Effect of laser shock peening on elevated temperature residual stress, microstructure and fatigue behavior of ATI 718Plus alloy

Accepted Manuscript Effect of Laser Shock Peening on Elevated Temperature Residual Stress, Microstructure and Fatigue Behavior of ATI 718Plus Alloy Micheal Kattoura, Seetha Ramaiah Mannava, Dong Qian, Vijay K. Vasudevan PII: DOI: Reference:

S0142-1123(17)30335-3 http://dx.doi.org/10.1016/j.ijfatigue.2017.08.006 JIJF 4431

To appear in:

International Journal of Fatigue

Received Date: Revised Date: Accepted Date:

21 June 2017 28 July 2017 5 August 2017

Please cite this article as: Kattoura, M., Mannava, S.R., Qian, D., Vasudevan, V.K., Effect of Laser Shock Peening on Elevated Temperature Residual Stress, Microstructure and Fatigue Behavior of ATI 718Plus Alloy, International Journal of Fatigue (2017), doi: http://dx.doi.org/10.1016/j.ijfatigue.2017.08.006

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Effect of Laser Shock Peening on Elevated Temperature Residual Stress, Microstructure and Fatigue Behavior of ATI 718Plus Alloy Micheal Kattoura1, Seetha Ramaiah Mannava1, Dong Qian2, Vijay K.Vasudevan1 1

2

Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH45221-0072, USA

Department of Mechanical Engineering, University of Texas at Dallas, Richardson, TX 750803021, USA Email address: [email protected]

ABSTRACT Laser Shock Peening (LSP) is a mechanical surface treatment that induces large compressive residual stresses and microstructural changes in the material by using repetitive shocks from laser pulses. In this study, we investigate the use of LSP to improve the fatigue life of ATI 718 Plus (718Plus) at high temperature of 650 °C. LSP led to severe surface plastic deformation, which, in turn, led to a high magnitude of surface compressive residual stresses and changes in the near-surface microstructure which caused high surface hardening. This change in the near-surface microstructure was in the form of high dislocation density forming dislocation entanglements and slip bands and formation of near-surface nanoscale sub-grains/crystallites that remained stable at elevated temperatures. In addition, LSP retained ~ -470 MPa residual stress (68% of its initial residual stress) even after 140 hours exposure to 650 °C. The retained residual stresses and the stable microstructure from the LSP increased the yield strength by ~14% (~ 140 MPa) and endurance limit by ~10% (~ 90 MPa) in corresponding tests at 650 °C. This improvement in fatigue life was attributed to near-surface microstructure, hardening and high 1

compressive residual stress. The estimated crack growth rates were 72% lower for LSP-treated 718Plus as compared with untreated material. The thermal-mechanical residual stress relaxation indicates the effectiveness of LSP in improving the fatigue life of 718Plus at 650 °C. Keywords: Laser Shock Peening (LSP); Nickel-based superalloys; High temperature fatigue; Residual stresses; Electron microscopy 1

Introduction Aircraft and power generation turbines demand high strength at high temperatures alloys.

In these applications, the service life is expected to be years, while maintaining a stable structure. In addition, higher operation temperatures increases the efficiency of the combustion cycle which yields substantial economic benefits. In order to increase the performance and maximum operating temperature of the commonly used nickel-based superalloy Inconel 718 (IN718), ATI 718 Plus (718Plus) was developed for improved thermal stability by 50 °C enhancing its high temperature capabilities up to 650 °C and ability to retain strength up to 704 °C [1,2]. 718Plus includes composition changes such as an increase to 9.13% Co from 0.16% (with correspondingly less Fe), the addition of 1%W and a higher Al content, see Table 1. The higher Al content promotes the formation of the L12 ' phase (Ni3(Al,Ti)), which has higher thermal stability than the DO22 ’’ phase (Ni3Nb) [2]. The coarsening rate of ' in 718Plus is much lower than the ’’ phase in IN718, which results in a reduction in the associated formation rate of the stable orthorhombic  phase (Ni3Nb). The addition of 1% W further increases the high temperature strength [3]. The compositional changes addressed above led to great improvements in the properties of 718Plus. With regards to tensile properties, 718Plus and IN718 have comparable strength at room temperature, but these are higher for 718Plus at 649°C and 704°C [4]. 718Plus also has good low cycle fatigue (LCF) properties and low crack propagation rates at 2

