Mechanical alloying and milling

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Abstract

Mechanical alloying (MA) is a solid-state powder processng technique involving repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Originally developed to produce oxide-dispersion strengthened (ODS) nickel- and iron-base superalloys for applications in the aerospace industry, MA has now been shown to be capable of synthesizing a variety of equilibrium and non-equilibrium alloy phases starting from blended elemental or prealloyed powders. The non-equilibrium phases synthesized include supersaturated solid solutions, metastable crystalline and quasicrystalline phases, nanostructures, and amorphous alloys. Recent advances in these areas and also on disordering of ordered intermetallics and mechanochemical synthesis of materials have been critically reviewed after discussing the process and process variables involved in MA. The often vexing problem of powder contamination has been analyzed and methods have been suggested to avoid/minimize it. The present understanding of the modeling of the MA process has also been discussed. The present and potential applications of MA are described. Wherever possible, comparisons have been made on the product phases obtained by MA with those of rapid solidification processing, another non-equilibrium processing technique.

Introduction

Scientific investigations by materials scientists have been continuously directed towards improving the properties and performance of materials. Significant improvements in mechanical, chemical, and physical properties have been achieved through chemistry modifications and conventional thermal, mechanical, and thermomechanical processing methods. However, the ever-increasing demands for “hotter, stronger, stiffer, and lighter” than traditional materials have led to the design and development of advanced materials. The high-technology industries have given an added stimulus to these efforts.

Advanced materials may be defined as those where first consideration is given to the systematic synthesis and control of the structure of the materials in order to provide a precisely tailored set of properties for demanding applications [1]. It is now well recognized that the structure and constitution of advanced materials can be better controlled by processing them under non-equilibrium (or far-from-equilibrium) conditions [2]. Amongst many such processes, which are in commercial use, rapid solidification from the liquid state [3], [4], mechanical alloying [5], [6], [7], [8], [9], plasma processing [2], [10], and vapor deposition [2], [11] have been receiving serious attention from researchers. The central underlying theme in all these techniques is to synthesize materials in a non-equilibrium state by “energizing and quenching” (Fig. 1). The energization involves bringing the material into a highly non-equilibrium (metastable) state by some external dynamical forcing, e.g., through melting, evaporation, irradiation, application of pressure, or storing of mechanical energy by plastic deformation [12]. Such materials are referred to as “driven materials” by Martin and Bellon [13]. The energization may also usually involve a possible change of state from the solid to liquid or gas. The material is then “quenched” into a configurationally frozen state, which can then be used as a precursor to obtain the desired chemical constitution and/or microstructure by subsequent heat treatment/processing. It has been shown that materials processed this way possess improved physical and mechanical characteristics in comparison with conventional ingot (solidification) processed materials.

The ability of the different processing techniques to synthesize metastable structures can be conveniently evaluated by measuring or estimating the departure from equilibrium, i.e., the maximum energy that can be stored in excess of that of the equilibrium/stable structure. This has been done by different groups for different non-equilibrium processing techniques [12], [14], [15], [16]. While the excess energy is expressed in kJ/mol in Refs. [14], [15], [16], Turnbull [12] expressed this as an “effective quenching rate”. The way the departure is calculated is different in these different calculations and therefore the results do not correspond exactly in all the cases. However, it is clear that vapor deposition and ion implantation techniques have very large departures from equilibrium (or effective quench rates). It is also clear that mechanical alloying is a technique that allows the material to be processed much farther from equilibrium than, e.g., rapid solidification, which has been shown to have a tremendous potential in developing non-equilibrium materials [2], [3], [4]. Table 1 summarizes the departures calculated for the different processing techniques.

