Abstract
Ti-17 titanium alloy was treated with laser shock processing (LSP) and tested for impact toughness. Residual compressive stress, cross-section hardness, microstructure, and the impact toughness properties of the treated alloy samples were evaluated. The average impact toughness of the samples subjected to LSP increased by 10 J compared with that of the base material. The samples subjected to LSP remained connected after an impact test, whereas untreated samples completely ruptured. The location of fracture initiation shifted from the center to the side to the Charpy-V notch region because of the stress concentration induced by LSP. The stress concentration was induced by the uneven microhardness of the samples treated with LSP. The hardness of the irradiated surface increased compared with that of the base metal because of the higher dislocation density and smaller grains produced by LSP.
Introduction
Titanium alloy is widely used in the aircraft industry for landing gears, large springs, and structural components in airplane wings because of its low-density, high tensile strength, and excellent high-temperature performance. For example, the Airbus A380 contains approximately 9 % titanium alloy by weight [1]. Ti-17 titanium alloy (Ti-5Al-2Sn-2Zr-4Mo-4Cr), which was primarily developed by General Electric aircraft engineers, is a “beta-rich” α+β titanium alloy with 8 %-content of β-stabilizer alloying elements, such as molybdenum and chromium [2]. These titanium materials include various structural defects such as keyway, thread, tool mark gaps, induced during the manufacture process and through environmental exposure. Owing to the existence of these gaps, a stress concentration develops at gaps in components under load, which may cause the component to become prone to sudden rupture during operation, particularly components subjected to impact load [3]. Thus, it is important to improve the impact toughness of these materials. Most previous work in this area has focused on changing the material composition or using heat treatments to improve the impact toughness of materials [4, 5, 6, 7]. However, these methods are complicated and uneconomical.
Laser shock processing (LSP) is a novel approach for improving the microhardness, impact toughness, and other mechanical properties of metals. When a high-power (GW/cm2), short-pulse (ns) laser passes through a transparent confining layer and hits the surface of the absorbing material, the absorbing material vaporizes and forms a high temperature and high pressure plasma over an ultrashort time period, as illustrated in Figure 1 [8, 9]. The rapidly expanding plasma is trapped by the confining layer, which creates a shock wave that propagates into the material [10]. The plastic layer at the surface layer of the material deforms and compressive residual stresses are generated on the material surface [11, 12]. This effect results in more uniform compressive residual stress, which improves the mechanical performance of the material [13, 14].

Illustration of process for LSP treatment.
Many researchers have intensively studied LSP treatments with a particular focus on the effects of LSP on the fatigue performance of metals [15, 16, 17]. However, there has been little research on changes in the impact toughness of materials after LSP. Zhou et al. [18] reported that the impact toughness of 2Cr13 martensite stainless steel increased after LSP. A maximum impact toughness of 10.5 J/cm2 was attained at a laser power density of 8.57 GW/cm2. Ge et al. [19] found that the average impact energy of AZ31B changed from 5.38 J to 9.23 J, increasing by 71.5 % after LSP. LSP improves impact toughness through a combination of surface nanocrystallization and induction of residual compressive stress. However, the impact toughness of Ti-17 titanium alloy after LSP treatment has not been widely reported.
In the present work, Ti-17 titanium alloy was treated with LSP and its impact toughness was evaluated. The fracture morphology and microstructure of the samples were observed and analyzed. Finally, the relationship between the impact toughness and microstructure of the samples was examined.
Experimental procedure
Test sample
A 5-mm thick Ti-17 titanium alloy plate was used in the experiments. Ti-17 titanium alloy materials, with chemical compositions (see Table 1) of Ti-5Al-2Sn-2Zr-4Cr-4Mo (wt%), were annealed at 55 °C for 2 h and air cooled. The mechanical and physical properties of the Ti-17 titanium alloy are listed in Table 2. The microstructures of the Ti-17 titanium alloy are shown in Figure 2. The figure illustrates that Ti-17 titanium alloy is an 
Chemical compositions of Ti-17 titanium alloy.
| Alloy element (wt.%) | |||||
|---|---|---|---|---|---|
| Al | Sn | Zr | Mo | Cr | Ti | 
| 4.5–5.5 | 1.6–2.4 | 1.6–2.4 | 3.5–4.5 | 3.5–4.5 | Bal | 
Basic material properties of Ti-17 titanium alloy.
| Density | 4650 kg/m3 | 
| Poisson’s ratio | 0.28 | 
| Elastic modulus | 108 GPa | 
| Yield stress | 1075 MPa | 
| Hardness | 400  | 

