INTERNAL FRICTION IN COLD WORKED IRON

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INTERNAL FRICTION IN COLD WORKED IRON

L. Magalas, P. Moser

To cite this version:

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CoZZoque C5, suppZ6ment au no1O, Tome 4 2 , octobre 1981 page C5-97

INTERNAL FRICTION IN COLD WORKED IRON

L.B. ~ a ~ a l a s * and P. Moser

Centre d f E t u d e s NucZe'aires de GrenobZe, D6partement de Recherche FondmentaZe S e c t i o n de Physique du SoZide, 85 X - 38041 GrenobZe Cedex, France

Abstract.- Internal friction spectrum for CEN-G and Johnson Flatthey iron has been measured as a function of preannealing temperature after identical deformation procedure. It is found that the spectrum is different when preannealing treatment was made below and above recrystallisation temperature. Ploreo- ver, two recovery stages exist after low temperature deformation, and a large

interaction peak between flexible dislocations and carbon atoms is observed after, small, 0.1 % deformation in CEN-G iron preannealed at 1073 K. The experimental data on a', Ba, y, and 6 peaks are discussed.

Y

1. Introduction.- In spite of the fact, that the general classification of the internal friction peaks in pure iron is accepted and the results are now confirmed by different groups working in this field El-111 there is still some uncertainty concer- ning B 2 , and the so-called a' (or a2) peak [2, 3, 5,

9,

101. Needless to say,

B Y ,

that the temperature range where y peak appears seems to be controversial as well. In this study the effect of different preannealing temperatures in a range of 483 to 1243 K on the internal friction spectrum obtained for two kinds of iron, namcly :

CEN-G and Johnson Matthey iron, has been investigated. The problem of iron purity is to be discussed by emphasizing similarities and dissimilarities between CEN-G and Johnson Matthey iron ; both of them were subjected into identical heat treatment and deformation procedure. A few remarks on the interaction between dislocations and point defects are presented. Furthermore, it should be pointed out that the present work was carried out in an attempt to investigate the influence of different preannealing temperatures on the internal friction spectrum in a-Fe. This means that some confirmations of previously reported phenomena are briefly discussed and a few as- pects are supposed to clarify the point.

2. Experimental procedure.- The CEN-G high purity iron (99,998

X)

was prepared by zone refining in an atmosphere of hydrogen, swaging and wire-drawing [121. The sam- ples were wires 0.6 mm in diameter, 100 mm in length of CEN-G and Johnson Elatthey iron ; both of them were preannealed for 2 hrs at different temperatures in an atmosphere of purified hydrogen and furnace cooled. Actually, seven preannealing temperatures were chosen namely : 483, 573, 673, 773, 923, 1073 and 1243 K. Let us allow to use in the text some transparent abbreviations. We shall call CEN-G iron -"G" and Johnson Matthey iron

-

"JM"

.

In addition, G673 means CEIJ-G iron preannealed at 673 K, JM 673 K etc... The mean grain size is 0.05 rmn for G670 and is greater

- - - -

*permanent address : Akademia Gdrniczo-Hutnicza, Instytut Metalurgii, Al. Mickiewiczz 30, 30-059 Krakbw, Poland

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C5-98 JOURNAL DE PHYSIQUE

than 1 mm for G1073. In this case a strong s110> texture parallel to the specimen axis was observed. Microscopic observations (made on lFW HVEM) were performed for G specimens. A tangled dislocation structure was observed for G670 while the dislocation structure of G1073 exhibits long, flexible edge dislocations with very low dis-

3

location density of 2 x

lo7

cm/cm

.

They tend to move even under small bending of the specimen in the microscope. By successive bendings these dislocations move towards <Ill> direction.

