The activation energies for creep of zircaloy-2

THE ACTIVATION

Hanford Laboratories,

ENERGIES FOR CREEP OF ZIRCALOY-2t

Qeneral Electric Company, Received

The

activation

determined

energies

for creep

by the ~m~r~ture

were obtained

for stresses of 20000,

at temperatures the activation

of zircaloy-2

change

method.

30 January

were Values

25000 and 30000 psi

energy is stress, strain rate and temperature dislocation

intersection

01:

Washington,

1964

Au-dessus

de 285 “C environ,

cd/mole,

being

typical

of dislocation

creep. In addition an activation

climb

-Die Aktivierungsenergien

58500

lay-2-Legierung

controlled

methode bestimmt.

energy peak was observed.

pour fluage du zircaloy 2 furent

furent obtenues

psi bei Tempemturen

Unterhalb

aux tEmp6ratures

verhalten bestimmen.

von

van 50 bis

285 “C ist die Aktivierungs-

und temperaturabhingig.

auf Versetzungszwischenprozesse

pour des efforts de 20 000 et

einer Zirka-

Es wurden Werte fiir Sp~~ungen

energie spannungs-

dPtermin6es par la m&hode de ohangement de temp&ture. 30 000 psi (soit 24, 17,Ei et 21 kg/mm2)

fiir das Kriechen

wurden mit Hilfe der Tempzraturwechsel-

500 “C: ermittelt.

Lea valeurs

fluage oontr616 par la

vation a ktk obsfrv&e.

20000, 25000 und 300~ Lt?s Energies d’activation

d’un

px~r

soit 58 500

En outre un pie d’6nergie d’acti-

process is controlling creep. Above about 285 “C the activacal/mole,

l’&n!n-rgied’activation

cs qui est typiqx

montbe des dislocations.

the Peierls

tion energy for creep is equal to that of self-diffusion,

USA

le flnoge est &gals & oellz de I’~utodi~usion,

from 50 “C to 500 “C. Below about 285 “C

dependent indicating

Richland,

Dies w&t

hin, die das Krieohver-

Oberhalb 285 ‘C ist die Aktivierungs-

energie fiir das Krieohen genau so grol3 wie diejenige die Yelbstdiffusion,

l’tinergie d’activation

in diesem Temperaturbereich

ist typisc’n fiir Versetzungs-

w~nderungen.

Beob~chtu~on

deformation

dbpend

processus d’interseotion

1.

de l’effort,

et de la tem~rature,

de la vitesse de

ce qui indique que les

de dislocations

contrtftlent le fluage.

Introduction

niimlich 58500 kal/Mol.

fiir

comprises entre 50 et 500 ‘C. En dessous de 285 “C environ,

Au&x

diesen

of zircaloy-2

creep and thus establish

have been stu-

died ext,ensively in order to provide data for nuclear

time-t,emperature

reactor design. lm3) Creep tests at service conditions

paper describes

for times as long as the life of a reactor are impracti-

for creep of zircaloy-2.

cal. Appropriate rived

design data can t,herefore be ar-

at only by extrapolation

temperature conditions.

This is usually accomplished

of one of the t~ime-temperature parameters

Activation

data to long time service temperature

are generally

ment of the activation

parameters

This

energies

energies were determined

for both an-

was established by heating cold rolled

stock to 750 “C, holding for two and one-half hours and furnace cooling. The cold worked material was

control creep in

produced

change. Direct measure-

20%

energies for creep can be used

and

t This work was performed under contract No. AT(4501)1350 between the U. S. Atomic Energy Commission end the Richland,

can be applied.

a study of the activation

by cold rolling the annealed material to

reduction

in area. The zircaloy-2

study was manufactured

General Electric Company,

controlling

the range over which the

nealed and 20 o/o cold worked material. The annealed condition

These

valid in the temperature

range where diffusion ~necha~ms the absence of metallurgical

by the use

parameters.

ein

Material

2.

of short time high

wurde

Maximum der Aktivierung~en~r~i~beobachtet.

to reveal the nature of the mechanism

The creep properties

Das Krieohen

contained

the

used in this

from reactor grade sponge following

alloy

additions:

1.39 wt y. Sn, 0.14 wt y. Fe, 0.10 wt y.

