Microstructures and mechanical properties of β forging Ti17 alloy under combined laser shock processing and shot peening

Accepted Manuscript Microstructures and mechanical properties of β forging Ti17 alloy under combined laser shock processing and shot peening

Junfeng Wu, Shikun Zou, Yongkang Zhang, Shuili Gong, Guifang Sun, Zhonghua Ni, Ziwen Cao, Zhigang Che, Aixin Feng PII: DOI: Reference:

S0257-8972(17)30879-4 doi: 10.1016/j.surfcoat.2017.08.069 SCT 22630

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

13 May 2017 29 August 2017 30 August 2017

Please cite this article as: Junfeng Wu, Shikun Zou, Yongkang Zhang, Shuili Gong, Guifang Sun, Zhonghua Ni, Ziwen Cao, Zhigang Che, Aixin Feng , Microstructures and mechanical properties of β forging Ti17 alloy under combined laser shock processing and shot peening, Surface & Coatings Technology (2017), doi: 10.1016/j.surfcoat.2017.08.069

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Microstructures and mechanical properties of β forging Ti17 alloy under combined laser shock processing and shot peening Junfeng Wua, b, Shikun Zoub, Yongkang Zhangc, Shuili Gongb*, Guifang Suna*, Zhonghua Nia, Ziwen Caob, Zhigang Cheb, Aixin Fengd Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, School of Mechanical

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Engineering, Southeast University, Nanjing, Jiangsu, 211189, China

School of Electro-mechanical Engineering, Guangdong University of Technology, Guangzhou, Guangdong, 510000, China d

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Key Laboratory for High Energy Density Beam Processing Technology, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing, 100024, China

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College of mechanical &Electrical Engineering, Wenzhou University, Wenzhou, Zhejiang, 325035, China

ABSTRACT

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The aim of this paper was to utilize combined laser shock processing (LSP) and shot peening (SP) to study the grain refinement mechanism and mechanical properties modification of β forging Ti17 alloy. Firstly, LSP experiments were conducted on the surface of Ti17 alloy by a YAG laser system and square spots of 8 %

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overlapping rate. LSP process parameters were pulse width of 15 ns, pulse energy of 30 J and spot size of 4 mm × 4 mm. Secondly, SP experiments were carried on the surface of LSPed Ti17 alloy. ASH230 steel shots were used to SP with a diameter of 0.3 mm and an intensity of 0.3 mmA (A-type Almen stripe). Lastly, the microstructures and mechanical properties in the surface layer of Ti17 alloy were investigated by surface topography and

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roughness, micro-hardness, X-ray diffraction (XRD) analysis, residual stress, scanning electron microscopy (SEM)

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and transmission electron microscope (TEM). Results showed that surface topography and surface roughness amplitude were increased by combined LSP and SP. The amplitude and depth of the micro-hardness in the surface layer were also significantly improved. No new phase was formed after combined LSP and SP. High amplitude compressive residual stress with -613.5 MPa was induced in the surface with combined LSP and SP. The smaller

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phase sizes in β phase and more obvious crystal defects (deformed twining and dislocations in α phase and dislocations in β phase) were generated by combined LSP and SP. Grain refinement mechanisms were attributed to high density dislocations in α and β phases and multidirectional twin intersections in α phase.

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Keywords: Ti17 alloy; laser shock processing; shot peening; microstructure; mechanical properties

1. Introduction

Ti17 alloy has been extensively utilized for compressor blades and the integrated blisk rotators (IBRs) in aero-engines due to its excellent strength-weight ratio, high ductility and outstanding intermediate temperature performance [1]. However, the blades undergo severe mechanical and thermal loading conditions in service and may be in danger of foreign object damage (FOD) [2]. The fatigue failure of the blades can result from crack faults and splits during poor working conditions, which limits the further application of titanium blades. To solve this * Corresponding author. E-mail addresses: [email protected] (Shuili Gong), [email protected] (Guifang Sun) 1

