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Laser shock peening induced surface nanocrystallization and martensite transformation in austenitic stainless steel
Journal of Alloys and Compounds 655 (2016) 66e70
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Laser shock peening induced surface nanocrystallization and martensite transformation in austenitic stainless steel Liucheng Zhou, Weifeng He, Sihai Luo, Changbai Long, Cheng Wang, Xiangfan Nie, Guangyu He, XiaoJun Shen, Yinghong Li* Science and Technology on Plasma Dynamics Lab, Air Force Engineering University, Xi'an 710038, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 August 2014 Received in revised form 27 June 2015 Accepted 30 June 2015 Available online 4 September 2015
Surface nanocrystallization and deformation-induced martensite in AISI 304 stainless steel subjected to multiple laser shock peening (LSP) impacts were investigated by means of EBSD and TEM observations. A layer of isometric nanocrystalline with a size of 50e300 nm has been formed on the surface after three LSP impacts. And a grain refinement mechanism induced by ultra-high strain rate plastic (>107 s 1) deformation during multiple LSP impacts in AISI 304 stainless steel (AISI 304ss) is proposed based on the microstructural observations. Both multidirectional mechanical twins and multidirectional martensites bands led to grain subdivision at the top surface during multiple LSP impacts. © 2015 Published by Elsevier B.V.
Keywords: AISI 304ss Laser shock peening Surface nanocrystallization Martensite
1. Introduction Laser shock peening (LSP) can significantly improve the fatigue performance of metallic components by forming a considerable residual compressive stress and even grain refinement occurs on the surface of metal through the action of laser shock wave [1e3]. AISI 304 stainless steels (AISI 304ss) are widely used when both high strength and good corrosion resistance are required. The high pressure plasma shock wave was induced by laser irradiation on the materials and caused severe plastic deformation in the material [4]. In this work, a surface nanostructured layer was formed on the AISI 304ss after multiple laser shock peening treatments, and the reason of grain refinement was discussed. There are a number of reports for the grain refinement mechanism and martensite transformation of AISI 304ss by severe plastic deformation [5,6]. For example, according to Lu et al. [7], the ultrahigh strain rate plastic deformation leads to the generation of dislocation lines, dislocation tangles, dislocation walls and mechanical twins (MTs) in the original coarse grains when subjected to multiple LSP. Then the dislocation structures are transformed into subgrain boundaries, which finally evolve into nanoscale grain boundaries through the intersections of mechanical twins and
* Corresponding author. E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.jallcom.2015.06.268 0925-8388/© 2015 Published by Elsevier B.V.
dynamic recrystallization. Chen at el [8]. discussed the deformation mechanism of AISI 304ss subjected to surface impacts over a wide range of strain rates (10~105 s 1) and they found that the strain rate between 10 and 103 s 1 only activated dislocation motions and martensite transformations, resulting in nanocrystallines and ultrafine grains. However, higher strain rates (104~105 s 1) produced a high density of twin bundles with nanoscale thickness in the bulk material. Ye et al. [9] investigated the microstructure evolution of AISI 304ss by LSP at room and cryogenic temperature (liquid nitrogen temperature) respectively. They found that a nanostructured surface layer was synthesized after LSP at both room and cryogenic temperature and indicated that the deformation-induced martensite (DIM) was generated by LSP at room temperature only when the laser-generated plasma pressure is sufficiently high (>5.56 GPa). Luo et al. [10] indicated that LSP couldn't cause deformation-induced martensite. These phenomena may be due to the fact that there is an absorbing layer which avoids the thermal effect from heating the surface by the laser beam during LSP. Gerland and Hallouin [11] investigated the microstructure evolution in AISI 304ss by LSP with a very short laser pulse (0.6 ns) and extremely high laser intensity (250e1620 GW/cm2), and they found that the martensite embryos were formed at the intersections of deformation twins within the pressure range of 15e25 GPa. However, these studies do not agree on the conditions for DIM, neither on the role of DIM for grain refinement. The aim of this paper was to
L. Zhou et al. / Journal of Alloys and Compounds 655 (2016) 66e70
