Abstract New materials such as metastable phases, ordered intermetallic compounds and amorphous alloys that are difficult to fabricate using normal metallurgical techniques can be produced using the mechanical-alloying technique, this being a powder-processing technique consisting of repeated cold welding, fracturing and re-welding of powders in a dry, high-energy ball mill machine. Ultrafine microstructures with grain sizes down to the nanometer level can be produced using this method. At least four events of collision can be identified in the ball-milling process: (a) direct collision between balls; (b) collision with sliding between balls; (c) direct collision between balls and the inner surface of the rotating container; and (d) collision with sliding between balls and the inner surface of the rotating container. Since the balls normally move in the same direction, the most efficient impact event for welding is direct collision between the balls and the inner surface of the container. In the present study, a model based on dynamics and cold-welding theory is used in calculations related to collision events. Occurrence of cold welding in the following cases is considered: welding between two different alloys: between the same alloy; and between the same alloy, with the inclusion of another interposed alloy which is not cold-welded. Because cold welding is an essential condition for mechanical alloying, the critical deformation required to achieve cold welding is evaluated with the model. It is proposed that the minimum bonding strength of the powders to be cold welded be considered as a criterion for mechanical alloying. With this, the critical inner diameter of the milling container at a particular rotational speed can be calculated.