Irreversible plastic deformation of pure iron during phase changes

634

ACTA

METALLURGICA,

It can be shown from the nucleation

rate equations

priate

given in Ref. 2 that the relative error in the computed quantity (yl, - Y,,), embryo statistics, is given by

711, -~ Ysn

nkT l)] In S ( I,h i

’

When

equation

for

the

(7) is evaluated

for

chosen

of discussion-we

liquid

by

tin

Sundquist

obtain

for

the

relative error A&,

YlP -

(8)

Because 6 = A(l)(i*) $ 1f2)(i*) + A(1)(i*)il(2)(i*), and

up the required

even locally. therefore, Ag,Sn;

duced using

(8) suffices to prove that the errors intro-

into

the

the

calculated values of ylll - ysp (by embryo statistics) are certainly

“usual”

negligible

(see also Table

procedures

shown here demonstrate

Sundquist’s

statements,

arr, satisfactory

1).

More generally, that, contrary

the “usual”

approximations

levels of supercooling

(AT

to Frenkel’s

incorrect,

the

systems

stating:

in equilibrium

unlikely.

Ag, as originally

assume.”

Pt-Sn

was be

surface

was made, or implied;

paragraph

stated that, for the systems substrate

and

“pure Ag and pure Pt cannot

ru’o such assumption

(catalyst)

phase

seems,

was most

stated in Ref. 2, and not no complications

of our data

arise

as suggested

by

.

The author gratefully assistance

and

Mackliet,

Metal

acknowledges

contributions Physics

given

Branch,

the extensive by

Dr.

Naval

C. A.

Research

31. E.

GLICKSMAN

Naval Research Laborator!/

tI.8.

Washington 25, D.C. References B. E:. SUNDQUIST, Acta Met. This issue p. 630 (1963). 8: M. E. GLICKSMAN and W. J. CHILDS, Acta Met. 10, 925 (1962). 3. J. FRENKEL, J. Physics (U.S.S.R.), 1,315 (1939). 4. M. HANSEN and K. ANDERKO, Constitution of Binary Alloys, 2nd Ed., pp. 52-53. McGraw-Hill, New York (1958). * Received

February

Irreversible

1, 1963.

of our paper it is

considered, “neither

the foreign

enters actively

plastic during

of the nucleation

Ag-Sn

of Ag,

of Ag,Sn

The catalytic

thus, in this respect,

Sundquist

concentration

statistics

with liquid Sn as Glicksman and Childs

in fact, in the opening

of the

at even modest

out that in Ref. 1 Sund-

quist argues that our description for

to

>_ 5°C).

It should also be pointed catalysts

the

embryo statistics

for unary, metallic liquids nucleating

7 at.“/;

The alleged formation

is no more than about 0.21 (for i, even as low as 50), equation

(below one-third

melting point of Ag) it is difficult to believe

Laboratory.

Y,,) _ ln (1 + 6) 2134 . YSP

-

at the low relative-temperature

experiment

in the interpretation

contact angle.

at AT = 5”C-as

purpose

absolute

probably (7)

nucleating

here because

build

3 cos Y) In (1 + b)

[3 cos Y (cos2 Y -

where ‘F is the equilibrium

1963

that a small Ag crystal could, in less than one minute,

Y,,) ~ (2 + 00s~ Y -

-

II,

of the nucleation

arising from an error 6 in the

4(&l

VOL.

Irreversible

deformation phase

deformation

phase transformation alloys

of a number

It is the purpose

direct

observations

these

were

y +

occurring during the M + y

in pure iron and some of its

has been the subject

papers.o-*)

of pure iron

changes*

made

transforming

S phase changes.

of recent

of this note to record

on fine iron

wires while

at

u+ y

both

the

and

Metallic purity was estimated

into the reaction nor, in general, is in equilibrium with

at better than 99.97 per cent and wires of 0.00508,

the system”.

0.01218 and 0.0254 cm dia were used.

