Mechanical Alloying and the Milling

¶ … Mechanical Alloying and the Milling Process Facilitate the Production of Supersaturated Solids and Solutions of Two or More Elements

Today, nanomaterials hold an enormous amount of promise in delivering innovations in construction, healthcare and manufacturing process of all types, but in order to yield these positive outcomes, they must first be produced in a cost-efficient fashion. Although there have been several thousand nanomaterial-related patents issued in the United States in the past few years, the vast majority of these products remain outside the commercial realm. To date, mechanical alloying techniques have been used to produce supersaturated solid and solutions of two or more elements. This paper provides a review of the relevant literature to determine how the milling process facilitates this as well as a description of the various applications of such non-equilibrium materials. A summary of the research and salient findings are presented in the conclusion.

Review and Discussion

According to Nickols-Richardson (2008), nanotechnology involves the practical application of scientific knowledge of matter that varies in size from 1 to 100 nanometers (nm), with one nm equaling one-billionth of a meter. In order to appreciate the diminutive sizes involved, the diameter of deoxyribonucleic acid (DNA) is just 2.0 nm; likewise, one human eyelash is just 800,000 nm in size [1]. Generally speaking, nanotechnologists manipulate atoms and molecules on this exceedingly tiny scale [1]. The peculiar behaviors of materials at the nanoscale hold the potential for the manufacture of inexpensive rare molecules, the production of light and microfibers that are stronger than anything available currently, as well as the production of ultrasensitive detectors that can be used in a wide range of industrial settings [2, 3]. In many ways, though, nanotechnology largely remains in a developmental phase and just a few actual nanotechnological inventions have been developed that have reached the commercial marketing stage [4], although a growing number of consumer products contain nanomaterials [5] and there are growing concerns about the adverse impact that these manufactured nanosubstances may have on the environment [6, 7].

In spite of these constraints and concerns, the expectations involving nanotechnology, though, are enormous and span the spectrum from free energy to plentiful materials that will be one of the primary drivers of economic growth in the future [8, 9]. For instance, current estimates indicate that more than 3,700 nanotechnology patents have already been issued in the United States, which is an impressive number given that the supporting technology has produced few actual industrial products to date [4]. There are some success stories, though. For instance, in recent years, companies such as M.B.N. Srl have developed proprietary methods for producing mechanomade powders through the use of high-energy milling processes. These processes are extremely flexible and provide the ability to produce nonphased materials through a combination of reaction milling, mechanical alloying and high-energy mixing. In this regard, mechomade materials include high-speed steels, copper and intermetallic alloys, as well as ceramics, metal mix nanocomposites and metal flakes [10]. Moreover, nanomaterials are appearing in an increasing number of consumer products [11], currently estimated at more than eight hundred such products [5].

Mechanical alloying is a highly flexible technique that has been used successfully to create a wide range of commercially useful and materials that are of scientific interest, including intermetallics and amorphous, nanocrystalline, and nanocomposite materials [12]. Moreover, mechanical milling or mechanical alloying methods have also been employed successfully to refine the grain size and to synthesize non-equilibrium structures [13, 14]. At its most fundamental level, mechanical alloying is the process whereby materials are forced to mix on a nanoscale in ways that increase their interfacial energy and thereby raise their internal energy to a point where they become energetically predisposed to mixing [15]. The studies to date have confirmed that mechanical alloying represents an efficient method for synthesizing nanostructured and non-equilibrium titanium aluminides, for example, and while fine-grained structures have been successfully created, the point is made throughout the literature that more research is required in order to reduce contamination and to consolidate nanostructural powders [13].

The processes involved in the creation and manipulation of nanomaterials include mechanical attrition. This process uses a solid-state powder processing method that employs welding fracturing and rewelding of powder particles in a repetitive fashion using a high-energy ball-mill for the mechanical impacting function [15]. The mechanical attrition process produces reductions in particle size and operates by making non-equilibrium materials more amenable to mixing with other substances, and ball-milled mixtures of two or more such substances can produce solid solutions that are beyond their respective equilibrium solubility limits [15].

Despite being developed about four decades ago to provide oxide-dispersion strengthened nickel and iron-base superalloys for use in the aerospace industry with the goal of producing superior strength and resistance to corrosion in ways that are not possible using conventional casting methods, there remains a lack of understanding concerning how these processes operate at the nanoscale level [15]. Researchers posit that they are related to the significant plastic deformation and nanoscale mixing that mechanical alloying provides [15].

