**3.4 Morphological study on mechanically alloyed nanocrystalline MTO powders**

### *3.4.1 SEM analysis*

SEM micrographs were obtained to see the influence of mechanical activation on the evolution of microstructure of MTO powders milled for different hours (**Figure 6**). It is well-known that milling processes yield a significant modification

**Figure 5.** *TGA and DSC curves of 35 hours milled powders.*

**Figure 6.**

*SEM micrographs of the MTO powders milled for (a) 0 (b) 5 (c) 20 and (d) 35 hrs (adapted with permission from Bhuyan et al., 2020, @ Springer [19]).*

in the morphology of composite materials due to severe plastic deformation of the particles during the milling process [38, 39]. Generally, the microstructure evolution is controlled by the processing parameters, such as composition of the materials, rise of temperature and milling intensity. From the morphological study, it is noticed that the starting powders consists of spherical particles with extreme agglomerated morphology. After a short period of milling up to 5 hrs, there is significant effect on the morphology of the MTO powders was observed. The particles were distributed over a wide range from sub-micrometer to few micrometers with spherical in morphology. As the milling time increased up to 35 hrs, cold welding of particles was activated and the size of the particles reduced into nanometer range due to the high impact collision of the balls. At this stage more distinct granular structure particles are observed as compared to the initial stages of milling along with the presence of new phases in the shape of agglomerates covered with many smaller nanosized particles of starting powders. These clustering of MTO nanoparticles are typically mechanically alloyed powders that are resulted from repeated cold welding and fracture of powders during the process of high energy mechanical alloying. The surface morphology of the nanocrystalline MTO powders are in support as evidence to the XRD results that the crystalline nature enhances with the increase of milling durations.