This present review article will discuss some of the recent advances that have occurred during the past few years in the synthesis of equilibrium and metastable alloy phases by a simple and inexpensive processing technique — mechanical alloying/milling of metal powders. The outline of the review will be as follows. In Section 2 of this review, we will briefly discuss the historical background that has led to the development of the technique. This will be followed by the nomenclature of the different mechanical alloying methods explored so far (Section 3) and then a description of the process, processing equipment, and process variables in Section 4. The mechanism of mechanical alloying will be discussed in Section 5 and Section 6 briefly describes the different methods of characterizing the mechanically alloyed powders. The temperature rise observed during milling of powders is discussed in Section 7. The synthesis of stable and metastable phases (supersaturated solid solutions and intermediate phases) are discussed in 8 Solid solubility extensions, 9 Synthesis of intermetallics, respectively. Disordering of ordered intermetallics is discussed in Section 10, while the synthesis of amorphous alloys by solid-state amorphization techniques is described in Section 11. Formation of nanostructured materials is considered in Section 12, while reduction of oxides, chlorides, etc. to pure metals and synthesis of nanocomposites by mechanochemical reactions is discussed in Section 13. The ubiquitous problem of powder contamination is discussed in Section 14. Recent developments in understanding the process of mechanical alloying through modeling and milling maps is briefly described in Section 15. The applications of mechanically alloyed products are described in Section 16 and the problem of safety hazards in handling fine powders such as those produced by mechanical alloying are discussed in Section 17. The last Section will present the concluding remarks and possible future research directions in this area.

Section snippets

Historical perspective

Mechanical alloying (MA) is a powder processing technique that allows production of homogeneous materials starting from blended elemental powder mixtures. John Benjamin and his colleagues at the Paul D. Merica Research Laboratory of the International Nickel Company (INCO) developed the process around 1966. The technique was the result of a long search to produce a nickel-base superalloy, for gas turbine applications, that was expected to combine the high-temperature strength of oxide dispersion

Nomenclature

Two different terms are commonly used in the literature to denote the processing of powder particles in high-energy ball mills. Mechanical Alloying (MA) describes the process when mixtures of powders (of different metals or alloys/compounds) are milled together. Material transfer is involved in this process to obtain a homogeneous alloy. On the other hand, milling of uniform (often stoichiometric) composition powders, such as pure metals, intermetallics, or prealloyed powders, where material

The process of mechanical alloying

The actual process of MA starts with mixing of the powders in the right proportion and loading the powder mix into the mill along with the grinding medium (generally steel balls). This mix is then milled for the desired length of time until a steady state is reached when the composition of every powder particle is the same as the proportion of the elements in the starting powder mix. The milled powder is then consolidated into a bulk shape and heat treated to obtain the desired microstructure

Mechanism of alloying

During high-energy milling the powder particles are repeatedly flattened, cold welded, fractured and rewelded. Whenever two steel balls collide, some amount of powder is trapped in between them. Typically, around 1000 particles with an aggregate weight of about 0.2 mg are trapped during each collision (Fig. 9). The force of the impact plastically deforms the powder particles leading to work hardening and fracture. The new surfaces created enable the particles to weld together and this leads to

Characterization of powders

The powders obtained after MA or MM need to be characterized for their size, shape, surface area, phase constitution, and microstructural features. Additionally, one could also characterize the transformation behavior of the mechanically alloyed powders on annealing or other treatments. The measurement of crystallite size and lattice strain in the mechanically alloyed powders is very important since the phase constitution and transformation characteristics appear to be critically dependent on

Temperature rise during milling

The intense mechanical deformation experienced by the powders leads to generation of crystal defects and this plus the balance between cold welding and fracturing operations among the powder particles is expected to affect the structural changes in the powder. Another important parameter, the temperature experienced by the powder during milling, dependent on the kinetic energy of the balls, can also determine the nature of the final powder product. If the temperature generated is high, the

Solid solubility extensions

Solid solubility extensions have been achieved in many alloy systems by non-equilibrium processing methods such as RSP [219] and vapor deposition [2]. Similarly, mechanically alloyed powders also exhibit extension of equilibrium solid solubility limits. In addition to synthesizing stable (equilibrium) solid solutions, it has also been possible to synthesize metastable (non-equilibrium) supersaturated solid solutions by MA starting from blended elemental powders in several binary and higher

Synthesis of intermetallics

The ordered nature of intermetallics leads to attractive elevated temperature properties such as high strength, increased stiffness, and excellent corrosion/oxidation resistance. These attributes are a result of the reduced dislocation motion (since pairs of dislocations — superdislocations — need to move together to retain the ordered nature of the lattice) and low diffusivities. Also associated with the reduced dislocation activity is the low ambient temperature ductility and fracture

