M A T E R I A L S C H A R A C T E R I Z A T I O N 6 8 ( 2 0 1 2 ) 6 3 – 7 0

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Microstructural evolution of P92 ferritic/martensitic steel underAr + ion irradiation at elevated temperature

Shuoxue Jina, Liping Guoa,⁎, Tiecheng Lia, Jihong Chena, Zheng Yanga, Fengfeng Luoa,Rui Tangb, Yanxin Qiaoc, Feihua Liuc

aKey Laboratory of Artificial Micro- and Nano-structures of Ministry of Education and School of Physics and Technology, Wuhan University,Wuhan 430072, ChinabNational Key Laboratory for Nuclear Fuel and Materials, Nuclear Power Institute of China, Chengdu 610041, ChinacSuzhou Nuclear Power Research Institute, Suzhou 215004, China

A R T I C L E D A T A

⁎ Corresponding author at: School of Physicsfax: +86 27 6875 2569.

E-mail address: guolp@whu.edu.cn (L. Gu

1044-5803/$ – see front matter © 2012 Elseviedoi:10.1016/j.matchar.2012.03.009

A B S T R A C T

Article history:Received 17 August 2011Received in revised form6 March 2012Accepted 17 March 2012

Irradiation damage in P92 ferritic/martensitic steel irradiated by Ar+ ion beams to 7 and12 dpa at elevated temperatures of 290 °C, 390 °C and 550 °C has been investigated bytransmission electron microscopy, scanning electron microscopy and atomic forcemicroscopy. The precipitate periphery (the matrix/carbide interface) was amorphized onlyat 290 °C, while higher irradiation temperature could prevent the amorphization. Theformation of the small re-precipitates occurred at 290 °C after irradiation to 12 dpa. Withthe increase of irradiation temperature and dose, the phenomenon of re-precipitation be-came more severe. The voids induced by irradiation were observed after irradiation to7 dpa at 550 °C, showing that high irradiation temperature (≥ 550 °C) was a crucial factorwhich promoted the irradiation swelling. Energy dispersive X-ray analysis revealed thatsegregation of Cr and W in the voids occurred under irradiation, which may influence me-chanical properties of P92 F/M steel.

© 2012 Elsevier Inc. All rights reserved.

Keywords:Irradiation damageVoid swellingP92 ferritic/martensitic steelSuper-critical water reactor

1. Introduction

The super-critical water reactor (SCWR) has been selected asone of the Generation IV advanced nuclear reactor concepts be-cause of its high thermal efficiency and simple design as com-pared to current light water reactors [1–3]. Viability of theconcept requires operation conditions at a temperature rangeof 280–620 °C, at pressure up to 25 MPa and core materials thatcan withstand neutron dose of 15 dpa (E>1 MeV). The require-ments such as radiation, high temperature, high pressuresand a corrosive water environment make the SCWR one of themost challenging reactors from the standpoint of material se-lection [4]. Ferritic/martensitic (F/M) steels are being considered

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as candidate materials for the structural components in theSCWR, due to their several attractive merits such as low activa-tion, resistance to void swelling, low thermal expansion coeffi-cients and high thermal conductivity [2,3,5]. F/M steels suchas T91, HT9 and P92 are candidate structural materials forthe SCWR because of their wide use in supercritical fossilplants [4,6]. The P92 F/M steel possesses good oxidation resis-tance, strong corrosion resistance, excellent high temperaturestrength and good mechanical properties. Recently, potentialapplications of P92 F/M steel in the SCWR have attracted muchattention [6,7]. The mechanical properties of P92 steel [8–11]and the performance of corrosion resistance [6] have beenreported. However, investigations on irradiation properties of

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Table 1 – Chemical composition of P92 F/M steel (wt.%).

