INTERNAL FRICTION STUDY OF THE γ PEAK IN COLD-WORKED NIOBIUM

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INTERNAL FRICTION STUDY OF THE γ PEAK IN COLD-WORKED NIOBIUM

F. de Lima, W. Benoit

To cite this version:

F. de Lima, W. Benoit. INTERNAL FRICTION STUDY OF THE γ PEAK IN COLD- WORKED NIOBIUM. Journal de Physique Colloques, 1981, 42 (C5), pp.C5-85-C5-90.

�10.1051/jphyscol:1981512�. �jpa-00220989�

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JOURNAL DE PHYSIQUE

CoZZoque C5, supple'ment au nO1O, Tome 42, octobre 1981 page C5-85

INTERNAL FRICTION STUDY O F THE y P E A K IN COLD-WORKED NIOBIUM

I n s t i t u t de Ge'nie Atomique, Swiss Federal I n s t i t u t e o f Technology, EcubZens CH - 1015 Lausanne, SwitzerZand

A b s t r a c t . - The y peak due t o t h e double k i n k generation (DKG) along screw d i s l o c a t i o n s has been observed i n h i g h p u r i t y niobium. This peak appears a t 270K ( 1 Hz) and i s unstable. Some s p e c i a l thermomechanical treatments have been r e a l i z e d i n order t o develop and s t a b i l i z e t h i s peak.

The e v o l u t i o n o f t h e i n t e r n a l f r i c t i o n spectra has been s t u d i e d as a f u n c t i o n o f these treatments. The screw d i s l o c a t i o n s network c r e a t e d by low temperature deformation i s unstable and rearranges i n two stages: a W stage (180K-240K) and a X stage (280K-350K).

The t r a n s f o r m a t i o n accomplished i n t h e W stage and t h e e f f e c t o f t h e y peak s t a b i l i z a t i o n treatment have been observed by transmission e l e c t r o n

microscopy.

1. I n t r o d u c t i o n . - The p l a s t i c deformation o f a r e a l s o l i d i s t h e macroscopic consequence o f d i s l o c a t i o n motion. The m o b i l i t y o f these d i s l o c a t i o n s i s i t s e l f connected w i t h d i f f e r e n t microscopic processes. These processes t o overcome obstacles can occur simultaneously. Some s o l i d s , due t o t h e s p e c i a l c h a r a c t e r i s t i c s o f c e r t a i n defects, present a few processes t h a t occur almost i n an i s o l a t e d manner. I t i s t h e case of t h e bcc t r a n s i t i o n metals, i n which t h e s p e c i a l p r o p e r t i e s o f screw d i s l o c - a t i o n s (1) a r e r e s p o n s i b l e f o r an increase o f t h e c r i t i c a l shear s t r e s s (CSS) w i t h decreasing temperature (2). The temperature from which t h e CSS becomes athermal i s c a l l e d t r a n s i t i o n temperature (Tc). A t Tc t h e screw d i s l o c a t i o n s g e t m o b i l e under a low a p p l i e d s t r e s s through t h e t h e r m a l l y a c t i v a t e d DKG. During i n t e r n a l f r i c t i o n measurements such DKG along screw d i s l o c a t i o n s i s r e s p o n s i b l e f o r a r e l a x a t i o n phe- nomenon c a l l e d t h e y peak. This peak and i t s r e l a x a t i o n c h a r a c t e r i s t i c s have recent- l y been e x t e n s i v e l y s t u d i e d i n d i f f e r e n t bcc metals (3-6). The i n s t a b i l i t y o f t h i s peak i s r e l a t e d t o t h e rearrangement o f the screw d i s l o c a t i o n network, which i s t h e s u b j e c t o f t h i s study.

