Microstructural Changes during Annealing of Work-Hardened Mechanically Milled Metallic Powders (Overview)

The behavior of work-hardening which occurs during mechanical milling (MM) treatment in metallic powders, and the process of recovery and recrystallization which occurs during annealing in the MM powders were over-viewed showing the results obtained by the authors using an industrial pure iron powder. Through the MM treatment, metallic powders stores extremely large strain energy, and this results in the marked work-hardening and the formation of a fine structure with nanocrystalline grains. In the case of iron, the hardness of powder can be increased to DPH1024 in practice, and the crystalline grain size is to be reduced to the limiting value of 3.4 nm in principle. The polycrystallization of dislocation cells and subgrains also proceeds on the grain refining process. When the MM powders are annealed, the powders undergo different microstructural changes depending on the degree of work-hardening subjected by the prior MM treatment. In the case that powders are not work-hardened so much, usual recovery and recrystallization occur with raising annealing temperature. However, when powders are extremely work-hardened to the level where crystalline grain size nearly reaches the limiting value, only the process of normal grain growth occurs during annealing and this results in the softening of MM powders. Under the usual milling conditions, the degree of work-hardening of powders is in the middle stage, so that both of the above two processes are possible with overlapping each other. It was confirmed in both of MM iron powders and annealed iron powders that the relation between hardness and polycrystalline grain size gives a good fit to the Hall-Petch relationship in a wide grain size range up to 6 nm. In addition, some examples are introduced at the end, in order to excellent properties of materials produced from MM powders.