STACKING FAULT FORMATION IN A FAULTED Cu-Zn-Al - MARTENSITE

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STACKING FAULT FORMATION IN A FAULTED Cu-Zn-Al - MARTENSITE

R. Gotthardt

To cite this version:

R. Gotthardt. STACKING FAULT FORMATION IN A FAULTED Cu-Zn-Al - MARTENSITE.

Journal de Physique Colloques, 1982, 43 (C4), pp.C4-667-C4-671. �10.1051/jphyscol:19824108�. �jpa-

00221963�

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JOURNAL DE PHYSIQUE

CoZZoque C4, suppldment au n o 12, Tome 43, ddcembre 1982 page c4-667

STACKING FAULT FORMATION I N A FAULTED Cu-Zn-A1 - MARTENSITE

R. Gotthardt

I n s t i f u t de G6nie Atomique, EcoZe PoZytechnique FdddraZe de Lausanne, 1025 Lausanne. Switzer land

(Revised text accepted 18 October 1982)

Abstract : During martensitic transformation a high number of dislocations are created in the interfaces in an ordered arrangement as well as inside the martensitic phase. Both types of dislocations can be mobile for appro- priate temperature or applied stress variation. Their mobility has been studied during and after in situ-transformations by Transmission Electron Microscopy (TEM) and the results are compared with internal friction measurements.

Introduction.- The martensitic transformation in Cu-Zn-A1 alloys with compositions of about 70% Cu, 15% Z n and 15% A1 (at%) exhibit a thermoelastic behaviour. It means that the interfaces between the parent phase and the martensitic phase migrate when the temperature or the applied stress is changed. The martensitic phase is in-

ternally faulted and its cristallin structure is in general a 9R or a 18R structure, depending whether it was transformed from a B2 or a DO3 structure of the parent phase.

As shown by Chalcravorty and Wayman there must exist so-called random faults in the martensite phase, because the shift on every third basal plane alone (shuffling) can not produce the experimental determined orientation of the invariant habit phane. In fact they observed dislocation contrasts in the interfaces between martensite and parent phase with a spacing which can explain the missing angle (I).

In addition to these dislocations, a relatively high number of dislocations is always created inside the transformed phase at the edges of stacking faults. In some early studies such dislocations have already been observed within martensite crys- tals in Copper-Aluminium alloys ( 2 , 3 ) . So their existence has been known for a long time but their influence on mechanical properties is still discussed (4).

The mechanical properties of the martensitic phase are quite different from those of the parent phase. For instance the internal friction (Q-l) or damping capacity is always higher in the martensite and its dependence on the amplitude of vibration is not simple (5,6).

It was shown that the higher damping in the martensite phase compared to the matrix phase can be explained qualitatively by the movement of a large number of parallel dislocations (7). In martensitic phases such an ordered network of dislocations ap- pears as dislocations in the interfaces. In addition the dislocations inside the mar-

tensite have also a preferential orientation and can be mobile, as will be shown in the following paragraphs.

The purpose of this work is to study the mobility of both types of dislocations especially those at the edges of stacking faults by TEM during and after an in-situ transformation and to relate the results to Q-I measurements.

Experimental Procedures.- The Cu-Zn-A1 alloys were melted in a graphite crucible in an argon atmosphere. They were casted under small pressure in a cold mould through a hole in the bottom of the crucible. In this way the different metal oxides floated

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19824108

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always above the melted alloy especially during casting. The alloys were remelted two times, in order to increase homogeneity. The specimens for TEM observation were cutted by spark machining to the desired dimensions. They were heat treated to about 900°C in order to eliminate the introduced defects and then water-quenched. The samples were prethinned by jet polishing (20% H2SO4 in methanol) and thinned to perfora- tion by the conventional electropolishing technique (H3P04 saturated with CrO3 and diluted 1 : 1 by H3P04) (1).

The TEM studies were made in a Hitachi H-700 and a JEOL JSEM-200 operating at 200 kV and a Philips EM300 operating at 100kV. Some of the observations were filmed by a TV camera with E.I.C. intensifier and recorded on video tape. The difficulty was that whenever a part of the specimen transformed in the neighbourhood of the observed region, the diffraction conditions changed and the quality of the photographs t a k e n f r o m t h e T V s c r e e n w a s t h e r e f o r e o f t e n q u i t e p o o r . T o i l l u s t r a t e t h e m o v e m e n t , t h e positions oftherespectivedislocationshavebeencopiedonseperatedrawingsmadefrom thephotographs.Experimental details on the internal frictionmeasurements andthepre- parationof the specimen forthose experiments are givenelsewhere (5,6).

Results and Discussion : For a better understanding of the TEM micrographs and the respective positions of the different types of dislocations within a martensite plate a simplified three dimensional representation of a martensite plate is shown in figure 1.

