New Book
Mechanical Alloying

By
Dr.Eng. M. Sherif. El-Eskandarany
Mechanical alloying (MA) process using ball-milling and/or rod-milling techniques, has received much attention as a powerful tool for fabrication of several advanced materials, including equilibrium, nonequilibrium (e.g., amorphous, quasicrystals, nanocrystalline, etc.), and composite materials. In addition, it has been employed for reducing some metallic oxides by milling the oxide powders with metallic reducing agents at room temperature. The MA is unique process in that a solid state reaction takes place between the fresh powder surfaces of the reactant materials at room temperature. Consequently, it can be used to produce alloys and compounds that are difficult or impossible to be obtained by the conventional melting and casting techniques.
This book intended primarily to serve as an introduction to the MA process, including general description of the process, starting material requirements, the equipment, characterizations of the milled powders, and consolidation techniques, which used to compact the powder into fully-dense bulk materials.
The book contains several typical examples of selected advanced materials that have been fabricated by MA. This book is aimed at either senior undergraduate/post graduate students or materials scientists/metallurgists.
M. Sherif El-Eskandarany
February 2000
Cairo - Egypt

Chapter One


Fundamentally, the term milling may be referred to breaking down the relatively coarse materials to the ultimate fineness. Apart from the milling of ores, milling is also used for preparing the materials for some industrial applications, such as milling of quartz to fine powder (under 70 mm in diameter), milling of talc to produce body powder, milling of iron ore for preparation of pellets, and many others. Over the past three decades, ball milling has evolved from being a standard technique in mineral dressing and powder metallurgy, used primary for particle size reduction, to its present status as an important method for the preparation of either materials with enhanced physical and mechanical properties or, indeed, new phases, or new engineering materials. Accordingly, the term mechanical alloying (MA) [1.1] is becoming increasingly common in the materials science and metallurgy literatures [1.2].
So far, the MA process, using ball-milling [1.3] and/or rod-milling techniques [4], has received much attention as a powerful tool for fabrication of several advanced materials (Fig. 1.1), including equilibrium, nonequilibrium (e.g., amorphous, quasicrystals, nanocrystalline, etc.), and composite materials [1.5-1.7]. In addition, it has been employed for reducing some metallic oxides by milling the oxide powders with metallic reducing agents at room temperature [1.8-1.10]. In fact, MA is unique process in that a solid state reaction takes place between the fresh powder surfaces of the reactant materials at room temperature. Consequently, it can be used to produce alloys and compounds that are difficult or impossible to be obtained by the conventional melting and casting techniques [1.11]
1.2. HISTORY OF MECHANICAL ALLOYING
The MA process was developed in 1966 at International Nickel Company (INCO) as part of a program to produce a material combining oxide dispersion strengthening with gamma prime precipitation hardening in a nickel-based superalloy intended for gas turbine applications. In fact, the original MA process was the by-product research into different subjects. In the early 1960's, INCO had developed a process for manufacturing graphite aluminum alloys by injection nickel-coated graphite particles into a molten bath by argon sparging. A modification of the same technique was tried to inoculate nickel-based alloys with dispersion of nickel-coated, fine refractory particles. The reason of nickel coating was to render the normally unwetted oxide particles wettable by a nickel-chromium alloy. Early experiments used metal-coated zirconium oxide purchased from an outside vendor. The examinations of these materials revealed no differences between the inoculated materials and uninoculated alloys. Examinations of the inoculants revealed that they were zirconia-coated nickel rather than nickel-coated zirconia. Attention was directed to ball milling as a means of coating oxide particles with nickel. Ball milling had been used to the coating of tungsten carbide with cobalt for well over 70 years [1.12]. Small amounts of nickel-coated thoria and zirconia were successfully produced in a small high-speed shaker mill. This process in particular was used to coat oxides with metals that could not be applied by chemical process due to their reactivity. Since the apparatus employed, a small high-energy ball mill, could produce only 1 cm3 of powder per single milling run, these powders were used only for studies of the rated of rejection of oxide powders from molten alloys. Compacts of the composite powders were partially melted in an arc melter, sectioned and examined metallographically.
In mid-1966, attention was turned to the ball milling process that had been used to make metal powders for the wetting studies as a means of making the alloy itself by powder metallurgy. The reason was attributed to the capability of this process to coat hard phases (e.g. WC or ZrO2) with a soft phase (Co or Ni) [1.13].
