Materials Science & Engineering A 576 (2013) 346–355

Contents lists available at SciVerse ScienceDirect

Materials Science & Engineering A

0921-50http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/msea

Comparison of mechanisms of advanced mechanical surface treatments innickel-based superalloy

Amrinder Gill a,n, Abhishek Telang a, S.R. Mannava a, Dong Qian b, Young-Shik Pyoun c,Hitoshi Soyama d, Vijay K. Vasudevan a

a School of Dynamic Systems, University of Cincinnati, Cincinnati, OH, USAb Department of Mechanical Engineering, The University of Texas at Dallas, Dallas, TX, USAc Department of Mechanical Engineering, Sun Moon University, Cheonan, South Koread Department of Nanomechanics, Tohoku University, Sendai, Japan

a r t i c l e i n f o

Article history:Received 18 February 2013Received in revised form4 April 2013Accepted 5 April 2013Available online 13 April 2013

Keywords:Ni-based superalloysLaser shock peeningCavitationResidual stressesX-ray diffraction

93/$ - see front matter Published by Elsevierx.doi.org/10.1016/j.msea.2013.04.021

esponding author. Tel.: +1 513 556 3119; fax:ail address: gillah@mail.uc.edu (A. Gill).

a b s t r a c t

Mechanical surface enhancement techniques are used to introduce compressive residual stresses toenhance the fatigue life and corrosion resistance of metallic components. In this study, we compare theeffects of three advanced mechanical surface enhancements treatments: laser shock peening, cavitationshotless peening and ultrasonic nano structure modification on residual stress, hardness, plasticdeformation and changes in near surface microstructure introduced in a Ni-Base superalloy, IN718 SPF(Super plastic forming). Coupons of the alloy were peened using two different conditions of eachtreatment and results compared to achieve a better understanding of underlying mechanisms of thesetechniques. Results indicate that there are significant differences in mechanisms of these surfacetreatments leading to differences in material response.

Published by Elsevier B.V.

1. Introduction

Mechanical surface enhancement treatments like Shot Peening(SP), Laser Shock Peening (LSP), Cavitation Shotless Peening(CSP),Ultrasonic Nanocrystal Surface Modification (UNSM), low plasti-city burnishing(LPB) among others introduce compressive residualstress to achieve improvement in fatigue life. Each of thesetechniques has some unique advantages and disadvantages andaffects materials in different ways. The selection of a technique forapplication requires analysis of its effects on residual stressincluding magnitude and depth, surface roughness, microstructuresand extent of plastic deformation.

IN718 is the most widely used Ni-base superalloy in gas turbineengines. IN718 super plastically formed (SPF) alloy is a relativelynew fine-grained aero engine alloy that is capable of super plasticforming. The ability to super plastically deform enables easier andcheaper manufacturing of complex parts.

LSP uses a Q-switched laser for ablating a thin coating (usuallya vinyl or Aluminum tape) on a material surface, with a highenergy (several joules), short duration (o50 ns) laser pulse. Thematerial is covered with a transparent confining medium like

B.V.

+1 513 556 3773.

water. The plasma produced on ablation is confined by water layer,allowing it to absorb more energy. The plasma expands rapidlyand finally blows off, introducing a shock wave into the material.The volume affected by the shockwave is plastically deformedduring its propagation to a depth beyond which the peak pressurepulse does not exceed the Hugoniot Elastic limit (HEL) of thematerial. The surrounding material in the sub-surface region isopposed to this lateral straining, resulting in a biaxial compressivestress near surface. A number of studies have demonstrated thebeneficial effect of LSP in improvement of fatigue life. Recently, LSPat atypical temperature have been reported to enhance fatigue life[1–3]. The process is controlled by varying the energy of the laser,spot size and pulse width to create a patch as per requirement.A typical application of the process, utilizes a Q-switched Nd:Glass laser (λ¼1.054 μm) or Nd:YAG laser (λ¼1.064 μm). A numberof studies provide excellent description of the process and physicsbehind it [4].

Cavitation shotless peening is a peening process which makesuse of impacts of cavitation bubbles to introduce plastic deforma-tion in near surface regions. Cavitation is a phase change phenom-enon (from liquid to gas phase) and refers to formation andimplosion of bubbles in a liquid. On imploding, these bubblesgenerate shock waves which introduce plastic deformation in nearsurface regions of materials. Cavitation bubbles are well known tocause erosion in hydraulic equipment, but in cavitation peening

Table 2Mechanical Properties of IN718 SPF Alloy in the As-received Condition.

Yield Stress (Mpa) U.T.S(Mpa) Young's Modulus (GPa) Poisson's ratio

1070 1270 205 0.285

Table 3Laser Shock Peening Parameters.

Energy(J)

Pulsewidth(ns)

SpotDiameter(mm)

Power density(GW/cm2)

Peened patchsize(mm)

Overlap

8.23 28.6 2 9 10�10 50%15.24 25.3 2.18 16 10�10 50%

A. Gill et al. / Materials Science & Engineering A 576 (2013) 346–355 347

the conditions are controlled to cause deformation without anyremoval of material.

