Fabrication and Characterization of Tungsten Heavy Alloys Using Chemical Reduction and Mechanical Alloying Methods ()
1. Introduction
Tungsten heavy alloys (WHA’s) illustrate the advantages of microencapsulated powders. WHA’s are a kind of composite materials, consisting of two-phase microstructure of spherical tungsten particles embedded uniformly in a metallic matrix, such as W-Ni-Fe or W-NiCu or even W-Ni-Cu-Fe solid solution [1,2]. Some elements are added to WHA’s such as Co, Mo, Cr, etc. The addition of cobalt to a W-Ni-Fe alloy is a common approach for slight enhancement of both strength and ductility. The presence of cobalt within the alloy provides solid-solution strengthening of the binder and slightly enhanced tungsten-matrix interfacial strength. Nickel, iron and copper serve as a binder matrix, which holds the brittle tungsten grains together and makes the alloys ductile and easy to machine. These alloys provide a unique combination of properties, such as high density, excellent mechanical properties and good corrosion resistance, making them increasingly attractive in many practical applications, such as radiation shielding, mass balance for aerospace, vibrating masses for cell phones and kinetic energy penetrators [3-8]. Tungsten content in conventional heavy alloys varies from 90 to 98 weight percent and is the reason for their high density (between 16.5 and 18.75 g/cc).
There are several conventional methods for fabrication of tungsten heavy alloys, including mechanical alloying [9-11], electro-deposition technique [12,13], metal injection molding [10] and spray drying [14-16]. L. DongWon et al. [16] studied a novel reduction method to fabricate ultrafine tungsten heavy alloy powders, using cost effective source materials. The advantages of mechanical alloying method have the simple equipment and process and easy operation that make it suitable for mass production.
Using fine powder raw materials and/or novel techniques to prepare fine-or ultrafine-grained WHA’s are the current trends. Fine-or ultrafine grained WHA’s are usually prepared via sintering ultrafine/nano tungsten composite powders and controlling the growth of the tungsten grains [10,15-21]. By refining the microstructure of the tungsten heavy alloys, the mechanical properties can be improved.
The conventional processing route includes mixing the desired amount of elemental powders, followed by cold pressing and sintering. There are two essential phases during the sintering process: solid state phase, and liquid state phase. At melting, the liquid phase of the matrix, wet and infiltrate the solid grain structure by a combination of reaction and capillary forces, followed by a dissolving and growth of the solid particles by coalescence and Ostwald-ripening. The driving force for liquid phase sintering is not only the reduction of surface energy by capillary forces but also the reduction of chemical potential by dissolving of original and growth of equilibrium solid phase [22].
Liquid phase sintering is generally easier and more cost-effective than solid state. However, a certain degradation of the raw material will occur that is not the case with the solid state process. The sintering process occurs by mass transport at high temperature and sometimes a high pressure is also required as a driving force. The driving force for solid-state sintering is the reduction of surface free energy of the particles, which is achieved either by atomic diffusion transporting matter from inside the grains to fill the pores (densification) or rearrangement of the matter between different parts of the pore surfaces (without reducing pore volume) coarsens the microstructure.
In this study, a novel process was developed to synthesize nano-sized tungsten heavy alloys (WHA’s) composite powders by chemical reduction method. For comparison, the same compositions of the investigated powders were also processed by mechanical alloying method. The prepared powders from both methods were cold compacted and sintered under vacuum. The fabricated composites have been evaluated in terms of density, microstructure, hardness, transverse rupture strength, thermal and electrical conductivities.
2. Materials and Methods
Heavy tungsten alloy powders with composition of 95W- 3.5Ni-1.5Fe, 93W-4.5Ni-1.0Fe-1.5Co, and 90W-6Ni-4Cu in wt% were prepared by chemical reduction and mechanical alloying (MA) techniques. Tungsten, iron, cobalt and nickel have been characterized in terms of flow rate, average grain sizes and apparent density. Table 1 summarizes these characterizations. A scanning electron microscope (SEM) equipped model JEOL, JSM-5410 was used to study surface morphologies of the powders. A JEOL (JEM 2010) transmission electron microscope was used to analyze the powder size. The phase structure was analyzed by X-ray diffractometer (XRD) model PAN analytical X’pert PRO with Cu Kα1 target with wavelength 1.5046 A and operated at 45 kV and 40 mA. The step size, rate and range were 0.02˚, 0.05˚ s−1 and 10˚ - 100˚, respectively.
