The Austenite stability and the Martensite burst

Scripta M E T A L L U R G I C A

Vol. 8, pp. 527-532, 1974 P r i n t e d in the United States

P e r g a m o n Press,

Inc.

THE A U S T E N I T E S T A B I L I T Y AND THE

R.M.Brito,

MARTENSITE

BURST

R.B.C. da Silva and J.R.C. G u i m a r a e s

INSTITUTO M I L I T A R DE E N G E N H A R I A C E N T R O DE P E S Q U I S A DE M A T E R I A I S Pqa. Gen. T i b ~ r c i o s/n9 - ZC-82 URCA- Rio de J a n e i r o , G B -Brazil (Received March

13, 1974)

The effects of plastic d e f o r m a t i o n and thermal s t a b i l i z a t i o n upon the m a r t e n s i t e burst have been r e c e n t l y studied by W o l l m a n n and Guimaraes (I) and by G u i m a r a e s and Brito (2) .

At that time it was r e p o r t e d that there would

exist a linear r e l a t i o n s h i p between the extent of thermal s t a b i l i z a t i o n imposed upon the a u s t e n i t e and the o b s e r v e d increase in the initial m a r t e n s i t e burst. The present note d e s c r i b e s the results of a work u n d e r t a k e n to further explore the c o r r e l a t i o n b e t w e e n thermal s t a b i l i z a t i o n

(TS) and the burst phenomenon.

In a d i t i o n to this matter the c o n d u c t e d e x p e r i m e n t s have yielded i n f o r m a t i o n about both "mechanical s t a b i l i z a t i o n " and "mechanical s e n s i t i z a t i o n "

of

austenite. The m a t e r i a l used was a high purity Fe-27%

Ni-0.23% C alloy.

It was

subjected to the same t h e r m o - m e c h a n i c a l treatments d e s c r i b e d in ref.l, w h i c h y i e l d e d a u s t e n i t i c strips w i t h a .2 mm average grain diameter. this m a t e r i a l was then 50% cold rolled.

of

Specimens of each set were given

e q u i v a l e n t sequences of c o o l i n g to burst and aging, of Fig.l.

A part

i l l u s t r a t e d in the d i a g r a m

This sequence was d e s i g n e d to study the p o s s i b i l i t y of having the

r e a c t i o n o c c u r i n g by r e p e a t e d bursts.

The c o o l i n g m e d i u m was a liquid n i t r o g e n

r e f r i g e r a t e d ethyl alcohol bath and the r e a c t i o n temperature was d e t e r m i n e d by the r e s i s t o m e t r i c method e x h a u s t i v e l y d i s c u s s e d e l s e w h e r e (3) on in a heated silicone bath at 150°C for 15 minutes. 527

A g i n g was carried

528

AUSTENITE STABILITY AND THE MARTENSITE BURST

Vol.

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5

Optical m e t a l l o g r a p h y and X-ray d i f f r a c t i o n were used to o b t a i n the amount of m a r t e n s i t e in the specimens.

The D i c k s o n ' s (4) t e c h n i c q u e was e m p l o y e d

to

c o r r e c t the X - r a y data for p r e f e r r e d o r i e n t a t i o n effects.

u)

Specimen

Aging T r e a t m e n t

/

/

/

T! " " ~"

T2-,..

×/

o 4--

o

o.

E X-Mortensite

Fraction

Volume Determination

Time (arbitrary

scale)

Fig.l Experimental Sequence

It was o b s e r v e d that the e x p e r i m e n t a l p r o c e d u r e s c h e m a t i z e d in Fig.l was able to cause the m a r t e n s i t e t r a n s f o r m a t i o n to proceed in both d e f o r m e d and a n n e a l e d specimens by s u b s e q u e n t bursts.

However in the a n n e a l e d specimens the

burst t r a n s f o r m a t i o n d i s a p p e a r e d after 3 e x p e r i m e n t a l sequences while it could be o b s e r v e d after 5 sequences in the d e f o r m e d ones.

This b e h a v i o r is p r o b a b l y

due to the size of the initial burst w h i c h was about 5 times larger in the a n n e a l e d than in the d e f o r m e d specimens.