Internal friction and changes in the dynamic shear modulus were simultaneously measured using an automatic inverted torsion pendulum operated near 1.5 Hz and over a temperature range from 4.2 to 470 K during linear warm up (for details, see [ 3 ] ) -

-6

The shear strain amplitude of the measurements was 5 x 10 at the surface of the specimen. In order to suppress magneto-elastic phenomenon the specimens were subjected to an axial magnetic field of 100 Oe. The sequence of internal friction measure- Dents is the following : (i) After preannealing at different, already mentioned, temperatures and mounting in the pendulum at room temperature (RT). (ii) After annealing at 400 K for 17 hrs. (iii) After 2.5 X deformation at RT in torsion. (iv) The specimen was annealed at 400 K (because of previous measurement) and subsequently cold worked of 1.3 % at 77 K. (v) The last run was made again after annealing at 400 K.

3. Experimental results.- The main difference between G and JM iron in the first measurement after different preannealing treatments consist in the behaviour of 8

peak. This peak is only observed in G iron and is also substantially bigger for higher preannealing temperatures. Then, after additional annealing at 400 K for 17hrs the internal friction background is markedly decreased and the modulus is increased in all investigated temperature range of 4.2 - 400 K having approximately the same value at 4.2 K.

The influence of different preannealing temperatures on the internal friction spectra for specimens deformed 2.5 % at RT and 1.3 % at 77 K is shown, for G iron, on Fig. la and Fig. lb, respectively. The outstanding difference between G and JM iron after deformation performed at ambient temperature is also, as in the previous case, con- cerned with B peak. The discussion on the modulus recovery after deformation at 77 K

a

will be postponed to one of the following sections.

Let us describe, now, the most characteristic features and give some additional details.

Peak a.- The amplitude of a peak after cold work at RT (after background sub- straction) is approximately constant for the preannealings under 923 K in both sets i.e. G and JX iron. However, a peak in JM iron is always 50 % lower than in G iron. When the preannealing temperature of 923 K is exeeded a rapid decrease in the amplitude of a peak is observed in both sets. Comparatively small a peak is also observed in heavely deformed G 673 specimen i.e. after 99.8 % of the specimen's areareduction,

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served at it's shoulder [5,9,101. The shape of the complex a-a' peak is convincingly asymmetric in a d<'/d~ and 1/T plot. An exact fit of normalised a-a' peak is esta- blished between a peak obtained in this study (f

"

1 Hz) and the peak measured at

100 kHz by Takita and Sakamoto 191. Similarly, a good fit is obtained in the normalised modulus curves. It is also discovered that the shape of the a-a' peak is independent of the degree of cold work at RT, the frequency of measurement and also of the kind of pure iron (under the condition that carbon concentration < 10 atm ppm). The influence of low temperature deformation is misleading because after such a deformation

BI

peak is rising at the expence of the initial a peak. Therefore only existence of a' peak is more evident. A further evidence on the existence of a' peak has been recently discovered in microdeformation experiments by J. San Juan D61.

Peak y.- A similar relationship between Y peak and the preannealing temperature is found as in the case of

a

peak. In order to obtain a comparatively stable Y peak (for G 673) it is suggested to deal with the deformation procedure in a special manner : after 2 % prestrain at RT and 2 % CTJ at 77 K several annealings at 400 K

followed by 0.7 % JC! in torsion at 77 K should be performed. Under these exprimental circumstances the temperature of Y peak, after annealings up to 450 K, is shifted towards low temperature no more than 2 K. A small y peak is observed in G 1073

(48 hr-preanneal) after 0.1 S: CrJ performed at temperaWres helcw 200 K ; Y peak (Tmax = 290 K, f = 1.5 Hz) is accompanied here by a strong modulus defect (Fig. 3).

