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Effect of He pre-implantation and neutron irradiation

E?ect of He pre-implantation and neutron irradiation on mechanical properties of SiC/SiC composite

Shuhei Nogami a,*,Akira Hasegawa a,Lance L.Snead b,Russell H.Jones c,

Katsunori Abe a

a Department of Quantum Science and Energy Engineering,Tohoku University,01Aramaki-aza-Aoba,

Aoba-ku,Sendai980-8579,Japan

b Oak Ridge National Laboratory,Oak Ridge,TN378316138,USA

c Paci?c Northwest National Laboratory,Richland,WA99352,USA

Abstract

Mechanical property changes of SiC/SiC(Hi-Nicalon/C/SiC)composite caused by uniform He pre-implantation up to about170at.ppm at400–800°C followed by neutron irradiation up to about7.7·1025n/m2(E n>0:1MeV)at800°C in HFIR were investigated by the three-point bend tests and nano-indentation tests.Degradation of the composite bend properties due to neutron irradiation was observed.The hardness increased after neutron irradiation for both the SiC-matrix and the Hi-Nicalon?ber.There was almost no change in the elastic modulus of the SiC-matrix,but there was an increase in the modulus of the Hi-Nicalon?ber after neutron irradiation.He pre-implantation had almost a negligible e?ect on the mechanical properties of the composite specimen.

1.Introduction

Silicon carbide(SiC)?ber-reinforced SiC-matrix composites(SiC/SiC composites)are being considered as a structural material for components in future fusion reactor blankets[1,2].Displacement damage and trans-mutation products such as helium(He)will be produced in these materials during high energy(about14MeV) neutron irradiation.In a full power year,the displace-ment damage and He concentration in a SiC component of the blanket of the ARIES-IV concept reactor were calculated to be approximately57dpa and15380 at.ppm,respectively[2].The He concentration per dpa in SiC will be about10times larger than that in other candidate materials(ferritic steels and vanadium alloys).

A relatively large number of studies of the displace-ment damage e?ect on the mechanical properties of SiC/ SiC composites have been carried out.Advanced SiC/ SiC composites,developed in recent years using stoi-chiometric and high crystalline SiC-?bers such as Hi-Nicalon Type-S[3]and Tyranno SA[4],have exhibited improved radiation resistance up to about7.7·1025n/ m2[5].There have also been some studies of the e?ect of transmutant He on the mechanical properties of SiC/SiC composites[6–8].Using bend tests,the ultimate fracture strength of a Nicalon CG/C/SiC composite(C:carbon) [6]and a Hi-Nicalon/C/SiC composite[7]decreased after He-implantation up to2500at.ppm at900°C and up to 150–170at.ppm at400–800°C,respectively.For com-posite studies utilizing the nano-indentation test,both hardness and elastic modulus of the SiC-matrix and SiC-?bers(Hi-Nicalon and Hi-Nicalon Type-S)decreased after He-implantation up to20000at.ppm below100°C

*Corresponding author.Present address:Research Unit for Structural Strength of Materials,Department of Materials Research for Power Plants,Hitachi Research Laboratory,

Hitachi Ltd.,1-1,Saiwai-cho3-chome,Hitachi317-8511, Japan.Tel.:+81-294558195;fax:+81-294559969.

E-mail addresses:sngm@gm.hrl.hitachi.co.jp(S.Nogami),

akira.hasegawa@qse.tohoku.ac.jp(A.Hasegawa),sneadll@

https://www.sodocs.net/doc/f13650211.html,(L.L.Snead),rh.jones@https://www.sodocs.net/doc/f13650211.html,(R.H.Jones),katsun-

ori.abe@qse.tohoku.ac.jp(K.Abe).

doi:10.1016/j.jnucmat.2004.04.121

https://www.sodocs.net/doc/f13650211.html,/locate/jnucmat Journal of Nuclear Materials329–333(2004)577–581

[8].However,the e?ects of He,and of displacement damage,and their synergistic e?ect on the mechanical property changes of SiC/SiC composites were not clearly distinguished in all cases.The purpose of this study is to investigate the e?ect of He pre-implantation followed by neutron irradiation on the mechanical properties of a SiC/SiC composite.

2.Experimental

A SiC/SiC composite with Hi-Nicalon?bers(0°/90°plain-weave)manufactured by DuPont Lanxide[9]was examined in this study.The SiC-matrix(crystalline b-SiC)was fabricated by an isothermal chemical vapor in?ltration(ICVI)process.The interface material be-tween SiC-?bers and SiC-matrix was a pyrolytic carbon (%150nm thickness),which was fabricated by a chemi-cal vapor deposition(CVD)process.Hi-Nicalon is composed of b-SiC grains whose size is about5–10nm and have some of residual oxygen(<0.5wt%),but substantial amounts of carbon(C/Si atomic ratio$1.39) [3].The geometry of the specimens for irradiation and bend tests was approximately4w?1t?20l mm3(ma-chined from specimens with an original geometry of 6w?2t?20l mm3).The compression side of the bend specimens was machined to1mm thickness,while ten-sion side was not machined.As a result,a CVI b-SiC overlayer remained on the tension side.Fig.1shows the schematic illustration of the specimen dimensions and test con?gurations in this study.