In cavitation peening, liquid phase is turned into gas phase by areduction in static pressure, till the saturated vapor pressure isreached, by increasing the flow velocity. The cavitation nuclei aretiny bubbles present in high speed region of jet. On increasing thestatic pressure (by decreasing flow velocity), the bubbles implode.On implosion, these bubbles produce micro jets of high velocity(∼1500 m/s) which causes plastic deformation in the material. Asthe cavitation bubble shrinks, they rebound causing a shock wave,which also causes plastic deformation. The cavitation is normallyproduced by injecting a high-speed water jet in a water chamber(CSP in water) [5], but now it is possible to produce cavitation inair (CSP in air), by injecting a high speed water jet into a low speedwater jet which is then injected into air as shown in [6,7]. This haswidened the scope of application of cavitation peening as it doesnot require a water filled chamber for peening. The parameterswhich can be to control the process include pressure, traverse rateof jet, standoff distance, exposure time, and types of jets used[8,9]. The mechanisms behind both the techniques i.e. CSP in airand water have been discussed by Soyama [6,7].

The effectiveness of CSP in enhancing fatigue strength, retard-ing hydrogen assisted fatigue crack growth and stress corrosioncracking have been reported recently [10]. Cavitation peening hasbeen applied in nuclear power plants on a variety of componentsincluding Stainless Steel and Nickel-Base alloys as preventivemaintenance against stress corrosion cracking.

Ultrasonic nanocrystal surface modification (UNSM) is a surfaceenhancement process developed and patented in Sun MoonUniversity (S. Korea) by Dr. Young Pyun. The process uses vibrationenergy by striking the material surface at the rate of severalthousand strike sper second (20–40 KHz) as a constant pressureis applied. This method uses a piezo electric transducer (forvibrations) and a tungsten carbide ball to transmit the ultrasonicvibrations (with a magnitude of ∼25 μm) to the material. In UNSM,the total energy applied on the material is the sum of the staticenergy due to constant pressure imposed the ultrasonic vibratorydevice and the dynamic energy (∼2.5–5 times the static energy) asa result of the ultrasonic vibration. Process variables include totalstriking force, feed (S), ball radius (r), amplitude of dynamic load(P), and speed (V (m/min)). The mechanism and the device hasbeen described in more detail in literature [11].This methodintroduces severe plastic deformation and nano-crystal structurein the near surface region of the material. Improvements in wearand fatigue properties after UNSM process attributed to theincreased hardness and large compressive residual stresses inthe modified layer have been reported [12].

In this study, we characterize the effects of three surfacetreatments: LSP, CSP and UNSM on an aero alloy IN718 SPF interms of the nature of residual stress fields, hardness modification,plastic deformation and near surface microstructural changesintroduced by these techniques. This study aims to compare andcontrast the fundamental mechanisms responsible for the changesin material properties introduced by these techniques. To this end,special care was taken to obtain thin foils for Transmissionelectron microscopy from areas very close to surface. Doing soenables a better observation of mechanisms of plastic deformationas the changes brought about by these techniques are most severeat surface. To best of our knowledge no such study has been

Table 1Nominal Composition of IN718 SPF.

Element Ni Cr Nb Mo Al

Wt.% 50–55 17–21 4.75–5.25 2.8–3.3 0.2–0.8

performed which compares the fundamental mechanisms of thesethree techniques. This paper aims to fill that gap.

2. Experimental

The Inconel 718 SPF alloy used in the current study wasobtained from Special Metals Corporation in a sheet form ofdimensions 250�250 mm2 and thickness of 2 mm. The nominalcomposition of Alloy IN718SPF is given in Table 1.

The as-received sheet had been annealed and heat-treated toachieve AMS 5950 specifications for mechanical properties(Table 2). Coupons of dimensions 38�38 mm2�2 mm thick weresectioned from this sheet using electrical discharge machining(EDM) and were then heat treated to 750 1C for 5 h to remove anymachining stresses and also to cause precipitation of γ" tostrengthen the material. For metallographic studies, a samplewas polished to mirror finish. Part of polished sample was etchedusing a solution of 100 mL HCl and 0.5 mL H2O2 to perform opticalmicroscopy.

2.1. Laser shock peening (LSP)

A patch of 10�10 mm was peened at center of the couponsusing a GEN I Q-switched Nd:Glass laser (λ¼1.054 μm.) at OhioCenter for Laser Shock Processing for Advanced Materials andDevices at University of Cincinnati using the parameters listed inTable 3.

2.2. Cavitation Shotless peening (CSP)

One surface of the coupon (38 mm�38 mm) was ere cavitationpeened at the Department of Nano-mechanics, Tohoku University(Japan) with the process variables as tabulated in Table 4. Twotype of cavitation peening was done: CSP in water and CSP in air.

2.3. Ultrasonic nanocrystal surface modification (UNSM)

Samples were treated with UNSM at the Institute of Manufac-turing Systems Technology, Sun Moon University, Republic ofKorea as per conditions shown in Table 5.

Ti C Si Mn Cu Fe

0.65–1.15 0.05 0.35 0.35 0.3 17–19

Table 5UNSM Treatment Parameters.