Ti-17 titanium alloy microstructure.
A sketch of the impact sample, fabricated according to the national standard of the People’s Republic of China GB/T 229-2207, is shown in Figure 3. The Charpy impact test was performed with a SANS-2000 (MTS) pendulum bob impact testing machine at room temperature. One operation mode of this apparatus is to place a Charpy V-notch specimen (shown in Figure 3) across the parallel jaws of the machine. In the impact test, a heavy pendulum released from a known height strikes and fractures the sample on its downward swing. The energy absorbed by the fracture can be calculated by determining the mass of the pendulum and the difference between its initial and final heights.

Schematic of the impact toughness sample.
Laser shock processing
In this research, laser shock processing was performed with an Nd: glass laser operating at 1064 nm with a pulse duration of 10 ns and a spot diameter of 3 mm. The laser energy was approximately 5 J and the repetition rate of the laser was 2 Hz; 0.1 mm of black tape was used to protect the titanium alloy from the heating effects of the laser. To increase the peak pressure of the laser shock wave, a layer of flowing water with a layer thickness of approximately 2-mm was used as the transparent confining layer. The overlap rate of the laser spot was 50 %. The 25-mm×10-mm impact zone was subjected to two passes of LSP on both sides of the impact sample, as shown in Figure 3.
Characterization
After the LSP treatment, the tape coating was removed and the workpiece surface was cleaned with ethanol. The residual stress was measured by X-ray diffraction with the sin2
The microstructures of the untreated and LSP-treated samples were examined with a transmission electron microscope (TEM) (JEM-2100, JEOL). The fracture morphologies of the samples were examined before and after LSP using a scanning electron microscope (SEM) (JSM-6010LA, JEOL).
Results and discussion
Surface characteristics
The surface morphology of a single spot subjected to the laser pulse energy is shown in Figure 4. When the laser pulse was 5 J, a circular dent was created after the LSP treatment because of plastic deformation at the shock wave loading region. The plastic deformation produced by the laser pulse energy is a consequence of the shock wave pressure, which is proportional to the root of the laser pulse energy [21]. Line profiles of the peened surface treated at 5 J are shown in Figure 4; the depth of the microdents was about 15.3 μm. After multiple LSP treatments, the surface exhibited a regular array of microdents, which were caused by the plastic deformations induced by successive laser irradiations at different locations [22, 23, 24]. The dent spacing was controlled by the laser shot-to-shot offset.

Microdents on material surface after LSP treatment.
The residual compressive stress distributed on the sample before and after LSP is shown in Figure 5. The plasma generated in the breakdown water layer prevented the laser from directly influencing the material surface. Multiple impacts could produce a higher surface residual stress than a single impact because of increased plastic strain and cyclic hardening [25]. In many metal materials, plastic deformation can enhance microhardness of the base material by inducing high-density dislocations or grain refinement [10]. The microhardness distribution on the Ti-17 titanium alloy was measured after the LSP treatment (see Figure 6). The depth of the gradient hardened layer (GHL) was determined by calculating the microhardness distribution along the cross-section of the laser-peened sample. According to Chen et al. [26] and Osamu et al. [27], induced residual compressive stress can enhance the hardness of materials. A residual stress field was produced by the uneven plastic deformation of the deformed layer. After LSP, the surface layers of the samples featured severe elastic deformations. Residual stress fields were introduced and the domain sizes were refined. In addition, dislocations and dislocation cells were formed. These effects could increase the microhardness of the samples. The impact toughness is also related to the compressive residual stress [28, 29]. Thus, we hypothesized that the gradients of hardness and impact toughness had similar distributions. Hence, the hardness along the cross-section of the laser-peened sample can be used to determine the GHL depth. The GHL depth of the Ti-17 titanium alloy treated with 5-J laser pulse energy was measured with the microhardness test, as shown in Figure 6. At a laser energy of 5 J, the cross-section showed that the microhardness increased at the surface of the GHL, and then gradually decreased to the value of the untreated region at a depth of (1.00 ± 0.05) mm, as shown in Figure 6. This result indicates that the GHL thickness was (1.00 ± 0.05) mm at laser pulse energy of 5 J because the laser-induced shock pressure is proportional to the square root of the laser energy according to Fabbro’s model [30]; namely, higher laser energy produces a higher laser-induced shock pressure. According to Ballard et al. [31], the plastically affected depth is proportional to the shock pressure. Thus, a higher shock pressure induces a deeper plastically affected depth.

