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Mechanical alloying, abbreviated as MA, is a powder processing technique that produces alloy or composite powders with balanced or non-equilibrium phases from elemental powders. This process takes place in a high-energy ball mill, where intense grinding occurs between the powder particles and between the particles and the grinding balls. As a result, the powders are broken apart, and their newly exposed surfaces undergo cold welding, leading to gradual alloying. Through repeated cycles, this process ultimately achieves the goal of mechanical alloying.
Developed by Benjamin and his team at the United States International Nickel Corporation (INCO) in the late 1960s, mechanical alloying was initially used to produce nickel- and iron-based superalloys with both precipitation hardening and oxide dispersion hardening effects. In the early 1980s, American scientist Koch successfully created Ni60Nb40 amorphous powder using this method, marking a significant breakthrough. Since then, the technique has evolved rapidly. Following extensive research by W. Schlü and H. Grewe, it was proposed in 1988 that mechanical alloying could produce nanocrystalline materials. Later, Fecht successfully fabricated ultrafine-grained alloys on the nanoscale using this approach, opening up a new field in material science.
Today, mechanical alloying is widely applied in the production of nano-scale ultra-fine grain dispersion-strengthened materials, magnetic materials, superconductors, amorphous materials, nanocrystalline materials, lightweight high-strength metals, and supersaturated solid solutions. Countries like the United States, Germany, and Japan have invested heavily in research and development, achieving remarkable results and even industrial-scale production. For example, INCO has already established a mechanical alloying line for iron, nickel, and aluminum oxide dispersion-strengthened alloys, with an annual production capacity of 350 tons.
In China, research on mechanical alloying began in 1988 and has made significant progress over the past decade. The method continues to attract attention due to its ability to create advanced materials with unique properties.
The basic principles of mechanical alloying were first proposed by Shinmiya Hideo from Japan in 1988. He introduced the rolling and refolding model, where after multiple rolling cycles, the thickness of the powder decreases exponentially. For instance, if two elements are rolled 10 times with a reduction rate of 1/a ≈ 31,6296, the particle size can be reduced to one hundred thousandth of its original thickness, forming a very thin layered structure. Further calendering can lead to a nanometer-sized microstructure, enabling solid-state alloying.
In 1990, Atzmon proposed another mechanism: the mechanically induced self-propagating reaction. According to this theory, intermetallic compounds form not through nucleation and growth but rather through a sudden reaction. The ignition temperature for such reactions depends on the size of the powder particles and grains. As the particle size decreases, so does the ignition temperature. When the particles reach a certain small size, the high temperatures generated during ball milling can trigger a "combustion-like" reaction, resulting in rapid alloy formation.
Currently, it is generally accepted that most mechanical alloying processes are diffusion-controlled. The core process involves repeated mixing, crushing, and cold welding of the powder particles. During ball milling, mixtures of different metallic or non-metallic powders develop high-density dislocations, leading to grain refinement down to the nanoscale. This creates fast diffusion pathways for atoms, allowing the formation of alloy nuclei under suitable conditions. As the milling continues, all the elemental powders gradually transform into a single alloy phase and grow over time.