482
T. Balusamy et al.
formation of equiaxed nanograins (100–300 nm). The top surface reveals uniform
distribution of nano-sized grains (50–250 nm) with a random orientation. The variation in microstructure is mainly due to the level of strain experienced at different
depths. Based on the microstructural features observed at various depths as a function of strain, Zhu et al. [46] have suggested that the evolution of microstructure of
Ti during SMAT occurs as follows: (i) onset of twins and the intersection of twin
systems; (ii) formation of low angle disoriented lamellae with a high density of dislocations; (iii) subdivision of microbands and dislocation cells into disoriented blocks
and polygonal submicronic grains; and (iv) further breakdown of submicronic polygonal grains into randomly oriented nanograins. Due to the higher SFE (>300 mJ/m
2 ),
twinning is identified as the preferred mode of deformation of Ti at low strain. At
moderate strain, intersection of the twins is promoted, which hinders their movement.
Hence, at moderate strain, a high density of dislocations facilitates the formation of
lamellae. At higher strain, the lamellae are subdivided into dislocation cells or low
angle disoriented blocks, which is subsequently converted to polygonal grains with
a random orientation. Equiaxed nanograins are formed at the top surface. The final
stage of grain refinement is governed by rotational recrystallization process.
The evolution of dislocations during SPD of Ti by SMAT is addressed by Wen
et al. [47]. According to them, the dislocation density is low at a depth of 180 μm
from the treated surface. An increase in dislocation density at a depth of 150 μm
has lead to the formation of dislocation bands. Some of these dislocation bands
change to equiaxed subgrains at a depth of 100 μm, while the shape of the subgrains
becomes more uniform at a depth of 60 μm. Equiaxed nanograins (103 nm) with a
random orientation predominates at a depth of 15 μm below the top surface. The
microstructures reveal that at low strain, the formation of dislocation tangles and
dislocation bands predominates. An increase in grain boundary density and triple
junctions promotes nucleation of new subgrains. With an increase in strain and strain
rate, the size of the subgrains becomes smaller and the subgrains transform into
dislocation bands. Dynamic recrystallization plays a critical role in conversion of
dislocation bands to equiaxed nanograins until formation of nanocrystalline Ti.
During SMAT, the multidirectional impingement of the flying balls on the surface
of Ti and Ti alloys results in the generation of numerous dimples and craters on
the surface, which leads to an increase in surface roughness. SMAT using 8 mm
Ø stainless steel balls at 50 Hz for 60 min has increased the R a of Ti6Al4V alloy
from 0.19 ± 0.07 μm to 3.93 ± 0.26 μm [20]. Similarly, SMAT using 8 mm Ø
alumina balls at 50 Hz for 45 min has increased the R a of Ti from 0.15 ± 0.12
μm to 3.27 ± 0.98 μm [18]. The R a of the treated Ti surface is increased with an
increase in treatment time from 15 to 45 min. However, Anand Kumar et al. [45, 48]
have observed a slight decrease in R a with an increase in treatment time from 30 to
60 min for Ti6Al4V alloy subjected to SMAT using 5 mm Ø SAE 52100 steel balls
at 50 Hz. This is due to the severe surface damage with the generation of numerous
microcracks after treatment for 60 min, which could have contributed to material
removal from the top surface.
The XRD pattern of untreated coarse-grained Ti exhibits the presence of sharp
diffraction peaks pertaining to (100), (002), (101), (102), (110), (103) and (112)
T. Balusamy et al.
formation of equiaxed nanograins (100–300 nm). The top surface reveals uniform
distribution of nano-sized grains (50–250 nm) with a random orientation. The variation in microstructure is mainly due to the level of strain experienced at different
depths. Based on the microstructural features observed at various depths as a function of strain, Zhu et al. [46] have suggested that the evolution of microstructure of
Ti during SMAT occurs as follows: (i) onset of twins and the intersection of twin
systems; (ii) formation of low angle disoriented lamellae with a high density of dislocations; (iii) subdivision of microbands and dislocation cells into disoriented blocks
and polygonal submicronic grains; and (iv) further breakdown of submicronic polygonal grains into randomly oriented nanograins. Due to the higher SFE (>300 mJ/m
2 ),
twinning is identified as the preferred mode of deformation of Ti at low strain. At
moderate strain, intersection of the twins is promoted, which hinders their movement.
Hence, at moderate strain, a high density of dislocations facilitates the formation of
lamellae. At higher strain, the lamellae are subdivided into dislocation cells or low
angle disoriented blocks, which is subsequently converted to polygonal grains with
a random orientation. Equiaxed nanograins are formed at the top surface. The final
stage of grain refinement is governed by rotational recrystallization process.
The evolution of dislocations during SPD of Ti by SMAT is addressed by Wen
et al. [47]. According to them, the dislocation density is low at a depth of 180 μm
from the treated surface. An increase in dislocation density at a depth of 150 μm
has lead to the formation of dislocation bands. Some of these dislocation bands
change to equiaxed subgrains at a depth of 100 μm, while the shape of the subgrains
becomes more uniform at a depth of 60 μm. Equiaxed nanograins (103 nm) with a
random orientation predominates at a depth of 15 μm below the top surface. The
microstructures reveal that at low strain, the formation of dislocation tangles and
dislocation bands predominates. An increase in grain boundary density and triple
junctions promotes nucleation of new subgrains. With an increase in strain and strain
rate, the size of the subgrains becomes smaller and the subgrains transform into
dislocation bands. Dynamic recrystallization plays a critical role in conversion of
dislocation bands to equiaxed nanograins until formation of nanocrystalline Ti.
During SMAT, the multidirectional impingement of the flying balls on the surface
of Ti and Ti alloys results in the generation of numerous dimples and craters on
the surface, which leads to an increase in surface roughness. SMAT using 8 mm
Ø stainless steel balls at 50 Hz for 60 min has increased the R a of Ti6Al4V alloy
from 0.19 ± 0.07 μm to 3.93 ± 0.26 μm [20]. Similarly, SMAT using 8 mm Ø
alumina balls at 50 Hz for 45 min has increased the R a of Ti from 0.15 ± 0.12
μm to 3.27 ± 0.98 μm [18]. The R a of the treated Ti surface is increased with an
increase in treatment time from 15 to 45 min. However, Anand Kumar et al. [45, 48]
have observed a slight decrease in R a with an increase in treatment time from 30 to
60 min for Ti6Al4V alloy subjected to SMAT using 5 mm Ø SAE 52100 steel balls
at 50 Hz. This is due to the severe surface damage with the generation of numerous
microcracks after treatment for 60 min, which could have contributed to material
removal from the top surface.
The XRD pattern of untreated coarse-grained Ti exhibits the presence of sharp
diffraction peaks pertaining to (100), (002), (101), (102), (110), (103) and (112)