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Materials Science and Engineering A 474 (2008) 112–119

Reassessment of the effects of laser surface melting on IGC of SUS 304

Sen Yang a,∗, Zhanjie Wang b, Hiroyuki Kokawa b, Yutaka S. Sato b

a Department of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, PR Chinab Department of Materials Processing, Graduate School of Engineering, Tohoku University,

6-6-02 Aramaki-aza-Aoba, Aoba-ku, Sendai 980-8579, Japan

Received 14 May 2006; received in revised form 26 March 2007; accepted 29 March 2007

bstract

Laser surface remelting (LSM) experiments were conducted on surface of 304 stainless steel using a 2 kW CW Nd: YAG laser and the effects ofSM on the intergranular corrosion (IGC) resistance of 304 stainless steel were reassessed from view point of grain boundary engineering (GBE).SM could make the sensitized microstructures locally desensitize, and could improve the IGC resistance. The improved IGC resistance of the

aser-surface-melted specimens could be attributed in part to Cr redistribution at the boundaries of the cells and grains and in part to existence oflarge amount of low energy Σ(1 ≤ Σ ≤ 29) boundaries and the formation of 〈0 0 1〉(1 0 0) texture. However, the laser-surface-melted specimens

ecame much more susceptible to IGC in the sensitization temperature region, and the corrosion rate of the resensitized specimen was even higherhan that of the base materials under the same sensitization condition. A subsequent annealing treatment changed the grain boundary characteristribution (GBCD) remarkably and the IGC resistance of the processed specimens was improved. 2007 Elsevier B.V. All rights reserved.

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eywords: Laser surface remelting; Intergranular corrosion resistance; Grain b

. Introduction

304 Austenitic stainless steels are extensively used in nuclear,hemical and other industries owing to their relatively good cor-osion and mechanical properties. But the sensitization problemenders this type of stainless steel susceptible to intergranularorrosion attack, which usually leads to the premature failuref components. Several remedies were used to prevent sensiti-ation, such as reduction carbon-content [1], addition of carbontabilizing elements [2], rapidly cooling throughout the sensi-ization temperature range [3], and so on. However, there existome limitations for these methods in actual application. As mostorrosion generally occurs at the surfaces of materials exposedo corrosive media, the corrosion resistance would be improvedhrough modifying their surfaces without altering the bulk prop-rties. An alternative aggressive approach to improving the IGCesistance of 304 stainless steel is laser surface melting (LSM).

SM has been extensively used to modify the surface perfor-ance of the various metallic alloys since high power laser

ecame available in 1960s for its special advantages [4]. LSM is

∗ Corresponding author. Tel.: +86 471 6575752; fax: +86 471 6575752.E-mail address: yangsen@tsinghua.edu.cn (S. Yang).

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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2007.03.103

ry character distribution; Austenite stainless steel

typical rapid solidification processing which generally resultsn the extended solid solution of the alloy system, formationf metastable phases and fine microstructure. This techniquenvolves the melting of a thin surface layer of the alloy usinglaser beam, which creates a more corrosion-resistant barrier

etween the corrosive environments and underlying materialsusceptible to localized corrosion attack. Also, because of easyransmission of a high-power laser beam through fiber or optics

irrors without any appreciable power loss, LSM can be car-ied out in situ on complicated or inaccessible components ifequired. In addition, this technique can be easily used to repairhe damaged surfaces that are difficult to reach by conventional

ethods. In fact, during the last two decades, LSM has beenidely used in improving or modifying the corrosion resis-

ance of many kinds of materials, such as SUS304 [5], 304L6], 316 [7], 316L [8], and Alloy 600 [9]. However, almostll of the researchers attributed the improving of corrosionesistance by LSM to the fined and homogenous microstruc-ures, decomposition of chromium carbides and disappearancef the chromium-depleted zone, and restrain of precipitation of

hromium carbides resulted from the high cooling rate. Andhere is no report on susceptibility of the laser surface meltedpecimens to IGC when the laser surface melted specimens aregain used in the sensitization temperature region.