Washington.

0.05 wt y. Ni. 137

Cr, and

3.

Experimental

3.3. PROCEDURE

5. I. METH~I) Activation perature Lytton

Two experimental energies were determined

cha,nge

method

described

techniques

by the tem-

reproducible

by

stressed at, temperature

Sherby,

and Thorn.*) In t,his method t,he creep rates

results. Above

were used to obtain

300 “C specimens

Ft was allowed to decrease with time t,o a ~on~~e~~ierlt

just before and just, aft,er an abrupt change in t,em-

value after which the temperature

perat,urr are substituted

decreased

int,o t,he equation,

was increased or

15 t,o 25” and & measured.

After measurement dH = (Rln &/&)

of Ezthe temperature

/(;,-

a,

and t#heactivat8ion energy calculated.

and the procedure repeated.

F1 is the creep

rate just, before an abrupt ten~pera~,ure change from

Below 300 “C t,he act,iIn this region

values were det,ermined only when t,hr creep rat,e decreased to preselected values between 1 x 1O-*/min

T, t,o T, and d, is the creep r&e just after the

and 4 x 10W4/min. Only one activabion

change. The above equat,ion is valid only when the

det’ermined on each specimen.

modifications

occurring

t’emperature change are insignificant. changes

can be nearly

or s:tress

was changed to again obtain a convenient’ creep rat#e vation energy is strain rate dependent.

substruct#ural

were

T,. The resulting creep rat,e

during

energy was

the

Substructural

eliminat,ed if t,he time re-

Results

4.

The

effect

of

temperature

ou

the

act,ivation

quired to change temperature is short,. In t#hisstudy.

energy is shown in figs. 1, 2 and 3 at stress levels of

F; and & were determined

20000.25000.

by graphical

(~ifferent,ia-

and 30000 psi. respectiveiy.

The plots

tion of t,he creep curves. 100 /

lnstron

high vacuum

creep machines

for testing at, temperatues

I

were used

in excess of 325 “C. Be-

low 325 “C temperat~~re changes in t,he Instron machines could not be completed

with sufficient’ speed

to eliminate significant substructural Activation

modifications.

energy studies below 325 “C were there-

fore conducted

in a specially

much less thermal

designed unit having

inertia than the Instron

unit.

0

50

This creep machine utilized a liquid bath and several immersion heaters for t,emperature control. Silicone oil wa,s used as the bath below 250 “C and a 50-50

I

100

I

150

I

,

1

I

200 250

3cQ

I

I

350

4w

1 450

500

Temperature.%

Fig. 1. ~~ctivation energies for zircaloy-2 creep at 2~0~ psi.

mixt,ure of sodium nitrate and potassium nitrite was used above

250 “C. No reaction

of the zircaloy-2

specimen with the silicone oil was noted;

however,

a slight, surface reaction with the salt, mixture

_._~

1

Region

Number

I

Region

Number

2

was

observed. The stress applied

by both

to 500 psi and temperatures

unit,s was accurate were controlled

to

-i_ : “C. Strain was measured by pull rod separation with a dial indicator inches.

Temperature

chines were completed Temperature

changes

in the InsLron ma-

in 45 minutes to an hour.

changes in the liquid bath apparatus

were completed

0 0

capable of resolving 40 micro-

in two to three minutes.

/

0 50

loo

I

150

1

I

200

250

Cold

Worked

Annealed

I,

Temperature,

I 300

350

4.8

450

500

%

Fig. 2. Activation energies for zircaloy-2 creep at 25000 psi.

