As a rule of thumb, it can be estimated that a process that takes only a few minutes in the SPEX mill may take hours in an attritor and a few days in a commercial low-energy mill even though the actual details can be different depending on the efficiency of the different mills and the powder characteristics. Full details of the process including the effect of different variables on the alloying behavior may be found in two recent references [ 16 , 17 ]. The effects of a single collision on each type of constituent powder particle are shown in Figure 2 a.
The initial impact of the grinding ball causes the ductile metal powders to flatten and work harden. The severe plastic deformation increases the surface-to-volume ratio of the particles and ruptures the surface films of adsorbed contaminants. The brittle intermetallic powder particles get fractured and are refined in size.
The oxide dispersoid particles are comminuted more severely. Whenever two grinding balls collide, a small amount of the powder being milled is trapped in between them. Typically, around particles with an aggregate weight of about 0. During this process, the powder morphology can be modified in two different ways, depending on whether one is dealing with ductile—ductile, ductile—brittle, or brittle—brittle powder combinations.
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If the starting powders are soft metal particles, the flattened layers overlap and form cold welds. This leads to formation of layered composite powder particles consisting of various combinations of the starting ingredients. The more brittle constituents tend to become occluded by the ductile constituents and trapped in the composite. The work-hardened elemental or composite powder particles may fracture at the same time.
These competing events of cold welding with plastic deformation and agglomeration and fracturing with size reduction continue repeatedly throughout the milling period. Eventually, at the steady-state condition, a refined and homogenized microstructure is obtained and the composition of the powder particles is the same as the proportion of the starting constituent powders.
A thin layer of the coating is beneficial in preventing wear and tear of the grinding medium and also in preventing contamination of the milled powder with the debris. But, a too thick layer will result in lower yield and also possible compositional inhomogeneity of the powder, which should be avoided.
The generally accepted explanation for alloying to occur from blended elemental powders and formation of different types of phases is that a very fine and intimate mixture of the components often lamellar if the constituent elements are sufficiently ductile is formed after milling, if not the final product. Alloy formation is facilitated through crystalline defects grain boundaries, dislocations, stacking faults, vacancies, and others introduced into the material due to the intense cold working operation, which act as fast diffusion paths, and a slight rise in the powder temperature during milling, as a result of frictional forces and impact of the grinding balls against other balls and the surfaces of the container.
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If the final desired phase had not been formed directly by MA, then a short annealing treatment at an elevated temperature was found to promote diffusion and consequently alloy formation. Accordingly, it was shown that, by a proper choice of the process parameters during MA and by choosing an appropriate alloy composition, it is possible to produce a variety of alloys starting from metal powders. A significant attribute of MA has been the ability to alloy metals even with positive heats of mixing that are difficult or impossible to alloy otherwise [ 18 , 19 ].
The actual type of phase formed could be different depending on the alloy system, its composition, and the milling conditions employed.
High-energy grinding of FeMo powders
A number of metastable alloys have been produced this way. Microstructural Changes : As a result of MA, the processed materials could develop ultrafine-grained and nanostructured phases. In addition, uniform dispersion of a large volume fraction of very fine oxide or other ceramic particles can be achieved in different matrices which is either impossible or difficult by other processes. These novel nanocomposites can exhibit properties far superior to those that have conventional grain sizes and sometimes even completely new behavior [ 16 , 17 , 20 ].
In many cases, chemical reactions have also been found to occur leading to the production of pure metals from their ores, synthesis of alloys, and formation of novel materials and microstructures. Such a process has been referred to as mechanochemical processing [ 21 ]. All these effects have been well documented in the literature, and Table 1 summarizes the important attributes of MA.
Before discussing the synthesis of advanced materials and their applications, let us first describe the constitutional changes that can take place in MA materials. The literature on MA has several examples of the different equilibrium and nonequilibrium phases produced in different alloy systems.
An exhaustive list of the results up to date at the time of compilation is available in [ 16 , 17 ], and the list is constantly growing. We will, however, briefly highlight here some of the more recent and salient features in the following paragraphs. Solid solution alloys exhibit high strength, changes in density, and suppression or elimination of undesirable second phases. Increasing the strength of alloys through precipitation of a second phase from a supersaturated solid solution during aging precipitation hardening has been known to materials scientists for over a hundred years.
Further, since the strengthening effect is related to the size and volume fraction of the second phase particles and since both of them could be controlled by the extent of supersaturation and the aging parameters, achievement of extended solid solubility limits in alloy systems is very desirable. Solid solutions—both equilibrium and supersaturated—are still being produced [ 22 — 24 ]. Rules similar to those developed by Hume-Rothery for solid solution formation in binary noble-metal alloy systems under equilibrium conditions also seem to be generally applicable to supersaturated solutions obtained by MA, even though exceptions have been noted in some cases [ 25 , 26 ].
As will be described later, the mechanically alloyed materials exhibit very fine grain sizes, often reaching nanometer levels. Since these nanocrystalline materials contain a large volume fraction of atoms in the grain boundaries, the diffusivity of solute atoms is significantly enhanced [ 27 ], and this would lead to increases in the solid solubility levels.