nd Engineering A 474 (2008) 112–119 113

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Table 2Laser processing parameters

Scanning velocity (mm/s) 5–50Output power (W) 721–1415Beam diameter (mm) 3SO

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S. Yang et al. / Materials Science a

Numerous studies showed that low-Σ coincidence site lat-ice (CSL) grain boundaries (usually Σ ≤ 29), especially Σ3 or

3-related boundaries display a high resistance (and, in manyases, immunity) to sliding, cavitations, and fracture; corrosionnd stress-corrosion cracking; sensitization; solute segregation.herefore, “grain boundary design and control” became into aiable means of enhancing the bulk properties of conventionalolycrystalline materials, which has evolved the concept of grainoundary engineering (GBE) [10,11]. Since then, the influencef grain boundary (GB) on the mechanical properties of poly-rystalline materials has been extensively studied during the pastwo decades and still attracts a great of attention [12–15].

Therefore, the aims of this study are to reassess the laserurface melting, and to investigate the relationship betweenmprovement of corrosion resistance of SUS304 and grainoundary characteristic distribution using electron back scat-ering diffraction (EBSD) and corrosion behavior of the laserurface melted specimens after sensitization again.

. Experimental procedure

.1. Raw materials

Commercial type 304 austenite stainless steel plate was usedn this investigation. Table 1 lists the chemical composition. Theectangular specimens of 50 mm × 8 mm × 8 mm were cut fromlloy plate and solution annealed at 1323 K for 0.5 h followed byater quenched to eliminate any carbide precipitate formed dur-

ng processing, which were named as base material (BM). Andhen, these materials were further sensitized at 923 K for 20 hollowed by water quenched prior to LSM. In order to minimizehe reflection for the laser beam and obtain a similar surfaceuality for each specimen, all the specimens were ground up to00 grit SiC paper and cleaned in ethanol prior to LSM.

.2. Laser processing

LSM experiments were carried out using a 2 kW continuousave Nd: YAG Laser (NEC 850). The normally incident laseream was defocused to a spot with diameter of 3 mm. Duringaser processing, a continuous flow of argon gas was blown to

he melted zone to prevent heavy oxidation. The detailed laserrocessing parameters are list in Table 2. In order to obtain aarger melting area, overlapping technique was adopted, and theptimal overlapping ratio was 30%.

able 1hemical composition of 304 austenite stainless steel (wt%)

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.3. Microstructure characteristics

Specimens were cut along the laser-scanning track, and wererepared using the standard metallographic technique. In ordero remove residual surface deformation and stress caused by

echanically grinding and polishing, specimens were electro-olished in a solution containing 10% perchloric acid in ethanolt a voltage of 30 V for 60 s. At last, the specimens werelectro-etched in an oxalic acid solution at room tempera-ure for 30 s to show GBs. A Philips XL30 scanning electron

icroscopy equipped with a field emission gun (FEG) and aolid-state backscatter detector was used to obtain orientationmage microscope (OIM), and the TSLTM analyzing softwareas used to analyze the GBCD. GBs were categorized in threeroups: LABs (low angle boundaries) with Σ = 1, CSLs with< Σ ≤ 29, and HABs (high angle boundaries) with Σ > 29,here the Σ number is the reciprocal of the fraction of shared

attice sites from each grain at the boundary. Brandon’s cri-erion [16] was applied to determine the Σ number for alloundaries.

.4. Corrosion test

In order to detect susceptibility to intergranular attack, a dou-le loop electrochemical potentiokinetic reactivation (DL-EPR)est [17] was performed. The electrolyte was 0.5 M H2SO4 with.01 M potassium thiocyanate (KSCN). The reactivation cur-ent ratio (Ra) was used to quantify the degree of sensitizationDOS) of alloys. The larger Ra value is, the higher the suscep-ibility of the material to intergranular attack. Each sample wasepeated at least two times in different solutions to ensure repro-ucibility. The surface morphologies of the samples after EPRest were observed by scanning electronic microscopy. EPR testnly reflects the local area of chromium depletion near sensi-ized GBs on the test surface. In fact, IGC propagates alongBs from the surface into the interior of materials and leads toass-loss caused by grain dropping. In order to further evaluate

he effects of GBE on corrosion resistance of 304SS, a ferriculfate–sulfuric acid test [18] was carried out as well.

