ELASTIC AFTER-EFFECT AND INTERNAL FRICTION IN HIGH PURITY IRON

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ELASTIC AFTER-EFFECT AND INTERNAL FRICTION IN HIGH PURITY IRON

J. San Juan, Gilbert Fantozzi, J. Rubianes, C. Esnouf, M. No

To cite this version:

J. San Juan, Gilbert Fantozzi, J. Rubianes, C. Esnouf, M. No. ELASTIC AFTER-EFFECT AND INTERNAL FRICTION IN HIGH PURITY IRON. Journal de Physique Colloques, 1985, 46 (C10), pp.C10-305-C10-308. �10.1051/jphyscol:19851068�. �jpa-00225452�

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JOURNAL DE PHYSIQUE

C o l l o q u e C10, supplBment a u n 0 1 2 , Tome 4 6 , decembre 1985 page C10-305

ELASTIC AFTER-EFFECT AND INTERNAL FRICTION IN HIGH PURITY IRON

J. SAN JUAN', G. FANTOZZI, J. RUBIANES, C. ESNOUF AND M.L. NO ' D e p t o . F i s i c a d e l E s t a d 0 S o l i d o , F a c u l t a d d e C i e n c a s ,

U.P.V. A p t d o 6 4 4 , B i l b a o , S p a i n

GEMPPM - INSA d e L y o n , B Z t . 5 0 2 , 6 9 6 2 1 V i l l e u r b a m e C e d e x , F r a n c e

R6sum6

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Nous avons dtudi6 la structure du pic a + a ' par mesures de frot- tement interieur, de microfluage et de relaxation anelastique. Quatre compostantes apparaissent : la composante basse temperature est attribuee au mouvement des d6crochements g6om6triques sur les dislocations vis tandis que les composantes haute temperature sont dues i la creation des decrochements g6omGtriques sur les dislocations non-vis.

Abstract

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The structure of the a + a ' peak has been studied by internal friction (IF), strain relaxation and microcreep experiments. Four components appear. The low temperature component should be due to the motion of geometrical kinks on screw dislocations ; the three other components are attributed to double-kink generation (DKG) on non-screw dislocations.

I - INTRODUCTION

The interpretation of the low temperature IF spectra in the bcc metals in always a controversial question : the a peak shows a complex structure /1-3/. Recently, we have studied the structure of the a peak in pure iron by microcreep experiments /4/.

The high 'temperature components al, a 2 and a4 are attributed to the double-kink formation on non-screw dislocations on (110) and (112) planes whereas the low temperature component a 4 situated near 10 K is attributed to the geometrical kink motion on screw dislocations /5/.

The stress used during the microcreep experiments are much higher than the one used for the IF measurements ; thus, it i s difficult to compare the obtained activation energies and the proposed interpretation is still controversial. So, we have performed some relaxation strain experiments to specify this point.

I1

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EXPERIMENTAL METHODS

The measurements are made on an inverted torsion pendulum described elsewhere /5/, on plates (0.5 x 5 x 40 mm3) of CENG pure iron /6/. Specimens are firstly predeformed by 3 % tension at room temperature in order to suppress the 6, peak /5/

; then, they are deformed by torsion in situ. The elastic after-effect experiments are conducted following the sequence : a) a torsion bias stress is applied at high temperature (30 K) during a time enough for obtaining the complete anelastic strain,

b ) with the bias stress, the temperature is decreased up to 4 K, c) the temperature

is increased up to the wanted value Tm and then stabilized at f 0.1 K,d) the bias stress is removed and the strain relaxation is measured. The obtained relaxation curves are analyzed by the method of decomposition of exponentials /7/.

I11 - RESULTS AND DISCUSSION

The elastic after-effect experiments have been performed on a sample plastically deformed by tension of 5 % and torsion of 5 % at 290 K (the a peak is well

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19851068

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C10-306 J O U R N A L D E PHYSIQUE

developed). We have realized ten tests between 6 K and 23 K with a bias stress a s =

2 X 10-4 p

.

The analyze of the curves show the existence of three relaxation times and the results are shown by the Arrhenius diagram of the Fig. 1. The obtained activation energies are compared with the ones obtained by microcreep /4/.

