Effect of Room Temperature Fine Particle Peening Pretreatment on Grain Refinement of Fe–Cr Alloys by AIH-FPP

Abstract

Fine particle peening (FPP) at room temperature was introduced to create finer grains at the surfaces of Fe–Cr alloys that were then treated by atmospheric controlled induction heating FPP (AIH-FPP). The effect of the proposed treatment on the grain refinement behavior of the alloys was investigated using optical microscopy and micro-Vickers hardness testing. The FPP pretreatment at room temperature increased the effectiveness of the grain refinement via dynamic recrystallization during subsequent AIH-FPP. Grain refinement at the surface of Fe–Cr alloys progressed with an increase in the cycles of the treatment. The addition of Cr in Fe–Cr alloys promotes the formation of fine grains during the treatment. Specimens with fine grains created via static recrystallization were also prepared and compared to grains formed via dynamic recrystallization by the room temperature FPP pretreatment and subsequent AIH-FPP to evaluate the effectiveness of the proposed treatment. The grains formed through dynamic recrystallization by the proposed treatment were finer than those formed through static recrystallization. These results indicate that the proposed combination of room temperature FPP and AIH-FPP is effective for the formation of fine grains at the surfaces of Fe–Cr alloys.

Fig. 9 Relationship between grain size of Fe–Cr alloys and number of cycles of room temperature FPP pretreatment and subsequent AIH-FPP (heating temperature: 1073 K).

1. Introduction

Grain refinement is an effective procedure to improve the fatigue strength of metallic materials. Various technologies, such as cold rolling,^1,2) warm rolling,³⁾ equal channel angular pressing,⁴⁾ and high-pressure torsion^5,6) are applied to obtain fine grains, and their effects on the mechanical properties of metals have been investigated. However, grain refinement generally leads to a reduction of ductility, and compatibility between ductility and fatigue strength is necessary.

One procedure to increase fatigue strength while maintaining ductility is the application of a surface modification that can produce fine grains at the surfaces of metals because fatigue cracks are generally initiated at the surface. Surface modification techniques through severe plastic deformation, such as shot peening,^7–11) laser shock peening,^12,13) wet peening,¹⁴⁾ ultrasonic nanocrystal surface modification,^15,16) and ultrasonic peening^17–21) have been employed to form surface layers with fine grains. Among these techniques, fine particle peening (FPP) is one of the most effective treatments, which involves the collision of fine particles (<200 µm diameter) with a material surface at high velocity.²²⁾ FPP has been shown to form grain-refined layers at the surfaces of metals.^23–32) Cho et al.²⁹⁾ demonstrated that the grain refinement during FPP occurred due to severe plastic deformation and the generation of numerous dislocations.

To effectively form fine grains at metal surfaces, the authors have developed a surface modification technique that combines FPP with induction heating (IH), which is referred to as IH-FPP.³³⁾ In the IH-FPP system, FPP can be performed for induction-heated metals, and the thermomechanical process can deform the metal surface at high temperatures. This has resulted in quenching and the formation of fine grains at the surfaces of AISI 440C³⁴⁾ and AISI 4135H³⁵⁾ alloy steels through dynamic recrystallization. Moreover, we have modified the system to control the processing atmosphere and inhibit oxidation of the treated material, which is referred to as atmospheric-controlled IH-FPP (AIH-FPP).³⁶⁾ AIH-FPP can also form fine grains at the surfaces of Fe–Cr alloys.³⁷⁾ In the AIH-FPP system, FPP can be performed at both room temperature and high temperature in the same chamber. FPP at room temperature can create dislocations with high density and fine grains at the metal surface.^22–32) It has also been demonstrated that the initial grain size affects the dynamic-recrystallized grain size, i.e., as the initial grain size becomes smaller, the dynamic-recrystallized grain also becomes finer.³⁸⁾ The AIH-FPP system is suitable to realize a procedure that effectively forms fine grains before the dynamic recrystallization; therefore, a treatment that combines FPP at room temperature and AIH-FPP is expected to be more effective than AIH-FPP alone.