In 1970, Benjamin [1.1] has introduced a pioneering development on ball-milling technique for producing complex oxide dispersion-strengthened (ODS) alloys that are used for high temperature structural applications such as jet engine parts. This unique method could be successfully used for preparing fine, uniform dispersions of oxide particles (Al2O3, Y2O3, ThO2) in nickel-base superalloys. It is worth noting that these materials cannot be obtained by the conventional powder metallurgy method.
During the 1970's, the research programs concerned the nature and mechanism of MA process itself and the design of special equipment for carrying out the process. At that time, the MA was well known process for fabrication of several ODS alloys [1.14-1.22].
Apart from the fabrication of ODS alloys by ball milling technique, for their subsequent beneficiation, White [1.23] observed the formation of an amorphous phase by ball-milling elemental Nb and Sn powders at room temperature. In 1983, Koch et al. [1.24] have reported the first novel technique for formation of Ni60Nb40 amorphous alloy by high-energy ball milling of elemental Ni and Nb powders. Since then, MA method has been successfully employed for formation of large number of amorphous alloys. This technique leads to the formation of several alloys that can not be prepared by liquid metallurgy, such as Al-Ta [1.25] and Al-Nb [1.26] binary systems.
An attractive application of the ball milling technique has been demonstrated by El-Eskandarany et al. for preparing nanocrystalline metal nitrides (e.g., Fe4N [1.27] AlTaN [1.28], TiN [1.29], and NbN [1.30]) by milling the elemental powder under nitrogen gas flow. This method which so- called reactive ball milling has been employed for preparing several metal nitrides and hydrides [1.31].
Within the last five years, the ball-milling technique has been proposed for formation of nanocrystalline materials at room temperature [1.32-1.35]. The end-product of the milled powder was consolidated into fully-dense nanocrystalline compacts which have unusual unique physical and mechanical properties [1.36].
In 1998, El-Eskandarany [1.37] prepared homogeneous nanocomposite Al/SiCp materials by milling the elemental powders of Al and b-SiC in a high-energy ball mill. It should be notified that this composite material is difficult to be obtained by the conventional liquid metallurgy method due to the poor wettability between molten Al (or Al alloys) and the reinforcement material of SiC. In addition, the liquid metallurgy method usually leads to an undesirable reaction between SiC and molten Al, producing brittle phases of Al4C3 and Si.
More recently ceramic/ceramic nanocomposite WC-14 % (at.) MgO material that combines two interesting properties of high hardness and fracture toughness values [1.38], has been fabricated by the ball-milling technique [1.39].
1.3. MILLING
As mentioned above, the objectives of milling are being particle size reduction (broken down the minerals until every particle is either fully mineral or fully gangue), mixing and blending and particle shaping. In this book, we focus only on the application of milling (ball milling and rod milling) for fabrication of engineering materials via MA process. Benjamin [1.13] has defined the MA process as a method for producing composite metal powders with a controlled fine microstructure. It occurs by the repeated fracturing and rewelding of a mixture of powder particles in a highly energetic ball mill. As originally carried out, the process requires at least one fairly ductile metal to act as a host or binder [1.17]. Other components can consist of other ductile metals, brittle metals and intermetallic compounds or nonmetals and refractory compounds.
The major process in MA for producing quality powders of alloys and compounds with well-controlled microstructure and morphology, is the repeated welding, fracture, and rewelding the reactant mixed powders. Several types of mills have been employed for such purpose. The MA process can be successfully performed in both high-energy mills (attritor-type ball mill, planetary-type ball mill, centrifugal-type ball mill, and vibratory-type ball mill), and low energy tumbling mills (e.g., ball and rod mills).
1.3.1. FACTORS AFFECTING THE MECHANICAL ALLOYING
The MA process is affected by several factors that are playing very important rules in the fabrication of homogeneous materials [1.40]. It is well known that the properties of the milled powders of the final product, such as the particle size distribution, the degree of disorder or anorphization, and the final stoichiometry, depend on the milling conditions and, as such, the more complete the control and monitoring of the milling conditions, the better end-product is obtained[1.2, 1.40, 1.41]. These factors can be listed as follows:
� Type of mills (e.g. high-energy mills and low energy mills).
� The materials of milling tool (e.g. ceramics, stainless steel, and tungsten carbide).
� Types of milling media (e.g. balls or rods).
� Milling atmosphere (e.g. air, nitrogen and an inert gas).
� Milling environment (e.g. dry milling or wet milling).
� Milling media-to-powder weight ratio.
� Milling temperature.
� Milling time.
A summary of these chief factors that control the mechanical alloying process, is schematic presented in Fig. 1.2.
