Anti-phase domain size in quenched beta-brass

Scripta METALLURGICA

Vol. 2 , PP. 491-494, 1968 Printed in the United States

Pergamon Press, Inc.

SUMMARY* ANTI-PHASE DOMAIN SIZE IN QUENCHED BETA-BRASS E. Michael Moore, Jr. Department of Mechanical Engineering Division of Materials Science Vanderbilt University Nashville, Tennessee 37203

(Received June 3, 1968) Abstract: The results of this investigation indicate that the growth rate and final size of anti-phase domains (APD) in quenched beta-brass are determined by two factors: (a) the rate of formation and (b) the mobility of dislocation loops nucleated at and trapped in domain boundaries during the quench. It is believed that most of the domain growth occurs in the first 200°K below the critical temperature for ordering.

Experimental Procedure Disks of annealed 51.5 Cu - 48.5 gn beta-brass measuring 0.125 inches in diameter and 0.009 inches in thickness were quenched from both above and below the critical temperature for ordering (Tc) at rates varying from I00 - 100,000°F/set.

Chromel-alumel thermocouple wires

were spot welded to either side of the specimens, and the cooling curve during the quench was shown as a trace on an oscilloscope screen. from above

T

tive areas.

c

The resultant APD size for specimens quenched

= 738°K was determined from electron micrographs taken over several representa-

An average intercept distance between domain boundaries was used as the measure

of the APD size. Results Electron micrographs of annealed specimens showed them to be structureless, contain one or two dislocations per grain, and have an average grain size of about 250 microns.

All

specimens quenched from just below T c were likewise structureless and devoid of dislocations, whereas every specimen quenched from above T

c

contained plentiful dislocation networks clearly

outlining APD. The effect of quenching rate on APD size in beta-brass can be separated into three regions:

(a) for rates greater than 105 °F/set, the APD size must decrease rapidly with

increasing quenching rates because for this rate the domain size is already 0.9 microns

491

ANTI-PHASE DOMAIN SrZE IN QUENCHED BETA-BRASS

492 (b)

for

effect

rates

b e t w e e n 10 4 - 10 5 ° F / s e c ,

on t h e APD s i z e ,

each grain

contains

a variation

a s shown i n F i g .

a single

of quenching

1, a n d ( c )

for rates

rate

Vol, 2, No. 9

has practlcally

on t h e o r d e r

no

o f 10 2 ° F / s e c ,

APD.

-16 x[v:

,5

dilt~cl r ~ .

-t 4

m,vl,

-q3 ,.,,eaf sc..e, i~l ,.tl m . t l ol

"J2

. ivl. wIech,. 9 ,or.

i i ,i.O

09

r~

' ~,o~o'

' ' ~

zo.ooo

O U ( N C . I N S ~ATE tN t H E W C I ~ I I Y

~,ooo

OF "~

,oo,ooo

zoc~o

FIG. 1 Variation of the APD size of beta-brass with the quenching rate in the vicinity of T • c

(UNITS, "F/~Cl

Discussion When b e t a - b r a s s observed

to nucleate

two r e a s o n s :

(a)

is quenched from above Tc, a t t h e APB.

large

numbers of dlslocation

C u p s c h a l k a n d Brown ( 1 ) h a v e s h o w n t h a t

the vacancy concentration

loops this

are

occurs

a t t h e APB c a n b e m o r e t h a n an o r d e r

for

of

magnitude greater than in the interior of the domain and (b) a dlslocation loop at an APB erases part of the boundary by the removal of

a

layer of atoms.

Consequently,

as beta-brass

is quenched, excess vacancies anneal out preferentially at the APB and condense to form dislocation loops. A slmple plane strain calculation shows that the maximum posslble strain energy induced by quenching stresses in the specimens used is three orders of magnitude less than the binding energy between a dislocation loop lying in an APB and the APB itself. as noted previously, electron micrographs of specimens quenched from below T appreciable numbers of dislocations induced by quenching stresses alone.

c

Furthermore , showed no

Thus, the

appearance of dislocation networks outlining essentially dlslocatlon-free regions in every specimen quenched from above T c is the result of the nucleation of dislocation loops in the APB.

Since (a) there is insufficient quenching strain energy to break the dislocation loops

away from the APB and (b) there are large numbers of loops at the boundaries, the rate of

Vol. 2, No. 9

ANTI-PHASE D O M A I N SIZE IN Q U E N C H E D

BETA-BRASS

493

domain growth and the final domain size are limited by the rate of formation and the mobility of these loops. During a quench excess vacancies condense into dislocation loops at the APB, and unless these loops can move, the APB in that locality is pinned.

There are two possible means by

which the loops can move in such a manner that the APB will not be pinned: (b) prismatic glide.

(a) climb and/or

It is doubtful that many dislocation loops move appreciable distances

by climb in the few milliseconds during which the temperature is high enough so that climb is possible.

Prismatic glide may be a significant factor affecting the growth rate and

final domain size, however, since the observations of Silcox and Hirsch (2) indicate that a large number of the dislocation loops may be prismatic. For temperatures between T

c

and about 500°K, a decrease in temperature should result in

an increased mobility of prismatic loops because these loops can glide without the creation of a cylindrical APB trail only when there is perfect long range order.

On the other hand,

as the temperature drops there is a concomitant decrease in diffusivity, and at some temperature roughly about 500°K this factor will become dominant.

For temperatures below

this, it will be increasingly difficult for a growing APB to drag a prismatic loop along with ~t. Once a dislocation loop is nucleated at an APB, the boundary will be locally pinned unless (a) the loop is prismatic and (b) the direction of boundary motion is in the prismatic glide direction.

The rate of domain growth will be determined by (a) the rate at which

dislocation loops are nucleated by the condensation of vacancies on APB and (b) the mobility of the prismatic loops. the system falls

The final APD size will probably be reached when the temperature of

below about 500°K.

The excess vacancies that anneal out between 500°K and

the bath temperature cause the loops to spiral into the observed tangled helices. The proposed theory explains the fact that for quenching rates on the order of 100°F/set each grain contains a single APD.

During rapid quenches, the excess vacancies condense into

dislocation loops on the APB because this is the quickest way they can be eliminated.

The

free energy of the system can be further lowered, however, if the vacancies have time to anneal completely out, and for slower quenches the brass remains at high enough temperatures for sufficiently long times so that this is possible.

Hence, dislocation loops are not

formed at the APB, and the domain growth is not impeded. is limited only by the grain size of the specimen.

The APD size under these conditions

494

ANTI-PHASE DOMAIN SIZE IN QUENCHED BETA-BRASS

Vol. 2, No. 9

References I.

Cupschalk, S. G. and Brown, N., "Observations of Defects in Beta Brass," Acta Met. 15, 847 (1967).

2.

Silcox, P. B. and Hirsch, P., "Dislocation Loops in Neutron-irradiated Copper," Phil. Mag. ~, 1356 (1959).

*This is a sununary of a M.S. thesis with the same title (56 pages) submitted to the Faculty of the Graduate School of Vanderbilt University.

The research was supported

jointly by the Atomic Energy Comission and the National Science Foundation under grant numbers AT-(40-I)-3091 and GZ-604.

Copies may be obtained from the author st the cost of

reproduction also from the publisher.