Figure 1 : Martensitic plate schematically presented as a parallelepipedal volume (a) and a horizontal section through the plate (b) presen- ting the thin sample as used for TEM observations

The faults are parallel to the basal plane ABCD. The corresponding dislocations in the interfaces, e.g. in CDEG, are indicated by "St'. The dislocations at the edges of the stacking faults which terminate in the martensitecryst& are indicated by

t t p t t

.

D_ue to the special geometry of martensite plates, AB and AF are much bigger

than AD (lens shape I), there will be more dislocations P parallel to AB than to AD.

-

Consequently there is a preferential orientation in the arrangement of those dislocations; then their movement can also create a higher damping than in a disor- dered network in the matrix phase. A typical micrograph of a completely transformed Cu-Zn-A1 alloy is shown in f i ~ u r e 2.

Little is known about the parameters controling the exact position of the dislocations P. Certainly the stresses appearing during the martensitic transformation have an influence. In order to study this interaction, in-situ transformations have been observed in Cu-Zn-A1 alloys of different concentrations, all within an inter- val of about ? 3% around the above mentioned concentrations. In all samples the martensite plates are developtd in the same way. These transformations, especially the evolution of the stacking fault dimensions have been recorded on video tape and important events have been photographed from the TV screen.

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F i g u r e 2 : E l e c t r o n microscopy micrograph of m a r t e n s i t i c p l a t e s of a Cu- Zn-A1 a l l o y . The i n t e r n a l s t r u c t u r e of t h e upper p l a t e i s s c h e m a t i c a l l y presented i n f i g u r e l b .

F i g u r e 3 : Schematical pre- s e n t a t i o n of a micrograph of a m a r t e n s i t i c p l a t e c o n t a i - ning s t a c k i n g f a u l t s . T h e d i s - l o c a t i o n s a t t h e end of stackin:

f a u l t s a r e i n d i c a t e d by t h i c k l i n e s and t h e i n t e r f a c e s by h a t c h i n g ( a )

.

Successive posi- t i o n s o f - t h e d i s l o c a t i o n s between t h e two arrows i n ( a ) du- r i n g a change of i n t e r n a l s t r e s - s e s : b) t o e ) . The g r a p h i c s a r e drawn from photographs taken from a v i d e o f i l m ( f o r de-

t a i l s , s e e t e x t ) .

(b) (c) (dl (e)

F i g u r e 3 i s t h e r e s u l t of an o b s e r v a t i o n made a t a temperature of about l l O K a t which n e a r l y a l l t h e specimen was a l r e a d y transformed ( t h e Ms temperature i s a l - ways d r a s t i c a l l y lowered i n t h i n f o i l s ) . The s u c c e s s i v e p o s i t i o n s of t h e d i s l o c a - t i o n s a t d e c r e a s i n g temperature a r e drawn from photographs taken from a video f i l m . Some of t h e o r i g i n a l photographs a r e shown elsewhere ( 8 ) .

A v a r i a t i o n of t h e background i n t e n s i t y i n t h e TEM p i c t u r e s i n d i c a t e s a r o t a t i o n of t h e observed a r e a i n t h e t h i n sample which i s due t o a change i n t h e i n t e r n a l s t r e s s d i s t r i b u t i o n . T h i s change was s u f f i c i e n t t o r e a r r a n g e t h e d i s l o c a t i o n s P . Each movement of t h e s e p a r t i a l d i s l o c a t i o n s r e s u l t s i n an i n c r e a s e o r a d e c r e a s e of t h e r e s p e c t i v e s t a c k i n g f a u l t . The f i g u r e 3e shows t h e s i t u a t i o n where a s t a c - king f a u l t c o n t r a s t has completely disappeared. The r e s o l u t i o n of t h e used technique was n o t high enough t o r e v e a l what happened i n s i d e t h e i n t e r f a c e when t h e p a r t i a l d i s l o c a t i o n a r r i v e d .

Not a l l t h e d i s l o c a t i o n s i n s i d e t h e p l a t e a r e mobile, a s can be seen a t t h e lower l e f t d i s l o c a t i o n i n t h e f i g u r e s 3. A s i m i l a r o b s e r v a t i o n of moving p a r t i a l d i s l o - c a t i o n s has been made i n t h e Copper Aluminium a l l o y by varying t h e temperature with d i f f e r e n t e l e c t r o n beam i n t e n s i t y ( 2 ) .

The number of migrating d i s l o c a t i o n s and t h e d i s t a n c e s they move d e c r e a s e w i t h i n c r e a s i n g number of t r a n s f o r m a t i o n c y c l e s .

I t was of s p e c i a l i n t e r e s t t o know, whether t h e P d i s l o c a t i o n s s t a r t t o m i g r a t e b e f o r e o r a f t e r t h e d i s l o c a t i o n s i n t h e i n t e r f a c e s and i f t h e r e a r e p r e f e r e n t i a l s i t e s i n t h e specimen, where t h e m i g r a t i o n o c c u r s . T h e r e f o r e a second s e r i e s of experiments has been c a r r i e d o u t i n which t h e t r a n s f o r m a t i o n was s t r e s s induced.

