Mechanical Alloying for Advanced Materials

MECHANICAL ALLOYING FOR ADVANCED MATERIALS C. Suryanarayana Department of Mechanical, Materials, and Aerospace Engineering University of Central Florida, Orlando, FL 32816-2450 and E. Ivanov Tosoh SMD, Inc., 3600 Gantz Road, Grove City, OH 43123-1895 Keywords: Mechanical alloying, Supersaturated solid solutions, Metastable intermetallic phases, Amorphous alloys, Applications

Abstract Mechanical alloying is a powder metallurgy powder processing technique involving cold welding, fracturing, and rewelding of powder particles in a high-energy ball mill. This method is now an established technique to commercially produce oxide-dispersion strengthened (ODS) nickel- and ironbased materials. Additionally, this method can be exploited to synthesize a variety of alloys with novel constitutional effects such as supersaturated solid solutions, metastable quasicrystalline and crystalline intermetallic phases, and amorphous alloys. The greatest advantage of mechanical alloying is that this technique, which is carried out at or near room temperature, is capable of producing alloys in systems, which are difficult or impossible to produce by other techniques. Mechanical alloying is now applied to produce high-temperature intermetallics, ceramic materials, electronic materials, and composites. The present paper presents an overview of the process of mechanical alloying, the types of novel phases that can be obtained by this technique, and the present and potential applications of mechanically alloyed products.

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Introduction It has been recognized for quite some time that processing of materials under non-equilibrium (or far-from-equilibrium) conditions can significantly change their constitution, structure and properties [1]. Amongst many such processes, rapid solidification from the melt [2, 3], spray forming [4], plasma processing, vapor deposition, and mechanical alloying have been receiving the serious attention of researchers. The materials processed by these above methods have improved mechanical and structural characteristics and their performance has been found to be superior to those of others processed by conventional ingot metallurgy methods. Mechanical Alloying (MA) is a dry, high-energy ball milling technique involving cold welding, fracturing, and rewelding of powder particles in a high-energy ball mill. This technique has been used to synthesize both equilibrium and non-equilibrium phases of commercially useful and scientifically interesting materials. It is a simple and versatile technique and at the same time an economically viable process with important technical advantages. One of the greatest advantages of MA is in the synthesis of novel alloys, e.g., alloying of normally immiscible elements that are not possible by any other technique. This is because MA is a completely room- or near-room temperature solid-state processing technique and therefore limitations imposed by phase diagrams do not apply here. The technique is being used extensively to produce advanced materials for different applications. The present article presents an overview of this technology.

The Process of Mechanical Alloying The technique of MA was developed by John S. Benjamin and his co-workers around 1966 at INCO’s Paul D. Merica Research Laboratory [5]. The idea was to develop an alloy combining oxide dispersion strengthening with γ′ precipitation hardening in a nickel-base superalloy intended for gas turbine applications. Since fine oxide particles cannot be dispersed in the liquid state, a solid-state processing technique was necessary. Thus, the birth of MA owes to an industrial necessity and the “science” of the formation of metastable phases and other attributes of MA came much later. This is in contrast to several other non-equilibrium processing techniques, e.g., rapid solidification, which started as an academic curiosity and the industrial applications were realized subsequently. The process of MA consists of loading the powder mix and the grinding medium (generally hardened steel or tungsten carbide balls) in a stainless steel or tungsten carbide container sealed under a protective argon atmosphere (to avoid/minimize oxidation and nitridation during milling) and milling for the desired length of time. About 1-2 wt% of a process control agent (PCA) is normally added to prevent excessive cold welding amongst the powder particles, especially when powders of ductile metals are milled. The process variables include the type of mill, intensity of milling, size and type of milling medium, milling atmosphere, ball-to-powder weight ratio, milling time, milling temperature, and nature and amount of the PCA used. The effects of these variables on the constitution, microstructure, and properties of materials are discussed in some of the reviews on the subject [6-8]. The types of mills generally used are SPEX mills (wherein about 10 grams of the powder can be processed at a time), Attritors (where a large quantity of about a few pounds of powder can be processed at a time), or Fritsch Pulverisette mills (where powder in more than one container can be processed simultaneously). The times required for processing are short in the SPEX mills whereas they are longer in the attritors or Fritsch mills [7].