2. I n t e r n a l f r i c t i o n . - 2.1.- hmpl$-p[epamaAon.- The niobium samples from M a t e r i a l s Research Co. have 100 atppm Ta as t h e main m e t a l l i c i m p u r i t y and 400 atppm o f i n t e r - s t i t i a l i m p u r i t i e s (H,O,C and N). The sample p u r i f i c a t i o n has consisted i n a decar- b u r i z a t i o n treatment ( 3 hours a t 2173K under an oxygen pressure o f T o r r ) f o l l o w - ed by a h i g h temperature annealing ( 3 hours a t 2500K under a vacuum pressure o f about 7x10-lo T o r r ) . A f t e r t h i s p u r i f i c a t i o n treatment t h e i n t e r s t i t i a l c o n c e n t r a t i o n

*On leave from IPEN, Sao Paulo, B r a z i l

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981512

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C5-86 JOURNAL DE PHYSIQUE

should be about 100 atppm (3).

2.2. _Exewjmw_t.- The i n t e r n a l f r i c t i o n and frequency measurements were c a r r i e d o u t i n an i n v e r t e d t o r s i o n pendulum under a frequency o f 1.3 Hz and i n a temperature i n t e r v a l from 77K t o 347K. The maximum surface s t r a i n amplitude was and t h e h e a t i n g r a t e was 1°/min.

2.3. Results.- - - - - - - - 2.3.1. y, al anda2 peaks .- The y peak associated t o t h e t h e r m a l l y a c t i v a t e d DKG along t h e screw d i s l o c a t i o n s have been observed r e c e n t l y i n niobium

(3). I t appears a t 270K a t low frequency ( 1 Hz) according t o t h e p l a s t i c deformation observations (TC=230K).

A systematic study o f the e f f e c t o f deformation amplitude and deformation tem- p e r a t u r e has shown t h e optimum deformation c o n d i t i o n s t o observe an i m p o r t a n t r e l a x - a t i o n . They are: 160K < Tdef < 200K and E~~~ = 5%.

The y peak g e n e r a l l y anneals o u t i n a temperature range s i t u a t e d near t h e peak temperature. A s p e c i a l thermomechanical treatment has been performed i n order t o s t a b i l i z e the d i s l o c a t i o n network: a room temperature predeformation o f 3% i n t r a c t - i o n f o l l o w e d by a s e r i e s (about 15) o f successive 5% t o r s i o n d e f o r m a t i o r s a t 200K w i t h i n t e r c a l a t e d l i n e a r annealings up t o room temperature (RT) and t h e l a s t 5 a t 347K.

The r e l a x a t i o n c h a r a c t e r i s t i c s o f the peak have then been determined:

E = (0.61 +0.02) eV, -rm= l o - " sec and i t i s 1.3 times wider than a s i n g l e Debye peak.

The al and a2 peaks have been observed a t 150K and 120K, r e s p e c t i v e l y , a t 1.3

Hz. I t i s g e n e r a l l y accepted t h a t these peaks are due t o the i n t e r a c t i o n o f hydrogen w i t h d i s l o c a t i o n s (7) .

2.3.2. I n f l u e n c e o f deformation temperature.- To p u t i n evidence the i n f l u e n c e o f t h e deformation temperature on t h e i n t e r n a l f r i c t i o n spectra, two deformation temperatures have been chosen, below and above Tc. I n f i g u r e 1 a r e presented t h e curves o f i n t e r n a l f r i c t i o n as a f u n c t i o n o f the measurement temperature, corresponding t o d i f f e r e n t treatments r e p o r t e d on t h e f i g u r e . The fundamental d i f f e r e n c e i s t h a t t h e

low temperature deformation develops

Q-I 103 1 = I 3 H z m a i n l y t h e a2 and y peaks ( c u r v e c ) ,

a A l t e r drgasrlnp b A t f e r 3 % fraotmon R T

c r \ l t e r I % forrlan 160K w h i l e t h e RT deformation i s r e s p o n s i b l e

f o r t h e al peak. The p a r t i c i p a t i o n o f the screw d i s l o c a t i o n s i n t h e a 2 and y r e l a x a t i o n s and o f t h e 71° d i s l o c - a t i o n s i n t h e a, peak seems c l e a r (3).

