INTERNAL FRICTION OF SINGLE AND POLYVARIANT MARTENSITES OF Cu-Zn-Al

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INTERNAL FRICTION OF SINGLE AND POLYVARIANT MARTENSITES OF Cu-Zn-Al

M. Morin, G. Guénin, P. Gobin

To cite this version:

M. Morin, G. Guénin, P. Gobin. INTERNAL FRICTION OF SINGLE AND POLYVARIANT MARTENSITES OF Cu-Zn-Al. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-685-C4-689.

�10.1051/jphyscol:19824111�. �jpa-00221967�

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JOURNAL DE PHYSIQUE

CoZZoque C4, suppZ6ment au n o 12, Tome 43, d6cembre 1982 page C4-685

INTERNAL FRlCTION OF SINGLE AND POLYVARIANT MARTENSITES OF Cu-Zn-A1

M. Morin, G. Guenin and P.F. Gobin

Groupe d'Etudes de Me'taZZurgie Physique e t de Physique des Matlriaurc, L.A.

341, I n s t i t u t NationaZ des Sciences AppZiqu&es, Bctiment 502, 69621 V i Z Zeurbanne Ceder, France

(Revised text accepted 11 October 1982)

Abstract.- The alloys which exhibit a thermoelastic martensitic transformation are characterised by a high damping in the martensitic phase.In the case of 18R faulted martensites,two explanations can be proposed:the movement of the interfaces between the variants or the movement of the partial dislocations present at the edge of the faults.Thesemovements can be both detected by electron microscopy.To cut between these two hypothesis,the internal friction ofa single then pblyvariant martensite has been measured.A very low internal friction is obtained in the martensite single variant whereas high internal friction is as usually obtained for the polyvariant one.This proves that the internal friction of martensite is probably due to the movement of interfaces between the' variants.

Introduction

The alloys which exhibit a thermoelastic martensitic transformation as for example Cu-Zn-Al, have a high damping in the martensitic phase (1,2,3). During the direct or reverse transformation a very high internal friction peak is observed. Besides, the damping in the martensitic phase is very small. The present paper is especial- ly devoted to the martensite damping. The martensite phase possesses two types of defects,which can move by applied stress and which can generate a high internal friction (fig. 1). These defects are the interfaces between the variants,which are very mobile,and the imperfect dislocations which limit the staking faults inside the variants ; these imperfect dislocations are also very mobile and can be seen to move when submitted to intense electron beam ( stress are thereforegenerated in thin foil due to local temperature increase ) (4).

We have compared the internal friction of a single variant martensite sample which consequently does not contain any interface,with the internal friction of the same sample,but in the polyvariant state,in order to establish what type of defect contribute to the high damping.

Experimental conditions.- The polycristalline alloy was obtained by melting in an induction furnace vitt argon protection. The 6 phase single crystal was prepared by a Bridgman technique. The single crystal was then homogeneized at 1073 K for one hour and subsequently water-quenched. The composition was Cu-13.8 atX 211-17 at; Al, which give a Xs temperature of 283K. The martensite had a 18R faulted structure.The sample,lx4x50 mm,was cut from the single crystal with a slow diamond blade saw and electrolytically polished.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19824111

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J O U K N A L D E PHYSIQUE

Fig. I - Martensite with interfaces between the variants and imperfect dislocations which limit the stacking faults.

F

Pig. 2- Tortion pendulum.

C= Counterweight F= Fly-wheel S = Sample

I I

Figs. 3 and 4- Internal friction& and

%

the change in vibration period ,as a functixn of temperature for martensite single variant (fig. 3 ) and polyvariant (fig. 4).

0

1 DO 2 00 300

-

Fig.4

.

^. ^<

'-

-4 T:K,

I-.2

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The internal friction,measured as logarithmic decrement,was studied by means of an inverted tortion pendulum. The strain amplitude was

E N =I. 1 x 1 0 - ~ and the heating rate was IKImn. Additional masses have been put on the counterweight of the pendulum (fig. 2) to apply a vertical tensile stress, o ,on

v the sample. This was done in order to obtain a martensite single variant "in situ"

by the following method

.

The sample was mounted at room temperature in the appara- tus ( f phase single crystal); a tensile stress n v of I .25 x 10-'~a was applied by the above described system,this stress was not sufficient to induce the martensite by superelasticity. The temperature is then decreased below M f temperature and a martensite single variant was thus obtain if the orientation of the sample is favou- rable. It must be noticed that this single variant is destroyed if the sample is heated an cooled down again without stress. This method is however not alway success-

ful,occasionaly,polyvariants are formed in the vicinity of the grips.

Results.- The internal friction spectrum of the same sample has been measured twice, once in the single martensite state and a second time in the polyvariant condition.

This was achieved as follow:

I - Production of a martensite single variant by cooling while appling the tensile stress 0,

.

2- Measurement of the internal friction of tile single variant during the heating cycle ( crv =0) : fig. 3.

3- Production of a martensite polyvariant by cooling again but without an applied tensile stress ( o v =O).