Fe Cr W Mo Mn Si Ni Cu V Nb Al B P S N C

Bal. 9.50 2.0 0.60 0.50 0.50 0.40 0.25 0.25 0.09 0.04 0.001 0.02 0.01 0.03 0.13

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P92 steel have been very limited. The response of P92 steel to in-tensive irradiation of energetic particles is an important issuefor their use in an advanced nuclear power plant [12]. Previous-ly, we have investigated the response of P92 steel to energeticparticle irradiations at room temperature [7]. In the presentwork, we investigated the microstructural evolution of P92steel under argon ion irradiation at elevated temperatures of290 °C, 390 °C and 550 °C, a commonmethod used in irradiationdamage research in materials [13,14].

2. Experimental Details

The testmaterial of P92 F/M steel (supplied by Panzhihua Iron&Steel Corporation in China) was annealed at 1050 °C for 30minand tempered at 780 °C for 60 min. The nominal chemical com-position of the material is shown in Table 1. The P92 plate wascut into specimens with a dimension of 10×10×0.5 mm andthen mechanically polished with silicon carbide paper withthe grades of 800–2000 and 0.5 μm diamond spray until amirror-like surface appeared. Finally, the polished bulk speci-mens were cleaned with acetone and ultrasonically rinsed inde-ionized water for 5 min. Disk specimens of 3 mm diameterwere punched out from a thickness of 50–100 μm thick foils,which was prepared from a thick plate of the P92 F/M steel bylinear cutting and thinned to its final thickness. Final transmis-sion electron microscope (TEM) specimens were polished by aMTPA-5 twin-jet electro-polishing machine (produced byShanghai Jiaotong University, China), using 10% HClO4 and90% CH3COOH polishing solution at room temperature.

Already prepared TEM thin foils and polished bulk specimenswere irradiatedwith argon ions using an ion implanter in the Ac-celerator Laboratory of Wuhan University. Sample temperaturewas maintained at 290±5 °C, 390±5 °C and 550±5 °C during

Fig. 1 – SRIM simulation of damage profiles produced by120 keV Ar+ ion irradiation to a dose of 1×1019 ions/m2 in P92F/M steel.

irradiation, whichwasmonitored by a thermocouple throughoutthe experiment process. Irradiations were performed using120 keV Ar+ to a fluence of 3×1019 and 5×1019 ions/m2, corre-sponding to a peak damage dose of 7 and 12 dpa, respectively.The damage profiles calculated by SRIM 2008 are shown inFig. 1, where the displacement energy (Ed) was set to 40 eV asrecommended in ASTM E521-89 [15]. The surface morphologiesof irradiated bulk specimens were measured using a FEI Sir-onMP scanning electron microscope (SEM) system and a SPM-9500J3 atomic forcemicroscope (AFM). Themicrostructural evo-lution of the P92 F/M steel was observed in a JEM-2010HT TEM.The accelerating voltage of the TEM was 200 kV, and the mostoften used image conditions were bright field. The composi-tions of matrices and voids in matrices were measured using aJEM-2010FEF field-emission-gun analytical TEM equipped withan energy dispersive X-ray (EDX) analyzer whose nominal spotsize is 0.5 nm.

3. Results and Discussion

3.1. Microstructure Changes

The bright-field micrograph of unirradiated P92 steel speci-men is shown in Fig. 2, in which we can see a variety of car-bide precipitations. Electron diffraction patterns revealedthat M23C6 dominated the precipitate microstructure of theP92 steel, according well with the results reported in Refs.[11,16,17].

Fig. 3 shows themicrograph of P92 F/M steel after irradiationat 290 °C. Compared to the unirradiated specimen (see Fig. 2),the carbides had obvious changes. The precipitate periphery(the matrix/carbide interface) was amorphized under irradia-tion dose of 7 dpa at 290 °C, as indicated by the selected-area

Fig. 2 – Microstructure of P92 F/M steel prior to irradiation.

Fig. 3 – Microstructure of P92 F/M steel after irradiation at290 °C. (a) — 7 dpa, (b) — 12 dpa. The selected-area electrondiffraction (SAED) pattern of the precipitate periphery isshown in the inset of panel (a).

Fig. 4 – Microstructure of P92 F/M steel after irradiation at390 °C. (a) — 7 dpa, (b) — 12 dpa.

