Microstructure of boron carbide after fast neutron irradiation

JOURNAL

OF NUCLEAR

MICROSTRUCTURE

MATERIALS

44

OF BORON

(1972)

91-95.

0

CARBIDE

NORTH-HOLLAND

AFTER

FAST

PUBLISHING

NEUTRON

CO., AMSTERDAM

IRRADIATION

*

A. JOSTSONS ** and C. K. H. DUBOSE Metals

and Ceramics Division,

Oak Ridge

Received

National

Laboratory,

8 March

Oak Ridge,

Tennessee

37830

1972

Boron carbide is used extensively as a neutron absorber in various types of nuclear reactors and currently is considered for use in the safety and shim rods of the Fast Test Reactor and eventually for fast breeder reactors. Unfortunately, little is known about the nature of radiation damage which may arise from displacement damage and from He and Li from the transmutation reaction iaB(n, a)7Li. Recently developed 192) ion thinning techniques for preparation of transmission electron microscopy samples have enabled the direct observation of radiation induced defect structures in B&. Ashbee 3) observed small defects parallel to traces of (111) plane in boron carbide irradiated in a thermal reactor at 400 “C. The nature of the defect clusters was not determined. In this letter we describe the results of an electron microscope investigation of the nature of defects

Fig.

1.

regions

Defect clusters and grain boundary in

twinned

boron

N 500 “C to 1.7%

carbide,

lOB burnup.

denuded

irradiated

at

x12000

in boron carbide irradiated in a fast reactor. Specimens were prepared from pellets of Argonne National Laboratory “higher worth”

resembles closely the defect clusters formed by

control rod, capsule G, subassembly L-400@, irradiated in Row 5 of the Experimental Breeder Reactor (EBR-II) at an estimated temperature of 500 “C to a burnup of 1.7% of iOB (5470 Mwd). The microstructure of an as-irradiated pellet, fig. 1, is characterized by a high density of black-spot defects except in denuded regions near grain boundaries. Twin boundaries, however, do not exhibit denuding of defects. To

the agglomeration of point defects created by displacements in neutron irradiated metals at temperatures near 0.3 T, which corresponds to the present irradiation temperature of boron carbide whose melting point is about 2450 “C. Diffraction contrast studies of these blackspot defects show them to be resolvable into loops which often exhibit a line of no contrast, or more correctly, a line of minimum contrast, fig. 2. This line of minimum contrast is

*

Research sponsored by the U.S.

Corporation. ** On attachment Heights,

from

the

Atomic

Australian

a first glance, the radiation

Energy

Atomic

Commission

Energy

N.S.W. 91

damage

structure

under contract with the Union Carbide

Commission,

Research

Establishment,

Lucas

C.

H.

K.

DUBOSE

The nature of the defects responsible strain contrast was deduced through-focus images taken kinematic

or absorption

fig. 3. The change bright

image

in image

fringe at “underfocus”,

Fig.

2.

contrast

surrounded

by

for the

from a series of under undefined conditions,

contrast

from

a dark

a

Fresnel

fig. 3b, t’o dark image

Bright field image of defect clusters showing

strain fields in irradiated boron carbide. The arrows denote the line of no contrast,.

x 100 000

independent of the diffraction vector, indicating that the defects do not possess a spherical strain field 4). This contrast behavior is consistent with prismatic dislocation loops or plate-like precipitates which strain the matrix. Prismatic dislocation loops were considered unlikely, however,

because changing the diffraction from +g to -g with uq:,O did not change the apparent width of the image 5). vector

Fig.

3.

Defects in boron carbide

after a

1 h post-

irradiation anneal at 1150 “C, the same area imaged under

(a) bright

field two-beam

condition

(b and c) bright

field absorption

contrast

under-focused

and (c) over-focused.

w > 0, with

x 100 000

(b)

MICROSTRUCTURE

surrounded focus”,

fig.

