Jinyu Zhng ,Yongxin Jin ,Xuzh Zho ,Dn Mng ,Fushng Pn ,Qingyou Hn,*

a School of Engineering Technology,Purdue University,West Lafayette,IN 47906,USA

b State Key Laboratory for Mechanical Behavior of Materials,Xi’an Jiaotong University,Xi’an 710049,PR China

c B?llhoff Inc,Troy,MI 48084,USA

d School of Automobile,Chang’an University,Xi’an 710064,PR China

e College of Materials Science and Engineering,Chongqing University,Chongqing 400045,PR China

Abstract A magnesium alloy AZ31 sheet was processed by ultrasonic shot peening treatment to fabricate a surface nanocrystalline,and a ballon-disk dry sliding wear test was performed to evaluate the tribological behavior after treatment.The microstructure observation indicated a gradient nanocrystalline structure was formed after USSP treatment.The microhardness at the top surface was improved from 60 HV to 145 HV after treatment.The formed nanocrystalline resulted in an easy formation of MgO patches on the surface and reduced the coefficien of friction.Moreover,the formed nanocrystalline leaded to a retard of delamination with increasing the sliding speed and applied load,which was due to its stronger sub-surface.Under high sliding speed (0.5m/s) and high applied load (50N),it was firstl found that the formed nanocrystalline prevented the happening of thermal softening and melting.The possible reasons accounting for the prevention of thermal softening and melting were discussed accordingly.

Keywords: AZ31Mg alloy;Ultrasonic shot peening;Surface-nanocrystallization;Wear mechanism.

1.Introduction

The friction and wear behavior cannot be forbidden for structural parts in automobile field such as automotive brakes and engine components [1-3].Meanwhile,sliding wear is also an important factor in material processing by the rolling,extrusion and forging [4].The wear loss and macroscopic friction of the metallic are always the challenges that determine the active time during contact conditions [5].As a future structural material,Mg alloy is promising to replace other metals because of its relatively low density and high specifi strength [6-10].Compared with other structural materials,such as iron and aluminum alloys,the wear resistance of Mg alloy is relatively poor,which strongly limits its further application [11-13].It is necessary to develop methods to reduce the wear loss of Mg alloy and substantially prolong its lifetime.

According to Archard classic model,the wear rate of alloy is directly related to its hardness [14].Several plastic deformation (SPD) is regarded as an effective method to improve the hardness of metal by refinin the grain to ultrafin sized (100nm-1000nm) or nanosized (<100nm),thus improves the wear resistance [15,16].According to the classicHall-Petchformula,the fine the obtained grain,the higher the hardness would be [17,18].The improved wear resistance has been reported in steel or Aluminum alloys fabricated by different SPD methods,such as surface nanocrystallization[19-21],equal channel angular pressing (ECAP) [22-24],high pressure torsion (HPT) [25,26] and accumulative roll bonding (ARB) [27,28].In Mg alloys,the improved wear resistance after SPD is also widely reported,such as AZ31 Mg alloy after ECAP [29],AM70 Mg alloy after ECAP [30],AZ31 Mg alloy after HPT [31],AZ91 Mg alloy after surface nanocrystallization [32] and AZ31 Mg alloy after laser shot peening [33].

Meanwhile,some reports also imply the wear resistance was insensitive to the grain size.Wang et al.studied the tribological behavior of pure Ti after HPT treatment and found that the coefficien of friction of all the samples was about 0.6,implying there was limited effect of grain size on the coefficien of friction [34].The effect of ECAP,HPT as well as their combination on the wear behavior of copper was reported by Zhilyaev et al.[35].Their result showed there was no advantage in reducing the wear rate in copper after the grain refinement The coarse grain copper presented a better wear resistance compared with all the ultrafin grained(UFG)copper samples and the brittleness of the UFG copper led to the development of crack networks at the bottom of the wear track.Jamaati et al.also reported a reduced wear resistance of Al/Al2O3composite after ARB [36].The wear rate was increased after ARB due to the easy nucleation and propagation of cracks.Similar reduced wear resistance after SPD in Mg alloy was also reported [37].They found the nanocrystalline exhibited a poor wear resistance because of the low ductility of the nanocrystalline and a high risk of fracture during the wear process.To sum up,the brittle nanocrystalline or UFG grains has a great potential to pile off and increase the wear rate under high load force and high sliding speed.It is necessary to investigate the wear behavior of Mg alloy after grain refinin by SPD method under high load force and high sliding speed.However,the detailed wear behavior of nanocrystallized or ultrafin grained Mg alloys under high sliding speeds is still scare,especially their wear mechanisms.