Sample Load(N) Amplitude(μm) Scanning speed(mm/min)

Patch size(mm)

UNSM-S1 70 30 3000 34�34UNSM-S2 80 30 3000 34�34

Table 6XRD Parameters for residual stress measurement.

Item Description

Detector PSSD (Position sensitive scintillation detector),201 2θ range

Radiation Mn Kα1 (λ¼2.10314 A1)Tilt angles 01, 72.581, 79.071, 712.451, 718.81, 723.01)Aperture size(dia) 1 mmPlane(Bragg's Angle) {311} set of planes. Bragg's angle: 1521X-ray elastic constant S2/2: 6.37�10−6 MPa−1

Table 4Cavitation Shotless Peening Parameters.

Type Injectionpressure(MPa)

Injectionpressure of lowspeed water jet(MPa)

Nozzlediameter(mm)

Standoffdistance

Scanningspeed s/mm

Patchsize(mm)

Water 30 0.42a 2 85 10 38�38Air 30 0.05 1 45 20 38�38

a Pressure of tank

A. Gill et al. / Materials Science & Engineering A 576 (2013) 346–355348

2.4. Surface roughness, hardness and microstructure

Surfaces of the treated samples were observed under an opticalInterferometer (Wyko NT1100) using vertical step interferometryto characterize roughness. In each case, a total of 5 measurementswere made at different areas and average value reported.

A small piece (5 mm�8 mm section) was sectioned fromvarious coupons and mounted in cross section in a conductiveepoxy to make hardness measurements as well study near surfacemicrostructure. These samples were polished using successivelyfine grit size abrasives media to eliminate damage from sectioningas well as subsequent grinding. Final chemical-mechanical polish-ing was done by 0.05 μm colloidal silica to obtain a mirror finish.After polishing, the mounting material was ground off to exposethe bottom of samples. This was done as mounting materials cancreep when load is applied to them. Micro hardness measure-ments were made using LECO M400-H1 hardness testing machinewith a Knoop indenter. Knoop indenter was preferred as indentwidth is small allowing more measurements in a given area.Measurements were then made on the polished cross sectionsgoing from peened edge to the unpeened edge. A total of six suchruns were made on each sample to get good statistics and theaverage value was reported at each depth. A load of 50 g was usedfor 15 sec and a distance of 30 μm was kept between indents toensure that the adjacent measurements were not affected by eachother. Measurements were made in accordance with ASTM E 384–05a (Standard Test Method for Micro indentation Hardness ofMaterials).

TEM was used to study near surface microstructure. To obtainfoils from close to the peened surface, a thin section was sliced offparallel to peened surface and thinned (dimpled and ion milled)from one side to a thickness of 15 μm, leaving the peened surfaceintact. The sample was then ion milled from only one side, at lowangle (121) to ensure that there is no damage induced by ion beam.This enables one to obtain thin areas which are very close tosurface (within a micron). The near surface specimens are impor-tant as they help in understanding the fundamental deformationchanges introduced by these surface treatments.

2.5. Residual stress and FWHM

Residual stresses were analyzed in two orthogonal directionsusing conventional X-ray diffraction using sin2Ψ technique with

electrolytic layer removal with a Proto LXRD instrument (singleaxis goniometer using Ω geometry). Alignment of instruments waschecked before each set of measurements using a standard sample(316 stainless steel powder in this case) in accordance with ASTME915-96 (“Verifying the Alignment of X-ray Diffraction Instrumen-tation for Residual Stress Measurement”). X-ray elastic constantsused in these study were measured

Strains were measured in depth by layer removal on a portionof the surface treated region using a solution of sulfuric acid andMethanol (12.5: 87.5% by volume). The step size for layer removalwas small in the first 100 μm for detailed mapping of stress fieldsin near surface regions. The data was corrected for stress gradientsand layer removal. The X-ray elastic constants were measured inaccordance with ASTM E1426-94 (“Determining the EffectiveElastic Parameter for X-ray Diffraction Measurements of ResidualStress”). FWHM values were also recorded through depth for thesame peak to compare the extent and depth of plastic deformationafter the surface treatment in the same material. Details of theresidual stress measurement parameters are provided in Table 6.

3. Results

Fig. 1 shows the microstructure of the heat treated IN718 SPFalloy (baseline sample). The grain size was determined to be ASTM10 or higher (∼6 μm or smaller) and will be referred to as baselinehenceforth. Fig. 1a and Fig. 1b is an optical micrograph and aninverse pole figure (IPF) map obtained using Electron backscat-tered diffraction (EBSD)/ orientation imaging microscopy (OIM) inan FEI XL-30 scanning electron microscope (SEM). The IPF mapsuggests that grains are randomly oriented in the material. Finegrain size and absence of texture create good conditions forreliable stress analysis using X-ray diffraction.

3.1. Roughness

As received samples showed roughness of ∼120 nm. Fig. 2shows the surface profiles of unpeened, laser shock peened,cavitation peened and UNSM treated samples. Clearly, LSP intro-duced highest surface roughness in the material with Averageroughness (Ra)¼4.18 μm, with UNSM¼320 nm and Cavitationshotless peening¼425 nm. The surface treatments patterns areclearly visible in each case. In LSP treated sample, rows of over-lapping dimples created by laser shots can be made out, while incase of cavitation shotless peening small random indents causedby implosion of bubbles are visible and in UNSM treated samples,the rows of ridges created by the by the tool can be observed. InCSP, a large number of shallow impacts can be made out. This isindicative of impacts from a large number of cavitation bubbles,whose size can vary from micrometers to millimeters.

