Surface Nanostructuring of Metallic Materials …
483
planes. However, after SMAT, the peaks are broadened due to refinement of grains by
plastic deformation induced during SMAT and increase in atomic level microstrain.
Lai et al. [49] have identified an additional peak ~44° 2θ for SMAT Ti, which is
believed to be due to the formation of TiO due to increasing in penetration of oxygen
during SPD. The average grain size of the treated Ti lies in the range of 25 to 43 nm
while the microstrain is in the range of 0.14–0.16% [44, 45, 48, 50, 51].
The SPD induces compressive residual stress on SMAT Ti and Ti alloys. The
compressive residual stress is much higher at their surface and it is decreased with
an increase in the depth from the top surface. This is due to the variation in the level
of strain and strain rate experienced at different depths of the Ti and Ti alloys during
treatment. Anand Kumar et al. [45, 48] have reported an increase in the compressive
residual stress of Ti6Al4V alloy from −679 ± 20 MPa to −706 ± 14 MPa with an
increase in treatment time from 30 to 60 min.
SMAT of CP-Ti using 3 mm Ø stainless steel balls at 20 kHz for 30 min has lead to
an increase in hardness. The maximum hardness of 3.8 GPa is found at a depth of 50
μm from the top surface. The hardness profile exhibits a steady decrease in hardness
with an increase in depth from the surface to the bulk, supporting the formation of
a graded layer structure in line with the inference made from the cross-sectional
microstructural features. Accordingly, a hardness of 3.0 GPa and 2.8 GPa is found
at a depth of 200 and 400 μm from the top surface. The increase in hardness is due
to the grain refinement caused by plastic deformation induced during SMAT. The
maximum hardness of 3.8 GPa at a thickness of 50 μm is due to the high strain rate
of the order of 10
2 –10
3 /s near the top surface [52]. SMAT using 8 mm Ø stainless
steel balls at 20 kHz for 60 min increased the hardness of CP-Ti to 6.9 GPa at a depth
of 50 μm from the surface while it is decreased to 2.4 GPa at a depth of 500 μm from
the surface [44]. Jelliti et al. [7] have reported a hardness of 4.4 GPa for Ti-6Al-4 V
alloy subjected to SMAT using 3 mm Ø stainless steel balls at 20 kHz for 20 min.
SMAT using 8 mm Ø alumina balls at 50 Hz has increased the hardness of Ti from
3.1 to 4.4 GPa with an increase in treatment time from 15 to 45 min [18]. Similarly,
SMAT using 5 mm Ø SAE 52100 steel balls at 50 Hz has increased the hardness of
Ti6Al4V alloy from 5.63 to 8.64 GPa with an increase in treatment time from 30 to
60 min [51]. Hence, it is clear that strain rate experienced by the treated surface, the
extent of plastic deformation, increase in lattice strain, amenability of the Ti/Ti alloy
for nanocrystallization and reduction in grain size determines the degree of increase
in hardness.
The mechanical properties of Ti subjected to SMAT using 8 mm Ø stainless steel
balls at 50 Hz for 60 min are compared with its untreated coarse-grained counterpart
by Wen et al. [50]. According to them, after SMAT the YS and UTS are increased
to 920 ± 40 MPa and 970 ± 30 MPa, respectively. Ti6Al4V alloy has become the
material of choice instead of Ti due to its higher YS (902 MPa) and UTS (975 MPa).
Since SMAT enables an increase in the YS and UTS of Ti that is highly comparable
with the Ti6Al4V alloy, it would be possible to use Ti in place of Ti6Al4V alloy,
particularly for biomedical applications. Such a choice would eliminate the problems
associated with the leaching of aluminum and vanadium from the Ti6Al4V alloy into
the body fluid. Zhao et al. [44] have also observed an improvement in YS (39.5%)
483
planes. However, after SMAT, the peaks are broadened due to refinement of grains by
plastic deformation induced during SMAT and increase in atomic level microstrain.
Lai et al. [49] have identified an additional peak ~44° 2θ for SMAT Ti, which is
believed to be due to the formation of TiO due to increasing in penetration of oxygen
during SPD. The average grain size of the treated Ti lies in the range of 25 to 43 nm
while the microstrain is in the range of 0.14–0.16% [44, 45, 48, 50, 51].
The SPD induces compressive residual stress on SMAT Ti and Ti alloys. The
compressive residual stress is much higher at their surface and it is decreased with
an increase in the depth from the top surface. This is due to the variation in the level
of strain and strain rate experienced at different depths of the Ti and Ti alloys during
treatment. Anand Kumar et al. [45, 48] have reported an increase in the compressive
residual stress of Ti6Al4V alloy from −679 ± 20 MPa to −706 ± 14 MPa with an
increase in treatment time from 30 to 60 min.
SMAT of CP-Ti using 3 mm Ø stainless steel balls at 20 kHz for 30 min has lead to
an increase in hardness. The maximum hardness of 3.8 GPa is found at a depth of 50
μm from the top surface. The hardness profile exhibits a steady decrease in hardness
with an increase in depth from the surface to the bulk, supporting the formation of
a graded layer structure in line with the inference made from the cross-sectional
microstructural features. Accordingly, a hardness of 3.0 GPa and 2.8 GPa is found
at a depth of 200 and 400 μm from the top surface. The increase in hardness is due
to the grain refinement caused by plastic deformation induced during SMAT. The
maximum hardness of 3.8 GPa at a thickness of 50 μm is due to the high strain rate
of the order of 10
2 –10
3 /s near the top surface [52]. SMAT using 8 mm Ø stainless
steel balls at 20 kHz for 60 min increased the hardness of CP-Ti to 6.9 GPa at a depth
of 50 μm from the surface while it is decreased to 2.4 GPa at a depth of 500 μm from
the surface [44]. Jelliti et al. [7] have reported a hardness of 4.4 GPa for Ti-6Al-4 V
alloy subjected to SMAT using 3 mm Ø stainless steel balls at 20 kHz for 20 min.
SMAT using 8 mm Ø alumina balls at 50 Hz has increased the hardness of Ti from
3.1 to 4.4 GPa with an increase in treatment time from 15 to 45 min [18]. Similarly,
SMAT using 5 mm Ø SAE 52100 steel balls at 50 Hz has increased the hardness of
Ti6Al4V alloy from 5.63 to 8.64 GPa with an increase in treatment time from 30 to
60 min [51]. Hence, it is clear that strain rate experienced by the treated surface, the
extent of plastic deformation, increase in lattice strain, amenability of the Ti/Ti alloy
for nanocrystallization and reduction in grain size determines the degree of increase
in hardness.
The mechanical properties of Ti subjected to SMAT using 8 mm Ø stainless steel
balls at 50 Hz for 60 min are compared with its untreated coarse-grained counterpart
by Wen et al. [50]. According to them, after SMAT the YS and UTS are increased
to 920 ± 40 MPa and 970 ± 30 MPa, respectively. Ti6Al4V alloy has become the
material of choice instead of Ti due to its higher YS (902 MPa) and UTS (975 MPa).
Since SMAT enables an increase in the YS and UTS of Ti that is highly comparable
with the Ti6Al4V alloy, it would be possible to use Ti in place of Ti6Al4V alloy,
particularly for biomedical applications. Such a choice would eliminate the problems
associated with the leaching of aluminum and vanadium from the Ti6Al4V alloy into
the body fluid. Zhao et al. [44] have also observed an improvement in YS (39.5%)