As previously mentioned, the main process which takes place in a mill
during the MA method to produce quality powders with controlled
microstructure is the repeated welding, fracturing, and rewelding of a
mixture of powders of the diffusion couples. It is critical to
establish a balance between fracturing and cold welding in order to
mechanically alloy successfully. Two techniques are proposed by Gilman
and Benjamin[22] to reduce cold welding and promote fracturing. The
first technique is to modify the surface of the deforming particles by
addition of a suitable processing control agent (PCA) (wet milling)
that impedes the clean metal-to-metal contact necessary for cold
welding. The second technique is to modify the deformation mode of the
powder particles so that they fracture before they are able to deform
to the large compressive strains necessary for flattening and cold
welding. Cooling the mill chamber is an approach to accelerate the
fracture and establishment of steady-state processing (effect of
milling temperature).[23]
We should emphasize that milling the powders of certain metals which cold-weld easily (e.g., Ti, Zr, Al, Pb, Zn, Ag, etc.) with an organic agent (PCA),[53] may lead to a undesired reaction between the PCA and the milled powders, specifically those pure metals of the 4f and 5f elements.