1 wt% of Co addition and above may be due to the formation of intermetallic compounds that retards the diffusion mechanism during the sintering process. The X-ray diffraction patterns of the alloys are shown in Fig. 2. The appearance of bcc tungsten phase is seen as major peaks (JCPDS 89-4900) in all the alloys, with relatively weak peaks of Ni-Fe solid solution (JCPDS 47-1417) in WNF and WNFC4 alloys. The presence of small amount of intermetallic phase of Ni-W (JCPDS 47-1172) in WNFC4 alloy may be the possible reason for reduced density for this alloy. With increase in addition of cobalt, the diffraction peaks of the tungsten phase is found to decrease. The maximum density, obtained in the literatures for different compositions of WNF alloys using nano powders and sintered through spark plasma sintering and vacuum sintering at identical temperature conditions, ranges from 93% to 95% 68
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Fig. 3. Optical and SEM images of WNF(a, b), WNFC1(c, d), WNFC2(e, f), WNFC3 (g, h) and WNFC4(i, j) respectively.
and matrix strengthening effect [33,41]. The strength of the WHA's depends on the sintering behaviour and microstructural factors such as tungsten grain size, cohesion between tungsten and matrix phase, contiguity and other microstructural factors [35,38]. The yield and tensile strengths of WNFC alloys are higher than that of WNF alloy. The yield strength increases from 686 MPa for WNF alloy to 770 MPa for WNFC1 alloy. The alloy is strengthened through solid solution hardening which is the major advantage in cobalt added heavy alloys. The binder Ni-Fe-Co provides a ductile bonding matrix for the heavy alloy and also strengthens the interfacial strength between the tungsten and the matrix phase. The grain size also influences the yield strength of the alloy. When the grain size decreases, there is a lower possibility of finding energetic atom clusters to move the dislocations formed at the grain boundaries to initiate plasticity [42]. This leads to Hall-Petch strengthening, resulting in higher yield stress for the alloy. According to Hall-Petch relation, the grain size is found to influence the yield strength of the alloy by inverse correlation [43]. The alloy with 1%
tungsten in the matrix phase [31]. The dissolved tungsten in WNFC alloys is found to vary in the range of 33–44%.
3.3. Mechanical properties The strength properties and the ductility of tungsten heavy alloys are found to be optimal with Ni/Fe ratios ranging from 2 to 4 [7]. The ratio of 7:3 is suggested to provide good properties of the alloy by avoiding the formation of intermetallic compounds [13]. The influence of cobalt, as an additive to the tungsten heavy alloy of W-Ni-Fe, on the microstructure and mechanical properties of the alloy are analysed while keeping the ratio of Ni:Fe constant. The experimental results of the mechanical properties of the alloys are listed in Table 4. The hardness is noted to increase with increase in cobalt addition. Further, the hardness of WNFC alloys is greater than the WNF alloy. The alloy with 2 wt% cobalt addition gives the highest hardness of 499Hv50. The increase in hardness is attributed to the cobalt related surface activation 69
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Fig. 3. (continued)
depends on the diffusion of atoms which is a function of sintering temperature and time. The other possible reason for lower ductility in W-Ni-Fe system is the presence of residual porosity in the sintered alloy [32]. The W-W interfaces are the low energy preferred path for crack nucleation. A lesser contiguity value implies that such crack nucleation sites are less in WNFC alloys thereby resulting in greater ductility. The cobalt addition leads to increase in tungsten-matrix and tungstentungsten interface cohesive strength thereby enhancing ductility [13]. Generally, in tungsten heavy alloys, larger grain size with lower contiguity contributes to higher ductility, but with some loss in strength of the alloy [33]. However, the SPS processed alloys show controlled grain growth, giving a finer grain size and higher contiguity compared to the liquid phase sintered (LPS) W-Ni-Fe and W-Ni-Fe-Co alloys by different sintering techniques, but, provides good ductility and strength for the alloys. Table 5 gives an overview of the mechanical properties of tungsten heavy alloys obtained through various sintering processes from reference literatures for comparison with the results obtained in the present work. As sintering in SPS is done in vacuum, there is no problem of hydrogen embrittlement and also impurity segregation in the matrix and interface is avoided due to high heating rate.