Ultrasonic shot peening (USSP) treatment is an effective SPD way to fabricate nanocrystalline on the surface of metals [38-40].The USSP process is consisted of a generator of ultrasonic signals,a transducer which translates the generated signals into mechanical motion,and a simple metal rod which propels the shots.During USSP treatment,a surface nanocrystalline is easily formed and the hardness is obviously promoted [41].In this study,a AZ31 Mg alloy sheet was processed by ultrasonic shot peening treatment,and the tribological behavior before and after USSP treatment was investigated by a ball-on-disk dry wear test.A sliding distance of 100 m was selected and two sliding speeds (0.1m/s and 0.5m/s) and three applied load values (10N,30N,and 50N)were applied.The detailed wear mechanisms under different wear conditions were discussed.

2.Experimental setup

The as-rolled Mg-3Al-1Zn-0.3Mn alloy sheet with a thickness of 3mm was selected in this study.The sheet was cut by CNC machining with a size of 55×55 mm2for ultrasonic shot peening treatment.The normal plane (transversal direction-rolling direction plane) was shot peening treated.The detailed experimental set up of USSP process can be found in previous works [38-40].The sample was USSP treated at a vibration frequency of 20kHz by stainless steel balls with a diameter of 3mm.The distance between the horn and the treated sample was 10mm and the treated duration was 15min.

X-ray diffraction tests of the as-received and the USSP treated surface were performed on the Riguku D/max 2500PC X-ray diffractometer with Cu-K radiation.The test was conducted by a scan step of 0.02° per step in the 2θranging from 10° to 90°.The grain size of the nanocrystalline was evaluated according to Williamson-Hall formula [42]:

wheredis the grain size,Kis the Scherrer constant(0.9),λis the x-ray wavelength,βis the full-width-half-max (FWHM),andθis the diffraction angle.

Electron-back scattering diffraction (EBSD) method was applied to study the microstructure change after ultrasonic shot peening,including the twinning,the distribution of residual stress and grain’s orientation.Sample preparation for EBSD analysis consisted of mechanical polishing by SiC paper to 2000 grit and electro-polishing in a commercial AC2 electrolyte (Struers,Germany),using a voltage of 20V for 60 s at -20 °C.EBSD was conducted on a JEOL 7800F fiel emission gun scanning electron microscope (SEM) with HKL Channel 5.0 software.The characterization of the nanograined structure was conducted by a FEI Tecnai G20 transmission electron microscope (TEM) and operated at 20kV.The TEM sample was prepared by the FIB Lift-out method using a Zeiss AURIGA FIB/SEM dual beam system.The TEM sample cut by FIB was at a position of～20μm from the top surface.Bright-Field (BF) TEM images were captured to characterize the nano-grained structure.

The wear resistance of as-received AZ31 Mg alloy sheet before and after USSP treatment was tested by a ball-on-disk dry sliding wear test on a computerized ball-on-disk wear testing machine at room temperature and in the air with a relatively humidity of～45%.Disks of AZ31 Mg alloy sheet were cut by CNC machining to a diameter of 44mm.The surface of the test samples was mechanically polished to a surface roughness of～0.05μm and then cleaned by acetone solution in ultrasonic clean machine.The GCr15 ball with a surface roughness of～0.1μm and a hardness of～700 HV was selected as the friction pair.The ball was cleaned every time before using by an acetone solution in ultrasonic clean machine.Two sliding speeds (0.5m/s and 0.1m/s) and three applied loads (10N,30N,and 50N) were selected.The sliding distance was 100 m.The rotational speeds were 800 r/min for 0.5m/s and 160 r/min for 0.1m/s,respectively.Each test condition was tested at least by three times for replicability.The wear rate was calculated as

Fig.1.Microstructure of as-received AZ31 sheet (a) and USSP treated (b) from the TD direction.

where w is the wear rate,M1is the mass before wear test and M2is the mass after wear test,L is the sliding distance.The loss of mass was calculated after the removal of loss debris from the worn surface.The surface topography of the worn surface was analyzed using a color 3D laser scanning microscope (VK-970,Keyence Corporation,Japan).The worn surface was also analyzed by SEM (TESCAN,Vega II XMU)equipped with an energy dispersive X-ray spectrometer(EDX).

