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Deforming TC6 titanium alloys at ultrahigh strain rates during multiple laser shock peening
Materials Science & Engineering A 578 (2013) 181–186
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Deforming TC6 titanium alloys at ultrahigh strain rates during multiple laser shock peening Liucheng Zhou n, Yinghong Li, Weifeng He, Guangyu He, Xiangfan Nie, Donglin Chen, Zhilin Lai, Zhibin An Science and Technology on Plasma Dynamics, Laboratory Air Force Engineering University, Xi'an 710038, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 November 2012 Received in revised form 1 March 2013 Accepted 18 April 2013 Available online 29 April 2013
The deformation mechanism of TC6 titanium alloys at ultrahigh strain rates (4106 s−1) during multiple LSP was investigated. When nanosecond pulse and 1000 MW laser irradiated on the materials, the high pressure plasma shock wave was induced and ultrahigh strain rates response was caused in the material. The TEM observation of the surface layer indicates that a layer of nanocrystalline has been formed and is unevenly distributed after a single impact. Increasing impact times could provide longer time and more energy to dislocation movement which refines the grains into smaller size and makes the distribution uniform. The hardness of TC6 titanium alloy has been improved by a single laser shock peening, forming a severe plastic deformted layer with a depth of 200 μm. Increasing impact times will improve the hardness and effective depth. The XRD test shows that the position of diffraction peak did not show any change, but the Bragg diffraction peak was broadened. Dislocation activity is a prevalent deformation mechanism of TC6 titanium alloy in the grain refinement study. & 2013 Elsevier B.V. All rights reserved.
Keywords: TC6 titanium alloys Laser shock Ultrahigh strain rates High density dislocation Surface nanocrystalline
1. Introduction The use of shockwaves allows us to investigate materials behavior under extreme conditions and presents a great research interest about its applications, such as Laser shock peening (LSP) [1]. When the nanosecond pulse and 1000 MW laser irradiate materials, a thin layer of the material surface vaporizes and plasma develops firstly. Under continuing laser irradiation, the pressure and temperature of the plasma increase dramatically which lead to the plasma explosion [2]. The explosion drives a high pressure shock wave from the irradiated surface. Due to the high pressure (2–10 GPa) and short impact time(20–100 ns) of shock wave, shock-loading provides not only very high deformation rates 107 s−1, but also a unique combination of high hydrostatic pressure and shear stress in the shocked material. It would lead to deep compressive residual stresses and grain refinement in the surface of materials, and also improve the mechanical property of the metals and alloys [3–6], as is shown in Fig. 1. In this paper, the TC6 titanium alloy is treated by laser shock peening. Titanium alloys are very attractive with low weight, high strength and outstanding intermediate temperature performance. Therefore, titanium alloys have been widely used to manufacture compressor blades and IBRs in aero-engines to decrease weight
n
Corresponding author. Tel.: +86 15319996476; fax: +86 84787527. E-mail address: [email protected] (L. Zhou).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.04.070
and increase thrust–weight ratio. However, titanium alloys have their weaknesses, such as low fatigue resistance, low hardness and wear-resisting property, etc. The reasons above often lead to fatigue induced crack faults and splits, and thus restrict the application of titanium blades. There are a number of shock simulations and experiments reported for the improvement of the mechanical properties of titanium alloy by LSP. Spanrad [7] reported the titanium alloy FOD specimens which were treated by LSP to generate protective residual stresses in the leading edge region prior to the impact. Results indicated that the LSP could inhibit the crack initiation and early crack growth in stress concentration region, and increase the fatigue strength of FOD specimens. Fretting fatigue experiments were conducted using LSP treated Ti–6Al–4V. At different stress levels, the fatigue lives was increased after LSP for 5, 10 and 25 fold separately compared to the lives of untreated specimens. While LSP treatment increases the fatigue life, it does not mitigate the formation of fretting cracks [8]. Zhang reported that LSP treatments with one single laser shock and two successive laser shocks respectively provide a 22.2% and 41.7% increase in the fatigue strength as compared with the asreceived specimens of Ti–6Al–4V. The maximum value and influence depth of the residual stress increase with the increase of the number of laser shock [9]. These works mainly researched on the influence of LSP on the residual stress and fatigue property of titanium alloy. Results showed that LSP could not only significantly improve the mechanical properties of materials, but also induce obvious deformation of microstructures.
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Fig. 1. Schematic illustrations of the plasma shock wave generated by nanosecond pulse and 1000 MW laser.
