Figure 6.01 shows typical room temperature XRD patterns of as-mixed and 40 hours milled Fe80(Al,Cr)10Co5Si15 alloy powders. It is observed that as-mixed powders exhibit sharp characteristic Bragg reflections corresponding to all four constituting elements, i.e., Al (face centered cubic), Co (face centered cubic and hexagonal close-packed structures), Si (diamond cubic) and Fe (body centered cubic) for Fe70Al10Co5Si15 powders and Cr (body centered cubic), Co (face centered cubic and hexagonal close-packed structures), Si (diamond cubic) and Fe (body centered cubic) for Fe70Cr10Co5Si15 powders. However, the Bragg reflections corresponding to Si, Co, Al and Cr disappear after milling for 40 hours. This is mainly due to the diffusion of Si, Co, Al and Cr in Fe matrix resulting the formation of non-equilibrium body centered cubic (bcc) solid solutions of α-Fe(Al,Co,Si) and α-Fe(Cr,Co,Si). It is also clear that no additional peaks corresponding to any other phases or compounds were observed within the resolution limit of high power XRD.
Figure 6.02. Room temperature XRD patterns of (a) Fe80-zAlzCo5Si15 and (b) Fe80-zCrzCo5Si15
alloy powders milled for 40 hours.
To understand the effect of substituting elements on the structural properties systematically, XRD patterns were collected for all the samples milled for 40 hours and depicted in Figure 6.02.
A close observation of the XRD patterns reveals that with increasing Al and Cr contents, all the peaks exhibit fairly large peak broadening along with minor shift in peak positions to lower angles.
and internally strained grains with increasing substituting elements, the later one could be attributed to the occurrence of atomic disorder due the dissolution of substituting elements in Fe matrix leading to a change in lattice constant. To study the solid solubility level, the lattice constant was calculated from the peak positions for all the compositions. It is observed that lattice constant increases from 0.28658±0.00005 nm to 0.28698±0.00007 nm and 0.28658±0.00005 nm to 0.28689±0.00004 nm with increasing Al and Cr contents in milled Fe80-z(Al,Cr)zCo5Si15 alloy powders from 0 to 10 at.%, respectively. These results suggest a maximum lattice change of 0.00004 nm per at.% Al and 0.000031 nm per at.% Cr occurred by the substitution of Al and Cr, respectively. Although the observed variation is in close agreement with the earlier reports on similar systems [SHIG1974, PETR2002, BOUK2012], the increase of lattice constant in the presently investigated alloys is slightly lower than those reported for binary Fe-Al and Fe-Cr alloys. This could be possibly due to the change in the dissolution process in the multicomponent materials as compared to the binary alloys.
Figure 6.03. Variations of D, ρ and fGB of 40 hours milled Fe80-z(Al,Cr)zCo5Si15 alloy powders.
In order to separate the contributions of average crystallite size (D) and strain through density of dislocations (ρ) to peak broadening, XRD patterns were analyzed carefully by MWHP method [UNGA19991, UNGA19992] using eqn.(4.04) as demonstrated in Chapter 4. Figure 6.03 depicts the variations of D, ρ and fraction of grain boundaries (fGB) calculated using eqn.(4.07) as a function of Al and Cr contents for as-milled Fe80-z(Al,Cr)zCo5Si15 alloy powders. It is revealed that Al and Cr substitutions interestingly reduce the values of D from about 11 nm to about 6 nm and increase the ρ values largely from 4.2×1017 m-2 to 13.6×1017 m-2. This indicates that the Al and Cr substitution helps in improving the size refinement and the existence of nearly 0.42 to 1.36 dislocations for every one nm2 area. This could be attributed to the enhancement in milling intensity with increasing substituting elements [KALI20081, KALI20082, BOUK2012].
Saturated value of dislocation density is achieved more rapidly in these alloys indicating that the solute concentration can strongly affect the mechanical properties. The formation of solid solution with fine crystallite size suggests that the solid solution would be harder and stronger than pure metal [AGUI2010]. The observed results confirm that the behavior of the dislocation density is inversely related to the crystallite size (see eqn.(4.06)), as the large dislocation density occurring in the smallest crystallite size. Furthermore, grain boundary which has a strong effect on the structural properties of nanocrystalline materials increases with decreasing the average crystallite size. It is well understood that the basic underlying mechanism of grain refinement involves an increase in the dislocation density by heavy deformation process. These dislocations form an order arrangement giving rise to dislocation walls, which transform in to grain boundaries, when the formation continues with milling. The grain boundaries affect the movement of dislocations and strain hardening significantly.
