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Presence of coherent interfaces within crystallites, referred to as "annealing
twins", has many effects on the properties of several fcc metals and alloys.
The mechanism of the formation of these twins
have been subject of numerous studies over fifty years. Although much progress
has been made in recent
years, a universally accepted view on the formation of these twins has not yet
emerged. This brief review
will describe recent studies at Naval Research Laboratory to understand several
aspects of the annealing
twins including nature of these twins, the mechanism of their formation,
and the compositional parameters
which affect their densities. These studies have established a relation
between twin density and grain size,
temperature and material properties. A model of the mechanism of their
formation based on the emergence
of Shockley partial loops on consecutive {111} planes during grain migration
has also been developed. It is
argued that various experimental and theoretical results obtained over the
years can be consistently and
satisfactorily explained by this model. This study has analyzed two aspects
of annealing twins in details
viz, a) role of boron additions in reducing twin density in nickel and b)
the effect of annealing twins in Hall-Petch
relation.
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