CREEP OF ZIRCALQP-2

130

transition between Region I and II. The temper-

100 Region Number

Region Number

1

2

ature at which the 58500 callmole value is first observed is stress dependent and is about 250 “C at 20000 psi and 280 “C at 25000 psi. The transition temperature at 30000 psi is obscured by an activa,tion energy peak which is discussed

l

Cold Worked

0

iillwaled

I 50

100

150

200

250

3w

350

4w

later. c) A decrease in activation energy with i~~reasi~~g

I

I

450

500

stress at constant temperature and strain rate can be seen in fig. 4. The effect of stress on the

Temperature.?

activation energy increases from 50 “C to about

Fig. 3. Activation energies for zircaloy-2 creep at 30000 psi.

200 “C. Above 200 “C the stress dependence decreases and disappears altogether at the onset of Region II. 4.2. REGION II The activation energy of Region II is independent of stress, strain rate, and temperature. The average value in this region is 58500 callmole. No effect from cold work is observed. 4.3, ACTIVATION ENERGY PEAK

Fig. 4. Effect of stress on actiwtion energies at g = = 1 x lo-” min. These values are taken from fig. 1,2 and 3.

of temperature 21s.activation energy are separated into two regions which show two distinct activation energy behaviors. 4.1.

REGION I

The activation energies of Region I depend on strain rate, stress and temperature. Relationships between these variables and the activation energy of Region I are as follows:

4 The activation energy decreases with increasing strain rate at constant stress and temperature. As the temperature increases the effect of stra,in rate becomes less being insignificant near the onset of Region II.

b) Activation energies increase with temperature at constant strain rate and stress from values near 20000 Cal/mole at 50 “C to 58500 cal/mole at the 3

A third behavior appearing as an abrupt rise followed by an abrupt fall in the H versus T plot is seen at 20000 psi stress and 350 “C. Two data points near 240 “C at 30000 psi indicate that a similar peak occurs here. However, these two points might be only data scatter. No peak was observed at 25000 psi. It is possible that such a peak does occur at 25000 psi, but at temperatures not included in this experiment. 5.

Discussion

5.1.

CBEEl THEOREM

Modern dislocation theory has not provided a complete description of creep because of the complexity of controlling processes. However, the nature of several of the rate controlling processes has been revealed experimentally. Two mecha~sms have been shown to be operative at low temperatures. In FCCs+ b, c metals, low temperature creep is control led by the rate of intersection of glide dislocations with forest dislocations which act as barriers to dislocation motion. In HGP~) and BCW) metals, the controlling process is the overcoming the Peierls stress hills.

Both of these mechanisms

follow the form of bhe

5.3.

Seeger equation :

i

.Z creep rate

d

-= a constant

1H (0) = activation

of stress and strain on , I B in Region I

suggests that at least, one of the low t,empcraturc

- ,I II (0)

mechanisms is operative.

RT

0

1

The elect

c: = A cxp where

REGION

Seeger’s equation predicm that A H (rr) should extrafor con&ant

“state”

energy

polate to zero at zero temperature. data presented evidence

R

:= gas constant = absolut,e temperature

Near the onset, of Region

?’ The above decrease

equateion predicts

with decreasing

with decreasing

that

that more than one process

mechanisms d fiT (G) will

st’rain rate and increase

stress. roughly

the absolute melting temperature,

above one half creep is control-

led either by tJhe climb of dislocation7) les of various types or the motion

by arrival

or departure

of vacancies

at jogs in

dislocations.

Here that activation

temperature,

rate dependence

or interstitials

energy is equal

and is independent

and varies

very

diffusion

controlled

in creep.

This

by t~he decrease in strain

observed

near Region

further assumes that an increasing processes,

to zero is

is involved.

II. If one

stress enhances

processes with respect to diffusion

the stress dependence

of Region

Region II transition temperatures

I to

can be explained.

over obstac-

of jogged screw

dislo~atioIls.*) The creep rat~eis controlled

t,o that, for self-diffusion

II,

must be participating is supported

low temperature

At higher temperatures,

The fact that, t,he

here do not extrapolate

= stress

assumption

and

It is not possible to ascer-

tain from these data which of these is involved.

of strain

slightly

w&h

st,rrss.

5.4.

