Introduction

Most data about high-cycle fatigue in metals and alloys are concerned with the nominal stress required to cause failure over a given number of cycles. However, smooth or notched specimens are typically used in these tests and it is difficult to distinguish between the fatigue crack initiation life and fatigue crack propagation life. When the stress applied to a component with existing cracks reaches a critical value, crack extension failure occurs. However, in the vast majority of cases, the macro-critical crack is partially initiated by small cracks, which gradually develop under cyclic loading, a process known as crack propagation. Thus, it is important to study the propagation processes of fatigue cracks, to ensure the security of components. Various methods have been developed to measure the fatigue life associated with pre-existing flaws in materials (Ref 1).

Ti-17 titanium alloy is widely used in aircraft components such as compressor disks and fan blades owing to its high strength, superior fracture toughness, and good hardenability (Ref 2-4). However, it is becoming increasingly difficult for titanium alloys to meet the fracture property requirements of structural materials currently demanded by the aerospace industry. As a result, the application of titanium alloys has been greatly limited. Two approaches may be used to address this issue. First, a new class of titanium alloys based on the damage tolerance design concept could be developed. Second, the fracture toughness of existing titanium alloys could be optimized by controlling features of their surface microstructure. The design of a new material is a complicated and expensive process, which needs many verification experiments; however, the second approach is more cost-effective and convenient (Ref 5, 6).

Laser shock processing (LSP) is a promising surface treatment technique to improve the fatigue properties of some metals and alloys (Ref 7-9). Improved fatigue, wear and anti-corrosion properties can be attained from the compressive residual stress and grain refinement induced by LSP (Ref 10, 11). In an LSP process, a sample is irradiated by a laser pulse with a nanosecond pulse width and a high intensity, on the order of GW/cm2. Rapid evaporation takes place on the irradiated surface as a result of the high-power laser beam (Ref 12, 13), and a plasma consisting of a partially ionized gas is formed on the irradiated surface. The irradiated surface is generally coated with an opaque over-layer, such as a black paint or metallic foil tape, to increase absorbance of laser energy and avoid overheating on the surface. A transparent overlay (tamping layer) is applied to prevent the plasma from expanding away from the surface, thereby increasing the intensity of the shock wave. Water, quartz, and glass can be used for these overlays, also known as a confining medium. Other materials used for confining media include K9 glass, Pb glass, Perspex, and silicon rubber (Ref 14, 15). As a recyclable resource, water is both convenient and cost-effective. The plasma continues to absorb laser energy and expand generating a high pressure at the sample surface. The pressure is transmitted into the materials through shock waves. When the pressure exceeds the dynamic yield strength of the materials, plastic deformation occurs in the irradiated area, which changes the microstructure and properties of the materials. In some cases, plastic deformation induced by the shock waves may result in strain hardening at the surface (Ref 16).

Most studies on laser shock processing have focused on issues such as fatigue performance, microstructure, residual stress, and the hardness variation and grain refinement mechanisms in aluminum alloys after LSP (Ref 17-19). Huang et al. (Ref 20, 21) have shown that LSP improves the fatigue crack performance of 6061-T6 aluminum alloy. Zou et al. (Ref 22, 23) and Lin et al. (Ref 24) have also reported that the fatigue crack performance of Ti-6Al-4V is improved after LSP. Grain refinement in metals by LSP has recently become an area of research focus (Ref 25, 26). Cellard et al. (Ref 27) have reported the influence of LSP parameters on Ti-17 titanium alloy. However, there have been few studies on the crack growth life of Ti-17 titanium alloy treated by LSP. The objective of the present work is to evaluate the effects of LSP on crack growth in Ti-17 titanium alloy. The hardness and residual stress before and after LSP are determined and compared. The fracture properties and microstructure are examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Experimental Procedures

Sample Preparation

The Ti-17 titanium alloy materials used in this study were bought from Jiaming Group Corporation (Shenzhen, China). The mechanical and physical properties of the Ti-17 titanium alloy are listed in Table 1. The samples were etched in a mixture of 90 ml of H2O, 8 ml of HNO3, 2 ml of HF for 15 s at room temperature. The morphologies of precipitated phases were monitored by SEM, and the microstructures of the Ti-17 titanium are shown in Fig. 1. These images show that the Ti-17 titanium alloy is an α+β two-phase alloy, which consists of fine α plates embedded in β matrix. Plates of the Ti-17 titanium alloy (10-mm thick), with the composition detailed in Table 2, were used in the experiments. The dimensions of the compact tension (CT) samples used in the fatigue crack growth (FCG) test are shown in Fig. 2 according to the standard GB/T 6398-2000. The CT samples were processed with the loading axis parallel to the rolling direction and then cut by low speed one-way walk wire cut electrical discharge machining. Prior to LSP, the intended peening surfaces of specimens were ground with 1200-grit sandpaper followed by the final polishing to a surface roughness of 0.05 μm, and a fatigue pre-crack 2.5 mm long (from notch tip) was formed on each sample using a MTS-880 servo-hydraulic system at room temperature (25 °C) in air.