1.3.1.1. TYPES OF MILLS
1.3.1.1.1. HIGH ENERGY BALL MILLS
1.3.1.1.1.1. ATTRITOR OR ATTRITION BALL MILL
Szigvari introduced this type of mill to industry in 1922, in order to quickly attain fine sulfur dispersion for use in vulcanization of rubber [1.22]. The illustration of this mill that is known also as Szigvari attritor grinding mill is shown in Fig. 1.3. In this mill, the milling procedure takes place by the stirring action of an agitator which has a vertical rotating central shaft with horizontal arms (impellers). The capacity (volume) of the attritor used for MA process is ranging between 3.8 x 10-3 m3 to 3.8 x 10-3 m3. The rotation speed of the central shaft is about 250 rpm (4.2 Hz).
Szigvari introduced this type of mill to industry in 1922, in order to quickly attain fine sulfur dispersion for use in vulcanization of rubber [1.22]. The illustration of this mill that is known also as Szigvari attritor grinding mill is shown in Fig. 1.3.
Kimura and his coworker [1.42] at the National Defense Academy of Japan, developed an attritor ball mill with a higher rotation speed of about 500 rpm. In addition, they have equipped the mill with several devices to control and measure the applied torque during the MA process. They could minimize the oxygen contamination content during the MA experiments by continuous evacuation (using rotary & diffusion pumps) of the vial and introducing continuous flow of an argon gas. In addition, the milling temperature could be controlled the by flushing the outermost shell of the vial with current water. They have proposed this milling tool for synthesizing of several amorphous alloy powders.
1.3.1.1.1.2. PLANETARY BALL MILL
Planetary ball mill is one of the most popular mills that used in the MA researches for synthesizing almost all of the materials, which presented in Fig. 1.1. In this type of mills, the milling media have considerably high energy, because milling stock and balls come off from the inner wall of the vial (milling bowl) and the effective centrifugal force reaches up to twenty times gravitational acceleration.
The centrifugal forces caused by the rotation of the supporting disc and autonomous turning of the vial act on the milling charge (balls and powders). Since the turning directions of the supporting disc and the vial are opposite, the centrifugal forces alternately are synchronized and opposite. Therefore, the milling media and the charged powders alternatively roll on the inner wall of the vial, and are lifted and thrown off across the bowl at high speed (360 rpm), as schematically presented in Fig. 1.4.
One advantage of this type of mills is being the ease of handling the vials (45 ml to 500 ml in volume) inside the glove box (see Fig. 1.5).
1.3.1.1.1.3. VIBRATORY BALL MILL
Vibratory ball mill is an another kind of high-energy ball mill that is used mainly for preparing amorphous alloys. The vial's capacities of the vibratory mills are smaller (about 10 ml in volume) comparing the previous types of mills. In this mill, the charge of the powder and milling tools are agitated in three perpendicular directions (Fig. 1.6.) at very high speed, as high as 1200 rpm.
Another type of the vibratory ball mill, which is used at the Van der Waals-Zeeman Laboratory, consists of a stainless steel vial with a hardened steel bottom, and a single hardened steel ball of 6 cm in diameter (Fig.1.7).The mill is evacuated during the milling down to a pressure of 10-6 Torr in order to avoid reactions with a gas atmosphere [1.44]. Subsequently, this mill is suitable for mechanical alloying some special systems that are highly reacted with the surrounding atmosphere, such as rare earth elements.
1.3.1.1.2. LOW ENERGY TUMBLING MILL
The tumbling mills are defined as cylindrical shape shell, which rotate about a horizontal axis. Loads of balls or rods are charged into the mill to act as milling media. The powder particles of the reactant materials meet the abrasive and/or impacting force which reduce the particle size and enhance the solid state reaction between the elemental powders.
1.3.1.1.2.1. TUMBLER BALL MILL
The tumbler ball mills date back to 1876 [1.45] and are characterized by the use of balls (made of iron, steel or tungsten carbide) as milling media. The capacities of these mills are governed by several variables (ratio of mill length to diameter, speed of mill, size of balls, particle size, etc.) that should be adjusted and balanced.
In these mills, the useful kinetic energy can be applied to the powder particles of the reactant materials (see Fig. 1.8.) [1.46]; by
� Collision between the balls and the powders.
� Pressure loading of powders pinned between milling media or between the milling media and the liner.
� Impact of falling the milling media.
� Shear and abrasion caused due to dragging of particles between moving milling media.
� Shock wave transmitted through crop load by falling milling media
The tumbler ball mills have been successfully used for preparing several kinds of mechanically alloyed powders [see for example Ref. 1.41]. However, this kind of low-energy mill may lead to increase the required milling time for a complete MA process, it produces homogeneous and uniform powders [1.47]. In addition, it is cheaper than those of the high-energy mills and can be self-made with lower costs. Moreover, the tumbling mills are operated simply with low maintenance requirements.