reveal the formation mechanism of surface nanocrystallines of AISI 304ss induced by multiple LSP treatment by means of Transmission Electron Microscopy observations (TEM), X-ray Diffraction (XRD) and Electron Back Scattered Diffraction (EBSD), and to study the effect of DIM for grain refinement. 2. Experimental procedure 2.1. Material Samples were cut by a water jet cutter from a plate (thickness 9.0 mm) of AISI 304ss and the sample was annealed in vacuum condition at 1080 C for 1 h. Prior to the LSP treatment, the surface of samples should be polished with SiC paper with the roughness ranging from 500 to 2400 to make the surface roughness at Ra 0.3 mm. Chemical composition of 304ss are shown in Table 1. 2.2. LSP experiments LSP experiments were performed using a Q-switched Nd:YAG laser operating at 1 Hz repetition rate with a wavelength of 1064 nm and the full width at half maximum of the pulses was about 20 ns. The laser beam size was 4 mm and the laser intensities used was 4.3 GW/cm2. Samples were submerged in a water bath, then processed by LSP. A water layer with a thickness of about 1 mm was used as the transparent confining layer and professional aluminum tape with a thickness of 100 mm was used as the absorbing layer. The application of the protective layer is to protect the surface of the metal from direct ablation and to promote a better coupling with the laser energy [12]. The water confinement is to confine the diffusion of the high temperature plasma produced by laser irradiation and to increase the pressure of shock wave [13]. 2.3. Measurement equipment & methods The microstructure change of the different layers in the treated samples subjected to LSP impacts was characterized by transmission electron microscopy (TEM). The cross-sectional TEM samples were made as follows: both sides of the samples were grinded to make its thickness less than 20 mm. By means of lowering the Ion Milling (Gatan691) from 4.8 kv to 3.2 kv and decreasing the angle from 15 to 4 to prepare the thin zone. This step takes 30 mins. The TEM foils at the surface were prepared by a combination of single and twin-jet electro polishing. The observation equipment is TEM-3010. The XRD qualitative analysis of phases of 304 alloy before and after LSP is conducted. The XRD analysis was obtained via MFS7000 X-ray diffraction equipment using Cu-Ka radiation, a takeoff angle of 6 . The generator settings were 40 kV and 35 mA. The diffraction data were collected over a 2q range of 30e80 , with a step width of 0.02 and a counting time of 5 s per step. Residual stress measurements were performed by a standard Xray diffraction technique according to the sin2J method in the equipment X-350A, using Cu-Ka radiation, the diffraction plane was {220} and the speed of the ladder scanning was 0.10 s 1. The Xray beam voltage and electricity were 26.0 kV, 6.0 mA, respectively. The micro-hardness change of the samples before and after laser shock processing was measured by a MVS-1000JMT2 micro-hardness test machine with a 200 g load and 10s holding time. An Table 1 Chemical composition of 304ss. Composition
C
Si
Mn
Cr
Ni
Fe
Percentage (wt.%)
0.08
1.0
2.0
18.0e20.0
8.0e10.5
balance
67
average of five measurements was used for each reported data point. 3. Results and discussion The changes of microstructure and mechanical performance after different LSP impacts as are shown in Fig. 1. Fig. 1(a) and (b) demonstrate the residual stress within a surface depth of 1600 mm and the Vickers microhardness within a surface depth of 1000 mm of the 1 and 3 LSP impacted AISI 304ss samples. The dotted lines in these two figures indicate the stress and the hardness of the non-impacted sample, namely, matrix stress and matrix hardness. It is clear that the LSP impact has a powerful effect on the mechanical properties of the AISI 304ss stainless steel, and a significant increase in residual stress and hardness is observed. Compared to 1 LSP impact, the effect of the 3 LSP impacts is stronger. In addition, the material surface has the maximal residual compressive stress and the maximal microhardness. For example, the 3 LSP impacted sample has a maximum compressive stress of 435 MPa and a maximum hardness of 279 HV0.5 in the surface, much larger than those of the non-impacted sample (about 15 MPa and 216 HV0.5, respectively). However, an increased depth gives rise to a sharp reduce in the stress and hardness, and they tend to be stable at a certain depth, 1400 and 800 mm, respectively. The enhanced mechanical properties of the impacted samples can be attributed to the deformation-induced martensite and surface nanocrystallization, as discussed later. Fig.1(c) shows the TEM image of the surface of AISI 304ss after 3 LSP treatments. It is observed that the laser shock has formed the 30e500 nm nanocrystallites on the surface of the sample. The corresponding diffraction pattern confirms the presence of the nanocrystallites with random orientation, which is dominated by circles, as is shown in Fig.1(d). Many literature consider that the twining is a prevalent deformation mechanism of AISI 304ss by severe plastic deformation for the grain refinement [4,6]. However, besides mechanical twining, we found that the DIM is an important deformation mechanism for grain refinement in the top surface at ultra-high strain rates in AISI 304ss subjected to multiple LSP impacts. The XRD qualitative analysis of phases of AISI 304ss treated by different impacts is conducted, as is shown in Fig. 2. Without regard to the influence of instrumental broadening, the Bragg diffraction peak of AISI 304ss have broadened, which indicates that the grain refinement, lattice deformation and the increasing of microstress have occurred in the