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electron diffraction (SAED) pattern [Fig. 3(a)] inwhich the amor-phous diffraction halos appeared. With the irradiation dose in-creased to 12 dpa, the phenomena of periphery amorphizationbecame more severe [Fig. 3(b)]. The amorphization of the ma-trix/carbide interface could also be observed in P92 steel irradiat-ed by argon ions at room temperature in our previous study [7].The similar phenomenon of amorphization of M23C6 precipitatesin ferritic/martensitic steels under irradiation was also reportedby other researchers, and the lower irradiation temperature(below 230 °C) was the controlling parameter for the amorphiza-tion [18–21]. This is consistentwith our experimental results. Theamorphization phenomenon of the matrix/carbide interfaceshowed that the peripheries of carbides were more easilydamaged by irradiation than other regions. The amorphizationof the interfacewould affect the irradiation-resistance perform-ance of the P92 F/M steel.

With the increase of irradiation dose from 7 dpa to 12 dpa,a variety of black dots inmatrix could be seen [Fig. 3(b)], whichmight be carbide re-precipitates. Comparing Fig. 3(b) withFig. 3(a), higher irradiation dose could lead to the formationof small re-precipitates at 290 °C. On the contrary, no small

carbide re-precipitate was found in matrix when irradiatedto 11.5 dpa at room temperature as reported in Ref. [7],which indicated that the irradiation temperature would bean important factor for the formation of re-precipitates.The carbides re-precipitated and appeared in the matrix ofP92 steel when the irradiated dose increased to as high as34.5 dpa at room temperature [7], which meant that high ir-radiation dose was another important factor leading to re-precipitations. Both the elevated irradiation temperature andthe high irradiation dose played an important role for the for-mation of small carbide precipitates in the matrix. The forma-tion of the small precipitates might be due to the elementsegregation [22]. Solute atom diffusivity in the matrix was en-hanced during high-energy ion irradiation, which was calledradiation enhance diffusion (RED) [23,24]. Both the elevatedtemperature and the irradiation dose could enhance the diffu-sion of the solute atom, and hence enhance the element seg-regation. Therefore, the formation of re-precipitates needed ahigher dose irradiation at a lower temperature or a lower doseat a higher temperature.

The microstructure of P92 steel irradiated by Ar+ ions at390 °C was shown in Fig. 4. A large number of small re-precipitates caused by irradiation could be observed. With the

66 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 8 ( 2 0 1 2 ) 6 3 – 7 0

irradiation temperature increasing from 290 °C to 390 °C, boththe density and the size of the re-precipitates increased. Onthe other hand, when the irradiation dose increased from7 dpa [Fig. 4(a)] to 12 dpa [Fig. 4(b)] at 390 °C, the phenomenonof re-precipitation became more severe. High irradiation dosepromoted the precipitation, whichwas consistentwith our pre-vious experimental results in P92 F/M steel irradiated with Ar+

ions at room temperature [7]. These further indicated thatboth larger irradiation dose and higher irradiation temperaturecould accelerate the precipitation of small carbides in thematrix, as explained by the mechanism of RED.

A few very small voids, about 2.5 nm in diameter, were ob-served after irradiation to 7 dpa at 550 °C, which was shown inFig. 5(a). With the irradiation dose increasing to 12 dpa, the sizeof the voids kept almost unchanged, but the density becamelarger [Fig. 5(b)]. The appearance of the voidmeant that swellinghad taken place in the specimen. When irradiated to 7 dpa at550 °C, the void swelling was about 0.03%, while for irradiationto 12 dpa the swelling increased to 0.53%. The swelling rate perdpa rapidly increased from 0.0043%/dpa (7 dpa) to 0.044%/dpa(12 dpa). The phenomena of void swelling had also been foundin other F/M steels when irradiated at high temperatures. Highnumber density cavities were observed in the Fe–Cr–Mn alloy

Fig. 5 – Microstructure of P92 F/M steel after irradiation at550 °C. (a) — 7 dpa, (b) — 12 dpa.