3.Result

3.1.Microstructure

The optical microstructure of the AZ31 sheet from the transverse direction (TD) before and after USSP treatment is shown in Fig.1.The as-received sheet is of a uniform microstructure.The majority of grains are homogeneous and equiaxed with an average grain size of～9μm.After USSP treatment,the grains at the top area are refined and the thickness of the refine area is more than 100μm.

The XRD patterns of the as-received and the USSP treated surface are seen in Fig.2.Before USSP treatment,the asreceived AZ31 Mg alloy shows a typical basal texture.After USSP treatment,the peak of (0001) is reduced greatly and the peaks ofbecome relatively strong,indicating a random grain orientation is formed to replace the strong basal texture.Meanwhile,the full width half maximum of the peaks after USSP is enlarged.Such a larger full width half maximum is related to a smaller grain size,and the grain size is evaluated according to Williamson-Hall formula [42].The calculated average grain size on the surface after USSP treatment is 37nm.

Fig.2.XRD patterns of as-received and USSP treated.

Fig.3.The TEM sample cut by FIB (a) and Bright-fiel TEM image of nanocrystalline (b).

The bright fiel TEM image at the position of 20μm from the top surface is present in Fig.3.An equiaxed nanocrystalline of less than 200nm has been developed.The TEM result indicates the grain size is changed along the depth.According to previous study,a gradient strain and strain rate along the depth was formed during the shot peening process,which resulted in the formation of a gradient microstructure along the depth [43,44].In this study,the gradient strain and strain rate formed during USSP process lead to this gradient microstructure with a nanocrystalline of 37nm at the surface and an equiaxed nanocrystalline of less than 200nm at depth of 20μm.

The cross section of the USSP treated sample from the TD was also characterized by EBSD to show the microstructure change along the depth,seen in Fig.4.The inverse pole figur (IPF) maps,the twins,low angle grain boundaries and high angle grain boundaries,the Kernel Average Misorientation (KAM),and the band contrast map are presented from left to right.From the IPF map,the top refine region is white and uncharacterizable,which is due to the nanocrystalline nature and the residual stress.From the KAM map,the residual stress is also reserved at the area beneath the white region and it is as far as 400μm from the top surface.Here,the gradient structure along the depth after USSP treatment can be divided into three regions:(1) the refine region near the surface,(2) the deformed region with residual stress beneath the refin region and (3) the unaffected matrix.The thickness of refine region was about 200μm.The deformed region is located at 200-400μm from the top surface.

Fig.4.The EBSD characterization of the microstructure from the TD direction after ultrasonic shot peening.

Fig.5.The distribution of the Vickers microhardness as a function of distance to the top surface.

3.2.Microhardness

Fig.6.The variation of coefficien of friction of AZ31 Mg alloy before and after USSP treatment.

Fig.5 shows the Vickers microhardness distribution along the depth before and after treatment.The microhardness of as-received is～60 HV.After USSP treatment,the microhardness at the top surface is improved to 145 HV.Meanwhile,due to the gradient structure,the microhardness is gradually decreased along the depth.Besides the refine region,the deformed region also shows an improved microhardness,which is due to the induced compressive residual stress [45,46].At the depth of～400μm,the microhardness of USSP treated is close to that of as-received.

Fig.7.The average COF values:under a sliding speed of 0.1m/s (a) and under a sliding speed of 0.5m/s (b).

3.3.The tribological behavior

The tribological behavior after USSP treatment was conducted by a ball-on-disk dry wear test under different sliding speeds (0.1m/s and 0.5m/s) and different applied loads(10N,30N,and 50N).In Fig.6,the coefficien of friction(COF) is present as a function of the sliding distance.The COF curves are fluctuatin during the wear process and the fluctuation of COF values are reduced with increasing the sliding speed and applied load.Moreover,the COF value is reduced obviously after USSP treatment.For the further comparison,the average COF values were calculated,shown in Fig.7.Under the sliding speed of 0.1m/s,the average COF value with applied load of 10N is reduced from 0.35 to 0.31 after USSP treatment.With increasing the applied load to 30N,the average COF values are reduced while the USSP treated is still 0.04 smaller than that of as-received.Under the applied load of 50N,the COF value is reduced from 0.28 to 0.25 after USSP treatment.Under the sliding speed of 0.5m/s,the COF is smaller.Meanwhile,the COF is quite stable under different applied loads.A clear reduction of the COF is seen with applied load of 30N and 50N.