Concerning t h e i n t e r p r e t a t i o n o f these

T(K)

c

⁴⁰ ^zw ³⁰⁰ peaks, some r e c e n t observations a r e

F i g . 1 : I n t e r n a l f r i c t i o n s p e c t r a o f 3 s i g n i f i c a t i v e . R i t c h i e e t a1. (4) i n a- degassed samples: a) a f t e r degassing; i r o n and Yaul e t a1 . (7) i n niobium b ) a f t e r cold-work 3% i n t r a c t i o n a t

RT and c ) a f t e r cold-work 1% i n t o r s i o n a t 160K

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have observed an internal f r i c t i o n peak a t 30K (0.5 Hz) and 70K (1 KHz), respective- l y . They have been interpreted by a DKG along 71° dislocation. Moreover, Ritchie e t a1 . (4) observed a frequency variation a t 4K t h a t i s associated t o the geometrical kink migration. The connection between the influence of hydrogen on the ul and a2 peaks, and these l a s t i n t e r p r e t a t i o n s have lead us t o the following conclusion: the hydrogen i n t e r a c t s with DKG along the 71° dislocations f o r al and with geometrical kink migration in screw dislocations f o r a,. The y peak i s due t o the thermally activated DKG along the screw dislocations (3) .

2.3.3. Recovery stages.- The difference of mobility between 71° and screw dislocations i n bcc metals favours the study of the evolution of the internal f r i c t i o n spectrum as a function of the annealing treatments. As mentioned before, the screw dislocations g e t mobile under a low applied s t r e s s a t temperatures near Tc, while the 71' get mobile a t much lower temperature ( ~ 4 0 K ) . Then i t i s i n t e r e s t i n g t o inserl screw dislocations i n t h e i r "froze-in" s t a t e ( T < Tc) and follow t h e i r evolution

def

with anneal ings a t temperatures near Tc . The 1 i near anneal i ngs real ized between 180K and 350K showed two recovery stages, t h a t i s , two annealing temperature ranges durinI which the internal f r i c t i o n spectrum changes: W stage (180K-240K) and X stage (280K- 350K). Figure 2 shows the r e s u l t s obtained with a degassed sample deformed 1% i n torsion a t 160K (curve a ) and then submitted t o the l i n e a r annealings indicated on

f = 1.3 H z

Q-1 . 1 03 a : r i t e r cola-work 1% 16011 b. After annealing 200K

3 0 ^{C :}~ i t e r annealmg 240K

d ' After annealing 260 6

the figure. The arrows under the curves show the position and evolution of the u2

1 0 -

0.0

and a1 peaks during the W stage. The curve a of figure 3 has been obtained with Fig. 2: Internal f r i c t i o n spectra

?.'-y

^of^{t h e}^{1% a}^torsiontreatments: degassed a t sample 160K a ) a f t e r and submitted a f t e r cold-work anneal t o ings a t : b) 200K; c ) 240K and

d ) 260K.

- ¹ ^I ^I ^T(K) *

a degassed sample submitted to the y peak s t a b i l i z a t i o n treatment (para.2.3.1.) and

100 200 300

then deformed 5% in torsion a t 200K. After a l i n e a r annealing until 347K, t h a t i s , i n the temperature corresponding t o the X stage the curve b has been obtained. The temperature i n t e r v a l s corresponding t o the W and X stages are shown. The dashed curves marked a ' and a" show the influence of the W stage while the X stage induces the t r a n s i t i o n from curve a t o curve b. The i n s e t shows the creep stage t h a t takes place simultaneously with the W stage (curve c ) .

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A _0-1. ¹⁰³

mi

1 0 -

. lo5 a . After cold-work 5 % 200K

b Aner annealing 347K

T(K)

C

100 200 300 400

F i g . 3: I n t e r n a l f r i c t i o n s p e c t r a o f a specimen cold-worked a t RT and then submitted t o t h e y peak s t a b i l i z a t i o n treatment (para.2.3.1.). The i n s e t shows t h e creep stage ( c r = r o t a t i o n deformation measured a t t h e specimen s u r f a c e ) t h a t accompanies always t h e M stage.

