The technique of MA has been shown to produce a variety of materials. The most important reason for the development of MA was the production of ODS materials in which fine particles of Y2O3 or ThO2 were uniformly dispersed in a nickel-or iron-base alloy. In the mid-1980s, it was realized that MA is also capable of producing true alloys from elements that are not either easy to form by conventional means or sometimes even impossible to prepare, e.g., elements which are immiscible under equilibrium conditions. Investigations have revealed that metastable phases, such as supersaturated solid solutions, non-equilibrium crystalline or quasi-crystalline intermediate phases, and amorphous alloys, can be synthesized by MA (Ref 7, 8, 9). In addition, nanostructures with a grain size of a few nanometers, typically <100 nm, are produced. These metastable phases have interesting combinations of physical, chemical, mechanical, and magnetic properties and are being widely explored for potential applications. It is impossible to provide details of each of these developments, so only a very brief survey of the ODS alloys is presented here. Details of the structure and properties of metastable alloys are in Ref 7, 8, 9, and 10.

Ever since it was realized that MA could synthesize metastable phases, attempts have been made to compare the results of MA with those of rapid solidification, another non-equilibrium processing technique, even though the mechanisms by which metastable phases form by these two techniques are different. For example, solid solutions have been formed in the whole composition range in the copper-silver system by both techniques. A similar situation was obtained in several other systems by rapid solidification methods but not MA. Solid solutions have been obtained in the full composition range in the Cu-Fe, AlSb-InSb, and Cu-Co systems by MA but not by rapid solidification. Similarly, the levels of solid solubility achieved differ in various systems using the two techniques.

A similar situation exists with respect to the formation of metastable intermediate phases and amorphous alloys. Formation of an amorphous phase has been reported in the titanium-aluminum system by MA, which is not possible by rapid solidification methods. An important difference between these methods is that while amorphous phases form in the vicinity of eutectic compositions by rapid solidification, they form near the equiatomic composition by MA. Further, the glass formation range (the composition range over which glasses can form) is wider in the mechanically alloyed condition than in rapid solidification.

Formation of nanostructures has been reported in many alloy systems by mechanical alloying methods, but not by rapid solidification methods. But, devitrification of amorphous phases obtained by rapid solidification has led to the synthesis of nanostructure composites useful for magnetic applications (Ref 19).

Another important difference between mechanically alloyed and rapidly solidified alloys containing dispersoids appears to be in the size and distribution of the dispersoids. Figure 9 shows a pair of transmission electron micrographs comparing the matrix grain sizes, and size and distribution of dispersoids in HIP compacts of Ti3Al-base alloys containing Er2O3 dispersoids. The rapidly solidified alloy was Ti3Al to which 2 wt% Er was added and then hot isostatically pressed at 850 °C. The mechanically alloyed material was Ti-25Al-10Nb-3V-1Mo (at.%) to which 2 wt% Er was added and the alloy powder was hot isostatically pressed at 1000 °C. Even after HIP at a higher temperature, in comparison to the rapidly solidified alloy, the mechanically alloyed material showed a finer matrix grain size, more uniform distribution of the dispersoids, absence of large dispersoids at the grain boundaries, and absence of dispersoid-free zones near the grain boundaries.

Fig. 9 Transmission electron micrographs showing the difference in the matrix grain size, and size and distribution of dispersoids. (a) Rapidly solidified and hot isostatically pressed at 850 °C. (b) Mechanically alloyed and hot isostatically pressed at 1000 °C

The majority of these and other differences between materials processed by the two techniques could be traced to the fact that rapid solidification is a liquid-to-solid transformation while MA is a fully solid-state transformation.

The major industrial applications of mechanically alloyed materials have been in thermal processing, glass processing, energy production, aerospace, and other industries. These applications are based on the oxide-dispersion strengthening effect achieved in mechanically alloyed nickel-, iron-, and aluminum-base alloys.

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