Table 1 Basic material properties of Ti-17 titanium alloy
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Fig. 1
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Microstructures of Ti-17 titanium alloy

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Table 2 Chemical composition of Ti-17 titanium alloy (wt.%)
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Fig. 2
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Geometry of the CT specimen

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Laser Shock Processing

A schematic diagram and an image of the LSP equipment are shown in Fig. 3. A water layer about 2 mm thick was used as the transparent confining layer and 3-mm-wide black tape served as the ablating coating to protect the target from thermal effects. The water flow could be varied to control the thickness of water layer, which was measured with vernier calipers. In our experiment, the black tape, composed mainly of polyvinyl chloride, was bought from NITTO Co. Ltd. A Q-switched Nd:YAG laser (2-Hz repetition rate, 1064-nm wavelength, and 10-ns pulse duration) was used with a 3-mm-diameter laser shock spot, 7-J pulse energy, and 50% overlapping ratio in the LSP experiment. Prior to LSP, the samples were polished by SiC paper and a polishing cloth and then ultrasonically cleaned in alcohol.

Fig. 3
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Schematic diagram and image of equipment for LSP

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FCG Test

The FCG tests were performed on a MTS-880 ± 100 KN fatigue test machine at room temperature (25 °C) in air. The parameters were controlled by a computer to ensure a maximum load of 5 KN, stress ratio of 0.1, and frequency of 20 Hz with a tensile sinusoidal form. The crack length was monitored by a crack opening displacement silicon chuck, and the final lives were calculated for the final instantaneous fracture length of the CT sample.

Measurements of Residual Stress and Microstructure

After LSP, the tape was removed and the sample was cleaned with ethanol. The residual stress was determined by x-ray diffraction with the sin2 \(\varphi\) method (x-ray diffraction tester X-350A, Handan Stress Technology Co. Ltd., China). The x-ray source was CuKα, and the x-ray beam diameter was about 1 mm. The voltage and current of the x-ray source were 26 kV and 6.0 mA, respectively. The measurements were collected at different locations across the LSP region, and the micro-hardness of the untreated and treated samples was determined on a micro-hardness tester (DHV-1000) with a 100 g load and 10 s holding time.

The specimen fracture was cut from the specimen and then cleaned ultrasonically in acetone for 20 min. SEM (JSM-6010LA, JEOL) was performed to observe the fracture morphology. The microstructural change of the samples subjected to LSP impacts was characterized by TEM. The TEM samples were prepared as follows: the substrate side of the sample was ground to a thickness less than 20 mm. A thin zone was achieved by lowering the ion milling (Gatan691) from 4.8 to 3.2 kV and decreasing the angle from 15° to 4°, over 30 min. The TEM foils at the surface were prepared by a combination of single- and twin-jet electropolishing. A JEM-2100 JEOL system with 20 kV was used for the observations.

Results and Discussion

Surface Morphology

The surface morphology of a single spot subjected to a laser pulse energy of 7 J is shown in Fig. 4. The circular dent after the LSP treatment is caused by plastic deformation in the region of the shock wave loading. This plastic deformation increases with increasing laser pulse energy owing to the higher shock wave pressure, which is proportional to the square root of the laser pulse energy (Ref 28). A line profile of the peened surface subjected to a 7-J pulse is shown in Fig. 4. After LSP, the treated surface exhibits a regular array of micro-dents, which can be attributed to the plastic deformations induced by successive laser irradiation at different locations (Ref 29-31). The spacing of dents depends on the shot-to-shot offset of the laser.

Fig. 4
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Micro-dent on material surface after LSP treatment

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Micro-hardness Distribution

For most metal materials, plastic deformation enhances micro-hardness of the base material, by inducing a high density of dislocations and/or grain refinement (Ref 32). After LSP treatment, the micro-hardness distribution of the Ti-17 titanium alloy samples was measured (see Fig. 5). The depth of the gradient hardened layer was determined by analyzing the micro-hardness distribution along the cross section of the laser-peened sample. According to Chen et al. (Ref 33) and Carlsson and Larsson (Ref 34), the induced residual compressive stress can enhance the hardness of materials. The hardness along the cross section of the peened sample can be used to determine the depth of the gradient hardened layer. The micro-hardness test was used to measure the depth of the gradient hardened layer of the Ti-17 titanium alloy sample treated with LSP, as shown in Fig. 5. For the sample subjected to a laser energy of 7 J, the micro-hardness along the cross section increases at the surface of the gradient hardened layer and then gradually decreases to the value of the untreated region at a depth of (1.00 ± 0.05) mm, as shown in Fig. 5.