In this study, pretreatment with FPP at room temperature and subsequent AIH-FPP were employed to produce fine grains at the surfaces of Fe–Cr alloys. The effects of the temperature during AIH-FPP, the number of cycles of the proposed treatment, and the Cr concentration of the alloy were investigated, and the effectiveness of the proposed treatment as a surface modification technique to produce fine grains is discussed.

2. Experimental Procedures

Two Fe–Cr alloys with 1.98 mass% Cr and 10.00 mass% Cr were used, with the balance being Fe; these alloys are referred to as Fe–2%Cr and Fe–10%Cr, respectively. The alloys were hot-rolled at 1273 K with a rolling reduction of 50% and air-cooled. The alloys were then heated at 1273 K for 1.8 ks and cooled in a furnace. The average grain sizes after these procedures were 241 µm in Fe–2%Cr and 137 µm in Fe–10%Cr. The average value and standard deviation of the Vickers hardness from five measurements were 90.6 ± 2.7 HV for Fe–2%Cr and 107.4 ± 1.0 HV for Fe–10%Cr. Plates of the alloys were machined into disk-shaped specimens with a 15 mm diameter and 4 mm thick. One end surface of a specimen was polished with emery paper and then mirror-finished using colloidal silica suspension.

Figure 1 shows a schematic illustration of the treatment system.³⁶⁾ The apparatus consists of an FPP nozzle and an IH coil in a sealed chamber. The combined treatment was performed using mirror-polished specimens. The specimen was set in the IH coil, and nitrogen gas was supplied through the nozzle to replace the atmosphere inside the chamber. The treatments were performed when the oxygen meter (measurement tolerance: 0.3 vol%) in the chamber showed 0.0 vol%. The conditions for the FPP treatment were as follows: particle supply rate, 1 g/s; peening pressure, 0.5 MPa; internal diameter of the nozzle, 4 mm; and nozzle distance, 100 mm. The shot particles were made of high-speed steel, and three different particle diameters were used, <45 µm, <150 µm and <250 µm. Figure 2 shows the thermal history during the treatment. The FPP time at room temperature and at high temperature was 30 s. The temperature during AIH-FPP was measured using a K-type thermocouple welded to the center of the specimen surface and controlled at 973 K or 1073 K by adjusting the power of the IH unit. For comparison, tensile tests and subsequent IH were performed to promote static recrystallization, which is described in section 3.3.

Fig. 1

Schematic illustration of the treatment system and magnified image of treatment region with the specimen and IH coil.

Fig. 2

Thermal history during the pretreatment with FPP at room temperature and subsequent AIH-FPP.

The microstructure at the surface of the specimens was observed using optical microscopy. Prior to observations, the specimens were cut, molded with resin, and polished to a mirror-finish. The polished surfaces of Fe–2%Cr and Fe–10%Cr were etched with Nital solution (a mixture of nitric acid (3 mL) and ethanol (97 mL)) and Vilella solution (a mixture of picric acid (1 mL), hydrochloric acid (4 mL) and ethanol (95 mL)), respectively. The grain size was calculated by the intercept method based on the optical micrographs. In the intercept method, lines were drawn at the positions where the grain boundaries were clearly visible near the surfaces. When a line with a length of L was drawn and the points where the line and grain boundaries crossed were P, the grain diameter D was calculated from the following equation:   

\begin{equation} D = L/P, \end{equation} (1)

temperature gradients in the radial direction of the disk-shaped specimen exist due to the skin effect of IH and the cooling effect of gas blowing. Accordingly, one optical micrograph, taken at the center of the specimen where the temperature during AIH-FPP was measured, was used to measure the grain diameter.

The hardness distributions were measured via micro-Vickers hardness tester at the cross-sections of the specimens. The force and holding time were 0.25 N and 20 s, respectively.

3. Results and Discussion

3.1 Effect of room temperature FPP on the surface characteristics of Fe–Cr alloy

To produce finer grains through dynamic recrystallization, the effect of the room temperature FPP pretreatment on the microstructure of the Fe–Cr alloys was examined. In particular, the effect of the shot particle diameter on the surface characteristics of the Fe–Cr alloys was investigated.