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Attributes of Mechanical Alloying The formation of an amorphous phase by mechanical grinding of a Y-Co intermetallic compound in 1981 [9], and in the Ni-Nb system by ball milling of blended elemental powders in 1983 [10] brought about the recognition that MA is a potential non-equilibrium processing technique. Since the mid1980s, a number if investigations have been carried out to synthesize a variety of stable and metastable phases including equilibrium and supersaturated solid solutions, crystalline and quasicrystalline intermediate phases, and amorphous or glassy alloys. Nanocrystalline materials (with a grain size of a few nanometers, usually < 100 nm) are also produced by MA of powder mixtures. Additionally, it has been recognized that MA can be used to induce chemical (displacement) reactions in powder mixtures at room temperature or at much lower temperatures than normally required to synthesize pure metals [11]. Thus, pure copper powder was produced by ball milling a mixture of CuO and Ca powders at room temperature. Subsequently, this technique has been used to produce nanocomposites and a variety of alloys and compounds [12, 13]. Because of these special attributes, there has been tremendous research activity in this area of metallurgical research in recent years. A number of stand-alone conferences, such as the annual ISMANAM (International Symposia on Mechanically Alloyed and Nanocrystalline Materials) are organized on this topic in addition to being covered in the general and powder metallurgy conferences, and the RQMM (Rapidly Quenched and Metastable Materials) series. The literature available up to 1994 has been collected together in an annotated bibliography [14]. As mentioned above, MA has been used to produce oxide-dispersion strengthened (ODS) nickeland iron-based superalloys on an industrial scale. Some lightweight aluminum alloys containing dispersions of Al2O3 or Al4C3 and magnesium alloys for supercorroding applications have also been produced on an industrial scale. Mechanically alloyed materials have been used in thermal processing, glass processing, energy production, aerospace, and other industries. These include gas turbine vanes, turbine blades, and sheets for use in corrosive/oxidizing atmospheres.

Novel Alloy Phases Mechanical alloying of blended elemental metal powders has been shown to result in the formation of a variety of novel alloy phases. These include supersaturated solid solutions, metastable quasicrystalline and crystalline intermetallic phases, and amorphous alloys. Supersaturated Solid Solutions Extended solid solubility limits have been achieved in a number of alloy systems by MA. The magnitude of increase is different in different alloy systems. But, a general observation is that the solid solubility is higher if the equilibrium room temperature solid solubility is very low. Rationalization of the solid solubility levels on the basis of milling energy (or efficiency?) has not been done so far. This situation is different from that in RSP studies where the solid solubility could be related to the To temperature, at which the free energies of the solid and liquid phases are equal to each other for a given alloy composition . Table I lists some selected values. It may be noted that the solid solubility extension is quite extensive in some cases. In fact, complete solid solubility of one component in the other has been achieved by MA in AlSb-InSb, Bi2O3-Nb2O5, Bi2O3-Y2O3, Co-Cu, Cu-Ag, and Fe2O3-Cr2O3 systems. There has been some discussion in the literature whether the Hume-Rothery rules that have been so widely employed to predict the maximum solid solubility limits in alloy systems under equilibrium conditions could be applied under the non-equilibrium conditions of MA. It was observed [15] that

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Table Ι: Solid solubility limits (at.%) achieved by MA in some alloy systems. Solvent

Solute

Ag

Cu Ni Fe Mg Mn Ru Ti Ag Fe Hg Ag Cu

Al

Cu

Fe

Equilibrium Value At room temperature Maximum 0.0 14.1 0.0 0.0 0.0 0.025 1.2 18.6 0.4 0.62 0.0 0.008 0.0 0.75 0.0 4.9 0.3 11.0 0.0 5.0 0.0 0.0 7.2 15.0

Extended Value 100 44 4.5 40 18.5 14 50 100 68 17 10 40

favorable size factor (