high temperatures (650 °C) [1]. In addition, few studies on fatigue crack growth showed that crack growth rates for 718Plus alloy were lower than that for either IN718 or Waspaloy [5–7]. Several studies have been conducted to understand the behavior of 718Plus at elevated temperatures. Some aimed to study the microstructure, thermal stability, oxidation behavior, hardness, strength, stress rapture life, fatigue crack growth and low cycle fatigue at elevated temperatures but little to no elevated temperature high cycle fatigue life data was reported in the open literature [3,5,6,8–12]. Other studies aimed to understand the effect of thermal-mechanical treatments on the fatigue crack propagation at room and elevated temperatures [13,14]. Several studies have been conducted to understand the microstructural phenomena behind the improvement of fatigue life at elevated temperature for ’- hardened nickel-based superalloys [15,16]. These studies attributed this improvement to the strengthening of ’ precipitates with the increase of temperature and the change of the slip mode from planar to wavy and thus more homogeneous distribution of slip creating more hardening in the alloy and giving rise to higher fatigue life. Fatigue begins with micro-crack initiation at a region of high stress intensity, such as a structurally weak spot or a region of tensile stress, followed by crack growth leading to final failure when the material cross section can no longer bear the load [17]. The fatigue micro-crack initiation often lies in the Bauschinger effect and the formation of extrusions and intrusions by a dislocation-based mechanism. Early work of Mughrabi and Laird successfully identified the formation of persistent slip bands (PSB), created by dislocations during fatigue loading, to be the basis of intrusions and extrusions formation [18]. Crack propagation with each cycle leaves a microscopic fingerprint of fatigue crack growth in the form of striations on the fracture surface indicating the advance of the crack with each cycle [19]. Surface treatments like shot-peening,

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deep-rolling, roller-burnishing, and laser shock peening (LSP) have been used to increase resistance to early fatigue crack initiation and growth, and to improve the fatigue life of various materials at room and elevated temperatures including LSP of 718Plus at room temperature [20– 28]. As shown in our recent report on 718Plus alloy [26], LSP introduces near-surface microstructural changes and compressive residual stresses that improve fatigue resistance by delaying crack initiation and lowering growth rates. However, few works have studied the effectiveness of these surface treatments on 718Plus at elevated temperatures. Studies of thermal relaxation that were done on several materials, including LSP of IN718 SPF, showed that the residual stresses’ relaxation behavior depends on the temperature but the microstructure created by these surface treatments tend to be much more thermally stable and still effective [23,27,29– 33]. A few studies have considered the effects of both cyclic plasticity and temperature on the residual stresses and work-hardened surface layers during elevated temperature fatigue, but none addressed 718Plus [23,34,35]. The purpose of this study was to evaluate and understand the improvement in strength and fatigue behavior of 718Plus due to LSP treatment at this temperature (650 °C). The thermal relaxation of the LSP residual stresses was studied at 650 °C. Corresponding uniaxial tension tests and uniaxial tension-tension fatigue tests were conducted at 650 °C to produce stress-strain and stress-life (S-N) curves to evaluate the effect of LSP on strength and fatigue behavior of 718Plus at 650 °C. The compressive residual stress relaxation under the combined effect of temperature and cycles (thermal-mechanical relaxation) at different stress levels was investigated. In addition, changes in the microstructure after LSP and fatigue testing were characterized using Scanning Electron Microscopy (SEM), Electron Back-Scatter Diffraction (EBSD)/ Orientation Imaging Microscopy (OIM), energy dispersive x-ray spectroscopy (EDS)

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and Transmission Electron Microscopy (TEM). Fracture modes, mechanisms, and estimated crack growth rates were also characterized by SEM to explain the improvement in fatigue life due to LSP treatment. 2

Experimental details 2.1 ATI 718Plus 718Plus sheets of dimensions 130 mm x 100 mm x 2 mm thick were sectioned using

electrical discharge machining (EDM) from 30 cm × 30 cm and thickness 12.7 mm plates obtained from ATI. The plates had been hot-rolled and solution treated at temperature 954 – 982 ˚C for an hour then air-cooled, and the nominal composition of 718Plus compared to IN718 is given in Table 1. A heat treatment and aging process is recommended by ATI for the formation and growth of γ’ precipitates for strengthening of the material. Thus, all the sheets underwent this heat treatment and aging process and served as the baseline material. This heat treatment and aging process was conducted in a vacuum furnace at 788 °C for 8 hours, furnace cooled at 38 °C per hour to 704 °C, held at 704 °C for 8 hours, then air cooled [36]. Table 1: Nominal Composition of 718Plus Compared With IN718. Element IN718 718Plus

Ni Bal Bal

Cr 17.9 17.42

Co 0.16 9.13

Mo 2.86 2.72

Al 0.49 1.46

Ti 1.01 0.71

Nb 5.22 5.48

Fe 18.08 9.66

W 0.03 1.04

P 0.008 0.013

C 0.025 0.028

B 0.004 0.005

2.2 Laser Shock Peening (LSP) and sample preparation LSP was performed using a Q-Switched, Nd:YAG laser with infrared wavelength (λ = 1064 nm) and at a frequency of 10 Hz. The target (718Plus sheet) movement was controlled using a programmed robotic arm synchronized with the laser during peening to produce a 30 mm × 90 mm patch with 50% overlap in the center of the sheet from both sides to cover the gauge area of the samples. Flowing water was used as confining medium and vinyl tape was used as the 5

sacrificial ablative coating. Based on a series of experiments involving variations in the LSP parameters and counterpart measurements of the surface residual stress, the conditions of pulse energy, spot size and pulse width shown in Table 2 were used for treatment of samples. The laser power density for each pulse based on these parameters was calculated to be ~ 4.7 GW/cm2. Rectangular cross section samples with continuous radius between ends according to ASTM E466-07 [37] (Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials) of dimensions 120 mm long x 3 mm wide in the center x 2 mm thickness and radius 30 mm were cut using EDM from the base material and the LSP-treated sheets, and is displayed in Figure 1. Figure 2 (a) shows a LSP-treated sheet with the peened patch area and a sample that is cut out of the sheet. Figure 2 (b) is a close up showing the pattern and overlap of the first three rows of LSP spots on the gauge area of the sample. The entire surface including the edges of the samples’ gauge area from both sides were treated by this technique, the two remaining side surfaces of the samples were not treated. Table 2: LSP condition utilized on 718Plus alloy. Condition LSP

