Comments on “thermodynamics of the martensite transformation”

1110

ACTA

e.m.f.

in measurements

identical

made

preparations.

through

METALLURGICA,

from

two

The deviations

a series of measurements

or

more

may persist

made at different

VOL.

10,

Calculation

1962

of

AF”“‘ILW8

yields a value of -290

iron-carbon

alleys(2)

cal/mole (independent

for

of carbon

content), and extrapolation

to zero carbon leads to the

electrolyte concentrations, and such effects have been recorded for lead dioxide2p5 and manganese dioxide

same value of AF>yz at M, for pure iron.t3) From these points, Singh and Parr have calculated that in

electrodes.6

pure iron:

Thus E, should reversibility,

not he a necessary

criterion

for

since it may vary from one experiment

to another even though the cells are sensibly reversible. in E, have been attributed

These variations

to the

physical nature of the solid phase, and the relationship of E, to the criterion of reversibility

has been discussed

At this temperature,

AF$TX =

is in accord with calculations K. P.

Department of Mining

The following doubts

Edmonton, Alberta

-285

cal/mole which

of AFY-a’l_+f8 for iron-

difficulties would seem to raise grave

concerning

the

validity

of

the

(1) The questionable

1. H. H. UHLIG, Proc. Nat. Accd Sci. Wash. 40, 276 (1964). 2. K. P. SINGH, Ph.D. Thesis, University of Durham (1956). 3. W. H. BECK, K. P. SINGE and W. F. K. WYNNE-JONES, l’mns. Faradaw sot. 55. 331 (19591. 4. W. J. HAMER,>. Ar&.‘Ch&. Sot: 57, 9 (1935). 5 W. C. VASBUXGH and D. N. CRAIG, J. Amer. Chem. Sm. 51, 2009 (1929). 6. K. H. MAXWELL and H. R. THIRSK, J. Chwn. Sot. 4054 (1955). i. 1~. G. BATES, E. A. GUGGENHEIM, H. 8. HORNED, D. J. G. IVES, G. J. JANZ, C. B. MONK, R. A. ROBINSON, R. H. STOKES and W. F. K. WYNNE-JONES, J. Chem. Phys. - 25. 361 (19,5(i). I

February

out the problems Although

involved

with the type of cell used.

the authors have informed us privately

the observed is generally

e.m.f.

was reproducible,

not sufficient

to prove

that the cell is

difference calculated from the e.m.f. is actually AF%T~‘. (2) The large difference in energy between u and cc’ (i.e. 300 cal/mole)

suggested

by these measurements

in that both

the linear carbon-dependence on

“Thermodynamics

martensite

of the

M and a’ appear to be

ment and density of imperfections.

Yet, the measured

trans-

values of defect energies which can be stored in metal lattices by a variety of methods are usually much less

temperature

where austenite,

have equal free energies, interpretation

could

it states

y, and martensite,

i.e. AF)‘+a’1338 = 0.

have

that

of martensitic

far-reaching

austenite

tc’, This

implications

will transform

to

on cooling just as soon as the free energy

of the bulk martensite imperfections,

to be formed, including

all its

drops below the free energy of the bulk

austenite. In other words, supercooling and there is no problem of nucleation. Owing to several typographical the printed version,(l) is first appropriate at room

does not occur

as confirmed by the authors, it

to restate their case briefly.

temperature

that

than 300 cal/mole.(g) (3) If AF’““l,, tion barrier should

go

= 0, suggesting

exists, to

Evi-

cell measure-

the free energy

that no nuclea-

the martensitic

completion

Instead. these reactions

transformation

immediately

below

occur characteristically

a range of temperatures.

The question

M,. over

also arises as

to why the reverse (u’ + y) reaction should not occur when martensite is heated to just above M,. Although such a rapid-heating

errors appearing in

dence is presented(l) from electrochemical ments

The only known

in which M, is considered to represent the

evidence for a new interpretation

martensite

from

structural difference between x and cc’ is in the arrange-

In a recent letter,(l) Singh and Parr have presented

because

judging

of the lattice parameter

ofFe-CY7)andFe-Ni-CWmartensites.

transformation”*

that

this condition

reversible, i.e. that the free-energy

thermodynamically

seems unlikely,

16, 1962.

significance of the e.m.f. meas-

urements(l) has been discussed by Uhlig@) who points

b.c.c. with the same lattice parameter,

formations

Singh-Parr

hypothesis : References

Comments

Gilbert and

carbon alloys.(2s5)

SINGH

J. G. PARR

and A4etallurgy

of Alberta

* Received

ments (at cooling rates up to 5500’C/sec),

Owen’*) have found M, in pure iron to be 545 & 5°C.

in detail elsewhere.7

University

It may be noted that in recent rapid cooling experi-

experiment

formed on pure martensitic tensiticiron-carbon transformation

has not been

per-

iron, work on three mar-

alloys(lO) has shown that thereverse

occurs

only

on heating

300”400”C

above MS.(ll) Furthermore, in iron-nickel martensites, it is well known that A,, the temperature at which the

difference, AFT,<“‘, between (highly imperfect) martensitic ferrite, tl’, and (relatively perfect) ferrite, M, is

tc’ - y reaction higher than M,.