Sundquist

(Italics added here.) argues

phase Ag,Sn-not “equilibrium”

that

Ag-should

the

intermediate

be considered

as the

catalytic

The equilibrium ponents

further

phase in the system Ag-Sn. diagram (*) between the system com-

Ag and Sn shows that, under the conditions

of the nucleation

experiment

Ag,Sn

only

can form

in two

in Ref. ways:

2, the phase by

eutectic

reaction: after cooling the Sn solution to 221°C; and by precipitation, after isothermally crossing into the two-phase field (Ag,Sn-liquid) at about 7 at.yb Ag. The first mechanism is, clearly, unsuitable because the

system

was

never

cooled

below

the

eutectic

reaction temperature during these catalysis experiments. The second mechanism also seems inappro-

The observations determine

were made during experiments

the surface

energy

and

self-diffusion

to of

solid iron.c5) The wires were suspended under stresses in the range minus 2 dyn

cm-2

which

x

lo5 dyn cm-2 to plus 9

resulted

from

x

lo5

a balance

between

surface energy effects and the applied load.

Experi-

ments were carried out in purified argon and the specimens were bright and ductile at the end of the wires were heated to normal tests. Typically, some chosen temperature at which they were held for up to 250 hr. During the first hours at the test temperature the wires straightened, recrystallisation and grain growth oped

a

“bamboo”

occurred

and the specimens

structure

in

which

the

develgrain

LETTERS

boundaries

occupied

TO

about

8 hr in the 6 range,

lengths

lengths

63.7

were from

This

20 hr in the y

range and 50 hr in the ct range at 850°C. gauge

EDITOR

the whole of the cross-sectional

area and were at right angles to the wire axis. required

THE

Specimen

1.5 to 3.0 cm and grain

are given in the table.

During

the heating

Typical grain sizes for wires of “bamboo” configuration

TABLE.

Phase

Grain lengths, cm

c((85O”C)

Wire dia., cm

0.0034 0.015 0.044-O. 14

x

0.00508 0.01218 0.01218

At room temperature the 0.01218 cm dia wire contained about 8 grains to the cross-section.

period the heating rate varied from a few “C to 500°C per hour at the phase transformations.

The cooling

rate was due to the heat losses from the furnace with the power off and was 550°C and 240°C per hour at the 6 -y

and y +

Observations

time experiments after tests.

u transformations,

respectively.

were made during these tests, in short and on the specimens

These may be summarised

before

and

as follows:

(1) On heating the cold drawn wires to the chosen temperature.

without

temperatures,

a few specimens

at the transformation

annealing

at

intermediate

vibrated

temperatures

very slightly

but most gave no

indication of a phase change. (2) During cooling after a test, kinks developed the originally

straight

change

the test temperature.

below

wires during

shown in Fig. 1. Examination

FIG. 1. 0.01218 rm dia wire after cooling from lQdO”C, illustrating a typical kink in the originally straight sperimrn. i 90.

in

the first phase The kinks are

of the cold specimens

showed that the kinks were associated

with deforma-

tion bands, Figs. 2 and 3. Wires cooled from the 6 range kinked at t’he 6 - y but not at the y - GC change temperature. (3) Holding in either the CI or y range and then heating

through

kinking

i.e. generally,

transformation,

the next

grain growth

the pre-existing phase. (4) Deformation could a specimen min.

phase

for kinking

at different

Any particular

change

produced

to occur during a

had to take place

occur at different

parts of

times over a period

kinking

operation

in

of 2-3

was usually

completed in less than a second. (5) Deformation bands could be straight or wavysome wires had a gnarled appearance in the kinked region and both types of band have been seen on the same specimen. The kinks were localised to a small section of a wire and could appear at any part of a grain. (6) Kinking heavily

occurred

stressed

wires

less frequently and

deformation

on the most was

most’

FIG. 2. 0.01218 cm dia wire after cooling from 1385”C, showing straght deformation lines. The other lines are thermally etched grain boundaries. >: 700.