Disordering of intermetallics

It has been long known that partially ordered phases are stronger than those wholly disordered or fully ordered (because at a certain value of the long-range order parameter, S, superdislocations separate into unlinked singles). Thus, it is of interest to study the mechanical behavior of materials in various states of partial order. Disordering phenomena of ordered alloys have also been studied to understand the mechanism of disordering and also to produce the disordered material that has a

Solid-state amorphization

Amorphous alloys were first synthesized by vapor deposition in the form of thin films by Buckel and Hilsch [651]. However, the synthesis of a non-crystalline phase by rapid solidification from the liquid state by Pol Duwez and his associates in 1960 [652] paved the way for an enormous amount of activity during the last four decades. These amorphous alloys (also referred to as metallic glasses) have an unusual combination of properties such as high strength, good bend ductility, high fracture

Nanostructured materials

Nanocrystalline materials are single-phase or multi-phase materials, the crystal size of which is of the order of a few (typically 1–100) nanometers in at least one dimension. Because of the extremely small size of the grains, a large fraction of the atoms in these materials is located in the grain boundaries (Fig. 35) and thus the material exhibits enhanced combinations of physical, mechanical, and magnetic properties (compared to material with a more conventional grain size, i.e., >1 μm).

Mechanochemical synthesis

It was first reported in 1989 that MA could be used to induce a wide variety of solid–solid and even liquid–solid chemical reactions [850], [851]. It was demonstrated that CuO could be reduced to pure metal Cu by ball milling CuO at room temperature with a more reactive metal like Ca. Milling together of CuO and ZnO with Ca has resulted in the direct formation of β′-brass [851]. But, it has been reported that such types of chemical reaction were observed as early as 1894 where the conversion of

Powder contamination

A major concern in the processing of metal powders by MA is the nature and amount of impurities that get into the powder and contaminate it. The small size of the powder particles, availability of large surface area, and formation of new fresh surfaces during milling all contribute to the contamination of the powder. Thus, it appears as though powder contamination is an inherent drawback of the technique, unless very special precautions are taken to avoid/minimize it.

As mentioned earlier, in

Modeling studies and milling maps

From the description of the process it is easy to realize that MA is a process involving a number of both independent and interdependent variables. Like in any other process, modeling of MA is also carried out to identify the salient factors affecting the process and establish process control instrumentation. By modeling the process effectively, it is possible to bring down the number of actual experiments to be conducted to optimize the process and achieve a particular application. A highly

Applications of mechanical alloying

The technique of MA has been shown to produce a variety of materials. The most important reason for the invention and development of the MA process was the production of oxide dispersion strengthened (ODS) materials in which fine particles of Y2O3 or ThO2 were uniformly dispersed in a nickel- or iron-based superalloy. In the mid-1980s, it was realized that MA was also capable of producing true alloys from elements that are not either easy to form by conventional means or sometimes even

Safety hazards

The processing of powders in mechanical alloying equipment has special safety hazards due to the fine size of the powders and additional factors. Although handling of powders is well regulated, the safety hazards are accentuated during MA wherein conversion processes occur in temperature–time–stress regimes beyond those of conventional powder metallurgy. These concerns have been addressed by Weber et al. [932].

The safety hazards related to the MA process include heat evolution, reaction rates,

Concluding remarks

Mechanical alloying is a simple, elegant, and useful processing technique that continues to attract the serious attention of researchers. Even though the technique was originally developed to produce ODS superalloys, the synthesis of a variety of alloy phases including solid solutions, quasicrystalline and crystalline intermetallic phases, and amorphous phases has spurred lots of research investigations in recent years. It is estimated that so far about 4000 research/review papers have been

Acknowledgements

The author is grateful to Professor Brian Cantor of Oxford University, UK for the invitation to write this review and to Professor John Moore of the Colorado School of Mines for his generous support and constant encouragement. He also would like to thank the researchers who have provided the figures used in this review.

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