1.3.1.1.2.2. TUMBLER ROD MILL
Our current knowledge of the materials which are fabricated by MA has shown that almost all ball-milled alloy powers are contaminated with iron, when stainless steel balls and vial are used; this is a natural consequence of the collision between milling media. Therefore, MA method is faced with this serious problem which has impeded progress. Since the ball-powder-ball collision in a tumbling, planetary or vibrating mills can be considered as the main source of iron contamination, different kinds of mills in which there is no collision between the milling media should be used.
In 1990, El-Eskandarany et al. [4] have employed a laboratory scale rod-mill for preparing large amount (30 g) of homogeneous amorphous Al30Ta70 powder. In their experiments, they made a stainless steel (SUS 304) cylindrical shell and used 10 stainless steel (SUS 304) rods as milling media. In order to prevent jamming of the rods inside the shell, the shell has been designed so that its length (250 mm) is greater than its diameter (120 mm) and the rods have been cut to lengths (200 mm) less than the full length of the shell. The movement of the rods inside the shell was directly observed through a thick and transparent plastic plate sealing the window of the shell. This observation has shown that the milling occurs by the line contact of rod-powder-rod extending over the full length of the shell. The results have shown that a single phase of amorphous AlxTM100-x (TM; Ti, Zr, Hf, Nb and Ta) powders with low iron contamination content can be formed via rod-milling technique [1.48-1.50]. They have reported that rod-milling technique leads to the formation of high thermal stable, homogeneous and low iron-contaminated amorphous alloys.
Figure 1.9 shows the concentration content of iron concentration in mechanically alloyed Al30Ta70 as a function of (a) rod-milling and (b) ball-milling time, as well as the effect of repeating the MA process. At the beginning of the first milling run the milling media were used in the absence of mechanically alloyed powder coatings. After this run the milling media which had been coated by the powders were used again in the second and the third milling runs. A drastic decrease in iron content from the first to the third milling runs were observed. It is also shown that the amount of iron contamination in the rod-milled powders is lower than in the ball-milled powders.
In the ball-milling (BM) process the starting elemental powders usually agglomerate at the early stage of milling to form powder particles of grater diameters, as large as several hundred microns, and this is followed by continuous disintegration until the particle size is less than a few microns. As shown in Fig. 1.10, the rod-milling (RM) leads to a similar behavior for the variation in powder diameters. In RM, however, the average diameter of the agglomerate powders is very small and the subsequent disintegration into fine powders proceeds at a high rate to provide a narrow size distribution.1.3.1.2. EFFECT OF BALL-TO- POWDER WEIGHT RATIO
The effect of the ball-to-powder weight ratio (Wb:Wp) on the amorphization reaction of Al50Ta50 alloy powders in a low energy ball mill, was studied in 1991 by El-Eskandarany et al. [1.51]. They have used 90 g, 30 g, 20 g, 10 g and 3 g of powders to obtain Wb:Wp ratios of 12:1. 36:1, 54:1, 108:1 and 324:1, respectively.
The x-ray diffraction patterns (XRD) of mechanically alloyed Al50Ta50 powders as-ball milled for 1440 ks (400h) as a function of the Wb:Wp ratio. Single phase of amorphous alloys are obtained when ratios 36:1 and 108:1 were used. The Bragg peaks of elemental Al and Ta crystals still appear when the Wb:Wp ratio is 12:1, indicating that the amorphization reaction is not completed. In contrast, when the Wb:Wp ratio is 324;1, the amorphous phase coexists with the crystalline phases of AlTa, AlTa2 and AlTaFe.
Based on their results [1.51], it is concluded the rate of amorphization depends strongly on the kinetic energy of the ball mill charge and this depends on the number of opportunities for the powder particles to be reacted and interdiffused. Increasing the Wb:Wp ratio accelerates the rate of amorphization which is explained by the increase in the kinetic energy of the ball mill charge per unit mass of powders. It has been shown in that study that the volume fraction of the amorphous phase in the mechanically alloyed ball milled powders increases during the early stage of milling, 86-173 ks (24-48 h) with increasing Wb:Wp ratio. It is noted that further increasing this weight ration leads to the formation of crystalline phases and this might be related to the high kinetic energy of the ball mill charge which is transformed into heat. When the Wb:Wp ratio was reduced to 12:1, however, the amorphization reaction was not completed. This indicates that the kinetic energy of the mill charge is insufficient for complete transition from the crystalline to the amorphous phase.