irradiated with Ar+ ions at 450 °C [14]. Void swelling in a 9Cr F/M steel irradiated with energetic Ne+ ions was found at elevatedtemperatures (≥440 °C) [12]. In the F/M steel F82H, cavities alsoformed at 470 °C under irradiation by dual beams of Fe3+ andHe+ ions to 50 dpa [25]. Small voidswith the size of approximate-ly 6 nm in diameter were observed in the China Low ActivationMartensitic steel irradiated to 1.4 dpa by electron beam at450 °C [26]. However, in Ref. [27], no cavity was formed in F82Hsteel irradiated at 360 °C to 50 dpa under irradiation by dualbeams of Fe+ and He+ ions. These studies suggested that thehigh temperature (>400 °C) might be an important factor forthe formation of cavities in F/M steels. The increasing of theswelling rate with the irradiation dose also implied that thedose would also affect the formation of the voids.

However, the precipitate peripheries in Figs. 4 and 5 werenot amorphized when the irradiation temperature was390 °C and 550 °C. This implied that higher irradiation temper-ature (≥390 °C) could prevent the amorphization of carbide pe-ripheries. The reason may be that the high temperature couldplay an annealing action and reduced the disorder of the lat-tice, and the amorphous peripheries would recrystallize at550 °C. An early publication on low-activation Fe–Cr–Mnalloy irradiated with Ar+ ions at 450 °C indicated that thecarbide/matrix interfaces supplied favorable sites for growthof cavities [14]. Hu et al. also reported that the interfaces ofcarbides/matrices in Fe–Cr–Mn steel irradiated to 10 dpa at400 °C by electron-helium ions were the priority formation lo-cations of the voids [28]. However, in the present study, nocavity was found in carbide peripheries. This result showedthat the carbides in P92 F/M steel exhibited good resistanceto swelling induced by irradiation at elevated irradiation tem-perature (≥390 °C).

3.2. TEM–EDX Analysis

Compositions of matrix and cavity from the 12 dpa-irradiatedspecimen at 550 °C were investigated with TEM–EDX. In Fig. 6,the regionofmatrix andcavitymarkedby black ringswas inves-tigated by EDX. The energy spectra and the element distribu-tions are shown in Figs. 6 and 7, respectively. The results ofEDX also showed that the concentration of argon element incavity was less than that in matrix (Fig. 7), implying thatirradiation-induced cavities may be the voids instead of argonbubbles. Irradiation-induced Cr and W segregation behavior incavities occurred, which was the non-equilibrium segregationof alloying elements at defect sinks (such as cavities) [7,29,30].The discussions for the radiation-induced Cr segregation be-havior at defect sink (grain boundary, void and carbide) werereported by Lu et al., and they gave a possiblemechanism to ex-plain the radiation-induced Cr segregation behavior [30]. How-ever, the explanation of the radiation-induced W segregationwas rarely reported, and the fundamental studies concerninganalytical models and molecular dynamics simulations maybe proper approaches to illustrate the problem. The segregationof elements could influence mechanical properties of P92 F/Msteel. For example, segregation of Cr will affect the creepstrength of the alloy and lead to re-precipitation of carbides asshown in Figs. 3(b), 4(a),(b), and 5(a) and (b). In addition, Cr ele-ment possesses the capability of high strength corrosion-resistance, thus, the segregation of Cr will degrade corrosion-

Fig. 6 – EDX spectra of matrix and cavity in P92 F/M steel irradiated to 12 dpa at 550 °C.

Fig. 7 – Compositions of matrix and cavity after 12 dpa argonirradiation at 550 °C.

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resistant performance, while the enrichment of W will reducethe high temperature strength [29].

3.3. SEM and AFM Analysis

Fig. 8 shows the SEM images of specimen surfaces irradiated atdifferent irradiation conditions. The images shown in Fig. 8(a)and (b) were taken from the surface of the specimens irradiatedto the fluence of 5×1019 ions/m2 at 390 °C and 5×1019 ions/m2

at 550 °C, respectively. Almostno irradiationdamage couldbeob-served in Fig. 8(a). However, a large number of nanometer-sizedhillocks were formed in the surface irradiated at 550 °C, asshown in Fig. 8(b). The formation of thenanometer-sizedhillocksmight be due to the voids that appeared as shown inTEM images[Fig. 5(b)]. The surface can be regarded as a larger effective defectsink that could absorb the point defects (interstitials and vacan-cies) [31]. The elevated irradiation temperature (550 °C) could in-crease the migration of the vacancies, and they would migrate

Fig. 8 – SEM images of P92 F/M steel at different irradiationconditions.

