Diffusion and creep in beryllium oxide

JOURNALOFNUCLEARMATERIALS12,No.3

DIFFUSION

(1964)337-339,NORTH-HOLLANDPUBLISHINGCO.,AMSTERDAM

AND

CREEP

IN

BERYLLIUM

OXIDEt

S. B. AUSTERMAN

Atomics International, Canoga Park, California, USA and R. CHANG

North American Aviation

Science Center, Canoga Park, California, USA Received 5 February 1964

The interrelationship

the creep diffusion

between creep and diffusion

in Be0 was discussed by one of the authors several years agol).

A recapitulation

time appears appropriate

of the subject

deformation Herringa),

A model of diffusion-controlled

based

on a stress-directed

flow of lattice vacancies. tal

evidence19 4*5, show

creep of polycrystalline

Accumulated that the Be0

activation

diffusional

tempe-

in polycrystalline larger than

from single crystals and have lower

energies, suggesting that 0 diffuses along in polycrystalline

BeO. The

data are not included in fig. 1 for reasons of clarity. The “steady-state”

experimenupon

coefficients

orders of magnitude

the grain boundaries

and

creep of polyorystalline

could be controlled

“steady-state”

depends

are several

those obtained

creep

by Nabarro2)

d) The 0 diffusion Be0

since more accurate mea-

has been proposed

at a given

rature.

at this

surements of both the diffusion of Be and 0 in Be0 are available.

coefficients

by the lattice diffusion of Be or

the grain boundary

grain

diffusion

of 0. Since the creep

size and stress in a manner predicted by the Nabarro-

activation

Herring

agree quite well with the Be lattice diffusion

mechanism.

and 0 is involved

Since the motion

in mass transport,

of both Be

energy

and creep diffusion

ving species should control the creep rate. Compari-

data, it is concluded

that “steady-state

sons of the chemical diffusion data with creep diffu-

polycrystalline

is controlled

sion data are shown in fig. 1. The following

diffusion

points

are noted:

Be0

of the Be ions.

data

diffusion ” creep of

by

the

It is assumed

lattice that

0

diffuses much more rapidly along the grain bounda-

a) The activation “steady-state”

energies for Be diffusion and for

ries and hence is not the rate controlling

creep all lie within the range of 4.0

similar mechanism

to 5.0 electron volts. calculated

has been proposed

species. A

by Paladin0

and CobleT) for the creep of polycrystalline

b) The Be diffusion coefficients coefficients

coefficients

while not at all with the 0 grain boundary

the slower mo-

Be0

and the diffusion

from creep data agree within

comes very large.

an order of magnitude. c) The 0 diffusion coefficients

The Be ion diffuses is practically (single crystal data)

are much lower than the Be diffusion (single crystal as well as polycrystalline

Al,O,.

The model does not apply when the grain size be-

temperature

coefficients

range

isotropic

in the

1500 to 2000 “C, as shown

in

fig. 2. This suggests that the diffusion of Be in Be0

data) and

is by the vacancy diffusion

t This work was supported in part by the U. S. Atomic Energy Commission.

would

mechanism,

give

than is experimentally 337

as interstitial

rise to a geater observed.

Be

anisotropy

This is consistent

S. B. AUSTERMAN

338

Fig. 1.

Comparison

of chemical

diffusion

AND

R. CHANG

coefficients and creep diffusion Be7 chemical diffusion.

coefficients

in BeO. Creep diffusion

and

DIFFUSION AND CREEP IN BERYLLIUM OXIDE DIFFUSION

.

0

PARALLEL

n

PERPENDICULAR

To

339 COEFFICIENT

Q IMPURITY

c-Ax6

TO C-AXIS

Be0

IMPURITY-EXCESS

VALENCE

PRODUCT

Fig. 3. Effect of impurities on diffusion penetration of Be7 in BeO. (Abscissa, in arbitrary units, is plotted as ZC~ (nip2), where Ciis the concentration of cation impurity i

. 1

4.4

I,

Id

1

4.6

l/TX

Fig. 2.

I

5.0

4.0

I1

I

5.2

54

I

T'8,

represent of valency “i. Symbols in figure, circles and data points from Be0 of three different origins.)

I

5.6

104(+)

Diffusion penetration of Be’ in monocrystalline and polycrystalline BeO.

are partly

bound

extremely

interesting

to the impurities.

It should

to undertake

be

similar studies

of high purity Be0 in order to confirm the present findings.

with the observed

enhancement

impure BeO, illustrated Interpretation

of Be diffusion

in

in fig. 3.

of the Be diffusion in Be0 in terms

of a vacancy mechanism is, however, not completely satisfactory. represent

It is believed

diffusion

the variations vation

region

in the diffusion coefficients

data where

and acti-

energies are due to impurity-vacancy

ciation and to impurity phenomena

NaCi

that the present

in the extrinsic

described

containing

interpretation

precipitation, by Dreyfus

divalent

‘) 2,

and Nowick

impuritiess).

This

Y 4,

cation impurity

and Be

is quite large such that in the tempera-

ture range 1000 to 1700 “C the Be ion vacancies

1 (1959) 174-81

Conference

on Strength

of Solids,

1948) p. 75

C. Herring, J. Appl. Phys. 21 (1950) 437 R. R. Vandervoort

and W. L. Barmore, J. Am. Ceram.

Sot. 46 (1963) 1804

7

for new

R. Chang, J. Nucl. Mat. F. R. N. Nabarro,

The Physical Society (London,

similar to the

suggests that the energy of associa-

tion between a polyvalent ion vacancy

asso-

References

“)

Second Annual Report -

High Temperature

and Reactor

Development

Component

Materials

Program.

I. - Materials. GEMP-177A,

General Electric

sion Laboratory

(Feb. 28, 1963)

J. B. Holt, Radiation

Department

UCRL-6940

(Nov. 28,

1962),

Vol.

Propul-

Lawrence

Laboratory

‘)

A. E. Paladin0 and R. L. Coble, J. Am. Ceram. Sot. 46

?

R. W. Dreyfus

(1963) 133-6 and A. S. Nowick,

Suppl. (1962) 473-477

J. Appl.

Phys. 33