Mechanistic alloying mechanisms in high-energy ball milling

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Mechanical alloying (abbreviated as MA) is a new technique for the solid-state, non-equilibrium processing of materials, which was first proposed by Benjamin in the United States at the end of the 1960s. In 1983, American scientist Professor Koch was the first to use mechanical alloying technology to produce Ni-Nb-based amorphous alloys, thereby sparking a surge in research on mechanical alloying worldwide. Mechanical alloying involves mechanically mixing elemental powders to be alloyed in a specific ratio. In equipment such as high-energy ball mills, rotational mechanical energy is transferred to the powders over an extended period of time. Meanwhile, under the repeated impacts from the ball milling media, the powders are subjected to forces such as impact, shear, friction, and compression, undergoing repeated processes of extrusion, cold welding, and crushing. This leads to the mutual diffusion of atoms among the powders or to solid-state reactions, resulting in the formation of a super-fine particle alloy powder with a dispersed structure. Due to the complexity of the reaction process in mechanical alloying, its reaction mechanism is also very complex. After decades of theoretical exploration and research, understanding of its mechanism has gradually matured. Today, mechanical alloying, as an important method for preparing new materials, is attracting increasing attention from the global materials community; therefore, it is crucial to understand its reaction mechanism. To date, many reaction mechanisms have been proposed centered around a certain major phenomenon in the reaction. This article mainly introduces several relatively mature mechanisms for study and reference. 1 Reaction mechanism dominated by interfacial reactions. Generally, a multiphase chemical reaction process involving a solid phase is one in which the reactants achieve atomic-level bonding and overcome the reaction barrier to undergo a chemical reaction; its characteristic is the presence of an interface between the reactants. When the activity of the powder system reaches a sufficiently high level during the ball milling process, the temperature rise at the interface caused by the collisions between the balls and the powder particles triggers a chemical reaction there (similar to the combustion synthesis reaction SHS observed in mechanical alloying processes as reported by some material scientists). The reaction products separate the reactants, and the reaction rate depends on the diffusion rate of the reactants within the product layer. During the ball milling process, as the powder particles continue to break apart, a large number of new surfaces are created, and the reaction products are carried away, thereby maintaining the continuous progress of the reaction until the end of the entire process. In the literature, the authors prepared Fe-Al feedstock in a ratio of 28% Al (atomic fraction) and subjected it to high-energy ball milling. Tests and analyses of the powder showed that as the milling time increased, the peak intensity of aluminum gradually decreased; after 20 hours of milling, the diffraction peak corresponding to aluminum was very weak, and after 30 hours of milling, this diffraction peak was almost undetectable. Exothermic analysis of the powder after 30 hours of milling revealed a very gradual exothermic process, indicating that as milling time increased, most of the aluminum reacted with iron to form intermetallic compounds. This result is similar to those obtained by Cardellini. After the powder is finely ball-milled to a certain extent, its particles become extremely small; and as the surface area increases, the likelihood of direct reactions occurring at the interfaces between the particles rises. Consequently, on a macroscopic level, interfacial reactions dominate. The mechanical alloying of pure Fe and Al powders to form FeAl or Fe3Al is primarily driven by this mechanism: during ball milling, the powder undergoes continuous collisions that generate numerous new surfaces, and when the atomic distances between the particles become sufficient, they fuse together, resulting in atomic bonding. Continuous collisions create a large number of new bonding surfaces, allowing the reaction to proceed continuously and ultimately resulting in the formation of a compound. Some researchers have also found that Fe and Al powders begin to alloy after 25 hours of ball milling, and complete alloying to form a FeAl alloy occurs after 100 hours of ball milling. 