TABLE OF CONTENTS The MA technique is being used mainly for three types of processing: alloying, metastable phase formation and activation of chemical reactions. The underlying mechanisms involved in these processes to the extent known today are discussed here.
To discuss the mechanism of MA, it is convenient to divide the powder charge into three systems:
ductile-ductile
ductile-brittle
brittle-brittle
When both components are ductile, according to Benjamin and Volin [1], a balance of plastic deformation, cold welding and fracture lead to the final microstructure. The alloying occurs in five stages as shown in Fig. 5.1. In the first stage, when the particles start getting comminuted, the malleable components are deformed into long lamellae by the impact of the balls while the more friable components are comminuted. This is followed by growth in the number of lamellae due to cold welding; these composite lamellae in the coarser particles have a multilayered, oriented (plate-like) structure. The next stage is associated with the reduction in the aspect ratio of the lamellae and the plate-like coarse particles becoming equiaxed. In the fourth stage, the welding orientation in the composite particles becomes random and convoluted. The final stage is characterized by a narrow particle size distribution and the composition becoming uniform. At this stage, the individual lamellae cannot be resolved in an optical microscope. A saturation level of hardness of the particle is attained and the particles are in the heavily cold worked state. Regarding the atomistic mechanisms that could be responsible for...