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problem, shot peening (SP) since 1970s [3] and laser shock processing (LSP) around 2002 [4] have been widely and successfully applied in the aerospace industry. SP imparts near-surface compressive residual stress, which improves fatigue life of metals [5]. However, shallow compressive residual stress layer (tens of a millimeter) induced by SP may be less than the depth of initial damage caused by large FOD. Then compressive residual stress of metal was rapidly released due to SP high work-hardening during loading [6]. LSP is a highly effective technique for enhancing mechanical properties of metals due to some advantages with deep compressive residual stress (1 mm to 2 mm), flexibility in dealing with the complex shapes and no lower surface roughness (only several microns impact crater) [7-9]. When high power density (GW/cm2) laser beam irradiates the surface of the target, the plasma is formed by vaporization and ionization of ablative layer and the plasma happens to blast due to the constraint effect of water overlay. Then laser shock wave with high pressure (GPa) and short pulse (ns) would be generated and spread into the target [10]. Compressive residual stress and refined grains would be introduced in the surface layer of the target [7, 8]. It should be pointed out that LSP process parameters i.e. laser power density, pulse width, spot size, scan speed and impact times play a role in the improvement of the microstructure and mechanical properties of metals for example residual stress, hardness, refined grains and fatigue performance [11, 12]. The combination of LSP and SP can enhance the depth and the amplitude of compressive residual stress [13]. Meanwhile, the combination can enlarge plastic deformation in the surface and sub-surface layer and obtain more refined grains, which are beneficial to analyze the microstructure evolution of the titanium alloy. Many reports have shown that SP induced surface grain refinement and superficial compressive residual stress to improve the mechanical properties of titanium alloy. Liu et al. [14] investigated the nanocrystallite surface layer in a coarse TC17 alloy (approximately corresponding to ASTM Ti17 alloy) by high energy SP. Li et al. [15] reported the gradient nanocrystallite structure of TC17 alloy via high energy SP and the gradient variation of the micro-hardness. The micro-hardness increased by 43% from the topmost surface to the matrix. Kumar et al. [16] obtained the surface nanostructure of TC4 alloy with approximately 100 μm thickness through SP. The surface micro-hardness was increased by 34% with 30 minutes SP. In addition, most of studies in published literatures also pay intensive attention to the surface nanocrystallite and good mechanical properties of titanium alloy after LSP. Hua et al. [17] studied the nanostructure in the surface layer of TC11 alloy with LSP and its effect on the corrosion rate. Compared with the as-received material, the average corrosion rate of TC11 alloy with LSP was lower than 50%. Zhou et al. [18] noted the grain refinement mechanism and its influence on the mechanical properties of TC6 alloy under multiple LSP. The micro-hardness and the depth of plastic deformation layer would improve with increasing impact times. Ren et al. [19] showed the surface nanocrystallite evolution mechanism and surface micro-hardness of TC4 alloy with LSP. The surface micro-hardness increased with laser energy. Nie et al. [20] studied the nanocrystallite, improvement in micro-hardness, and high compressive residual stress and its good stability of TC17 alloy with LSP. However, there still is lack of understanding about microstructures and its mechanical properties (micro-hardness, residual stress and surface topography) of β forging Ti17 alloy with basket weave structure under combined LSP and SP. From the point of microstructure mechanism, the study results reveal the fatigue modification mechanism of β forging Ti17 blades of IBRs after combined LSP and SP. The results are 2

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beneficial to industry application of combined LSP and SP, especially the turbine blades with service temperature not exceeding 500°. This paper aims at acquiring the grain refinement mechanism and the mechanical property improvement in β forging Ti17 alloy with basket weave structure under combined LSP and SP. The surface topography and roughness, micro-hardness distribution at cross section, X-ray diffraction (XRD) and residual stress were analyzed. Furthermore, the microstructure changes in the surface layer were investigated by emission scanning electron microscope (SEM) and Transmission electron microscopy (TEM). Based on the analysis of the microstructure, the grain refinement mechanism was proposed.