. Experimental results and discussions

.1. Microstructures and GBCD of BM

Fig. 1(a) shows the typical microstructure of the BM afterensitization at 923 K for 20 h. It can be seen that nearly con-inuous Cr-rich carbides are precipitated at most of the grainoundaries, and the average grain size is approximately 15 �m.

114 S. Yang et al. / Materials Science and Engineering A 474 (2008) 112–119

Fig. 1. Optical micrograph of the BM sensitized at 923 K for 20 h (a) and corresponding GBCD (b). Random and low Σ CSL boundaries indicated by black line andgray line, respectively.

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ig. 1(b) shows the corresponding GBCD of the BM after sensi-ization at 923 K for 20 h. In the BM, the fraction of low CSL GBss 57.6%, which is composed of 12.0% Σ1, 34.8% Σ3, 4.0%

9, and 1.0% Σ27, and the random grain boundaries (RGB)how a continuous network (see Fig. 1(b)). The fact that the Cr-ich carbides easily precipitate at the RGBs led to the formationf the continuous Cr-rich carbides, which means that the sensi-ized BM is susceptible to intergranular attack. Fig. 2 shows the

ole figure of the BM after sensitization. It can be seen that theres no particular regions of apparent texture and the red regionsn the pole figures only indicate intensity five times above theandom background.

ig. 3. Macrographs of the molten pool from the longitudinal cross-section.

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res of the BM.

.2. Effects of LSM on microstructure characteristics

After laser surface remelting, both of the microstructures andBCD have been remarkably changed. Fig. 3 shows a typicalacroshape of the molten pool from the longitudinal cross-

ection. It can be found that there existed three distinctive zones,amely, laser-melted zone (LMZ), heat-affected zone (HAZ),nd the substrate. Fig. 4(a) shows the microstructure of HAZ.n the HAZ, no carbides were observed at the grain bound-ries, which meant that desensitization occurred in this zone.n the one hand, heat conducted from the melted zone during

he laser-melting process made the pre-existed Cr-rich carbidesompletely dissolve. On the other hand, abnormal grain growthccurred under the effects of the heat input, which led to the grainoundary characteristic distribution being changed. Fig. 4(b)hows GBCD of HAZ. It can be seen that a large amount of lownergy Σ GBs (primarily Σ3 or Σ3-related boundaries) wereormed in the HAZ after laser surface remelting. The fractionf CSL GBs was 74.8%, which was more than that of the BM,7.6%. Owing to a large amount of special CSL GBs existed inAZ; they restrained the Cr-rich carbides from precipitating at

he GBs during the following cooling process.Fig. 5(a) shows the microstructures of the LMZ. As observed

n this figure, the microstructures of LMZ became very fine, andhe average primary cellular/dendritic spacing was only about

�m. During laser melting processing, the pre-existed Cr-richarbides had been completely dissolved due to the high heat fluxensity of the laser beam and will not reprecipitate in the follow-ng solidification processing due to the very rapid solidification

S. Yang et al. / Materials Science and Engineering A 474 (2008) 112–119 115

Fig. 4. The typical microstructure (a) and GBCD (b) of HAZ. Random and low Σ GBs indicated by black line and gray line in (b), respectively.

Fig. 5. (a) Microstructure of the LMZ and (b) GBCD. Random and low Σ GBs indicated by black line and gray line in (b), respectively.