Helium pre-implantation was performed using the cyclotron accelerator at Tohoku University.The accel-eration energy of He-ions was36MeV.The projected range of36MeV He-ions in SiC was calculated to be about470l m by the TRIM code[10].Tandem type energy degrader wheels were used to obtain uniform depth distribution of He-atoms.The nominal He con-centration was about170at.ppm.The displacement damage in the He implanted region was calculated to be about0.009dpa using a threshold displacement energy of40eV.The implantation temperature was400–800°C.He-ions were implanted to the tension side of the bend specimens within an area of about4w?4l mm2 located at the center of the specimens as shown in Fig.1.

Neutron irradiation was performed in the High Flux Isotope Reactor(HFIR)at Oak Ridge National Labo-ratory.Specimens were irradiated at800°C up to about 7.7·1025n/m2(E n>0:1MeV),which corresponds to about7.7dpa using an assumption that the displace-ment energy of40eV for both the Si and C sublattices.

Three point bend tests were performed at room temperature in air.The support span length and cross-head speed were10mm and0.5mm/min,respectively. Con?gurations#1–1and#1–2in Fig.1were used for the bend test measurements of the non-He-implanted, but neutron-irradiated region.Con?guration#2was for the measurements of the He-implanted and neutron-irradiated region.Fractography observations were per-formed using a scanning electron microscope(SEM).

Cross-sectional nano-indentation tests were per-formed at room temperature using a NANO IN-DENTERaII(Nano-instruments)equipped with a Berkovich diamond tip.The indentation depth and loading(unloading)rate were constant,50nm and50 m N/s,respectively.Specimens were mechanically sliced perpendicular to the He-implanted surface as shown in Fig. 1.The cross-section of the cut specimens for indentation was mechanically polished with a0.5l m diamond slurry.

Hardness H and elastic modulus E were evaluated using an unloading curve and the method proposed by Oliver and Pharr[11].H is calculated from the following formula:

H?P max=Aeh cT;e1T

where P max is the maximum indentation load and A, which is a function of constant indentation depth h c,is the contact area of the indenter with the specimen sur-face.E is de?ned as follows:

E?1

àm2

=1=E r

te1àm2

T=E i

;e2Twhere m s;m i;E r and E i are the Poisson’s ratio of specimen, the indenter(0.07for diamond),the reduced modulus and the elastic modulus of the indenter(1141GPa for diamond),respectively.Further details of this method are given in Ref.[11].

3.Results

Fig.2shows the summary of the bend test results (ultimate fracture strength,elastic modulus and proportional limit stress–PLS)for the as-received,the non-He-implanted and neutron-irradiated(Non-He/ Neutron),and the He-implanted and neutron-irradiated

578S.Nogami et al./Journal of Nuclear Materials329–333(2004)577–581

(He+Neutron)composites.The elastic modulus was calculated using the slope of the linear range of the stress–strain curves.The PLS was the stress value when o?set strain was about0.0005.The ultimate fracture strength,elastic modulus and proportional limit stress decreased due to neutron irradiation by about50%,10% and55%,respectively.The‘He+Neutron’specimens had similar bend properties to the‘Non-He/Neutron’specimens.

Fig.3shows a typical observation of the fracture surface by SEM for the‘as-received’,the‘Non-He/ Neutron’,and the‘He+Neutron’composites.He-ions were implanted from the bottom side of the pictures.The pull-out lengths of the Hi-Nicalon?bers for the ‘Non-He/Neutron’composite were longer than that for the‘as-received’composite.Almost no di?erences of the pull-out lengths for the Hi-Nicalon?bers were observed between‘Non-He/Neutron’and‘He+Neutron’com-posites.

Figs.4and5show the hardness and elastic modulus of the SiC-matrix and Hi-Nicalon?ber of the‘as-received’,‘Non-He/Neutron’,and‘He+Neutron’composites.The data for non-He-implanted,but neu-tron-irradiated region and data for He-implanted and neutron-irradiated region were the average of the data points at the depth of0–470and470–1000l m from the He-implanted surface,respectively.The hardness in-creased by about30%after neutron irradiation(no

Fig.3.Typical fracture surface observations by SEM for as-received,non-He-implanted and neutron-irradiated(Non-He/Neutron) and He-implanted and neutron-irradiated(He+Neutron)composites.He-ions were implanted from the bottom side of the pictures.