Notwithstanding the lack of a complete understanding concerning how these processes operate at the nanoscale, mechanical alloying has been demonstrated to be capable of synthesizing a wide range of equilibrium and non-equilibrium alloy phases beginning with blended elemental or prealloyed powders [16]. The non-equilibrium phases that are capable of being synthesized using the mechanical alloying processes include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys [16]. The research that has been conducted in recent years has provided the scientific community with an improved understanding in these areas, though, as well as the disordering of ordered intermetallics and mechanochemical synthesis of materials involved in mechanical alloying processes [16].

As to the operation of the mechanical milling process, it is the actual mechanical force of the impact that takes place during the milling process itself that creates deformation in plastic and the hardening of the powder particles through repeated flattening, cold-welding, fracturing and rewelding. The majority of the mechanical force that is applied during the mechanical milling process is transformed into heat; however, what is left over is communicated to the milled material in ways that increase its internal energy, thereby making it more amenable to mixing with other substances [15]. Reducing the size of already incredibly small powder particles may require a number of repetitions of the mechanical milling process in order to achieve powder particle sizes of the desired diameters which is typically achieved within a few minutes. Ductile composite materials tend to lump together in larger particles during the initial stages of mechanical milling, and these composites are characterized by a layered structure that is comprised of various permutations of the constituent materials. As the mechanical milling process continues, these larger particles tend to fracture and fragment in ways that predominate over cold welding [15]. Ultimately, though, the size of the particles achieves a steady state in which the fracturing and cold welding rates are comparable because smaller particles tend to deform without fracturing and to weld together. Taken together, the mechanical milling process provides a mechanism whereby any particle size distribution is reduced to an extremely fine state by welding and larger particles are forced into an intermediate size through fracturing [15].

The manner in which narrow particle size distribution is created as a result of the tendency of small particles to weld together and large particles to fracture under steady-state conditions is illustrated in Figure 1 below.

Figure 1. Narrow particle size distribution caused by tendency of small particles to weld together and large particles to fracture under steady-state conditions.

Source: Mechanical Alloying and Mechanical Milling

In tandem with the reduction in particle size, the mechanical milling process also produces defects into the crystal particles that ultimately alter their internal microstructure in significant ways. The mechanical impact involved in the mechanical milling process results in plastic deformation that initially occurs via the glide of dislocations in ways that are not completely understood [17]. What is known is that within the particles, there is a congregation of dislocations of high density, and the grain structure that results has been shown to develop in a three-step fashion as follows:

1. The dislocations congregate in general shear bands within the particles; these types of bands are evident in cold rolling metals and are congruent with the high strain rate that is used; the shear bands that are produced are increasingly communicated throughout the particles.

2. By the time a given strain level is achieved within the shear bands, the dislocations that are produced disintegrate and are reformulated as small-angle grain boundaries in which individual grains are separated within the particle. The sub-grains that are produced through this process have already achieved a size that is in the nanometer range (generally 10 -- 20 nm); however, this penultimate phase of the process is restricted by the size of the sub-grains. This restriction is due to the fact that when crystals are reduced to the nanosize range, their yield stress for plastic deformation is substantially increased. Although the precise mechanics involved remain unclear, it is likely that during this phase, the high defect boundaries serve as attractors for any imperfections that already existed in the crystal lattice and that the nanocrystals which are, by contrast, relatively defect -- free, become increasingly refined and near crystalline perfection.

3. The final step in the process involves random reorientations of the single-crystalline grains with regards to their neighboring grains. At the point where the grain structure achieves its limiting size (this size limit relates to the particles' crystal symmetry and the energy and amount of mechanical milling employed), the material become amenable to plastic deformation through grain boundary sliding. In fact, this type of deformation mechanism has been discerned in superplasticity in which a high diffusion rate stage is capable of accommodating such forces at any strain rate. Researchers have posited that in the case of the nanocrystalline, the high defect-density crystal interfaces are responsible for producing the rapid diffusion paths to provide the means by which the self-organization and rotation of the grains is achieved, thereby increasing the ability of the grain boundaries to store energy based on the reorientation of the grains with respect to their neighboring particles and the boundary's excess volume. The research to date suggests that to the extent that this reorientation process is allowed to continue is the extent to which it eventually releases some of the strain as the grains relax during the reorientation stage [15].