Residual compressive stress on material surface before and after LSP.

Microhardness of the sample before and after LSP.
Impact energy
Table 3 shows the impact energies of the Ti-17 titanium alloy before and after LSP treatment. The average impact energies of the base material and LSP-treated titanium alloy were 123.225 and 133.175 J, respectively. Thus, the average impact energy of the Ti-17 titanium alloy increased approximately by 10 J after the LSP treatment.
Effect of LSP treatment on the impact toughness of Ti-17 titanium alloy.
| Impact energy (J) | Average (J) | |
|---|---|---|
| Before LSP ( | 117.8, 120.6, 128.1, 126.4 | 123 | 
| After LSP( | 134.3, 131.2, 134.1, 133.1 | 133 | 
We used statistical analysis to verify to what the degree LSP affected the impact toughness of Ti-17 titanium alloy. The average impact toughness of the LSP treated Ti-17 titanium alloy was 
where m and n represent the numbers of test samples.
The sample variance of 
The average impact toughness of the Ti-17 titanium alloy after LSP is 
The sample variance of 
Supposing that 
Therefore, the alternative hypothesis 
Under the conditions of the original hypothesis, the statistical power 
The rejection region:
According to the principle of the statistical test:
At the 
The original hypothesis was rejected because 
Fracture analysis
As shown in Figure 7(a), the untreated impact samples all completely fractured into two parts, whereas the LSP samples were not fractured and remained connected at the reverse of the impact site, as shown in Figure 7(b). Figure 8 shows a photograph of the impact sample after fracture. The crack source of the base material and the sample subjected to LSP are marked by circles. The location of the crack source of the impact fracture shifted from the center to the side of the Charpy-V notch region after the LSP treatment. The LSP treatment caused an increase in the microhardness and uneven plastic deformation of the material, which induced stress concentration on the laser-irradiated surface. The crack source was primarily determined by the stress concentration. In addition, under an external force, the expansion of the crack path in the LSP samples (Figure 8b) featured more twisting than that of the original samples (Figure 8a). This suggests that more energy was consumed to initiate cracking for the LSP samples [32]. Figure 9 shows SEM images of the crack source of the untreated and LSP samples. The untreated material fracture did not show any notable plastic deformation, and the fracture cracks were of an intercrystalline type, where the intergranular binding force is weak (Figure 9a). Figure 9(b) shows the many dimples on the LSP sample fracture, which displayed features of plastic fracture.

Photographs of samples after impact test. (a) Impact sample before LSP; (b) Impact sample after LSP; (c) Sample marked in Figure 7(a).

SEM images of impact sample fracture. (a) Fracture of impact sample before LSP; (b) Fracture of impact sample after LSP.

SEM images of crack source. (a) Fracture of impact sample before LSP; (b) Fracture of impact sample after LSP.
Microstructures
Typical TEM images of the base material and the LSP-treated samples are shown in Figure 10. The density of dislocations in the sample subjected to LSP treatment (Figure 10[b]) increased considerably compared with that in the base material. The laser shock wave displaced materials and plastic deformation occurred through dislocation slip, shock wave reflection, and refraction at grain boundaries. The shock waves had various effects on grains. Thus, dislocation after complex slip, agglomeration, and annihilation led to formation of new grain boundaries and small grains, and also improved the dislocation density [33].