Peak

Ba

and

By.-

A detailed study on G 1073 preannealed for 48 hrs revealed a large interaction peak between dislocations and carbon atoms. After mounting a specimen in the pendulum a 2hr anneal at 470 K was employed. Conseqi~ently, the internal friction spectrum was perfectly flat since flexible dislocations were completely pinned by carbon atoms. After 0.1 % deformation a broad, large interaction peak is ri-

3

sing

(Q-h

= 8 x 10 ) and well defined pinning stage is observed on the modulus curve. After subsequent annealing at 470 K a flat spectrum is observed (Fig. 2). This cycle, measured at two amplitudes and marked by arrows, is reversible and the loop can be repeated dozen times. Actually, depinning can be obtained by applying rhe same 0.1 % deformation at any temperature between 4.2 and 400 K. Above this temperature a rapid pinning masks depinning. Yoreover, the results obtained after the same, 0.1 % deformation performed at different temperatures are presented on Fig. 3. This investigation on CEN-G iron preannealed above recrystallisation tempe- rature have led to general conclusion that 0.1 % deformation generate large and complex peak which consist of

6

and

6

peaks. Although, low temperature deformation

a Y

generate

6

peak and Y peak with strong modulus defect. Y

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C5-100 JOURNAL DE PHYSIQUE

tedon CEN-G iron and exactly the same peak at the same temperature let alone activa- tion energy of 0.78 eV and pre-exponential factor were found. On the strength of that we would like to put into consideration that the peak found by Shimada might have been a 6 peak.

Y

&. Secovery phenomenon on the modulus.- On the modulus curves, for G iron deformed at 77 K, there are apparently two distinct, irreversible recovery stages ; an initial stage at 270 K followed by a second at 330 K (Fig. lb). The phenomenon at 270 K was only observed after 77 K deformation of G iron ; JPI iron did not exhibit this stage. The 330 K step was observed in both sets after 77 K and 9T deformation. Similar results have already been obtained by Shimida I81 and the interpretation on 270 K

stage is consistent with

B

-a reconversion I41.

1

5. Discussion.- A satisfactory explanation for a' peak in a-Fe has not been esta- blished hitherto. Also, however, there are so far no experimental data available on the dependence of a' peak on one evident factor. Since the frequency change by a

factor of does not induce any change in the shape of a-a' peak we can implicitly assume that it is not the case as a-a' peak in Nb (Klam et al. [14] found a shift of a' peak when the frequency was changed from 0.5 to 26 Hz). In consequence, it appears more likely that a and

a'

relaxations in a-Fe have the same pre-exponential factor. This rules out any explanation in terms of the migration of geometrical kinks on dislocations, which is expected to appear at lower temperature. One has to consider the results on electron microscopy. It is confirmed that in round numbers 60 % of the lenghts of dislocations are found to have 112 <ill> Burgers vectors, 20 % <loo>, and 20 % <110> [131. On the supposition that the possibility that <loo> and/or <110> dislocations are mobile is acceptable, we are prompted to argue in favour of double kink relaxation on one of already mentionad non screw (<loo>, <110>) dislocations to account for the a' peak in a-Fe.

It is expected that the CEN-G iron has higher purity in comparison with the Johnson Matthey iron. This is because, in JM iron the a peak is always lower, no first recovery step at 270 K was observed after 77 K deformation and B peak was

a not found.

For the specimens preannealed below 923 K a tangled dislocation structure with a high dislocation density was observed. In this case a high level of internal stres- ses acts in favour of a peak. When the preannealing treatment was performed at higher temperatures long, flexible non screw dislocations were observed. The interaction of these dislocations with carbon atoms gives the broad, large peakas itwas already discussed. In fact, a flat spectrum is obtained when the flexible dislocations are pinned by carbon atoms. A deformation of 0.1 % is sufficient to push the dislocations into regions where a gradient of carbon concentration exist. This gives rise to an interaction peak

;

an additional 470 K anneal cause the pinning again

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worthwhile to emphasize it's dependence on the preannealing temperature. Fig. la further exemplifies that B peak increases gradually with increasing preannealing

-

1 C1 3

temperature up to Q = 6.6 x 10 for G923 after 2.5 % CW at RT.

A

detailed inter- max

pretation and amplitude dependence is given in [3, 11, 151.

It is to be noted that, in all cases, after an additional CB of 1.3 % at 77 K

B

peak is rising at the expence of a peak. After final annealing at 400 K a peak

1

is restaured with it's amplitude reduction of 50 %. The mechanism of the reconversion of a-f3 peak and the amplitude dependence of and 6 peak has been discussed

1 1

elsewhere[4]. Furthermore, this investigation revealed that the mechanism is %irtu- ally independent of the preannealing temperature.

