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Submitted on 1 Jan 1985
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INTERNAL FRICTION TIME DEPENDENCE OF
Cu-Zn-Al MARTENSITE
M. Morin, A. Vincent, G. Guenin
To cite this version:
JOURNAL D E PHYSIQUE
Colloque C10, suppl6ment au na12, Tome 46, decembre 1985 page C10-625
INTERNAL FRICTION TIME DEPENDENCE OF Cu-Zn-A1 MARTENSITE
M. MORIN, A. VINCENT AND G. GUENIN
Groupe dlEtudes de Metaflurgie Physique et de Physique des Materiaux, LA 341, INSA de Lyon, Villeurbanne,France
6 s d .
-
Dans une r6cente publication, nous avons relev6 une dbpendance de llamrtissement du Cu-Zn-A1 avec le nombre dloscillations de mesure. ~ o u s proposons maintenant un dcanisme bas5 sur la migration des d6fauts ponctuels quirend m p t e de cet ef fet.
Abstract.
-
In a previous paper we have reported the internal friction dependence of Cu-Zn-A1 with the number of measuring oscillation. m explain this effect, we propose a mechanism based on migration of point defects.When measured at constant temperature below %, the internal friction of Cu-Zn-Al alloys depends on measuring time. This effect has been observed by the present authors at 0.05 Hz and 1 HZ /1,2/ and by J. Van HUMEiEECK at higher frequency
(40 Hz) /3/. In this paper, the characteristics of the internal friction will be briefly reviewed and a mechanism based on point defects migration will be proposed to take these characteristics into account.
All the measurements have been done at constant temperature to suppress the internal friction component depending on temperature rate
.
The characteristics of the remaining damping (IF2) are the following:1
-
When measured at low frequency (1 Hz or 0,05 Hz) the damping decreases with the oscillation nmber to reach a steady state which has been called IF1 (Fig. 1) /1/. At higher frequency (40 Hz) /3/ the damping first increases before the decrease to reach IF1 (Fig. 2).2
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The elastic mcdulus follows the same evolution but in the opposite way (Fig. 1).3
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The damping and the elastic mcdulus can been partially restored when the measuring oscillation is stopped during some time (fig. 2) or when an amplitude jumpis temporarily done /3/.
4
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At high frequency (40 Hz) the time corresponding to the damping maximum and the decay time decrease when the temperature increases (fig. 2).5
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When the frequency increases the decay time increases (fig. 1 and 2). 6-
When the anplitude increases, the decay time increases (fig. 3). The internal friction general increase is due to the IF1 increase /I/.7
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The internal friction decrease can be observed either in the full martensite state or in the transformation temperature domain /1/.C10-626 JOURNAL DE PHYSIQUE
The following proposed mechanism is based on the progressive pinning of the interfaces by point defects which are probably vacancies. Indeed it has been experimentally shown /4/ that the interfaces between variants are responsible for the high damping in martensite. Before the internal friction measurement, the sample being at rest, the interfaces are partially pinned by the vacanchs. h e n the measuring stress is applied the interfaces move and drag the vacancies which are on their swept volume. During the successive oscillations the vacancies of the swept volume will gather in the regions which correspond to the extreme positions occupied by the mobile interfaces. After a given number of oscillations, the vacancy concentration in these regions can be sufficient to pin the interfaces.
The process is therefore divided in two steps :
a
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Gathering of the. vacancies in the regions corresponding to the extreme positions of the interfaces. This gathering will induce an increasing phase lag between the stress and the strain and therefore an increase of the damping.b
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When the vacancy concentration is sufficient at the extreme position, the interfaces can be pinned which induces an abrupt decrease of the damping and increase of the modulus.The consequence on internal friction of this process for an unique interface is schematized on fig. 4a. In fact this curve is not observed due to the distribution of size and orientation of the interfaces. This distribution will provide a progressive pinning and will induce a behavior schematized on figure 4b which has been observed by V. HlM3EECK (Fig. 2). The other characteristics of the internal friction are qualitatively well explained by this process :
1 and 2- At low frequency a single decrease of damping (and increase of modulus) is observed. This can be explained by the fact that the main part of interfaces are pinned after the first half cycle. Their low velocity allows to drag a large quantity of va-cies (fig. 4c).
3- The damping restoration when the sample is at rest can be explained by the rearrangement of vacancies inside the interfaces. In the case of amplitude jump, the vacancies are gathered in different places which start again a new process.
4- The curves obtained by Van EKMEEKX /3/ are displaced towards long times when the measuring temperature decreases. This characteristic is explained by the fact that the vacancy diffusion cccurs in the process'. Therefore a lower temperature makes the process slower.
5 and 6- The process is slower when the amplitude is higher. The measuring stress being sinusoXa1, the movement of the interfaces must also be sinusoIda1. The dragging process being vacancy diffusion controlled is certainly more efficient when the velocity of the interfaces is snall. If the dragging occurs significantly only if the interface velocity is lower than a given value vo (for a given temperature), the effective relative time during a period is given by :
t 4 Yo I
tr=
-
=-
arcsin-
T 2K
T is the oscillation period.
k c o is the maximum interface displacement.
This expression also predicts a faster process when the oscillation per id increases. This is coherent with the fact that at low frequency the maximum is not observed.
Conclusion :
The proposed mechanism reas~~ably explains all the characteristics of the component IF2 of the Cu-Zn-Al martensite. More quantitative experiments are on the way to elaborate a quantitative model and to verify the nature of the pinning defects.
References:
(1) M. MORIN, G. GUENIN, S. ETIENNE, P.F. GOBIN, Trans. T.I.M.,
3,
no 1, (1981) ,l. (2) M. MORIN, G. GUENIN, J. Physique,44,
(1983), C9-250.(3) J. VAN HWBEECK, L. D E W , J. Physique, 43, (1982), c4-691. (4) M. MORIN, G. GUENIN, P.F. GOBIN, J. Physique,
43,
(1982), C4-685.C10-628 JOURNAL' DE PHYSIQUE
Fig.3.- I n t e r n a l f r i c t i o n versus t i m e f o r d i f f e r e n t a m p l i t u d e measurement /3/.
Frequency = 40 Hz.