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ate. These results are constant with the other researches [5–8].n addition, laser surface melting made the grain boundary char-cteristic distribution alter obviously. Fig. 5(b) shows the GBCDf LMZ. As there existed an ultra-high temperature gradient inhe molten pool (generally >106 K/m), microstructures direc-ionally grew from the bottom to the top surface along 〈1 0 0〉

rientation for cubic crystal, which resulted in a large amount ofow angle boundaries (Σ1), 46%, and the amount of special CSLBs also increased and reached 84.1%. Table 3 lists the detailedata about the GBCD in different zones. It is difficult for Cr-rich

able 3raction of the special CSL GBs and average grain size

osition Σ1 (%) Σ3 (%) Σ(1–19) (%) Average grainsize (�m)

ubstrate 12.0 34.8 57.6 15.47AZ 12.1 55.3 74.9 22.14MZ 46.0 36.7 84.1 5.0

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arbides to precipitate at the low Σ CSL GBs, especially at Σ1rain boundaries, because low Σ boundaries are less susceptibleo impurity or segregation and possess greater resistance to grainoundary sliding and intergranular degradation phenomena suchs fracture, cavitations and localized corrosion [19–21]. Fig. 6hows the (1 1 1), (1 1 0), (1 0 0) pole figures for the LMZ. Theseere taken from the longitudinal cross-section of the moltenool. One can see that (1 0 0) pole is strongly aligned, althought is not a typical (1 0 0) pole figure, which maybe resulted fromhe fact these grains had their long axis far enough from the exact1 0 0〉 direction. This strong alignment of (1 0 0) pole then leadso the texture observed in (1 1 1) and (1 1 0) pole figures. Theed color in the contour plots in Fig. 6 corresponds to a valuehat is about 30 times the random background. Therefore, LSM

ade the isotropic BM become an anisotropic one. Given that

aterials performances are strongly dependent on texture [22],

ne would expect this texture could have a strong effect on theocal corrosion resistant response of the laser surface remeltedpecimens.

116 S. Yang et al. / Materials Science and Engineering A 474 (2008) 112–119

Table 4The ratio of the reactive current in EPR test

EPR sample no. Power (W) Velocity (mm/s) Ecorr Imax (mA) Imin (mA) Ratio (%)

1 1415 8 −0.456 6.27 1.05 × 10−2 0.1672 1415 12 −0.453 5.31 1.19 × 10−2 0.2243 1009 12 −0.454 6.6 1.4 × 10−2 0.2124 1415 20 −0.454 6.6 1.05 × 10−2 0.1595 1415 5 −0.458 5.78 0.54 × 10−2 0.0936 1415 15 −0.459 4.58 2.43 × 10−2 0.5317 −2

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.3. Corrosion resistance

Table 4 lists the ratios of the reactive current of the laserurface remelted specimens without sensitization. Owing to theapid remelting and the rapid cooling during LSM, the surfaceayer of the laser surface melted specimens (including LMZ andAZ) was in the solid solution state. Therefore, compared with

he value of the sensitized BM, the reactive current ratios ofhe laser surface melted specimens were very small, generallyess two orders of magnitudes. This meant that susceptibility tontergranular attack of the sensitized BM has been remarkablyecreased after laser surface remelting.

It is noted that, when the laser surface melted specimens wereesensitized at the sensitization temperature region, the reactiveurrent ratios of the specimens even became higher than thosef BM under the same sensitization condition, as showed inig. 7 (marked as LSM), which meant that the laser surfaceelted specimens became much more susceptive to intergranu-

ar corrosion after resensitization. As above mentioned that theicrostructure of the molten pool grew from the substrate along

he heat flux anti-direction, which resulted in a large amountf subgrain boundaries, and these boundaries generally runhrough the whole melted layer from the substrate to the top sur-ace. Therefore, in the following sensitization experiments, these

rain boundaries became the ways for intergranular corrosion,nd sensitization even became much easier to happen than BM.

EPR test only reflects the local area of chromium depletionear sensitized GBs on the test surface. In fact, IGC propagates

ig. 7. Relationship between the ratios of the reactive current and sensitizationime under different processing conditions.