To characterize the deformation microstructures driven by multiple laser shock peening at high strain rates, Mordyuk et al. [10] conducted multiple LSP tests with the laser power density of 0.35 GW/cm2 on AISI 321 stainless steel and found the formation of dislocation-cell structures and highly tangled and dense dislocation arrangements in the austenitic surface layer of about 10 μm thick. Ye et al. [11] studied the dislocation structures induced by overlapping warm laser shock peening (WLSP) impacts on AISI 4140 steel and showed that increasing the work piece temperature to 250 1C produced fewer dislocation pile-ups but more dislocation tangles (DT) and a higher dislocation density. Lu et al. [12] investigated the plastic deformation behaviors of the LY2 aluminum alloy subjected to multiple LSP of laser pulse duration of 20 ns and energy of 5 J in the water confinement regime and studied the material micro structural strengthening mechanisms due to grain refinement induced by high pressure laser shock waves. Ding et al. [13] studied the prediction of the microstructural evolution of metallic components subjected to single or multiple LSP impacts. As discussed above, the majority of the reports are on the microstructural deformation induced by LSP. However, both the deformation mechanism and the effect of the impact time are seldemly studied. As one of the key parameters in LSP, the impact time can influence the mechanical property as well as the microstructural deformation. The study on the influence of impact time would help optimize parameters and improve strengthening effect. In this work, a nanostructured layer was found on the surface of TC6 titanium alloy after multiple laser shock peening. Meanwhile, we studied the plastic deformation behaviors and the grain refinement mechanism under the condition of high strain rates.
Table 1 Composition of TC6 titanium alloy. Composition
Al
Mo
Cr
Fe
Si
Ti
Percentage (wt%)
5.5–7.0
2.0–3.0
0.8–2.3
0.2–0.7
0.15–0.40
Bal
Table 2 The LSP parameters. Parameters Laser wavelength (nm) Pulse energy (J) Pulse duration (ns) Spot diameter (mm) Repetition-rate (Hz) System ASE energy (mJ) Export laser energy stability Lapping rate
1064 6 20 ns 3 1 Hz o 50 o 7 5% 60%
schematic plan of shock path, and also gives the pattern of Al foil after laser shock peening. In this paper, the samples were shocked with the same layer, but with the impact times of 1/5, respectively. During multiple LSP impacts the laser beam was perpendicular to the sample surface and the multiple laser pulses impacted at the same location on the sample, and the aluminum tape was replaced after each impact during multiple LSP impacts.
2.2. Measurement equipments and methods 2. Experiments 2.1. Principle and experimental procedure of LSP TC6 is primarily composed of the hexagonal close-packed (hcp) α phase with some body-centered cubic β (bcc) phase. The composition of TC6 titanium alloy was shown in Table 1. The samples of TC6 titanium alloy were cut into rectangular shapes with dimensions of 30 30 4 mm3 (width length thickness). Prior to the LSP treatment, the surface of samples were polished with SiC paper with different grades of roughness (from 500 to 2400), followed by cleaning in deionized water. Ultrasound in ethanol was used to degrease the surface of samples, and LSP experiments were conducted shortly after preparation. 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 μm was used as the absorbing layer to protect the sample surface from thermal effects. The processing parameters used in LSP are shown in detail in Table 2. Fig. 1 also illustrates the
Vickers's microhardness on the cross-sections of LSP specimens was measured with the indentation load of 500 g and the dwell time of 10 s, and with MVS-1000JMT2 microhardness tester. The micro-structural evolution of the different layers of the treated samples subjected to multiple LSP impacts was characterized by transmission electron microscopy (TEM). The cross-sectional TEM samples were made as follows: both sides of the samples were ground to make its thickness less than 20 μm. By means of lowering the Ion Milling (Gatan691) from 4.8 kv to 3.2 kv and decreasing the angle from 151 to 41 to prepare the thin zone. This step takes 30 mins. The TEM foils at different depths were made as follow: TEM foils at different depths into the surface were prepared by a combination of single and twin-jet electro polishing. The XRD qualitative analysis of phases of the TC6 titanium alloy before and after the LSP treatment is conducted. The XRD analysis was obtained via MFS-7000 X-ray diffraction equipment using Cu Ka radiation, with a take-off angle of 61. The generator settings were 40 kV and 35 mA. The diffraction data were collected over a 2θ range of 30–801, with a step width of 0.021 and the time for each step is 5 s.
L. Zhou et al. / Materials Science & Engineering A 578 (2013) 181–186
3. Results and discussion 3.1. Hardness Hardness is a basic mechanical property of material and it can be aptly defined as the resistance offered by the material to indentation. The Vickers microhardness in the cross section of the TC6 titanium alloy sample with different shock impacts is shown in Fig. 2. The basic hardness of TC6 is 336. After one impact, its surface hardness becomes 396, improved by 17.8%; after five impacts, the surface hardness becomes 406, improved by 20.8%. LSP is effective to improve the hardness of titanium alloy materials, but increasing the numbers of impact would not obviously improve the hardness. From Fig. 2 we know that the gradient of hardness has changed on the cross section, it decreases from the surface vertically and tends to be stable at a certain depth. This depth is called the affected layer. In the affected layer, the hardness value will increase with more impacts at the same depth, and the affected layer becomes larger with the increasing impact times. The depth of the affected layer of TC6 titanium alloy is about 200 μm after a single impact, while this is increased to 600 μm after five impacts.