surface layer of the alloy. Meanwhile, non-processed AISI 304ss only consisted of an austenite phase, but the DIMs took place on the surface layer subjected to different LSP impacts treatment. After LSP with laser intensity of 4.3 GW/cm2 and single impact, the two peaks (A200 and A220) corresponding to austenite disappeared, while the three peaks (M110, M200 and M211) corresponding to martensite shown up, indicating that the laser shock induced the formation of martensite. In the research of Ye [6], it also found that the DIM of AISI 304ss subjected to LSP, with the laser power density is higher than 5.6 GW/cm2 was needed. However, in our work, the DIM takes place after single impact with laser intensity of 4.3 GW/cm2. The number of impacts will also greatly affect DIM. After three impacts, three peaks (M110, M200 and M211) were intensified. On the other hand, the peaks (A111) corresponding to the austenite phase decrease in intensity with an increase in laser impacts, indicating that more and more austenite phase has transformed to martensite at more impacts. The purpose of multiple laser shock impacts is to provide longer time and more energy to plastic deformation and thus induces more martensite, and the volume fraction of martensite also increased.
68
L. Zhou et al. / Journal of Alloys and Compounds 655 (2016) 66e70
Fig. 1. (a) Residual compressive stress and (b) Vickers's microhardness of the surface layer of the 1 and 3 LSP impacted AISI 304ss samples. (c) Bright-filed TEM and (d) diffraction pattern images of the surface nanocrystallines of the 3 LSP impacted AISI 304ss sample
To further reveal the effect of DIM on the mechanism of grain refinement, we used EBSD to analyze the microstructure of AISI 304ss surface with and without LSP. The EBSD observations in the surface subjected to three LSP impacts are shown in Fig. 3. In which two typical deformation-induced microstructural
Fig. 2. XRD patterns of the AISI 304ss surface after LSP at different impacts.
features are identified: MTs and DIM. Fig. 3(aeb) shows the EBSD images of the top surface without LSP treatment, which shows the surface of AISI 304ss is mainly composed of austenite and small amount of martensite, which is consistent with Chen [8], this is because martensite transformation was generated at the process of annealed in 304ss. Fig. 3(ced) shows the EBSD images of the top surface subjected to three LSP impacts. It is found that the number of MTs and DIMs were induced by multiple LSP impacts. Fig. 3e and f are a magnified image of circle [A] and [B] in Fig. 3(c) and (d), showing the typical microstructure observed on the top surface. It can be seen that the martensite was induced by severe plastic deformation and its size was decided by the MTs, and DIM at different orientations were intercrossed, and then refine the grains to be between 50 and 300 nm. Meanwhile, the lath-shaped martensite was embossed and stratified at different orientations are shown in Fig. 3f. Such phenomenon was induced by the shearing, which was caused by the DIM. Owing to the presence of habit plane during phase transformation, there was slight orientation relationship between the impact-formed martensite phases. The formation mechanism of surface nanocrystalline was induced under multiple LSP impacts can be shown in Fig. 4. Each state will be discussed in terms of the experimental observations and literature. The high density dislocations are rapidly generated at the wave front by the multidirectional loads which are formed by reflection and refraction of the shock wave [14] as can be seen in Fig. 4 state(I). When the stress caused by the pile-up of dislocation increased to a certain level, MTs will be generated. The boundaries of these
L. Zhou et al. / Journal of Alloys and Compounds 655 (2016) 66e70
69
Fig. 3. EBSD maps of AISI 304ss after 3 LSP impacts; In the phase distribution map, different color represents different phases, red and blue represent austenite and martensite, respectively. In the grain orientation map, different color represents different grain orientation; (a) phase distribution without LSP, (b) grain orientation without LSP, (c) phase distribution with 3 LSP impacts; (d) grain orientation with 3 LSP impacts; (e) and (f) are the magnification of local area A and B in fig(c) an fig(d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
MTs are almost parallel, and they divide the original coarse grain into thin twin matrix lamellae at the top surface [15], as can be seen in Fig. 4 state (II). As the wave pressure further increases, the distortion become
stronger and stronger, finally the accumulated distortion make the original twins into intercrossed multiple twins [16]. The intercrossed MTs were accompanied by martensite transformation, whose size was decided by the size of the intercrossed twins. The DIMs were further accompanied by the deformation and intercrossing of twins, as can be seen in Fig. 4 state (III). With the interaction of MTs and DIMs induced by multiple LSP impacts, the coarse grain evolve continually and eventually a nanocrystalline layer with a size of 50e300 nm is formed in the surface of the AISI 304ss, as can be seen in Fig. 4 state (IV).