OF

BORON

CARBIDE

AFTER

FAST

NEUTRON

Fig.

Crystallographically

IRRADIATION

93

by a bright Fresnel fringe at “over3c,

unambiguously

establishes,

according to the image calculations of Riihle a), that the defects responsible for the strain images are small cavities. The shape of these cavities, deduced from stereo electron microsoopy, appears to be ribbon like with the largest face most frequently parallel to the trace of the (111) plane and less frequently parallel to (100) and (110) traces, indexed on the rhombohedral unit cell. The images shown in fig. 3 refer to foils from specimens given a postirradiation anneal at 1150 “C. Similar effects were observed in as-irradiated foils but the cavity size was smaller and the contrast not suitable for good photographic reproduction. The number of cavities in the irradiated boron carbide is estimated to be about 1016/cma. It is difficult to choose a parameter to describe the size of these nonequiaxed cavities but the largest dimension apparent on electron micrographs was found to be in the range 30 to 200 A. The lattice strains around the oavities were eliminated in specimens annealed at or above 1450 “C (0.63 Tm). Annealing reduced the number density of cavities but increased the cavity size. An example of the crystallographically faceted cavities in annealed specimens is shown in fig. 4. Although the complete crystallography of the cavities has not been established it is clear from fig. 4 that the largest faces of the cavities are parallel to (11 l), {loo}, and (110) planes as was the case in the asirradiated boron carbide. The denuded zone near grain boundaries was observed to increase in width with increasing annealing temperature and only occasionally were cavities observed on grain boundaries. The observations, of small irregular cavities with strong lattice strains and denuded zones at grain boundaries in the irradiated B& together with the coalescence and growth of these cavities on annealing above N 0.6 Tm, are unusual if compared with the behavior of irradiated metals. Except for the large lattice strains, the creation of cavities together with denuding near grain boundaries for an ir-

4.

after a postirradiation bubble

faceted

gas

bubbles

anneal of 1 h at 1900 “C. The

edges a, b, and c are parallel

{ 11 l} , { 1CO}, and (0 11) planes respectively.

to traces

of

x 100 000

radiation temperature near 0.3 T, is suggestive of radiation voids. On the other hand, the coalescence and growth of these cavities on annealing at higher temperatures suggests gas bubbles although the absence of bubbles on grain boundaries is puzzling. Assuming ideal gas behavior, the number of gas atoms required to maintain a bubble population in thermal equilibrium is given by N = SAy/rkT where N = number of gas atoms/cm3 of solid, A =fractional volume occupied by gas bubbles, r = bubble radius, y = surface energy, and kT has its usual meaning. This relationship is expected to hold only for bubbles showing small or negligible lattice strains and hence can be applied to estimate the helium content required to stabilize the cavities in irradiated boron carbide annealed above 1400 “C. Values of A were determined from measurements of cavity size in thin foils where the thickness was measured using stereo

A.

94

techniques.

JOSTSONS

The value of r was derived

AND

from

C.

K.

limited

H.

DUBOSE

observations

8) of helium release from

the measured cavity volume divided by the number of cavities. Typical values of d, r, and

B& during irradiation. helium release is rapid

the cavity number density were l.S%, 440 A, and 4.6 x 1013 cavities/cma, respectively, in

further burnup. On the basis of our microstructural observations, we suggest that the