Laser Energy (J) 3

Shot Diameter (mm) 2

Pulse width (ns) 20

Power Density (GW/cm2) ~4.7

Figure 1: Schematic of the sample used in the uniaxial tensile and fatigue tests. All dimensions are in mm.

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(b)

(a)

LSP Patch

Figure 2: (a) An LSP-treated sheet showing the peened patch area and a sample, (b) close-up image showing the pattern and overlap of the first 3 rows of LSP spots on the gauge area of the sample. All dimensions are in mm. 2.3 Residual stress and thermal relaxation Residual stresses were analyzed in two orthogonal directions using conventional X-ray diffraction using sin2Ψ technique with a Proto LXRD instrument (single axis goniometer using Ω geometry, Mn Kalpha x-rays). The two directions will be denoted as X and Y for the simplicity of analyzing the data. Alignment of instruments was checked before each set of measurements using a standard sample (316 stainless steel powder in this case) in accordance with ASTM E915-10 [38] (Verifying the Alignment of X-ray Diffraction Instrumentation for Residual Stress Measurement). The X-ray elastic constants were measured in accordance with ASTM E1426-94 [39] (Determining the Effective Elastic Parameter for X-ray Diffraction Measurements of Residual Stress). Details of the residual stress measurement parameters are provided in Table 3.

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Thermal residual stress relaxation study was conducted on two LSP-treated samples to investigate the relaxation of residual stresses with time due to the exposure to elevated temperature 650 °C. Interrupted heat exposure and residual stress measurements were performed at specific periods of time, then the tests were resumed at the same exact conditions prior to the interruption. The results reported are the averages of the duplicated test. Table 3: Parameters for XRD residual stress measurement. Item Detector

Description PSSD (Position sensitive scintillation detector), 20˚ 2θ range Mn Kα1 (λ = 2.10314 A˚) 0˚, ±2.58˚, ±9.07˚, ±12.45˚, ±18.8˚, ±23.0˚ 1 mm {311} set of planes. Bragg's angle: 152˚

Radiation Tilt Angles Aperture size (diameter) Plane (Bragg's Angle)

2.4 Tension and fatigue tests An MTS 810 testing machine, multipurpose servo hydraulic testing system for static and dynamic tests, with MTS 409.83 temperature controller and MTS 653 furnace, capable of reaching 1400 °C, was used to conduct elevated temperature (650 °C) tension and fatigue tests on the samples shown in Figure 1. The tension tests were conducted according to ASTM E21-09 [40] (Standard Test Methods for Elevated Temperature Tension Tests of Metallic Materials) with cross-head speed control at a rate of 0.005 mm/mm/min in order to allow proper determining of yield and ultimate strengths. The fatigue tests were conducted according to ASTM E466-07 [37] (Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials). The fatigue tests were force controlled tension-tension with stress ratio R = 0.1 and were conducted at frequency of 10 Hz. All the results shown have been at least duplicated and averaged to give a good statistical representation of the results. 2.5 Thermal-mechanical residual stress relaxation testing

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In order to understand the combined effect of temperature and number of cycles on the residual stress relaxation induced by LSP, experiments on three LSP samples tested at three different fatigue stresses (endurance strength, intermediate stress, and high stress) were performed at 650 °C. The residual stress measurements were performed with a Proto LXRD instrument with similar conditions to those described above. Interrupted elevated temperature fatigue testing and residual stress measurements were performed at specific cycles then the tests were resumed at the same exact conditions prior to the interruption. The cycles at which the test was stopped, and at which the residual stress measurements were done, were chosen to be in a logarithmic manner and up to 200,000 cycles. 2.6 Microstructure Fractured surfaces were analyzed under FEI Scois SEM to give fractured surface striation images. After fatigue testing, striation spacing on the fracture surface were recorded in an area 750 μm away from crack initiation site and close to the surface. These samples were also analyzed by EBSD to study the near-surface crack microstructure, misorientations and plastic deformation. EBSD samples were prepared by electro-polishing with 875 mL methanol and 125 mL sulfuric acid in an ElectroMet 4 system from Buehler with a voltage of 24 V. EBSD scans were carried out using a TSL OIM analysis system in the SEM operating at 30 kV. Characterization of the near-surface regions of the LSP samples is important for understanding the fundamental microstructural changes introduced by this surface treatment. Thin foils TEM lamella were obtained from the LSP-treated samples and fatigued samples using a Focus Ion Beam (FIB) starting from the surface and into the material using an FEI Scios with Sidewinder HT FIB and Easylift Nano-manipulator. TEM thin foils were observed with a Phillips/FEI CM-20 TEM operated at 200 kV and photographs from relevant regions were