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2. Experimental procedures

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2.1. Experimental material and parameters

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The investigation has been carried out on β forging Ti17 alloy with alpha-beta two phases basket weave structure from the blade of IBRs. The chemical composition and mechanical properties of the investigated material are given in Table 1 and Table 2. The material was performed at 800℃/4h solution and 630℃/8h aging treatment to obtain β forging Ti17 alloy with basket weave structure. In order to study the microstructure changes and mechanical property of Ti17 alloy under combined LSP and SP, the as-received material was cut into samples with the dimensions of 50 mm × 50 mm × 5 mm (length × width × thickness). Before SP and/or LSP treatment, the sample surfaces were polished with SiC paper from 400# to 2000# and then cleaned by acetone and anhydrous alcohol. The SP experiments were conducted by ASH230 steel shots with the dimension of 0.3 mm, shot velocity of 500 mm/min, shot angle of 85° and the intensity of 0.3 mmA (A-type Almen stripe). LSP experiments were performed by a Nd:YAG laser system with a wave-length of 1064 nm. LSP process parameters are laser energy of 30 J, pulse width of 15 ns, square spots of 4 mm×4 mm, overlapping rate of 8% and repetition rate of 1 Hz. The material process parameters are aluminum foil ablative layer of 0.12 mm thickness and water overlay of about 1 mm thickness [21]. For convenience, the three kinds of samples would be referred to as SP, LSP and combined LSP and SP, respectively. The first sample was conducted by SP-0.3 mmA on one side. The second sample was conducted by LSP-30 J on one side. The third sample was conducted by LSP prior to SP. Table 1 Chemical composition of Ti17 alloy

Composition

Al

Sn

Zr

Mo

Cr

Ti

Percent (wt. %)

4.5-5.5

1.6-2.4

1.6-2.4

3.5-4.5

3.5-4.5

Bal.

Table 2 Mechanical properties of Ti17 alloy Mechanical properties

Tensile strength Rm/MPa

Yield strength Rp 0.2/MPa

Elongation/A%

Value

956.05

878.24

18.19

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ACCEPTED MANUSCRIPT 2.2. Measurements and observations

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In order to quantitatively characterize the surface plastic deformation of Ti17 alloy after LSP and/or SP, two-dimensional surface topography was measured by Talysurf PGI 1230 with the line trace length of 20 mm. For studying the mechanical performance changes after LSP and/or SP, the micro-hardness distributions of the cross section were measured by HXD-1000TMC LCD Vickers indenter with a load of 300 kgf and a dwell time of 10 s. The measurement step was 0.1 mm between successive points. The XRD qualitative analysis of phase of Ti17 alloy after LSP and/or SP was conducted. The XRD analysis was carried out on a D8 Advance XRD instrument with a Cu-Kα radiation. The generator settings were 40 kV and 40 mA. The diffraction results were collected over a 2θ range of 30°~80°, with a step width of 0.02° and a counting time of 5 s per step. The residual stress was measured by laboratory X-ray diffraction with sin2ψ-method. The X-ray beam diameter was about 2 mm and the diffracted Cu-Kα characteristic X-ray from hexagonal α-phase {213} plane was detected with a diffraction angle (2θ) of 142°.

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After mechanically grinding, polishing and chemically etched with a solution of 10% HF and 20% HNO3 and 70% H2O, the microstructural observations were carried out on a ZEISS SUPRA 55 field SEM at a voltage of 15 kV. The grain sizes were measured by Nano Measurer 1.2 analysis software. TEM was applied to investigate the microstructural features, which can't be resolved by SEM. For in-depth microstructural analysis, TEM features were obtained from 15 μm and 160 μm below the topmost surface. TEM observation was performed on thin foils, which were obtained by firstly mechanically grinded to 50 μm and then perforated by using dimpling and ion thinning. These foils were examined by a JEM 2100 transmission electron microscopy operated at a voltage of 200 kV.

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3. Results and discussions

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3.1. Surface topography and roughness The two-dimensional surface topographies of Ti17 alloy with different processes are presented in Fig. 1. It clearly shows that the height of peak and valley of the as-received material scatters from 1 μm to -2 μm, as shown in Fig. 1(a). For LSP treated sample, surface topography is close to that of the as-received material (about from 1.25 μm to -1.75 μm), as shown in Fig. 1(b). This can be ascribed to no large deformation and optimized LSP process parameters. However, the surface topography is changed much after SP. SP treated one fluctuates from 7.5 μm to -8.5 μm, as shown in Fig. 1(c). Combined LSP and SP treated one scatters from 5 μm to -8.5 μm, as shown in Fig. 1(d). The results are in agreement with previous research [22]. The reason is that two phases with different properties in Ti17 alloy show the large and inconsistent deformation under SP. The surface roughness of Ti17 alloy with different processes is listed in Table 3. The surface roughness of the as-received material is Ra 0.0742 μm and Rz 1.1 μm. For LSP treated sample, the surface roughness is Ra 0.124 μm and Rz 1.3 μm. SP treated one is Ra 1.21 μm and Rz 7.45 μm. 4

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Combined LSP and SP treated one is Ra 1.22 μm and Rz 7.62 μm. It is clear that compared to surface roughnesses of original state and LSP, it obviously increases after SP and combined LSP and SP. Because of severe plastic deformation and uneven distribution shots during SP, the maximum value of the height of peak and valley obviously increases, which increases the surface roughness.