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−0.456 5.03 1.88 × 10 0.374−0.456 6.5 2.22 × 10−2 0.342−0.455 6.02 1.63 27.08

long GBs from the surface into the interior of materials andeads to mass-loss caused by grain dropping. In order to fur-her evaluate the corrosion resistance of 304SS under variousonditions, ferric sulfate–sulfuric acid tests were conducted onM and laser surface melted specimens, respectively. Fig. 8

hows the corrosion rates under various processing conditions.or comparison, a ferric sulfate–sulfuric acid test was also con-ucted on laser surface melted specimens without sensitization.t can be seen that the corrosion rate of the laser surface meltedpecimens without sensitization was much lower than that ofM sensitized at 923 K for 2 h. Whereas the corrosion rate of

he laser surface melted specimen resensitized at 923 K for 2 has much higher than that of BM under the same sensitiza-

ion, and the specimens was completely decomposed within 3ays. Fig. 9 shows the surface morphologies and the transverseross-section macrostructures of the specimens boiled in theerric sulfate–sulfuric acid solution for 3 days. It can be seenhat, owing to there existed continuous random grain boundaryetwork in BM, grain boundaries of BM were easily attackednd IGC propagated from surface into interior of the speci-en, which subsequently resulted in a large amount of grain

ropping and a higher corrosion rate. In contrast, owing to the

as not easy to propagate from surface to interior of the spec-mens (see Fig. 10), which subsequently led to less mass-loss

Fig. 8. Corrosion rates of 304SS under various processing conditions.

S. Yang et al. / Materials Science and Engineering A 474 (2008) 112–119 117

Fig. 9. Surface morphologies (a) and cross-section macrostructures (b) of BM sensitized at 923 K for 2 h after ferric sulfate–sulfuric acid test 72 h.

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ig. 10. Surface morphologies (a) and cross-section macrostructures (b) of theest 72 h.

nd a lower corrosion rate. However, the resensitized laser sur-ace melted specimens showed a much higher corrosion ratehan that of BM under the same sensitization condition. The

ain reason could be ascribed to the metastable and direction-lly solidified microstructure. The microstructure of the laserurface melted specimens was unstable due to the rapid solidi-cation rate. During the following sensitization processing, theandom grain boundary generally run through the remelted layerrom the bottom to the top surface. On the one hand, these high-nergy boundaries are easily attacked by the corrosion media.n the other hand, IGC attacking is easily to propagate to

he substrate along these high-energy boundaries, which sub-equently led to a great amount of grain-dropping, and a muchigher corrosion rate. Fig. 11 shows the surface morphologynd cross-section macrostructure of the resensitized specimen.

n apparent and serious grain-dropping phenomenon could bebserved. Owing to the directionally solidified microstructures,pecimens became much easier to be attacked, and IGC easilyenetrated from the surface into interior of specimens.

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ig. 11. Surface morphologies (a) and cross-section macrostructures (b) of the laser scid test 48 h.

surface melted specimen without sensitization after ferric sulfate–sulfuric acid

.4. Effects annealing treatment on the LSMed specimens

A lot of research [23] showed that special low energy Σ

oundaries (1 ≤ Σ ≤ 29), especially Σ3 or Σ3-related bound-ries, was correlated with the abnormal grain growth (AGG)ccurred in polycrystalline. Koo et al. [24–26] reported that therexists a critical strain for most of the metals. At the critical strain,ome of the grain boundaries are distorted by extrinsic disloca-ion, thus inducing the rapid growth of some grains. If the initialrain size is very small, AGG will occur rapidly at high tem-eratures, even without any deformation, and the deformationffect will be less pronounced.It is well known that laser surfaceelting can remarkably fine the microstructures. On the other

and, owing to the rapid cooling rate, there exists certain stressn the molten zone after solidification. Indeed, when the laser

urface melted specimens were given an annealing treatmentt a certain temperature, the GBCD were apparently modified.ig. 12(a) shows the microstructure of the laser surface meltedpecimens when it was annealed at 1200 K (927 ◦C) for 30 h.

urface melted specimen resensitized at 923 K for 2 h after ferric sulfate–sulfuric

118 S. Yang et al. / Materials Science and Engineering A 474 (2008) 112–119

Fig. 12. (a) Microstructure of specimen annealed at 1200 K for 30 h; (b) GBCDof zone b in (a). Random and low Σ GBs indicated by black line and gray linein (b), respectively.