4. Conclusions
Fig. 4. Schematic illustration showing the microstructural evolution process of the AISI 304ss.
Conclusions can then be drawn as follows. Both multidirectional MTs and DIMs are crucial factors in the formation of refined grains during ultra-high strain rate plastic deformation of AISI 304ss under the multiple LSP impacts, as has been validated through both experiments and theoretical analysis. In summary, a surface nanocrystalline layer was formed in AISI 304ss samples by means of multiple laser shock peening. The intersectional MTs and DIMs aligned in multiple directions plays an important role in the grain refinement of coarse grains in the surface layer of AISI 304ss.
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Acknowledgments The authors acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51405507, 51205406) and National Basic Research Program of China (Grant No. 2015CB057400). The authors would like to thank Mr. HS. Jiao for EBSD sample preparation. References [1] B. Nikitin, H.J. Scholtes, I. Maier, Altenberger, Scr. Mater 50 (2004) 1345. [2] L.C. Zhou, Y.H. Li, W.F. He, G.Y. He, X.F. Nie, D.L. Chen, Z.L. Lai, Z.B. An, Mater. Sci. Eng. A 578 (2013) 181. [3] Patrick J. Golden, Alisha Hutson, Vasan Sundaram, H. James, Int. J. Fatigue 29 (2007) 1302. [4] J.-P. Cuq-Lelandais, M. Boustie, AIP Conf. Proc. 1426 (2012) 1167. [5] B.N. Mordyuk, Y.V. Milman, M.O. Lefimov, Surf. Coat. Technol. 202 (2008)
4875. [6] H.W. Zhang, Z.K. Hei, G. Liu, J. Lu, K. Lu, Acta Mater. 51 (2003) 1871. [7] J.Z. Lu, K.Y. Luo, Y.K. Zhang, G.F. Sun, Y.Y. Gu, J.Z. Zhou, X.D. Ren, X.C. Zhang, L.F. Zhang, K.M. Chen, C.Y. Cui, Y.F. Jiang, A.X. Feng, L. Zhang, Acta Mater. 58 (2010) 5354. [8] A.Y. Chen, H.H. Ruan, J. Wang, H.L. Chan, Q. Wang, Q. Li, J. Lu, Acta Mater. 59 (2011) 3697. [9] C. Ye, S. Suslov, D. Lin, G.J. Cheng, Philos. Mag. 92 (2012) 1369. [10] K.Y. Luo, J.Z. Lu, Y.K. Zhang, J.Z. Zhou, L.F. Zhang, F.Z. Dai, L. Zhang, J.W. Zhong, C.Y. Cui, Mater. Sci. Eng. A 528 (2011) 4783. [11] M. Gerland, M. Hallouin, J. Mater. Sci. 29 (1994) 345. [12] B.J. Demaske, V.V. Zhakhovsky, N. Inogamov, C.T. White, I.I. Oleynik, AIP Conf. Proc. 1426 (2012) 1163. [13] B.X. Wu, Y.C. Shin, Appl. Phys. Lett. 88 (2006) 041116. [14] M.A. Meyers, Dynamic Behavior of Materials, A Wiley-interscience Publication, New York, 1994. [15] M. Wen, G. Liu, J.F. Gu, W.M. Guan, J. Lu, Appl. Surf. Sci. 255 (2009) 6097. [16] X. Wu, N.R. Tao, Y. Hong, G. Liu, B. Xu, J. Lu, K. Lu, Acta Mater. 53 (2005) 681.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Laser shock peening induced surface nanocrystallization and martensite transformation in austenitic stainless steel Liucheng Zhou, Weifeng He, Sihai Luo, Changbai Long, Cheng Wang, Xiangfan Nie, Guangyu He, XiaoJun Shen, Yinghong Li* Science and Technology on Plasma Dynamics Lab, Air Force Engineering University, Xi'an 710038, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 August 2014 Received in revised form 27 June 2015 Accepted 30 June 2015 Available online 4 September 2015
Surface nanocrystallization and deformation-induced martensite in AISI 304 stainless steel subjected to multiple laser shock peening (LSP) impacts were investigated by means of EBSD and TEM observations. A layer of isometric nanocrystalline with a size of 50e300 nm has been formed on the surface after three LSP impacts. And a grain refinement mechanism induced by ultra-high strain rate plastic (>107 s 1) deformation during multiple LSP impacts in AISI 304 stainless steel (AISI 304ss) is proposed based on the microstructural observations. Both multidirectional mechanical twins and multidirectional martensites bands led to grain subdivision at the top surface during multiple LSP impacts. © 2015 Published by Elsevier B.V.