We can note that the energies obtained by microcreep are lower than the ones obtained by relaxation experiments, which is in agreement with the proposed interpretation /4/ (stress effect on the activation energy). The IF spectrum has been measured on a sample deformed by tension of 3 % and torsion of 4 % at 290 K. From the shift of the high temperature part of the a peak as a function of frequency, we can measure the activation energy of the high temperature component, after removing the background /5/ : E( al) = 0.048 eV. This activation energy is higher than the value obtained by relaxation strain ; when the stress is applied before the elastic after effect, the dislocation moves through a number of valleys higher than for an IF experiment. When the stress is removed, the energetic diagram of the dislocation tilts and the dislocation moves towards the minimum energetical position correspon- ding to a zero applied stress, or to the IF experiments. So, the activation energy for the relaxation strain is smaller than for IF. Furthermore, the activation energies obtained by relaxation strain for the very long times of relaxation are nearer the ones obtained by IF, the dislocations drawing near to the energetic minimum.

The IF activation energies for the two other components are calculated by conside- ring that the ratio is the same than for the relaxation energies. (Table 1). The three components seem to present the same preexponential factor T o in all cases ; so, we choose for TO the value dbtained for the a peak /5/.

We have studied the decomposition of the IF spectra by supposing that the 1/T peaks are symmetrical and have the same broadening factor 6 . An example of decomposition is shown on Fig. 2 : besides the three components al, "2 and "3, One can note the presence of another component "4 around 10 K which seems equally to show a substructure. Thisaq component can be attributed to the geometrical kink motion on screw dislocations. It disappears when a static stress is applied at high temperature /5/ (fig. 3). Concerning the a l l a 2 and a 3 components, they are attributed to the DKG on non-screw dislocations gliding on different slip planes.

TEM observations /8/ show that the glide plane of <Ill> non-screw dislocations are (110), (123) and (112). Therefore, two hypothesis can be proposed for the interpretation of the three IF components :

-i- The DKG on (110) planes for the a 3 component and the DKG on (112) planes with twinning and anti-twinning for the a1 a n d a 2 components,

- i i - The DKG on (110) planes for a 3 and DKG on (112) and (123) planes for a 1 and a2.

The first hypothesis seems reasonable because there is a good correlation between the activation energies and the Peierls stresses calculated by Yamaguchi and Vitek /4,9/. Furthermore, the relation between the relaxation strength of the a 1 and a components can be interpreted by the asymmetry of the Peierls barrier on (112f planes /5,8/. Nevertheless, the second hypothesis must be taken into account and complementary TEM observations are necessary to specify the dislocation motion on (123) planes. On the other hand, some atomistic simulations of the Peierls stress in these planes are needed.

Finally, the a 3 component is developed by deformation at low temperature and annealing (in order to remove the $ 1 component) and this result agrees with the increase of the dislocation density on (110) planes observed by TEM observations.

REFERENCES

/1/ Fantozzi, G., Ritchie, J.C., ICIFUAS-7, C5 42 (1981) 3.

/2/ Grau, R., Schultz, H., ICIFUAS-7, J. Phys., 42 ^{(1981) C5}

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^49.

/3/ Ziebart, U., Schultz, H., ICIFUAS-4, J. Phys. C9, 3 (1983) 691.

/4/ San Juan, J., Fantozzi, G., Esnouf, C., Vanoni F., Bernalte, A., ICIFUAS-4, J.

Phys., 3 (1983) C9

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^685.

/5/ San Juan, J., These INSA DE LYON (1984).

/6/ Vanoni, F., These Universitk de Grenoble (1973).

/7/ Gibala, R., Wert, C.A., Acta Met., 14 (1966) 1095.

/8/ San Juan, J., Tesis Universidad delTais Vasco, Bilbao (1985).

/9/ Yamaguchi, M., Viteck, V., J. Phys. F : Metal Phys. 5 (1975) 11.

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Fig.1.- Arrheni us diagram f o r t h e r e l a x a t i o n : r e s u l t s o b t a i n e d from I F

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r e s u l t s deduced from mi c r o c r e e p

+

r e s u l t s o b t a i n e d by e l a s t i c a f t e r - e f f e c t

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JOURNAL DE PHYSIQUE

Fig.2.- i n t e n s

Decomposition o f t h e a peak f o r a sample deformed i o n o f 3% and i n t o r s i o n o f 4% a t room temperature.

Fig.3.- E f f e c t o f b i a s s t r e s s on t h e l o w temperature s i d e o f t h e a peak: as a p p l i e d a t 4K; ( 2 ) o s a p p l i e d a t 78K;

( 3 ) d i f f e r e n c e between (1) and ( 2 ) ( os = 5 x 1 0 - = ~ )