specimens given a postirradiation

initial

anneal of 1 h

rapid

at 1900 “C. Assuming y= 1000 ergs/cmz, substi-

migration

tution

release

of A and r in the equation

gives

an

estimated value for N of 2.5 x 1019 helium atoms/cm3 of solid compared with 3.5 x lOaO/cma obtained from the iOB burnup. The order of magnitude disagreement between the two values of helium concentration is larger than can be attributed to uncertainties in the value of y, the surface energy, in the equation. This behavior suggests gas release for which no chemical analysis is available at this stage. Gas release is also implied by the increasing denuded zone at grain boundaries with increasing annealing temperature. The results of these observations on irradiated as well as postirradiation annealed B& suggest strongly that the defect structure, for irradiation temperatures near 500 “C, consists of small accompanied by strong helium precipitates lattice strain fields. The nature of the lattice strains, that is vacancy or interstitial, could not be established by dark field techniques using anomalous images near foil surfaces 4) because of too much image overlap. Nevertheless, the behavior of the helium bubbles on postirradiation annealing, which eventually eliminated the lattice strains, suggests strongly that the helium pressure in the planar cavities in irradiated B4C exceeds the surface tension restraint. Such behavior is possible according to Speight 7) under conditions where the gas content is high and the helium atom diffusion coefficient is much higher than the self-diffusion coefficient, or more exactly, the vacancy coefficient. diffusion The grain boundary denuding and failure to observe helium bubbles on grain boundaries is not understood but the observations imply that helium atoms reaching grain boundaries diffuse rapidly to the surface instead of forming bubbles. These conclusions are consistent with the

is due

to

helium

atom

from the bulk specimen and the gas

rate

nucleated.

release

Initially the rate of but then drops with

decreases

once

gas clusters

At the low irradiation

are

temperature

of N 0.3 T,, gas bubbles are expected to be relatively immobile and the only gas release occurs along grain boundaries involving helium not trapped in bubbles, that is helium formed within a volume comparable to the grain boundary denuded regions. The observation of small helium bubbles in B& irradiated at about 500 “C suggests that gas release will be limited due to bubble trapping and is encouraging for the design of nonvented absorber rods for use in fast reactors. Nevertheless, the shape of the helium bubbles in irradiated boron carbide resembles closely small cracks. At higher burnups, helium pressure could build up to levels sufficient to propagate these cracks and lead to disintegration of the pellet. At higher irradiation temperatures, bubble coalescence may be expected and this could limit the useful life of boron carbide because of increased swelling and gas release. The present observations do not provide an indication of the fate of the Li atoms produced from the transmuted iOB although the studies of Secrist and Childs 9) suggest that lithium boride and lithium carbide may form at higher burnups. No microstructural evidence was found for clustering of displaced atoms in the form of dislocation loops. Acknowledgement The authors thank K. Farrell, F. W. Wiffen and J. 0. Stiegler for critically reviewing the manuscript and G. L. Copeland for supplying the irradiated specimens, References 1) G. L. Copeland, C. K. H. DuBose and D. N. Brrtski, Trans. Am. Nucl. Sot. 14 (1971)

171

MICROSTRUCTURE 2)

R.

N.

Katz

and

87

3)

K.

H.

G. Ashbee,

4)

M.

F.

Ashby

5)

A.

0.

King,

Acta

and

L.

Met.

P. B. Hirsch,

M.

A. Howie,

of Thin Crystals M. R.

CARBIDE

Metallography

AFTER

Riihle,

FAST

1971

4

19 (1971)

Brown,

Phil.

Meg.

R. B. Nioholson, Electron

(Butterworths,

8

International

Transmission

of Radiation-Induced

Electron

Defects,

1965)

Microscopy

presented

Voids

IRRADIATION Conference

in Metals,

7)

M. V. Speight,

Met.

8)

W.

Oak Ridge

at the

R. Martin,

private

D. W.

Microscopy

London,

NEUTRON

8)

D.

R.

on

Albany,

June 1971, to be published, USAEC

1079

1649

Paahley and M. J. Whelan, 6)

BORON

Induced

(1971)

(1963)

OF

95 Radiation-

New

CONF.

Sci. J. 2 (1908) National

York, 710601

73 Laboratory,

communication Secrist and W.

Carbide Reaction Laboratory,

J. Childs,

Studies,

USAEC

Knolls

Report

Lithium-Boron Atomic

Power

TID- 17 149 ( 1962)