Figure 3 shows longitudinal optical micrographs of Fe–10%Cr treated with FPP at room temperature using different particle diameters. Stratification patterns were observed at the surfaces of the FPP-treated specimens, especially with particle diameters under 150 µm and 250 µm (Figs. 3(b) and (c)). The patterns were formed because of a concentration of plastic deformation and through a folding mechanism.³⁹⁾ In this stratification pattern, it is inferred that fine grains with several hundreds of nanometers were created^30,40) through the formation of dense dislocation walls and dislocation tangles, and the transformation of sub-boundaries with highly misoriented grain boundaries.⁴¹⁾ In contrast, no stratification patterns were observed on the surface treated with FPP using particles with diameters less than 45 µm. Figure 4 shows cross-sectional Vickers hardness distributions of Fe–10%Cr treated with FPP at room temperature. High hardness layers were created at the FPP-treated surfaces due to an increase of the dislocation density and grain refinement.²⁹⁾ The hardness value and thickness of the hardened layer increased when the particle diameter was increased from 45 µm to 150 µm, while the hardness value and thickness were almost the same for FPP with particle sizes of 150 µm and 250 µm. This implies that the increase of the dislocation density and grain refinement proceeds as the particle diameter is increased up to 150 µm.

Fig. 3

Longitudinal optical micrographs of Fe–10%Cr treated with FPP at room temperature with particle diameters under (a) 45 µm, (b) 150 µm and (c) 250 µm.

Fig. 4

Cross-sectional Vickers hardness distributions of Fe–10%Cr treated with FPP at room temperature.

The roughness of the FPP-treated surface generally increases with the particle diameter. However, the increase of surface roughness generally results in a reduction of the fatigue strength. Therefore, particles with diameters less than 150 µm were used as shot particles in the next section, which allowed the hardness to become saturated and prevented an increase of surface roughness.

3.2 Effect of AIH-FPP with room temperature FPP pretreatment on the microstructure of Fe–Cr alloys

After room temperature FPP pretreatment, AIH-FPP was performed at different heating temperatures (973 K or 1073 K) for the two Fe–Cr alloys to examine the effects of the heating temperature during AIH-FPP and the Cr concentration of the alloy on the grain refinement via dynamic recrystallization. Figure 5 shows longitudinal optical micrographs of Fe–Cr alloys after AIH-FPP at different heating temperatures. Fine grains with sizes smaller than that of the substrate (241 µm) were observed at the surfaces of Fe–2%Cr at both heating temperatures. This is because plastic deformation at high temperature due to IH and the high strain rate due to FPP leads to the occurrence of dynamic recrystallization.^34,35,37) The grains formed at the surface of Fe–2%Cr at 973 K were smaller than those obtained at 1073 K. This is because the Zener-Hollomon (Z) parameter, as discussed later, was higher at 973 K than at 1073 K. In the case of Fe–10%Cr, grain refinement occurred at the heating temperature of 1073 K during AIH-FPP whereas no recrystallization could be found at 973 K. Suzuki⁴²⁾ reported that the addition of Cr to ferrous materials increases their recrystallization temperatures. Therefore, the heating temperature at which fine grains can be formed through dynamic recrystallization increases with the Cr concentration in Fe–Cr alloys. Moreover, it was also revealed that grain sizes of the Fe–10%Cr were smaller than those of Fe–2%Cr when AIH-FPP was performed at 1073 K. This indicates that the addition of Cr to ferrous materials promotes the formation of fine grains during AIH-FPP with the room temperature FPP pretreatment, and this is the same trend as that when only AIH-FPP was performed for the Fe–Cr alloys.³⁷⁾

Fig. 5

Longitudinal optical micrographs of Fe–Cr alloys performed with AIH-FPP at different heating temperatures (treatment cycle: 1).

