Materials consist of
large variety of grains with different crystallographic orientations are known
as poly-crystal grain. Various metal working techniques involve very large
strains and strain rates and results in an ultra-fine grains (UFG) with size
d<500 nm. This mechanism involved is known as SPD or Severe Plastic
Deformation. Severe plastic deformation includes various process which includes
Equal Channel Angular Pressing or ECAP, Hot Compression, Hot Torsion
experiments etc. ECAP process consists of a die that has an internal channel
bent at an angle. The Plunger presses the billet (i.e. the work piece) through
the channel which undergoes large shear deformation in the tube and is
collected on the other side of the channel. The cross section area of the
billet remains the same during this process. An important factor is the use of
multiple or several passes, i.e. repeat the process for multiple times until
the desired microstructure is achieved. For each pass, the grain size reduces,
hardness and strength increases, and the ductility is reduced. In the high
pressure torsion or hot torsion tests, the specimen undergoes extensive shear
deformation without losing its original dimensions. The specimen is placed
between 2 anvils. A large compressive force is applied and a torsional effect
is created by the rotation of one of the anvils, while the other anvil is
remained fixed. Whereas hot compression tests is similar to upsetting of a
billet in which specimen is placed between two dies that compresses the
material (work piece) at high strain rate.
All SPD processes
ultimately yields significant reduction in grain size as compared to the parent
material or material before the deformation takes place. The smaller grain size
leads to higher material strength as per Hall-Petch relation 2,3, also higher the dislocation density, higher impedance
or resistance to dislocation motion i.e. plastic deformation and thus higher
strength (Taylor equation 4). The occurrence of ultra-refined grains is due
to processes known as recovery and dynamic recrystallization.