Table 3 Microstructural parameters of the tungsten heavy alloys. Alloy
Grain size µm
Contiguity
Matrix volume fraction (vol%)
Dissolved tungsten in matrix (wt%)
WNF WNFC1 WNFC2 WNFC3 WNFC4
12.3 ± 5.2 11.56 ± 3.5 9.48 ± 3.2 9.68 ± 4.1 11.1 ± 6.3
0.64 ± 0.04 0.54 ± 0.02 0.47 ± 0.03 0.43 ± 0.03 0.50 ± 0.04
18.7 ± 2.6 7.9 ± 2.7 8.9 ± 1.3 15.4 ± 3.1 12.3 ± 2.3
28.5 ± 3.7 33.6 ± 2.8 35.2 ± 1.6 43.7 ± 3.2 38.4 ± 3.5
Table 4 Observed mechanical properties of the tungsten heavy alloys. Alloy
Yield strength MPa
Tensile strength MPa
% Elongation
Microhardness Hv50
WNF WNFC1 WNFC2 WNFC3 WNFC4
686 770 1300 1080 1000
975 961 1508 1330 1256
12 16 20 21 18
385 ± 14 455 ± 11 467 ± 08 471 ± 16 499 ± 11
3.4. Fractography
cobalt addition shows yield strength of 1300 MPa with a fine tungsten grain size of 9.48 µm. Further, the obtained yield strength results for WNFC alloys are greater than the values found in other research works [31,41,44] processed through different sintering techniques. The lower tensile strength in the WNF alloy may be due to the high contiguity and relatively lesser cohesive strength between the tungsten grains. The cobalt containing alloys show higher elongation than WNF alloy. This is attributed to several reasons such as higher solubility of tungsten in the matrix phase, strength of the tungsten-tungsten and tungsten-matrix interfaces, lower contiguity and connectivity [6]. Ductility depends on bonding between the particles and phases. The degree of bonding
The addition of cobalt and the sintering through SPS process have a significant effect on the fracture modes of the alloys. The SEM micrographs of the fractured surfaces of the tensile tested specimens are shown in Fig. 4. The WNF fractured structure is observed to exhibit predominantly local matrix and interface failure. The 0.5 wt% cobalt alloy (WNFC1) shows more of matrix interface fracture mode and local matrix ductile tearing with few regions of W-grain cleavage features. Therefore, these two alloys confirm the lower tensile behaviour nature [38,45]. The tungsten heavy alloy with 1.0 wt% cobalt (WNFC2) indicates the presence of more regions of transgranular tungsten cleavage 70
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Table 5 Mechanical properties of sintered tungsten heavy alloys obtained in reference literatures. Composition
Processing method
Yield Strength MPa
Tensile strength MPa
% Elongation
Reference
92.5W–6.4Ni–1.1Fe
– –
[10]
90W–5Ni–2Fe–3Co
Sintering at 1480 °C in H2 for 120 min
90.5W–7.1Ni–1.65Fe–0.50Co–0.25Mo
Sintering at 1460 °C in H2 atmosphere for 90 min
93W–3.0Ni–2.0Fe–2.0Co
Sintering under vacuum condition at 1400 °C for 60 min
805 ± 14 642 ± 23 750 (At 1400 °C) 1185 (At 1500 °C) 1200 989 1050 675 993 608 976 608 920 (Heat treated) 980 ± 40
11.2 ± 1.1 3.5 ± 0.8 4.2 16.4
90W–7Ni–3Fe 90W–6Ni–2Fe–2Co 90W–7.2-Ni–1.8Fe–1Co 90W–6Ni–2Fe–2Co
Microwave sintering at 1500 °C for 20 min in H2 atmosphere. Conventional sintering at 1500 °C for 20 min in H2 atmosphere. Microwave sintering at 1400 °C and 1500 °C for 5 min in reducing atmosphere Sintering under vacuum condition at 1350 °C for 60 min Microwave sintering at 1470 °C for 60 min in H2 atmosphere Sintering at 1470 °C in H2 atmosphere for 90 min Sintering at 1480 °C in H2 for 120 min
93W–4.9Ni–2.1Fe
673 667 (Heat treated) 603 658 (Heat treated) 542 642 (Heat treated)
16 4 27 1 24 5
[15] [30] [13] [19] [31] [31] [41] [44]
Fig. 4. Fracture surfaces of the tungsten heavy alloys. (a) WNF (b) WNFC1 (c) WNFC2 (d) WNFC3 and (e) WNFC4. W-tungsten grain cleavage, W-W – tungsten decohesion, M-Matrix rupture, MC-Microcracks and P-pore.