Fig. 1. The two-dimensional surface topographies of Ti17 alloy, (a) the as-received material, (b) with LSP, (c) with

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SP, (d) combined LSP and SP Table 3

The surface roughness of Ti17 alloy Ra (μm)

Rz (μm)

As-received material

0.0742

1.10

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0.124

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7.45

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Condition

combined LSP and SP

Ra=arithmetic average roughness, Rz=ten point height

3.2. Micro-hardness Micro-hardness distributions at the cross section of Ti17 alloy with different processes are shown in Fig. 2. The micro-hardness of the as-received material along the depth direction is constant about 386 HV. However, the surface micro-hardnesses of Ti17 alloy after LSP and/or SP are increased. The surface micro-hardnesses are 444.8 HV for LSP, 469.3 HV for SP and 438.6 HV for combined LSP and SP, respectively. They are increased by 15.2 %, 21.6 % and 13.6 % compared to that of the substrate material. The results could be attributed to compressive residual stress and refined grains induced by LSP and/or SP. High pressure shock wave in the level of GPa 5

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H v  H 0  KV d 1 / 2 [23], where H0 is the intrinsic hardness, Kv is the Hall-Petch coefficient

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and d is the average grain size, it could be inferred that the smaller the grain size d is, the higher the micro-hardness is. Due to the comprehensive effect of compressive residual stress and refined grains, the materials in the surface layer are strengthened [8]. From Fig. 2, the affecting layer depths are about 0.7 mm for SP, 1.1 mm for LSP and 1.6 mm for combined LSP and SP, respectively. The affecting layer depths of Ti17 alloy with combined LSP and SP are larger than that of Ti17 alloy with LSP alone and SP alone. The reason is more severe plastic deformation induced by two loadings of combined LSP and SP. The micro-hardness of Ti17 alloy with LSP and/or SP is higher than that of the as-received material in the affecting layer depth, as shown in Fig. 2. Over to the affecting layer depth, the micro-hardness of Ti17 samples with LSP and/or SP is equivalent to that of the as-received material. Li [15] and Nie [24] reported similar results. Due to shock wave attenuation and increasement of the grain size, the micro-hardnesses of Ti17 alloy with LSP and/or SP decreases rapidly with the increasement of the depth and tend to be the same level of that of the matrix. FOD resistance relies intensively on the substrate hardness of the material [25], which implies that the surface hardness is one of the most important parameters of FOD resistance. It can be clear from Fig. 2 that the amplitude value and depth of the micro-hardness of Ti17 alloy with LSP and/or SP are greater than those of the original material. Therefore, LSP and/or SP can effectively improve the micro-hardness of Ti17 alloy, which is beneficial to increase the FOD resistance of the blades manufactured by Ti17 alloy.

Fig. 2. Micro-hardness distributions at the cross section of Ti17 alloy, (a) SP and combined LSP and SP, (b) LSP and combined LSP and SP

3.3. XRD analysis The XRD patterns of Ti17 alloy surface with and without LSP and/or SP are shown in Fig. 3. As seen from Fig. 3, all of the Bragg diffraction peaks of Ti17 alloy before and after LSP and/or SP are well agreed with those of α-phase and β-phase standards, which imply that no new phases occur or disappear in Ti17 alloy after LSP and/or SP. However, Bragg diffraction peak has broadened from original state to LSP, SP and combined LSP and SP, without respect to the effect 6

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(002), (110)and (101). Li [15] also found that peak broadening and serious overlaps of TC17

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alloy between the adjacent peaks after high energy SP. The results could be ascribed to the grain refinement, lattice deformation and the increasement of micro-stress under LSP and/or SP. In addition, the peak broadening degree of Ti17 alloy with SP is more than that of Ti17 alloy with LSP. It implies that the grain refinement increases on the surface of Ti17 alloy with SP compared to that of Ti17 alloy with LSP.

