Martensitic transformations between martensites in a CuAlNi alloy

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

Vol. 9, pp. 491-498 Printed in the United States

MARTENSITIC

Pergamon Press,

Inc.

T R A N S F O R M A T I O N S BETWEEN MARTENSITES IN A Cu-A1-Ni A L L O Y

K. Otsuka*, H. S a k a m o t o and K. Shimizu Institute of Scientific and Industrial Research, Osaka university, Yamadakami, Suita, Osaka 565, Japan (* Presently with the D e p a r t m e n t of M e t a l l u r g y and Mining Engineering, U n i v e r s i t y of Illinois, Urbana, Illinois, USA) (Received February 12, 1975) (Revised March 13. 1975) Introduction Cu-Ai-Ni tensitically

alloys w i t h composition from the 81 matrix phase

m a r t e n s i t e phase M

near Cu-14.2Ai-4.3Ni(wt%)

temperature

(2H

type stacking

(2,3).

However,

(DO 3

type ordered

order structure)**

the present

authors

stress-induced

above M

t r a n s f o r m mar-

structure)

to the ¥i'

when cooled b e l o w the

recently

found that the

S

structure

of the martensites

(4,5,6).

That is, if the temperature

(18R type long period temperature

is lower than Af,

the same crystal 81

i

stacking

martensites

structure

is temperature dependent s is higher than Af, the 81' martensite

order structure) the Vl

l

martensite

are stable only under stress,

is lowered keeping

the stress

crystal has been stress-induced was initiated

direct m a r t e n s i t i c was

transformation

found to occur under certain Although

These

after a 81

i

The present

if the tem-

martensite

single

investigation

as a result of the investigation,

from the 81

l

martensite

to the Y1

t

a

martensite

conditions.

there have been a few reports

from one martensite

formed martensites.

Then, what will happen

constant,

and,

which has

and they transform back to the

by tensile test?

to test this h y p o t h e s i s

while the

is stress-induced,

as that of the thermally

81 matrix phase when the stress is removed. perature

is stress-induced,

on the crystal

to another as induced by deformation

structure

change

in such alloy systems,

**The m a r t e n s i t e was termed y' in the authors' previous papers, but it will be termed Y]' hereafter in order to show the ordered structure explicitly, after W a r l i m o n £ ' s notation (i). 491

492

MARTENSITIC

Cu-Al

(7,8,9,10)

TRANSFORMATIONS

and A u - 4 9 ( a t % ) C d

not been fully i n v e s t i g a t e d . b r i e f l y but explicitly morphologically

The purpose

process

9, No.

5

itself has

of the present paper is to report

on how the transformation

and stress

Vol.

(ii), the transformation

and crystallographically,

in the temperature

IN Cu-AI-Ni

between martensites

and to discuss

space b e t w e e n

proceeds

the phase relationship

the two martensite phases and the

matrix phase in the present alloy. Procedure The methods been described

of specimen preparation

elsewhere

Cu-14.1AI-4.2Ni

(3,12).

and single crystal fabrication have

Two single crystals with the composition

(wt%) have been used and

their orientations

are shown in Fig.

The final h e a t - t r e a t m e n t

I.

of the speci-

mens were such that 1000°C x 2 hr. - W.Q. After shaping by a Servomet M a c h i n e

into

tensile test specimens with the gauge length of 20 mm, they were electropolished for sufficient

time to remove

strains introduced by the spark cutting.

FIG. 1 Orientations of the single crystals used.

The specimens were tensile tested with an Instron Machine of T T - C M - L type, the m o r p h o l o g i c a l

changes

and

during the

tests were recorded m a c r o s c o p i c a l l y

by a camera with a telescope

m i c r o s c o p i c a l l y by an optical microscope. also used to determine

unambiguously Results

The process

of realizing tensite to the Y1 t martensite produce the 81 perature

Region I in Fig.

2.

and Discussion

the martensitic consists

transformation

from the 81

of three steps as follows.

As reported p r e v i o u s l y

!

mar-

First we

and the microscopic

clearly showed that they were 81 is completed

curve of

(6), the curve between B and C is

of the 81 to 81 ' transformation,

point C the transformation (Fig.

Laue method was

the phase of concern.

This step is represented by the stress-strain

of acicular martensites

dition

and

martensite by stressing the 81 matrix single crystal at a tem-

above Af.

characteristic

The back reflection

lens

I

and the micrograph

observation

martensites.

At

taken for this con-

3a) clearly shows that it is the 81' single crystal martensite.

Vol.

9, No.

