MECHANICAL ALLOYING
Initially, larger particles are produced this way. Increased defect structures such as dislocations, gaps and tension in the crystal
lattices of the individual
particles lead to an elevated diffusion rate of their atoms, resulting in increased embrittlement which promotes the for- mation of cracks and a subsequent breaking of the particle. The diffusion is supported by a temperature rise gener- ated by frictional heat in the grinding
jar. A model calculation showed that temperature peaks of 700 – 1,800 K and pressure peaks of a few thousand atmo- spheres occur in a planetary ball mill [1]. The process of fusion and folding con- tinues until complete homogenization is achieved after a few minutes or several hours. Diminutive crystalline sections of adjacent initial components are formed in the powder particles which are called “nano crystallites” (fi g. 2 and 3 ).
MECHANOCHEMISTRY
The mechanical effects generated by a planetary ball mill are also highly suitable for the so-called mechano- chemistry. Mechanical impact pro- vides the activation energy required for chemical reactions. These complex reactions can be car- ried out without the use of solvents. The types of reactions vary greatly, from oxidative halogenations or Diels-Adler reactions to the formation of enamines, syntheses of glycosides or even simple regio-selective reac- tions. Mechanochemistry can be used, for example, for the dehalogenation of waste (DMCR) which would hardly be possible with conventional meth- ods.
Fig. 2: Cross sectional optical micro- graph of a mechanically alloyed iron-tan- talum-copper (FeTaCu) powder particle after 5 h
In this way it is possible to produce alloys which cannot be obtained by melting and casting. Any mixing ratio can be selected. RETSCH’s ball mills provide the required energy input for mechanical alloying and can be operat- ed with high speed ratios.
Fig. 3: Scanning electron microscopical (SEM) picture of a mechanically alloyed FeTaCu powder after 20 h (top view)
For reactions under controlled atmo- sphere or working under inert conditions the “comfort” grinding jars of the PM series can be equipped with safety clo- sures and aeration lids. The high ener-
gy ball mill Emax also meets all require- ments for mechanical alloying.
[1] Urakaev FK (2000), Powder Technology 200, 107, 93
Hydrogen Storage, Magnetic Materials, Catalysts, Sensors, etc.
Production of Pure Materials
MECHANICAL ALLOYING
Nanostructured Materials
Synthesis of Novel Phases,
PVD Targets, Solders, Carbides, Nitrides, Silicides etc.
Source: Suryanarayana et al. (2001), The science and technology of mechanical alloying, Materials Science and Engineering, A304–306 p151–158
www.retsch.com | the sample 39 | page 11 Supercorroding Alloys ODS Superalloys
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