2.3.4. Conclusion.- Concerning t h e thermomechanical treatments these experiments have shown t h a t the a2 and y peaks evolve l i k e w i s e even when a 1 peak presents t h e opposite o f t h i s e v o l u t i o n , w i t h samples predeformed a t RT. Then i t seems t o e x i s t one connection between the t h r e e peaks and t h e i r e v o l u t i o n .

Two d i f f e r e n t phenomena may be r e s p o n s i b l e f o r t h e e x i s t e n c e o f t h e two recovery stages, and as consequence f o r t h e y peak e l i m i n a t i o n . The f i r s t one c o u l d be a r e - arrangement o f the screw d i s l o c a t i o n s network and t h e second one, a p i n n i n g o f these d i s l o c a t i o n s by the i n t r i n s i c p o i n t d e f e c t s c r e a t e d a l s o by t h e low temperature deformation. We b e l i e v e t h a t the rearrangement i s t h e more probable process because a d i s l o c a t i o n p i n n i n g cannot be r e s p o n s i b l e f o r : a) t h e creep stage t h a t occurs simultaneously w i t h the W stage, and which i s due t o a d i s l o c a t i o n movement, and b ) the increase o f t h e a, peak amplitude w i t h annealings i n t h e temperature range corresponding t o the X stage. A screw d i s l o c a t i o n rearrangement may be an a n n i h i l a t - i o n o f screw d i s l o c a t i o n s o r a screw network change w i t h l o s t o f the screw character (8). We w i l l t r y t o r e p l y t o t h i s q u e s t i o n by transmission e l e c t r o n microscopy (TEM) observations .

3. E l e c t r o n microscopy.- 3.1. CIbJeAjye.- The main o b j e c t i v e o f t h e TEM observations was the v e r i f i c a t i o n o f t h e W stage i n t e r p r e t a t i o n : p a r t i a l a n n i h i l a t i o n o f the screw d i s l o c a t i o n s network. The i n t e r n a l f r i c t i o n measurements w i t h s p e c i a l l y t r e a t e d samples (para.2.3.1.) show a screw d i s l o c a t i o n network s t a b i l i t y ( f i g u r e 3, curve b).

The second o b j e c t i v e o f t h e TEM observations was t o v e r i f y t h i s s t a b i l i t y .

3.2. S?mplg-prwpayg_tjon.- C y l i n d r i c a l samples ( g = 3 mm, R = 5 0 mm) have been degassed by magnetic i n d u c t i o n h e a t i n g i n vacuum c o n d i t i o n s mentioned i n s e c t i o n 2.1. Some samples have been prepared a f t e r t h i s p u r i f i c a t i o n t r e a t m e n t (A), w h i l e o t h e r s were submitted t o t h e y peak s t a b i l i z a t i o n treatment (para.2.3.1.)(B). A f t e r e l e c t r o n e r o s i o n c u t t i n g we have obtained some p l a t e s (7x3x0,2 mm3) p r e s e n t i n g a <110> normal t o the surface. A f t e r t h i n n i n g treatment t h e samples thickness was about 1500 t o

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2000 A i n a r e g i o n s i t u a t e d a t 3 t o 6 pm away from t h e h o l e

3.3. Results-ayd-discujslon.- The experiment has c o n s i s t e d i n t h e c r e a t i o n o f a d i s - l o c a t i o n network by " i n s i t u " deformation a t low temperature i n t h e e l e c t r o n microscope and i n observation o f i t s e v o l u t i o n d u r i n g t h e h e a t i n g r u n . F i g u r e 4 shows

q "W -m

1

'L* 4

. ^*a

F i g . 4: D i s l o c a t i o n arrangements observed d u r i n g 1 ow temperature

" i n s i t u " deformation i n t h e e l e c t r o n microscope .

c d

f o u r photographies t h a t have been taken from a f i l m which presents more d e t a i l s . This sequence has been chosen between 40K and 222K. The f i g u r e 4a shows t h e d i s l o c a t i o n network a t 40K a f t e r two s e r i e s (deformation a t 40K and l i n e a r annealing a t 245K).

