Mechanism of Alloying

During high-energy milling, the powder particles are repeatedly flattened, fractured, and rewelded. Whenever two steel balls collide, some amount of powder is trapped between them. Typically, around 1000 particles with an aggregate weight of —0.2 mg are trapped during each collision. The force of the impact plastically deforms the powder particles, creates new surfaces, and enables the particles to weld together, which leads to an increase in particle size. In the early stages of milling, the particles are soft (using either ductile-ductile or ductile-brittle material combination) and their tendency to weld together and form large particles is high. A broad range of particle size develops, with some as large as three times bigger than the starting particles. The composite particles at this stage have a characteristic layered structure consisting of various combinations of the starting constituents. With continued deformation, the particles become work hardened and fracture by a fatigue failure mechanism and/or by the fragmentation of fragile flakes. Fragments generated by this mechanism can continue to reduce in size in the absence of strong agglomerating forces. At this stage, the tendency to fracture predominates over cold welding. Due to the continued impact of grinding balls, the structure of the particles is steadily refined, but the particle size continues to be the same. Consequently, the interlayer spacing decreases and the number of layers in a particle increases. As previously mentioned, the rate of refinement of the internal structure (particle size, crystallite size, lamellar spacing, etc.) is roughly logarithmic with processing time and therefore the size of the starting particles is relatively unimportant. In a few minutes to an hour, the lamellar spacing usually becomes small and the crystallite (or grain) size is refined to nanometer (1 nm = 10~9 m or 10 A) dimensions (Fig. 7). The ease with which this can be achieved is one reason why MA has been extensively employed to produce nanocrystalline materials (Ref 8, 9, 16).

Milling time, h

Fig. 7 Refinement of particle and grain sizes with milling time. Rate of refinement increases with higher ball-to-

powder weight ratios.

After milling for a certain length of time, steady-state equilibrium is attained when a balance is achieved between the rate of welding, which tends to increase the average particle size, and the rate of fracturing, which tends to decrease the average composite particle size. At this stage, each particle contains substantially all of the starting ingredients in the proportion they were mixed together, and the particles reach saturation hardness due to the accumulation of strain energy. The particle size distribution at this stage is narrow, because particles larger than average are reduced in size at the same rate that fragments smaller than average grow through agglomeration of smaller particles (Fig. 8). The average particle size obtained at this stage depends on the relative ease with which agglomerates can be formed by cold welding, fatigue and fracture strength of composite particles, and resistance of particles to deformation.

Fig. 8 Narrow particle size distribution caused by tendency of small particles to weld together and large particles to fracture under steady-state conditions

From the foregoing explanation, it is clear that during MA, heavy deformation is introduced into the particles. This is manifested by the presence of a variety of crystal defects, such as dislocations, vacancies, stacking faults, and increased number of grain boundaries. The presence of this defect structure increases the diffusivity of solute elements into the matrix. Further, the refined microstructural features decrease the diffusion distances. Additionally, the slight rise in temperature during milling further aids the diffusion behavior, and consequently, true alloying takes place among the constituent elements. While this alloying generally takes place nominally at room temperature, sometimes it may be necessary to heat treat the mechanically alloyed powder for alloying to be achieved. This is particularly true when formation of intermetallics is desired.

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