Fig. 9 – AFM images of P92 F/M steel irradiated to 5×1019

ions/m2 at 550 °C.

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towards the surface, leading to the formation of larger voids atthe region which is more close to the surface. Finally, the voidsnear the surface might lead to the formation of the nanometer-sized hillocks. Therefore, these surface modification results ob-served by SEM are consistent with the results of TEM analysis,i.e. both SEM and TEM observations showed that voids formedand irradiation swelling occurred when the P92 steel was ir-radiated at high temperature (≥550 °C).

Changes in surface morphology of the P92 F/M steel in-duced by ion beam irradiation at 550 °C were also analyzedwith AFM, as shown in Fig. 9. Significant modifications werediscerned in the surfaces irradiated to 5×1019 ions/m2 at550 °C [Fig. 9(a) and (b)]. The appearance of nanometer-sizedhillocks observed by AFM was also consistent with the resultsof SEM analysis, which showed again that the voids wereformed at 550 °C.

The surface roughness is represented by the root-mean-square (RMS) roughness, and the changes of the RMS rough-ness along with temperature under different doses areshown in Fig. 10. The surface roughness slightly decreasedfrom 3.5 nm for unirradiated specimen to 3.0 nm for the spec-imen irradiated to 3×1019 ions/m2 at 390 °C and to 3.3 nm forthe specimen irradiated to 5×1019 ions/m2 at 290 °C. The re-sults mean that the surface of the specimen could be polishedat low irradiation temperature and low irradiation dose [32].However, with the irradiation temperature increase to550 °C, the RMS values increased rapidly to 7.0 nm (3×1019

ions/m2) and 8.6 nm (5×1019 ions/m2), respectively. The incre-ment of RMS roughness was due to that the nanometer-sizedhillocks formed in the surface at 550 °C as observed by AFMand SEM.

4. Summary

In the present study, microstructural evolution of the P92 F/Msteel irradiated by Ar+ ions to 7 dpa and 12 dpa at elevatedtemperatures of 290 °C, 390 °C and 550 °C was investigatedby TEM, SEM, AFM and EDX. The irradiation damage suchas void swelling, amorphization of precipitate periphery, re-precipitation and radiation-inducedsegregationwasdeterminedand discussed. The main results are summarized as follows.

1. The precipitate periphery (thematrix/carbide interface) be-came amorphous under irradiation at 290 °C to a dose of7 dpa, and the interface amorphization became more se-vere with the irradiation dose increasing. However, theamorphization phenomena did not occur when the ir-radiation temperatures rose to 390 °C and 550 °C.

2. High density of small carbide re-precipitates was observedin matrix irradiated to 12 dpa at 290 °C, and the increasing

Fig. 10 – Variations of the surface roughness for P92 steelirradiated under different conditions.

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of irradiation temperature and dose could accelerate there-precipitation.

3. High irradiation temperature (≥550 °C) was a crucial factorfor the formation of void swelling. When irradiated to7 dpa at 550 °C, the void swelling was about 0.03%, whilefor irradiation to 12 dpa the swelling increased to 0.53%.

4. The segregation of Cr and W elements occurred under ir-radiation, which could influence mechanical properties ofthe P92 steel. Especially, the segregation of Cr will affect thecreep strength of the alloy, lead to precipitation of carbidesand degrade corrosion-resistant performance.

Acknowledgments

The financial supports from the National Basic Research Pro-gram of China (Grant no. 2007CB209800), the National NaturalScience Foundation of China (Grant no. 11075119) and theFundamental Research Funds for the Central Universities(Grant no. 20102020201000013) are gratefully acknowledged.The authors would like to thank the Center for Electron Mi-croscopy in Wuhan University, China.

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