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The most important observations can be described as follows :

1) Under stress first the dislocations P start to migrate before the interface it- self moves. They migrate individually in the local stress field (like in figure 3) in contrast to the displacement of the interface dislocations, which always move as a whole (figure 4)

.

2) The movement of the P dislocations has been observed preferentially near the border of the specimen and at those points, where different plates meet each other

.

Figure 4 : TEM - micrographs of martensite plates induced by an increasing applied stress from a to c (in situ transformation).

Figure 5 : TEM-micrograph of martensite plates growing into the parent phase.

At the tip of the two self accommodating variants (left side) a very thin single variant continues growing (right side).

Due to these observations the existence and the behaviour of the P dislocations can be explained, as follows : During growth, stress inhomogeneities can be accommoda- ted by the creation and the displacement of P dislocations. In the caseof self accom- modatingvariants ( 9 ) t h e t r a n s f o r m a t i o n s t r a i r . i ~ lower than for a single variant and as a consequence less P dislocations are present. In TEM micrographs such a diffe- rence in the transformation strain can be seen by the variation in the background intensity (figure 5). If an increasing external stress is applied, first for small strains, the partial dislocations P move and at higher strains the interfaces migrate (figure 4)

.

In the case of internally twinned martensite the twin dislocations inside the twin boundaries(lO),thatmeans dialocationswithin the martensite, canplay the same role as the above mentioned P dislocations in a faulted martensite and they should react as those.

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Comparison with internal friction measurements.- As mentioned in the introduction a characteristic property of all thermoelastic martensites is the high internal friction

.

The measurements as a function of the strain amplitude show a characteristic two stade dependence. Koshimizu and Benoit (11) explain the stade I at low amplitudes by a mechanical break away of dislocations and the stade I1 at higher

( E > amplitudes by the displacement of the interfaces.

During Q - ~ measurements a sinusoydal stress is applied to the specimen and regar- ding the above mentioned results 1 a n d 2 , the low amplitude internal friction is mainly due to the movement of the individual dislocations P (stade I), whereas at higher amplitude the interfaces move (stade 11). Morin et a1 concluded from their Q-I measurements in single- and polycristals that only interface migration contri- butes to Q-l inmartensites (4). The strain amplitude they used for their measurement was relatively high (stade 11), so that their conclusion is in agreement with our result.

The explanation given above canalsobeused for internal twinned martensites,because, again, two types of dislocations are present : the interface dislocations and the twin dislocations situated inside the martensite. In fact Koshimizu found in an internal twinned martensite (an equiatomic NiTi alloy) the same strain amplitude dependence of the internal friction as for an internal faulted Cu-Zn-A1 martensite (6)

.

Conclusion :

By the formation of stacking faults ending inside a martensite and the movement of the partial dislocations at their edges the crystal can accommodate small changes in the stress distribution, whereas at higher stress variations the interfaces migrate. The partial dislocation movement is probably the mechanism responsible for the high damping capacity even for vibrations with relatively small strain amplitudes.

Assuming that the dislocations in the twinboundaries of a twinned martensite correspond to the dislocations inside the faulted martensite, then the movement of these dislocations can be the general mechanism for the characteristic high damping capacity of thermoelastic martensites at low strain amplitudes.

References :

1 CHAKRAVORTY S. and WAYMAN C.M., Acta Met.

5

(1977) 989.

2 NISHIYAMA Z. and KAJIWARA S., Trans. Jap. Inst. Met.

2

(1962) 127.

3 SWANN P.R. and WARLIMONT H., Acta Met

11

(1963) 511.

4 MORIN M., GUENIN G. and GOBIN P.F., see contribution at the same conference.

5 TIRBONOD B. and KOSHIMIZU S., J. Physique

41

(1981) C5-1043

6 KOSHIMIZU S., doctor thesis, Ecole Polytechnique FGd6rale de Lausanne, Lausanne (1981).

7 GOTTHARDT R. and MERCIER O., J. Physique

9

(1981)C5-995.

8 GOTTHARDT R., Proc. 1OthInt.Congr. onElectron Microscopy,Hamburg (1982),Vol.II, p.337 9 SABURI T. and WAYMANN C.M., Acta Met

7

(1979) 979

10 HULL D.,Proc. 5thInt.Congr. onElectronMicroscopy ,Philadelphia, Academic Press,New York (1962) B-9.

11. KOSHIMIZU S. and BENOIT W., see contribution at the same conference.

Acknowledgements :

The partial support of the Swiss ~ational Science Fund through grant No. 2.835-0.80 is gratefully acknowledged.