Structural phenomena preceding martensitic phase transformation in CuAlZn alloy

Scripta METALLURGICA

Vol, i0, pp. 895-899, 1976 Printed in the United States

Pergamon Press,

Inc.

STRUCTURAL PHENOMENA PRECEDING MARTENSITIC PHASE TRANSFORMATION A.Nagasawa,

IN Cu-AI-Zn ALLOY

A.Gyobu +, K.Enami*,

S.Nenno* and N.Nakanishi**

Department of Physics, Nara Women's University, 630 Nara, Japan *Department of Materials Engineering, Osaka University, 565 Osaka, Japan **Department of Chemistry, Konan University, 658 Kobe, Japan (Received July 15, 1976)

INTRODUCTION In the B 1 phases of AuCuZn2[1],

(Cii-C12)/2 for

the

NiAl[2]

and others,

the elastic constant

(iI0) [i~0] type shear becomes soft as a preceding phenomenon

of the martensitic phase transformation. Such a lattice instability is due to the softening of a transverse, acoustic phonon mode(TA mode or shear mode) as observed in the case of AuCuZn2[3]. Consequently, we can accept that the softening of the shear mode plays very important role as the trigger to cause the. martensitic phase transition in the 8 phase alloys. However, we must take the followings into consideration. Firstly, when the shear mode is softened, the 81 lattice will be very slightly distorted. Secondly, a large amount of the lattice deformation is necessary to form the martensite: The martensites in the 8 phase alloys have commonly the close packed layer types of the structures such as 2H and 9R(or 18R). In other words, the lattice distortion due to the soft phonon mode is so small that it can not supply the deformation to form the martensite. Based on the above consideration, one of the authors[4] has presented a formation model of the martensite in the 8 phase alloys. The model predicts that the martensitic transformation proceeds through the two successive processes. That is, the matrix phase is slightly distorted by the soft phonon mode(formation of the distorted lattice) and then the distorted lattice is largely deformed into the close packed layer type martensite. The distorted lattice has the same symmetry as that of the finally produced martensite. If such a schema of the martensitic transformation is realized in the 8 phase alloys, we can expect to detect the existence of the distorted lattice. As reported previously by one of the present authors[4], the distorted lattice has been observed in AgCd. In the case of AuCuZn^[3],anomalous satellite reflections have been observed on the neutron diffractio~ pattern of the Heusler type 81 phase just above the M s point: We suppose that this anomaly is also due to the formation of the distorted lattice in AuCuZn~. These experimental results [3,4] support strongly a hypothesis that the distorted lattice will commonly +Present adress: Department of Mechanical Engineering, 792 Niihama, Japan.

895

Niihama Technical College,

896

STRUCTURAL PHENOMENA IN Cu-Al-Zn

Vol.

I0, No. I0

appear in the 8 phase alloys before the martensite formation. To prove the validity of this hypothesis is very important for understanding the nature of the martensitic phase transformation in the 8 phase alloys. The present study is concerned to this problem. In the present paper, we report briefly that the distorted lattice is formed in a Cu-AI-Zn alloy as the structurally preceding phenomenon of the martensitic phase transformation. EXPERIMENTAL An ingot of Cu-AI-Zn alloy was prepared by melting together the component metals with purity of 99.99% in an evacuated quartz capsule. During the preparation, the weight loss of about 0.25% was produced. We regarded this as vapourization of Zn metal and estimated the composition of the alloy to be Cu:69.1, AI:I7.1 and Zn:13.8(at%). The ingot was hot forged and then homogenized at 850°C for 24 hrs in argon atomosphere. Thin plates, cut from the ingot, were hot rolled into sheets with thickness of about 0.3 mm. The sheets were annealed at 850°C for 1 hr in quartz capsules sealed under about 1 atm of argon, followed by quenching into cold water. The M s temperature of the alloy was found to be about 15°C. The sheets were electrolytically thinned by Bollmann's method using an electrolyte of a solution of 50% phosphoric acid in water. The thinned specimens were examined by selected area electron diffraction method using a 100 kV type of electron microscope with a cooling device attached in a tilting stage. RESULTS AND DISCUSSION The 81 phase with the Fe3AI type of structure is transformed into the 2H or 18R type martensite by cooling below the M s point. Figure 1 shows typical electron diffraction patterns of the both types of the martensites formed in a specimen. Before the martensite formation, however, anomalies are sometimes observed on the electron diffraction patterns of the B 1 phase. Figure 2 is an example of the anomalous diffraction pattern appearing before the formation of the 2H type martensite. The anomalies are characterized by considerably strong and sharp extra spots with the indices of ii0, 112 and others as indicated by arrows. In the case of the formation of the 18R type martensite, the anomalies also occur as a precursor of the martensitic phase transformation. Figure 3 shows an example of this case. In this pattern, the strong and sharp extra spots with the indices of 2/3 10/3 0, 4/3 8/3 0 and others are shown by the arrows. The extra spots in Figs.2 and 3 are sharp and they appear only at definite positions. Such characteristics of the diffraction patterns suggest strongly that a certain kind of a symmetry change occurs in the 81 phase before the formation of the close packed layer type martensite. This situation is very similar to that of the case of AgCd[4]. Consequently, we can easily suppose that a distorted lattice is also formed as a precursor of the martensite formation in the Cu-AI-Zn alloy. The formation model of such a distorted lattice has been already proposed by one of the present authors[4]. According to this model, the transformation from the Fe3AI type 81 phase to the 2H or 18R type martensite will occur through the two successiv~ processes as shown in Fig.4. 13 or C 2h 1 type structure is formed Firstly, the distorted lattice of the D2h by introducing periodic shears as indicated by the arrows in this figure. It is considered that the production of the shears is due to a soft mode of the [ii0] (ii0) type TA phonons. In this model, the phonons with the wave vector q = (1/2,1/2,0) or (1/3,1/3,0) are responsible to the formation of the distorted lattice. Such phonons can exist in the Fe3AI type 81 phase[5,6]. After the disr torted lattice is formed, a large deformation to form the martensite is finally introduced without any symmetry change. It must be noted that the symmetry change in the martensitic phase transformation is finished by the first process. This predicted transformation schema seems to interpret well the observed results. However, whether the above model on the martensitic phase transformation is valid or not must be inferred by comparing the anomalous diffraction patterns in Fig.2 and 3 with the intensity distributions calculated on the distorted lattices in Fig.4. The calculated intensity formula are as follows:

Vol.