TEM images of impact sample. (a) Untreated material; (b) Impact sample after LSP.
The higher dislocation density hindered the plastic flow of metal and improved the microhardness of materials. The relationship between the microhardness and the density of dislocations can be defined as [34]:
where 
Formula (1) shows that Hv is proportional to ρ1/2. Furthermore, increasing the dislocation density ρ helps to improve the microhardness of the material.
In addition, Hall–Petch found the relationship between the rupture stress and the diameter of the material grain [35].
where σ is the rupture stress, 
Formula (2) shows that rupture stress is inversely proportional to the diameter of grains in the material. After LSP treatment, the grain diameter became smaller than that of the original material. Therefore, LSP improves the rupture stress of metals and consequently, the samples subjected to LSP required more energy to fracture.
The stress was released through dislocation slip and through propagation of stacking faults under stress because of the fine plastic nature of the titanium alloy. Therefore, the uniform plastic deformation had a high capacity as indicated by the macroscopic analysis. Furthermore, the titanium alloys had a high work hardening capacity. Deformation and microvoids were formed because of the stress concentration at the intersection of the slip bands at the different slip surfaces and along the different slip directions. The microvoids expanded into microcracks, which then fractured [8, 36]. Determination of optimal grain sizes has attracted research attention because of the profound effects of grain size on the impact toughness of materials. In general, greater plastic deformation allows for more energy to be absorbed at turning sites [37], meaning that more energy can be absorbed during impact leading to better impact toughness. Hence, the improved impact toughness in our samples can be attributed to the more effective small surface grains induced by the LSP treatment.
Conclusions
In summary, the following conclusions can be drawn:
The average impact energy of Ti-17 titanium alloy increases by 10 J after LSP treatment.
After LSP treatment, the crack source shifts because of the altered stress concentration. The stress concentration is caused by the uneven microhardness of the samples subjected to LSP and leads to improved surface microhardness of the LSP-treated samples. The LSP-treated samples had higher microhardness because of the high dislocation density produced by LSP.
The smaller grain size produced by during the LSP treatment hindered the propagation of cracks, resulting in the excellent impact toughness of the LSP-treated samples.
Funding statement: Supported by International Science and Technology Cooperation Program of China (Grant No. 2013DFR50590), National Natural Science Foundation of China (No. 51501219, No. 51675030, No. 11372019, No. 51605019), Natural Science Foundation of Liaoning Province (No. 2015020115).
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Articles in the same Issue
- Frontmatter
 - Research Articles
 - Effect of Heat Treatment on Microstructure and Thermal Fatigue Properties of Al-Si-Cu-Mg Alloys
 - Effect of Laser Welding Parameters on Weld Bowing Distortion of Thin Plates
 - Effect of the Tundish Gunning Materials on the Steel Cleanliness
 - Effects of Laser Shock Processing on Impact Toughness of Ti-17 Titanium Alloy
 - Effect of Prior Thermomechanical Treatment on Annealed Microstructure and Microhardness in Cobalt-Based Superalloy Co-20Cr-15W-10Ni
 - Oxidation Resistance of Austenitic Steels under Thermal Shock Conditions in an Environment Containing Water Vapor
 - Comparative Evaluation of Spark Plasma and Conventional Sintering of NiO/YSZ Layers for Metal-Supported Solid Oxide Fuel Cells
 - Present Situation and Prospect of EAF Gas Waste Heat Utilization Technology
 - Formation of Nano-porous Structure in a Cathode at the Interface between Pt Electrode and YSZ during CO2 Electrolysis at 1,000 °C
 - Numerical Simulation of Inclusion Capture in the Slab Continuous Casting Considering the Influence of the Primary Dendrite Arm Spacing
 - Effect of Surface Fe-Sn Intermetallics on Oxide Films Formation of Stainless Steel in High Temperature Water
 
Articles in the same Issue
- Frontmatter
 - Research Articles
 - Effect of Heat Treatment on Microstructure and Thermal Fatigue Properties of Al-Si-Cu-Mg Alloys
 - Effect of Laser Welding Parameters on Weld Bowing Distortion of Thin Plates
 - Effect of the Tundish Gunning Materials on the Steel Cleanliness
 - Effects of Laser Shock Processing on Impact Toughness of Ti-17 Titanium Alloy
 - Effect of Prior Thermomechanical Treatment on Annealed Microstructure and Microhardness in Cobalt-Based Superalloy Co-20Cr-15W-10Ni
 - Oxidation Resistance of Austenitic Steels under Thermal Shock Conditions in an Environment Containing Water Vapor
 - Comparative Evaluation of Spark Plasma and Conventional Sintering of NiO/YSZ Layers for Metal-Supported Solid Oxide Fuel Cells
 - Present Situation and Prospect of EAF Gas Waste Heat Utilization Technology
 - Formation of Nano-porous Structure in a Cathode at the Interface between Pt Electrode and YSZ during CO2 Electrolysis at 1,000 °C
 - Numerical Simulation of Inclusion Capture in the Slab Continuous Casting Considering the Influence of the Primary Dendrite Arm Spacing
 - Effect of Surface Fe-Sn Intermetallics on Oxide Films Formation of Stainless Steel in High Temperature Water