3381

Sensors and Materials, Vol. 36, No. 8 (2024) 3381–3393
MYU Tokyo
S & M 3738

Functional Characteristics of Ultrasonic Shot-peened
Ti–Ni Shape Memory Alloy with Functionally Graded Property
Ryosuke Matsui*
Department of Mechanical Engineering, Aichi Institute of Technology,
1247 Yachigusa, Yakusa-cho, Toyota, Aichi 470-0392, Japan(Received March 25, 2024; accepted May 23, 2024)

Keywords: shape memory alloy, Ti-Ni alloy, functionally graded material, ultrasonic shot peening, digital
image correlation, powder metallurgy

The author aimed to enhance the functional characteristics of the proposed Ti–Ni shapememory alloy (SMA) with a functionally graded property by ultrasonic shot peening (USP)
treatment. To the author’s knowledge, this was the first study in which USP treatment was
utilized for manufacturing functionally graded SMAs; previous studies relied on powder
metallurgy and hot rolling techniques. By examining the various properties of the specimens,
the author confirmed the effects of hot rolling and USP treatment. Hot rolling and USP treatment
led to an increased Vickers hardness. Hot rolling primarily contributed to densification, while
USP treatment induced work hardening, resulting in increased hardness. The surface of TiNi
SMA sintered bodies experienced compressive residual stress up to 300 MPa because of the USP
treatment. The transformation peaks in differential scanning calorimetry curves shifted toward
lower temperatures as the Ni content increased. Therefore, the transformation properties of thefunctionally graded shape memory alloys were unaffected by the USP treatment. The USP
treatment work-hardens the sintered TiNi SMA produced through hot rolling. The effect wasmore pronounced in the high-Ni-concentration region, resulting in the development of afunctionally graded behavior of deformation resistance.

1.

Introduction

Shape memory alloys (SMAs) exhibit two characteristics known as the shape memory effectand superelasticity, which are affected by temperature.(1,2) The shape memory effect enables therecovery of large deformations through heating, while superelasticity enables the recovery oflarge deformations without heating. Consequently, SMAs exhibit functional properties that are
not present in other metal materials. These shape memory properties are observed in various
series of alloy, with Ti–Ni based SMAs (TiNi SMAs) being particularly notable for their fatigue
strength, corrosion resistance, and biocompatibility. Consequently, they are widely employed invarious fields, ranging from medical devices, such as stents and guidewires, to a diverse range of
coil-spring-shaped actuators equipped with temperature sensors.(3,4)*Corresponding author: e-mail: r_matsui@aitech.ac.jp

https://doi.org/10.18494/SAM5092

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

3382

Sensors and Materials, Vol. 36, No. 8 (2024)

In TiNi SMAs, the shape memory characteristics fundamentally arise from the martensitic
transformation of the parent austenite phase and its reverse transformation. For instance, when
TiNi SMAs are deformed, stress-induced martensitic transformation occurs. During thesubsequent heating process, shape recovery starts at the beginning of the reverse transformationand continues until the completion of the reverse transformation. Therefore, the transition
temperatures and functional properties of TiNi SMAs are strongly correlated to transformation
temperatures.(5–9)
In the previous research, the author proposed a manufacturing process for functionally
graded TiNi SMAs (FGSMAs), in which the transformation temperatures change gradually
within a material.(10) It is widely recognized that the Ni concentration can significantly affect thetransformation temperatures in TiNi SMAs,(7,11) and this property is actively utilized. For
example, if the entire material exhibits superelasticity and its deformation resistance can be
graded, a guidewire with both flexibility and operability could be developed. Conversely, if thematerial demonstrates a shape memory effect with a gradual change in shape recovery
temperature throughout, it could enable the creation of an actuator that operates independentlyof a power supply or sensor, with a stroke that adapts to the ambient temperature. Consequently,
FGSMAs have the potential to exhibit advanced functional properties not found in conventional
TiNi SMAs.
Several studies on imparting functionally graded properties to TiNi SMAs have beenreported.(12) For example, Mahmud et al.(13) proposed a method of applying different heat
treatments to various parts of cold-worked SMA wires. However, there are a few examples ofrealizing this in bulk form, and to the best of the author’s knowledge, there are no reports
detailing with the local deformation characteristics of FGSMAs.
In this study, the transformation properties, density, and hardness of the FGSMA generated
by the previously indicated procedure are elucidated. In addition, while applying tensile load to
the FGSMA, digital image correlation (DIC) was utilized to determine the local strain
distribution throughout the material.(14) Moreover, the gradient functional characteristics ofdeformation resistance were examined in detail. Through these efforts, it was discovered thatultrasonic shot peening (USP) can effectively induce the gradient functional characteristics.(15)2.