Optical micrographs from the cross-section samples of LSPtreated, CSP treated and UNSM treated samples are shown inFig. 3(a), (b) and (c) & (d) respectively. The near surface micro-structure in LSP treated and CSP treated samples show no change

Fig. 1. (a) Optical Micrograph and (b) IPF map of IN718 SPF.

Fig. 2. Surface Profiles from samples with peening conditions.

A. Gill et al. / Materials Science & Engineering A 576 (2013) 346–355 349

in grain size as compared to the baseline samples. No deformationbands were observed in either LSP or CSP treated samples.

The UNSM treated samples showed a layer (∼5 μm thick) innear surface region which had a fine grain size as shown in Fig. 3c.This fine structure was too fine to be studied by optical microscopyor Scanning electron microscope, so transmission electron micro-scopy was performed to study this layer and results will bediscussed in later sections. Higher magnification images of UNSMtreated sample reveal severe plastic deformation with multipledeformation bands visible, below the aforementioned layer(Fig. 3d).

3.2. Hardness

Fig. 4 shows the hardness as a function of distance from peenedsurface for three peening treatments. UNSM treated sampleshowed highest hardness at the surface, followed by LSP treated

and CSP treated samples. UNSM samples showed an increase of∼35% from baseline, with surface hardness of 356 HK but thehardness increase was only seen to a depth of 300 μm or so. LSPand CSP enhanced the surface hardness by ∼20% with surfacevalues of 322 HK and 315 HK respectively. In case of CSP, themodification was rather shallow (∼250 μm), but in case of LSP thehardness modification extended to ∼700 μm. The change fromsurface to baseline was also most gradual in LSP samples, withboth UNSM showing highest gradient and CSP showing a gradientsomewhere in between LSP and UNSM.

3.3. Residual stress

Residual stress values(from one direction) obtained from XRDfor each condition of LSP, CSP and UNSM treatments are shown inFig. 5(a), (b) and (c) respectively.

Fig. 3. Cross section optical micrographs of (a) LSP (b) CSP (c) and (d)UNSM.

Fig. 4. Hardness Vs. Depth for various surface treatments.

A. Gill et al. / Materials Science & Engineering A 576 (2013) 346–355350

The results showed that in all the treatments, changing para-meters lead to differences in residual stresses introduced in thematerial. In case of LSP with 16 J energy, residual stress at surfacewas −690 MPa while in case of 8 J, stress was −750 MPa. Thehigher energy showed higher stress in near surface regions, up to adepth of 120 μm or so. After that, both treatments had similarstress profiles.

In case of CSP, residual stresses in near surface regions (first10 μm) were slightly higher in case of CSP in water with surfacestress of −800 MPa, compared to −705 MPa in case CSP in air. Insub surface regions CSP in air showed higher magnitude ofstresses as well as deeper compressive residual stress field withcompressive stress field of extending to a depth of ∼400 μm ascompared to only ∼250 μm in case of CSP in water.

Both UNSM treatments showed very high residual stress valuesin surface regions. In case of UNSM, S2 treatment showed higherstresses (−1860 MPa) at surface as compared to S1 treatment(−1570 MPa). This difference in stress fields only extended to

∼120 μm deep. After that both treatments showed rather similarstress profiles, with a compressive stress field of ∼250–300 μm.

Fig. 6 shows the residual stress as function of depth in twoprincipal directions(X and Y) for one condition (which introducedhigher stress levels) for each surface treatment. In cases of LSP andCSP, residual stresses were equi-biaxial and while UNSM introducedthe highest residual stresses in near surface region, they weresignificantly different in two directions with stress at surface in twodirections being −1060 MPa and −1860 MPa. The direction of traverseof tool tip showed significantly lower stresses than transverse to it.These differences in stress field were only seen up to a shallow depth(150 μm), after which the profiles in two directions became similar.

Fig. 7 is a comparison of residual stresses and FWHM as afunction of depth from all surface treatments (only the conditionsproducing higher magnitude between two directions were con-sidered). The depth of residual stresses which is a critical factor inmany applications of surface treatments was maximum in LSPpeened samples (∼600 μm), followed by CSP and UNSM (in bothcases ∼250 μm). Fig. 7b shows the FWHM of diffraction peaks as afunction of distance from surface for different treatments. FWHMof diffraction peak can provide a qualitative indication of plasticstrain introduced in a material. Here FWHM values were normal-ized (1 being assigned as the value of FWHM of virgin material) soas to allow for as easier comparison. Clearly, UNSM and CSP howhigher values of FWHM near surface as compared to LSP. In case ofUNSM, FWHM drops to a level shown in virgin material within250 μm. CSP shows similar trend; although in CSP the drop inFWHM within first few microns is rather steep.