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Acknowledgements
fracture and few features of matrix interface failure. This is in consistent with the higher tensile strength obtained for the alloy. The higher amount of intergranular matrix phase in WNFC alloys compared to the WNF alloy contributed to better densification of the cobalt added alloys [38]. In SPS technique, due to the intense heat and high heating rate, the impurities get homogeneously distributed instead of segregating at the matrix interface. Moreover, the cobalt addition increases the cohesive strength between the tungsten-matrix and W-W interface, thereby enhancing ductility and strength [13]. The liquid phase sintered W-Ni-Fe-Co alloy in the work by Jiten das et al. and ravi kiran et al. [31,38] through conventional sintering process showed primarily matrix or interface failure mode, that is related to poor tensile properties which transforms to transgranular W-cleavage fracture only after post-sintering heat treatment or swaging operations. The requirement of post sintering operations can be negated by obtaining near-net shape samples using SPS technique through proper control of the alloy composition and SPS process parameters. The alloys, with 1.5 wt% and 2 wt% cobalt (WNFC3 and WNFC4), show more features of tungsten-tungsten and tungsten-matrix debonding compared to 1.0 wt% cobalt alloy in addition to the W-cleavage fracture modes. This could be the reason for lower tensile properties for these alloys in comparison with WNFC2. The micro-cracks are not predominant in the fracture modes of WNFC alloys, while large number of micro-cracks are visible in WNF alloy as realized in this work and also in consistent with other works [21]. This can be attributed to the improved tungsten-tungsten cohesive strength of the WNFC alloys due to cobalt addition. The presence of micro-cracks in the tungsten grain boundaries is due to the morphology of the alloy containing polygon W-grains with high contiguity that may result in stress concentration at the triple tungsten boundary junctions. But, the addition of cobalt minimizes this effect.
The authors gratefully acknowledge VIT University, Vellore, India for the support through Seed Grant for Research. References [1] Nicolas. G, Voltz. M, Tungsten heavy metal alloys – relations between the crystallographic texture and the internal stress distribution, in: Proceedings of the 15th International Plansee Seminar, 1, 2001, pp. 780–781. [2] Y. Sahin, Recent progress in processing of tungsten heavy alloys, J. Powder Technol. (2014) 1–22. [3] Don-Kuk Kim, Sunghak Lee, Joon Woong Noh, Dynamic and quasi-static torsional behavior of tungsten heavy alloy specimens fabricated through sintering, heattreatment, swaging and aging, Mater. Sci. Eng. A 247 (1998) 285–294. [4] Yang Yu, Wencong Zhang, Yu Chen, Erde Wang, Effect of swaging on microstructure and mechanical properties of liquid phase sintered 93W-4.9(Ni,Co)-2.1Fe alloy, Int. J. Refract. Met. Hard Mater. 44 (2014) 103–108. [5] Nuri durlu, Kaan Caliskan, Sakir Bor, Effect of swaging on microstructure and tensile properties of W-Ni-Fe alloy, Int. J. Refract. Met. Hard Mater. 42 (2014) 126–131. [6] U. Ravi kiran, A. Panchal, M. Sankaranarayana, G.V.S. Nageswararao, T.K. Nandy, Effect of alloying addition and microstructural parameters on mechanical properties of 93% tungsten heavy alloys, Mater. Sci. Eng. A 640 (2015) 82–90. [7] R.M. German, Microstructure and impurity effects on tungsten heavy alloys, final report, U.S. Army Research Office, 1990, pp. 1–19. [8] D.P. Xiang, L. Ding, Y.Y. Li, X.Y. Chen, T.M. Zhang, Fabricating fine-grained tungsten heavy alloy by spark plasma sintering of low-energy ball-milled W–2Mo–7Ni–3Fe powders, Mater. Sci. Eng. A 578 (2013) 18–23. [9] Zeinab Abdel Hamid, Sayed Farag Moustafa, Walid Mohamed Daoush, Fatema Abdel Mouez, Moha Hassan, Fabrication and characterization of tungsten heavy alloys using chemical reduction and mechanical alloying methods, Open J. Appl. Sci. 3 (2013) 15–27. [10] A. Upadhyaya, S.K. Tiwari, P. Mishra, Microwave sintering of W–Ni–Fe alloy, Scr. Mater. 