This e f f e c t can be easily r a t i o n a l i s e d

if one r e a l i s e s that the a u s t e n i t e grains p a r t i t i o n i n g by m a r t e n s i t e plates and the a m o u n t of a u s t e n i t e a v a i l a b l e to t r a n s f o r m are l i m i t i n g factors for the burst p r o p a g a t i o n and i n t e n s i t y in a p a r t i a l l y t r a n s f o r m e d material.

Both effects are

n a t u r a l l y i m p o r t a n t after a big burst and are assumed to be r e s p o n s i b l e for the

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AUSTENITE

STABILITY AND THE MARTENSITE

BURST

529

experimental observations described above. It was also possible to confirm the existence of the reported

(i)

linear

relationship between the amount the of martensite, ~v • formed in a burst caused by aging a partially transformed specimen and the correspondent extent, thermal stabilization,

Fig.2,

e

(i)

where ~ is a proportionality constant.

The quantity @ is given by the difference

between two subsquent bursts temperatures.

The existence of the relationship

Eq. (i) can be confirmed by plotting the total amount of martensite ~v '

against the cooling temperature T, Fig.3.

if the transformation Eq. (i) holds,

is forced to occur by

then V M

of

i.e

~v =

specimen,

e,

(subsequent)

in the

It can be shown that

bursts only,

and

should be linearly related to T, i.e,

V

~v = ~

(T - T )

(2)

and

. T

where MB, O ~,0 V

VvB'O = ~,0

-

~-

(3)

is the experimentally determined martensite

the associated burst intensity.

It is clearly seen in Fig.3 that the

experimental data support this hypothesis. slopes of the lines

~v

vs.

This plot also discloses that the

T are essentially equal in the range

temperatures where the observations were carried-on. Eqs. (i) and

start temperature and

(2) would be strain independent,

caused by thermal stabilization

of

Thus, the coeficient

, @(b

i.e, equal amounts of "super-cooling"

(or an equal excess of Gibbs Free-Energy, which

is nearly a linear function of the temperature) of the specimen initial conditions.

produce equal bursts,

independent

On the other hand it was observed that

specimens from both sets, continuously cooled to a given temperature,

T, below

M B, displayed nearly the same amount of martensite as the specimens subjected to the sequential transformations,

Fig.l.

This suggests that for a given set

of

530

AUSTENITE

X

S T A B I L I T Y AND THE M A R T E N S I T E BURST

>

•

Vol.

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5

- Annealed

.15

:>> .,o .05

0

b

=/" III 0 5

t I0

I 15

I 20

I 25

I :50 e (oK)

I :35

Fig.2 The linear r e l a t i o n s h i p o b s e r v e d between the i n t e n s i t y of a burst, degree

of

thermal

~v

' and the a s s o c i a t e

stabilization,

@ .

initial c o n d i t i o n s the e x t e n t of t r a n s f o r m a t i o n would depend upon the t e m p e r a t u r e only, a s i t u a t i o n named

"athermal".

F u r t h e r m o r e the same e x p e r i m e n t a l fact also

suggests that the bursts caused by the s t a b i l i z i n g treatments would occur

to

c o m p e n s a t e for the associated r e a c t i o n arrest. Another

fact w h i c h is c l e a r l y d e p i c t e d in Fig.3 is that the curve

c o r r e s p o n d e n t to the d e f o r m e d set is d i s p l a c e d d o w n w a r d s r e l a t i v e l y to the one a s s o c i a t e d w i t h the a n n e a l e d specimens.

In terms of "athermal kinetics" this

implies that at a given t e m p e r a t u r e there would be less d r i v i n g force available for the r e a c t i o n in the d e f o r m e d than in the a n n e a l e d specimens.

This effect

can be a c c o u n t e d for if one r e c o g n i z e s that the plates m e c h a n i c a l c o u p l i n g can be thought of as e f f e c t i v e l y i n c r e a s i n g

(locally)

the d r i v i n g force for the r e a c t i o n

This e f f e c t earlier suggested by C h r i s t i a n (5) was found to be a p l a u s i b l e e x p l a n a t i o n for some of the o b s e r v a t i o n s r e p o r t e d by G u i m a r a e s and Brito (2) Then it is p r o p o s e d that the i n h i b i t i o n of the o v e r - a l l t r a n s f o r m a t i o n k i n e t i c s o b s e r v e d in the d e f o r m e d set w o u l d be a result of the i n t e r f e r e n c e of the austenite s u b s t r u c t u r e upon the plates coupling.