It is worth noting that the powder particles reached the minimum of extreme fineness when using a high Wb:Wp ratio. One disadvantage of using such a high weight ration is being the high concentration of iron contamination which is introduced to the milled powders during the MA process, as presented in Fig.1.12
1.3.1.3. EFFECT OF MILLING ATMOSPHERE
The atmosphere of the mill is considered as a one of the most important factors during ball and/or rod milling the elemental powders. It has been shown [1.27] that the very fine powders have relatively large surface areas and thus are highly reactive not just with oxygen but also with other gases, such as hydrogen or nitrogen [2]. This so-called reactive ball milling [1.29] will be discussed in another chapter of this book.
It has bee suggested by Yavari et al. [1.51] that the amorphization reaction which takes place in a ball mill between immiscible A-B binary couples with DHmix<0 is attribute to the presence of some oxygen (~5 at. %) and the ternary mixtures are found to be miscible with DHmix<0.
1.4. MECHANISM OF MECHANICAL ALLOYING
As previously mentioned, the main process which takes place in a mill during the MA method to produce quality powders with controlled microstructure is the repeated welding, fracturing and rewelding of a mixture of powders of the diffusion couples. It is critical to establish a balance between fracturing and cold welding in order to mechanically alloy successfully. Two techniques are proposed by Gilman and Benjamin [1.22] to reduce cold welding and promote fracturing. The first technique is to modify the surface of the deforming particles by addition of a suitable processing control agent (PCA) (wet milling) that impedes the clean metal-to-metal contact necessary for cold welding. The second technique is to modify the deformation mode of the powder particles so that they fracture before they are able to deform to the large compressive strains necessary for flatting and cold welding. Cooling the mill chamber is an approach to accelerate the fracture and establishment of steady-state processing (effect of milling temperature) [1.23].
We should emphasis that milling the powders of certain metals which cold-weld easily (e.g. Ti, Zr, Al, Pb, Zn, Ag, etc.) with an organic agent (PCA) [1.53], may lead to a undesired reaction between the PCA and the milled powders, specially those pure metals of the 4f and 5f elements.
1.4.1. BALL-POWDER-BALL COLLISION
The starting material powders that are mechanically alloyed can be two (or more) metallic powders, powders of intermetallic compound(s) or dispersoid powders. The MA process starts by blending the two (or more) individual powder constituents in order to obtain the final or so called end-product after certain hours of milling (dry or wet). The morphology of the powders is modified when they are subjected to ball collisions (see Fig.1.13). It is worth noting that the effects of collisions on the milled powders depend on the type of the constituent particles.
It is has been shown that the initial ball-powder-ball collision causes the ductile metal powders to flatten and work harden. When these metallic powders are cold welded and heavily mechanically deformed. They therefore, flattened overlap, atomically clean metal interfaces are brought into intimate contact, forming layered structure composite particles consisting of various combinations of the starting ingredients, as schematically shown in Fig.1.13. Further milling results in cold welding and deformation of the layered particles and a refined microstructure is obtained. Due to the initially low hardness of the starting elemental powders, the lamellar spacing of the agglomerated particles fast reduced upon further milling. Increasing the MA time leads to increase the hardness and this leads to fracturing of the agglomerated powders in smaller particles. Further milling time leads to an interdiffusion reaction that takes place at the clean or fresh surfaces of the intimate layers in the powder particles to form an alloy.
1.5. NECESSITY OF MECHANICAL ALLOYING
Mechanical alloying is a unique process for formation of several alloys and compounds that are difficult or impossible to be produced by the conventional melting and casting technique. For example, Al-Ta binary system (see Fig.1.14) shows a remarkable gap difference between the melting points of Al (933 K) and Ta (3293 K). This gap difference restricts the production of such promising advanced materials that are used as capacitors in the industrial applications. The MA method leads to the fabrication of such new amorphous material with wide range of formation [1.11].
The MA has been also used as a promising method for fabrication of many nanocrystalline materials (Chapter 4), specially the refractory materials of metals carbides (Chapter 5) and metal nitrides (Chapter 6), using very simple technique.
Furthermore, it can be used also for fabrication of many nanocomposite materials (Chapter 7) at room temperature.
More recently, the MA method has been used for reducing several metal oxides (Chapter 8) at room temperature.
REFERENCES OF CHAPTER ON
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1.39. M. Sherif El-Eskandarany, M. Omori, and T. J. Konno, K. Sumiyama, T. Hirai and K. Suzuki, Met. Trans. A (2000) in press.
1.40. S.J. Campbell and W.A. Kaczmarek, "M?ssbauer Spectroscopy Appl
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