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impacts. The SAED (selected area electron diffraction) shows a random orientation of nano-grains (Fig. 3a), and large angle orientations between adjacent grains. From the structural image, we may also see many nonlinear grain boundaries which are bent and unequal. Meanwhile, in the image the contrasts of many grain boundaries are unclear, and the diffraction contrasts of the grains are also inconsistent and complex, which indicates a high level of both internal stress and distortion of lattice. Fig. 4 shows a high-resolution image which further amplifies the adjacent structures of the impacted surface. From the image, we find an even amorphous layer with its depth of 10 nm where the atoms are out of order. In some works about the grain refinement that induced by severe plastic deformation, the mosaic structure of nanocrystalline/amorphous appeared in some materials. This work assumes that the amorphous is formed from the interactions between many dislocations and the high-density stacking faults. Dislocations bring deformations to surrounding lattices, while the arrangement of atoms is out of order under the effect of high-density stacking faults, and thus the amorphous phase will be formed. The XRD tests have been finished on the surface of specimens treated by different impacts. As seen in Fig. 5, without regard to the influence of instrumental broadening, the
3.2. TEM observation and XRD The size of the cross-section TEM is 500 nm 1000 nm 200 nm, the parameter of laser is 6 J/20 ns/3 mm/5-times. As is known from Fig. 3 that an even layer of nanostructure has been formed on the surface of the titanium alloy after five laser shock
Fig. 2. Vickers's microhardness as a function of depth for the laser peening with 1/5 impacts
Fig. 4. An Ambios white-light profilometry image of the surface after 5 times laser shock treatment. Cross-sectional HRTEM image of the amorphous and nanocrystalline layers at the shock surface of the TC6 titanium alloy.
Fig. 3. Cross-section microstructure of TC6 titanium alloy after 5 impacts, (a) bright-filed images, (b) SAED patterns for A region, and (c) SAED patterns for B region.
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the dislocation deformation. When the deviatoric elastic stresses at the front reach a critical level, dislocations are generated. And the high density dislocation relaxes the deviatoric stresses that elastically distort the original lattice. It is proposed that to generate the homogeneous dislocation the shock pressure need to meet the following condition [14]. τh =G ¼ 0:054
Fig. 5. XRD analysis of surface microstructure for the laser peening with 1/5 impacts.
Fig. 6. The laser-induced shock decay inside the material. The regions of A–B and B–C along the shock decay represent the typical corresponding microstructures at different depths. The severe plastic deformation is induced by laser shock wave.
Bragg diffraction peak of TC6 titanium alloy has broadened, which indicates that the grain refinement, lattice deformation and the increasing of microstress have occurred in the surface layer of the alloy. Meanwhile, with the increasing of impact times, the spectral peak of X-ray diffraction and its position are basically unchanged, indicating that the surface of TC6 titanium alloy still consists of α phase and β phase, and no new phase is generated. In the previous TEM observation the thickness of amorphous phase is only 10 nm, while in this test the XRD reflects the statistical information of the surface layer with a depth of 10 μm, which is hard to be characterized. 3.3. Discussion on grain refinement mechanism We further make the depth-directional TEM foils, and observe the microstructure of TC6 titanium alloy at different depths. As is shown in Fig. 6, layer A–B is mainly consisted of nanocrystallines, layer B–C is mainly consisted of different structures of high density dislocations. These different structures along the depth are related to the pressure of the shock wave. Fig. 6 also shows a typical laser-induced pressure wave propagating a sample of titanium alloy. The pressure of shock wave decreases exponentially along the depth of material. For the grain refinement mechanism that induced by laser plasma shock wave, dislocation is its main formation reason. Meyers put forward the homogeneous nucleation theory about
ð1Þ
τh and G are the shearing stresses needed to generate homogeneous dislocation and shear modulus, respectively. When the maximum shearing stress caused by shock wave (τ¼6GP(1−2υ)/E) is greater than τh, the homogeneous dislocation will be generated. For TC6 titanium alloy (E ¼113 GPa), the threshold pressure is about 2.99 GPa. According to the Fabbro shock wave pressure calculation model and the PVDF piezoelectric film experiment [15], when the laser power density is about 6 GW/cm2, the surge pressure induced by laser is about 4 GPa, which is larger than the threshold pressure. For TC6 titanium alloy, when the pressure of the shock wave decays to 2.9 GPa, its corresponding depth is 5 μm, and the TEM image can be seen in Fig. 7(a). The dislocations are deformed at the wave front and the multidirectional loads are formed by reflection and refraction of the shock wave. During the propagation of the shock wave, the slip system of some titanium alloy grains which reach the critical resolved shear stress also contributes to the formation of the dislocation. With the rise of the wave pressure, high density dislocations changed into dislocation walls and dislocation cells by slip, accumulation, and interaction as is shown in Fig. 7(b) and (c). During the LSP of the titanium alloy, because of the reflection and refraction of the shock wave at boundaries and defects, the multidirectional load is generated, which will change the directions of the slips between grains as well as those inside the grains. Interactions of the dislocations occur not only in a same slip system but also between different slip systems. Therefore, grains can be refined more effectively with the formations of dislocation walls and tangles when compared to other simple deformation ways. Treated by the laser shock wave, a layer of nanocrystalline with a size of 100 nm is eventually formed on the surface of the titanium alloy, with a round but unevenly-distributed diffraction, as is shown in Fig. 7(d). Meanwhile, in this titanium alloy many typical microstructures in Fig. 6 have been formed at different depths. The purpose of multiple shock is to provide longer time and more energy to dislocation movement which saturates the plastic deformation and thus refines the grains into smaller size. Fig. 7 (e) and (f) are the bright-field image and dark-field image of the surface microstructures after five impacts. It can be seen that the crystalline grains in the surface layer of different samples are equiaxed. The grain size is about 30–60 nm. The SAED patterns of titanium alloy after 5 times of laser shock treatment present continuous, homogeneous and broadened concentric rings, which indicates that the grain size is very tiny and the grains are random in crystallographic orientation. 3.4. Influence of microstructure on mechanical properties It is widely understood that mechanical properties of titanium alloys strongly depend on the microstructure. Both the increase of dislocation density and the change of crystal structure will improve the hardness of a material. For polycrystalline metals, the typical Taylor formula about the hardness (Δh) and dislocation density (ρ) is given [13,16,17] pffiffiffi Δh ¼ kh MαGb ρ ð2Þ
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Fig. 7. TEM images of different depths after 5 impacts; (a) high density dislocation at the grain boundary at the depth of 5–20 μm; (b) high density dislocation pileup at the depth of 5–20 μm; (c) dislocation cell at the depth of 1–5 μm; (d) surface nanocrystallines after 1 impact; (e) bright-filed image of surface nanocrystallines after 5 impacts; and (f) dark-field image of surface nanocrystallines after 5 impacts.