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recorded under bright field (BF), dark field (DF), weak-beam dark field (WBDF) and selected area diffraction (SAD) modes. 3 3.1

Results Baseline and LSP-treated material From the room temperature part of this study [26], the microstructure of the Baseline

material was analyzed prior to LSP-treatment and revealed grain size of ASTM 7 (around 32 μm diameter) that did not change with the LSP treatment. In addition, the microstructure within the grains is composed of more-or-less spheroidal, homogeneous distribution of ’ precipitates with size in the range of 30 - 50 nm, and δ precipitates at the grain boundaries. LSP treatment changed the near-surface microstructure in the form of high dislocation density forming dislocation entanglements and slip bands, and formation of a few near-surface sub-grains 30-50 nm in size. The near-surface hardness of 718Plus increased from 6.71 ± 0.25 GPa for the Baseline to 8.33 ± 0.34 GPa for the LSP-treated material. Through-the-depth analysis showed that the hardness is highest on the surface and then decreases with increase in distance from the surface and becomes more or less constant and equivalent to the Baseline hardness beyond about 450 μm. 3.2

Thermal relaxation of residual stress

To study the effect of LSP on 718Plus at high temperature of 650 °C, a thermal residual stress relaxation study was conducted to investigate the relaxation of surface residual stresses with time due to the exposure to elevated temperature of 650 °C and is presented in Figure 3. For LSP, the direction of laser peening (X-Direction) showed almost similar values to its transverse (Y-Direction) and that is due to the biaxial nature of the process. In both directions, there is an increase in magnitude of around 600 MPa of surface compressive stresses when compared to the

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Baseline [26]. Thermal relaxation of the residual stresses is drastic in the first few hours but the values of the residual stresses become more or less constant after 10 hours with very slight drop with time up to 140 hours. In the first hour, the residual stress relaxed at a very high rate from around -700 MPa to around -575 MPa. Afterwards, the rate of relaxation decreased and the residual stress relaxes to around -500 MPa after 10 hours. From there on, the rate of relaxation decreases drastically and the magnitude of the residual stresses are almost constant with slight drop to around -470 MPa after 140 hours. The LSP-treated 718Plus residual stresses stabilizes and retains around 68% of its initial residual stresses even after exposure to 650 °C for 140 hours.

Figure 3: Thermal relaxation of LSP residual stresses at 650 °C 3.3

Elevated temperatures uniaxial tension tests and stress-strain curves

Elevated temperature uniaxial tension tests were used to evaluate the effect of LSP treatment on the strength of 718Plus at high temperature of 650 °C. Figure 4 shows the results from the uniaxial tension tests conducted on Baseline and LSP samples at 650 °C. Comparison of the LSP and Baseline samples shows an increase of ~14% in the yield strength but slight

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reduction of ~3.5% in the failure strain. The increase of the near-surface hardening and the retained compressive residual stress caused by the LSP treatment gave rise to higher strength. The serrations in the stress-strain curves are a product of the temperature and strain rate at which these tests were conducted. Serrations in the stress-strain curves at elevated temperatures have been reported previously in nickel-base superalloys, including 718Plus, and associated with two mechanisms that are both temperature and strain rate dependent. The first mechanism is the Portevin–Le Chatelier (PLC) effect, i.e. interaction of randomly formed slip bands with the precipitates [41,42]. The second mechanism, oxygen-induced intergranular and dynamic embrittlement, involves diffusion of oxygen to the grain boundaries during the high temperature test in air and associated weakening [9,43–46]. A full map of temperature versus strain rate explaining the regions dominance of these phenomena was constructed for IN718 [47].

Figure 4: Uniaxial tension stress-strain curves for Baseline and LSP samples tested at 650 °C.

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3.4

Elevated temperatures uniaxial fatigue tests and stress-life (S-N) curves The stress-life (S-N) fatigue behavior of 718Plus is plotted in Figure 5 in terms of the

maximum stress as a function of number of cycles to failure (at R = 0.1, 10 Hz frequency) for Baseline and LSP at both room temperature (23 °C) [26] and high temperature (650 °C). The fatigue strength of Baseline 718Plus at high temperature (650 °C) has increased compared to that of room temperature (23 °C). Comparing the Baseline at 650 °C samples with Baseline at 23 °C samples, it is apparent that there is an increase in the fatigue endurance strength at 5,000,000 cycles by around 20%, i.e. an increase in the endurance strengths by around 145 MPa at high temperature compared to room temperature. This increase of fatigue life is also prevalent in the lower life cycles (105 cycles’ regime). This is attributed to the strengthening of ’ particles with the increase of temperature and the change of the slip mode from planar to wavy and thus more homogeneous distribution of slip creating more hardening in the alloy and giving rise to higher fatigue life [15,16]. Comparing the LSP at 650 °C to the Baseline at 650 °C, it is clear that the LSP treatments increased the endurance strength at 5,000,000 cycles by around 10%, i.e. an increase in the endurance strengths by around 90 MPa after the surface treatment. The beneficial influence of LSP treatment is maintained with slight increase in percentage improvement in the lower life cycles (105 cycles’ regime). The fatigue performance improvement in 718Plus by LSP at high temperature of 650 °C is a synergistic effect of surface work hardening and retained surface compressive residual stresses at this elevated temperature. Enhanced fatigue resistance at room and elevated temperatures due to LSP has been shown in several materials subjected to different types of fatigue [20–27].

