Inﬂuence of spark plasma sintering parameters on magnetic properties of FeCo alloy

Equiatomic FeCo alloys with average particle size of 24 µ m were sintered using spark plasma sintering (SPS) system at sintering temperatures of 1100, 800, and 850 ◦ C for heating rates 50, 100, 300 ◦ C/min by applying pressure of 50 MPa instantly at room temperature for sintering time of 5 and 15 minutes. The highest saturation induction was achieved at SPS conditions of 50 MPa, 50 ◦ C/min, 1100 ◦ C, without dwelling, of value 2.39 T. The saturation induction was improved with extending sintering time, the coercivity was higher in samples sintered at a fast heating rate in comparison to the slowest heating rate. © 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.4993529

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I. INTRODUCTION
Equiatomic FeCo alloy is one of the most important soft magnetic materials due to its high magnetic properties. 1 Spark plasma sintering (SPS) process is relatively new sintering technique with many advantages over conventional sintering processes such as high density accompanied with minimal grain growth, short time for sintering, high heating rate, high sintering temperature, binderless process, and applying pressure during sintering. This process improves different properties of various materials. 2 SPS recently used for sintering FeCo alloy composites, showing an advantage for maintaining the nanostructure features and significant improvement in mechanical properties. [3][4][5] Newly, the mechanical properties of FeCo alloy were significantly improved by SPS at certain sintering conditions. 6 Therefore, the current study aims to evaluate the magnetic properties of FeCo alloy sintered at different sintering conditions in order to optimise the best sintering conditions for the best combination of magnetic and mechanical properties.

A. Spark plasma sintering technique
A gas atomised Fe-50Co-0.2Si alloy powder average particle size of 24 µm was supplied by Sandvik Osprey Powder Group. A twenty gram of equiatomic FeCo alloy powder was charged in a 30 mm graphite die, lined with graphite foil, which was the compacted by using a Specac manual cold press, the prepared die was then moved to the SPS furnace and exposed to a pre-programmed sintering  procedure. All samples were sintered under vacuum (5 hPa), at a constant 50 MPa uniaxial sintering pressure. An optical pyrometer was inserted through a 10 mm diameter channel in the SPS tooling and into a top punch of the graphite die at a distance of 4 mm from the top surface of the sample, in order to measure the sintering temperature. Therefore, it is expected that the measured temperature is overestimated or underestimated according to the electrical property of the sintered sample with respect to die-punch assembly. The processing temperatures considered are thus insignificant. A total of three different heating rates (50, 100 and 300 • C.min -1 ) were investigated. Therefore, the 'optimum' sintering temperature was selected for each heating rate based on the point at which maximum shrinkage happened (+50 o C to ensure good densification). Then, the second group of samples were densified at that 'optimum' temperature for 15 min to assess the time taken before shrinkage accomplishes to give the 'optimum' sintering time. Finally, of samples were sintered for each heating rate under the 'optimised' sintering conditions (temperature and time). A summary of the resulting sintering conditions for all samples are seen in Table I, the sintering conditions were given alphabetical identities for simplicity during referring hereafter.

B. Evaluation of sintered compacts
Archimedes' immersion technique in water was used to measure the density of the sintered samples within an error of ± 0.5%. Electron discharge machining (EDM) was used to cut rectangular samples of cross section 24 × 5 mm from the 30 mm diameter sintered discs, which were ground with silicon carbide papers to eliminate the machining scratches. An automatic universal measurement system was used to evaluate the quasi DC magnetic response of the samples by adjusting the magnetic field up to 25 kA.m -1 .

A. Relative density and grain size
The relative density of FeCo alloys sintered at various sintering conditions is seen in Table II. The density was reduced with increasing heating rates, the highest density value of 99.8 % was achieved at sintering conditions 50 • C.min -1 , 850 • C and 15 min, at sintering pressure of 50 MPa. In order to consider the influence of grain size on magnetic properties, the measured sizes are listed in Table II. The detailed of microstructure study of FeCo alloy sintered following the above-mentioned sintering conditions are reported in our previous study on mechanical properties in Ref. 6.

B. Magnetic properties
The upper halves of hysteresis curves of sintered FeCo alloys at different sintering conditions are presented in Figure 1, a summary of the magnetic properties of the sintered FeCo alloys are seen in Table II. Regardless of the sintering time, the saturation induction shows a linear relationship with sintering temperature for heating rate 50 • C.min -1 . Inconsistent behaviour is observed for the other heating rates. The highest value of saturation induction for all sintering temperature was achieved  at heating rate 50 • C.min -1 , due to improved densification at this rate. The magnetic properties of FeCo alloy fabricated by powder metallurgy are mainly affected by grain size and density of the component. The former has a significant influence on coercivity, while saturation magnetisation of powder metallurgy product is more sensitive to density. 7,8 Increase the dwelling time at a sintering temperature of 850 • C and 800 • C was very effective in increasing the density of the sintered FeCo alloys, therefore a significant improvement in saturation induction was achieved at these sintering conditions Table II. Further, a change in the fraction of ordered state may happen due to sintering in lower temperature, which rationalises that even reduction in density in some samples does not because a reduction in saturation induction value since the saturation induction in fully ordered structure exceeds the disordered structure by 2-3%. 9 An increase in coercivity is observed for the samples sintered at 800 • C of heating rates 300 • C.min -1 compared with samples sintered at temperature 850 • C for lower heating rate 50 • C.min -1 . However, for all heating rates, the coercivity was lower at the highest sintering temperature of 1100 • C 6 than other sintering temperature as shown in Figure 2. Generally, most types of defects such as grain boundaries, dislocations, precipitates effect on coercivity. Order-disorder transformation has also an influence on the final value of coercivity since the coercivity of ordered state is higher than the disordered state at room temperature. 9 The ordered structure may increase in samples sintered at 800 and 850 • C, giving a reason for increasing coercivity in comparison to samples sintered at 1100 • C. Furthermore, it is claimed by Ref. 1 that the coercivity is inversely proportion to grain size in the micron size order. An increase in grain size was observed at the high sintering temperature. However, the fast heating rate of 300 • C.min -1 was effective in reducing grain size, therefore an increase in coercivity value was observed. Conflicting behaviour is observed when the  sintering temperature is increased to 1100 • C, due to the reduction in volume fraction of the ordered state at faster heating rates, 6 and increase the grain size. Moreover, sintering at high temperature helps in releasing the coercivity dependent on residual stresses in the material which are introduced during mechanical pressing and causes more purification for structure. Table III is a comparison of magnetic density and saturation induction of the current FeCo alloy with attempts were made by powder metallurgy PM to improve the magnetic properties of the alloy. [10][11][12] Spark plasma sintering process shows a significant change in magnetic properties in comparison to other sintering processes due to approximately full densification in sintered materials.

IV. SUMMARY
The SPS parameters to achieve maximum saturation induction for equiatomic FeCo alloy were found to be 1100 • C, 50 MPa, 50 • C/min without dwelling, showing significant improvement in comparison to other PM processes to sintering FeCo alloy. The coercivity was significantly decreased at a sintering temperature of 1100 • C due to the increase in grain size. In comparison between dwelling for heating rates 100 • C/min and 300 • C/min, the highest value for the relative density was only obtained by sintering at a temperature of 1100 • C without dwelling. However, near full densification of relative density, 99.8 % was achieved by sintering at 850 • C, 50 MPa, 15 min for the slowest heating rate of 50 • C/min. Fast heating rates were not suitable to obtain full densification, therefore the magnetic properties were deteriorated with increasing heating rate.