Keywords: AISI 304ss Laser shock peening Surface nanocrystallization Martensite
1. Introduction Laser shock peening (LSP) can significantly improve the fatigue performance of metallic components by forming a considerable residual compressive stress and even grain refinement occurs on the surface of metal through the action of laser shock wave [1e3]. AISI 304 stainless steels (AISI 304ss) are widely used when both high strength and good corrosion resistance are required. The high pressure plasma shock wave was induced by laser irradiation on the materials and caused severe plastic deformation in the material [4]. In this work, a surface nanostructured layer was formed on the AISI 304ss after multiple laser shock peening treatments, and the reason of grain refinement was discussed. There are a number of reports for the grain refinement mechanism and martensite transformation of AISI 304ss by severe plastic deformation [5,6]. For example, according to Lu et al. [7], the ultrahigh strain rate plastic deformation leads to the generation of dislocation lines, dislocation tangles, dislocation walls and mechanical twins (MTs) in the original coarse grains when subjected to multiple LSP. Then the dislocation structures are transformed into subgrain boundaries, which finally evolve into nanoscale grain boundaries through the intersections of mechanical twins and
* Corresponding author. E-mail address: [email protected] (Y. Li). http://dx.doi.org/10.1016/j.jallcom.2015.06.268 0925-8388/© 2015 Published by Elsevier B.V.
dynamic recrystallization. Chen at el [8]. discussed the deformation mechanism of AISI 304ss subjected to surface impacts over a wide range of strain rates (10~105 s 1) and they found that the strain rate between 10 and 103 s 1 only activated dislocation motions and martensite transformations, resulting in nanocrystallines and ultrafine grains. However, higher strain rates (104~105 s 1) produced a high density of twin bundles with nanoscale thickness in the bulk material. Ye et al. [9] investigated the microstructure evolution of AISI 304ss by LSP at room and cryogenic temperature (liquid nitrogen temperature) respectively. They found that a nanostructured surface layer was synthesized after LSP at both room and cryogenic temperature and indicated that the deformation-induced martensite (DIM) was generated by LSP at room temperature only when the laser-generated plasma pressure is sufficiently high (>5.56 GPa). Luo et al. [10] indicated that LSP couldn't cause deformation-induced martensite. These phenomena may be due to the fact that there is an absorbing layer which avoids the thermal effect from heating the surface by the laser beam during LSP. Gerland and Hallouin [11] investigated the microstructure evolution in AISI 304ss by LSP with a very short laser pulse (0.6 ns) and extremely high laser intensity (250e1620 GW/cm2), and they found that the martensite embryos were formed at the intersections of deformation twins within the pressure range of 15e25 GPa. However, these studies do not agree on the conditions for DIM, neither on the role of DIM for grain refinement. The aim of this paper was to
L. Zhou et al. / Journal of Alloys and Compounds 655 (2016) 66e70
reveal the formation mechanism of surface nanocrystallines of AISI 304ss induced by multiple LSP treatment by means of Transmission Electron Microscopy observations (TEM), X-ray Diffraction (XRD) and Electron Back Scattered Diffraction (EBSD), and to study the effect of DIM for grain refinement. 2. Experimental procedure 2.1. Material Samples were cut by a water jet cutter from a plate (thickness 9.0 mm) of AISI 304ss and the sample was annealed in vacuum condition at 1080 C for 1 h. Prior to the LSP treatment, the surface of samples should be polished with SiC paper with the roughness ranging from 500 to 2400 to make the surface roughness at Ra 0.3 mm. Chemical composition of 304ss are shown in Table 1. 2.2. LSP experiments LSP experiments were performed using a Q-switched Nd:YAG laser operating at 1 Hz repetition rate with a wavelength of 1064 nm and the full width at half maximum of the pulses was about 20 ns. The laser beam size was 4 mm and the laser intensities used was 4.3 GW/cm2. Samples were submerged in a water bath, then processed by LSP. A water layer with a thickness of about 1 mm was used as the transparent confining layer and professional aluminum tape with a thickness of 100 mm was used as the absorbing layer. The application of the protective layer is to protect the surface of the metal from direct ablation and to promote a better coupling with the laser energy [12]. The water confinement is to confine the diffusion of the high temperature plasma produced by laser irradiation and to increase the pressure of shock wave [13]. 