A homemade ball mill was constructed and optimized in order to prepare nano crystallite size of tungsten heavy alloys, with composition of 90W-7Ni-3Fe and 90W-7Ni-3Co in wt%. The samples were mechanically alloyed under high purity of argon atmosphere and were sintered under high vacuumat 1200°C, 1300°C and 1400°C. X-ray diffraction (XRD), Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), Energy Dispersive X-ray (EDX), Vickers, ultrasonic techniques and SQUID magnetometer were all used to characterize the studied samples. The sintering temperature and the milling time at which the heavy tungsten alloys were obtained, are discussed in details. The results showed that the tungsten heavy alloys were synthesized and sintered at lower temperature than those prepared by the conventional techniques. Moreover, the strains and relative densities increased with milling time up to 100 hrs; then decreased with further milling. On the other hand, the elastic moduli and hardness increased with milling time up to 200 hrs; then decreased with further milling. The hardness calculated from ultrasonic and measured from Vickers exhibited a similar trend though with different values. The saturated magnetization decreased by increasing the milling time and decreasing the crystallite size.
Tungsten heavy alloys consist essentially of tungsten (W), with contents (88 - 90) wt% as a main component, in association with a binder phase containing transition metals (Ni, Fe, Cu, Co) [
The properties of tungsten heavy alloys are found to be dependent on the amount of tungsten content and on Ni/Fe or Ni/Cu ratio [
WHAs with W content 90% - 95% possess good combination of tensile properties (tensile strength―1000 MPa and elongation―30%), as well as, high density of 17 - 18 g/ml [
Numerous investigations have been carried out in order to improve the properties of tungsten heavy alloys as well as lowering the sintering temperature by refining the microstructure of tungsten alloy. Previous studies have shown that sintering temperature is related to the powder size; such that when the size is in nano-scale, the sintering temperature is decreased up to several hundreds of degrees [
An optimized homemade ball mill, suitable for producing homogenized nanocrystallite sizes from tungsten heavy alloys was designed and constructed in our laboratory. The microstructural parameters (crystallite size and strains) were investigated in the aim of reaching reasonable nanocrystallite size. The hardness measurements were used as indictor for the mechanical properties of the sintered material using Hardness Vickers technique. In addition, the ultra sonic technique was used to determine the elastic moduli beside the hardness. All the obtained results were correlated with each other.
The as received Tungsten powder of 2.46 μm, Nickel powder of 2.3 μm, Cobalt powder of 2.4 μm and Iron powder of 2.1 μm (in average size)were used in this study. Two samples of tungsten heavy alloy powder with composition 90W-7Ni-3Fe and 90W-7Ni-1.5Fe-3Co in wt% were mechanically alloyed by using the home made ball millfor 50, 100, 150, 200 and 400 hrs. The mill consists of two rotators connected to a motor of maximum rotation speed 1000 rpm and the vial rotates at the same speed as the tray, but in the opposite direction,
The milled powder of the two tungsten samples, obtained at different milling times, were cold pressed into a cylindrical die by using a hydraulic press at ambient temperature and under pressure up to 600 Mpa. The final green compacts were cylindricalin shape with dimensions: diameter = 8.0 mm and length = 6.0 mm, with tolerance of ±1 mm.
The green compacts were divided into three groups, the first group was sintered at a temperature of 1200˚C, the second was sintered at 1300˚C and the third was sintered at 1400˚C. The sintering process was performed in vacuum furnace and the samples were held for a constant time of 60 minutes.
A PHILIPS® X’Pert diffractometer, which has the Bragg-Brentano geometry and Copper tube, was used to collect the XRD patterns for the different samples. The operating voltage was kept at 40 kV and the current at 30 mA. The divergence-slit angle = 1˚, the receiving slit = 0.1˚, the step scan size = 0.03˚ and the scan step time = 5 seconds. The Kβ radiation was eliminated using the secondary monochromator at the diffracted beam.