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Figure 5: Uniaxial tension-tension fatigue testing stress-life (S-N) curves for Baseline and LSP at room (23 °C) [26] and high (650 °C) temperatures 3.5

Thermal-mechanical relaxation of the residual stress from LSP Mechanical surface treatments like shot peening, deep rolling, LSP and others modify the

nature of residual stresses in the near surface region from tensile to compressive. The presence of deep compressive residual stresses after surface treatments has been shown to be beneficial against fatigue cracks that initiate at the surface. Thus, understanding the change in the residual stress after cyclic loading at elevated temperatures is important. LSP treated samples were subjected to cyclic loading at 650 °C at different stress levels and residual stresses were measured after a set number of cycles as shown in Figure 6. The initial compressive stresses relax drastically in the first few cycles, dropping from -750 MPa to around -580 MPa slightly varying depending on the applied stress. With the sample tested at 1110 MPa, for which the life to failure is around 455,000 cycles, there is a gradual decrease in the residual stress between 10 and 10000 cycles. After that, the stress relaxation becomes larger until it reaches -225 MPa at 200,000 cycles, which is close to its failure life. Similar initial behavior can be seen for the 14

sample tested at 1065 MPa, for which the life to failure is around 725,000 cycles. However, the rate of decrease in the residual stress after 10,000 cycles is less than that of the previous sample reaching a residual stress of -390 MPa at 200,000 cycles. This indicates that the number of cycles that this sample has already been subjected to is still much less than its life to failure. Finally, the third sample tested at stress of 965 MPa, which is the endurance limit, didn’t fail before the limiting 5,000,000 cycles mark. These results show that the stress initially relaxes in the first few cycles but only slightly relaxes up to the 200,000 cycles. This behavior indicates that the residual stress will only slightly relax due to thermal effect after the initial cyclic relaxation before becoming more or less constant after a period of time as discussed in 3.2, which is the reason that the latter samples did not fail.

Figure 6: LSP residual stress relaxation with number of cycles at 650 °C for samples tested at applied maximum fatigue stresses of 965, 1065, and 1110 MPa. 3.6

Fracture behavior and estimated fatigue crack propagation rates SEM observations of the fracture surfaces of the failed samples were carried out to better

understand the improvement of fatigue life by LSP at 650 °C. Baseline and LSP samples were

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analyzed under the SEM to understand the crack initiation, propagation, and failure mechanism in the samples. Figure 7 shows the cracks’ initiation sites and these cracks propagate through the samples to their critical length followed by ductile failures. All of the samples that were subjected to fatigue loading failed similarly due to a crack that initiated at the edge corner. Images of fatigue striations on the fracture surface were recorded at the same distance 750 μm from the crack initiation site and from almost the same area shown in Figure 7. Images of the striations in the Baseline and LSP samples tested at maximum stress of 977 MPa at 650 °C are shown in Figure 8. The striations per unit length were measured and crack growth rates were subsequently estimated [48]. These values were determined using multiple SEM images for each sample from the area shown in Figure 7. The results for the number of striations per micron and the estimated crack growth rates are shown in Table 4. One Baseline and LSP samples tested at maximum stress of 977 MPa had lives of 610,000 and 3,365,000 cycles respectively. This can be related to the lower estimated rate of crack propagation for the LSP sample (0.078 μm/Cycle) as contrasted with the Baseline (0.275μm/Cycle). This shows that the LSP treatment had improved the fatigue life of the material by not only delaying crack initiation but also reducing the crack growth rate.

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Figure 7: Stages in fatigue failure of the elevated temperature fatigue samples showing crack initiation site, crack propagation, and ductile failure region for both Baseline and LSP sample tested at 977 MPa.

(a)

(b)

Baseline

LSP

Figure 8: SEM micrographs of striations on the fracture surface after elevated temperature fatigue loading at 977 MPa: (a) Baseline, and (b) LSP. Table 4: Striations per micron and the estimated crack growth rates for the Baseline and LSP samples tested at 977 MPa and 650 °C. Sample Condition

Striations/μm

Baseline at 650 °C LSP at 650 °C

3.63 ± 0.02 12.81 ± 0.01

Estimated Crack Growth Rates (μm/Cycle) 0.275 ± 2.0x10-3 0.078 ± 0.1x10-3

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Cycles to Failure 610000 3365000

3.7

Microstructure of the fractured samples near the fractured surface under EBSD The top surfaces of the Baseline and LSP samples tested at 977 MPa were mapped using