The a p p l i c a t i o n o f an i n c r e a s i n g f o r c e u n t i l 1.4N creates t h e screw d i s l o c a t i o n n e t - work shown on f i g u r e 4b. The h e a t i n g r u n has been r e a l i z e d a t constant small force ( F = O.1N) i n order t o keep t h e specimen f l a t and w i t h a h e a t i n g r a t e o f 1°/min.

Figures 4c and 4d show t h e d i s l o c a t i o n network a t temperatures i n the temperature range corresponding t o t h e W stage. We can note t h a t the l o n g screw d i s l o c a t i o n s created by the low temperature deformation have disappeared. I n t h e f i l m we observe t h a t some screw d i s l o c a t i o n s are n o t e l i m i n a t e d by t h e l i n e a r annealing a t 245K. We b e l i e v e t h a t these d i s l o c a t i o n s are r e s p o n s i b l e f o r t h e y peak.

F i g u r e 5 shows t h e e x i s t i n g d i s l o c a t i o n network a t 270K i n a sample submitted t o the y peak s t a b i l i z a t i o n treatment. We can observe a d i s l o c a t i o n network composed

F i g . 5: D i s l o c a t i o n arrangement o f a sample submitted t o t h e y peak s t a b i l i - z a t i o n t r e a t - ment (para.2.3.1.) observed i n t h e e l e c t r o n microscope a t 270K.

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by long and medium d i s l o c a t i o n s anchored i n some d i s l o c a t i o n agglomerations.

The TEM observations have shown t h a t : a ) some screw d i s l o c a t i o n s created by the 1 ow temperature deformation disappear a t the temperature range corresponding t o t h e W stage and, b ) a s t a b l e d i s l o c a t i o n network t h a t contains some screw d i s - l o c a t i o n s e x i s t s a t 270K. The p o s s i b i l i t y o f p i n n i n g may be excluded, b u t i t i s very d i f f i c u l t t o d e f i n e t h e rearrangement process.

4. Conclusion.- The i n t e r n a l f r i c t i o n r e s u l t s which have been confirmed by TEM observations are: a ) t h e W recovery stage corresponds t o a screw d i s l o c a t i o n n e t - work rearrangement and b ) t h e thermomechanical treatment t h a t c o n s i s t s i n a s e r i e s

(about 15) o f low temperature (200K) deformation i n t e r c a l a t e d by l i n e a r annealings a t RT and t h e l a s t 5 a t 347K, i s e f f i c i e n t t o s t a b i l i z e t h e screw d i s l o c a t i o n n e t - work.

References

(1) A.Seeger and C.Wuthrich, I 1 Nuovo Cimento g , 38 (1976) (2) J.E.Dorn and S.Rajnak, Trans. Met. Soc. AIME - 230, 1052 (1964) (3) F.De Lima and W.Benoit, submitted t o Phys. S t a t . S o l .

(4) 1 .G.Ritchie, J.F.Dufresne and P.Moser, Phys. S t a t . Sol. ( a ) 52, ³³¹(1979) (5) P.Asti&, J.P.Peyrade and P.Groh, S c r i p t a M e t a l l . 3, 611 (1980)

(6) H.Schultz, U.Rodrian and M.Mau1, i n : Proc. 3 r d European Conf. on I n t . F r i c t i o n and U l t r a s o n i c Attenuation, Manchester, J u l y 1979

(7) M.Maul and H.Schultz, t o be published

(8) P.Asti@, J.P.Peyrade and P.Groh, t o be published

The authors want t o thank Ph.Buffat, A.Steiner and R.Gotthardt f o r t h e TEM observations which have been made i n " I n s t i t u t Interdepartemental de M e t a l l u r g i e "

of t h e Swiss Federal I n s t i t u t e o f Technology.

This work has been supported by t h e "Fonds N a t i o n a l Suisse de l a Recherche S c i e n t i f i q u e " , subsidy n r . 2.472-0.79.