LSP shows only a minor increase in FWHM at the treatedsurface. The decrease in FWHM is gradual in case of LSP andextends far deeper than the other two treatments (up to a depth of∼550 μm or so). Although FWHM provides qualitative informationon amount of plastic strain in a material, FWHM is function ofmany other parameters including instrument broadening andcrystallite size. Hence, it is important to look at near surfacemicrostructure which gives a better idea of mechanisms of plasticdeformation and hence provides a better understanding ofFWHM data.

Fig. 5. Residual Stress as function for different peening conditions in (a)LSP (b)CSP and (c) UNSM treated samples.

Fig. 6. Residual Stress vs. Depth for two principal axes, in (a)LSP (b)CSP and (c) UNSM treated samples.

A. Gill et al. / Materials Science & Engineering A 576 (2013) 346–355 351

3.4. Microstructural analysis

Fig. 8 shows TEMmicrographs of near surface region from a LSPtreated sample. It is clear that no grain refinement was introduced

by LSP as original grain boundaries can be clearly discerned(Fig. 8a). There is a moderate increase in dislocation density. Athigher magnification (Fig. 8b), Moiré fringes are clearly visible.Moiré fringes are observed when the interplanar spacing changes

A. Gill et al. / Materials Science & Engineering A 576 (2013) 346–355352

locally or when two adjacent crystals are rotated with respect toeach other [13].

Fig. 9 shows the TEM micrograph of near surface region from aUNSM treated sample. There is a clear evidence of dislocation pileup and formation of sub grain boundaries. The selected areadiffraction (SAD) pattern confirms the sub grain formation witha spotty SAD pattern. If there was a fine grain structure, one wouldexpect a complete ring pattern. The dislocation density is muchhigher than that in case of LSP treated specimen. Other areasshowed presence of deformation twins (Fig. 9 (b)) which were notseen in LSP as well as CSP treated samples.

Fig. 7. (a)Residual stress and (b) FWHM Vs. Depth for Different Surface treatments.

Fig. 8. TEM micrograph of near surface re

Fig. 10 shows the TEM micrograph of near surface region from aCSP treated sample. The near surface area shows grain refinement,with formation of nano sized grains of varying size(tens of nm).SAD pattern confirmed the fine grain size with a sharp ringpattern. An interesting observation in this case is lack of highdislocation density. Very few dislocations were observed even athigh magnifications.

4. Discussion

The differences in roughness, hardness, residual stresses andmicrostructure in all three techniques is dependent on the wayplastic deformation takes place. LSP uses a shockwave of severalGPa [14] peak pressure to introduce deformation in a material inform of dimples. These dimples have pile ups on the edges. Thesepileups introduced by high pressure shock waves are the cause ofhigher roughness in case of LSP sample. It must be noted thatroughness introduced by LSP can be reduced by using a higheroverlap (50% overlap was used in current study) between theimpacts which serves to flatten the pileups, although necessitatinghigher number of laser impacts. In case of CSP, the pressureproduced by an individual bubble is not known, although a verylarge number of bubbles of varying size are produced providinghigh surface coverage, resulting in lower roughness values. In caseof UNSM specimen, the severe plastic deformation caused by toolscoupled with small step size between successive rows(7 μm) leadsto materials flow and flattening of surface. The higher plasticdeformation caused by UNSM as compared to other techniques isalso observed in cross section images (Fig. 3), where grainrefinement as well as deformation bands were seen in UNSMtreated specimen.

Hardness results also reflect this difference in the amount ofplastic deformation (Fig. 4). UNSM, with high number of disloca-tion pile ups and sub grain formation characterized by deforma-tion twins shows higher hardness in near surface areas than CSPand LSP. The hardness in UNSM drops rather sharply and reachesthe baseline in first 300 μm indicating the shallow nature ofdeformation introduced by UNSM. This is related to the fact thatUNSM is a contact treatment, with tool tip striking with a load tensof Newton, with a frequency of 20–40 KHz, giving a high numberof strikes per unit area (100,000 shots per mm2 in current study).A large number of repetitive strikes lead to accumulation ofcold work.

In contrast, LSP showed similar hardness as CSP but the depthof modification in case of LSP was much higher than in CSP orUNSM. This seems to indicate that although LSP is causing lessplastic deformation in near surface regions but its affects materialto much greater depths (700 μm). This is due to the fact that LSPgenerated shock wave can affect material up to a larger depthunlike UNSM which uses impact energy generated using ultrasonic

gion of laser shock peened specimen.

Fig. 9. TEM micrograph of UNSM surface region Showing (a).Dislocation pile ups (b) Deformation twins (Marked by arrows).

Fig. 10. TEM micrograph of near surface region of CSP treated specimen.

A. Gill et al. / Materials Science & Engineering A 576 (2013) 346–355 353

vibration. No shockwave is produced in the UNSM process and thedepth of the modified layer is thus lower than LSP. The moderateincrease in hardness observed in case of LSP suggests that theunderlying mechanism in this case is increase in dislocationdensity[15,16]. Other mechanisms would influence hardness to amuch higher degree [17]. The plastic deformation in case shockwaves is a result of dislocations being generated at the shock front[16].Shock front moves fast and leaves dislocations behind, with wholeprocess lasting for tens of nano seconds. So essentially dislocationgeneration, and not motion, is the mechanism responsible for plasticdeformation in LSP. Also, in case of LSP, much less laser impacts arerequired to cover a given area(around 60 in our study), leading tolower accumulation of cold work.