To investigate the effect of the number of cycles of the proposed treatment, the Fe–Cr alloys were treated at 1073 K with various cycle numbers. One cycle is equivalent to the thermal cycle shown in Fig. 2. Figure 6 shows longitudinal optical micrographs of Fe–Cr alloys pretreated with FPP at room temperature and then with AIH-FPP at 1073 K for different cycles. Micrographs obtained from the specimens treated with only AIH-FPP at 1073 K (without room temperature FPP pretreatment) are also shown for comparison. The grains formed at the surfaces treated for 1 cycle were 22.5 µm in Fe–2%Cr and 12.3 µm in Fe–10%Cr, which were finer than those obtained via only AIH-FPP at 1073 K (30.1 µm in Fe–2%Cr and 23.2 µm in Fe–10%Cr). This result indicates that a pretreatment with FPP at room temperature increases the effect of AIH-FPP on grain refinement. The sizes of grains generated by dynamic recrystallization are generally determined by the Z parameter, which is calculated using the strain rate, temperature and activation energy. Dynamic recrystallization with a higher Z parameter results in the formation of finer grains, although dynamic recrystallization does not occur in thermomechanical treatment with Z parameter higher than a critical value (Z_e). For AIH-FPP only and AIH-FPP with the room temperature FPP pretreatment, the strain rate, heating temperature and activation energy are expected to be same because AIH-FPP was performed under the same conditions. Therefore, the grain sizes seem not to change between these treatments. However, in the pretreatment with FPP at room temperature, grain refinement occurs by an increase of the dislocation density and the formation of cell structures. Maki et al.³⁸⁾ reported that the initial grain size has an effect on the dynamic-recrystallized grain size because Z_e increases with a decrease in the initial grain size, so that dynamic recrystallization was achieved with a higher Z parameter. Therefore, it is predicted that dynamic recrystallization with a higher Z parameter occurred during AIH-FPP with the room temperature FPP pretreatment, which resulted in a smaller dynamic-recrystallized grain size than that with only AIH-FPP.

Fig. 6

Longitudinal optical micrographs of Fe–Cr alloys performed with room temperature FPP pretreatment and AIH-FPP for different cycles (heating temperature: 1073 K).

Figure 6 shows that the grain sizes in both alloys decreased with an increase in the treatment cycles; therefore, an increase of the treatment cycle promotes grain refinement in the Fe–Cr alloys. When the grain sizes created at the surfaces of two Fe–Cr alloys treated with the same number of cycles were compared, Fe–10%Cr had smaller grains than Fe–2%Cr. This also suggests that the addition of Cr accelerates the formation of fine grains during the proposed treatment.

The results obtained in this section indicate that the room temperature FPP pretreatment is effective to produce fine grains during AIH-FPP, and the grain size is dependent on the Cr concentration of the Fe–Cr alloy, the number of treatment cycles, and the temperature during AIH-FPP.

3.3 Effectiveness of AIH-FPP with the room temperature FPP pretreatment on the grain refinement of Fe–Cr alloys

In this section, the effectiveness of AIH-FPP with the room temperature FPP pretreatment as a technique to produce fine grains in Fe–Cr alloys was investigated by a comparison with fine grains produced by static recrystallization. Static recrystallization was occurred via tensile tests with subsequent IH. The Fe–Cr alloys were first fractured by tensile test based on JIS Z 2241 to continuously vary the working ratio near the fracture area by necking, and the fractured specimens were then treated with IH for 30 s at various temperatures (893 K to 993 K) in air, which resulted in the occurrence of static recrystallization. The grains formed via static recrystallization were compared with the grains formed via dynamic recrystallization with room temperature FPP pretreatment and subsequent AIH-FPP to demonstrate the effectiveness of the proposed treatment.

To analyze the grains near the fracture surface, which would be refined through static recrystallization, the fractured specimens were cut, etched and observed using optical microscopy. Figure 7 shows longitudinal optical micrographs of Fe–Cr alloys treated with IH at 993 K after fracture by tensile tests. The white dashed lines show the fracture surfaces of the specimens. The grains were refined near the fracture surfaces due to static recrystallization. The relationship between the grain size and the working ratio (W_r) was obtained from micrographs of the specimens heated at various temperatures. W_r is defined as   

\begin{equation} W_{\text{r}} = (d_{0}-l)/d_{0}\times 100, \end{equation} (2)

where d₀ is the initial diameter of the tensile specimen at the gauge section (6 mm) and l is the length of the section where the grain sizes were calculated based on the intercept method, which was obtained from optical micrographs. W_r increased toward the fracture surface. Figure 8 shows the relationship between the grain size of the Fe–Cr alloys and W_r obtained for several heating temperatures. In Fe–10%Cr, static recrystallization did not occur by heating under 973 K. In both alloys, the grain sizes decreased with an increase in W_r, and the variation in the grain sizes was similar, regardless of the heating temperature. The results suggest that the minimum grain size shown in Fig. 8 is the minimum size that can be produced via static recrystallization. The minimum grain sizes of the Fe–2%Cr (D_min(2%Cr)) and Fe–10%Cr (D_min(10%Cr)) obtained via static recrystallization were 15.0 µm and 12.4 µm, respectively.

