Directional Recrystallisation and its Exploitation in
Mechanically Alloyed Metals
H. K. D. H. Bhadeshia
Abstract
Mechanical alloying involves the severe deformation of mixtures of
powders until they form the most intimate of atomic solutions. Inert oxides can also be
introduced to form a uniform dispersion of fine particles which strengthen the
consolidated product. Significant quantities of iron and nickel-base
alloys with unusual properties are produced commercially using this
process.
The total true strain during mechanical alloying can be as large as 9;
there is proof that this leads to mixing on an atomic
scale and to the development of a uniform grain structure which is
submicron in size. The large stored energy,
approaching 1 J/g, ought to make it
easy to induce recrystallisation, but in practice the alloys fail to
recrystallise except at very high temperatures close to melting. On the
other hand, the recrystallisation temperature can be dramatically reduced
by a small additional deformation prior to heat treatment. When
recrystallisation does occur, the grains that evolve can be enormous and
anisotropic in shape. In some cases, the principal growth direction can be
controlled by applying thermal gradients; in other cases that direction
obstinately remains parallel to the extrusion direction, irrespective
of the nature of the heat treatment. These and other anomalous aspects of
the recrystallisation behaviour of these remarkable materials are assessed
and discussed in the context of applications.
21st Riso International Symposium on Materials Science, eds N. Hansen, X.
Huang, D. Juul Jensen, E. M. Lauridsen, T. Leffers, W. Pantleon, T. J.
Sabin and J. A. Wert, published by Riso National Laboratory, Denmark,
2000, pp. 15-28.
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