Surface Nanostructuring of Metallic Materials …
503
Mg alloy is increased from 276.5 MPa to 323.7 MPa after SMAT for 6 min. The
elongation to a fraction is slightly decreased from 17.23 to 10.44% after SMAT for
6 min [93]. An improvement in YS and UTS after SMAT of AZ31 Mg alloy is also
observed by Peng et al. [94]. Hence, it is evident that the weakening of (0002) basal
texture of AZ31 Mg alloy after SMAT has no deleterious effect on its mechanical
property. The improvement in YS and UTS of AZ31 Mg alloy after SMAT is due to
the grain refinement and work hardening.
SMAT increased the hardness of AZ31 Mg alloy [91–94]. Grain refinement and
a high density of dislocations are considered responsible for the observed increase
in hardness [94]. The hardness profile displays a depth-dependent gradation in hardness in accordance with the graded layer structure, decreasing with an increase in
deformation depth [92, 93]. For AZ91D Mg alloy, dissolution of the β-Mg 17 Al 12
phase is also considered as a reason for the improvement in hardness after SMAT
besides grain refinement and dislocations [95].
Laleh and Kargar [95] have evaluated the corrosion behavior of untreated AZ91D
Mg alloy and those subjected to SMAT using 2, 3 and 5 mm Ø stainless steel balls at
20 kHz for 30 min in 3.5% NaCl. SMAT increased the surface roughness, refined the
grain size, introduced a high density of crystalline defects and promoted dissolution
of the β-Mg 17 Al 12 phase into the matrix. AZ91D Mg alloy treated using 2 mm Ø
balls has offered an improvement in corrosion resistance, which is evidenced by a
shift in E corr towards the noble direction from −1509 mV vs. SCE to −1334 mV
vs. SCE and a decrease in i corr from 3.91 μA/cm
2 to 0.24 μA/cm
2 . In contrast, the
alloy treated using 5 mm Ø balls has drastically reduced the corrosion resistance. A
shift in E corr from −1509 mV vs. SCE to −1561 mV vs. SCE and an increase in i corr
from 3.91 μA/cm
2 to 24.83 μA/cm
2 validates it. In spite of an increase in surface
roughness and defect density, the corrosion resistance of AZ91D Mg alloy treated
using 2 mm Ø balls is much higher than the untreated alloy. This is believed to be due
to the dissolution of β-Mg 17 Al 12 phase into the matrix. The galvanic couple formed
between the matrix and the β-Mg 17 Al 12 phase results in poor corrosion resistance of
the untreated AZ91D Mg alloy. The poor corrosion resistance of AZ91D Mg alloy
treated using 5 mm Ø balls is due to the higher surface roughness and high density
of crystalline defects Laleh and Kargar [95].
The corrosion behavior of untreated AZ31 Mg alloy and those subjected to SMAT
using 2 mm Ø zirconia balls for 40–90 min, in Hanks’ solution at 37 °C, is studied
by Li et al. [91]. SMAT has increased the surface roughness, refined the grain size
introduced high density of crystalline defects and promoted dissolution of the Mg 2 Ca
phase into the matrix. Grain refinement does not seem to exert a positive influence
on the corrosion resistance since the passivation tendency for the AZ31 Mg alloy in
Hanks’ solution is poor. Dissolution of the Mg 2 Ca phase into the matrix, which would
otherwise promote the formation of galvanic cells, fails to exhibit any improvement in
corrosion resistance. The higher surface roughness and the high density of crystalline
defects deleteriously influence the corrosion resistance. Based on the inferences
made, Li et al. [91] have suggested that SMAT is not a suitable method to improve
the corrosion resistance of Mg-based biodegradable implants.
503
Mg alloy is increased from 276.5 MPa to 323.7 MPa after SMAT for 6 min. The
elongation to a fraction is slightly decreased from 17.23 to 10.44% after SMAT for
6 min [93]. An improvement in YS and UTS after SMAT of AZ31 Mg alloy is also
observed by Peng et al. [94]. Hence, it is evident that the weakening of (0002) basal
texture of AZ31 Mg alloy after SMAT has no deleterious effect on its mechanical
property. The improvement in YS and UTS of AZ31 Mg alloy after SMAT is due to
the grain refinement and work hardening.
SMAT increased the hardness of AZ31 Mg alloy [91–94]. Grain refinement and
a high density of dislocations are considered responsible for the observed increase
in hardness [94]. The hardness profile displays a depth-dependent gradation in hardness in accordance with the graded layer structure, decreasing with an increase in
deformation depth [92, 93]. For AZ91D Mg alloy, dissolution of the β-Mg 17 Al 12
phase is also considered as a reason for the improvement in hardness after SMAT
besides grain refinement and dislocations [95].
Laleh and Kargar [95] have evaluated the corrosion behavior of untreated AZ91D
Mg alloy and those subjected to SMAT using 2, 3 and 5 mm Ø stainless steel balls at
20 kHz for 30 min in 3.5% NaCl. SMAT increased the surface roughness, refined the
grain size, introduced a high density of crystalline defects and promoted dissolution
of the β-Mg 17 Al 12 phase into the matrix. AZ91D Mg alloy treated using 2 mm Ø
balls has offered an improvement in corrosion resistance, which is evidenced by a
shift in E corr towards the noble direction from −1509 mV vs. SCE to −1334 mV
vs. SCE and a decrease in i corr from 3.91 μA/cm
2 to 0.24 μA/cm
2 . In contrast, the
alloy treated using 5 mm Ø balls has drastically reduced the corrosion resistance. A
shift in E corr from −1509 mV vs. SCE to −1561 mV vs. SCE and an increase in i corr
from 3.91 μA/cm
2 to 24.83 μA/cm
2 validates it. In spite of an increase in surface
roughness and defect density, the corrosion resistance of AZ91D Mg alloy treated
using 2 mm Ø balls is much higher than the untreated alloy. This is believed to be due
to the dissolution of β-Mg 17 Al 12 phase into the matrix. The galvanic couple formed
between the matrix and the β-Mg 17 Al 12 phase results in poor corrosion resistance of
the untreated AZ91D Mg alloy. The poor corrosion resistance of AZ91D Mg alloy
treated using 5 mm Ø balls is due to the higher surface roughness and high density
of crystalline defects Laleh and Kargar [95].
The corrosion behavior of untreated AZ31 Mg alloy and those subjected to SMAT
using 2 mm Ø zirconia balls for 40–90 min, in Hanks’ solution at 37 °C, is studied
by Li et al. [91]. SMAT has increased the surface roughness, refined the grain size
introduced high density of crystalline defects and promoted dissolution of the Mg 2 Ca
phase into the matrix. Grain refinement does not seem to exert a positive influence
on the corrosion resistance since the passivation tendency for the AZ31 Mg alloy in
Hanks’ solution is poor. Dissolution of the Mg 2 Ca phase into the matrix, which would
otherwise promote the formation of galvanic cells, fails to exhibit any improvement in
corrosion resistance. The higher surface roughness and the high density of crystalline
defects deleteriously influence the corrosion resistance. Based on the inferences
made, Li et al. [91] have suggested that SMAT is not a suitable method to improve
the corrosion resistance of Mg-based biodegradable implants.