Materials and Methods

2.1

Fabrication process for FGSMA

Powder metallurgy and thermomechanical treatments constitute the production process for
the FGSMA proposed in this study and are essentially identical to the procedures outlined inRef. 10. Figure 1 presents a general overview of this process. Initially, pure Ti and Ni powders
with particle sizes of less than 45 µm and 3–5 µm, respectively, were utilized to produce thesintered body. The two powders were mixed using a planetary ball mill. The mixture ratio of Ti
and Ni powders was adjusted such that after alloying, the Ni concentration would vary from 49.8
to 51.0 at%, in increments of 0.3 at% (a total of five types). These mixed powders were stacked
in sequence in a graphite die. Subsequently, spark plasma sintering was performed at a sinteringSensors and Materials, Vol. 36, No. 8 (2024)

3383

Ti powder
Ni powder

Mixing51.0 50.7
50.7 50.4
50.4 50.1
51.0
50.1 49.8
49.8

Ni composition [at%]Spark Plasma
Sintering (SPS)

Filling
Wire Electric
Discharge Machining(WEDM)

1023 K，，1.8 ks
Solution Heat
Treatment(SHT)
1273 K，，43.2 ksSHT

Hot Rolling (HR) 30%
873 K

Shape
MemoryTreatment(SMT)
773 K，，3.6 ks

HR-SMT
HR-SMT-USP
Ultrasonic Shot
Peening (USP)

Fig. 1. (Color online) Fabrication process for FGSMA by a combination of powder metallurgy, hot rolling, and
USP treatment.

temperature of 1023 K and a holding time of 1.8 ks. During sintering, a compressive load of 10.4
kN was constantly applied in the axial direction of the powder compact to promote compaction.
The resulting cylindrical sintered compact had a diameter of 20 mm and a length of 29 mm
following sintering. Finally, solution heat treatment (SHT) was performed to homogenize thestructures and solid solutions of Ti and Ni. The SHT was maintained at 1273 K for 43.2 ks in Ar
atmosphere, followed by furnace cooling. Afterward, wire electrical discharge machining wasused to remove plate-shaped specimens, which measured a total of 29 mm in length, 5 mm inwidth, and 1.1 mm in thickness, from the sintered compact. The Ni concentration in the plateshaped specimen progressively varied along its longitudinal direction. The length of each region
corresponding to a Ni concentration was set to 3 mm, and the ends were set to 7 mm to serve as
grips during tensile testing. The specimens produced by the aforementioned procedure arecollectively referred to as SHT specimens.
Each SHT specimen was subsequently subjected to hot rolling for densification, which was
conducted at 873 K. The reduction rate (thickness reduction rate) R is defined ast −t
R = 0 1 × 100 (%) .
t0(1)Here, t0 represents the thickness before rolling (SHT specimen) and t1 the thickness after rolling.
In this study, R = 30% was achieved through multiple hot rolling processes. The shape memory
properties were then added to the hot-rolled plate by shape memory heat treatment. During thisheat treatment, the shape that was to be memorized was restrained to be flat. The temperature
for the shape memory heat treatment was 773 K, the holding time was 3.6 ks, and the cooling3384

Sensors and Materials, Vol. 36, No. 8 (2024)

method was water cooling. The material treated up to this point is referred to as the HR-SMT
specimen. To achieve work hardening and compressive residual stress in the plate material that
underwent shape memory heat treatment, levels for the amplitude of the vibration tool and thecoverage of USP are also set under the conditions summarized in Table 1. The material subjected
to USP treatment is denoted as the HR-SMT-USP specimen.
2.2