56 (2007) 5–8. [11] Avijit Mondal, Anish Upadhyaya, Dinesh Agrawal, Microwave sintering of tungsten based alloys, in: Proceedings of the International Conference on tungsten, refractory and hard materials VII, 3, 2008, pp. 122–133. [12] Avijit Mondal, Anish Upadhyaya, Dinesh Agrawal, Microwave and conventional sintering of 90W-7Ni-3Cu alloys with premixed and prealloyed binder phase, Mater. Sci. Eng. A 527 (2010) 6870–6878. [13] G. Prabhu, N. Arvind kumar, M. Sankaranarayana, T.K. Nandy, Tensile and impact properties of microwave sintered tungsten heavy alloys, Mater. Sci. Eng. A 607 (2014) 63–70. [14] K.H. Lee, S.I. Cha, H.J. Ryu, S.H. Hong, Effect of two-stage sintering process on microstructure and mechanical properties of ODS tungsten heavy alloy, Mater. Sci. Eng. A 458 (1) (2007) 323–329. [15] Jiajia Zhang, Wensheng Liu, Yunzhu Ma, Xiaoshan Ye, Yayu Wu, An improved twostage sintering method for tungsten heavy alloys: conventional solid-phase sintering followed by microwave heating, Int. J. Mater. Res. 106 (1) (2015) 80–82. [16] M. Saiprasad, R. Atchayakumar, K. Thiruppathi, S. Raghuraman, Consolidation of copper and aluminium powders by spark plasma sintering, Mater. Sci. Eng. 149 (2016) 012057. [17] K. Hu, X. Li, S. Qu, Y. Li, Spark plasma sintering of W-5.6Ni-1.2Fe heavy alloys: densification and grain growth, Metall. Mater. Trans. A 44A (2013) 923–933. [18] S.H. Islam, M.A. Al-Eshaikh, Z. Huda, Synthesis and characterization of high energy ball milled tungsten heavy alloyed powders, Arab. J. Sci. Eng. 38 (9) (2013) 2503–2507. [19] K.B. Povarova, M.I. Alymov, O.S. Gavrilin, A.A. Drozdov, A.I. Kachnov, N.L. Korenovskii, I.O. Bannykh, Structure and properties of W-Ni-Fe-Co heavy alloys compacted from nano powders, Russ. Metall. (2008) 52–55. [20] H. Elshimy, Z. Heiba, K. El-Sayed, Structural, microstructural, mechanical and magnetic characterization of ball milled tungsten heavy alloys, Adv. Mater. Phys. Chem. 4 (2014) 237–257. [21] X. Li, K. Hu, S. Qu, Li. Li, C. Yang, 93W–5.6Ni–1.4Fe heavy alloys with enhanced performance prepared by cyclic spark plasma sintering, Mater. Sci. Eng. A 599 (2014) 233–241. [22] Mxolisi Brendon Shongwe, Saliou Diouf, Mondiu Olayinka Durowoju, Peter Apata Olubambi, Munyadziwa Mercy Ramakokovhu, Babatunde Abiodun Obadele, A comparative study of spark plasma sintering and hybrid spark plasma sintering of 93W–4.9Ni–2.1Fe heavy alloy, Int. J. Refract. Met. Hard Mater. 55 (2016) 16–23. [23] ASTM E112-96, Standard Test Methods for Determining Average Grain Size, 16 ASTM, Philadelphia, PA, 1996. [24] J. Gurland, The measurement of grain contiguity in two-phase alloys, Trans. Met. Soc AIME 212 (1958) 452–455. [25] Ke Hu, Xiao-qiang Li, Chao Yang, Yuan-yuan Li, Densification and microstructure evolution during SPS consolidation process in W-Ni-Fe System, Trans. Nonferrous Met. Soc. China 21 (2011) 493–501. [26] M. Lungu, V. Tsakiris, E. Enescu, D. Patroi, V. Marinescu, D. Talpeanu, Development of W-Cu-Ni electrical contact materials with enhanced mechanical properties by spark plasma sintering process, Acta Phys. Pol. A 125 (2014) 327–330. [27] N.S. Ermakova, M.S. Yurlova, Grigoryev, Properties of tungsten heavy alloys prepared by spark-plasma sintering, IOP Conf. Ser. Mater. Sci. Eng. 130 (2016) 1–6.
4. Conclusions 1. The maximum relative density of 93.36% is observed for the tungsten heavy alloy with 0.5 wt% cobalt addition. The relative density is found to decrease with increase in the percentage of cobalt addition. 2. The spark plasma sintered WNF and WNFC alloys show finer Wgrain size with respect to the initial particle size. The W-grain size of WNF alloy is 12.3 µm. The alloy with 1 wt% cobalt gives the least average grain size of 9.48 µm. The contiguity of cobalt added alloys are lower compared to the WNF alloy, which implies enhanced mechanical properties for WNFC alloys. The W-W contiguity is found to decrease with increase of cobalt percentage. 3. The addition of cobalt improves the dissolution of tungsten in the matrix phase. This is validated with increasing matrix volume fraction and decreasing contiguity with increase in the percentage addition of cobalt. 4. The tensile properties of the WNFC alloys are superior to the WNF alloy. The yield strength of the alloys is found to depend on the tungsten grain size. The alloy with 1 wt% cobalt gives the highest yield strength of 1300 MPa and tensile strength of 1508 MPa. 5. The mode of fracture in WNF alloy is intergranular with tungstenmatrix debonding dominating the fractograph. In contrast, the WNFC alloys, especially the alloys with 1 wt% and more, fail predominantly by transgranular failure mode with more of W-grain cleavage. This may be the reason for superior tensile properties for the cobalt added alloys. 6. The present investigations reveal the effectiveness of SPS as a sintering technique and cobalt as an effective alloying element to obtain higher strength alloys with good ductility. By proper control of the process parameters, it is possible to obtain tungsten heavy alloys with enhanced mechanical properties in as-sintered stage itself, thereby avoiding post sintering processes.