4- tleasurement of the internal friction of the polyvariant sample during the following heating cycle ( o v =0) : fig. 4.

During all these steps the sample remained in the pendulum.

From the spectrum showninfigure 4 and corresponding to the polyvariant sample the following general characteristics can be observed : a high damping in the martensitic phase,a very high damping peak associated with a large period anomaly during the transformation and a low internal friction in the phase. The same sample in the single variant state (fig. 3 ) exhibits on the contrary a very small damping at low temperature (about one tenth of that of the polyvariant state ) and sharper but smaller transformation peak ( one third ) . The period anomaly is also sma1ler.A small maximum in the internal friction around 200 K can be noticed which also is associated with a small change of the period ( modulus).

Disc:ussion.-

a) Internal friction in the martensitic phase.

'The results obtained in L h e figures 3 and 4 show that in the absence of martensite interfaces,the internal friction of the martensite becomes very small. It seems therefore that imperfect dislocations have a small influence on the martensite.

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JOUKNAL DE PHYSIQUE

It must, however,be verified that for the single variant state,these dislocations are well activated by the measuring StressT

.

To check this, the following p m c e s s was adopted :

-Calculation of the variant induced bya

.

-Determination of the slip plane and shear direction of the imperfect dislocations.

-Calculation of the corresponding reduced shear stress due to the measuring stressTi4 The habit plane and macroscopic shear strain were calculated by the phenomenolo- gical theory described by Kajiwara (5). By the symmetry operations the habit planes and corresponding macroscopic shear have been deduced for 24 variants. The highest algebraic Schmidt factor then determines the induced martensite variant. On the stereographic projection shown in figure 5 the characteristic directions and planes of that variant are displayed. The sample surface and the directions of the stres- ses'r and U are also put on this projection. The calculated habit plane trace G (fig. 5 ) agrees withing I " with the measured habit plane. The slip plane of the imperfect dislocations is the basal plane (001)9R and the corresponding shear direction [100] 9s

.

The reduced shear stress due to the applied measuring stress

$,

is calculated to be 1

-

^0.59^?

^.

This value of this stress is quite reasonable in order to excite the imperfect dislocations. It seems therefore,that the high internal friction of the martensite is primary due to the movement of the interfaces between the variants. However,the small maximum observed at 200 K in the single variant state could be due to the movement of the imperfect dislocations.

Figure 5 : Stereographic projection showing the calculated habit plane norna1,basal plane normal and martensite shear direction for the single variant.

b is the Burgers direction of

-

imperfect dislocation(// 100

[

^{1 9 ~ )}

G is the calculated habit plane.

-

Surface plane of the sample,and

are

also plotted.

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b) Internal friction during the martensitic transformation

The internal friction peak associated with the martensitic transformation of the single variant saclple is smaller than that of the polyvariant one. This peak is due to the appearance or disappearance of the martensite during one measuring stress cycle (2,7). Therefore, the incidence of the transformation strain on the measurement strain must be calculed : E ~ = 0.29 x E ~.This small value can easily be explained ~ ~ ~ ~ ~ . by the fact that the martensite single variant was induced by o which is completely

v

different in the stress mode from rM

.

It is probable that for the polyvariant sample some variants are much more active for T~ and this can explain the higher transformation peak.It is here interesting to mention the results obtained by De Jonghe and al. (6) who also have measured the internal friction on the martensite single variant. The internal friction measurement were done by flexural vibrations therefore the measuring stress a M was tensile or compressive stress in the same direction than

o v which serve to make the single variant. In that case,the transformation peak was much higher than for polyvariant which is well explained because a M is identical t o o v ,the influence of the induced variant is maximum.

Conclusion.- It has been shown in this paper that the high damping observed in the martensitic phase of Cu-Zn-A1 alloy is probably due to the hysteretic movement of the interfaces between the martensite plate variants. The imperfect dislocations present inside the variants play a minor role as demonstrated by the low damping observed for a martensite single variant sample. These imperfect dislocations might be responsible for the small maximum observed around 200 K

.

The transformation peak associated with the single variant is smaller than that with polyvariant,which can be explain by the fact that the measuring shear stress is different from the stress which has induced single variant.

References

I- R.R.HASIGUT1 and K.IWASAK1, J. of Appl. Phys.,

2

(1968) 2182.

2- W. DE JONGHE, R. DE BATIST and L. DELAEY, Scripta Met.,

10

(1976) 1125 3- M. MORIN, G.GUENIN, S.ETIENNE and P.F. GOBIN, Trans. J.I.M., (1981) 1 . 4- Z. NISHIYAMA and S. KAJIWARA, Trans. J.I.M.,

2

(1962) 127.

6- W. DE JONGHE, R. DE BATIST and L.DELAEY, Shape Memory Effect in Alloys, Plenum Press, New-York, (1975) 451.

5- S. KAJIWARA, Trans J.I.t4.,

17

(1976) 435.

7- J.F. DELORME and P.F. GOBIN, Met. Corr. I n d . , Z (1973) 185 ;

574

(1973) 209.