EBSD/OIM to study the extent of plastic deformation experienced by each sample and other features. Two mapping methods were applied to understand the plastic deformation: (1) kernel average misorientation (KAM), and (2) grain average misorientation (GAM) which is similar to grain orientation spread or intragranular misorientation deviation (IMD). Both methods rely on deformation strain to develop intragranular misorientations through substructural evolution, such that the resultant misorientations provide a measurable signature of strain. The KAM is a measure of the average misorientation of a point with respect to a selected number of its nearest neighbors. In contrast, GAM and IMD measure the average of all deviations between each point in a grain and the grain’s average orientation [49–51]. Figure 9 provides the comparison of the Baseline and LSP samples tested at 977 MPa and 650 °C. In all these images the fractured surface is at the right side edge of the image and the direction of loading is shown as a red arrow, with their locations shown in green boxes in Figure 7. Figure 9 shows that the LSP sample has higher misorientation values, this is reflected in the both GAM and KAM, which are around 1.74 and 1.34 for the LSP sample compared with 0.73 and 0.53 for the Baseline sample, respectively. The KAM maps show a larger fraction of misorientations (red areas) especially near the grain boundaries near the fractured surface. The LSP sample had a much higher number of cycles to failure (3,365,000 cycles) compared with the Baseline sample (610,000 cycles). This shows that the LSP sample accommodated much more plastic deformation before failure compared to the Baseline sample.

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(b)

(a)

GAM LSP

GAM Baseline

(d)

(c)

GAM Average = 1.74

GAM Average = 0.73

(f)

(e)

KAM LSP

KAM Baseline

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(h)

(g)

KAM Average = 0.53

KAM Average = 1.34

(i)

(j)

Figure 9: EBSD analysis for the elevated temperature fatigue samples tested at 977 MPa: GAM map for (a) Baseline and (b) LSP, GAM chart for (c) Baseline and (d) LSP; KAM map for (e) Baseline and (f) LSP; KAM chart for (g) Baseline and (h) LSP;and legends for (i) GAM and (g) KAM maps. 3.8

Secondary crack analysis and mixed mode of fracture In order to understand the nature of the cracks and mode of failure of 718Plus at 650 °C,

Baseline sample tested at 977 MPa and 650 °C was analyzed under the SEM and secondary cracks were mapped using EBSD/OIM. The cracks analyzed were a mix of trans-granular cracks similar to that at room temperature [26] and inter-granular cracks as shown in Figure 10. The regions near the secondary crack in the SE image in Figure 10 (a) were scanned using EBSD and the image quality (IQ) map is shown Figure 10 (b). The crack is indicated by the arrows, and the boundaries are highlighted in black in Figure 10 (b). These images show that the crack path was predominantly but not exclusively inter-granular in nature since it mainly followed the grain boundaries of the material. The grains and their orientations in that area can be more clearly seen in the inverse pole figure (IPF) image in Figure 10 (c) where the arrows point out the crack

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position. The orientation of the grains around the crack and their boundaries indicate that the crack mainly propagated through the grain boundaries. Finally, the crack and its nature can be further analyzed in the KAM map in Figure 10 (d). Variations in the local misorientation provide a good indicator of the net change in dislocation density caused by plastic deformation, as explained in the previous section. Comparing the local misorientation changes in the vicinity of the fatigue cracks and in the microstructure of the surrounding neighborhood provides a way to identify the locations experiencing cyclic plastic deformation and fatigue crack initiation grains. Figure 10 (d) shows that the misorientation are mainly around the grain boundaries where the crack propagated in this region of the sample. The crack nature and propagation behavior have been analyzed for nickel-based superalloys tested at different temperatures and shows the transition from trans-granular to mixed to inter-granular cracks with the increase in testing temperature [45–47,52,53].

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Figure 10: Secondary crack showing inter-granular behavior seen through: (a) SE micrograph, (b) IQ map showing the cracks and surrounding grain structure, (c) IPF map and (d) KAM map. 3.9

TEM observations of the microstructure of the fatigued samples The microstructure of 718Plus after elevated temperature fatigue testing for the Baseline

and LSP-treated samples were analyzed under TEM to understand the microstructure changes at

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elevated temperature. Thin foils/lamellas for TEM observations were prepared from the Baseline and LSP-treated elevated temperature fatigued samples using the FIB lift out technique covering regions from the surface to a distance of ~5-10 μm into the depth. The main features of the Baseline sample fatigued at 650 °C is the more homogeneous distribution of the slip that can be seen in Figure 11 (a). In addition, Figure 11 (b) shows the formation of fatigue cell structure. The LSP process induces an extreme increase in dislocation density from the plastic deformation caused by the laser-induced high energy shock wave. This increase in dislocation density leads to dislocation entanglements and slip bands. The extent of this increase drops as the shock wave moves deeper into the material due to natural damping. In addition, some sub-grains with 30-50 nm size are observed near the surface. These observations were reported for room temperature LSP of 718Plus [26]. As for the LSP-treated and elevated temperature fatigued sample, Figure 11 (c) – (f) shows the stability of the microstructural changes by LSP even after fatigue testing at 650 °C. Figure 11 (c) & (d) shows this huge increase in the dislocation density causing formation of dislocation entanglements and slip bands. In addition, Figure 11 (e) & (f) shows the near surface sub-grains layer with the sub-grains of the size 30-50 nm. In addition, the oxide layer formed from testing at 650 °C can be seen directly under the protective Pt-deposition layer.

