References

1. J.S. Benjamin, Sci. Amer, Vol 234 (No. 5), 1976, p 40-48

2. J.S. Benjamin, Met. Powder Rep., Vol 45, 1990, p 122-127

3. A.E. Yermakov, Y.Y. Yurchikov, and V.A. Barinov, Phys. Met. Metallogr., Vol 52 (No. 6), 1981, p 50-58

4. C.C. Koch, O.B. Cavin, C.G. McKamey, and J.O. Scarbrough, Appl. Phys. Lett., Vol 43, 1983, p 1017-1019

5. G. Heinicke, Tribochemistry, Akademie Verlag, Berlin, Germany, 1984

6. P S. Gilman and J.S. Benjamin, Ann. Rev. Mater. Sci., Vol 13, 1983, p 279-300

7. C.C. Koch, Mechanical Milling and Alloying, Materials Science and Technology--A Comprehensive Treatment, Vol 15, R.W. Cahn, Ed., VCH, 1991, p 193-245

8. C. Suryanarayana, Metals and Mater., Vol 2, 1996, p 195-209

9. C. Suryanarayana, Bibliography on Mechanical Alloying and Milling, Cambridge International Science Publishing, 1995

10. H. Bakker, G.F. Zhou, and H. Yang, Prog. Mater. Sci., Vol 39, 1995, p 159-241

Mechanical Alloying

C. Suryanarayana, Department of Metallurgical and Materials Engineering, Colorado School of Mines

The Process of Mechanical Alloying

Figure 1 is a schematic showing the path of raw materials using the MA process. The raw materials, the type of mill used, the process of consolidation, and the details of heat treatment differ depending on the type of product desired, but the processing route remains essentially the same. It is possible that some minor steps are either added or deleted in some special circumstances. The actual process of MA starts with mixing of powders in the right proportion and loading the powder 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. A steady state occurs when the composition of every powder particle is the same as the proportion of the elements in the starting powder mix. Sometimes the powder is milled to an intermediate state either to form metastable phases or to achieve certain desired properties. The milled powder is then consolidated into a bulk shape and heat treated to obtain the desired microstructure and properties.

Figure 1 is a schematic showing the path of raw materials using the MA process. The raw materials, the type of mill used, the process of consolidation, and the details of heat treatment differ depending on the type of product desired, but the processing route remains essentially the same. It is possible that some minor steps are either added or deleted in some special circumstances. The actual process of MA starts with mixing of powders in the right proportion and loading the powder 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. A steady state occurs when the composition of every powder particle is the same as the proportion of the elements in the starting powder mix. Sometimes the powder is milled to an intermediate state either to form metastable phases or to achieve certain desired properties. The milled powder is then consolidated into a bulk shape and heat treated to obtain the desired microstructure and properties.

Fig. 1 Processing path in producing a product from powders by mechanical alloying. Source Ref 1 Raw Materials

The raw materials used for MA are widely available commercially pure powders that have particle sizes from 1 to 200 t* m. The powder particle size is not critical, except that it should be smaller than the grinding ball size, because the powder particle size decreases exponentially with time and reaches a small value after a few minutes of milling. These powders fall into the broad categories of pure metals, master alloys, prealloyed powders, and refractory compounds. The oxygen content of commercially pure metal powders ranges from 0.05 to 0.2 wt%. Therefore, if one is interested in studying phase formation, it becomes important to choose the proper purity of the powder. This is important because, more often than not, the nature and amount of impurities in the system decide the nature of the final phase formed and the chemical constitution of the alloy. Dispersion strengthened materials usually contain additions of carbides, nitrides, and oxides. Oxides are the most common and these alloys are known as oxide-dispersion strengthened (ODS) materials. In the 1960s, the powder charge for MA consisted of at least 15 vol% of a ductile compressibly deformable metal powder to act as a host or a binder. However, in the 1990s, mixtures of fully brittle materials have been milled successfully resulting in alloy formation. Thus, the requirement of having a ductile metal powder during milling is no longer necessary. Accordingly, ductile-ductile, ductile-brittle, and brittle-brittle powder mixtures are milled to produce novel alloys. Mixtures of solid powder particles and liquids have also been milled (Ref 11).

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