A Study on Ni-Fe Ball Milled Spark Plasma Sintered Sample

Elemental powders of Nickel and Iron with the purity of 99.8% and 99.5%, particle size 80 µm and 25 µm and grain size 280 nm and 160 nm respectively were used as the starting material. Powder mixture of 40wt.% Ni-Fe was selected as the starting material and it was milled at 16, 32 and  64 hours duration. High Energy Planetary ball milling was performed in the laboratory planetary mill (RETSCH) with ball to powder mass ratio of 10:1 at a speed of 300 rpm, using stainless steel jars and balls. The jars were sealed under atmospheric pressure and temperature. Toluene was used as a process control agent. DR.SINTER.LAB^TM (SPS 515S), apparatus was used for sintering of samples. Graphite punches and die were used. Cylindrical shape of 12 mm diameter and 5 mm height samples were made. Thermal Cycle of SPS is shown in Figure 1 and time-temperature values of SPS are listed in Table 1.

After reaching 650°C the sample was kept for 5 minutes holding and then the sample was free cooled at inside the furnace. 30 MPa compression pressure was applied during the heating steps and used vacuum pressure was 6 Pa while sintering. Characterization was carried out for milled powders and sintered samples. Phases and micro structures were analyzed by X-Ray Diffraction (XRD) using SIETRONICS XRD SCAN Diffraction meter as Cu-Ká target was used with a radiation of 1.541841 A°. Powder morphological images were taken under varying milling hours by Scanning Electron Microscopy. The average grain size was calculated from XRD images using Scherrer’s equation^{13, 14} as D = Kλ / (β cos θ), where D is the average grain size, l is the X-ray wave length , θ is the Bragg angle, β is the finite-size broadening and K is a constant close to unity. Mastersizer Particle Size Analyzer was used to measure the particle size. Volume fraction of porosity was calculated by P = 1- ( α / µ ), where α is the actual density of the alloy and µ is the theoretical density of the alloy. The density was measured using Archimedes principle with water as the liquid medium.

Figure 1: Thermal Cycle of SPS

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Table 1: Time-Temperature values of SPS

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Results and Discussion

XRD Pattern

The XRD spectra of Fe and Ni are shown in Figure 2 and those of 40% Ni-Fe powder and  spark plasma sintered samples at various milling hours are shown in Figure 3. From Figures 2 and 3, in the case of powder sample, it could be observed that the X-ray diffraction peaks widen as the milling time increases and in case of sintered sample X-ray diffraction peak narrow down as the milling time increases. Former could be explained as due to the refinement of grain size and increases of micro strain during mechanical alloying and later is grain growth due to the application of temperature and pressure while sintering.

Figure 2: XRD diffraction pattern of pure Ni and Fe powder at various milling hours

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Figure 3: XRD diffraction pattern of 40%Ni-Fe powder and Spark Plasma sintered samples at various milling hours

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Grain Size

The average grain sizes were calculated from the super imposed XRD (111) peak of Ni and (110)  peak of Fe using Scherrer’s equation.^{13, 14} The variation of the grain size with milling hour is listed in Table-2 and graphically shown in Figure 3. It can be seen that in powder sample the grain size decreases with increase of milling hours and reaches 18.5 nm at 64 hours of milling, in sintered sample the grain size increases with increase of milling hours.

Table 2: Average grain size with milling time

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 Figure 4: Grain size of 40Wt. %Ni-Fe powder and sintered samples

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Morphological Study

Fig.5 and 6 shows the SEM images of pure Iron and Nickel respectively. Fig.5 exhibits the sub angular particle of various sizes, while Fig.6 shows large lumps of spherical particle of uniform size. The average particle size of Fe is 24 µm while that of Ni is 79 µm.

The morphology of 40% Ni-Fe powders at different milling times was investigated using SEM analysis. Fig. 7 A to 7 D shows the morphological development of the particles from 16 to 100 hours of milling. The values of average particle size of different milling hours are listed in Table 3. It can be seen from Fig.7 A that at 16 hours of milling the mixture consists of both fine and large size particles. Further milling up to 32 hours has resulted fracturing of only the largest particles and hence the particle size keep on decreasing. Figure 7 B to 7C show that the particle size decreases till the particle size is the lowest at 64 hours of milling is 1.3 µm. This is because of the fracturing force dominate the cold welding force while milling ranges from 16 to 64 hours.

Figure 5: SEM Micrograph of Fe

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Figure 6: SEM Micrograph of Ni

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 Figure 7: SEM Micrograph of 40wt.% Ni-Fe with different milling hours

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Table 3: Average particle Size

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Densification

Theoretical density, sintering density and % of densification are listed in Table 4 and graphically shown in Fig 8. The Calculated value of theoretical density of 40%Ni-Fe alloy is 8.2 g/cm³. It could be observed from Fig 8 milling hour increases (reduction in grain size) density of sintered sample and % of densification decreases. This indicates while grain size decreases more temperature and pressure required for better densification. This may be due to the reasons of increases in the surface area of the milled nanopowder (ie. Reduction of particle size), low green body density of long milling powders by agglomeration of powders and oxidation of powders during a milling process. Increases the surface area of the powder particle causes the more heat transfer.

Table 4: Density of alloy & % of densification

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Figure 8: Density of alloy and spark plasma sintered sample

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Porosity

Volume fraction of porosity is listed in Table 5 and graphically represented in Fig 8. It can be seen from Table 5 and Fig 9 milling hour increases the volume fraction of porosity also increases

Figure 9: Volume fraction of Porosity

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Conclusion

Mechanical alloyed 40wt.%Ni-Fe of Four different milling hours powder samples and SPS sintered samples were characterized by using X-ray Diffraction, Scanning Electron Microscope and Particle Size Analyzer. The main conclusions drawn from the study are:

In powder sample the grain size decreases with increase of milling hour and it reaches 18.5 nm at 64 hours of milling.

In Spark Plasma Sintered sample, the grain size increases with increase of milling hour.

Milling hour increases, the volume fraction of porosity also increases.

Milling hour increases, (reduction in grain size) density of the sintered sample and % of densification decreases. This indicates while grain size decreases more temperature and pressure required for better densification. This may be due to the reasons of increases in the surface area of the milled nanopowder (ie. Reduction of particle size), low green body density of long milling powders by agglomeration of powders and oxidation of powders during a milling process. Increases the surface area of the powder particle causes the more heat transfer. Densification of 85.4% was achieved for 16 hours of milled sample.

Acknowledgment

The authors wish to thank the management of Rajalakshmi Engineering College for permitting the author to do the experimental analysis in IIT.Kanpur on duty. Thanks are due to IIT, Kanpur for permitting the author to use their experimental facilities such as SEM, XRD, Particle size analyzer and Ball milling without which this work could not have been possible.

References