This i n f l u e n c e of the a u s t e n i t e *

s u b s t r u c t u r e over the r e a c t i o n k i n e t i c s would be r e p r e s e n t e d b y the p a r a m e t e r T in Eq.(3).

In fact,

it can be inferred from the data in Fig.3 that 50% plastic

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AUSTENITE

S T A B I L I T Y AND THE M A R T E N S I T E

BURST

531

=E >

>

¢0

[A

.8 0 I-

.7

E

.6

0

.5

seq. cooled cont cooled

I,o -

cooled seq. conT. cooled

~

%%

1:3

.3

*'-.

4-

E m 0

-

B Deformed / I

.4 o0 C 03

_

Annealed~.A

.2

"'.

Me

l i k . TD

.I ,

o 170

190

210

/

230

,-.!

250

,

,

,

270

290

310

330

Temperature ,T ( °K ) Fig.3 A m o u n t of m a r t e n s i t e in the material, as a function of temperature, deformation depressed T w h i c h can be e s t i m a t e d

about 80°K

~v

'

T .

(relatively to the annealed set)

an effect

from the data by Kaufman and Cohen (6) to be e q u i v a l e n t to

a Gibbs F r e e - E n e r g y change of about 100 cal/mol. s t a b i l i t y the effect just d e s c r i b e d could be named

In terms of austenite "mechanical stabilization".

On the other hand the data in Fig.3 also shows that 50% d e f o r m a t i o n raised the M B t e m p e r a t u r e of the m a t e r i a l about 12°K.

This effect w h i c h supports the

h y p o t h e s i s that m a r t e n s i t e n u c l e a t i o n occurs p r e f e r e n t i a l l y at some sites of the austenite

internal s t r e s s - f i e l d has been named

"mechanical s e n s i t i z a t i o n ''(7)

It is then implied that "mechanical stabilization" are not e x c l u s i v e phenomena. (autocatalitic)

and "mechanical sensitization"

The former would be a s s o c i a t e d with the

p r o p a g a t i o n of the r e a c t i o n and the latter with the initial

n u c l e a t i o n of martensite. However it is important to recall that a v a i l a b l e (1-2) data indicate that small smounts of d e f o r m a t i o n can both raise the M B t e m p e r a t u r e and increase the initial burst. Eqs.(1) T

and

(2) upon strain,

could also be raised

Then,

the n o n - d e p e n d e n c e of

~,

suggested by the data in Fig.3, would imply that

("negative m e c h a n i c a l stabilization").

532

AUSTENITE STABILITY AND THE MARTENSITE BURST

Vol.

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5

Conclusions. The experiments

(i)

the martensite

described

above disclosed

that in the alloy studied:

reaction occurs during cooling under conditions

named

"athermal"; (ii)

there is a linear relationship

between the extent of thermal stabilization

and the intensity of the associated

(Ni)

the burst associated with thermal

martensite

burst;

stabilization

seems to compensate

reaction arrest during cooling which is caused by the stabilizing (iN)

"mechanical austenite

(N)

stabilization"

substructure

"mechanical

would result from the interference

with the plates mechanical

sensitization"

for the

treatment;

of the

coupling;

would be primarely associated with the initial

nucleation of martensite.

Acknowledgements. This work has been supported Materials

Research Center.

Council,

CNPq.,

and PRO.

2257/72.

by the Ministry of Planning through IME

Special thanks are due to the Brasilian

for aditional

support through the grants.

Research

TC 12508, PRO. 241/73

References. 1--

D.R. Wollmann and J.R.C.

2-

J.R.C.

3-

D.R. Wollmann,

4-

J.M. Dickson,

5-

J.W. Christian,

Guimaraes

Guimaraes,

and R.M. Brito,

M.Sc. Thesis,

J. Appl. Crxst. , ~, 176 Physical Properties London

L. Kaufman and M. Cohen, Progress Pergamon Press, London

7-

J.R.C.

Scripta Met., !, 621

Guimaraes

(1973).

(1973).

Instituto Militar de Engenharia,

the Fron and Steel Institute, 6-

Scripta Met., !, 355

1972.

(1969).

of Martensite

and Bainite,

p.14,

(1965).

in Metal Physics,

vol.~,

p.190,

(1958).

and J.C. Shyne, Scripta Met., ~, 1019

(1970).