Fig.8.The variation of wear rate of AZ31 Mg alloy before and after USSP treatment under different applied loads and sliding speeds.

Similar to the reduction of COF values,the wear rates(the wear volume loss per unit sliding distance) also become lower after USSP treatment,shown in Fig.8.Under the low sliding speed (0.1m/s) and low applied load (10N),the wear rate of the USSP treated is close to that of the as-received.With increasing the applied loads to 30N and 50N,the wear rates are increasing for both as-received and USSP treated.Moreover,an obvious reduction of the wear rate after USSP treatment is seen.Under the high sliding speed (0.5m/s),the reduction of the wear rate is also observed after treatment with the applied loads of 10-50N.

The 3-D surface profile of the worn surface under the sliding speed of 0.1m/s are shown in Fig.9.Numerous ridges and grooves parallel to the sliding direction are found with a worn surface of as-received under applied load of 10N.For USSP treated sample,the worn trace is less obvious and its surface roughness is smaller.With the applied load of 30N,the worn surface roughness increases in both as-received and USSP treated.With the applied load of 50N,in as-received,the pile-up morphologies,such as wide and deep groove,are found at the bottom of the wear scar and the surface roughness of as-received increases to 18.12μm.For USSP treated,still numerous ridges and grooves but no the pile-up morphologies are seen.

Under the sliding speed of 0.5m/s and the applied load of 10N,the ridges and grooves are also found in as-received,seen in Fig.10(a).Pile-up morphologies begin to show off at the worn surface of as-received with the applied load of 30N.However,when the applied load increases to 50N,such pileup morphologies are not further observed.Instead,the worn surface becomes smoother and the worn surface roughness is decreased from 9.89μm to 9.12μm.For USSP treated,the worn track is still unconspicuous and the roughness continuously increases with the increase of applied load.

Fig.9.3D morphologies of the wear surfaces under a sliding speed of 0.1m/s for as-received with a load of:10N (a),30N (c) and 50N (e),for USSP treated with a load of:10N (b),30N (d) and 50N (f).

The worn morphology was also characterized by SEM and the width of the worn track was also measured to estimate the wear resistance,seen in Fig.11 and Fig.12.For asreceived in Fig.11(a),under the applied load of 10N and sliding speed of 0.1 m/s,abrasion wear characteristics such as grooves and ridges parallel to the sliding direction are found on the worn surface.The mapping result of the area highlighted by the red dash line indicates there are some MgO patches found on the surface.The MgO patches are caused by the frictional heat generated during the sliding motion and the oxidation of the worn surface from the wear process.For the USSP treated sample,it has a narrower width of the worn track (0.82mm),which indicates a better wear resistance.No such obvious grooves or ridges are seen.Instead,more MgO patches appears at the worn surface.Under the load of 30N,a typical delamination,the detachment of the surface layer,is observed.For the USSP treated,besides oxidation,the abrasive characteristics such as grooves and ridges are present but no delamination is found.With the applied load of 50N,the as-received shows a more serious delamina tion while still abrasion wear characteristic is found in USSP treated.

Fig.10.3D morphologies of the wear surfaces under a sliding speed of 0.5m/s for as-received with a load of:10N (a),30N (c) and 50N (e),for USSP treated with a load of:10N (b),30N (d) and 50N (f).

Fig.11.SEM micrographs of the worn surface under a sliding speed of 0.1m/s for as-received AZ 31Mg alloy with a load of:10N (a),30N (c) and 50N(e),for USSP treated with a load of:10N (b),30N (d) and 50N (f).

Fig.12.SEM micrographs of the worn surface under a sliding speed of 0.5m/s for as-received AZ 31Mg alloy with a load of:10N (a),30N (c) and 50N(e),for USSP treated with a load of:10N (b),30N (d) and 50N (f).