where M is orientation factor; G is shear modulus; and b is Burgers vector; a is a constant; kh is a constant slope. Increasing the dislocation density will improve the hardness of a material. It can be known from the above analysis that the laser shock wave will decay inside the material when it is impacting on the surface of the titanium alloy, and the pressure of the wave decreases. Low deformation rate leads to low dislocation, which means that the hardness decreases as the depth decreases. Besides, when the wave decays to a certain level that cannot induce the deformation between atoms, the hardness tend to reach a stable value. After one impact, the surface strain of the
titanium alloy reaches its maximal value, generating the high density dislocation and refining the grains, and plastic deformation tends to saturate. Increasing the impact times would not improve the hardness obviously but will provide more energy and time to the dislocation inside the material, thus plastic deformation will be further generated and microstructures at the same depth will be distributed more evenly. That is, reflected by hardness curve, the hardness will be improved as the numbers of impact increases. The surface nanostructure formed in this way is different from the surface nanocrystallization by mechanical processing
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mechanisms such as surface mechanical attrition treatment (SMAT) [18], ultrasonic shot peening (USSP) [19], and high energy shot peening [20], The principle of USSP is that some grains in the surface layer may reach a critical resolved shear stress by a single shot, and the slip system is activated and then generates dislocations. If the directions of the following shots are changed, other slip systems of the grains will be activated which will lead to grain refinement. This process involves hundreds of thousands shots. The accumulation of plastic deformation expands gradually to the inner layer of the material to refine grains. In this process, the deformation rate is low. While the surface grain refinement induced by laser plasma shock wave is a plastic deformation at a very high rate. 4. Conclusion The main conclusions obtained in this study are as follows: (i) The laser-induced high pressure plasma shock wave will lead to ultrahigh strain rate dynamic response in material. When the wave reaches a certain critical value, high density dislocations will form rapidly on or near the wave front. As the wave pressure rises the dislocations will move, and thus nanocrystallines can be formed rapidly. (ii) After several impacts, equiaxial nanocrystallines are formed and evenly distributed on the surface of titanium alloy, with the size of 30–60 nm. Meanwhile, an amorphous layer with a depth of 10 nm is formed on the surface. (iii) Laser shock could improve the hardness within a certain depth. Increasing impact times could improve its amplitude and effective depth. The refinement of grains and the formation of high density dislocations are primary reasons for improving the hardness.
