On the mechanism of hot deformation

ACTA

1136

screw a/2 (111) dislocation the other metals

METALLURGICA,

is likely to occur in any of

considered.

This conclusion

could

VOL.

14,

1966

tions into sub-grain he has derived

boundaries.

the form

Using this concept

of the stress-strain

be changed by the presence of an applied stress which

although quantitative

favours dissociation

experiment is not completely satisfactory. He has discounted earlier observations

or by the presence

of impurities

which would lower the stacking fault energy.

However,

agreement

curve

between theory and of ours(‘)

it should be noted that if a larger value is chosen for

which suggest that, at least in copper and nickel, the

the core radius of the undissociated

dynamic

probability

of dissociation

Teutonico(2)

has also considered

forming extended of dissociated

dislocation,

the

the possibility

of

barriers resulting from the reaction

dislocations

found that for tungsten

on

(112)

planes.

It is

in all cases the extension From

times the lattice parameter.

distance core interactions important,

obtained

controlling

become

additional

restoration

recovery. evidence

process

Recently that

in these

the

we rate-

metals

is

We have proposed a correlation for hot deformation datac3) which has the form: .G= A (sinh MCS)~’ exp (-Q/RT)

less

Since at this

would undoubtedly

process appears to be recrystalli-

different from that in aluminium.

Table 2 it can be

seen that this result implies a barrier extension than 34

have

of

the barriers considered is less than that of a dissociated a/2(1 11) edge dislocation.

restoration

zation rather than dynamic

is decreased still further.

(1)

where i is strain rate (in torsion, 0, the rate of rotation can be used) ;

it is likely that linear, first-order elasticity

cr is stress (in torsion, I’, the torque, can be used);

is insufficient to analyse fully the nature and extension

A, c(, n’ are constants independent of temperature ;

of these barriers.

Q is an activation

This work

was supported

Science Foundation

through

by

the U.S.

National

a grant for postdoctoral

study.

At

low

stresses,

equation

(1) reduces

to a power

relationship :

2 = A’@

C. S. HARTLEY

Dept. of Physical Metallurgy University of Birmingham Edgbaston, Birmingham

(1955).

6. J. FRIEDEL Dislocations, p. 155. Pergamon 7. C. N. REID, Acta Met. 14, 13 (1966). 8. R. BTJLLOUGH, Dislocations, p. 12. AERE \--I-~

cc and n’ can be simply determined 73

Press (1964). PGEC

L 33

1964). ~--

I

Phil. Mug. 8, 1467 (1963). C. LESLIE, Phil. Mug.

PRIESTNER and W.

February

(3)

from experimental

data at high and low stresses. Hot torsion data for a number of metals have been satisfactorily

correlated by equation (1) and further in

the case of nickel and aluminium are available, of

strain

where suitable data

it has proved possible to correlate creep

and hot torsion 11, 895

(1965). 11. C. S. HARTLEY, to be published (1966). 12. F. C. FRANK, Phil. Mag. 42, 809 (1951). * Received

relationship:

The constants CCand n’ are related by p = un’ so that

5. F. R. N. NABARRO, Adw. Phys. 1, 271 (1952).

(sent.

(2)

.C= A” exp (Do) exp (-Q/RT)

1. L. J. TEUTONICO, Acta Met. 13, 605 (1965). 2. L. J. TEUTONICO, Phys. Status Solidi 10, 535 (1965). 3. A. J. E. FOREMAN Acta Met. 3, 322 (1955). 4. A. J. E. FOREMAN and W. M. LOMER, Phil. Mug. 46,

9. A. W. SLEESWYK,

exp (-Q/RT)

and at high stresses to an exponential

Ei

References

10. R.

energy;

T is the absolute temperature.

rates.

data over an extremely Using

energies for the rate-controlling have been determined values

obtained

for hot torsion

for aluminium,

activation

restoration

creep in the case of aluminium

10, 1966.

wide range

this correlation,

process

and also for

and

nickel.

copper,

The

nickel

and

18/S stainless steel are given in Table 1 together with On the mechanism

of hot

When metals are deformed temperatures

and intermediate

deformation*

the results of other workers and 18/S stainless stee1.(5,6)

to large strains at high to high strain rates,

the flow stress at first reaches a maximum

up

to very high strains. This steady value indicates that there is a balance between work hardening and a restoration

process.

