Till now, we have prepared Ti-based binary TiC and ternary Ti–Al–C, Ti–Ni–C, and Ti–W–C metal carbides by MA. A comparative study between the binary and ternary Ti-based metal carbides ball-milled at
Figure 2.10 HRTEM (a) transmission micrograph, (b) histogram plot of 8 h ball-milled homogeneous mixture of TiWC powders, (c) SAED pattern of 10 h TiAlC ball-milled sample, (d) HRTEM image of 12 h ball-milled TiNiC nanoparticle, (e) micrograph containing (111) planes in a TiAlC nanocrystalline particle, and (f) (111) planes in TiWC nanocrystalline particle.
0 5 10 15 20 0
10 20 30
Frequency
Particle size (nm)
(c) (d)
(e) (f)
(111)
(111)
(111)
0.251nm
2 nm
~2.42 Å
~8.7 nm (200) 5 nm
(331) (400)
(222) (311) (220) (200) (111)
2 1/ nm
(a) (b)
different time period can be enlightened from the results obtained from the Rietveld analysis of XRD data. Figure 2.11a illustrates the variations of Ti-based carbide phase formation with and without the presence of any of the solute Al/Ni/W with progress of milling time. Nature of phase content variation evident that the stoichiometric mixture of -Ti and graphite pow- ders under mechanical milling at room temperature and argon atmosphere produced binary stoichiometric TiC after 35 min of milling. Critical obser- vation of Figure 2.11a points out that at the early stage of milling, formation
of metal carbide phase is retarded due to introduction of solute metal M atom in Ti–C metal matrix. Ternary Ti-based metal carbides needed more milling time compared to TiC for formation as well as becoming complete stoichiometric in composition (Table 2.3, Figure 2.11a). Among them, Ti0.9Al0.1C forms and also turns into stoichiometric powder in quickest mill- ing time (48 min and 3 h, respectively). Ti0.9W0.1C and Ti0.9Ni0.1C started to form after 50 and 55 min of milling time, respectively, and become com- pletely stoichiometric after same milling time (8 h). However, at the time of formation phase, contents of both Ti0.9Ni0.1C and then Ti0.9Al0.1C are less than that of Ti0.9W0.1C in their respective powder mixtures. It is clear that on the way of preparation of Ti–M–C, W atom and then Ni atom diffuse slowly in Ti–C metal matrix compared to Al atom.
Figure 2.11b depicts the nature of change in lattice parameter values for all cubic Ti-based metal carbides. It is evident from the illustration that at the time of formation, lattice parameter of ternary phases does not differ much from that of binary Ti–C phase. Lattice parameter of TiC remains nearly constant during milling process, but inclusion of other metal atom
Figure 2.11 Variation of microstructural parameters of binary and ternary Ti-based metal carbides for (a) mol fraction, (b) lattice parameters, (c) particle size, and (d) rms strain.
0 2 4 6 8 10 12
0.88 0.90 0.92 0.94 0.96 0.98 1.00
TiNiC
Mol fraction
Milling time (h) TiCTiWC TiAlC
0 2 4 6 8 10 12
0 10 20 30 40 50 60 400 450
TiNiC
Particle size (nm)
Milling time (h) TiC TiWC
TiAlC
1 2 3 4 5 6 7 8 9 10 11 12
0 1 2 3 4 5 6 7
TiAlC
strainx103
Milling time (h) TiC
TiWC
TiNiC 0 1 2 3 4 5 6 7 8 9 10 11 12 13 0.429
0.430 0.431 0.432 0.433 0.434
Lattice parameter (nm)
Milling time (h) TiC TiWC
TiAlC
TiNiC
(a) (b)
(c) (d)
in α-Ti-C matrix in the process of ternary phase formation leads to differ- ent nature of variation as plotted in Figure 2.11b. Obviously, expansion or contraction of ternary Ti-based metal carbide lattice depends upon com- parative atomic radius of solute metal atom and that of Ti atom. Also at relatively higher milling time cold working on ball-milled powder mixture may results in lattice expansion in some cases.
A comparison study between binary and ternary Ti–C system on the basis of particle size variation with increasing milling time can be described from the analysis of Figure 2.11c. Nature of variation of particle size clearly shows that TiC phase formed at 35 min of milling with larger particle size (~448 nm). Within 6 h of milling, rapid decrease in TiC particle size (~13 nm) indicates that TiC phase was formed through MISPR. All ter- nary Ti-based metal carbides under consideration formed via MISPR with a relatively small particle size than that of TiC at the time of formation.
Compared to the variation of particle size of binary metal carbide phase, same plots for ternary phases show a rather slow variation at the early stage of milling. It is interesting to note that to attend the same order of particle size value ternary phases needed more time than binary phase. After 2 h of milling, particle size values for all phases differ in the increasing order of particle sizes, i.e., TiC < Ti0.9Al0.1C < Ti0.9Ni0.1C < Ti0.9W0.1C (Figure 2.11c).
Evidently, in case of ternary metal carbide formation, inclusion of Al/Ni/W atoms in -Ti–C matrix plays a vital role in rate of reduction of particle size. Observation of particle size plots (Figure 2.11c) also reveals that all metal carbide phases have same particle size value after 10 h of milling (~10 nm).
Figure 2.11d shows that rms lattice strain value of all binary and ter- nary Ti–C increases nonlinearly with increasing milling time and a sudden enhancement in strain value is noticed after 2 h of milling. In the course of milling, both particle size and rms strain values of TiC, Ti0.Al0.1C, and Ti0.Ni0.1C tend to saturate at relatively higher milling time. However, rms lattice strain value of Ti0.W0.1C increases significantly till its particle size saturates after 8 h of milling. It is to be noted that Ti0.W0.1C phase acquires a higher rms strain value in comparison to nearly same strain value of other binary or ternary Ti-based metal carbides. From the plots of lattice strain variation it is evident that in particular, diffusion of W atom in TiC metal matrix leads to rapid accumulation of enormous amount of stored energy which is manifested in rapid increase in lattice strain value of Ti0.9W0.1C after 8 h of ball milling. Rapid increase in rms strain value compared to slow reduction of particle size value in general leads to the conclusion that peak broadening of metal carbide phase is basically controlled by the high degree of lattice strain induced by ball milling.