300 + 70 Cal/mole. This difference is assumed independent of temperature as a first approximation.

more than 20 per cent nickel, where the reactions are

starts on heating, is substantially In particular, for alloys containing

clearly martensitic, (12) A, is 400”-600°C

higher than

I,ETTERS

M,.

These observations

are in conflict

TO

with the con-

tention that P:’ = Pa’ at n/r,?,and that the mnrtensitic reaction occurs at a temperature where AF)‘“” (4) In iron-nickel

alloys containing

can be induced

at t,empera-

to an energy differ-

ence of about 250 cal/mole)

above M,T. Inasmuch

the Singh-Parr

suggests

hypothesis

as

that y is more

stable than CC’at temperat’ures above M,?, one would not expect, the conversion

of stable y into w&able

Q’.

(5) Finally it should be emphasized t,hat,the thermodyna,mic

calculations

difFerences discussed dynamics

of

free-energy

in connection

of martensnic

case of iron-nickel

enthalpy have been

measurements

for the

that, M,

we must conclude tha’t the

represents

to point out that we stated as an experimental vation

the potential

difference

generated

FeSO,, Ferrite 1Pt.

remarks about, the significance

obser-

by the cell

We prefaced our

of this measurement

our assuml~t,io~ls. We will now attempt

to answer the point,s of the

letter in the order in which they are made: Our own measurements iron agree sensibly

on the NM, temperature

the temperature

at,

of

with those of Owen and Gilbert

referred to. (1) We privately

informed

than the reproducibility However,

a complete

Uhlig‘s criticisms,

Radcliffe

et aE. of more

of our cell mea~surelIlents. reply

has been given

to Dr.

and we will not, repeat it here.

We

believe the cell to be sensibly reversible, (2) We agree that the difference

martensitjes.(13,14)

In view of this evidence. hypothesis

and

with t,he t,hermo-

transformat,ions

confirmed by direct calorimetric

1111

Pt 1Martensite,

= 0.

29-33 per cent,

by deformat,ion

t;\mesup to 200°C (corresponding

EDITOR

t-hat, the y -+ CC’ by carefully chosen words t,hat indicated the extent of

nickel, it has been demonstrat,ed(ll) reaction

THE

large.

in free energy is

However,

one cannot reject, a figure because it does not, fit, a preconceived theory. One might equally

which the austenitic and mart,ensitic phases have equal

argue that the depression of Jf, below !7’,, in pure iron

free energies cannot, be sustained.

is extremely S. V.

L. KAUFMANt

~Munlabs, In,c: Cambridge,

BA~CI,IFFE~

Mass.

MORRIS

large compared t#oot,her pure meta mart-

ensites, and t,herefore that value is dubious. (3) We a’re not, aware of any experimental

ComN$

shows

a range

of temperature

reaction in pure metals.

$.Massachusetts Institute oj’

for

work that

the martensite

Further. the initial formation

of martensit’e may change the syst’em so that, a temp-

l’ech~mlogy

erature range of formation

References 1. K. P. SIXca and J. 0. l’.wit, AC&Z Xet. 9, 107-t (1961). 2. iv. COHEN. E. S. MACHLIN rtnci V. G. PARANJPE, Thermorlynrwnt~sin Physiml Mettrllurgy, Amrr. lYce. Xetcxls pi 242, Clewland, Ohio (1949). 3. J. C. FISHF:R,Truns. Amer. .In~f. met. (metdl.) Engrs. 185, 688, (1948).

are we yet convinced t,hat the reverse specifically,

Nor

transformat,io~~ occurs

above

iM,:

we do not know whet#her the heat,ing rates

are sufficiently fact,

would be expected.

by the experimentsal evidence

rapid that the transformation

martensite

t’o austenit’e,

and

not

is, in

the corre-

sponding ferritic phase (produced from the martensite during

heating)

that

is transformed.

However,

it

seems likely (we must, agree) that, the reverse transformatior~ will occur above

To. However,

sure t,hat, this damages our case.

\ve a,re not

f?.fter all: the use of

the term "free energy” applied to mart,ensitic processes is a misnomer. produce

for the reactions

a non-equilibrium

under consideration

phase.

Our argument

rather this b\vofold one : first. conventional theory product

implies

that, martensitic

iron and diffuxion-

ferrite have the sa8me free energies-and

work. we submit. computation

disproves

of an activation

to the confusion

this.

is

martensite our

~onseq~~entl~-. the

barrier (which.

to add

is also called the driving force) ba,sed

upon the reaction austenite to ferrite cannot be applied to the reaction austenite to martensite,

Reply to comments on “Thermodynamics martensite transformation”*

of the

We very much appreciate the comments by Radcliffe et uZ.(~) As a preamble to our reply, we wish

in pure iron.

Second, if our ideas about the free energy difference be-

tween martensitic and diffusion product. a,re meaningful, t~hen a parallel behavior other pure -metal systems. collaboration

should be observed in We have, in fact, (in

with L. P. Srivastava)

measured

the