494
T. Balusamy et al.
X-ray diffraction pattern of untreated coarse-grained NiTi alloy exhibits the presence of only the austenite B2 phase. SMAT of the NiTi alloy using 2 mm Ø stainless steel balls for 60 min has resulted in a decrease in intensity of the B2 (110)
and (211) planes along with broadening of the diffraction peaks, suggesting refinement of grains following SPD during SMAT [70]. Broadening of the diffraction
peaks is also observed for NiTi alloy subjected to SP using 250 μm Ø spherical
alumina balls at a peening angle of 90° under an air pressure of 45 psi for 20 min.
Besides the austenite B2 phase, B19’ martensite phase is also observed for the shotpeened samples. However, the B19’ martensite phase is completely transformed
to the austenite B2 phase after heat-treatment at 300 °C in an argon atmosphere
for 1 h [74]. Hence, it can be inferred that the type of phases formed after SMAT
of NiTi alloy is largely a function of the conditions, which determines the extent
of plastic deformation and phase transformation from austenite to martensite and
reverse transformation.
The evolution of microstructure at various depths from the top surface to the
maximum depth of the deformed region of the SMAT NiTi alloy is addressed by
Hu et al. [71, 72]. The variation in microstructure at different depths and the type of
phases formed is determined by the level of strain and strain rate. At a depth of 180–
220 μm from the top surface wherein the strain level is much lower, high-density
dislocation appears to be the main mechanism of strain accommodation. At a depth
of 150–200 μm, the extent of strain and strain rate experienced by the alloy will be
much higher. Accordingly, the dislocation promotes a sequential formation of dislocation lines (DLs), dense dislocation walls (DDWs), sub-boundaries, and refinement
of coarse-grained structure into a nanocrystalline structure. At a depth of 150 μm
from the surface, stress-induced martensite (SIM) transformation is found to be the
main strain accommodation mechanism. Hence, only the B19’ martensite prevails
at this depth. At a depth of 50–100 μm, a further increase in the extent of plastic
deformation would facilitate the formation of DLs and DDWs and sub-boundaries
that stabilizes the martensite plates, which in turn impedes the reverse transformation
of the martensite (B19’) to austenite (B2) phase. Hence, both martensite (B19’) and
austenite (B2) could prevail at this depth. At a depth of about 50 μm, the microstructure indicates formation of narrow lamellae, which is subdivided into ultrafine grains.
The top surface of the NiTi alloy could experience a very high level of strain and
strain rate of the order of 10
3 –10
4 /s. Hence, the high density of dislocations refines
the grain size, promoting the formation of nanocrystalline grains. However, the phase
content of the nanocrystalline region is only B2 austenite. It appears that with a very
high strain rate, a reverse transformation of martensite (B19’) to austenite (B2) has
occurred. The SPD at the top surface and temperature-induced during deformation
could have enabled such a phase transformation. Since the average grain size of the
nanocrystalline region is ~20 nm, the B2 austenite phase becomes thermally stable.
The increase in grain boundary density, higher dislocation densities and the energy
stored at grain boundaries facilitates crystalline-to-amorphous transformation. The
evolution of microstructure and phase content at various depths of the SMAT NiTi
alloy is schematically represented in Fig. 15.
T. Balusamy et al.
X-ray diffraction pattern of untreated coarse-grained NiTi alloy exhibits the presence of only the austenite B2 phase. SMAT of the NiTi alloy using 2 mm Ø stainless steel balls for 60 min has resulted in a decrease in intensity of the B2 (110)
and (211) planes along with broadening of the diffraction peaks, suggesting refinement of grains following SPD during SMAT [70]. Broadening of the diffraction
peaks is also observed for NiTi alloy subjected to SP using 250 μm Ø spherical
alumina balls at a peening angle of 90° under an air pressure of 45 psi for 20 min.
Besides the austenite B2 phase, B19’ martensite phase is also observed for the shotpeened samples. However, the B19’ martensite phase is completely transformed
to the austenite B2 phase after heat-treatment at 300 °C in an argon atmosphere
for 1 h [74]. Hence, it can be inferred that the type of phases formed after SMAT
of NiTi alloy is largely a function of the conditions, which determines the extent
of plastic deformation and phase transformation from austenite to martensite and
reverse transformation.
The evolution of microstructure at various depths from the top surface to the
maximum depth of the deformed region of the SMAT NiTi alloy is addressed by
Hu et al. [71, 72]. The variation in microstructure at different depths and the type of
phases formed is determined by the level of strain and strain rate. At a depth of 180–
220 μm from the top surface wherein the strain level is much lower, high-density
dislocation appears to be the main mechanism of strain accommodation. At a depth
of 150–200 μm, the extent of strain and strain rate experienced by the alloy will be
much higher. Accordingly, the dislocation promotes a sequential formation of dislocation lines (DLs), dense dislocation walls (DDWs), sub-boundaries, and refinement
of coarse-grained structure into a nanocrystalline structure. At a depth of 150 μm
from the surface, stress-induced martensite (SIM) transformation is found to be the
main strain accommodation mechanism. Hence, only the B19’ martensite prevails
at this depth. At a depth of 50–100 μm, a further increase in the extent of plastic
deformation would facilitate the formation of DLs and DDWs and sub-boundaries
that stabilizes the martensite plates, which in turn impedes the reverse transformation
of the martensite (B19’) to austenite (B2) phase. Hence, both martensite (B19’) and
austenite (B2) could prevail at this depth. At a depth of about 50 μm, the microstructure indicates formation of narrow lamellae, which is subdivided into ultrafine grains.
The top surface of the NiTi alloy could experience a very high level of strain and
strain rate of the order of 10
3 –10
4 /s. Hence, the high density of dislocations refines
the grain size, promoting the formation of nanocrystalline grains. However, the phase
content of the nanocrystalline region is only B2 austenite. It appears that with a very
high strain rate, a reverse transformation of martensite (B19’) to austenite (B2) has
occurred. The SPD at the top surface and temperature-induced during deformation
could have enabled such a phase transformation. Since the average grain size of the
nanocrystalline region is ~20 nm, the B2 austenite phase becomes thermally stable.
The increase in grain boundary density, higher dislocation densities and the energy
stored at grain boundaries facilitates crystalline-to-amorphous transformation. The
evolution of microstructure and phase content at various depths of the SMAT NiTi
alloy is schematically represented in Fig. 15.