2.3. Measurement equipment & methods The microstructure change of the different layers in the treated samples subjected to LSP impacts was characterized by transmission electron microscopy (TEM). The cross-sectional TEM samples were made as follows: both sides of the samples were grinded to make its thickness less than 20 mm. By means of lowering the Ion Milling (Gatan691) from 4.8 kv to 3.2 kv and decreasing the angle from 15 to 4 to prepare the thin zone. This step takes 30 mins. The TEM foils at the surface were prepared by a combination of single and twin-jet electro polishing. The observation equipment is TEM-3010. The XRD qualitative analysis of phases of 304 alloy before and after LSP is conducted. The XRD analysis was obtained via MFS7000 X-ray diffraction equipment using Cu-Ka radiation, a takeoff angle of 6 . The generator settings were 40 kV and 35 mA. The diffraction data were collected over a 2q range of 30e80 , with a step width of 0.02 and a counting time of 5 s per step. Residual stress measurements were performed by a standard Xray diffraction technique according to the sin2J method in the equipment X-350A, using Cu-Ka radiation, the diffraction plane was {220} and the speed of the ladder scanning was 0.10 s 1. The Xray beam voltage and electricity were 26.0 kV, 6.0 mA, respectively. The micro-hardness change of the samples before and after laser shock processing was measured by a MVS-1000JMT2 micro-hardness test machine with a 200 g load and 10s holding time. An Table 1 Chemical composition of 304ss. Composition
C
Si
Mn
Cr
Ni
Fe
Percentage (wt.%)
0.08
1.0
2.0
18.0e20.0
8.0e10.5
balance
67
average of five measurements was used for each reported data point. 3. Results and discussion The changes of microstructure and mechanical performance after different LSP impacts as are shown in Fig. 1. Fig. 1(a) and (b) demonstrate the residual stress within a surface depth of 1600 mm and the Vickers microhardness within a surface depth of 1000 mm of the 1 and 3 LSP impacted AISI 304ss samples. The dotted lines in these two figures indicate the stress and the hardness of the non-impacted sample, namely, matrix stress and matrix hardness. It is clear that the LSP impact has a powerful effect on the mechanical properties of the AISI 304ss stainless steel, and a significant increase in residual stress and hardness is observed. Compared to 1 LSP impact, the effect of the 3 LSP impacts is stronger. In addition, the material surface has the maximal residual compressive stress and the maximal microhardness. For example, the 3 LSP impacted sample has a maximum compressive stress of 435 MPa and a maximum hardness of 279 HV0.5 in the surface, much larger than those of the non-impacted sample (about 15 MPa and 216 HV0.5, respectively). However, an increased depth gives rise to a sharp reduce in the stress and hardness, and they tend to be stable at a certain depth, 1400 and 800 mm, respectively. The enhanced mechanical properties of the impacted samples can be attributed to the deformation-induced martensite and surface nanocrystallization, as discussed later. Fig.1(c) shows the TEM image of the surface of AISI 304ss after 3 LSP treatments. It is observed that the laser shock has formed the 30e500 nm nanocrystallites on the surface of the sample. The corresponding diffraction pattern confirms the presence of the nanocrystallites with random orientation, which is dominated by circles, as is shown in Fig.1(d). Many literature consider that the twining is a prevalent deformation mechanism of AISI 304ss by severe plastic deformation for the grain refinement [4,6]. However, besides mechanical twining, we found that the DIM is an important deformation mechanism for grain refinement in the top surface at ultra-high strain rates in AISI 304ss subjected to multiple LSP impacts. The XRD qualitative analysis of phases of AISI 304ss treated by different impacts is conducted, as is shown in Fig. 2. Without regard to the influence of instrumental broadening, the Bragg diffraction peak of AISI 304ss have broadened, which indicates that the grain refinement, lattice deformation and the increasing of microstress have occurred in the surface layer of the alloy. Meanwhile, non-processed AISI 304ss only consisted of an austenite phase, but the DIMs took place on the surface layer subjected to different LSP impacts treatment. After LSP with laser intensity of 4.3 GW/cm2 and single impact, the two peaks (A200 and A220) corresponding to austenite disappeared, while the three peaks (M110, M200 and M211) corresponding to martensite shown up, indicating that the laser shock induced the formation of martensite. In the research