6. Conclusions.- In this study the influence of preannealing treatment has been investigated. The test on the behaviour of a, Ba, and the first step on the modulus curve decidedly speak in favour of higher purity of CEN-G iron in comparison with Johnson Matthey iron. The origin of large, broad peak in well annealed CEN-G iron lies in the interaction between flexible dislocations and carbon atoms. It is sug-- gested that atpeak, in iron, can not be explained in terms of the diffusion of geometrical kinks on screw dislocations. Thus, the complex,

a'

and

a

peak is supposed to be associated with the presence of different types El31 of non screw dislocations Acknowledgments.- The authors are grateful to Dr. S. Talbot-Besnard for helpful discussion on recrystallisation in pure iron. The thanks are extended to Mr. P. Remy for his excellent technical assistance.

References.

[l] V. Hivert, P. Groh, P. ?loser, W. Frank, Phys. Stat. Sol. (a) 42, 517 (1977). [2] V. Hivert, P. Gro.h, W. Frank, I.G. Xitchie, P. Moser. Phys. Stat. Sol. (a) 46,

89 (1978).

[31 I.G. Ritchie, J.F. Dufresne, P. Ploser, Phys. Stat. Sol. (a) 59, 617 (1978). [4] I.G. Ritchie, J.F. Dufresne, P. Moser, Phys. Stat. Sol. (a) 61, 591 (1980). [5] P. AstiE, J.P. Peyrade, P. Groh, Proc. 111. Europ. Conf. Internal Friction and

Ultrasonic Attenuation in Solids, Yanchester, July 1979, p. 49. [6] P. AstiB, J.P. Peyrade, P . Groh, Scripta Plet. 14, 611 (1980). [7] P. AstiE, J.P. Peyrade, P. Groh, This Conference.

[8] "1. Schimada, K. Sakamoto Scripta Met. 13, 1177 (1979). [9] K. Takita, K. Sakamoto, Scripta Yet. 4, 403 (1970)

[lo]

I.G. 9itchie, J.F. Dufresne, P. !loser, ICIFUAS-6, Tokyo 1977, p. 701. [ l l ] J.F. Dufresne, I.G. Sitchie, P. Yoser, see [j],p. 37.

[12] F. Vanoni, ThPse, UniversitE de Grenoble 1973. [13] D. McLean, Hem. Sci. Rev. "let. LXV, 1968, p, 277.

[141 9. Klam, H. Schultz, H.E. Schaefer, Acta Vet. 27, 205 (1979). [I51 J.F. Dufresne, L.B. Magalas, P. 'loser, This Conference.

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C5- 102 JOURNAL DE PHYSIQUE

i

_--

(a) - d (b)

I

. - : d

4

:-a . , . . i L

.

. . . .

.

. . . .

;

I' . . ,: /

"

i /

'.

e ...

--__

.

e

100 200 300 LOO TIKI 103 200 300 ibO T~K:!

Fig.1-Internal friction and.normalised dynamic modulus of CEN-G iron deformed 2.5 %

at

(figure a), and 1.3 % at 77 K (figure b). 2 hr-preanneal at the following temperatures was employed : (a) 483 K ; (b) 573 R ; (c)773 K ; (d) 923 K ; (e) 1243K.

Fig.2-Internal friction and dynamic modu- =-Internal friction and normalised

-

1usofCENGiron preannealed at 1073 K for dynamic modulus of CENG iron preannealed 46 hrs after deformation of 0.1 % at RT. at 1073 K for 48 hrs and after 0.1 % deformation perforned at various temperatures : (a) 4 I;, 50 K, 100 K ; ( b ) 200 K;

(c) 300 K ; (d) 400 K.

-

1 3

Note-A Ba peak

(sax

= 14 x 10 ) is also observed in a specimen preannealed at1073K