References [1] J.P. Cuq-Lelandais, M. Boustie, L. Soulard, L. Berthe, J. Bontaz-Carion, T. de Resseguier, AIP Conf. Proc. 1426 (2012) 1167–1170. [2] P. Peyre, R. Fabbro, Opt. Quantum Electron. 27 (1995) 1213–1229. [3] Y. Sano, M. Obata, T. Kubo, N. Mukai, M. Yoda, K. Masaki, Y. Ochi, Mater. Sci. Eng. A 417 (2006) 334–340. [4] J.M. Yang, Y.C. Her, N.L. Han, A. Clauer, Mater. Sci. Eng. A. 298 (2001) 296–299. [5] K. Masaki, Y. Ochi, T. Matsumura, Y. Sano, Mater. Sci. Eng. A468–470 (2007) 171–175. [6] H. Luong, M.R. Hill, Mater. Sci. Eng. A 477 (2008) 208–216. [7] S. Spanrad, J. Tong, Procedia Eng. 2 (2010) 1751–1759. [8] P.J. Golden, A. Hutson, V. Sundaram, J.H. Arps, Int. J. Fatigue 29 (2007) 1302–1310. [9] X.C. Zhang, Y.K. Zhang, J.Z. Lu, F.Z. Xuan, Z.D. Wang, S.T. Tu, Mater. Sci. Eng. A 527 (2010) 3411–3415. [10] B.N. Mordyuk, Y.V. Milman, M.O. Iefimov, G.I. Prokopenko, V.V. Silberschmidt, M.I. Danylenko, A.V. Kotko, Surf. Coat. Technol. 202 (2008) 4875–4883. [11] C. Ye, S. Suslov, B.J. Kim, E.A. Stach, G.J. Cheng, Acta Mater. 59 (2011) 1014–1025. [12] J.Z. Lu, K.Y. Luo, Y.K. Zhang, C.Y. Cui, G.F. Sun, J.Z. Zhou, L. Zhang, J. You, K.M. Chen, J.W. Zhong, Acta Mater. 58 (2010) 3984–3994. [13] H.T. Ding, Y.C. Shin, Comput. Mater. Sci. 53 (2012) 79–88. [14] M.A. Meyers, Dynamic Behavior of Materials, A Wiley-interscience Publication, New York, 1994. [15] R. Fabbro, J. Foumier, P. Ballard, D. Devaux, J. Virmont, J. Appl. Phys. 68 (1990) 775–784. [16] B.L. Li, A. Godfrey, Q.C. Meng, Q. Liu, N. Hansen, Acta Mater. 52 (2004) 1069–1081. [17] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci. 45 (2000) 103–189. [18] K. Lu, J. Lu, J. Mater. Sci. Tech. 15 (1999) 193–197. [19] N.R. Tao, Z.B. Wang, W.P. Tong, M.L. Sui, J. Lu, K. Lu, Acta Mater. 50 (2002) 4603–4616. [20] G. Liu, S.C. Wang, X.F. Lou, J. Lu, K. Lu, Scr. Mater. 44 (2001) 1791–1795.
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Deforming TC6 titanium alloys at ultrahigh strain rates during multiple laser shock peening Liucheng Zhou n, Yinghong Li, Weifeng He, Guangyu He, Xiangfan Nie, Donglin Chen, Zhilin Lai, Zhibin An Science and Technology on Plasma Dynamics, Laboratory Air Force Engineering University, Xi'an 710038, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 November 2012 Received in revised form 1 March 2013 Accepted 18 April 2013 Available online 29 April 2013
The deformation mechanism of TC6 titanium alloys at ultrahigh strain rates (4106 s−1) during multiple LSP was investigated. When nanosecond pulse and 1000 MW laser irradiated on the materials, the high pressure plasma shock wave was induced and ultrahigh strain rates response was caused in the material. The TEM observation of the surface layer indicates that a layer of nanocrystalline has been formed and is unevenly distributed after a single impact. Increasing impact times could provide longer time and more energy to dislocation movement which refines the grains into smaller size and makes the distribution uniform. The hardness of TC6 titanium alloy has been improved by a single laser shock peening, forming a severe plastic deformted layer with a depth of 200 μm. Increasing impact times will improve the hardness and effective depth. The XRD test shows that the position of diffraction peak did not show any change, but the Bragg diffraction peak was broadened. Dislocation activity is a prevalent deformation mechanism of TC6 titanium alloy in the grain refinement study. & 2013 Elsevier B.V. All rights reserved.
Keywords: TC6 titanium alloys Laser shock Ultrahigh strain rates High density dislocation Surface nanocrystalline
1. Introduction The use of shockwaves allows us to investigate materials behavior under extreme conditions and presents a great research interest about its applications, such as Laser shock peening (LSP) [1]. When the nanosecond pulse and 1000 MW laser irradiate materials, a thin layer of the material surface vaporizes and plasma develops firstly. Under continuing laser irradiation, the pressure and temperature of the plasma increase dramatically which lead to the plasma explosion [2]. The explosion drives a high pressure shock wave from the irradiated surface. Due to the high pressure (2–10 GPa) and short impact time(20–100 ns) of shock wave, shock-loading provides not only very high deformation rates 107 s−1, but also a unique combination of high hydrostatic pressure and shear stress in the shocked material. It would lead to deep compressive residual stresses and grain refinement in the surface of materials, and also improve the mechanical property of the metals and alloys [3–6], as is shown in Fig. 1. In this paper, the TC6 titanium alloy is treated by laser shock peening. Titanium alloys are very attractive with low weight, high strength and outstanding intermediate temperature performance. Therefore, titanium alloys have been widely used to manufacture compressor blades and IBRs in aero-engines to decrease weight
n
Corresponding author. Tel.: +86 15319996476; fax: +86 84787527. E-mail address: [email protected] (L. Zhou).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.04.070
and increase thrust–weight ratio. However, titanium alloys have their weaknesses, such as low fatigue resistance, low hardness and wear-resisting property, etc. The reasons above often lead to fatigue induced crack faults and splits, and thus restrict the application of titanium blades. There are a number of shock simulations and experiments reported for the improvement of the mechanical properties of titanium alloy by LSP. Spanrad [7] reported the titanium alloy FOD specimens which were treated by LSP to generate protective residual stresses in the leading edge region prior to the impact. Results indicated that the LSP could inhibit the crack initiation and early crack growth in stress concentration region, and increase the fatigue strength of FOD specimens. Fretting fatigue experiments were conducted using LSP treated Ti–6Al–4V. At different stress levels, the fatigue lives was increased after LSP for 5, 10 and 25 fold separately compared to the lives of untreated specimens. While LSP treatment increases the fatigue life, it does not mitigate the formation of fretting cracks [8]. Zhang reported that LSP treatments with one single laser shock and two successive laser shocks respectively provide a 22.2% and 41.7% increase in the fatigue strength as compared with the asreceived specimens of Ti–6Al–4V. The maximum value and influence depth of the residual stress increase with the increase of the number of laser shock [9]. These works mainly researched on the influence of LSP on the residual stress and fatigue property of titanium alloy. Results showed that LSP could not only significantly improve the mechanical properties of materials, but also induce obvious deformation of microstructures.