S.Nogami et al./Journal of Nuclear Materials329–333(2004)577–581579

helium)for both the SiC-matrix and the Hi-Nicalon?-bers.Almost no change in the elastic modulus for the irradiated SiC-matrix was observed,but about a30% increase for the irradiated Hi-Nicalon?bers was ob-served.The‘He+Neutron’specimens exhibited almost the same hardness and elastic modulus as the‘Non-He/ Neutron’specimens.

4.Discussion

A10%reduction in the elastic modulus for the composite was observed.In contrast,almost no change in the elastic modulus for the SiC-matrix and about30% increase for the Hi-Nicalon?ber were observed.Thus, so called the sum rule de?ned as follows is not applicable for the neutron-irradiated composite in this study:

E c?E f?f ftE m?f m;e3Twhere E c;E f and E m are the elastic modulus of the composite,?ber and matrix,f f and f m are the volume fraction of the?ber and matrix in composite,respec-tively.Therefore,it can be predicted that the degrada-tion(e.g.crack and debonding)of the interface between the SiC-matrix and Hi-Nicalon?ber was occurred due to neutron irradiation.Shrinkage of Hi-Nicalon?bers during neutron irradiation was observed in previous studies[12–15].A previous study[16]also showed the presence of microcracks and debonding of the C-inter-face of Hi-Nicalon/C/SiC composite due to C-ions irradiation up to about10dpa at800°C.Moreover, tensile strength of Hi-Nicalon?ber increased with neu-tron dose[17].Thus,bend property degradations of Hi-Nicalon/C/SiC composite due to neutron irradiation might be related to the formation of microcracks and debonding of the C-interface induced by the shrinkage of the Hi-Nicalon?bers.

The e?ect of He pre-implantation treatment prior to neutron irradiation on mechanical properties was almost negligible(relative to the mechanical properties of the neutron irradiated specimens)up to a concentration of 170at.ppm for the composite and the individual com-ponents of the composite.In previous work[8],the total number of released He atoms from the room tempera-ture He-implanted Hi-Nicalon/C/SiC composite and carbon material during the annealing to1500°C were calculated to be about30%and80%of the original number of the implanted-He atoms,respectively. Moreover,a previous work[16]showed no cavity for-mation in the Hi-Nicalon/C/SiC composite due to simultaneous C-and He-ion irradiation up to about10 dpa and1000at.ppm-He at800°C.Therefore,it can be assumed that the pre-implanted-He in the Hi-Nicalon/C/ SiC composite in this study remained in the specimens (especially in the SiC-matrix and Hi-Nicalon?bers)as small clusters and single atoms after the neutron irra-diation at800°C.However,it may be indicated that the e?ect of the pre-implanted He($170at.ppm)on the mechanical properties of Hi-Nicalon/C/SiC composite in this study was relatively smaller than that of displace-ment damage caused by the He pre-implantation and neutron irradiation.

5.Summary

Mechanical property changes of SiC/SiC composite, its matrix and?bers(Hi-Nicalon)were studied for He pre-implantation up to about170at.ppm at400–800°C followed by neutron irradiation up to about7.7·1025n/ m2(E n>0:1MeV)at800°C in HFIR using bend tests and nano-indentation.The following results were ob-tained:

(1)The composite ultimate fracture strength,elastic

modulus and proportional limit stress decreased due to neutron irradiation by about50%,10%and 55%,respectively.Specimens that were He pre-im-planted and then neutron irradiated had similar bend properties to specimens irradiated with neu-tron only.

(2)The pull-out lengths of Hi-Nicalon?bers for non-

He-implanted,but neutron-irradiated composites were longer than that for as-received composites.Al-most no di?erences in pull-out lengths of Hi-Nicalon ?bers were observed between non-He-implanted and neutron-irradiated composites and He-implanted and neutron-irradiated composites.

580S.Nogami et al./Journal of Nuclear Materials329–333(2004)577–581

(3)The hardness increased by about30%due to neu-

tron irradiation for both SiC-matrix and Hi-Nicalon ?ber.The specimens that were He pre-implanted and neutron irradiated had almost the same hardness properties as the specimens that were only exposed to neutrons.

(4)Almost no change in the elastic modulus of the SiC-

matrix,but about a30%increase in elastic modulus of the Hi-Nicalon?bers was observed after neutron irradiation.The specimens that were He pre-im-planted and neutron irradiated had almost the same elastic modulus values as the specimens that were only neutron irradiated.

Acknowledgements

The authors are grateful to the sta?at Tohoku University relating to the implantation test and to the sta?at Oak Ridge National Laboratory relating to the irradiation program and the post-irradiation experi-ments.This work was supported by the JUPITER (Japan–USA Program of Irradiation Testing for Fusion Research)and JUPITER-II programs.

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Effect of He pre-implantation and neutron irradiation

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