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Fig. 3. XRD patterns of Ti17 alloy surface, (a) diffraction degree of 30~90, (b) magnitude of [A] in (a)

3.4. Residual stress

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Surface residual stress distributions of Ti17 alloy with different processes are presented in Fig. 4. It can be found that the surface residual stresses after different processes are markedly different. The surface of as-received material is introduced into a machining stress of -162.78 MPa. While, the high amplitude compressive residual stress state is generated in the surface of Ti17 alloy after LSP and/or SP. The surface residual stresses of Ti17 alloy are -523.25 MPa for LSP, -550.14 MPa for SP and -613.5 MPa for combined LSP and SP, respectively. It is reasonable to assume that residual stress improvement after LSP and/or SP is due to refined grains, high density dislocation and nanocrystallite in the surface layer induced by high pressure shock wave. Compared to LSP alone and SP alone, combined LSP and SP lead to maximum value of compressive residual stress. The reason is more severe plastic deformation in the surface layer, and deeper layer of refined grains and nanocrystallite under combined LSP and SP, which are corresponding to the increasement of plastically affected depth after multiple LSP impact times [7]. Accordingly, surface-related mechanical properties can be improved by optimized LSP and/or SP for example fatigue life [2, 5, 24].

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Fig. 4. Surface residual stress distributions of Ti17 alloy with different processes

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3.5. Microstructure observations

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3.5.1. Original microstructure

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Fig. 5(a) presents SEM microstructure of the as-received material. Ti17 alloy has duplex phase microstructure, which consists of colonies of α-plates (β-transformed structure) and α phase along grain boundaries [1, 26]. Fig. 5(b) reveals TEM microstructure of the original grains. The original grains have relatively large size and clear phase boundaries. There are no severe deformation defects in the original α phase and β phase. Only some dislocations appear in β phase and some dislocations pile up occur near grain boundaries.

Fig. 5. Microstructure features of the as-received material, (a) SEM microstructure, (b) TEM microstructure

3.5.2. SEM observations in the surface layer The cross-sectional SEM images of Ti17 alloy with combined LSP and SP are presented in Fig. 6. It can be seen from Fig. 6(a) that the severe plastic deformation appears in the surface layer of Ti17 alloy. Nie et al. [20] also reported that the severe plastic deformation layer was generated on TC17 alloy with LSP. Fig. 6(b) shows the high magnification image in the surface layer. The refined grains are generated within 5 μm below the topmost surface after combined LSP and SP. The length and the width of β phase in the substrate material are 3.87 μm and 0.55 μm, respectively, as shown in Fig. 5(a). While, it can be seen clearly from Fig. 6(b) that the length and 8

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the width of β phase with combined LSP and SP are 0.89 μm and 0.17 μm, respectively. TEM images are need to apply in further to investigate the microstructure evolution mechanism of Ti17 alloy with combined LSP and SP.

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Fig. 6. The cross-sectional SEM images of Ti17 alloy with combined LSP and SP, (a) in the surface layer, (b) high magnification image

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3.5.3. TEM observations at 160 μm below the topmost surface

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Fig. 7 shows TEM images at about 160 μm below the topmost surface of Ti17 alloy with combined LSP and SP. As seen from Fig. 7(a), large number of dislocations are observed in β phase and parallel dislocation lines (DLs) develop into dislocation walls (DWs). Some DLs annihilate near the grain boundaries. Dislocation multiplication is also observed in β phase. In Fig. 7(b), high density dislocations (HDDs) tangle together and develop into dislocation tangles (DTs). In Fig. 7(c), deformation twinnings are generated in α phase. Coarse α grains are divided into sub-grains by twin intersections (α-TIs), which makes further efforts to grain refinement [27]. In Fig. 7(d), dislocations are impeded to move by phase boundaries, and dislocations pile up and DWs are brought out at phase boundaries. Because of different lattice type and grain orientation from α phase and β phase, the dislocation slip direction in α phase is different from that in β phase at phase boundary. β phase with body-centered cubic crystal structure (bcc) has high stacking fault energy and more slip systems. Therefore, dislocation migration is easily achieved in β phase under severe plastic deformation, as shown in Fig. 7. As the plastic deformation intensities increase, multidirectional slip systems are achieved. Then, DWs and DTs are formed, as shown in Fig. 7. α phase with hexagonal close packed crystal structure (hcp) has low stacking fault energy with only four independent slip systems. According to the Von-Mises rule, five independent slip systems are necessary to maintain plastic deformation compatibility [28, 29]. Therefore, hcp grain needs other deformations to achieve the plastic deformation such as deformation twinning. In this paper, deformation twinning are observed in α phase, as shown in Fig. 7(c). Due to reflection and refraction of the shock wave in the material, different twin systems are initiated under the multidirectional loading and twin intersections are genereted. Therefore, multidirectional twin intersections (MTIs) subdivide coarse grains into sub-grain.

