502
T. Balusamy et al.
alloying elements should also be non-toxic, which limits the availability of elements
for alloying. Surface modification is a viable approach. It would be interesting to
see how SMAT could be useful in improving the performance of Mg towards the
development of degradable implants.
SMAT of AZ31 Mg alloy has enabled nanocrystallization at the surface along
with the formation of a graded layer structure [91–94]. An increase in treatment
time has led to a decrease in grain size at the surface and increased the deformation
thickness [92, 93]. However, no significant change in the deformation thickness could
be observed if the treatment time is increased beyond a threshold level [92]. SMAT
increased surface roughness [91, 95, 96] due to the formation of craters and dimples
on the surface. An increase in diameter of the balls used for treatment increased the
surface roughness [95, 96]. An increase in surface roughness decreased the water
contact angle, increased surface energy, and increased the degree of hydrophilicity
of the Mg alloy [91].
The evolution of the microstructure of AZ31 Mg alloy during SMAT is addressed
by Meng et al. [92] and Duan et al. [93]. According to Meng et al. [92], twinning is the
mechanism of deformation at lower strain while dislocation cells and subgrains with
misorientation start to emerge with a continuous increase in strain. With an increase
in treatment time, the level of strain and strain rate is increased, which decreases
the recrystallization temperature. The increase in surface temperature induced by the
repeated multidirectional impacts of the balls and the recrystallization temperature
promotes dynamic recrystallization, leading to the formation of a nanostructured
structure. According to Duan et al. [93], with reasonably applied stress, dislocations
start to accumulate near the grain boundaries, leading to the formation of dislocation
cells. With a further increase in strain, an increase in misorientation of the dislocation
cells leads to the formation of subgrains which gradually transform to recrystallized
grains having high angle grain boundaries (HAGBs). With an increase in treatment
time, more HAGBs are generated. SPD induced during SMAT promotes recrystallization and grain refinement and these stages continue until the grains at the surface
are reduced to nano-size. The refinement of grains to a nanoscale at the surface occurs
due to twinning and dynamic recrystallization [94].
Change in texture of AZ31 Mg alloy after SMAT could influence its mechanical
properties. Untreated AZ31 Mg alloy has a strong (0002) basal texture. SMAT using
3 mm Ø AISI 304 stainless steel balls at 20 kHz for 2 min fails to exhibit any change
in texture as the extent of plastic deformation is rather limited. However, a steady
decrease in the intensity of peak pertaining to (0002) basal texture with an increase in
treatment time to 4 min and 6 min suggest that the plastic deformation induced during
SMAT exert a strong weakening effect on the (0002) basal texture of AZ31 Mg alloy
[93]. Based on the inclination of (0002) poles, the decrease in maximum pole density,
and divergence of contour lines, Peng et al. [94] have validated such an occurrence.
The grain orientation differs from the surface to the substrate, which is evidenced by
the lower ratio of (0002) to (10, 11) at the surface [92]. SMAT increased the YS and
UTS of AZ31 Mg alloy and the extent of increase become much pronounced with an
increase in treatment time from 2 to 6 min. The YS of AZ31 Mg alloy is increased
from 187.4 MPa to 274 MPa after SMAT for 6 min. The tensile strength of AZ31
T. Balusamy et al.
alloying elements should also be non-toxic, which limits the availability of elements
for alloying. Surface modification is a viable approach. It would be interesting to
see how SMAT could be useful in improving the performance of Mg towards the
development of degradable implants.
SMAT of AZ31 Mg alloy has enabled nanocrystallization at the surface along
with the formation of a graded layer structure [91–94]. An increase in treatment
time has led to a decrease in grain size at the surface and increased the deformation
thickness [92, 93]. However, no significant change in the deformation thickness could
be observed if the treatment time is increased beyond a threshold level [92]. SMAT
increased surface roughness [91, 95, 96] due to the formation of craters and dimples
on the surface. An increase in diameter of the balls used for treatment increased the
surface roughness [95, 96]. An increase in surface roughness decreased the water
contact angle, increased surface energy, and increased the degree of hydrophilicity
of the Mg alloy [91].
The evolution of the microstructure of AZ31 Mg alloy during SMAT is addressed
by Meng et al. [92] and Duan et al. [93]. According to Meng et al. [92], twinning is the
mechanism of deformation at lower strain while dislocation cells and subgrains with
misorientation start to emerge with a continuous increase in strain. With an increase
in treatment time, the level of strain and strain rate is increased, which decreases
the recrystallization temperature. The increase in surface temperature induced by the
repeated multidirectional impacts of the balls and the recrystallization temperature
promotes dynamic recrystallization, leading to the formation of a nanostructured
structure. According to Duan et al. [93], with reasonably applied stress, dislocations
start to accumulate near the grain boundaries, leading to the formation of dislocation
cells. With a further increase in strain, an increase in misorientation of the dislocation
cells leads to the formation of subgrains which gradually transform to recrystallized
grains having high angle grain boundaries (HAGBs). With an increase in treatment
time, more HAGBs are generated. SPD induced during SMAT promotes recrystallization and grain refinement and these stages continue until the grains at the surface
are reduced to nano-size. The refinement of grains to a nanoscale at the surface occurs
due to twinning and dynamic recrystallization [94].
Change in texture of AZ31 Mg alloy after SMAT could influence its mechanical
properties. Untreated AZ31 Mg alloy has a strong (0002) basal texture. SMAT using
3 mm Ø AISI 304 stainless steel balls at 20 kHz for 2 min fails to exhibit any change
in texture as the extent of plastic deformation is rather limited. However, a steady
decrease in the intensity of peak pertaining to (0002) basal texture with an increase in
treatment time to 4 min and 6 min suggest that the plastic deformation induced during
SMAT exert a strong weakening effect on the (0002) basal texture of AZ31 Mg alloy
[93]. Based on the inclination of (0002) poles, the decrease in maximum pole density,
and divergence of contour lines, Peng et al. [94] have validated such an occurrence.
The grain orientation differs from the surface to the substrate, which is evidenced by
the lower ratio of (0002) to (10, 11) at the surface [92]. SMAT increased the YS and
UTS of AZ31 Mg alloy and the extent of increase become much pronounced with an
increase in treatment time from 2 to 6 min. The YS of AZ31 Mg alloy is increased
from 187.4 MPa to 274 MPa after SMAT for 6 min. The tensile strength of AZ31