In

a recent

paper,

Stiiwe’l) has proposed that the dynamic restoration process in aluminium, copper, nickel and 18/S stainless steel is dynamic

recovery

During hot torsion of aluminium,(2) of well developed

sub-grains

suggests

the formation that dynamic

value and

then drops to a steady value which is maintained

dynamic

for creep of topper(4)

by climbing of edge disloca-

TABLE 1. Activation

energies for hot deformation’3l Activation

Material Aluminium Copper Nickel 18/S stainless steel

energy (kcal/mole)

Hot torsion

Creep

30-43 7 ‘> 7; 99

37 48’4’ 58 75’5.6’

LETTERS

TO

THE

1137

EDITOR

energies reported been determined

for creep of these two metals have from steady-state

recovery.

activation

energies for creep and hot torsion for these

three

materials

The

rates and refer to

dynamic

are

differences

consistent

between

with

the

the

reported

differences in structural changes and support the view that recrystallization

is the rate-controlling

during their hot deformation. energies for hot

torsion

process

Further, the activation

of

copper

and

nickel

are

similar to those for grain growth in similar purities of these materials.(8,g) Also in tests at very low strain rates copper and nickel show a marked that

of

difference

aluminium.

Typical

in behaviour

results

by Rossard(l”) for copper and aluminium

-I

Figs. l(a) and (b). copper

exhibit

tions are reproducible

curves for

regular oscillations

which die out with increasing

by

are shown in

The torque-revolutions

marked

from

obtained

strain.

in torque

These oscilla-

and depend in both amplitude

and period on purity, e.g. for two batches of O.F.H.C. copper at 7; N 6 x 10-3/sec the periods are y = 0.15 and 0.2 which are similar to the strains to peak torque, y = 0.17 and 0.15 respectively. oscillations

Nickel shows similar

of period y = 0.3 when tested at 1100°C

at a similar revolutions

strain curves

undulations

rate.

In contrast

for aluminium

with a much

the torque-

show only

longer

period.

slight

We have

examined specimens of copper and aluminium quenched after various in grain from

strains and find significant

size between

the maxima

significant

SHEAR

STRAIN

(b)

is the rate-controlling

The similarity

cases is consistent

in the case of aluminium.

considered

as in creep.

energies for the two

as analogous

Recent

no

Such be-

if the oscillations

by are

to the changes in strain rate

studies of recrystallization

nickel’li) indicate that the conditions of recrystallization

during

during creep.

during creep of for the initiation

concurrent

deformation

may be met at small strains even in a commercial

of Rossard and Blairi

of 18/S stainless steel quenched

but

recovery model proposed

associated with repeated recrystallization

with this proposal.

In contrast the observations on specimens

process,

of the activation

quenched

in torque,

in the case of copper and nickel is not pre-

Stiiwe but is readily understood

FIG. 1. Torque-revolutions curves at slow strain rates at T/T,,, = 0.78 for: (a) copper, (b) aluminium. recovery

and minima

dicted from the dynamic

(a)

differences

specimens

difference in sub-grain size associated with

the undulations haviour

copper

rapidly

purity

material.

Thus

in torsion

tests

after hot torsion show that the restoration

process is

recrystallization.

on copper

below the strain at maximum torque. Since the torque depends mainly on the stress in the surface layers of

Similarly our observations

and nickel after hot torsion(2) show that recrystallization proceeds progressively with strain after the maximum torque. However, observations on creep of stainless steels of the 18/l@) and Type 316t6) varieties

to be initiated

.we would

expect recrystallization

the specimen, recrystallization need only propagate through a small volume below the surface to markedly influence

the

torque.

There

suggest that dynamic recovery is the rate-controlling In the case of copper and nickel, while process.

indicate

recrystallization,

stress. Further, the activation

strain

associated

rate can occur

during

with

rapid

creep,

changes

in

the activation

at st’rains well

that, once initiated,

is some

evidence

recrystallization

to

during

creep proceeds at a rate that increases with increased energy for recrystalliza-

tion during creep is similar to that for grain growth.

ACTA

1138

Since the rate of recrystallization stress

and temperature,

torque at

to occur

different

is a function

it is possible

at approximately

strain

recrystallization

rates

is

METALLURGICA,

the

and

of both

for the peak

the

same

Thus, we conclude,

in contrast

certainly

in hot deformation

ling process in aluminium, other metals, Department

as

the

restoration

an important

role

there is st’rong evidence for rate-controlling

process

in

The UGversity

W. J. MCTEGART

England

5. 6. 7. 8. 9. 10. Il.

