4.1 Structural Characterization of Milled Samples
4.1.2 XRD analysis
Figure 4.2 shows XRD patterns of 16 h milled powder sample milled in WC grinding media. From the phase analysis it has been found that peak intensity much reduced from initial, without milled powder and peak broadening occur. With SiC peaks SiO2
are also visible. This is mainly due to impurity content in SiC (purity >96%). Analysis of the XRD data revealed that average crystallite size of the milled powder was ~30 nm.
Fig. 4.2: XRD pattern of 16 h milled SiC powder sample.
Figures 4.3(a-d) show the modulation of the XRD patterns of Cu99Cr1, Cu94Cr6, Cu94Cr6–4 wt.% SiC and Cu99Cr1–4 wt.% SiC compositions with the progress of milling up to 50 h. It is interesting to note that Cu and Cr peaks are clearly visible after 50 h of milling. It is to be noted that the peak intensity gradually decreased and broaden with progress of milling in all the cases. But, broadening of peaks and decrease in the intensity was found to be higher in case of Cu99Cr1–4 wt.% SiC and Cu94Cr6–4 wt.% SiC compared to composition without SiC reinforcement. This is
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probably due to the faster reduction of particle size because of the presence of the harder, brittle SiC particles in the blend.
A few small peaks corresponding to Cr23C6 and Fe2C phases were detectable from the 50 h milled sample of Cu99Cr1 composition, which is shown in Fig. 4.3(a). The carbon contamination was occurred possibly due to from toluene, which was used as a process control agent during milling. In case of Cu94Cr6 (Fig. 4.3(b)), a few small peaks corresponds to Cr3C2 are visible along with the peaks of Cu, Cr and Fe. The peaks corresponding to Fe are possibly due to the contamination from milling container.
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Fig: 4.3: XRD pattern of (a) Cu99Cr1 and (b) Cu94Cr6 nanocomposites powder samples during milling.
Analysis of Cu99Cr1–4 wt.% SiC composition, showed the presence of SiC, CrSi2 and Cr23C6 phases after 2 h of milling (Fig. 4.3(c)). After 20 h of milling, SiC, CrSi, CrSi2
and Cr23C6 phases are observed. CrSi and Cr23C6 were gradually disappeared after MA of 20 h. No peaks corresponding to these phases were detectable by XRD analysis after 50 h of milling, whereas, SiC and CrSi2 phases were still persisted as shown in Fig. 4.3(c). The formation of CrSi, CrSi2 and Cr23C6 phases were possibly due to the reaction of elemental Cr with the contaminants from milling media and impure SiC, i.e., C from toluene, and Si from impure SiC.
From XRD analysis of Cu94Cr6–4 wt.% SiC as shown in Fig. 4.3(d) Cu and Cr peaks are visible from initial stage to 50 h of milling. CrSi2 and Si5C3 phases were detected after 50 h of milling only. The peak broadening and decrease in peak intensity is maximum for this composition compared to all other compositions, because of presence of higher percentage of Cr along with 4 wt.% SiC phases.
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Fig: 4.3: XRD pattern of (c) Cu99Cr1–4 wt.% SiC and (d) Cu94Cr6–4 wt.% SiC nanocomposites powder samples during milling.
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The stability of any new phase(s) formed/disappeared during continued deformation depends on kinetic factors and the structural factors like the extent of crystallite size refinement, lattice strain developed etc. (Mula et al., 2008). The possible reasons of the phase evolution in the Cu-Cr and Cu-Cr–SiC systems by mechanical milling could be analyzed in the light of Hume-Rothery’s rules of alloying. The atomic radii of Cr (0.1249 nm) and Si (1.17 nm) which are lower than that of Cu (1.275 nm) (Brandes et al., 1992). Here, the atomic size differences are within 15 % of the atomic radius of Cu. So it is expected that smaller size Cr and Si could enter into the solid solution of Cu. The solubility of Cr in Cu is less than 0.03% (at.%) at 400°C as per the equilibrium phase diagram (Liu et al., 2004). But, the solid solubility of solute element can be extended by high energy ball milling (Suryanarayana, 2001).
Therefore, analysis of the XRD data has been carried out to find out whether Cu-Cr solid solution was formed during milling up to 50 h.
The XRD data were analyzed for crystallite size refinement, lattice microstrain(%) and lattice parameter variation of Cu based solid solution of all the compositions. The crystallite size gradually decreases with progress of milling which was clearly visible in Fig. 4.4. Up to 20 h of milling rapid decrease in crystallite size can be noticed from the figure. After that a very slow trend in crystallite size refinement is observed. The refinement of crystallite size is much more prominent in case of composition having SiC compared to that of Cu99Cr1 andCu94Cr6 compositions for the same duration of milling. This could be corroborated with the XRD peak broadening and reduction peak intensity of the same composition (Figs. 4.3). In case of Cu99Cr1 composition after 50 h of milling, crystallite size was found to be in the range of 18–22 nm, whereas, it was in the range of 14–16 nm for Cu99Cr1–4 wt.% SiC.
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Fig. 4.4: Variation of crystallite size of different compositions with milling time.
From Fig. 4.5 it is observed that with increase in milling time the lattice microstrain (%) gradually increases rapidly up to 35 h of milling and after that it increases slowly and attains almost a constant value up to 50 h of milling. Due to the presence of harder and brittle SiC particles in Cu99Cr1–4 wt.% SiC and Cu94Cr6–4 wt.% SiC, crystallite size were found to be much finer (as shown in Fig. 4.4), which increases the lattice microstrain(%) to higher value than that of Cu99Cr1 and Cu94Cr6
compositions. The lattice microstrain(%) is in the range of 0.5–0.55 % after 50 h of milling in case of Cu-Cr and in case of compositions containing SiC, it is in the range of 0.56–0.7 %.
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Fig. 4.5: Variation in lattice microstrain(%) of different compositions with milling time.
Fig. 4.6 shows the variation of lattice parameters of Cu based solid solutions with progress of milling of Cu99Cr1, Cu94Cr6, Cu99Cr1–4 wt.% SiC and Cu94Cr6–4 wt.% SiC compositions up to 50 h. In all cases a little decrease in lattice parameter was observed up to 20 h of milling. After that there is no variation of lattice parameter was observed with further milling. The decrease in lattice parameter was possibly due to the formation of solid solution of Cr in Cu. As per the equilibrium phase diagram the solubility of Cr in Cu is less than 0.03% (at.%) at 400°C (Liu et al., 2004). In the present investigation also, it was observed that solid solubility of Cr in Cu is less than 1 at.% (Fig. 4.3(a)). With increase in Cr content in Cu, variation in solid solubility of Cr in Cu was not detected, which is possibly reflected from the constant lattice parameter of the Cu-based solid solutions after 20 h of milling in all the composition.
After 20 h milling, the alloying element either forms compounds with impurities from milling media or retains as mechanical mixture like SiC. Alloying element, when does not enter into solid solution, do not contribute to the lattice parameter variation of the Cu-based solid solution during milling.
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Fig. 4.6: Variation of lattice parameter of Cu-based solid solutions with milling time.