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Fig. 7. TEM images at about 160 μm below the topmost surface of Ti17 alloy with combined LSP and SP, (a) DWs and DLs in β phase, (b) DTs in β phase, (c) deformation twinning in α phase, (d) dislocation distribution at phase boundary

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3.5.4. TEM observations within 15 μm below the topmost surface

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Fig. 8 presents TEM images within 15 μm below the topmost surface of Ti17 alloy with combined LSP and SP. In order to reduce the energy of β grain induced by combined LSP and SP, HDDs will annihilate and rearrange near DTs or DWs. Then the DTs transform into dislocation cells (DCs) due to alignment of dislocations in different orientations. DCs and DWs develop into sub-grain boundaries (sub-GBs) with corresponding selected area electron diffraction (SAED) pattern between domains A and B and a misorientation about 6.96°, as shown in Fig. 8(a) ~ Fig. 8(c). It results in the grain refinement [30]. In Fig. 8(d) and Fig. 8(e), DLs, DTs, DCs, DWs and nanostructured grains are observed in α phase. DLs tangle together and develop into DTs in α phase and DTs tranform into DCs. DLs also develop into DWs in α phase and further develop into sub-GBs. Then nanostructured grains are generated in α phase. Finally nanocrystallite with a continuous diffraction ring and random in crystallographic orientations is generated in α phase and β phase, as shown in Fig. 8(f). The flow stress for dislocation slip can be modeled by the Zerilli-Armstrong model,

Y ( P ,P ,T )   g  k h  d 1 / 2  K pn  B exp(( 0  1 ln(p ))T )  [31],

B 0  p exp(( 0  1 ln(p ))T )

where σg is the contribution due to solutes and initial dislocation density, kh is the microstructure stress intensity, d is the average grain diameter, εp is the plastic strain,  p is the strain rate, T is 10

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 r is the critical twinning stress,  r 0 is initial twinning stress, kT is the slope of Hall-Petch

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relation curve, and d is the average grain size. It can be seen from Hall-Petch relationship that the critical twinning stress decreases with the increasement of grain size. Therefore, twinning can be formed in the coarse grains (Fig. 7(c)) and can be hardly formed in the refined grains. Then DLs are generated in refined grains in order to decrease the energy of refined grains, as shown in Fig. 8(e). With the increasement of the plastic deformation, DLs develop into DTs, DCs and DWs, which finally develop into nanostructured grains and nanocrystallite in α phase, as shown in Fig. 8(d) ~ 8(f).

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Fig. 8. TEM images within 15μm below the topmost surface of Ti17 alloy with combined LSP and SP, (a) ~(c)

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DTs, DCs, DWs and sub-GBs in β phase, (d)~(e) DLs, DTs, DWs, DCs, sub-GBs and nanostuctured grains in α phase, (f) nanocrystallite in α phase and β phase and corresponding selected area electron diffraction (SAED)

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pattern

3.6. Grain refinement process MTIs and sub-GBs are main reasons for grain refinement in Ti17 alloy with combined LSP and SP treatment. Based on experimental observation in Fig. 7 and Fig. 8, the grain refinement process can be schematically illustrated in Fig. 9. (i) Low density dislocations (LDDs) randomly distribute in original β phase, and dislocation activities lead to dislocation pile up in α/β phase boundary. (ii) After combined LSP and SP, high density dislocations (HDDs) are generated in β phase due to severe plastic deformation. (iii) HDDs develop into DLs and DTs, and DLs and DTs transform into DWs and DCs in β phase. Deformation twinning is formed in α phase and MTIs divide the coarse grain into sub-grains. (iv) With the increasement of plastic deformation, DWs and DCs transform into sub-GBs in β phase. 12

