4.4. Properties of Fe-Si binary alloy powders milled at 600 rpm
4.4.1. Evolution of structural properties
Figure 4.23 shows typical XRD patterns of Fe100-xSix (x = 5 – 40) powders milled for different milling periods such as (a) 40 hours, (b) 70 hours and (c) 100 hours. It is observed that (i) With increasing milling time to 40 hours, the reflections corresponding to Si observed in as-mixed powders (see Figure 4.01) disappear leaving behind only bcc Fe reflections for Si content up to 20 at.%. This confirms the formation of bcc α-Fe(Si) non-equilibrium solid solution. (ii) The samples with x > 20 do not form the solid solution completely within 40 hours of milling, (iii) On further increasing the milling time to 100 hours (see Figure 4.23 (c)), the formation of solid solution could be enhanced up to 40 at.% Si in Fe-Si alloy powders. (iv) However, a careful observation of XRD patterns reveals the existence of finite Fe5Si3 and/or Fe-Si intermetallic compounds (represented by filled circle) in Fe60Si40 sample. (v) The sharp diffraction lines observed for as-mixed powders are broadened with increasing milling time and a measurable shift in the peak positions to higher angle has also perceived. These results suggest that the mechanical alloying induces disordering of bcc Fe structure, dissolution of Si in Fe matrix, creation of large interfacial area and grain boundaries with randomly oriented and highly strained nanocrystalline grains and formation of solid solution and compounds during milling. As a result, Fe-Si solid solution could easily be obtained for x ≤ 20 at low milling hours and the milling time required for forming solid solution in samples with x ≥ 25 increases with increasing Si content. This is in good agreement with the earlier reports on similar systems [FECH1992, BENS2009]. In order to understand the effect of Si content on the structural parameters, the XRD patterns of the as-milled powders exhibiting non- equilibrium solid solutions were analyzed using eqn.(4.04). It is observed that the values of q and Chkl are in the range of 1.65 – 2.32 and 0.105 to 0.148, respectively for the presently investigated samples.
Figure 4.24 displays the changes in the lattice constant of non-equilibrium solid solution of α-Fe(Si) with increasing Si content. The lattice constant decreases gradually with increasing
Si content confirming the occurrence of atomic disorder during mechanical alloying, which is mainly due to the dissolution of Si in Fe matrix. This is expected since atomic radius of Si (0.118 nm) is smaller than Fe (0.125 nm) [KALI20081, MIRA2008]. A maximum lattice constant change of 0.5% was obtained by substituting 40 at.% Si in Fe-Si alloy powders.
Figure 4.25. Variations of D and ρ of Fe100-xSix solid solution as a function of Si content.
The average crystallite size of the powder decreases largely down to around 10 nm due to milling. However, the average values of crystallite size display a weak dependence on Si content as shown in Figure 4.25. This behavior is in close agreement with the earlier reports on similar systems [MIRA2008], where the average crystallite size of Fe-Si alloy powders exhibits oscillating behavior between 15 nm and 13.2 nm with increasing Si content from 6.5 at.% to 25 at.%. On the other hand, the dislocation density of the powder raises to a value of order of 1017 m-2 for the solid solution. In addition, dislocation density increases with increasing Si content up to 35 at.% and decreases slightly for Fe60Si40 sample. As discussed earlier, the decreasing of crystallite size after milling could be attributed to formation of defects such as dislocations that can appear in different ways such as formation of dense regions of these dislocations into the grains, pile up the grain boundaries or untidy clusters into the grain [SCHI1996]. All these possibilities guide to formation of sub-grain structures inside the original grain and therefore decreasing the effective size in crystalline region. On the other hand, the increasing of dislocation density of solid solution can be attributed to the enhanced
the Si particles. The decrease in dislocation density for Fe60Si40 sample might be due to the formation of finite Fe-Si intermetallic phases. Furthermore, the fraction of grain boundaries calculated using eqn.(4.07) increases from 19 % to 22.8 % with increasing Si content from 5 at.% to 35 at.% in α-Fe(Si) solid solution. This could be attributed to the enhanced work hardening rate of the matrix induced by the Si particles [BAHR2013].
Figure 4.26. SEM micrographs of pure (a) Fe, (b) Si, and Fe100-xSix solid solution with different Si content: (c) x = 5, (d) x = 10, (e) x = 20 and (f) x = 30.
The changes in the surface morphology of the non-equilibrium solid solution with different Si contents were investigated using SEM microscopy technique. Figure 4.26 shows the SEM micrographs of un-milled Fe and Si powders and milled Fe-Si alloy powders with increasing
sized particles. The average size of the particles in the Fe-Si solid solution decreased significantly with increasing Si content and relatively become homogenous. The average size distribution of the particles also becomes narrower with spherical shape in nature. Comparative studies between XRD and SEM reveal that the particles seen in the SEM micrographs consist of crystallites with a size of about 8 – 11 nm oriented randomly with respect to each other.
Composition analysis performed on as-milled powders using EDS showed the overall composition to be Fe95.1Si4.9, Fe90.4Si9.6, Fe85.2Si14.8, Fe80.5Si19.5, Fe75.9Si24.1, Fe71Si29 and Fe66.1Si33.9 for Fe100-xSix alloy powders with x = 5, 10, 15, 20, 25, 30 and 35, respectively. This confirms the presence of Si in the solid solution of α-Fe. These changes in the structural and surface morphology of the Fe-Si alloyed powders are expected to modify the magnetic properties considerably.
Figure 4.27. Room temperature (a) initial magnetization curves and (b) M-H loops of Fe100- xSix solid solution as a function of Si content.