Another indication of the extent of plastic deformation is givenby FWHM of diffraction peaks, which follows the trends ofhardness results to a large extent. The high number of impactsin UNSM, leading to accumulation of cold work along withformation of sub grains leads to a larger increase in FWHMcompared to other techniques Fig. 7a. This could be noted in crosssection image of UNSM too, where a layer of modified grainstructure was clearly observed. TEM observations, showing largenumber of dislocations and pile ups agree with this. The TEMmicrostructure seen here is similar to observation of Ahn et al. [18]in their study of effects of UNSM on an Aluminum alloy althoughtheir observations comes from a TEM specimen obtained from50 μm depth. Their study also noted grain size modification up to adepth of 180 μm, with grain size varying as a function of distance.This was not noted in our study, where grain size modification wasonly seen in top 10 μm or so, although deformation bands could beseen up to a much higher depth.

High FWHM in CSP at surface indicates that either CSP has largeaccumulation of plastic deformation (due to a large number ofcavitation bubbles which implode and generate shock waves) inregion very close to surface or there is a grain refinement in thatregion. The increase in FWHM in case of CSP samples is at oddwith studies by Soyama[19] where after CSP of a tool steel alloy,reduction in FWHM of various diffraction peaks was reported. That

study reported that CSP can relieve micro strain by introducingmacro stain in a polycrystalline metal surface, although the studydid not provide any microscopy observations. It was proposed thatcollapse of cavitation bubbles introduces high frequency vibrationsthat move dislocations and some of those dislocations get annihi-lated by moving to grain boundaries or by joining together. In ourstudy, no evidence of large deformation (e.g. deformation bands)was observed in cross section images. Hence, TEM analysis of nearsurface region becomes important. TEM observations showed thatat surface CSP specimen showed a modified grain structure, withfine nano size grain clearly visible as shown in Fig. 10c. As very fewdislocations were observed, it can be concluded that increase inFWHM in case of CSP came primarily from fine grain size. Thislayer of modified grain size must be very small (1–2 μm) as it couldnot be seen in optical micrographs and seems to be responsible forthe sharp decrease in FWHM which was noted within first fewmicrons from surface.

Very few TEM studies of CSP process have been carried out.Ju et al. studied the effects of cavitation peening on Titanium [20].The study observed that although no macro deformation wasapparent after peening. Deformation twinning and dislocationswere determined as primary mode of micro plastic deformation.An increase in number of deformation twins was seen as afunction of peening intensity in cross section optical micrographs.TEM micro graphs showed twinning interaction and a high densityof dislocations. No grain size modification was noted either inoptical or TEM micrographs. However, it must be emphasized thatin our study, TEM samples were made from regions very close topeened surface. In our opinion TEM samples from close to surfaceprovide a good representation of deformation mechanisms asimpacts whether from shock waves or from a tool have greatestmagnitude at surface. It is possible that in our specimen, highdislocation density is present at in sub surface regions. However,to study microstructure evolution as a function of distance, a crosssection sample or sample prepared using Focused ion beam wouldbe helpful. Sample preparation using those techniques is currentlytaking place and results of that study will be published shortly.

A. Gill et al. / Materials Science & Engineering A 576 (2013) 346–355354

LSP shows smallest increase in FWHM at surface. As pointedout earlier, this is a consequence of the fact that much fewerimpacts are needed to peen an area in LSP; hence the accumula-tion of cold work is significantly less. Also, due to very shortloading times in LSP, the dislocations do not get time to movemuch further lowering the amount of plastic strain. Also, no grainsize modification is noted even in TEM micrographs, which werefrom region very close to surface. The combination of both of thesefactors: low accumulation of plastic work and lack of grain sizerefinement, produces lowest increase in FWHM in case of LSPsamples.

Residual stress profiles also follow the similar trends as hard-ness values, with UNSM producing higher stresses but up to ashallower depth compared to LSP (Fig. 5). In LSP, both theconditions (8 J and 16 J) showed very similar residual stress profile.This is related to the peak pressures generated by different powerdensities. Fabbro et al. [21] modeled the confined ablation modeand related peak pressure of the plasma with laser power density.The relationship between peak plasma pressure and laser powerdensity in the water confinement mode is given by:

PðGPaÞ ¼ 0:01ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

α

2α þ 3

r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiZðg cm−2 s−1

qÞ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiI0ðGW=cm2Þ

qð1Þ

Where P¼ peak plasma pressure

Io¼ Incident laser power densityα¼ Fraction of energy used in creating plasma (typicallyα¼0.25 to 0.4)[22]Z¼ reduced shock impedance between water and the target.Z is given by:

2Z¼ 1

Zwaterþ 1

Ztargetð2Þ

where Zwater and Ztarget are the impedance of water overlay and thetarget.

