3.1 Evaluation by magnetic measurements
Figure 1 illustrates the vibrating sample magnetometer(VSM) a scientific instrument, which works on the basis of Faraday's Law of Induction, which tells us that a varying magnetic field generate an electric field. This electric field can be monitored and gives us information about the magnetic field changing. The total magnetic moment induction is generally measured as a function of an external magnetic field, which is referred to as magnetic hysteresis analysis [7–8], which gives information about magnetic characteristics, all of which play a vital role in the investigation of magnetic materials [9–11].
The variation of magnetization with the magnetic field at room temperature of the nanocrystalline TiAlV alloy in function of effect of collision of the balls is shown in Fig. 2. The elementary powder before milling is non-magnetic material. However, the samples after ball milling exhibit much larger value of Hc and also the saturation magnetization decrease during mechanical grinding.
Table 1 shows the variation of different magnetic parameters during mechanical alloying, The coercivity vary during milling, this a variation is due to reduction of crystallite size of iron worn off from the milling balls after milling due of ball collision. Saturation magnetization (Ms) is a characteristic of magnetic materials, MS depends on the variation of chemical composition of the local environment of atoms and their electronic structures, the apparition of iron worn off from the milling balls with Ti(Al) and Ti(Al,V) leads to a non-saturating magnetic behavior.
Table 1
Different magnetic parameters as function of milling time
Millig time
(h)
|
Mr
(emu/g)
|
HcOe
(Oe)
|
Hs
(Oe)
|
Ms
(emu/g)
|
Area
(Oe * emu/g)
|
S
|
10
|
0,02518
|
130,42
|
2728,5
|
0,065
|
40,9404
|
0,16
|
20
|
0,01834
|
166,49
|
3061,39
|
0,184
|
67,2069
|
0,16
|
40
|
0,00629
|
49,39
|
4075,65
|
0,121
|
22,0921
|
0,02
|
60
|
0,00696
|
136,72
|
3972,09
|
0,0325
|
36,5094
|
0,1
|
3.2. Morphology
Figure 3 shows the morphology of nanocrystalline TiAlV at the different grinding time of 0, 20, and 40 hours, which is indicates an important variation in size, shapes and nature of powder particle with changing of grinding time. This change is due to the grinding process, which leads to repeated welding and fracturing phenomenon during mechanical alloying.
Before grinding, Ti, Al and V particles appears in irregular shapes with different particle sizes. After 20h of grinding, the powder particles are spherical and irregularly shaped with an average size in the range (5–10) µm, while continuously grinding reduces the particle size at longer grinding times, this decrease is attributed to the fracturing phenomenon. After 40 h of grinding, the powder particles became more homogeneous while maintaining the granular morphology, and the two processes of fracturing and cold welding are in equilibrium and the powder particles appeared more homogeneous compared to the earlier grinding.
Figure 4a depicts the EDS micrograph of the nanocrystalline TiAlV alloy before grinding, the Ti, Al and V element composition was not homogeneously dispersed and we have no contamination caused by collisions of the balls and the vial during fabrication.
Figure 4a. EDS spectrum and EDS elemental distribution maps of the nanocrystalline TiAlV alloy before grinding
Figure 4b demonstrates the EDS micrograph of nanocrystalline TiAlV alloy during mechanical grinding at 40h, the repartition of Ti, Al and V elements is uniform and there is some contamination of iron worn off from the grinding balls after milling due to ball collision, the appearance of TiAlV alloy powder was previously formed in 40 hours
Figure 4c represents the EDS micrograph of the nanocrystalline TiAlV alloy through mechanical grinding at 80h, we observe that we have some particles of the iron residue dropped as a result of the collision between the grinding balls and that these elements are homogeneously dispersed.
An EDS study was performed on the powder particles during the grinding times in order to verify the formation of the nanocrystalline TiAlV alloy. It is found in the EDS results that there was no considerable change in the chemical composition and mass ratio of the powder and the TiAlV alloy is formed with some particle of iron after grinding.
3.3. Structural analysis
X-ray diffraction (XRD) patterns were performed in an XPERT PRO X-ray diffractometer utilizing CoKa radiation. XRD diagrams were taken in 2h with a step size of 0.10°. The size of crystallites (D) was calculated from the broadening (ß) by use of the Scherrer formula as described below [12]:
$$D=\frac{0.9\lambda }{\beta {cos}\theta } \left(3.1\right)$$
The lattice strain (ε) was determined for the similar diffraction lines from the formula below [13]:
$$=\frac{\beta }{4{tan}\theta } \left(3.2\right)$$
While λ is Co intensity (1.7889 Å),θ is the angle in radians, ß is the full width at half maximum and ε is the lattice strain
The X-ray diffraction (XRD) pattern of the grinded nanocrystalline TiAlV alloy is depicted in Fig. 5. XRD diagrams before mechanical grinding of TiAlV powder. It can be seen the presence of all Ti, Al and V peaks and the absence of any new phase. Following 10 h of grinding, certain Ti and Al peaks have been disappeared, some other Ti and Al peaks are enlarged, and all the peaks of V have been disappeared. This broadening of the principal Ti peaks extends to 20 h. This could be due to a progressive diffusion of Al and V atoms inside the Ti lattice, which suggests a continuing reduction in crystallite size resulting from the formation of new phases. After 40 h of grinding time the XRD pattern shows the formation of another phase as a result of crystallization of the amorphous phase. The increase in grinding time to 60 h resulted in a reduction in intensity and a broadening of the XDR peaks, which is signified the decrease in the crystallite size, which is a result of the chemical segregation produced by the high energy grinding. The repeated cold welding and fracturing of the particles and the increasing density of crystal defects and dislocations result in the dispersion of Al and V, which also resulted in the broadening of the XRD peaks [14].
Table 2 illustrates the impact of grinding time on the crystallite size and lattice strain of the nanocrystalline TiAlV alloy. Through 20 h, the crystallite size reduces from 48.73 to 13.93 nm, at this period; fracture process is dominant, which is due to severe plastic deformation of the powder particles that occurs during mechanical grinding. After 20h, a gradual reduction in crystallite size from 13.93 to 9.38 nm, which is caused by the equilibrium between the fracturing and the welding process. The lattice strain increases from 0.15–0.81%. This may be due with the introduction of dislocations, impurities, and other lattice defects during grinding [15].
Table 2
Crystallite size and lattice strain of TiAlV at different milling time.
Milling Time (h)
|
crystallite size (nm)
|
Latice strain (%)
|
0
|
48.73
|
0,15
|
10
|
25,48
|
0,3
|
20
|
13,93
|
0,45
|
40
|
10,96
|
0,71
|
60
|
9,38
|
0,81
|