Fig. 7

Longitudinal optical micrographs of Fe–Cr alloys induction heated at 993 K after tensile fractures.

Fig. 8

Relationship between grain size of (a) Fe–2%Cr and (b) Fe–10%Cr and working ratio at several IH temperatures.

Figure 9 shows the relationship between the grain size of Fe–Cr alloys and the number of cycles of AIH-FPP at 1073 K and pretreatment with room temperature FPP. In both alloys, the grain sizes obtained through multiple cycles of the proposed treatment was smaller than those obtained through static recrystallization. The room temperature FPP pretreatment and AIH-FPP at 1073 K successfully refined the grains. Therefore, subsequent AIH-FPP could be carried out with a higher Z_e and Z parameter by room temperature FPP pretreatment, which resulted in the formation of finer grains by dynamic recrystallization.

Fig. 9

Relationship between grain size of Fe–Cr alloys and number of cycles of room temperature FPP pretreatment and subsequent AIH-FPP (heating temperature: 1073 K).

The results obtained in this section indicates that the combined room temperature FPP pretreatment and subsequent AIH-FPP is effective for the refinement of grains in Fe–Cr alloys.

4. Conclusions

The proposed treatment consisted of a room temperature FPP pretreatment and subsequent AIH-FPP. The procedure was performed for Fe–Cr alloys, and the effect of the proposed treatment on the grain refinement behavior at the surfaces of the alloys was investigated. The effectiveness of the proposed treatment as a grain refinement process was also discussed. The main conclusions of this study are as follows:

1. (1)    FPP at room temperature forms fine grains at the treated surface, which resulted in the formation of finer grains through dynamic recrystallization during subsequent AIH-FPP.
2. (2)    Grains at the surfaces of the Fe–Cr alloys were refined by an increase in the number of treatment cycles of room temperature FPP pretreatment and subsequent AIH-FPP.
3. (3)    The addition of Cr to Fe–Cr alloys promotes the formation of fine grains during the room temperature FPP pretreatment and subsequent AIH-FPP.
4. (4)    The grains formed due to dynamic recrystallization by room temperature FPP pretreatment and subsequent AIH-FPP were finer than those produced due to static recrystallization. Therefore, the proposed treatment is effective for the refinement of grains in Fe–Cr alloys.

Acknowledgments

The Fe–Cr alloys used in this study were supplied by Dr. Keishi Matsumoto (NIPPON STEEL CORPORATION). The authors are grateful for the support. We also thank Ikko Tanaka (Graduate School of Keio University) for the help with the experiments.