I0, No.

I0

STRUCTURAL

I(hki)

= 2Io[ 1 +

(-i)

PHENOMENA

h+k+£

IN Cu-Al-Zn

897

cos(h¢)]

13 lattice and for the case of the D2h o

I(hk£)

= 2Io[ 1 +

(-i) h+k+£

] [ 3 + 2cos(120 k + h~)

O

+ 2COS(120

O

k - 2h~)

for the case of the C 1 lattice where 2h 2 £+ Io = 3f + fB + 2[(-1)

+ 2cos(240 k - h#)]

h o (-i) cos90 £ + (-i) h+£ cos90

o

£]

× (f~ + fAfB ) • fA and fB are the scattering

factors of the A(mainly

Cu) and B(mainly A1 and Zn)

atoms and ~ is shear d i s t o r t i o n in degree. Figure 5(a) and (b) are s c h e m a t i c a ~ 3 i n t e n s ~ t y d i s t r i b u t i o n s and [001] incidences calculated on the D2h and C2h type distorted

for the [ii0] lattices, re-

spectively. It is clear that the patterns in Fig.5(a) and (b) interpret very well the o b s e r v a t i o n s shown in Figs.2 and 3. This result leads to the conclusion that the anomalous d i f f r a c t i o n patterns are produced by the formation of the distorted lattices as p r e d i c t e d by the t r a n s f o r m a t i o n model of the 8 phase alloys[4]. The present study reveals that the distorted lattice is also formed in the Cu-AI-Zn alloy as a structurally preceding p h e n o m e n o n of the m a r t e n s i t i c phase transformation. Based on the present result and those of AuCuZn2[3] and AgCd[4], we can suppose that the formation of the distorted lattice will be observed as a general p h e n o m e n o n before the m a r t e n s i t e formation in the 8 phase alloys. This hypothesis will be proved if the soft phonon mode is the trigger of the m a r t e n s itic phase t r a n s f o r m a t i o n in the 8 phase alloys. We express sincere thanks to P r o f . J . K a k i n o k i using electron microscope.

of Osaka City U n i v e r s i t y

for

References i) N . N a k a n i s h i , T . M o r i , S . M i u r a , Y . M u r a k a m i 2) K . E n a m i , J . H a s u n u m a , A . N a g a s a w a 3) M . M o r i , Y . Y a m a d a

5) A . N a g a s a w a : P h y s . L e t t e r s 6) A . N a g a s a w a

and S.Nenno:Scripta

and G . S h i r a n e : S o l i d

4) A . N a g a s a w a : J . P h y s . S o c . J a p a n

and S.Kachi:Phil.Mag.28(1973)277.

40(1976)1021.

45A(1973)265.

and I.Sakata:to

metall. (to be published).

State Commun.17(1975)127.

be published.

898

STRUCTURAL

PHENOMENA

IN Cu-AI-Zn

Vol.

i0, No. i0

i ~!~¸~

(a) Fig.l.

(b)

Electron d i f f r a c t i o n patterns of the m a r t e n s i t e of Cu-AI-Zn. are the 2H and 18R type martensites, respectively.

(a) and

(b)

•c,S~

O

Q

(a) Fig.2.

(b)

E l e c t r o n d i f f r a c t i o n patterns of the [i[0] incidence. (a) is the pattern of the 81 phase. The pattern (b) shows anomalos spots in addition to those of the ~i phase.

The anomalies

appear before the formation of the

2H type martensite.

e e

(a) Fig.3.

(b)

Electron d i f f r a c t i o n patterns of the [001] incidence. (a) is the pattern of the 81 phase. (b) is an anomalous pattern which appears before the formation of the 18R type martensite.

Vol.

i0, No. i0

STRUCTURAL

Z = 0

z

=

PHENOMENA

IN Cu-Ai-Zn

899

1/2

0 h5 : B 1 L A T T I C E

Z = 0 D l h3

Z =

Z = 1/2

: DISTORTED

0

D 13 2h

LATTICE

Z = 1/2 : 2H

LATTICE

Y

~ D DQ oi 00, Do O1

t

Z

Clh

=

0

Z

: DISTORTED

=

Z ~ 0

1/2

Clh~

LATTICE

Z : 18R

I/2

LATTICE

Fig. 4. Formation model of the 2H and 18R type martensites phase. Large and z=0 (or 1/2) and mean the A and B same symmetry as

in the Fe3AI type 61

small circles correspond to the atoms on the planes z=i/4 (or 3/4), respectively. Solid and open circles kinds of the atoms. Note that the martensite has the the distorted lattice in each case.

FIG.

I•11" o 0 o 0

~ee 0 o ° 0

?oooo

?oOo o

o

v

000

o

--

220 (8 I ) (a)

o

Schematical intensity distributions calculated by the intensity formula in the text. Solid and open circles correspond to the fundamental and superlattice reflections. (a) and (b) are the patterns of the (IT0) andl3 {001) incidences of the D2h

o

v

[1101

000

400 (S1 ) (b)

5

H

1 type distorted and C2h tices, respectively.

lat-