Introduction

Mechanical alloying is a simple and useful technique to synthesize both equilibrium and non-equilibrium phases of commercially useful materials starting from elemental powders. It is also an economical process with important technical advantages. One of the greatest advantages of mechanical alloying is in the synthesis of novel alloys that are not possible by any other technique, such as alloying of normally immiscible elements. This is because mechanical alloying is a completely solid-state processing technique and therefore limitations imposed by the phase diagrams do not apply here.

Mechanical alloying (MA) is normally a dry, high-energy ball milling technique and has been employed to produce a variety of commercially useful and scientifically interesting materials. This technique was developed during the late 1960s by John S. Benjamin and his colleagues at the Paul D. Merica Research Laboratory of the International Nickel Company (INCO) essentially to combine the advantages of precipitation hardening and oxide-dispersion strengthening in several nickel- and iron-base superalloys (Ref 1, 2).

The formation of an amorphous phase by mechanical grinding of an yttrium-cobalt intermetallic compound in 1981 (Ref 3) and in the nickel-niobium system by ball milling of blended elemental powder mixtures in 1983 (Ref 4) brought about the recognition that MA is a potential nonequilibrium processing technique. Since the mid-1980s, a number of investigations have been carried out to synthesize a variety of stable and metastable phases including supersaturated solid solutions, crystalline and quasi-crystalline intermediate phases, and amorphous alloys. Efforts have been made since the early 1990s to understand the process fundamentals of MA. Additionally, it has been recognized that MA can be used to induce chemical (displacement) reactions in powder mixtures at room temperature or at least at much lower temperatures than normally required (Ref 5). This simple but effective processing technique has been applied to metals, ceramics, polymers, and composite materials. The important attributes of MA are:

• Production of fine dispersion of second phase particles

• Extension of solid solubility limits

• Refinement of grain sizes down to nanometer range

• Synthesis of novel crystalline and quasi-crystalline phases

• Development of amorphous (glassy) phases

• Disordering of ordered intermetallics

• Possibility of alloying of difficult to alloy elements

• Inducement of chemical (displacement) reactions at low temperatures

The different facets of MA have been reviewed periodically (Ref 6, 7, 8) and the literature available up to 1994 has been collected together in an annotated bibliography (Ref 9). Additionally, a number of conferences are devoted to the science and technology of mechanically alloyed materials and the proceedings of the annual International Symposia on Metastable and Mechanically Alloyed and Nanocrystalline Materials (ISMANAM) are regularly published in Materials Science Forum published by Trans Tech Publications, Switzerland.

Two different terms are used in the literature to denote the processing of powder particles in high-energy ball mills. Mechanical alloying describes the process when mixtures of powders (of different metals or alloys/compounds) are milled together. Material transfer is involved in this process to obtain a homogeneous alloy. On the other hand, milling of uniform (often stoichiometric) composition powders, such as pure metals, intermetallics, or prealloyed powders, where material transfer is not required for homogenization, has been termed mechanical milling (MM). Mechanical milling of ordered intermetallics has been shown to lead to disordering or amorphization, and the nature of these phase transformations and the reasons for their occurrence have been reviewed (Ref 10). Some investigators have referred to MM as mechanical grinding (MG). Since "grinding" is normally considered an abrasive machining process that involves mainly shear stresses and chip formation, the term milling is preferred because it includes the more complex triaxial, perhaps partly hydrostatic, stress states that can occur during ball milling of powders (Ref 7). It should be realized, however, that MA is a generic term, and some investigators use this term to include mechanical milling/grinding as well. This article uses the term MA only to include both MA and MM.

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