Tension test

Tension tests were conducted to investigate the overall deformation characteristics and local
deformation properties of FGSMAs. DIC was used to measure local strains during loading andunloading. The specimens were captured using a digital camera and DIC analysis was performed
using still images extracted from the recorded videos. Tension tests were conducted at room
temperature using a Shimadzu Ez-graph testing machine, and a Nikon D5300 camera was used
for image acquisition.
Specimens with random patterns were prepared for DIC analysis. Initially, each specimen
was completely coated with black spray paint, and then a random pattern was created by spraypainting in white. Using the digital images captured before and after deformation, this random
pattern made it possible to accurately analyze the strain distribution. A sample image of thespecimen used for DIC is shown in Fig. 2.
During the tension test, the crosshead displacement δ and gauge length L were used to
calculate the whole strain δ/L. The loading rate was set to a strain rate of 1%/min. The maximum
strain reached δ/L = 6%, after which unloading was performed at a rate of 1%/min. Subsequently,
the specimens were heated under stress-free conditions to examine shape-recovery strain, withthe heating temperature exceeding the reverse transformation finish temperature.

Table 1
USP treatment conditions.
Shot media
Material
52100 / AISI (SUJ2 / JIS)

Fig. 2.

Diameter
0.8 mmAmplitude of vibration tool

Coverage

70 μm

1000% (200 s)(Color online) Image of random-patterned specimen.

Sensors and Materials, Vol. 36, No. 8 (2024)

3.

Results and Discussion

3.1

Density and hardness

3385

The results of the investigation of density variations during the fabrication process, as
described in the previous section, are shown in Fig. 3. The plotted data represent the averagedensity and R of the ten test specimens. The horizontal error bars indicate the maximum andminimum R values during hot rolling, whereas the vertical error bars represent the maximum
and minimum densities. The secondary vertical axis shows the relative density calculated fromthe measured density of the cast TiNi SMA (6.44 g/cm³). Figure 3 shows that the density
increases steadily up to R of 20%. However, beyond 20%, the rate of increase slightly diminishes.
This correlation suggests a significant relationship between density and R. The results in Fig. 3
reveal a significant increase in density after hot rolling, from approximately 5.9 g/cm³ to 6.3 g/
cm³ (corresponding to a relative density of approximately 92 to 98%). This substantial increaseindicates the densification effect of hot rolling. After the USP treatment, there is a slight increasein the rolling reduction and density. In contrast, during shape memory heat treatment, R (in thiscase, corresponding to the reduction in specimen thickness after rolling) experiences a slightdecrease. This phenomenon results from the expansion of voids that were initially compressed
during rolling or USP treatment, triggered by shape recovery during heating.
The Vickers hardness of the specimens subjected to each procedure is shown in Fig. 4. The
Ni compositions shown in this figure are ideal values within a predefined region, and these areassumed to remain unchanged during each fabrication process. All fabrication procedures
resulted in an increase in the Vickers hardness within the regions that correspond to the various
amounts of Ni. Densification during hot rolling and work hardening during USP treatment were
the primary causes of this increase in hardness. Furthermore, focusing on the Vickers hardness
in each Ni concentration region, a phenomenon of increasing hardness with increasing Ni
concentration was observed at various stages of the manufacturing process. This phenomenon
was closely related to the local deformation characteristics discussed in Sect. 3.5.
3.2

Distribution of concentration

Micro X-Ray fluorescence (M4 TORNADO PLUS, BRUKER) analysis was performed toclarify the concentration distribution of Ni and Ti in the HR-SMT specimen. The results of thecompositional analysis are shown in Fig. 5. Figure 5(a) shows an optical image of the entire
specimen, and Fig. 5(b) shows an enlarged optical image of the analysis area. Figure 5(c) shows
the distribution of Ni and Fig. 5(d) shows that of Ti, with brighter areas indicating a higher
proportion of the corresponding element. Figure 5 qualitatively reveals that the Ni concentration
is lower and that of Ti is higher on the 49.8 at% Ni side, but the distributions of Ni and Ti arenonuniform even within the same concentration region. It is also observed that the Ti
concentration tends to be lower where the Ni concentration is locally higher.
This study indicates that the Ni concentration decreases globally towards the side with lower
Ni concentration. Conversely, it is necessary to achieve a more uniform concentration of each

3386

Sensors and Materials, Vol. 36, No. 8 (2024)

6.5

Density [g/cm3]

6.2
6.1
6.0
5.9
5.8

0

Fig. 3.

