TABLE OF CONTENTS While most MA is conducted in an inert atmosphere, there can be advantages to MA in a reactive atmosphere. In reaction milling, metal powders react extensively with the milling fluid that is reactive during milling. For example, processing powders in nitrogen atmospheres can result in the formation of nitrides or nitrogen can be trapped in the metal matrix [1,2,3,4], while processing in hydrogen atmospheres may form metastable hydrides [5].
The metal powder is comminuted to a particle size much smaller than the starting one. In general, chemical reactions between the fluid and the powder assist comminution of metal powders by not allowing them to weld together or to the balls/chamber walls.
Reaction milling can also be used to obtain the desired dispersoid by introducing an element that reacts during milling or during subsequent heat treatment, or in part during both. For example, MA of Al and C (lamp black or graphite) results in an Al-Al 4C 3 composite powder [6], and of Al-B-graphite in an Al-BC composite powder [7].
The process can also be used to produce nanostructured and amorphous materials, if milled for prolonged hours [1], e.g. when iron powder is mechanically alloyed with nitrogen at room temperature, it results in a highly disturbed, non-equilibrium, microfine microstructure with an enhanced nitrogen concentration entrapped in both the metal matrix and the highly disturbed regions. During MA, the impact deforms the particles plastically, creating a new surface. The newly exposed surface is highly reactive to...