(a)

(b)

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(c)

(d)

(e)

(f)

Figure 11: TEM images of fatigue samples tested at elevated temperature for: Baseline showing (a) low magnification BF image showing more homogeneous distribution of the slip, and (b) fatigue cell structure. LSP-treated showing (c) & (d) huge increase in the dislocation density causing formation of dislocation entanglements and slip bands, (e) & (f) near surface sub-grains layer with the sub-grains of the size 30-50 nm. 4

Discussion The results presented above have led to a number of new findings on the effects of LSP

on residual stress and near-surface microstructures in 718Plus alloy and in turn their effects on the tensile and fatigue properties and related mechanisms at high temperature (650 °C). These findings are discussed in detail below. The most important finding of this study is the large increase in strength (Figure 4) and fatigue resistance (Figure 5) of 718Plus at 650 °C following LSP, which is caused by both the retained high near-surface compressive residual stresses (Figure 3) and stable LSP 24

microstructure (Figure 11). LSP involves repeated laser shocks at high frequency onto the material surface. As discussed at room temperature [26], the material is deformed plastically to a distance governed by the peak pressure resulting from the shock relative to the Hugoniot elastic limit (HEL) of the material. The HEL is the magnitude of elastic precursor of the shock wave and is given by Johnson and Rohde [54]:

Where above

is the yield strength of the metal under uniaxial compression.

The material is deformed up to the depth where peak pressure no longer exceeds the HEL. This generates a gradient in plastic deformation with depth from the treated surface, and thus a gradient in residual stress and microstructure [26]. At the very top surface, intense and rapid plastic deformation results in huge increase in the dislocation density causing formation of dislocation entanglements and slip bands. In addition, sub-grains 30 nm to 50 nm are observed near the surface. In the subsurface layer, the dislocation density is high, but lower than that on the surface. In the interior, little plastic deformation occurs and the dislocation density is largely unaffected. Similar findings were presented in different works using LSP [55,56]. The analysis of the microstructure of the LSP-treated sample fatigue tested at 650 °C (Figure 11) showed that this microstructure is stable even at high temperature. The stability of the microstructure created by LSP at elevated temperatures was reported by other studies on various materials [23,33]. Theoretically, the alloy’s strength can be predicted by adding contributions from the reduction in grain size (grain boundary strengthening) and increase in dislocation density with plastic deformation (work hardening) [57,58] to the contribution of solid solution strengthening and precipitation hardening [59]:

25

Where above stress,

is the strength, is

a friction stress,

is the solid solution strengthening

is the precipitation hardening stress, k is the Hall–Petch constant,

is the mean

free path for dislocations, α is a constant, G is the shear modulus, b is the Burgers vector and ρ is the dislocation density. The improvement in the fatigue life of nickel-base superalloys at elevated temperatures has been addressed by several studies [15,16]. These studies contribute this improvement to ’ precipitates becoming stronger as the temperature increases via a cross-slip mechanism which greatly increase the dislocation drag. The increase in the strength of ’ particles increases the contribution of the precipitation hardening stress (

) at elevated temperatures. Thus, this

creates a more effective barrier that functions against the formation of single dislocation slip band. As the temperature increases, the change from planar to wavy slip mode results in a more homogeneous distribution of slip. Alloys deforming more homogeneously harden more than those deforming through planar glide which improves the fatigue life of the material [15,16]. This explains the improvement in the fatigue life of 718Plus at elevated temperature compared to that of room temperature (Figure 5) In the comparison of LSP to Baseline, the effect of the first three stresses (i.e. ) is constant. Thus, the highest work hardening and strength occurs at the surface where there are high dislocation density and sub-grains, and at the subsurface where there are high dislocation densities. The extent of plastic strain and dislocation density decreases with depth due to the dampening of the material, owing to which the hardness and strength decrease with depth to values of the unaffected matrix material. The enhanced hardening caused by the presence of the sub-grains and high dislocation density in the near-surface regions also leads to high values of the compressive residual stresses, which subsequently decrease in magnitude with 26