5

10

6.35

HR

6.31
6.29

98.6

USP

6.33

SMT
25
27
29
Rolling reduction [%]15

20

Rolling reduction [%]

25

98.3
98.0
97.7

Relative density [%]Density [g/cm3]

6.3

30

100
99
98
97
96
95
94
93
92
91

Relative density [%]6.4(Color online) Density changes of FGSMA with processing.

450
51.0

Vickers hardness[HV]

400

50.7

350

50.4
50.1
49.8

300
250

Ni composition [at%]

200
150

Fig. 4.

SHT

HR-SMT

HR-SMT-USP(Color online) Change in Vickers hardness of FGSMA with processing.

Ni composition

51.0 50.7 50.4 50.1 49.8

Analysis area

(a)

(b)

(c)

(d)
Fig. 5. (Color online) Concentration distributions determined by micro X-Ray fluorescence analysis: (a) opticalimage of entire specimen, (b) enlarged optical view of analysis area, and distributions of (c) Ni and (d) Ti.

Sensors and Materials, Vol. 36, No. 8 (2024)

3387

element to ensure a uniform deformation of the material within the same Ni concentration
region. This aspect will require further investigation in the future.
3.3

Transformation characteristics

For HR-SMT and HR-SMT-USP specimens, differential scanning calorimetry (DSC)
measurements were conducted to elucidate the transformation characteristics at positions
corresponding to each Ni concentration. The resulting DSC curves are shown in Figs. 6 and 7,
respectively. In both figures, (a) shows the cooling process, while (b) shows the heating process.
In addition, the transformation temperatures obtained from the DSC curves are summarized inTables 2 and 3.
In Figs. 6 and 7, particularly in the low Ni concentration region, the peaks associated with themartensitic transformation and its reverse transformation are separated into two distinct peaks.
To distinguish these peaks, the peak on the low-temperature side is labeled with the subscript 1
and the peak on the high-temperature side is labeled with the subscript 2. These figures show
that, for both materials, as the Ni concentration increased, the transformation temperatures
shifted toward lower values. This finding aligned with the general characteristics of TiNi SMAs.
Considering the phase separation into high- and low-temperature phases mentioned earlier,the increase in Ni concentration results in a larger volume of phase transformation at low
temperatures and a smaller volume of phase transformation at high temperatures. In addition,
both materials exhibit peaks associated with the rhombohedral phase (R phase), an intermediatephase between the martensitic and austenitic phases.

49.8

Cooling

50mW/gHeat flow per unit mass q [a.u.]

Heat flow per unit mass q [a.u.]

50mW/g

49.8
50.1
50.4
50.7

50.1
50.4
50.7

51.0
Heating

51.0

HR-SMT
150

200

Ni composition [at%]
250

300

350

Temperature T [K]

(a)

400

450

HR-SMT
150

200

Ni composition [at%]
250

300

350

Temperature T [K]

400

450

(b)

Fig. 6. (Color online) DSC curves of HR-SMT specimen during (a) cooling and (b) heating at various Ni
concentrations.

3388

Sensors and Materials, Vol. 36, No. 8 (2024)

Cooling

49.8
50mW/g

49.8

50.1

50.4
50.7

150

200

250

Ni composition [at%]
300

350

Temprature T [K]

(a)

400

50.4

50.7

51.0
Heating

51.0

HR-SMT-USP

50.1

Heat flow per unit mass q [a.u.]

Heat flow per unit mass q [a.u.]

50mW/g450

HR-SMT-USP
150

200

250

Ni composition [at%]
300

350

Temperture T [K]

400

450

(b)

Fig. 7. (Color online) DSC curves of HR-SMT-USP specimen during (a) cooling and (b) heating at various Ni
concentrations.
Table 2
Transformation temperatures of HR-SMT specimen at each Ni concentration position.
Transformation temperatures (K)Ni concentration
(at%)
M1f
M1s
Rf
Rs
M2f
M2s
A1s
49.8
—
—
—
—
331
348
—
50.1
232
269
285
294
334
348
289
50.4
233
268
285
296
—
—
290
50.7
235
259
284
296
—
—
290
51.0
236
262
284
294
—
—
292