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Powder Metall. Metal. Ceram. 46 (2007) 517–524. [37] Yu.V. Milman, The effect of structural state and temperature on mechanical properties and deformation mechanisms of WC-Co hard alloy, J. Superhard Mater. 36 (2014) 65–81. [38] G. Jiten das, Appa Rao, S.K. Pabi, Microstructure and mechanical properties of tungsten heavy alloys, Mater. Sci. Eng. A 527 (2010) 7841–7847. [39] H.E. Exner, J. Gurland, A review of parameters influencing some mechanical properties of tungsten carbide-cobalt alloys, Powder Metall. 13 (1970) 13–31. [40] B. Katavic, M. Nikacevic, Z. Odanovic, Effect of cold swaging and heat treatment on properties of the P/M 91W-6Ni-3Co heavy alloy, Sci. Sinter. 40 (2008) 319–331. [41] G. Jiten das, Appa Rao, S.K. Pabi, M. Sankaranarayanan, T.K. Nandy, Thermomechanical processing, microstructure and tensile properties of a tungsten heavy alloy, Mater. Sci. Eng. A 613 (2014) 48–49. [42] Ju-Young Kim, Dongchan Jang, Julia R. Greer, Tensile and compressive behaviour of tungsten, molybdenum, tantalum and niobium at the nanoscale, Acta Mater. 58 (2010) 2355–2363. [43] Hansen Niels, Hall-Petch relation and boundary strengthening, Scr. Mater. 51 (2004) 801–806. [44] C.C. Fu, L.J. Chang, Y.C. Huang, P.W. Wong, J.S.C. Jang, Microstructure and mechanical properties of solid phase sintered heavy tungsten alloy, Adv. Mater. Res. 15–17 (2007) 575–580. [45] M. Kaczorowski, P. Skoczylas, A. Krzynska, The influence of Fe content on spreading ability of tungsten heavy alloys matrix on tungsten surface, Arch. Foundry Eng. 11 (2011) 103–106.
[28] H.R. De Macedo, A.G.P. da Silva, D.M.A. de Melo, The spreading of cobalt, nickel and iron on tungsten carbide and the first stage of hard metal sintering, Mater. Lett. 57 (2003) 3924–3932. [29] James R. Spencer, James A. Mullendore, Relationship between composition, structure, properties, thermo-mechanical processing and ballistic performance of tungsten heavy alloys, Rep. U.S. Army Mater. Technol. Lab. (1991). [30] Akhtar Farid, An investigation on the solid state sintering of mechanically alloyed nano-structured 90W-Ni-Fe tungsten heavy alloy, Int. J. Refract. Met. Hard Mater. 26 (2008) 145–151. [31] U. Ravi kiran, M. Sankaranarayana, G.V.S. Nageswararao, T.K. Nandy, Effect of cobalt addition on microstructure and mechanical properties of tungsten heavy alloys, Trans. Indian Inst. Met. 70 (2017) 615–622. [32] R.M. German, K.S. Churn, Sintering atmosphere effects on the ductility of W-Ni-Fe heavy metals, Metall. Trans. A 15 (4) (1984) 747–754. [33] J. Wittenauer, T.G Nieh, Development of fine-grained ductile tungsten alloys for anti-armor applications, A Report for U.S. Army Research Office, by Lockheed Missiles and Space Company, Inc, 1991. [34] Xiao-qiang Li, Hong-wei Xin, Ke Hu, Yuan-yuan Li, Microstructure and properties of ultra-fine tungsten heavy alloys prepared by mechanical alloying and electric current activated sintering, Trans. Nonferrous Met. Soc. China 20 (2010) 443–449. [35] Yuanyuan Li, Ke Hu, Xiaoqiang Li, Xuan Ai, Shengguan Qu, Fine grained 93W5.6Ni-1.4Fe heavy alloys with enhanced performance prepared by spark plasma sintering, Mater. Sci. Eng. A 573 (2013) 245–252. [36] A.V. Laptev, Structure and properties of WC-Co alloys in solid phase sintering,
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