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L. Zhou et al. / Materials Science & Engineering A 578 (2013) 181–186
Fig. 1. Schematic illustrations of the plasma shock wave generated by nanosecond pulse and 1000 MW laser.
To characterize the deformation microstructures driven by multiple laser shock peening at high strain rates, Mordyuk et al. [10] conducted multiple LSP tests with the laser power density of 0.35 GW/cm2 on AISI 321 stainless steel and found the formation of dislocation-cell structures and highly tangled and dense dislocation arrangements in the austenitic surface layer of about 10 μm thick. Ye et al. [11] studied the dislocation structures induced by overlapping warm laser shock peening (WLSP) impacts on AISI 4140 steel and showed that increasing the work piece temperature to 250 1C produced fewer dislocation pile-ups but more dislocation tangles (DT) and a higher dislocation density. Lu et al. [12] investigated the plastic deformation behaviors of the LY2 aluminum alloy subjected to multiple LSP of laser pulse duration of 20 ns and energy of 5 J in the water confinement regime and studied the material micro structural strengthening mechanisms due to grain refinement induced by high pressure laser shock waves. Ding et al. [13] studied the prediction of the microstructural evolution of metallic components subjected to single or multiple LSP impacts. As discussed above, the majority of the reports are on the microstructural deformation induced by LSP. However, both the deformation mechanism and the effect of the impact time are seldemly studied. As one of the key parameters in LSP, the impact time can influence the mechanical property as well as the microstructural deformation. The study on the influence of impact time would help optimize parameters and improve strengthening effect. In this work, a nanostructured layer was found on the surface of TC6 titanium alloy after multiple laser shock peening. Meanwhile, we studied the plastic deformation behaviors and the grain refinement mechanism under the condition of high strain rates.
Table 1 Composition of TC6 titanium alloy. Composition
Al
Mo
Cr
Fe
Si
Ti
Percentage (wt%)
5.5–7.0
2.0–3.0
0.8–2.3
0.2–0.7
0.15–0.40
Bal
Table 2 The LSP parameters. Parameters Laser wavelength (nm) Pulse energy (J) Pulse duration (ns) Spot diameter (mm) Repetition-rate (Hz) System ASE energy (mJ) Export laser energy stability Lapping rate
1064 6 20 ns 3 1 Hz o 50 o 7 5% 60%
schematic plan of shock path, and also gives the pattern of Al foil after laser shock peening. In this paper, the samples were shocked with the same layer, but with the impact times of 1/5, respectively. During multiple LSP impacts the laser beam was perpendicular to the sample surface and the multiple laser pulses impacted at the same location on the sample, and the aluminum tape was replaced after each impact during multiple LSP impacts.
2.2. Measurement equipments and methods 2. Experiments 2.1. Principle and experimental procedure of LSP TC6 is primarily composed of the hexagonal close-packed (hcp) α phase with some body-centered cubic β (bcc) phase. The composition of TC6 titanium alloy was shown in Table 1. The samples of TC6 titanium alloy were cut into rectangular shapes with dimensions of 30 30 4 mm3 (width length thickness). Prior to the LSP treatment, the surface of samples were polished with SiC paper with different grades of roughness (from 500 to 2400), followed by cleaning in deionized water. Ultrasound in ethanol was used to degrease the surface of samples, and LSP experiments were conducted shortly after preparation. 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 μm was used as the absorbing layer to protect the sample surface from thermal effects. The processing parameters used in LSP are shown in detail in Table 2. Fig. 1 also illustrates the
Vickers's microhardness on the cross-sections of LSP specimens was measured with the indentation load of 500 g and the dwell time of 10 s, and with MVS-1000JMT2 microhardness tester. The micro-structural evolution of the different layers of the treated samples subjected to multiple LSP impacts was characterized by transmission electron microscopy (TEM). The cross-sectional TEM samples were made as follows: both sides of the samples were ground to make its thickness less than 20 μm. By means of lowering the Ion Milling (Gatan691) from 4.8 kv to 3.2 kv and decreasing the angle from 151 to 41 to prepare the thin zone. This step takes 30 mins. The TEM foils at different depths were made as follow: TEM foils at different depths into the surface were prepared by a combination of single and twin-jet electro polishing. The XRD qualitative analysis of phases of the TC6 titanium alloy before and after the LSP treatment is conducted. The XRD analysis was obtained via MFS-7000 X-ray diffraction equipment using Cu Ka radiation, with a take-off angle of 61. The generator settings were 40 kV and 35 mA. The diffraction data were collected over a 2θ range of 30–801, with a step width of 0.021 and the time for each step is 5 s.