5

MARTENSITIC

TRANSFORMATIONS

50

IN Cu-AI-Ni

493

C

4C s

E F 3C f

~ 2o 10 II

I I

A

I

I

I

I

I

I

1 2 3 4

5

6

7 8

Strain['/,]

1

I

I

1

m E

i

g 10 11 12

1 ) ) Decrc,~s ing Temperalure

2 3 4

5 6 7 8

(Neg~ive) Strain['/,]

FIG. 2 Stress-strain or stress-temperature relation for the process described in the text for Sp. No. 25. Region I: Loading with constant strain rate 2.5 x 10-2/min. at 49.2°C. Region II: Cooling from 49.2°C (C) to -194.5°C (D), keeping the stress constant. Region III: Unloading with constant strain rate 2.5 x 10-2/min. at -194.5°C.

FIG. 3 Optical micrograph showing the morphological change during the transformation from the 81' martensite to YI' martensite (Sp. No. 25). (a) 81 ' single crystal. (b) YI' band in the ~i' martensite. Fine striations in the Yl' band are [101]Yl, twins as a lattice invariant strain. (c) YI' martensite with [101~Yl, twins inside. (d) YI' single crystal.

494

MARTENSITIC TRANSFORMATIONS

IN Cu-AI-Ni

If the stress is relieved at this temperature, obtained,

Vol.

9, No.

the normal s u p e r e l a s t i c

5

loop is

and no m a r t e n s i t i c t r a n s f o r m a t i o n b e t w e e n m a r t e n s i t e s is observed.

Therefore,

as the second step,

the specimen was cooled keeping the stress con-

stant by moving the c r o s s h e a d slightly so that the m o v e m e n t compensates stress b u i l d u p due to the c o n t r a c t i o n of the p u l l rods upon cooling. step is represented by the region II in Fig.

2.

However,

the

This

no s t r u c t u r a l or mor-

p h o l o g i c a l change was observed during the cooling down to the liquid nitrogen temperature.

Then,

the stress was removed as the third step, w i t h the result

of the s t r e s s - s t r a i n curve as shown as the Region III in Fig.

2 (Note that the

sign of strain in R e g i o n III is opposite to that in Region I.

That is, the

increase of strains in Region III simply means the c o n t r a c t i o n of specimens). I n t e r e s t i n g l y enough,

there appears a stage b e t w e e n points s and f in Region

III, and m a c r o s c o p i c and m i c r o s c o p i c o b s e r v a t i o n s formation occurred b e t w e e n ~ and f. tion started at p o i n t ~,

showed that the real trans-

To speak more in detail,

the t r a n s f o r m a -

in the region near the grips w h e r e stress is lower

than in the p a r a l l e l portion,

and then the t r a n s f o r m a t i o n p r o c e e d e d to the

p a r a l l e l p o r t i o n b y the f o r m a t i o n and growth of the thick bands Fig.

3(b).

like those in

Fine striations are seen inside the martensite, w h i c h are identi-

fied to be twins as d i s c u s s e d later.

At p o i n t f the t r a n s f o r m a t i o n was com-

pleted, w i t h the specimen b e c o m i n g a single v a r i a n t of m a r t e n s i t e as shown in Fig.

3(c),

lower.

a l t h o u g h the density was m u c h

W i t h a further d e c r e a s e in stress from f to E,

gradually, tensite,

leaving twins inside the martensite,

the twins d i s a p p e a r e d

and at point E the specimen almost b e c a m e a single crystal of mar-

as shown in Fig.

3(d).

However,

there w e r e some indications that some

m i c r o - t w i n s w e r e still p r e s e n t inside the m a r t e n s i t e even at p o i n t E.

If the

crystal is then p u l l e d a little these twins d i s a p p e a r completely. The above process was also examined by the b a c k - r e f l e c t i o n

Laue patterns,

w h i c h w e r e taken from the single crystals of the initial and p r o d u c t phases. C o n s e q u e n t l y it was confirmed that the s t r u c t u r a l change d e s c r i b e d above is from the 81

l

m a r t e n s i t e to the Y1

b a s a l planes of the 81

!

I

martensite*.

m a r t e n s i t e and Y1

i

Furthermore,

the poles of the

m a r t e n s i t e w e r e found to be very

*The YI' single crystal m a r t e n s i t e may be o b t a i n e d d i r e c t l y from the 81 m a t r i x by s t r e s s - i n d u c e m e n t in the t e m p e r a t u r e range M s < T < A f ( 1 3 ). The o r i e n t a t i o n of the YI' single crystal thus o b t a i n e d was the same as that obtained i n d i r e c t l y through the 81' single crystal d e s c r i b e d above.

Vol.

9, No.

5

MARTENSITIC

close to each other. martensite

Thus,

TRANSFORMATIONS

it is obvious

to the YI' m a r t e n s i t e

that the lattice change

direction

3(b)

(c) was determined by the two surface analysis

the YI' martensite. variant

495

from the 81'

is carried out simply by the shuffling

[100181' and

on

IN Cu-AI-Ni

(001)81, plane.

in

The nature of the fine striations to be

{101}yl,

in F i g

twins in

It is to be noted that this twin system as a lattice in-

shear is different

from that of lattice

invariant

shear

(i.e.