2 Diffusion-dominated reaction mechanism: During high-energy ball milling, the powder is repeatedly broken and welded together, resulting in a large number of new bonding interfaces and the formation of fine, multi-layered composite particles. Continued grinding leads to further grain refinement due to the increase in internal defects (vacancies, dislocations, etc.) resulting from plastic deformation. At this point, solid-state reaction diffusion occurs between its constituent elements, and this diffusion has three characteristics: the temperature at which it takes place is low ； The diffusion distance is very short ； The system energy increases, and the diffusion coefficient rises. For solid crystalline materials, the macroscopic phenomenon of diffusion is the result of microscopic migration. For atoms to undergo transition, the system must reach a relatively high energy state; as shown in Figure 1(a), this additional energy is known as the activation energy ΔEa. Atomic transitions in the solid state are generally attributed to the vacancy mechanism, whose activation energy is the sum of the vacancy formation energy ΔEf and the migration energy ΔEm, as shown in Figure 1(b). Figure 1 Schematic diagram of the composition of diffusion activation energy. During high-energy ball milling, the powder undergoes numerous defects (vacancies, dislocations, etc.) due to collisions at high energies; therefore, the solid-state reactions induced by mechanical alloying are actually the result of the combined effect of defect energy and collision energy. Therefore, it no longer requires the formation energy of vacancies, and the total activation energy required for diffusion is reduced, as shown in Figure 1(c). According to Arrhenius’ law, the relationship between the diffusion coefficient D and the activation energy is given by: D = D0e^(-DEa/RT) (1), where D is the diffusion constant ； DEa is the diffusion activation energy, R is the gas constant, and T is the absolute temperature. For the vacancy mechanism, equations (1) D = D0e^(DEa/RT) and (2) D = D0e^(DEm/RT) hold. These equations show that for a given value of D, reducing the activation energy – such as the activation energy for vacancy formation – means that more vacancies will exchange positions with neighboring diffusing atoms. This reduces the diffusion barrier for atoms, increases the vacancy concentration, and thus increases the diffusion coefficient. Therefore, reducing △Ef may lead to a significant decrease in △Em. During high-energy ball milling, reducing the diffusion activation energy is the main way to enhance diffusion; for thermally activated diffusion, crystal defects are quickly eliminated by annealing, and these defects contribute little during the diffusion homogenization annealing process. For high-energy ball milling, the defect density increases as the ball milling time increases ； Thus, it plays a major role in the defects of the diffusion homogenization kinetics process during high-energy ball milling. Based on the above theoretical analysis, it can be concluded that during ball milling at room temperature, although the temperature rise of the powder itself is not high, the formation of a large number of defects (vacancies) enhances the diffusion capability of the elements, allowing processes that would normally only occur at high temperatures to take place at room temperature as well. Some researchers, using differential thermal analysis and X-ray combined methods on Al-Ti-C powder mixtures subjected to different levels of high-energy ball milling, concluded that high-energy ball milling reduces the activation energy for the Al-Ti-C synthesis reaction. Thus, composite materials with better performance can be obtained at lower temperatures. Some researchers have also synthesized nanoscale TiC grains from Ti and C powders at room temperature using high-energy ball milling. Experimental results show that TiC powder can be synthesized in a relatively short time using the mechanical alloying (MA) method. That is, the powder obtained through high-energy ball milling has a refined grain structure, which increases the reaction interface area and thus raises the surface energy. It also keeps the unreacted, fresh interface phases in contact dynamically. Additionally, the local temperature increase during collisions causes changes in certain structural parameters of the TiC powder: the diffusion distance decreases while the defect density increases, thereby promoting diffusion and enhancing the driving force for the solid-state reaction, and thus facilitating self-propagating reaction synthesis at low temperatures. 3 Mechanism of metal phase transformation controlled by activity: The metal phase transformation during mechanical alloying differs from conventional solid-state phase transformations, as it is characterized by its non-equilibrium nature and coerciveness. Phase transformation products are often non-equilibrium phases such as supersaturated solid solutions and amorphous materials; amorphous intermetallic compounds may also be formed. The literature provides a relatively detailed description of the metal phase transformations during mechanical alloying. The theory of metal phase transitions holds that the activity of solute atoms determines the level of chemical potential of the components. The activity can be expressed by the following formula: α=P/P0 (3) Where P and P0 represent the vapor pressures of the solute in the alloy and in its elemental state, respectively; under thermodynamic equilibrium conditions, 0