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3. Results and discussion 3.1. Hardness Hardness is a basic mechanical property of material and it can be aptly defined as the resistance offered by the material to indentation. The Vickers microhardness in the cross section of the TC6 titanium alloy sample with different shock impacts is shown in Fig. 2. The basic hardness of TC6 is 336. After one impact, its surface hardness becomes 396, improved by 17.8%; after five impacts, the surface hardness becomes 406, improved by 20.8%. LSP is effective to improve the hardness of titanium alloy materials, but increasing the numbers of impact would not obviously improve the hardness. From Fig. 2 we know that the gradient of hardness has changed on the cross section, it decreases from the surface vertically and tends to be stable at a certain depth. This depth is called the affected layer. In the affected layer, the hardness value will increase with more impacts at the same depth, and the affected layer becomes larger with the increasing impact times. The depth of the affected layer of TC6 titanium alloy is about 200 μm after a single impact, while this is increased to 600 μm after five impacts.
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impacts. The SAED (selected area electron diffraction) shows a random orientation of nano-grains (Fig. 3a), and large angle orientations between adjacent grains. From the structural image, we may also see many nonlinear grain boundaries which are bent and unequal. Meanwhile, in the image the contrasts of many grain boundaries are unclear, and the diffraction contrasts of the grains are also inconsistent and complex, which indicates a high level of both internal stress and distortion of lattice. Fig. 4 shows a high-resolution image which further amplifies the adjacent structures of the impacted surface. From the image, we find an even amorphous layer with its depth of 10 nm where the atoms are out of order. In some works about the grain refinement that induced by severe plastic deformation, the mosaic structure of nanocrystalline/amorphous appeared in some materials. This work assumes that the amorphous is formed from the interactions between many dislocations and the high-density stacking faults. Dislocations bring deformations to surrounding lattices, while the arrangement of atoms is out of order under the effect of high-density stacking faults, and thus the amorphous phase will be formed. The XRD tests have been finished on the surface of specimens treated by different impacts. As seen in Fig. 5, without regard to the influence of instrumental broadening, the
3.2. TEM observation and XRD The size of the cross-section TEM is 500 nm 1000 nm 200 nm, the parameter of laser is 6 J/20 ns/3 mm/5-times. As is known from Fig. 3 that an even layer of nanostructure has been formed on the surface of the titanium alloy after five laser shock
Fig. 2. Vickers's microhardness as a function of depth for the laser peening with 1/5 impacts
Fig. 4. An Ambios white-light profilometry image of the surface after 5 times laser shock treatment. Cross-sectional HRTEM image of the amorphous and nanocrystalline layers at the shock surface of the TC6 titanium alloy.
Fig. 3. Cross-section microstructure of TC6 titanium alloy after 5 impacts, (a) bright-filed images, (b) SAED patterns for A region, and (c) SAED patterns for B region.
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the dislocation deformation. When the deviatoric elastic stresses at the front reach a critical level, dislocations are generated. And the high density dislocation relaxes the deviatoric stresses that elastically distort the original lattice. It is proposed that to generate the homogeneous dislocation the shock pressure need to meet the following condition [14]. τh =G ¼ 0:054
Fig. 5. XRD analysis of surface microstructure for the laser peening with 1/5 impacts.
Fig. 6. The laser-induced shock decay inside the material. The regions of A–B and B–C along the shock decay represent the typical corresponding microstructures at different depths. The severe plastic deformation is induced by laser shock wave.
Bragg diffraction peak of TC6 titanium alloy has broadened, which indicates that the grain refinement, lattice deformation and the increasing of microstress have occurred in the surface layer of the alloy. Meanwhile, with the increasing of impact times, the spectral peak of X-ray diffraction and its position are basically unchanged, indicating that the surface of TC6 titanium alloy still consists of α phase and β phase, and no new phase is generated. In the previous TEM observation the thickness of amorphous phase is only 10 nm, while in this test the XRD reflects the statistical information of the surface layer with a depth of 10 μm, which is hard to be characterized. 3.3. Discussion on grain refinement mechanism We further make the depth-directional TEM foils, and observe the microstructure of TC6 titanium alloy at different depths. As is shown in Fig. 6, layer A–B is mainly consisted of nanocrystallines, layer B–C is mainly consisted of different structures of high density dislocations. These different structures along the depth are related to the pressure of the shock wave. Fig. 6 also shows a typical laser-induced pressure wave propagating a sample of titanium alloy. The pressure of shock wave decreases exponentially along the depth of material. For the grain refinement mechanism that induced by laser plasma shock wave, dislocation is its main formation reason. Meyers put forward the homogeneous nucleation theory about
ð1Þ