2.1. Tungsten Heavy Alloys Fabrication
2.1.1. Chemical Reduction Method
In the chemical reduction method, a novel process was developed to synthesize tungsten heavy alloys nanocomposite powders by controlled reduction of precursors obtained by spray drying of several appropriate aqueous solutions, which was made from salts containing tungsten, cobalt, nickel, and iron. Tungsten powder was obtained from solution of ammonium metatungstate
, whereas Co-powders was made from cobalt nitrate 
solution, and the Ni powder was made from nickel acetate
solution, Fe and Cu were formed from their oxides Fe2O3 and CuO respectively. Hydrogen gas was used as the reductant gas. By adjusting the stoichiometry of the component of the solutions and iron or copper oxide, it is possible to obtain the desired chemical composition of the tungsten heavy alloys powders.
Table 1. Characteristics of starting powders.
2.1.2. Mechanical Alloying Method
In the mechanical alloying method, highly pure elemental powders of tungsten of 1 - 3 µm particle sizes, nickel of 2.5 µm, cobalt of 2 µm, iron of 0.3 - 0.5 µm and copper of 0.2 - 0.4 µm in average diameter particles of the investigated chemical compositions of WHA’s have been mechanically alloyed by rod milling for different milling time. The elemental powders of W, Ni, Fe and Co weighed to the target compositions and milled in a milling machine. The diameter of the mill was 255 mm, and Cr-steel balls of 8 mm in diameter were used as the milling media. The milling speed was 120 rpm, milling time varied up to 72 h, rod-to-powder ratio was 20:1 by weight, and the ball filling ratio was 15% by volume.
The prepared powders from both methods were blended with 0.5 wt% paraffin wax and cold compacted using Hydraulic press at compaction pressure of 70 bar. The green compacts were sintered using solid-state sintering at a temperature 1400˚C for 1 h under vacuum as illustrates in the schematic diagram of the sintering cycle in Figure 1.
2.2. Characterization of Alloys
The evaluation testing of the powder and consolidated materials includes: density, hardness, transverse rupture strength as well as electrical and thermal conductivities. Moreover, extensive metallographic studies were performed using both optical and SEM microscopy for the sintered materials in order to measure the contiguity and structure homogeneity.
2.2.1. Densities Measurements
The densities of the sintered powders were measured according to MPIF Standards 42, 1998, using Archimedes rule. The density 
of the samples was calculated according to the following formula [22]:
(1)
Figure 1. The heating cycle for the vacuum sintering process at 1400˚C for the investigated tungsten heavy alloys.
where Wa and Ww are the weights in air and water respectively.
The compact sinterability was also expressed in the terms of densification parameter which is calculated as follows:
(2)
where δs is the sintered density, δg is the green density and δth is the theoretical density.
2.2.2. The Electrical Resistivity Measurements
The electrical resistivity of the sintered materials was measured using the four-probe method by using Omega CL 8400 micrometer device. The resistivity 
was calculated according to the following equation by using High Precision Micro-ohmmeter.
(3)
where, R is the resistance in micro ohm, L is the measured length in cm, A is the cross section area in cm2, and ρ is the resistivity in µW cm.
2.2.3. The Microhardness Measurements
The Vickers microhardness of the alloys was determined by an indentation technique at 500 gf load with a diamond pyramid indenter technique, Tukon Series B200 microhardness tester.
2.2.4. Transverse Rupture Strength (TRS) Measurements
The rupture test was performed using compression testing machine and a test fixture, according to MPIF Standard 41. All the samples of the powders were compacted into rectangular compacts of dimension of (30 × 10 × 6) mm3. In the transverse rupture fixture, the test bar was placed centrally located and perpendicular to the supporting rods with the top up. The fixture with the test bar was placed between the plates of the compression testing machine and the load was applied at constant rate of 2.5 mm/min, until the test bar fractured. The transverse rupture strength for sintered samples was calculated according to the following expression [22]:
(4)
where: TRS is Transverse rupture strength in 
P is the fracturing (rupture) load (N).
