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.
Materials consist of