Under the sliding speed of 0.5m/s and the applied load of 10N,some small cracks perpendicular to the sliding direction are found at the worn surface,seen in Fig.12(a).These cracks are reported as the initial stage of delamination [47].They would undergo a propagation,linking and subsequent shearing under further sliding,and remove the worn surface in the form of sheet-like laminates.For the USSP treated,still an abrasion is seen.With the applied load of 30N,both as-received and USSP treated present a typical delamination wear behavior but the width of USSP treated is smaller.With the applied load of 50N,the worn surface of as-received is a smoother surface and no delamination or abrasion features are observed.Meanwhile,its width of the wear track increases to 1.58mm.Those are the features of thermal softening and melting wear mechanism in Mg alloys,as reported in previous studies[48-50].Thermal softening and melting wear mechanism is one of the severe wear mechanisms in metals and the happening of severe wear mechanism indicates the wear test condition is beyond the safe operation window of this material.At the same time,the USSP treated still presents a delamination wear mechanism.

4.Discussion

In this study,a gradient nanocrystalline structure was formed at the surface after USSP treatment and the wear resistance was improved under the tested wear conditions.The role of the gradient nanocrystalline structure in the improvement of wear resistance is analyzed and the relationship between the wear mechanism and the formed gradient nanocrystalline structure is discussed.

4.1.The effect of surface nanocrystalline on the wear behavior

In this study,the USSP treatment results in the formation of nanocrystalline at the surface with a grain size of 37nm.This formed nanocrystalline at the surface leads to an easy formation of MgO patches on surface.It is because the nanocrystalline has a high activity to be oxidized during the wear process as the grain boundaries can act as the channel of atomic diffusion [32,51].The formed MgO patches on surface reduces the true metallic contact during the wear process and reduces the COF value.Moreover,the reduction of COF value is quite stable under different sliding speeds and applied loads.It indicates the easy formation of MgO happens under different sliding speeds and applied loads.

4.2.The effect of strong sub-surface layer on the wear behavior

The USSP treatment results in not only a surface nanocrystalline but also a gradient nanocrystalline along the depth.The formed gradient structure is as far as 400μm from the top surface,which indicates a strong sub surface layer is formed.Even though the sub surface layer doesn’t contact with the friction pair,it also contributes to the improved wear resistance.The strong sub-surface layer retards the initial of cracks during the wear process,seen in Fig.12(a) and (b).These cracks are the firs stage of delamination wear mechanism.They will prolong and link with each other when the increase of sliding speed and applied load,which results in the sheet-like fla es or laminates later[31,47,52].With this strong sub surface layer,the delamination is retarded and the wear resistance is improved consequently.It should be mentioned that some previous studies also reported a high risk of fracture of the nanocrystalline or ultrafin grains during the wear process as the ductility was reduced greatly after SPD treatment [36,37].In this study,no easy fracture is found in the formed gradient nanocrystalline structure,indicating a good balance between the reduced ductility and improved strength is achieved here.As a result,a retardation of delamination rather than an easy fracture is formed here.

4.3.The transition of wear mechanism after USSP treatment

Besides the reduction of COF value and the retardation of delamination,the formed gradient nanocrystalline also prevents the happening of thermal softening and melting under high sliding speed and applied load.Generally,thermal softening and melting is caused by the frictional heating at the sliding interface.The happening of thermal softening is related to the surface temperature of the worn surface as well as the specifi softening conditions of the material.The thermal softening and melting happens when the surface temperature is above the dynamic recrystallization (DRX) temperature [47].Consequently,there are two possible reasons for the retardation of thermal softening and melting in nanocrystalline here.The firs one is that the DRX temperature of the nanocrystalline formed by USSP treatment is improved,compared with that of as-received.General,the DRX temperature of the nanocrystalline is much lower than the coarse grain counter because of the higher density of grain boundaries and higher stored energy.It is reported that the DRX temperature of AZ31 Mg alloy is reduced to 150 °C after nanocrystalline while the grain growth of the AZ31 sheet with grains size of several micrometers usually happens above 250 °C [53-55].However,it is recently reported a notable thermal stability of nanograined copper and nickel below a critical grain size and they even remain stable above the recrystallization temperature of coarse grains [56].The thermal stability of the formed nanocrystalline by USSP treatment is necessary to verify in the future study.The other possible reason is that the worn surface temperature during the wear process may be reduced after treatment,which prevents thermal softening and melting.The surface temperatureTb(bulk temperature) during the wear process is proposed as [57]:

whereT0is the temperature of the heat sink where the heat fl ws,μ is the coefficien of friction,Fis the normal applied load,vis the sliding velocity,Anis the nominal contact area,αis the fraction of the heat diffusing in the pin,lbis the mean diffusion distance andkmpis the thermal conductivity of the pin or ball.Under the same wear conditions,the normal applied load (F),the sliding velocity (v),the mean diffusion distance (lb) and the thermal conductivity of the pin or ball (kmp)should be the same for both as-received and USSP treated.For the nominal contact area (An),in theory,the nanocrystalline should be smaller as its hardness is much improved after USSP treatment,which leads to an increment ofTbrather than a reduction.For the mean μ value,it is decreased from 0.28 in as-received to 0.25 after USSP treatment,which is beneficia for the reduction ofTb.It should be pointed out that the μ value difference between them is just 0.03,so its contribution to reduceTbmay be limited.It is worth mentioning that the fraction of the heat diffusing in the pin (α) may take a role.The nanocrystalline results in an easy formation of MgO and the thermal conductivity of MgO is smaller than that of Mg alloy.For example,at the temperature of～400K,the thermal conductivity of MgO is about 30 Wm-1K-1[58] while the thermal conductivity of AZ31 Mg alloy is about 90 Wm-1K-1[59].So the higher density of MgO on the surface of nanocrystalline will lead to a smallerαvalue,which may reduce the surface temperature and prevent thermal softening and melting.

Fig.13.Wear transition maps for as-received (a) and USSP treated (b).

The effect of USSP treatment on the transition of the wear mechanism transition maps at different sliding conditions are summarized in Fig.13.Lines are used to identify the boundaries of various wear regimes.For as-received AZ31,abrasion and oxidation happen under low applied load and slow sliding speed.With increasing sliding speed or applied load,the wear mechanism transfers from abrasion+oxidation to delamination+oxidation.When the sliding speed reaches 0.5m/s and the load reaches 50N,thermal softening and melting happens as well as the transition from mild wear to severe wear.For USSP treated,under lower sliding speed and low applied load,oxidation dominates the wear behavior.The increase of sliding speed and applied load lead to abrasion+oxidation.A further increase in sliding speed or applied load results in the delamination+oxidation.No transition from mild wear to severe wear is observed in USSP treated samples.The mild wear regimes are regarded as the safe operation regions for a material since the wear rate is typical low and the wear is proceeded at a steady-state condition [52,60].From the wear transition map,it is easily found that the surface nanocrystalline leads to the transition boundary between mild wear and severe wear to a higher sliding speed and a higher load,which enlarges the safe operation region of AZ31 Mg alloy as well as the possible application field

5.Conclusion

In this study,a ball-on-disk dry sliding wear study has been conducted to investigate the tribological behavior before and after USSP treatment under sliding speed from 0.1m/s to 0.5m/s and applied load from 10N to 50N.The wear resistance was evaluated by the coefficien of friction,the wear rate,the roughness of the worn surface and the width of the worn track.The wear mechanisms under different sliding speeds and applied loads were analyzed.The major conclusions from the experimental study are listed as follows:

(1) A gradient nanostructure was fabricated by USSP treatment at the surface of AZ31 Mg alloy.The top grains were refine to 37nm and the thickness of the refine region was 200μm.Beneath the refine region,a deformed region with residual stress was formed at depth of 200μm to 400μm.The microhardness at the surface was improve from 60 HV to 145 HV after USSP treatment,and it decreased gradually along the depth.

(2) The formed nanocrystalline resulted in an easy formation of MgO on the surface,which reduced the COF value and improved the wear resistance.

(3) The delamination wear mechanism was retarded after USSP treatment,which was due to the stronger sub-surface of this gradient nanostructure.

(4) Under high sliding speed (0.5m/s) and high applied load(50N),thermal softening and melting was reported in asreceived while it was prevented in the nanocrystalline,which enlarged the safe operation window of AZ31 alloy sheet.

Data availability

The raw/processed data required to reproduce these find ings cannot be shared at this time as the data also forms part of an ongoing study.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.

Acknowledgments

The current work was supported by the Center for Technology Development at Purdue University.The authors would like to thank Prof.Milan Rakita (Purdue University) for his assistance on the ultrasonic shot peening equipment setup.Dr.Yuan Xin and Dr.Xusheng Yang (Chongqing University)are also acknowledged for their assistance on the SEM and TEM work.