As the values of D decrease significantly with the substituting elements, we have tried to estimate the development of grain boundaries in these quaternary alloys using eqn.(4.07). While the values of fGB increase from 15 % to 23 % for the Al substituted powders, the Cr substitution enriches the fGB from 15 % to 28 %. This shows that Cr substitution enhances the formation of grain boundaries as compared to that of Al. Bahrami et al. [BAHR2013] reported the values of fGB as 13%, 18 % and 28 % for x = 0, 10 and 20, respectively in Fe80-xNi20Six system. In this case, the fraction of grain boundaries increases drastically from 13 % to 28 % with increasing the Si content up to 20 at.% and attributed to the enhanced process of microstructural refinement.
Similarly, Yousefi et al. [YOUS2014] reported almost 5 % increase in the grain boundaries with increasing 10 at.% Si in mechanically alloyed nanocrystalline (Fe65Co35)100-xSix powders.
Figure 6.04. SEM micrographs of pure (a) Fe, (b) Si, (c) Co, (d) Al and (e) Cr, and 40 hours milled (f) Fe78Al2Co5Si15, (g) Fe75Al5Co5Si15, (h) Fe78Cr2Co5Si15 and (i) Fe75Cr5Co5Si15 alloy powders.
To understand the modification of surface morphology of the powders carefully, we have analyzed the morphology of the powders using SEM technique. Figure 6.04 shows the surface morphology of the pure elements (Fe, Si, Co, Al and Cr) and the as-milled powders of Fe78Al2Co5Si15, Fe75Al5Co5Si15, Fe78Cr2Co5Si15 and Fe75Cr5Co5Si15. SEM micrographs show quasi similarity of particle morphology, i.e., the presence of particles with different sizes.
Aggregation of crystallites is a natural consequence of the mechanical alloying process due to repeated cold welding and fracturing. The shape of the aggregated particles is observed to be nearly spherical in nature for both Al and Cr substitution. However, the size distribution of the agglomerated particles is observed to be narrow in Fe-Al-Co-Si powders as compared to Fe- Cr-Co-Si powders. This could be attributed to the different properties of the substituting elements, i.e., Al is soft and ductile and the Cr is hard and brittle resulting different alloying process [CHRI2013]. Over all composition analysis was performed on as-milled powders using EDS attached to SEM unit. It is found that the composition of the as-milled powders to be Fe80.8Co4.6Si14.6, Fe79Al1.8Co4.8Si14.4 (Fe78.8Cr1.9Co4.6Si14.7), Fe75.9Al4.7Co4.7Si14.7
with Al (Cr) content of 0, 2, 5 and 10 at.%, respectively. These results evidently support the presence of Si, Co and Al or Cr in the form of solid solution in α-Fe.
Figure 6.05. BF-TEM micrographs and SAED patterns of 40 hours milled (a) Fe75Al5Co5Si15
and (b) Fe75Cr5Co5Si15 alloy powders.
In order to confirm the evolution of nanocrystalline microstructure, the as-milled powders were characterized using TEM technique. Figure 6.05 illustrates bright-field TEM (BF-TEM) micrographs and selected area electron diffraction (SAED) patterns for 40 hours milled Fe75Al5Co5Si15 and Fe75Cr5Co5Si15 powders. BF-TEM micrographs confirm the existence of fine-grain structure with the average grain size of about 7 nm having non-uniform contrast inside each grain and along the grain boundaries. This could be attributed to the large strain in the as-milled powders. On the other hand, the rings present in SAED patterns reveal a nanocrystalline microstructure and match with the Bragg reflections in the XRD patterns. The values of average crystallite size and lattice constant calculated from TEM analysis show nearby agreement with those obtained from XRD data as shown in Figure 6.03. These results show that the particles with sizes ranging from few hundred nanometers to few micrometers observed in SEM micrographs are agglomerations of fine nanosized crystallites oriented randomly with respect to each other. The nature of agglomeration, however, strongly depends on the alloy compositions.
Figure 6.06. Room temperature M – H loops of 40 hours milled Fe80-z(Al,Cr)zCo5Si15 alloy powders.