L is the distance between the supporting rods (25.4 mm).
t is the thickness of the sample in (mm).
W is the width of the sample in (mm).
2.2.5. Thermal Conductivity
The thermal conductivity was calculated from electrical resistivity measurement using Wiedemann and Franz equation which derived a relation equation between the thermal and electrical conductivities [23]. The Wiedemann-Franz equation is as follows:
(5)
where, l is thermal conductivity
, s is electrical conductivity
, T is absolute temperature in degree Kelvin, KB is Boltzmann constant, and L is Lorentz number.
3. Results and Discussion
3.1. Fabrication of Tungsten Heavy Alloys Using Chemical Reduction Method
Firstly, Salts of cobalt nitrate, ammonium metatungstate, and nickel acetate were used to prepare cobalt, tungsten and nickel powders by chemical method. The salt solution of each one was spray dries using the centrifugal atomization which was adjusted at rotating velocity of 11,000 rpm, and feeding rate of 30 CC/min. The spray dried powder of each composition was collected and heated up in air at 800˚C to form oxide powders. XRD spectra of each salt after heated are shown in Figure 2. XRD patterns illustrate the peaks of W oxide at different 2θ angles 24˚, 33˚, 42˚, 48˚, 55.5˚, 61˚ (Figure 2(a)), cobalt oxide at 18.98˚, 31.28˚, 36.86˚, 38.54˚, 44.82˚, 59.35˚ and 65.25˚ (Figure 2(b)) and nickel oxide at 26.6˚, 37.25˚, and 43.29˚, 62.88˚ and 75.40˚ (Figure 2(c)). The oxide powders were reduced in hydrogen atmosphere for 30 min at 750˚C to obtain pure metal powder.
Similarly, synthesis of tungsten-base nanocomposite alloy powders have been prepared by chemical reduction of precursors obtained by spray drying of several appropriate aqueous solutions, which were made from salts containing tungsten, cobalt and nickel, whereas iron and copper were added as oxides. Three tungsten-base alloy powders were obtained, namely, (95%W-3.5%Ni- 1.5%Fe), (93%W-4.5%Ni-1%Fe-1.5%Co), and (90%W- 6%Ni-4%Cu) alloys. Dried solution salts were further heated up in open air at 800˚C for 2 hrs in order to obtain corresponding alloy oxides. Figure 3 illustrates the XRD of the three investigated oxide alloys. The apparent crystallite size of the oxides of WHA’s prepared by chemical reduction technique reaches to the nanoscaled dimension as shown in TEM images (Figure 4).
The resulted oxide alloy powders were reduced in tube furnace using hydrogen gas as reductant. According to the analysis of the X-ray diffraction pattern (Figure 5) it can be concluded that the complete reduction was achieved at that condition. The apparent crystallite size of the WHA’s powders prepared by chemical reduction technique after reduction using TEM is shown in Figure 6. The images reveal that the crystal size reaches to the nano-scaled dimension.
The reduction of WO3 by H2 gas is a complex process, while Lassner and Schubert proved that, the reduction of WO3 can be divided into two steps: firstly, oxygen transport by solid state diffusion
, secondly, tungsten transport by chemical vapor transport 
as shown in the following chemical reactions [24]:
(6)
(7)
(8)
(9)
Figure 2. XRD of the oxide powders; where (a) tungsten oxide, (b) cobalt oxide, and (c) nickel oxide.
(a)
(b)
(c)
Figure 3. XRD of the three investigated oxide alloys.
Additionally, during the synthesis process, it can be assumed that nickel, cobalt and iron components are reduced before the tungsten due to the low reduction temperatures of their oxides.
3.2. Fabrication of Tungsten Heavy Alloys Using Mechanical Alloying Method
Figure 7 shows scanning electron microscopy images of the tungsten, nickel, iron and cobalt powder used in this study. The grain sizes of elemental powders were ranged from 2 up to 10 μm. In this investigation, three alloys composite powders were prepared from these highly pure elemental powders, namely; (95%W-3.5%Ni-1.5%Fe),