Mechanical Alloying

Mechanical alloying (MA) was originally developed for the manufacture of nickel-base superalloys strengthened by both an oxide dispersion and T precipitate. Now in its third decade of advancement since the first week of Benjamin (Ref 15), MA provides a means for producing a full range of complex dispersion-strengthened nickel-, cobalt-, iron-, titanium-, and aluminum-base powder systems.

Mechanical alloying is a dry, high-energy ball-milling process for producing composite metallic powders with a controlled, fine microstructure. It is carried out in a highly agitated ball charge by repeated cold welding and fracturing (Fig. 13) of a mixture of metal powders to which some nonmetal powders may be added. Its widest use has been in the production of dispersion-strengthened nickel- and iron-base superalloys for service at temperatures of 1000 °C (1830 °F) and above.

Fig. 13 Ball-powder-ball collision of powder mixture during mechanical alloying. (a) Cold welding. (b) Powder fracture. Courtesy of International Nickel Co., Inc.

Unlike mechanical mixing processes, mechanical alloying produces a material whose internal homogeneity is independent of starting powder particle size. Thus, ultrafine dispersions (less than 1 /'m interparticle spacing) can be obtained with relatively coarse initial powder (50 to 100 /'m average diameter).

Equipment. The machinery used for mechanical alloying consists of one of several types of high-energy ball mills. These are selected on the basis of given processing times, ranging from hours to tens of hours. The types of ball mills employed include shaker mills, vibratory mills, stirred ball mills, centrifugal ball mills, and conventional ball mills with diameters greater than about 1 m. The restriction on conventional ball mills arises from the relatively low-energy density of operation of smaller mills, which leads to excessive processing times.

Unlike the procedure used in ball milling for comminution, the ratio of balls to powder in mechanical alloying is relatively high. These ratios range from 6:1 by weight to as high as 30:1, but most commonly are in the range of 10:1 to 20:1. The balls themselves range from 4 to 20 mm (0.16 to 0.8 in.) in diameter, but are usually 8 to 10 mm (0.32 to 0.4 in.) in diameter and are made of a through-hardened steel, such as 52100. The environment within the grinding machine is controlled wherever practical; water cooling and atmosphere control are employed. The milling atmosphere consists of either nitrogen or argon with measured trace amounts of oxygen. Liquids can also be used.

Production of Oxide-Dispersion-Strengthened (ODS) Superalloy Powders. The raw materials used for mechanically alloyed dispersion-strengthened superalloys are widely available commercially pure powders that have particle sizes that vary from about 1 to 200 /'m. These powders fall into the broad categories of pure metals, master alloys, and refractory compounds. The pure metals include nickel, chromium, iron, cobalt, tungsten, molybdenum, and niobium. The master alloys include nickel-base alloys with relatively large amounts of combinations of aluminum, titanium, zirconium, or hafnium.

These master alloys are relatively brittle when cast and easily comminuted to powder. In addition, because they consist of relatively exothermic intermetallic compounds, the thermodynamic activity of the reactive alloying elements, such as aluminum and titanium, is considerably reduced compared to that of the pure metals.

A typical powder mixture may consist of fine (4 to 7 Z'm) nickel powder, -150 /,!m chromium, and -150 /' m master alloy. The master alloy may contain a wide range of elements selected for their roles as alloying constituents or for gettering of contaminants. About 2 vol% of very fine yttria, Y203 (25 nm, or 250 a) is added to form the dispersoid. The yttria becomes entrapped along the weld interfaces between fragments in the composite metal powders. After completion of the powder milling, a uniform interparticle spacing of about 0.5 /'m (20 /'in.) is achieved.

The oxygen contents of the commercially pure metal powders and the master alloys range from 0.05 to 0.2 wt%. The refractory compounds that can be added include carbides, nitrides, and oxides. For the production of dispersion-strengthened materials, such additions are limited to very stable oxides, such as yttria, alumina, or less frequently thoria. These oxides, which are prepared by calcination of oxalate precipitates, consist of crystallites of about 50 nm agglomerated into pseudomorphs of about 1 /'m.

The only restriction on the mixture of powder particles for mechanical alloying (other than the particle size range mentioned above and the need to minimize excessive oxygen) is that at least 15 vol% of the mix should consist of a compressibly deformable metal powder. The function of this component, which can consist of any one or all of the pure metals, is to act as a host or binder for the other constituents during the process.

Other similar metals, such as copper, zinc, and magnesium, are suitably ductile but not normally added to superalloys. For dispersion-strengthened superalloys, the amount of refractory oxide added ranges from about 0.4 to 1.5 wt% (1 to 2.7 vol% for yttria).

Powder Characteristics. A uniform distribution of submicron refractory oxide particles must be developed in a highly alloyed matrix for the production of ODS alloys. A given sample of mechanically alloyed superalloy powder may contain particles ranging from 10 to 500 /'m. with an average particle size between 50 and 200 /,!m. The internal structure of the powder is independent of particle size once the steady state is achieved.

Because of the severe plastic deformation that occurs during mechanical alloying, very high hardnesses are achieved in the powders. Hardness increases almost linearly during the initial stages of the process (Fig. 14), reaching a saturation value, after which time it is presumed that work softening balances further cold work.

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