depth because of the decreasing plastic strain and dislocation density. These results were reported previously at room temperature [26] and by several researches including Dane [60], Prevey [30], and Gill [61] in LSP-treated IN718. LSP increases the fatigue life and endurance limit of alloy 718Plus at 650 °C (Figure 5) due to the combined effects of retained compressive residual stresses and temperature-stable work hardening in the near-surface regions. The high retained near-surface compressive residual stresses at elevated temperatures following LSP (Figure 3) act as a shield and locally lower the stress intensity during fatigue by acting against the imposed axial tensile stresses, and thereby prevent or at least delay the initiation of cracks. The estimated crack growth rates are also reduced because the cracks advance into a compressive stress field. From a microstructural point of view, the high dislocation density, sub-grains, and work hardening of the surface creates a barrier that restricts the movement of dislocations to the surface. Thus, the required cycles to create extrusions and intrusions, which lead to crack initiation, is much higher for the LSP material compared with the base material and the estimated crack growth rates especially near the surface are much lower (Table 4). Consequently, the material withstands much more plastic straining (Figure 9) prior to failure. The residual stresses and their relaxation with the number of cycles at 650 °C (Figure 6) explain the shielding that the LSP treatment provides the material and improves the fatigue life of 718Plus at high temperature. In the first cycles, there is a large relaxation in the residual stresses due to combined effect of the applied forces and initial straining, and the large thermal relaxation of the first few minutes. Subsequently, the levels of these stresses only slightly drop due to thermal relaxation until the sample gets closer to the failure life where they start to relax due to local plasticity. In the interim cycle period, the residual stress is partially shielding the material from the applied stresses by reducing the

27

effective applied stress. Subsequently, the relaxation is mainly due to the plastic straining (Figure 9) that the material accumulates with the cycles and thus the amount of shielding is reduced. For the samples that showed a run-off and did not fail, however, the compressive residual stresses remained high even after 200,000 cycles. The effective applied stress in the material was lower than that required to create micro-plasticity and thus plastic straining in the material was prevented. Similar results for thermal-mechanical residual stress relaxation was reported for deep-rolling and laser-peening of various materials [23,34,35]. In summary, the increase in the fatigue life of alloy 718Plus at high temperature of 650 °C by LSP is due to the combined effects of high magnitude of retained compressive residual stresses and the microstructural resistance created in the material that delayed crack initiation and propagation. 4. Conclusions The following conclusions can be drawn from the results of this study: 1. 718Plus showed higher fatigue resistance at high temperature of 650 °C as compared to room temperature. 2. The residual stresses created by LSP process were largely retained -470 MPa (68% of initial -700 MPa residual stresses) even after exposure to 650 °C for 140 hours. 3. The near-surface microstructure created by severe surface plastic deformation of LSP was stable at 650 °C. The extreme increase in dislocation density causing formation of dislocation entanglements and slip bands, and sub-grains 30 nm to 50 nm were observed in LSP-treated fatigued sample at 650 °C. The amount of this increase in dislocation density drops as the shock wave moves deeper into the material due to natural damping.

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4. The retained residual stresses and the stable microstructure created by LSP influence the specimens' properties in the form of an overall increase in 718Plus yield strength by ~ 140 MPa and the fatigue endurance limit by ~ 90 MPa at 650 °C. 5. All fatigue samples investigated failed due to a crack initiation at the edge corner. LSPtreated sample survived longer due to the retained compressive residual stresses. This was analyzed through the study of the estimated crack growth rates and the thermalmechanical residual stress relaxation that investigated the stress shielding due to LSP. This stress shielding improved the fatigue life of 718Plus through inhibiting the nucleation and propagation of fatigue cracks. 6. The mechanism by which LSP helped improve the fatigue life of the material was also explained from a microstructural point of view. The increase in dislocation density causing formation of dislocation entanglements and slip bands, and sub-grains 30-50 nm due to LSP created a barrier for the dislocation motion to the surface thus delaying the creation of intrusions and extrusions which hindered the nucleation and propagation of fatigue cracks at the surface.

Acknowledgments The authors are grateful for financial support of this research by the National Science Foundation (Grant nos. CMMI-1334538, CMMI-1335204, and MRI-1531593) and ATI Specialty Metals for providing the 718Plus material used in this study. We also gratefully acknowledge the contribution of the State of Ohio, Department of Development and Third Frontier Commission (Grant no. TECH 10-014), which provided funding in support of the “Ohio Center for Laser Shock Processing for Advanced Materials and Devices” and the equipment in 29

the center that was used in this work. The collaboration with Cincinnati State Technical and Community College through the NSF-funded Louis Stokes Alliances for Minority Participation (LSAMP) Program (Grant no. HRD 1304371) that supported the undergraduate research work of Mr. Omar Eddins, Miss Luisa Anaya, and Mr. William Edwards on this project is also acknowledged. We would like to thank Miss Marianne Robison for her help proofreading this manuscript. Any opinions, findings, conclusions, or recommendations expressed in these documents are those of the author(s) and do not necessarily reflect the views of the NSF or the State of Ohio, Department of Development. References [1]

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LSP of ATI 718 Plus

Sacrificial layer: tape

718Plus

Near Surface Microstructure: High Dislocation Density with Dislocation Entanglements and Slip Bands

High Temperature (650 °C) Uniaxial Tension Tests

Water overlay ~2mm Focused Infrared Laser

Shock wave Confined plasma

High Temperature (650 °C) Uniaxial Fatigue Tests

Highlights:  Uniaxial tensile tests and uniaxial fatigue tests were conducted at 650 °C.  Laser shock peening improved the strength and fatigue life of ATI 718Plus.  Near surface microstructure changes due to laser shock peening were characterized.  Residual stress created by the laser shock peening hindered fatigue cracks.  Correlation of residual stress/microstructure and strength/fatigue life improvement.

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