The structural and the microstructural parameters (such as the lattice parameters, crystallite size, RMS lattice strain and the percentage of phases present) of all samples have been made by employing the Rietveld software MAUD [
where X is the sample thickness and Δt is the time interval.
The Elastic Moduli (Young E, Shear G and Bulk K), the Poisson ratio σ and the Hardness H were calculated from the following equations [
where ρ is the density, vl and vs are the longitudinal and transverse ultrasonic velocities, respectively. The ultrasonic velocities in the sample were measured using an ultrasonic flow detector, type USM 2 (Krautkramer, Germany). All measurements in this study were carried out at room temperature and at a frequency of 5 MHz for shear velocities and 10 MHz for the longitudinal velocities. The measurements were repeated five times to check the reproducibility of the data.
The X-ray diffraction patterns of the mechanically alloyed powder of 90W-7Ni-3Fe and 90W-7Ni-3Co are shown in
became gradually broadened and the peak intensities decreased with the increase of milling time. These observations indicate that both the crystallite sizes and microstrains of the mixed powders are significantly affected by the mechanical milling. From the figures, it appears that there are no new phases or foreign diffraction peaks are present at different milling times. Selected refined patterns are presented in
The volume fraction of the elemental composition (W, Ni, Fe or Co) calculated from XRD patterns as a function of milling time, is exhibited in
The TEM nanocrystallite size of the milled 90W-7Ni-3Fe powders at different times is shown in Figures 9(a)-(c).
The results of EDX analysis from different locations on the samples showed that the elemental percentage of the milled powders is very close to the initial composition. In turn this suggests that all elements were uniformly distributed throughout the matrix of the synthesized alloy. Eventually, this guarantees the minimization of the contamination. Moreover, it enhances the validity of the constructed homemade ball.
Green density is highly dependent on the friction forces among powder particles. These forces mainly result from electrostatic, Van der Waals and surface adsorption forces that become much more significant with decreasing particle size [
After 100 hrs of milling, the relative green density reached a maximum value of 67.8% for the 90W-7Ni-3Fe composition and 71.5% for the 90W-7Ni-3Co composition. While further increasing of the milling time, the green densities decreased due to of the effect of decreasing the particle size with milling time, thus causing an increase in the friction forces among the powder particles [
X-ray diffraction was used, in case of sintered samples, to identify the phase formation of the tungsten heavy alloy at the different sintering temperature. At sintering temperatures 1300˚C and 1400˚C, the qualitative analysis of the XRD patterns confirmed the formation of the tungsten heavy alloy for the two compositions, 90W- 7Ni-3Fe and 90W-7Ni-3Co, regardless of whether the powders are milled or not. Some of the selected XRD patterns are presented in
At sintering temperature 1200˚C, the tungsten heavy alloys are formed for powders milled at 100 hrs and above as a result of fine, smaller and homogenous powders, moreover the benefit of the high pressure [
To obtain further microstructural information, SEM is well suited to identify the existing phases, their shapes and their homogeneity. Investigations were carried on the two alloys 90W-7Ni-3Fe and 90W-7Ni-3Co alloys of unmilled powders as well as sintered at 1200˚C, 1300˚C and 1400˚C. In addition, the investigations included the alloys of powder milled for 200 hrs and sintered at the previously indicated temperature. The most prominent features of the micrographs are the size distribution and the homogeneity of the binder phase among the tungsten phase. Figures 14(a)-(f) and Figures 15(a)-(f) illustrate the micrographs of polished and etched specimens of the 90W-7Ni-3Fe and 90W-7Ni-3Co alloys, respectively. These specimens were fabricated from unmilled and 200 hrs milled powders, sintered at different temperatures. The SEM micrographs show that at the different sintering temperatures, the microstructures of the unmilled (90W-7Ni-3Fe) and (90W-7Ni-3Co) alloys consist of coarse primary particles and the distribution of nickel and iron (cobalt) are concentrated in certain areas while other areas are almost free. But for the 200 hrs milled alloys, the homogeneity of the binder phase is observed to be finer due to the fine grain size of powders; as a benefit of mechanical alloying.