Fig. 5
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Micro-hardness distribution on cross section

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Residual Stress

The residual stress components of \(\sigma_{xx}\) (parallel to pulse scan direction) and \(\sigma_{yy}\) (perpendicular to pulse scan direction) should be considered and are reported along paths 1 and 2, respectively, as shown in Fig. 6. The residual compressive stress results of the untreated and LSP-treated samples are shown in Fig. 7. The residual stress in the two orthogonal directions exhibits a similar trend because of the similar loading conditions during LSP.

Fig. 6
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Photograph of specimen after LSP

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Fig. 7
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Comparison of residual stress before and after LSP

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When a strong laser shock wave is applied to the surface, a permanent strain is produced. The permanent strained region is counteracted by the surrounding materials leading to compressive stress. For multiple applications of LSP, the residual stress increases with the number of laser pulses. Thus, a larger residual compressive stress is induced by LSP and this is the main factor that improves the fatigue limits and reduces the fatigue gap sensitivity (Ref 8, 18).

FCG Results

Figure 8 shows curves of the crack length a versus number of cycles N for the CT samples before and after LSP. The effective length of the initial crack is 15 mm, and the final fatigue life of the untreated sample is 191,736 cycles compared with 286,593 cycles for the treated sample. Thus, LSP increases the fatigue life of the Ti-17 titanium alloy by 49.47%. As shown in Fig. 8, when the crack length is about 15 mm, the two curves begin to separate, indicating that the crack in the LSP sample propagates slowly during this period.

Fig. 8
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Crack length vs. cycles before and after LSP

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According to the crack size versus elapsed cycle data (a-N), the crack growth rate d a /d n is calculated. The crack tip stress intensity factor range, Δ\(K = K_{\hbox{max} } - K_{\hbox{min} } ,\) is calculated from the maximum and minimum loads of the loading cycles. The fatigue crack growth data are expressed in terms of a Paris power-law expression, in which the Paris law parameters, C and m, are constants (Ref 23):

$$\frac{{d_{a} }}{{d_{n} }} = C(\Delta K)^{m}$$
(1)

The complete curve of the FCG rate can be qualitatively divided into three sections: near-threshold, stable expansion, and rapid expansion. Region II (Fig. 8) is the Paris region defined by a power-law relationship that corresponds to a straight line on a log(d a /d n ) versus log(ΔK) curve. This data reduction technique, explained in ASTM E647-08, gives the ΔK increase corresponding to regions II and III.

The rate of fatigue crack growth at the near-threshold Δ\(K_{\text{th}}\) is quite slow. The large scatter in the data and influence of pre-cracking conditions make it difficult to determine Δ\(K_{\text{th}}\) as observed by Motz et al. (Ref 35). Thus, these complications hinder the Δ\(K_{\text{th}}\)-decreasing test in region I.

In region III, the crack growth rate is large and obtaining data in this unstable region is also difficult. For fatigue life prediction, region III is usually not considered because the number of cycles in this region is insignificant compared with the total fatigue life. Here, regions I and III are not considered.

The ΔK-increasing test is performed, and the fatigue crack growth rate or d a /d n is obtained from the slope of the a-N curve under displacement-controlled conditions.

Before LSP, the Paris formulation is:

$$\frac{{d_{a} }}{{d_{n} }} = 7.98 \times 10^{ - 4} \times \left( {\Delta K} \right)^{2.70088}$$

After LSP, the Paris formulation is

$$\frac{{d_{a} }}{{d_{n} }} = 3.24 \times 10^{ - 4} \times \left( {\Delta K} \right)^{3.29127}$$

The Paris formula was fitted to the relationship of the curves, and changes in the constant values C and m are detailed in Fig. 9. Similar results have been reported in previous crack propagation studies (Ref 36, 37). LSP reduces the FCG rate compared with untreated samples as indicated by the decline in the d a /d n and ΔK curves. The reduction of d a /d n is clear in the initial period of FCG. However, when ΔK increases to large values in the final period of FCG, the FCG rates of the samples before and after LSP are almost the same.

Fig. 9
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Fatigue crack growth rates before and after LSP

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The improvement in the FCG resistance induced by LSP can be separated into a relatively large increase in the initial FCG stage (Stage II in Fig. 8) and a slight increase in the final FCG stage (Stage III in Fig. 8). In the initial FCG stage, the residual compressive stress induced by LSP causes crack closure and reduces the effective driving force, which leads to a reduction of ΔK and d a /d n (Ref 38). However, the residual compressive stress relaxes with increasing of crack length and the FCG rate decreases more slowly (Ref 18). In the LSP samples, the crack arrest effects in the final FCG stage are limited because the crack driving force is much larger than the resistance induced by the residual compressive stress.