A1f
—
306
309
303
306

A2s
361
362
—
—
—

A2f
381
380
—
—
—

Table 3
Transformation temperatures of HR-SMT-USP specimen at each Ni concentration position.
Transformation temperatures (K)Ni concentration
(at%)
M1f
M1s
Rf
Rs
M2f
M2s
A1s
A1f
49.8
—
—
283
296
—
348
290
301
50.1
231
266
285
296
324
351
290
307
50.4
233
266
285
296
—
—
291
311
50.7
234
262
285
298
—
—
291
307
51.0
234
264
285
296
—
—
291
307

A2s
360
351
—
—
—

A2f
383
383
—
—
—

Next, the changes in transformation characteristics due to the USP treatment are discussed.
From the DSC curves of the cooling and heating processes in Figs. 6 and 7, it is observed thatwhile significant changes do not appear on the high Ni concentration region upon USP treatment,
there is a slight broadening tendency in the low Ni concentration region. However, overall, theSensors and Materials, Vol. 36, No. 8 (2024)

3389

flattening of the peaks after USP treatment was minimal. Unlike conventional plastic
deformation, phase changes were not inhibited by the conditions employed for USP treatment.
Despite the improvement in Vickers hardness observed at each Ni concentration attributed to the
USP treatment (Fig. 4), the lack of significant changes in transformation characteristics suggests
differences in the evaluation regions in each experiment. The Vickers hardness test evaluates thesurface properties, whereas DSC evaluates the overall transformation characteristics, regardlessof the position in the thickness direction. Therefore, the results shown in Figs. 6 and 7 suggestthat the layer affected by the USP treatment remains confined to a shallow region near thesurface.
3.4

Residual stress

The effect of USP treatment on the residual stress distribution in the thickness direction at theposition corresponding to 51.0 at% Ni concentration was quantitatively examined. The obtained
results are shown in Fig. 8, indicating that the HR-SMT specimen, corresponding to the state
before USP treatment, does not exhibit compressive residual stress regardless of depth. However,for the HR-SMT-USP specimen, a compressive residual stress of approximately 300 MPa wasobserved at a depth of 20 µm, and this stress gradually decreased with increasing depth. These
results demonstrate that the USP treatment on the surface of the TiNi SMA produced by thisprocedure is effective in introducing compressive residual stress.
3.5

Deformation property

The stress–strain curves for HR-SMT and HR-SMT-USP specimens, as obtained through
DIC, are shown in Fig. 9. The horizontal axis in this figure represents the average local strain εxalong the specimen’s axial (longitudinal) direction, which corresponds to each Ni concentration
region. In these curves, stress plateaus are observed in both materials during loading; these are200

Ni composition : 51.0 at%

Residual stress [MPa]

100
0

-100
-200

HR-SMT-300

HR-SMT-USP-400

Fig. 8.

-20

0

20

40
60
Measuring depth [μm]

80

100

120

(Color online) Depth profiles of residual stress in HR-SMT and HR-SMT-USP specimens.

3390

Sensors and Materials, Vol. 36, No. 8 (2024)

350

HR-SMT
T=R.T.

300

50.4
50.7 51.0 50.1
49.8

250

Loading

200

Stress σ [MPa]Stress σ [MPa]350

Ni composition [at%]

150
100

250

Loading

Ni composition [at%]50.7 50.4

50.1 49.8

200

150

Unloading100

Unloading

50
0

300

HR-SMT-USP
51.0
T=R.T.

0

1

2
Local strain

(a)

3
[%]

4

50

5

0

0

1

2
Local strain

3
[%]

4

5

(b)

Fig. 9. (Color online) Stress–local strain curves at each area of Ni concentration in (a) HR-SMT and (b) HR-SMTUSP specimens, obtained by DIC.