As a general rule of thumb, solute elements which exhibit limited solubility under equilibrium conditions exhibit high solubility levels under the nonequilibrium conditions of MA. Even though solid solubility extensions have been achieved by other nonequilibrium processing techniques as well e. An important observation made in MA materials is that the solute atoms segregate to grain boundaries from where they could diffuse into grains and form solid solutions. Intermetallic phases or intermetallics, for short are typically ordered compounds existing at simple ratios of the two or more elements involved.
However, they usually suffer from low ambient temperature ductility, again due to the ordered nature of the compounds. A new family of intermetallics showing high ductility at room temperature, and based on rare-earth elements, has been recently developed [ 28 , 29 ]. Since MA is known to a disorder the ordered lattices, b refine grain sizes down to nanometer levels, and c also alter the crystal structures, all these effects could induce some amount of plasticity into the material.
In fact, the potential to ductilize the normally brittle intermetallics has been the driver for accelerated research into the MA of intermetallics.
A number of intermetallics both equilibrium and metastable and high-temperature or high-pressure phases have been synthesized by MA starting from elemental powder blends [ 16 , 17 , 24 , 30 ]. More interestingly, quasicrystalline phases with the forbidden crystal symmetries, first reported in rapidly solidified Al-Mn alloys, have also been synthesized by MA in a number of alloy systems [ 5 , 31 ].
Applications for these exotic materials have been limited.
Based on their high hardness, low friction, and low surface reactivity, quasicrystalline materials have been used to coat nonstick frying pans. Also, steels reinforced with small quasicrystalline particles are very strong and have been used for acupuncture and surgery, dental instruments, and razor blades [ 32 ]. The most significant change that occurs in MA materials is the formation of amorphous phases and this will be discussed in the next section. Amorphous alloys are materials that lack crystallinity and have a random arrangement of atoms.
Because of this extreme disorder, these materials exhibit a very interesting combination of high strength, good corrosion resistance, and useful electrical and magnetic properties. These novel materials have been traditionally produced starting from the vapor or liquid states. Amorphous or noncrystalline alloys based on metals are commonly referred to as metallic glasses. At present, there is worldwide activity in the area of metallic glasses and bulk metallic glasses [ 4 , 33 ].
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Subsequent developments include the synthesis of bulk metallic glasses with a large section thickness of a few centimeters. The current record is an mm diameter glassy rod produced by solidification in a Pd While solidification methods produce the metallic glasses either in thin ribbon or bulk form directly from the liquid, the amorphous phases are in powder form by MA methods. These need to be consolidated to bulk shape for subsequent characterization and applications.
Amorphous alloys were synthesized by MA first in an Y-Co intermetallic compound [ 36 ] and later in a blended elemental Ni-Nb powder mixture [ 37 ]. Amorphous phases have now been synthesized in a very large number of alloy systems starting from either blended elemental powder mixtures, intermetallic compounds, or mixtures of them [ 38 ].
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Comparisons have been frequently made between the amorphous alloys produced by MA and RSP techniques, and both similarities and differences have been noted [ 26 ]. Further, the mechanism of glass formation appears to be significantly different depending upon whether it is produced by MA or RSP methods. Several different criteria have been proposed to explain the glass-forming ability of alloys produced by solidification methods.
When the strain energy exceeds a critical value, determined by the size and amount of the solute atoms, it destabilizes the crystal lattice and results in the formation of an amorphous phase. With the introduction of bulk metallic glasses nearly thirty years ago, a very large number of new criteria, mostly based on the glass transformation temperatures , , and , where represents the crystallization temperature of the amorphous alloy , have been proposed.
In spite of a very large number of criteria proposed [ 4 ], it has not been possible yet to accurately predict the glass-forming ability of alloys [ 39 ]. There are some significant differences in the formation of metallic glasses by the MA and RSP methods. For example, metallic glasses are obtained by MA in almost every alloy system provided that sufficient energy has been stored in the powder.
But, metallic glasses are obtained by RSP methods only in certain composition ranges—about 20 at. Additionally, glass formation by MA occurs by the accumulation of crystal defects, which raises the free energy of the crystalline phase above that of the hypothetical glassy phase, under which conditions the crystalline phase becomes destabilized. On the other hand, for glass formation by the RSP methods, the critical cooling rate for glass formation needs to be exceeded so that formation of the crystalline nuclei is completely suppressed. Since MA is a completely solid-state powder processing technique, the criteria for amorphous phase formation should be different from those of solidification methods.
A new criterion was proposed; that is, the number of intermetallics present in the constituent phase diagrams determines the glass-forming ability of alloys processed by MA—the more the number of intermetallics in the constituent phase diagrams, the easier it is to amorphize the powder blend [ 40 ]. Amorphization by MA occurs when the free energy of the hypothetical amorphous phase, , is lower than that of the crystalline phase, G C ; i. A crystalline phase normally has a lower free energy than the amorphous phase.
But, its free energy can be increased by introducing a variety of crystal defects such as dislocations, grain boundaries, and stacking faults.