EDX analysis was carried out along with SEM in order to investigate the chemical composition of particles after sintering, and to ensure the absence of oxygen which could have been produced during the sintering process. The alloys of unmilled powder, those of milled for 200 and 400 hrs, all sintered at 1400˚C were chosen as test samples.
The EDX spectrum of the two alloys, the 90W-7Ni-3Fe and 90W-7Ni-3Co, after milling for 200 hrs and sintered at 1400˚C, are displayed in
Milling time | 90W-7Ni-3Fe | 90W-7Ni-3Co | ||||
---|---|---|---|---|---|---|
W% | Ni% | Fe% | W% | Ni% | Co% | |
Unmilled | 91.6 | 4.14 | 4.26 | 85.05 | 10.51 | 4.44 |
200 hrs | 89.16 | 7.62 | 3.22 | 89.10 | 7.06 | 3.84 |
400 hrs | 88.7 | 7.14 | 4.16 | 88.37 | 7.94 | 3.69 |
The effect of the milling time on the sintered densities for the two alloys 90W-7Ni-3Fe and 90W-7Ni-3Co are shown in
A comparison between the relative densities of 90W-7Ni-3Fe and 90W-7Ni-3Co at different temperatures, for the two alloys milled for 100 hrs, is given in
The ultra-sonic measurements are used as an indicator for the mechanical properties, as a function of milling time and sintering temperature. The measurements were carried out on alloys of unmilled powders as well as on alloys milled for 100, 200 and 400 hrs.
The effect of milling time on the elastic moduli and hardness for the 90W-7Ni-3Fe and 90W-7Ni-3Co alloys,
are displayed in Figures 20(a)-(c) and Figures 21(a)-(c), respectively, at different sintering temperatures. The variation in the elastic moduli (Bulk, Shear and Young modulus) as well as the changes in Hardness with milling time, exhibit almost similar trends for both the 90W-7Ni-3Fe and 90W-7Ni-3Co alloys at all sintering temperatures. On the other hand, the variation of the Poisson ratio as a function of milling time exhibits an opposite trend to that of the elastic moduli.
The elastic moduli and hardness increased with milling time up to 200 hrs and then decreased with further milling, up to 400 hrs; as shown in
The hardness values of the 90W-7Ni-3Fe and 90W-7Ni-3Co alloys appear in
The correlation between the hardness calculated from ultrasonic velocities and that measured from Vickers equipment is displayed in
The magnetization field dependence (M-H) for the 90W-7Ni-3Fe alloy, of unmilled powders and those of milled for 100 and 150 hrs, are shown in
The samples of 90W-7Ni-3Fe and 90W-7Ni-3Co in wt%, were mechanically alloyed under high purity of argon atmosphere and were sintered under high vacuum. The strains and the relative densities increased with milling time up to 100 hrs and then decreased with further milling. As the crystallite size becomes smaller, the ratio of the volume to the surface area of the crystallite decreases thus leading to a decrease in the density. The volume fraction of Fe, Ni and Co in the tungsten alloys disappeared for milling times of 400, 200 and 50 hrs re-
Sample | Unmilled | 100 hrs | 150 hrs |
---|---|---|---|
Saturation magnitization Ms (emu/g) | 7.31 | 2.34 | 2.05 |
spectively. Moreover, the Rietveld refinement indicated that the tungsten alloys are formed upon the transformation of Ni and Fe or Ni and Co into the amorphous phase acting as a binder to the tungsten grains. The tungsten heavy alloys were synthesized and sintered at lower temperature, 1200˚C, than those prepared by the conventional techniques. The elastic moduli and the hardness for the tungsten alloys were calculated using the ultrasonic technique. The elastic moduli and hardness increased with milling time up to 200 hrs and then decreased with further milling. Moreover, the hardness is measured using Vickers technique and the results obtained were correlated with those calculated from the ultrasonic technique. The hardness results obtained showed a similar trend but displayed different values.