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At the same time, DLs and DTs are generated in α phase. (v) DLs and DTs develop into sub-GBs and DCs in α phase. (vi) Finally, nanocrystallite is formed in α phase and β phase. Therefore, MTIs and division of sub-GBs lead to grain refinement. The similar mechanism can be seen in literature [14]. The α phase and β phase in Ti17 alloy have interactions with each other during combined LSP and SP. Further, phase boundaries play a major role in dislocation development. Dislocation pile up near phase boundaries or grain boundaries provide nucleation for new sub-grains due to pinning effect. Generally, the interactions between α phase and β phase are of great significance in grain refinement and metal strengthening during combined LSP and SP.

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Fig. 9. The grain refinement process of Ti17 alloy with combined LSP and SP

4. Conclusions

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In this paper, deformation twinning, dislocations and nanocrystallite in α phase and dislocations and nanocrystallite in β phase are generated in Ti17 alloy after combined LSP and SP. The mechanical properties of Ti17 alloy with combined LSP and SP are also obviously improved compared to LSP or SP alone. The main conclusions are drawn as follows: (1) The surface topographies and surface roughnesses of Ti17 alloy with LSP alone are close to those of the as-received material. But those of Ti17 alloy after LSP alone are less than those after SP alone and after combined LSP and SP. (2) The affecting layer depth of micro-hardness of Ti17 alloy after combined LSP and SP are larger than that after SP alone and that LSP alone. But the surface amplitude of micro-hardness of Ti17 alloy with combined LSP and SP is less than those of Ti17 alloy with SP alone and LSP alone and is greater than that of as-received material. (3) After LSP and/or SP, no new phases occur and/or disappear in Ti17 alloy. The serious overlaps are generated between the adjacent peaks. Bragg diffraction peak has broadened. The surface compressive residual stresses are obviously improved after LSP and/or SP, which are -162.78MPa for as-received material, -523.25MPa for LSP, -550.14MPa for SP and -613.5MPa for combined LSP and SP. 13

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(4) The severe plastic deformation appears in the surface layer of Ti17 alloy with combined LSP and SP. The refined grains in β phase are generated from 3.87 μm to 0.89 μm for the length and from 0.55 μm to 0.17 μm for width within 5 μm below the topmost surface. Dislocation migration and deformation twinning are the primary reasons of grain refinement. Different dislocation slip directions are distributed at phase boundary. (5) The grain refinement process of β phase under combined LSP and SP was proposed based on the microstructural observations. It involves the formation of DTs and DWs due to the pile up of DLs, transformation of DCs and DWs into sub-GBs, and division of sub-GBs to grain refinement and nanocrystallite. The grain refinement process of α phase involves the formation of deformation twinning, the formation of sub-GBs by MTIs, the formation of DWs and DTs by HDDs, the transformation of DTs and DWs into DCs and sub-GBs, and division of sub-GBs to nanocrystallite.

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Acknowledgements

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This project is supported by the National Science and Technology Major Project of China (20 16YFB1102705),the Joint Fund of Ministry of National Defense and Education of China (No.614 1A02033103), Postdoctoral Science Foundation of China (No.2015M570395,2016T90400), Indus try-University-Institute Cooperation Joint Research Project of Jiangsu Province of China (No.BY2 015070-05), Postdoctoral Science Foundation of Jiangsu Province (No.1501028A) and Six Talent Peaks of Jiangsu Province (2016-HKHT-001).

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Graphical abstract Microstructures and mechanical properties of β forging Ti17 alloy under combined laser shock processing and shot peening

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ACCEPTED MANUSCRIPT Highlights

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Combined laser shock processing and shot peening were used to β forging Ti17 alloy. Increased surface roughness amplitude and compressive residual stress was induced. Micro-hardness value in depth and its affecting layer depth were improved. Nanocrystallite and deformation twinning were generated at the topmost surface. Grain refinement mechanisms were dislocation movement and deformation twinning.

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