characteristics of TiNi SMAs and are attributed to stress-induced martensitic transformations or
the rearrangement of martensite variants. Notably, compared with the HR-SMT specimen, the
HR-SMT-USP specimen typically exhibits higher stress levels of plateaus. This difference isattributed to the work hardening induced by USP treatment. As shown in Fig. 9(a), the HR-SMT
specimen does not exhibit a functionally graded behavior of deformation resistance. However,for the HR-SMT-USP specimen, the stress level increases with the Ni concentration, and the
maximum strain tends to decrease, resulting in a clear manifestation of the functionally gradedbehavior of deformation resistance. This trend aligns with the well-known characteristics ofTiNi SMAs, where increasing the Ni concentration leads to greater deformation resistance. As
observed in Figs. 4 and 7, an increase in Ni concentration results in a higher Vickers hardness
and lower transformation temperatures. This pattern suggests that the material exhibits more
prominent superelastic deformation properties as the deformation resistance increases in thehigh Ni concentration region. Therefore, the local deformation characteristics of the HR-SMTUSP specimen align with these known properties and trends. Figure 9 also shows that elastic
deformation appears in the initial stage of tensile loading and that the elastic modulus increases
with the Ni concentration. This is due to the difference in phase at room temperature where thetensile tests were performed. As shown in Fig. 7, the material is in the austenite phase in the highNi concentration region and in the martensitic phase in the low Ni concentration region at room
temperature. The lower elastic modulus of the martensitic phase is believed to be the reason forthis difference.(7) However, there may be other reasons for such large differences, and clarifying
this point is a subject for future research. Notably, both materials exhibit nearly complete strain
recovery upon heating after unloading.
For a quantitative examination, the local strain distributions along the central axis aremeasured in the HR-SMT-USP specimen, and the results are shown in Fig. 10. This figure shows
the local strain distributions during loading (σ = 195 MPa), at maximum stress, and after
unloading. At maximum stress, the local strain changes within the material and decreases withhigh Ni concentration, which is consistent with the local deformation characteristics observed inFig. 9. The fluctuation in local strain is likely affected by the Ni concentration variations asSensors and Materials, Vol. 36, No. 8 (2024)

Local strain

[%]

3
2.5

HR-SMT-USP
T=333 K

3391

Ni composition [at%]

51.0

50.7

50.4

50.1

49.8

2

MPa

1.5
1

Unloading
MPa

0.5
0

MPa

-12

-10

-8

-6
-4
-2
0
2
4
6
Position along longitudinal direction [mm]

8

10

12

Fig. 10. (Color online) Longitudinal local strain distribution in HR-SMT-USP specimen, obtained by DIC.
400

T=R.T.

350

SHT

Stress σ[MPa]300

HR-SMT-USP
HR-SMT

Loading

250

200
150
100

Unloading

50
0

0

1

2
3
Average Strain [%]4

5

Fig. 11. (Color online) Stress–average strain curves for each material, obtained by DIC.

discussed in Sect. 3.2 and the distribution of small voids, but further detailed investigation isnecessary.
The stress–strain diagrams for SHT, HR-SMT, and HR-SMT-USP specimens are shown in
Fig. 11. The horizontal axis represents the average local strain εx across the entire specimen,
determined by DIC. Although the maximum strains for the three materials do not coincide, thisdiscrepancy arises because the maximum strain is defined using the strain δ/L obtained from thecrosshead displacement. As shown in Fig. 11, the HR-SMT specimen has a slightly higher stressthan the SHT specimen at the beginning, but there is little difference in stress level between thetwo during the stress plateau. Dislocations are not considered to exist because the hot rolling was
at a sufficiently high temperature of 873 K. Therefore, the primary difference between the twolies in their densities. This is because denser materials have fewer voids, preventing microscopic
stress concentrations. As shown in Fig. 3, the absolute density of the HR-SMT specimen isapproximately 0.4 g/cm³ higher than that of the SHT specimen (approximately 5 points higher in
relative density). However, in this study, the observed density difference did not significantly
affect the deformation characteristics.

3392

4.

Sensors and Materials, Vol. 36, No. 8 (2024)

Conclusions

In this study, the author proposed FGSMAs to expand the application range of TiNi SMAs.
The manufacturing process primarily involved powder metallurgy, hot rolling, and USP
treatment as the main steps. The author revealed the effects of hot rolling and USP treatment bycarefully examining the properties of every specimen fabricated by this proposed method. The
main findings obtained in this study are as follows.
(1) USP treatment and hot rolling increase the Vickers hardness. The maximum increase a inVickers hardness are approximately 80 for hot rolling and 60 for USP treatment. Hot rolling
mostly resulted in densification, while USP treatment induced work hardening, leading toincreased hardness.
(2) The USP treatment can introduce a compressive residual stress of up to 300 MPa on thesurface of TiNi SMA sintered bodies.
(3) The transformation peaks in DSC curves shifted toward lower temperatures as the Ni content
increased. This pattern was consistent with the well-known properties of TiNi SMAs. The
USP treatment had no effect on the transformation characteristics of the FGSMA.
(4) The USP treatment work-hardened the sintered TiNi SMA produced through hot rolling.
Martensitic transformation stress increases up to 100 MPa by USP treatment. The effect wasmore pronounced in the high Ni concentration region and resulted in the prominent
appearance of the functionally graded behavior of deformation resistance.