The acoustic impedance of water¼0.165 g cm−2 S−1 and fornickel¼4.14�106 g cm−2 S−1.[23] Using these values and α¼0.4(in literature, α¼0.25 to 0.4 has been reported31), Eq. (1) reduces to

PðGPaÞ ¼ 1:83ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiI0ðGW=cm2Þ

qð3Þ

Hence, increasing the power density should result in anincrease in peak plasma pressure and hence the plastic deforma-tion caused by shock wave. However, this increase in pressure islimited by a laser intensity threshold, beyond which the peakpressure gets saturated due to water breakdown and severalstudies have been done on plasma breakdown and resultantpressure saturation [4,22,24]. This breakdown generates parasiticplasma at the water surface, limiting absorption of laser energyand hence saturating the pressure pulse. Subsequent studies withour laser have shown that the threshold intensity for the laserparameters used is around 10 GW/cm2. This compares very wellwith the power density threshold reported in literature for lasersof similar wavelength. The results of that study will be reported ina forthcoming publication.

Residual stresses introduced by CSP at surface were similar inmagnitude to those by LSP (Fig. 5b) but the depth of the modifiedlayer was approximately half that of LSP. Both CSP techniques usedthe same injection pressure but standoff distance, nozzle diameterand scanning speed were different. Previous studies have shownCSP in air can be more effective at introducing higher compressiveresidual stresses than CSP in water, at same injection pressure andsame nozzle diameter, at atmospheric pressure[10]. In that study,CSP in air showed slightly higher residual stresses than CSP inwater, both in near surface as well as sub surface regions, as the

optimum cavitation s¼0.014 [[25], i.e., the ratio of tank pressureand injection pressure, was chosen at the present condition. It wasnoted that the depth of compressive residual stress layer increasedwith the processing time of CSP [26].

A comparison of the two UNSM treatments (Fig. 5c) indicatesthat higher the load, higher the magnitude of residual stresses in agiven direction. Severe plastic deformation introduced results in ahigh density of dislocations and formation of sub grains along withdeformation twins, resulting in an increase in yield strength of thematerials. This increase in yield strength leads to introduction ofresidual stresses whose magnitude is even higher than the originalyield stress of the material (1070 MPa). These stresses are highestin magnitude in near surface region where severe deformationtakes place. Below the surface, where the differences in deforma-tion might not be significant, the residual stress profiles are rathersimilar.

The residual stress fields generated are equi biaxial in LSP andCSP. This is due to the nature of impacts (close to circular)generated by these techniques, which produce symmetric plasticdeformation. In case of UNSM, the residual stress fields are notequi biaxial, especially in near surface regions. The residual stressin longitudinal direction (in direction of travel of tool tip) is muchlower than the transverse direction. This has been seen in othertechniques (like rolling) too and is not unique to UNSM.

5. Conclusions

Effects of three different mechanical surface enhancements viz.laser shock peening (LSP), Cavitation Shotless Peening (CSP), andUltrasonic Nanocrystal Surface Modification (UNSM) on hardness,residual stresses and microstructure of IN718 SPF superalloy werestudied and compared. The main conclusions arising from thisstudy are as follows:

1)

LSP treated samples showed a small increase in surface rough-ness whereas UNSM and CSP lead to only minor increases inroughness.

2)

No grain size modification was observed in case LSP treatedspecimen. UNSM treated sample surface showed formation ofsub grain in near surface regions of the sample (∼10 mm). CSPspecimen had a fine layer of nano sized grain on surface.

3)

UNSM treated sample showed highest increase in hardness atthe surface (∼35%) followed by LSP treated (∼20%) and CSPtreated samples (∼20%). The depth of hardness increase was250 μm in UNSM and CSP. In case of LSP the hardnessmodification extended to ∼700 μm.

4)

LSP showed maximum surface stress of −750 MPa, while CSPshowed maximum surface stress of ∼−800 MPa and UNSMshowed a maximum surface stress of −1860 MPa. Again theresidual stress field was deepest in LSP samples, extending upto 600 μm. In case of CSP the stress field was only ∼250–400 μmdeep while in case of UNSM compressive field extended to∼250–300 μm.

5)

The nature of residual stress fields was different in differenttechniques. LSP introduced residual stress fields which wereclose to being equi biaxial, with residual stresses profiles in twodirections differing by a small magnitude in first 150 μm or so.CSP introduced residual stress fields which were equibiaxial innature. UNSM introduced stress fields which non equi biaxial,with stress magnitudes at surface differing by a large magni-tude, although this difference in two directions was only seenin first 150 μm or so.

6)

UNSM and CSP how higher values of FWHM near surface ascompared to LSP. In case of UNSM, FWHM drops to a levelshown in virgin material within 250 μm against 550 μm in LSP.

A. Gill et al. / Materials Science & Engineering A 576 (2013) 346–355 355

In UNSM high FWHM is a consequence of high amount ofplastic strain as well as sub grain formation. In CSP, refinedgrain size in surface layer was a big contributor to increase inFWHM while in case of LSP it was result of only small amountof plastic strain.

7)

Surface microstructure of LSP showed moderate density ofdislocations with no grain refinement observed. Moiré fringeswere observed at high magnifications. In case of UNSM, a highdensity of dislocations was seen with arranged as cell structurewith pile ups. Deformation twins were also observed through-out the sample In CSP, nano size grain structure was observedat surface with few dislocations. Further studies are needed,with TEM samples from different depths to fully understandthe microstructure evolution in CSP.

