Mechanical processing can be used to obtain metastable crystalline phases, e.g., phases that can only be reached at equilibrium using high temperatures and/or pressures, or even amorphous phases from crystalline phases that are stable at room temperature and pressure. Such transformations, also called mechanical alloying, can be produced by ball milling.

 

W–Al2O3 nanostructured composite has been produced by mechanical milling of ammonium paratungstate (APT) and Al powder mixture at room temperature. The milled products have been characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). The XRD and EDS results confirmed that direct reduction of APT to α-W took place after 25 min milling. It was observed that α → β phase transition took place after 24 h mechanical milling. SEM micrographs showed that alumina matrix composite reinforced with W could be obtained by one step MSR (mechanically induced self-propagation reaction) process. X-ray diffraction peak broadening equation confirmed that the mean grain size of these particles was in nanometer range which gradually decreased with increasing the milling time.