RECOVERY, RECRYSTALLISATION AND GRAIN GROWTH


 RECOVERY,  RECRYSTALLISATION AND GRAIN GROWTH

When metal is subjected to hot working and
cold working processes, plastic deformation occurs which is an important
phenomenon. Plastic deformation of metal distorts the crystal lattice. It
breaks up the blocks of initial equiaxed grains to produce fibrous structure
and increases the energy level of metal.
Deformed metal, during comparison with its un-deformed state, is in
non-equilibrium, thermodynamically unstable state. Therefore, spontaneous
processes occur in strain-hardened metal, even at room temperature that brings
it into a more stable condition. When the temperature of metal is increased,
the metal attempts to
approach equilibrium
through three processes: (i)
recovery, (ii)
recrystallisation, and (iii) grain growth.
Fig. reflects
the recovery, recrystallisation and grain growth
and the main property
changes in each
region.


Recovery

When a strain-hardened metal is heated to a
low temperature, the elastic distortions of
the
crystal lattice are reduced due to the increase in amplitude of thermal
oscillation of
the atoms. This
heating will decrease the strength of the strain-hardened metal but there is an
increase in the elastic limit and ductility of metal, though they will not
react the
values possessed by the
initial material before strain-hardening. No changes in microstructure of metal
are observed in this period. The partial restoration of the original
characteristics, produced by reducing the distortion of the crystal lattice
without remarkable changes in
microstructure,
is called  recovery.
At the initial state, the
rate of the recovery is fastest
and   it drops off at longer times at given
temperature. Hence the amount of recovery that
occurs in a practical time increases with increasing temperature. The individual characteristic recover at different rates and gain various degrees
of completion in a given cold worked metal.


Recrystallisation


Formation of new equiaxed grains in the heating process
of metal, instead of the oriented fibrous
structure
of the deformed
metal,
is called recrystallisation. The process
of recrystallisation is
illustrated through Fig. The first effect of heating of metal is to form
new minute grains and these rapidly enlarge until further growth is restricted
by grain meeting another. The
original system of grains go out of the picture and the new crystallized structure is formed in the
metal. Recrystallisation does not produce new structures however it produces
new grains or crystals of the same structure in the metal. It consists in having  the
atoms of the deformed metal overcome the bonds of the distorted lattice, the
formation   of nuclei of equiaxed grains
and subsequent growth of these grains due to transfer of atoms from deformed to
un-deformed crystallites. Finer grains get refined and acquire a shape
resembling fibres. The temperature at which crystallization starts, that is new
grains are formed, is called
recrystallisation temperature. Recrystallisation temperature
is also defined as that temperature at which
half of the cold worked material will recrystallise in 60 minutes.



Formation of new equiaxed grains in the heating process of metal, instead of the oriented fibrous structure of the deformed metal, is called recrystallisation. The process of recrystallisation is illustrated through Fig. The first effect of heating of metal is to form new minute grains and these rapidly enlarge until further growth is restricted by grain meeting another. The original system of grains go out of the picture and the new crystallized structure is formed in the metal. Recrystallisation does not produce new structures however it produces new grains or crystals of the same structure in the metal. It consists in having  the atoms of the deformed metal overcome the bonds of the distorted lattice, the formation   of nuclei of equiaxed grains and subsequent growth of these grains due to transfer of atoms from deformed to un-deformed crystallites. Finer grains get refined and acquire a shape resembling fibres. The temperature at which crystallization starts, that is new grains are formed, is called recrystallisation temperature. Recrystallisation temperature is also defined as that temperature at which half of the cold worked material will recrystallise in 60 minutes.
Recovery, recrystalisation and grain growth




Grain Growth

On recrystallisation of metal, the grains are
smaller and somewhat regular in shape.
The grains
in metal will grow if the temperature is high enough or if the temperature is
allowed to exceed the minimum required for recrystallisation and this growth of
grain is the
result   of a tendency to return to more stable
and larger state. It appears to depend primarily on
   the shape of the grain.
For any temperature above the recrystallization temperature, normally there is practical maximum size at which the grains
will reach equilibrium and cease to grow
significantly.
However,
there are certain kinds of abnormal grains growth in metal that
occur as a result of applied or residual gradients of strain due to non-uniform
impurity distribution, and
which permits
growing very large single grain in metal.



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