Inst.Net& 90. 17 11961-621. C. M.‘ SELLAR$ and W. J. McG. TEGART, Journ& ~~~eiallurqiques d’A4utomne, Paris, Oct. (1965); Rev. Net., in p&s. C. K. BARRETT and 0. D. SHERBY, Trans. Am. Inst. M&L. metall. Engrs 230, 1322 (1964). F. G.~R~FAL~, W. F. DOMIS and F. VON GEMMIYGEN, Tmns. Am. Inst. Min. metall. Engrs 230, 1460 (1964). F. GAROFALO, 0. RICHMOND, W. F. DOMIS and F. VOX GE:RZDUKGEN, Proc. Joint tnt. Conf. Creep. 1, 31. Inst. of Moth. Eng., London (1963). C. Jtosn.4~~ and P. BLAIN, M&n. scient. Revue M&tall. 50, 285 (1959). K. L~‘CKE and H. P. STiiwE, Recovery and Recrystallization of Net&. p. 171. (1963). I. I. Kovrxov and I. L. ROZELBERG, Phys. Metals Metallogr. 6 (6), 175 (1958). C. R~SSARD, Private communication. G. .J.RI~ARDsoN,C.M.SELLARS~~~W.J.MCG.TEGART, Acts &let. (in press).

* Received

creep

January 25, 1966.

growth

sliding is causing

cavity

complicated.

We

creep

under

testing

direction original

during creep*

The growth of grain boundary creep has been attributed to (a) sliding(lz2) or (b) diffusion of vacant from

the

latticet3)

boundaries.(4*5) boundary

or from

The

sliding

the

relative

cavities during grain boundary lattice

surrounding

and diffusion

as growth

would seem to be easily distinguished creep tests in compression but

diffusion

growth

these conditions

now

two

of

grain grain

consider

stress

tensile

creep direction.

of grain boundary

(Fig.

cause cavities

a copper in.

specimens tension

of sliding direction or at least

cause no

sliding can

to grow in case (b). experiments

- 15.4 at. %

were machined

and

a gauge

were carried

aluminium

(26 hr) at 400°C

a diameter These of 2 in.

length

The microstructure

and

at a stress

on unloading

is quite

widespread

alloy.

having

were crept to the onset of tertiary

Cavitation

and

to grow they should tend to disappear

Creep specimens of 0.330

will be opposite

Thus if grain boundary

this hypothesis,

using

creep in of 7 tpsi.

is shown in Fig. 2. and

uniform

along

the gauge length. Compressive then

cut

has

creep

from

constant

the

stress

been

time.

specimens gauge

of $ in. cubes

length

compressive

described

and

creep

elsewhere.‘“)

were

tested

in a

machine

were

out at t’he same stress and temperature

that

The

density

These

which

tests

compression

and for the same period of

change

which

was measured

and

during

microstructure

examined.

The

results

tests

at 180’ and at 90” to the original

tensile

direction

microstructures Clearly,

are

after

occurred

the

compression

shown

in

carrying Table

out 1 and

the the

in Figs. 3 and 4.

when t’he sliding direction

is unaltered

processes I@

TV

by conducting

be suppressed.

no cavities

Under

are seen unless

barrelling of the specimen occurs(6) which appears to support diffusion growth mechanisms.(5) However, this is not conclusive since this result could be due to difficulties in nucleation conditions. Experiments can

under compressive be devised to avoid

this difficulty. Thus if compressive creep tests are carried out on materials in which cavities have been

a

whereas in case (b),

A reversal

in case (a) and continue To test

in

will be the same in tension

to close up a cavity

growth.

(a)

at 90” to the

In the first case, the sliding

in tension

1).

if

is more

compressive

systems

at 180’ and (b) in a direction

compression

out

However,

when sliding can still occur

should

of testing

sites either

importance

be

to occur under

growth, the situation

must

the sliding directions

carried

mechanisms

nuclei can

are important,

conditions.

had been used in tension, Cavity

cavity

growth

If diffusion processes

compressive

further

I. H. P. ST+,VE, Acta Met. 13, 1337 (1965). 2. D. HARDWICK and W. J. McG. TEGART,J.

4.

the

their

or even the reverse effect i.e. sintering

will tend

References

3.

creep,

so that

to that which occurred

C. M. SELLARS

of She@&

directly.

direction

such as copper and nickel. of Metallurgy

tensile

present

we would expect further cavity growth to be suppressed

and appears to be the rate-control-

recrysCallization

by prior

already

when

to Stiiwe, that while

plays

1966

studied

process.

Shefield.

developed are

all

dYynamic recovery

14,

strain

temperatures

rate-controlling

VOL.

X

10. i

x

Te

x

I

FIG. 1. Sliding directions on the grain boundary x x for creep stress o applied at (a) 0” (b) 180” and (c) 90” to the original tensile stress direction.

on