τh and G are the shearing stresses needed to generate homogeneous dislocation and shear modulus, respectively. When the maximum shearing stress caused by shock wave (τ¼6GP(1−2υ)/E) is greater than τh, the homogeneous dislocation will be generated. For TC6 titanium alloy (E ¼113 GPa), the threshold pressure is about 2.99 GPa. According to the Fabbro shock wave pressure calculation model and the PVDF piezoelectric film experiment [15], when the laser power density is about 6 GW/cm2, the surge pressure induced by laser is about 4 GPa, which is larger than the threshold pressure. For TC6 titanium alloy, when the pressure of the shock wave decays to 2.9 GPa, its corresponding depth is 5 μm, and the TEM image can be seen in Fig. 7(a). The dislocations are deformed at the wave front and the multidirectional loads are formed by reflection and refraction of the shock wave. During the propagation of the shock wave, the slip system of some titanium alloy grains which reach the critical resolved shear stress also contributes to the formation of the dislocation. With the rise of the wave pressure, high density dislocations changed into dislocation walls and dislocation cells by slip, accumulation, and interaction as is shown in Fig. 7(b) and (c). During the LSP of the titanium alloy, because of the reflection and refraction of the shock wave at boundaries and defects, the multidirectional load is generated, which will change the directions of the slips between grains as well as those inside the grains. Interactions of the dislocations occur not only in a same slip system but also between different slip systems. Therefore, grains can be refined more effectively with the formations of dislocation walls and tangles when compared to other simple deformation ways. Treated by the laser shock wave, a layer of nanocrystalline with a size of 100 nm is eventually formed on the surface of the titanium alloy, with a round but unevenly-distributed diffraction, as is shown in Fig. 7(d). Meanwhile, in this titanium alloy many typical microstructures in Fig. 6 have been formed at different depths. The purpose of multiple shock is to provide longer time and more energy to dislocation movement which saturates the plastic deformation and thus refines the grains into smaller size. Fig. 7 (e) and (f) are the bright-field image and dark-field image of the surface microstructures after five impacts. It can be seen that the crystalline grains in the surface layer of different samples are equiaxed. The grain size is about 30–60 nm. The SAED patterns of titanium alloy after 5 times of laser shock treatment present continuous, homogeneous and broadened concentric rings, which indicates that the grain size is very tiny and the grains are random in crystallographic orientation. 3.4. Influence of microstructure on mechanical properties It is widely understood that mechanical properties of titanium alloys strongly depend on the microstructure. Both the increase of dislocation density and the change of crystal structure will improve the hardness of a material. For polycrystalline metals, the typical Taylor formula about the hardness (Δh) and dislocation density (ρ) is given [13,16,17] pffiffiffi Δh ¼ kh MαGb ρ ð2Þ
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Fig. 7. TEM images of different depths after 5 impacts; (a) high density dislocation at the grain boundary at the depth of 5–20 μm; (b) high density dislocation pileup at the depth of 5–20 μm; (c) dislocation cell at the depth of 1–5 μm; (d) surface nanocrystallines after 1 impact; (e) bright-filed image of surface nanocrystallines after 5 impacts; and (f) dark-field image of surface nanocrystallines after 5 impacts.
where M is orientation factor; G is shear modulus; and b is Burgers vector; a is a constant; kh is a constant slope. Increasing the dislocation density will improve the hardness of a material. It can be known from the above analysis that the laser shock wave will decay inside the material when it is impacting on the surface of the titanium alloy, and the pressure of the wave decreases. Low deformation rate leads to low dislocation, which means that the hardness decreases as the depth decreases. Besides, when the wave decays to a certain level that cannot induce the deformation between atoms, the hardness tend to reach a stable value. After one impact, the surface strain of the
titanium alloy reaches its maximal value, generating the high density dislocation and refining the grains, and plastic deformation tends to saturate. Increasing the impact times would not improve the hardness obviously but will provide more energy and time to the dislocation inside the material, thus plastic deformation will be further generated and microstructures at the same depth will be distributed more evenly. That is, reflected by hardness curve, the hardness will be improved as the numbers of impact increases. The surface nanostructure formed in this way is different from the surface nanocrystallization by mechanical processing
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mechanisms such as surface mechanical attrition treatment (SMAT) [18], ultrasonic shot peening (USSP) [19], and high energy shot peening [20], The principle of USSP is that some grains in the surface layer may reach a critical resolved shear stress by a single shot, and the slip system is activated and then generates dislocations. If the directions of the following shots are changed, other slip systems of the grains will be activated which will lead to grain refinement. This process involves hundreds of thousands shots. The accumulation of plastic deformation expands gradually to the inner layer of the material to refine grains. In this process, the deformation rate is low. While the surface grain refinement induced by laser plasma shock wave is a plastic deformation at a very high rate. 4. Conclusion The main conclusions obtained in this study are as follows: (i) The laser-induced high pressure plasma shock wave will lead to ultrahigh strain rate dynamic response in material. When the wave reaches a certain critical value, high density dislocations will form rapidly on or near the wave front. As the wave pressure rises the dislocations will move, and thus nanocrystallines can be formed rapidly. (ii) After several impacts, equiaxial nanocrystallines are formed and evenly distributed on the surface of titanium alloy, with the size of 30–60 nm. Meanwhile, an amorphous layer with a depth of 10 nm is formed on the surface. (iii) Laser shock could improve the hardness within a certain depth. Increasing impact times could improve its amplitude and effective depth. The refinement of grains and the formation of high density dislocations are primary reasons for improving the hardness.
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