Acknowledgments

The authors (ASG, AT, SRM, DQ, YSP, HS and VKV) would like tothank the National Science Foundation (Grant no DMR-0706161,Dr. Alan Ardell Program Monitor) and Battelle-DOE-NEUP (contract #88635, Dr.Sebastien Teysseyre, Program monitor, Idaho NationalLaboratory) for financial support of this research and Special Metalsfor supplying the IN718 SPF alloy sheet for this study. We alsoacknowledge Dr. Young-Sik Pyun and his group at Sun MoonUniversity, Department of Mechanical Engineering, Cheonan, S. Koreafor performing UNSM treatments. We would also like to acknowl-edge Dr. Hitoshi Soyama and his group at Tohoku University Depart-ment of Nanomechanics, Sendai, Japan for performing CSPtreatments for this study. We also gratefully acknowledge thecontribution of the State of Ohio, Department of Development andThird Frontier Commission, which provided funding in support of“Ohio Center for Laser Shock Processing for Advanced Material andDevices” and the experimental and computational equipment in theCenter that was used in this work. Any opinions, findings, conclu-sions, or recommendations expressed in these documents are those

of the author(s) and do not necessarily reflect the views of the NSF orthe State of Ohio, department of Development.

References

[1] C. Ye, S. Suslov, B.J. Kim, E.A. Stach, G.J. Cheng, Acta Mater. 59 (2011)1014–1025.

[2] Y. Liao, S. Suslov, C. Ye, G.J. Cheng, Acta Mater. 60 (2012) 4997–5009.[3] C. Ye, S. Suslov, D. Lin, G.J. Cheng, Philos. Mag. 92 (2012) 1369–1389.[4] P. Peyre, C. Carboni, A. Sollier, L. Berthe, C. Richard, E. De Los Rios, R. Fabbro,

New trends in laser shock waves physics and applications, in: C.R. Phipps (Ed.),Proceedings of SPIE—The International Society for Optical Engineering, vol.4760, 2002, pp. 654–666.

[5] H. Soyama, T. Kusaka, M. Saka, J. Mater. Sci. Lett. 20 (2001) 1263–1265.[6] H. Soyama, Trans. ASME 126 (2004) 123–128.[7] H. Soyama, Singularity Of Cavitation Shotless Peening Compared With Shot

Peening, in: K. Tosha (Ed.), ICSP-10, Tokyo, Japan, 2008.[8] H. Soyama, J. Fluids Eng. 133 (2010).[9] S. Kunaporn, M. Ramulu, M. Hashish, J. Hopkins, Ultra high pressure waterjet

peening part I: surface texture, in: WJTA American Waterjet Conference, 2001.[10] O. Takakuwa, H. Soyama, Int. J. Hydrogen Energy 37 (2012) 5268–5276.[11] I.H. Cho, G.H. Song, C.S. Kim, A. Nobuhide, A. Combs, J. Park, C.M. Suh, J.H. Park,

Y.S. Pyoun, J. Mech. Science Technol. 19 (2005) 2151–2156.[12] A. Cherif, Y. Pyoun, B. Scholtes, J. Mater. Eng. Perform. 19 (2010) 282–286.[13] B. Fultz, J.M. Howe, Transmission Electron Microscopy and Diffractometry of

Materials, Springer, 2012.[14] P. Peyre, R. Fabbro, L. Berthe, C. Dubouchet, J. Laser Appl. 8 (1996) 135–141.[15] A. Clauer, B.P. Fairand, Interaction of laser-induced stress waves with metals,

in: Presented at Proceedings of ASM Conference Applications of Lasers inMaterials Processing, Washington DC, 1979.

[16] M.A. Meyers, K.-C. Hsu, K. Couch-Robino, Mater. Sci. Eng. 59 (1983) 235–249.[17] P. Peyre, R. Fabbro, Opt. Quantum Electron. 27 (1995) 1213–1229.[18] D. Ahn, Y. He, Z. Wan, I.S. Cho, C.S. Lee, I.G. Park, Y.S. Pyoun, J. Dong, K. Shin,

Surf. Interface Anal. (2012).[19] H. Soyama, N. Yamada, Mater. Lett. 62 (2008) 3564–3566.[20] D.Y. Ju, B. Han, J. Mater. Process. Technol. 209 (2009) 4789–4794.[21] R. Fabbro, J. Fournier, P. Ballard, D. Devaux, J. Virmont, J. Appl. Phys. 68 (1990)

775–784.[22] R. Fabbro, P. Peyre, L. Berthe, X. Scherpereel, J. Laser Appl. 10 (1998) 265–279.[23] D.J. Steinberg, Equation of state and strength properties of selected materials,

in: Lawrence Livermore National Laboratory, Livermore, CA, 1996.[24] A.S. Gill, Z. Zhou, U. Lienert, J. Almer, D.F. Lahrman, S.R. Mannava, D. Qian,

V.K. Vasudevan, J. Appl. Phys. 111 (2012) 084904.[25] H. Soyama, J. Soc. Mater. Sci., Jpn. 47 (1998) 381–387.[26] Y. Sekine, H. Soyama, Rev. Automot. Eng. 30 (2009) 393–399.