‘IHE

ACTIVATION

An act,ivation from Region nickolll)

ENERGY

energy

PEAK

peak near the t~ral~sition

I to Region

II has beon observed

and in aluminium-magnesium

in

alloys.12)

In both cases the peak has been attributed

to some

form of solute atom-dislocat,ion

Usually

tJhe peak is associat’ed

interaction.

with an incre:lse

st#rength. No large change

in creep

in creep st,rength was

uot.etf in this study, however, the method used would

‘l’hc~ a&vat& that observed average

energy in Region for diffusion

I I is typical of

cont3rolled creep. The

value here is 58500 cal/mole

pares favorable diffusion

which com-

with that given by Lyashenkos)

of zirconium

for

in a zirconium tin alloy hav-

obscure subtile increases that, might havth occ:ur~tl. Conclusion

At least t.hree creep mechanisms

are operative

t)he range of stresses and temperatures

in

studied. Be-

ing nearly 1.3 wt’ o/o t,in. In general, creep is diffu-

tween 50 “C and about 280 “C, the upper tempera-

sion cont,rolled only at, t,~mperatures above one half

ture limit depending on stress, creep is controlled

t,hc ahsotut~r melt]ing t,emperature. The melting point

dislocation intersection

of zirconium

285 “C creep is controlled

is 1845 ‘Y:. It is therefore

int,erest)ing

t30note that, at 20000 psi diffusion comrols temperatures McMullenl) haves

as low as 250 “C which is about. Tw/%. has suggested that alpha zirconium

as t7hough it,s melting

alpha-beta

transit,ion

temperature

tenlperature,

This gives an effective

The fact that zircaloy-2 in Region

865 “C.

behavior.

creep data can be extra-

II has been demonstrated

the Dorn parameter 60000 cal/mole.lO)

about

be-

is the

Tm/2 of 296 “C which is in

better agreement with the observed polated

creep st

using an activation

with

energy of

by dislocat,ion climb and is

charact,erized by an activation mole. Superimposed

by

or the Peierls process. Above energy of 58500 calj

on t,he in~rseetioll

mechanisms is a process characterized

and climb

by an activa-

tion energy peak. On the basis of previous work the activation

energy

atom-dislocation

peak is attribut3etl to a solute interaction.

References 1) W. D. ~~~~~len, B&is 2)

(USA)

TX-132, 1958 E’. R. Shober et al., Rattelle 1160, 1957

Report

No. VVAPD-

(USA) Report

No. WM-

141

CREEP OF ZIRCALOY-2 $)

P. J. Pankaski, 59 383 Rev.,

4,

Hanford

(USA)

Report

No.

fornia, Lawrence Radiation

HW-

1959

0. D. Sherby, J. L. Lytton

and J. E. Dorn, Acta Met.

‘)

G. Schoeck,

8,

1957), p. 199 P. B. Hirsch and

5 (1957) 219 5,

P. W. Osborne, sity

of

S. K. Mitra and J. E. Dorn,

California,

Materials

(USA), Tenth Technical S. K. Mitra,

Research

Univer-

(USA), UCRL

and

J. E. Dorn,

Creep and Recovery

(Cleveland,

D. H. Warrington,

Phil.

BSM, Mag.

!‘) Trans.

V. S. Lyashenko,

V. N. Bykov

Fiz. Metal. Metalloved,

and

1,. V. Pavlinov,

No. 3 (1959) 362-369

AIME 221 (1961) 1206

I”)

J. J. Holmes, Hanford (USA) Report HW-67 641(1961)

J. B. Mitchell, S. K. Mitra and J. E. Dorn, University

II)

P. R. Landon,

UCRL

Laboratory,

10 469 (August 1962)

J. E. Dorn and Stanley

Rajnak,

of Cali-

J. J. Lytton,

L. A. Shepard

and J. E.

Dorn, Trans. ASM 51 (1959) 900 12)

University

6

(1961) 735

Laboratory

Report (May 4, 1962)

P. W. Osborne

of California (USA), Lawrence Radiation B,

Laboratory

10 833 (July 3, 1963)

M. R. Borch,

L. A. Shepard

SSM 52 (1960) 494

and J. E. Dorn,

Trans.