Acknowledgments
I would like to thank K. Hattori for his technical assistance regarding USP treatment. This
work was supported by JSPS KAKENHI 19K05066.

References
1 J. A. Shaw and S. Kyriakides: J. Mech. Phys. Solids 43 (1995) 1243. https://doi.org/10.1016/00225096(95)00024-D
2 K. Tanaka, S. Kobayashi, and Y. Sato: Int. J. Plast. 2 (1986) 59. https://doi.org/10.1016/0749-6419(86)90016-1
3 J. M. Jani, M. Leary, A. Subic, and M. A. Gibson: Mater. Des. 56 (2014) 1078. https://doi.org/10.1016/j.
matdes.2013.11.084
4 H. Tobushi, K. Date, and K. Miyamoto: J. Solid Mech. Mater. Eng. 4 (2010) 1094. https://doi.org/10.1299/
jmmp.4.1094
5 E. A. Pieczyska, H. Tobushi, S. P. Gadaj, and W. K. Nowacki: Mater. Trans. 47 (2006) 670. https://doi.
org/10.2320/matertrans.47.670
6 E. A. Pieczyska, S. P. Gadaj, W. K. Nowacki, and H. Tobushi: Exp. Mech. 46 (2006) 531. https://doi.org/10.1007/
s11340-006-8351-y
7 K. Otsuka and X. Ren: Prog. Mater. Sci. 50 (2005) 511. https://doi.org/10.1016/j.pmatsci.2004.10.001
8 H. Miki, K. Tsuchiya, M. Ohtsuka, M. Gueltig, M. Kohl, and T. Takagi: Advances in Shape Memory Alloy, Q.
Sun, R. Matsui, K. Takeda, and E. A. Pieczyska, Eds. (Springer International Publishing, Cham, 2017) Chap.
11. https://doi.org/10.1007/978-3-319-53306-3_11
9 H. Tobushi, R. Matsui, K. Takeda, and E. A. Pieczyska: Mechanical Properties of Shape Memory Materials(Nova Science Publishers, New York, 2013) Chap. 1. https://novapublishers.com/shop/mechanical-propertiesof-shape-memory-materials/

Sensors and Materials, Vol. 36, No. 8 (2024)

3393

10 R. Matsui, K. Takeda, and H. Tobushi: Advances in Shape Memory Alloy, Q. Sun, R. Matsui, K. Takeda, andE. A. Pieczyska. Eds. (Springer International Publishing, Cham, 2017) Chap. 10. https://doi.org/10.1007/978-3319-53306-3_10
11 J. Frenzel, E. P. George, A. Dlouhy, Ch. Somsen, M. F.-X. Wagner, and G. Eggeler: Acta Mater. 58 (2010) 3444.
https://doi.org/10.1016/j.actamat.2010.02.019
12 B. S. Shariat, Q. Meng, A. S. Mahmud, Z. Wu, R. Bakhtiari, J. Zhang, F. Motazedian, H. Yang, G. Rio, T. Nam,and Y. Liu: Mater. Des. 124 (2017) 225. https://doi.org/10.1016/j.matdes.2017.03.069
13 A. S. Mahmud, Y. Liu, and T. Nam: Smart Mater. Struct. 17 (2008) 015031. https://doi.org/10.1088/09641726/17/01/015031
14 T. C. Chu, W. F. Ranson, and M. A. Sutton: Exp. Mech. 25 (1985) 232. https://doi.org/10.1007/BF02325092
15 M. Rakita, M. Wang, Q. Han, Y. Liu, F. Yin: Int. J. Comput. Mater. Sci. Surf. Eng. 5 (2013) 189. https://doi.
org/10.1504/IJCMSSE.2013.056948