636

ACTA

METALLURGICA,

VOL.

11,

1963

However,

if the heat evolved

formation

divided

few

“C only.

by the specific heat, it would be a

Further,

deformation

equals heat of trans-

occurs

this work

when

x 4

has shown

y and

when

that

y +

8,

when heat is absorbed. This work was carried out at the University of Swansea Greenough

and the author

College

wishes to thank

for helpful discussions

A. P.

and H. A. Ho11 for

the photographs. A. T.

PRICE

Central Electricity Research Laboratories Leather-head Xurrey, England References 1. P. LEHR, C.R. Acad. Sci., Paris, 242, 1172 (1956). 2. M. DE JONG and G. W. RATHENAU, Actu Met. 7.246 (1959). 3. B. G. LAZAREV, A. I. S~DOVTS~V and A. @. SM;RNO+, Physics rf Metals and Metallography, 7, 115 (1959). 4. 0. D. SRERBY and A. GOLDBERG, Acta Met. 9, 510 (1961). 5. A. T. PRICE, Ph.D. Thesis, University of Wales (1961). 6. B. A. BILBY and J. W. CHRISTIAN, Mechanism qf Phase Transformations in Metals. Inst. of Metals, Symp. (1956). 7. a. H. COTTRELL and D. F. GIBBON, Nature, Land. 162, 488 (1948). 8. G. C. SMITH and D. W. DEWHURRT, Resenrch 2, 492 (1949). FIG. 3. The same specimen deformation

severe

in the finest

as in Fig. 2, showing lines. x 700

wires and

least

wavy

severe

* Received

in the

thickest. (7) In experiments the specimens (presumably

using impure argon atmospheres

developed

a heavy blue-black

oxide and/or

the wires did not deformation

kink

nitride) during

tarnish

and in such cases phase

changes

and

bands were not present.

The difference

in volume

between

b.c.c.

and f.c.c.

Comments on “Irreversible tion of pure iron during In the preceding properties

of pure iron and some of its alloys during

the u-y transformation.(1-6)

during

transformation

formation

lines have been caused by stress relief due

to dislocation boundaries slip,

it

that the de-

is

movement.@)

Since pre-existing

or other transformation expected

that

greatest in the finest wires.

grain

nuclei will inhibit

deformation

should

Most interesting

be

perhaps

cross-sectional hundred).

movement,

this

a similar last

barrier

observation

to

dislocation

indicates

that

stresses due to the phase transformations in iron are not sufficient to account for the irreversible deformation previously reported. (1-4) In particular the explanation

proposed

by Lazarev

et uZ.t3) is not con-

sistent with these observations. They suggested that during the y - Q transformation, the transforming layer had a surplus of heat which increased mobil&y.

(in our

during

in polycrystalline phase

and Fe-N

case

15 up to

of a temperature

alloys.

transformation

macroscopically

homogeneous.

small

loads

external

(W-500

gradient,

In these cases, through

process, the dimensional

during

a few

of this type are pure iron trans-

in the absence

and Fe-C sample

area

Examples

an averaging

present

weakness

have to be distinguished.

grow at random and in which many grains occupy the

of the yield stress by surface films in certain metals(‘,@: tarnishes

Our results indicate that,

of mechanical

I. The first type is encountered

forming

and surface

types

of the mechanical

specimens in which the grains of the developing

is the observation that a heavy tarnish is sufficient to prevent de-formation. A similar effect is the raising If it is assumed that grain boundaries

letter Price presents work which

to our investigations

two different

and it appears

plastic deformaphase changes”*

is related

iron gives rise to stresses across the phase boundary transformation

October 8, 1962.

changes of the

were

found

By transforming g/mm2)

to

be

under

an excess

de-

formation was observed parallel to the applied stress. This deformation increases monotonically as transformation proceeds and depends linearly on stress.(334) On a microscopic

scale we observed

slip in bands as

well as along the grain boundaries, even in front of the moving phase boundary.(4) The easy yielding of the transforming material to small external loads is