of Ye [6], it also found that the DIM of AISI 304ss subjected to LSP, with the laser power density is higher than 5.6 GW/cm2 was needed. However, in our work, the DIM takes place after single impact with laser intensity of 4.3 GW/cm2. The number of impacts will also greatly affect DIM. After three impacts, three peaks (M110, M200 and M211) were intensified. On the other hand, the peaks (A111) corresponding to the austenite phase decrease in intensity with an increase in laser impacts, indicating that more and more austenite phase has transformed to martensite at more impacts. The purpose of multiple laser shock impacts is to provide longer time and more energy to plastic deformation and thus induces more martensite, and the volume fraction of martensite also increased.
68
L. Zhou et al. / Journal of Alloys and Compounds 655 (2016) 66e70
Fig. 1. (a) Residual compressive stress and (b) Vickers's microhardness of the surface layer of the 1 and 3 LSP impacted AISI 304ss samples. (c) Bright-filed TEM and (d) diffraction pattern images of the surface nanocrystallines of the 3 LSP impacted AISI 304ss sample
To further reveal the effect of DIM on the mechanism of grain refinement, we used EBSD to analyze the microstructure of AISI 304ss surface with and without LSP. The EBSD observations in the surface subjected to three LSP impacts are shown in Fig. 3. In which two typical deformation-induced microstructural
Fig. 2. XRD patterns of the AISI 304ss surface after LSP at different impacts.
features are identified: MTs and DIM. Fig. 3(aeb) shows the EBSD images of the top surface without LSP treatment, which shows the surface of AISI 304ss is mainly composed of austenite and small amount of martensite, which is consistent with Chen [8], this is because martensite transformation was generated at the process of annealed in 304ss. Fig. 3(ced) shows the EBSD images of the top surface subjected to three LSP impacts. It is found that the number of MTs and DIMs were induced by multiple LSP impacts. Fig. 3e and f are a magnified image of circle [A] and [B] in Fig. 3(c) and (d), showing the typical microstructure observed on the top surface. It can be seen that the martensite was induced by severe plastic deformation and its size was decided by the MTs, and DIM at different orientations were intercrossed, and then refine the grains to be between 50 and 300 nm. Meanwhile, the lath-shaped martensite was embossed and stratified at different orientations are shown in Fig. 3f. Such phenomenon was induced by the shearing, which was caused by the DIM. Owing to the presence of habit plane during phase transformation, there was slight orientation relationship between the impact-formed martensite phases. The formation mechanism of surface nanocrystalline was induced under multiple LSP impacts can be shown in Fig. 4. Each state will be discussed in terms of the experimental observations and literature. The high density dislocations are rapidly generated at the wave front by the multidirectional loads which are formed by reflection and refraction of the shock wave [14] as can be seen in Fig. 4 state(I). When the stress caused by the pile-up of dislocation increased to a certain level, MTs will be generated. The boundaries of these
L. Zhou et al. / Journal of Alloys and Compounds 655 (2016) 66e70
69
Fig. 3. EBSD maps of AISI 304ss after 3 LSP impacts; In the phase distribution map, different color represents different phases, red and blue represent austenite and martensite, respectively. In the grain orientation map, different color represents different grain orientation; (a) phase distribution without LSP, (b) grain orientation without LSP, (c) phase distribution with 3 LSP impacts; (d) grain orientation with 3 LSP impacts; (e) and (f) are the magnification of local area A and B in fig(c) an fig(d). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
MTs are almost parallel, and they divide the original coarse grain into thin twin matrix lamellae at the top surface [15], as can be seen in Fig. 4 state (II). As the wave pressure further increases, the distortion become
stronger and stronger, finally the accumulated distortion make the original twins into intercrossed multiple twins [16]. The intercrossed MTs were accompanied by martensite transformation, whose size was decided by the size of the intercrossed twins. The DIMs were further accompanied by the deformation and intercrossing of twins, as can be seen in Fig. 4 state (III). With the interaction of MTs and DIMs induced by multiple LSP impacts, the coarse grain evolve continually and eventually a nanocrystalline layer with a size of 50e300 nm is formed in the surface of the AISI 304ss, as can be seen in Fig. 4 state (IV).