{121]

,) Y1

for the direct t r a n s f o r m a t i o n

from 81 to YI' in the same alloy

(3,14).

The

habit plane of the m a r t e n s i t e was also determined by the two surface analysis approximately

{l13]yl,,

although

there are some scatters

from place to place

even in the same m a r t e n s i t e plate. The same procedure

as that described

out by changing

the temperature

as that of Fig.

2 when the temperature was

the stage

(between s and f) was

temperature,

above concerning

upon unloading.

lower than Af.

found to decrease

as shown by the solid

line AC in Fig. 4.

The stress

level for

This result seems to

B °

,3,~;/i~

-3a

(.-

2

/ /

020 u~

•

....

E

2 was carried

slightly with increasing

E

u

Fig.

The result was almost the same

m;'

I 100

/

/

¢

/

¢(, , "A

B,

/

r,' l 150

A

z

i '~'+- '?' 50

AsA# h i

0

I

I

50

100

Temperature [°C] FIG. 4 Phase r e l a t i o n s h i p b e t w e e n the three phases, 81 matrix, 81' martensite and YI' martensite. Solid lines AB and AC represent the measured critical stresses for the transformations from 81 ' to 81, and 81' to YI'" respectively. Dotted lines A'B' and A'D' represent the h y p o t h e t i c a l critical stresses for the transformations from 81 to 81', and 81 to Y1 l ' respectively. The path indicated by the arrow corresponds to the process described in Fig. 2.

496

MARTENSITIC TRANSFORMATIONS

indicate that the 81

!

IN Cu-AI-Ni

Vol.

9, No.

5

phase is the most stable phase at high stress levels,

even at temperatures below Af, while the Y1 the low temperature and low stress region A'D' and A'C in Fig. 4).

i

phase is the most stable phase in

(i.e. the region bounded by the lines

In order to check whether or not the 81

really a stable phase at high stress,

the y I

i

single crystal

i

phase is

(Sp. No. 25) ob-

tained by the procedure represented in Fig. 2 was tensile tested at the liquid nitrogen temperature.

Here the evidence of the transformation

from YI' to El'

was not detected up to the stress of 45 kg/mm 2 which is much higher than the line A'C in Fig. 4.

The authors believe that the transformation was not de-

tected because of the large hysteresis

associated with the transformation.

This

point is under intensive investigation in a continuing study. To conclude,

the martensitic transformation

single crystal martensite to the 71

i

from the stress-induced 81'

single crystal martensite was realized in

a Cu-AI-Ni alloy only when the specimens were cooled and then unloaded.

The

process of the transformation was found to be quite similar to that of the usual transformation

from matrix to martensite in morphology and crystallography

Although the crystal structures a common basal plane,

of the two martensites are simply related with

the habit plane was different from the basal plane,

the lattice invariant shear was

[i01]

and

twinning in the product martensite Yl'

phase. The authors would like to express their sincere appreciation to Professor C. M. Wayman,

University of Illinois,

for useful discussion.

work was partially supported by the Grant-in-Aid Research

(Ippan B, 1972-3)

This

for Fundamental Scientific

from the Ministry of Education of Japan.

Further

details will be published elsewhere. R@ferences i.

H. Warlimont, Special Report 93, Physical Properties of Martensite and Bainite, The Iron and Steel Institute (1965).

2.

M. J. Duggin and W. A. Rachinger,

3.

K. Otsuka and K. Shimizu,

Jap. J. Appl.

4.

K. Otsuka and K. Shimizu,

Phil. Mag.

5.

K. Otsuka, T. Nakamura and K. Shimizu,

6.

K. Otsuka,

7.

A. B. Greninger,

Acta Met.

K. Nakai and K. Shimizu, Trans. AIME,

12, 529

(1964).

Phys. 8, 1196

24, 481

(1969).

(1971).

Trans. JIM, 15, 200 (1974).

Scripta Met. 8, 913

133, 204

(1939).

(1974).

Vol.

9, No.

5

MARTENSITIC TRANSFORMATIONS

8.

I. Isaitschew,

9.

S. Kajiwara,

IN CuoAI-Ni

E. K a m i n s k y and G. Kurdjumov,

Trans JIM 9, suppl.

543

L. D e l a e y and A. Deruyttere,

497

Trans. AIME,

133,

204

(1939)

(1968).

i0.

H. Tas,

ii.

N. Nakanishi, Preprint for the S y m p o s i u m on T h e r m o e l a s t i c M a r t e n s i t e and Shape M e m o r y Effect (1974), Tokyo, Japan.

12.

K. Otsuka, M. T a k a h a s h i and K. Shimizu, Met. Trans.

13.

K. Otsuka, published.

14.

K. Otsuka and K. Shimizu,

K. Nakai,

H. Sakamoto,

Trans.

Scripta Met.

5, 1117

(1971).

4, 2003

(1973).

K. S h i m i z u and C. M. Wayman,

JIM 15, 109

(1974).

to be