REFERENCES

• 1)   N.  Ide,  T.  Morita,  K.  Takahashi and  H.  Seto: Mater. Trans. 56 (2015) 1800–1806. doi:10.2320/matertrans.M2015247
• 2)   S.  Kikuchi,  A.  Ueno and  H.  Akebono: Int. J. Fatigue 139 (2020) 105772. doi:10.1016/j.ijfatigue.2020.105772
• 3)   Y.L.  Hao,  Z.B.  Zhang,  S.J.  Li and  R.  Yang: Acta Mater. 60 (2012) 2169–2177. doi:10.1016/j.actamat.2012.01.003
• 4)   Y.  Zhao,  H.  Guo,  M.W.  Fu,  Y.  Ning and  Z.  Yao: Mater. Des. 46 (2013) 889–894. doi:10.1016/j.matdes.2012.11.047
• 5)   H.  Yilmazer,  M.  Niinomi,  K.  Cho,  M.  Nakai,  J.  Hieda,  S.  Sato and  Y.  Todaka: Acta Mater. 80 (2014) 172–182. doi:10.1016/j.actamat.2014.07.041
• 6)   P.  Chen,  H.  Liu,  M.  Niinomi,  Z.  Horita,  H.  Fujii and  T.  Hanawa: Mater. Trans. 61 (2020) 361–367. doi:10.2320/matertrans.MT-M2019148
• 7)   S.M.  Hassani-Gangaraj,  A.  Moridi,  M.  Guagliano,  A.  Ghidini and  M.  Boniardi: Int. J. Fatigue 62 (2014) 67–76. doi:10.1016/j.ijfatigue.2013.04.017
• 8)   M.  Novelli,  J.-J.  Fundenberger,  P.  Bocher and  T.  Grosdidier: Appl. Surf. Sci. 389 (2016) 1169–1174. doi:10.1016/j.apsusc.2016.08.009
• 9)   Y.G.  Liu,  M.Q.  Li and  H.J.  Liu: Mater. Charact. 123 (2017) 83–90. doi:10.1016/j.matchar.2016.11.020
• 10)   O.  Unal,  E.  Maleki and  R.  Varol: Vacuum 150 (2018) 69–78. doi:10.1016/j.vacuum.2018.01.027
• 11)   Y.  Lv,  B.  Zhao,  H.  Zhang,  C.  Su,  B.  Nie,  R.  Wang,  L.  Cao and  F.  Lyu: Mater. Trans. 60 (2019) 1629–1637. doi:10.2320/matertrans.M2019031
• 12)   S.J.  Lainé,  K.M.  Knowles,  P.J.  Doorbar,  R.D.  Cutts and  D.  Rugg: Acta Mater. 123 (2017) 350–361. doi:10.1016/j.actamat.2016.10.044
• 13)   X.  Jin,  L.  Lan,  S.  Gao,  B.  He and  Y.  Rong: Mater. Sci. Eng. A 780 (2020) 139199. doi:10.1016/j.msea.2020.139199
• 14)   K.  Li,  X.  Fu,  R.  Li,  W.  Zhou and  Z.  Li: Mater. Des. 86 (2015) 761–764. doi:10.1016/j.matdes.2015.07.117
• 15)   B.  Wu,  P.  Wang,  Y.  Pyoun,  J.  Zhang and  R.  Murakami: Surf. Coat. Technol. 216 (2013) 191–198. doi:10.1016/j.surfcoat.2012.11.033
• 16)   Q.  Wang,  Y.  Li,  Z.  Lu,  Y.  Zhang and  Y.  Zou: Mater. Trans. 61 (2020) 1258–1264. doi:10.2320/matertrans.MT-M2019314
• 17)   W.P.  Tong,  C.Z.  Liu,  W.  Wang,  N.R.  Tao,  Z.B.  Wang,  L.  Zuo and  J.C.  He: Scr. Mater. 57 (2007) 533–536. doi:10.1016/j.scriptamat.2007.05.017
• 18)   S.  Wu,  K.  Fan,  P.  Jiang and  S.  Chen: Mater. Sci. Eng. A 527 (2010) 6917–6921. doi:10.1016/j.msea.2010.06.085
• 19)   H.  Gao,  W.  Chen and  Z.  Zhang: Mater. Lett. 176 (2016) 181–184. doi:10.1016/j.matlet.2016.04.104
• 20)   I.  Ghamarian,  P.  Samimi,  A.  Telang,  V.K.  Vasudevan and  P.C.  Collins: Mater. Sci. Eng. A 688 (2017) 524–531. doi:10.1016/j.msea.2017.02.029
• 21)   S.  Kumar,  K.  Chattopadhyay and  V.  Singh: J. Alloy. Compd. 724 (2017) 187–197. doi:10.1016/j.jallcom.2017.07.014