Fracture Morphology

The fracture morphology of specimens with and without the LSP treatment is examined by SEM. In Fig. 10, the locations of fatigue crack initiation (FCI) are shown on the fracture surface under different conditions (as indicated by yellow circles). A FCI exists at each slot entrance on the untreated surface, whereas there are two crack initiations on the treated surface. After LSP, the fracture appears flatter owing to compressive stress generated in the surface layer, which counters the tensile stress from transforming to compressive stress and increases the resistance to fatigue crack nucleation.

Fig. 10
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Morphology of fatigue crack origins

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Figure 11 shows the micromorphology of the stable crack growth zone of samples treated under different conditions. The microstructure occurred at the same place on the fracture surface 5 mm away from the edge of the slot. Clear fatigue striations are observed from the fracture surface as wavy stripes. These parallel fatigue striations are characteristic of a fatigue fracture that occurs in the direction perpendicular to that of the crack growth. From Fig. 11(a), the fatigue striation spacing is determined as 0.6-0.7 μm/cycle for unLSP treated sample, while the average value of LSP treated is 0.4-0.5 μm/cycle as observed in Fig. 11(b). The spacing between the fatigue striations shows the distance of crack growth per cycle. Therefore, the crack growth rate of the untreated sample is faster than that of the LSP-treated sample in line with the FCG results.

Fig. 11
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Micromorphology of the fatigue striations in the stable crack growth zone: (a) untreated and (b) treated

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Microstructures

Figure 12 shows TEM images of the Ti-17 titanium alloy before and after LSP. The microstructure is refined after LSP, and the fatigue performance is improved. High-density dislocations can be seen on the irradiated surface. These high-density dislocations improve the yield strength of the titanium alloy. The existence of many dislocations and their movement can prevent initiation and propagation of fatigue cracks so that fatigue resistance is improved (Ref 39).

Fig. 12
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TEM images of the Ti-17 titanium alloy surface: (a) density of dislocations (untreated); (b) density of dislocations (treated); (c) sub-grain (untreated); (d) sub-grain (treated); (e) nanocrystalline (untreated); (f) nanocrystalline (treated)

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As shown in Fig. 12(a), deformed twins aligning in one direction are observed in the base material; however, no twinning intersections are observed. A high density of twins is formed after LSP, as shown in Fig. 12(b). Typical microstructures including twins and obvious dislocation movements are observed in an α phase. Figure 12(c) shows a typical TEM image of the β phase of the base material. Many dislocation lines can be observed in coarse grains, and some dislocation lines disappear near grain boundaries. In Fig. 12(d), dislocation multiplication is observed in the grains treated by LSP and their high density means that the dislocations tangle with each other. At high densities, the dislocations will annihilate and rearrange near tangles and walls to reduce the total energy of the grain system. These dislocation tangles and dislocation walls further develop into low-angle sub-grain boundaries that result in refinement of the coarse grains (Ref 40). The nanostructure of the titanium alloy surface after LSP is shown in Fig. 12(f). The uncertain orientation relationship between neighboring sub-grains in the nanostructure results in a sliding channel length lower than the grain length scale. Consequently, crack propagation is twisted and the strength of the materials is improved.

In the materials subjected to the laser-induced shock wave, dislocations and plastic deformation are produced by dislocation slip and shock wave reflection and refraction at grain boundaries. The shock wave induces various effects on the grains resulting in dislocations after complex slip events, agglomeration and annihilation to form new grain boundaries, smaller grains, and a higher density of dislocations. In addition, there is the Hall-Petch relationship between the rupture stress and grain diameter:

$$\sigma = \sigma_{0} + Kd^{ - 1/2}$$
(2)

where σ is the rupture stress, \(\sigma_{0}\) is the basic rupture stress, K is a constant related to the materials, and d is the grain diameter. According to Eq 2, the rupture stress is inversely proportional to the grain diameter. After LSP, the average diameter of grains is smaller than that of the original materials. Thus, LSP improves the rupture stress, and the samples subjected to LSP require more energy to fracture (Ref 41).

A grain refinement mechanism during LSP is proposed. The results show that ultra-high strain and strain rates are involved in the formation of dislocation lines. The accumulation of these features contributes to the formation of complex random structures such as dislocation tangles, dense dislocation walls, and dislocation cells. Increases in strain result in sub-grain formation through dislocation annihilation and formation of multiple shear bands, which leads to an ultra-fine nanograin structure (Ref 42).

Conclusion

LSP is an effective surface treatment technique to retard the propagation of fatigue cracks and improve the fatigue life of Ti-17 titanium alloy. Our results reveal that multiple LSP treatments have a beneficial effect on the residual stress in the superficial layers. The crack propagation rate slows because of the superficial residual compressive stress induced by LSP. After the laser treatment, the density of dislocations increases and the grain size decreases, consequently prolonging the fatigue life of the materials.