4. Conclusions
Fig. 4. Schematic illustration showing the microstructural evolution process of the AISI 304ss.
Conclusions can then be drawn as follows. Both multidirectional MTs and DIMs are crucial factors in the formation of refined grains during ultra-high strain rate plastic deformation of AISI 304ss under the multiple LSP impacts, as has been validated through both experiments and theoretical analysis. In summary, a surface nanocrystalline layer was formed in AISI 304ss samples by means of multiple laser shock peening. The intersectional MTs and DIMs aligned in multiple directions plays an important role in the grain refinement of coarse grains in the surface layer of AISI 304ss.
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Acknowledgments The authors acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51405507, 51205406) and National Basic Research Program of China (Grant No. 2015CB057400). The authors would like to thank Mr. HS. Jiao for EBSD sample preparation. References [1] B. Nikitin, H.J. Scholtes, I. Maier, Altenberger, Scr. Mater 50 (2004) 1345. [2] L.C. Zhou, Y.H. Li, W.F. He, G.Y. He, X.F. Nie, D.L. Chen, Z.L. Lai, Z.B. An, Mater. Sci. Eng. A 578 (2013) 181. [3] Patrick J. Golden, Alisha Hutson, Vasan Sundaram, H. James, Int. J. Fatigue 29 (2007) 1302. [4] J.-P. Cuq-Lelandais, M. Boustie, AIP Conf. Proc. 1426 (2012) 1167. [5] B.N. Mordyuk, Y.V. Milman, M.O. Lefimov, Surf. Coat. Technol. 202 (2008)
4875. [6] H.W. Zhang, Z.K. Hei, G. Liu, J. Lu, K. Lu, Acta Mater. 51 (2003) 1871. [7] J.Z. Lu, K.Y. Luo, Y.K. Zhang, G.F. Sun, Y.Y. Gu, J.Z. Zhou, X.D. Ren, X.C. Zhang, L.F. Zhang, K.M. Chen, C.Y. Cui, Y.F. Jiang, A.X. Feng, L. Zhang, Acta Mater. 58 (2010) 5354. [8] A.Y. Chen, H.H. Ruan, J. Wang, H.L. Chan, Q. Wang, Q. Li, J. Lu, Acta Mater. 59 (2011) 3697. [9] C. Ye, S. Suslov, D. Lin, G.J. Cheng, Philos. Mag. 92 (2012) 1369. [10] K.Y. Luo, J.Z. Lu, Y.K. Zhang, J.Z. Zhou, L.F. Zhang, F.Z. Dai, L. Zhang, J.W. Zhong, C.Y. Cui, Mater. Sci. Eng. A 528 (2011) 4783. [11] M. Gerland, M. Hallouin, J. Mater. Sci. 29 (1994) 345. [12] B.J. Demaske, V.V. Zhakhovsky, N. Inogamov, C.T. White, I.I. Oleynik, AIP Conf. Proc. 1426 (2012) 1163. [13] B.X. Wu, Y.C. Shin, Appl. Phys. Lett. 88 (2006) 041116. [14] M.A. Meyers, Dynamic Behavior of Materials, A Wiley-interscience Publication, New York, 1994. [15] M. Wen, G. Liu, J.F. Gu, W.M. Guan, J. Lu, Appl. Surf. Sci. 255 (2009) 6097. [16] X. Wu, N.R. Tao, Y. Hong, G. Liu, B. Xu, J. Lu, K. Lu, Acta Mater. 53 (2005) 681.