• 22)   T.  Ito,  S.  Kikuchi,  Y.  Hirota,  A.  Sasago and  J.  Komotori: Int. J. Mod. Phys. B 24 (2010) 3047–3052. doi:10.1142/S0217979210066069
• 23)   M.  Umemoto,  Y.  Todaka and  K.  Tsuchiya: Mater. Trans. 44 (2003) 1488–1493. doi:10.2320/matertrans.44.1488
• 24)   Y.  Hirota,  S.  Kikuchi and  J.  Komotori: J. Solid Mech. Mater. Eng. 2 (2008) 1330–1337. doi:10.1299/jmmp.2.1330
• 25)   S.  Kikuchi,  Y.  Nakahara and  J.  Komotori: Int. J. Fatigue 32 (2010) 403–410. doi:10.1016/j.ijfatigue.2009.07.019
• 26)   N.  Nakamura and  S.  Takagi: Mater. Trans. 52 (2011) 380–385. doi:10.2320/matertrans.L-M2010827
• 27)   T.  Morita,  H.  Nakaguchi,  S.  Noda and  C.  Kagaya: Mater. Trans. 53 (2012) 1938–1945. doi:10.2320/matertrans.M2012226
• 28)   S.  Kikuchi,  Y.  Yasutake and  J.  Komotori: J. Solid Mech. Mater. Eng. 6 (2012) 431–439. doi:10.1299/jmmp.6.431
• 29)   K.T.  Cho,  K.  Song,  S.H.  Oh,  Y.  Lee and  W.B.  Lee: Surf. Coat. Technol. 232 (2013) 912–919. doi:10.1016/j.surfcoat.2013.06.123
• 30)   S.  Kikuchi and  J.  Komotori: Mater. Trans. 56 (2015) 556–562. doi:10.2320/matertrans.M2014448
• 31)   Y.  Lv,  L.  Lei and  L.  Sun: Int. J. Fatigue 98 (2017) 121–130. doi:10.1016/j.ijfatigue.2017.01.017
• 32)   S.  Takesue,  S.  Kikuchi,  H.  Akebono,  Y.  Misaka and  J.  Komotori: Surf. Coat. Technol. 359 (2019) 476–484. doi:10.1016/j.surfcoat.2018.11.088
• 33)   A.  Sasago,  S.  Kikuchi,  Y.  Kameyama,  J.  Komotori,  K.  Fukazawa,  Y.  Misaka and  K.  Kawasaki: J. Japan Inst. Metals 72 (2008) 347–352. doi:10.2320/jinstmet.72.347
• 34)   S.  Kikuchi,  A.  Sasago and  J.  Komotori: J. Mater. Process. Technol. 209 (2009) 6156–6160. doi:10.1016/j.jmatprotec.2009.04.024
• 35)   T.  Harada,  S.  Kikuchi,  J.  Komotori,  K.  Fukazawa,  Y.  Misaka and  K.  Kawasaki: J. Soc. Mater. Sci. Jpn. 60 (2011) 1091–1096. doi:10.2472/jsms.60.1091
• 36)   T.  Ito,  S.  Kikuchi,  Y.  Kameyama,  J.  Komotori,  K.  Fukazawa,  Y.  Misaka and  K.  Kawasaki: J. Japan Inst. Metals 74 (2010) 533–539. doi:10.2320/jinstmet.74.533
• 37)   S.  Kikuchi,  I.  Tanaka,  S.  Takesue,  J.  Komotori and  K.  Matsumoto: Surf. Coat. Technol. 344 (2018) 410–417. doi:10.1016/j.surfcoat.2018.03.030
• 38)   T.  Maki,  K.  Akasaka,  K.  Okuno and  I.  Tamura: Tetsu-to-Hagané 66 (1980) 1659–1668. doi:10.2355/tetsutohagane1955.66.12_1659
• 39)   S.  Takagi,  M.  Kumagai,  Y.  Ito,  S.  Konuma and  E.  Shimodaira: Tetsu-to-Hagané 92 (2006) 318–326. doi:10.2355/tetsutohagane1955.92.5_318
• 40)   S.  Kikuchi,  Y.  Hirota and  J.  Komotori: J. Jpn. Soc. Abras. Technol. 54 (2010) 720–724.
• 41)   N.R.  Tao,  Z.B.  Wang,  W.P.  Tong,  M.L.  Sui,  J.  Lu and  K.  Lu: Acta Mater. 50 (2002) 4603–4616. doi:10.1016/S1359-6454(02)00310-5
• 42)   T.  Suzuki: Tetsu-to-Hagané 70 (1984) 1888–1897. doi:10.2355/tetsutohagane1955.70.15_1888