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ig. 13. Relationship between the fraction of CSL GBs and annealing time.

t can be seen that there was no longer obvious boundariesetween LMZ and HAZ and grain boundaries were mainly com-osed of low energy Σ boundaries (primarily Σ3 or Σ3-relatedoundaries). Accompanying the formation of high frequencyow Σ boundaries, the continuous random grain boundary net-ork was extremely dispersed by introduction of low energy

egments on migration random boundaries during twin emis-ion and boundary–boundary reactions in the grain growth, ashown in Fig. 12(b). Fig. 13 shows the relationship between

he fractions of low Σ(1 ≤ Σ ≤ 29) boundaries and annealingime. With increase of the annealing time, the fraction of theow Σ boundaries increased. It took about 48 h to attain the

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Fig. 15. Surface morphology (a) and cross-section macrostructure (b) of the p

ig. 14. Corrosion rates of 304SS under various processing conditions (sensi-ized at 923 K for 2 h).

aximum frequency of the Σ boundaries when the annealingemperature was 1200 K. As pointed out by the other researchers10–12,27], the highly frequent Σ boundaries (mainly composedf twin boundaries) could result in a much higher IGC resis-ance. In fact, the preliminary experimental results showed thathe reactive current ratios of the materials processed with LSMnd annealing treatment were lower than those of BM underhe same sensitization condition as showed in Fig. 7 (markeds LSM-1200 K-48 h), which meant that the processed materi-ls was much more resistant to IGC than BM under the sameondition. For comparison with the above-mentioned results andvaluating the effects of GBE on corrosion resistance, a ferriculfate–sulfuric acid test was also conducted on the processedpecimens under the same condition. Fig. 14 shows the relation-hip between corrosion rates and durations, and the specimensere sensitized at 973 K for 2 h in advance. It clearly shows that

orrosion rate of the processed specimens was much lower thanhat of BM, and was only one fourth of that of BM. Fig. 15hows the surface morphology and the transverse cross-sectionacrostructure of the specimens after ferric sulfate–sulfuric acid

ests. Owing to that there existed a large amount of CSL grainoundaries in the GBEed specimens, and the continuous randomrain boundary network had been dispersed by CSL segments,

he IGC was not easy to happen and IGC was not easy to propa-ate from surface to interior of the specimens (Fig. 15(b)), whichubsequently led to less mass-loss and a lower corrosion rate.

rocessed materials (GBEM) after ferric sulfate–sulfuric acid test 72 h.

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S. Yang et al. / Materials Science a

. Conclusions

1) LSM remarkably changed the microstructures and grainboundary character distribution. LSM made the microstruc-tures of the LMZ fine obviously; the average primarycell/dendrite spacing was only 5 �m. In addition, LSM madethe fraction of special grain boundaries in LMZ and HAZincrease, and GBCD of LMZ was mainly composed ofΣ1, which resulted in an obvious {0 0 1}〈1 0 0〉 texture inLMZ. LSM remarkably improved the local resistance tointergranular corrosion of the sensitized BM through localdesensitization and altering the GBCD of LMZ and HAZ.

2) However, resensitizing laser surface melted specimensincreased the specimens’ sensitivity to IGC and corrosionrate of the resensitized specimens was much higher than thatof BM under the same sensitization condition.

3) By combining LSM with annealing treatment at rela-tively lower temperature, the GBCD could be remarkablychanged. A high frequency of CSL boundaries and discreterandom boundaries network in materials were achieved,which subsequently resulted in an excellent resistance tointergranular corrosion of 304 stainless steel.

cknowledgements

One of the authors (S. Yang) is grateful to Japan Societyor the Promotion of Science for offering a JSPS fellowship.he authors would like to express their gratitude to Dr. Y.S.ang, Mr. A. Honda and Mr. Isago for their technical support.

he partial support of this work by Program for New Centuryxcellent Talents in University from Ministry of Education ofhina and Science Research Foundation of IMUT (ZD200521)re also acknowledged.

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gineering A 474 (2008) 112–119 119

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