THE TEMPERATURE DEPENDENCE OF DISLOCATION MOBILITY IN NaCl SINGLE CRYSTALS UNDER IMPACT STRESSES

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THE TEMPERATURE DEPENDENCE OF

DISLOCATION MOBILITY IN NaCl SINGLE

CRYSTALS UNDER IMPACT STRESSES

V. Kisel

To cite this version:

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THE TEMPERATURE DEPENDENCE OF DISLOCATION MOBILITY IN NaCl SINGLE

CRYSTALS UNDER IMPACT STRESSES

V . P . K I S E L

Institute of Solid State Physics, Academy of Sciences of the

USSR, Chernogolovka, Moscow district 142432, U.S.S.R.

Abstract

-

T h e dislocation mobility in NaCl a t temperatures

T

= 4.2, 77 a n d 298 K under impact s t r e s s e s i s d i s c u s s e d in terms of conservative and non-conservative motion of jogs on s c r e w components.

T h e standard dislocation theory s u g g e s t s that high-velocity dislocation motion /1 to 121 i s principally different from that with low velocity V 112 to 1 5 both

in the dependence on temperature, 9, and on resolved s h e a r s t r e s s

&

.

It i s characterized by the dislocation v i s c o u s d r a g coefficient

determined by the interaction of mobile dislocations with electrons and pho- n o n s

(

b i s the B u r g e r s vector modulus)/l to 121. T h e numerous experiments, however, indicate d e p a r t u r e s £ r o m the principal tenets of the theory:

1. At high velocites often

v ~ / Z ; ~ ,

where 0 . 9 6 m g 4.7 /2,4,5,14/.

2. E v e n if (1) i s fuifilled and the s t r e s s amplitudes a r e maximal c m a x - ( 1 . 5 to 600)

Tp

with the maximai duration t _m-^1

(

1 0 - ~ to

IO-^)

S. d B / d T a O

i s the yield point or the proportionality limit)

11

to 7,10,12,14,15/. 3: V in ( 1 ) i s still dependent on the concentration and the s t a t e of impu- rities, point defects /4,5,7/, the dislocation density /1 to 31, the type of mobile dislocations /2,4,12/ a n d the crystai orientations /IO/, the dislocation-loop s i z e s /2/ and the v a l u e s of

/I;

a n d tm 171, T h e metals exhibit sensitivity

max

of V to magnetic field a n d its orientation with r e s p e c t to the vector

b

191. 4. T h e starting s t r e s s e s

Est

/2,12,14/ a n d the delay times td 17,131 a r e

n

always characteristic of the dislocation motion. T h e mean dislocations always grow with up to the uitimate v a l u e s of starting with which a t -

a/C,,,

dislocations begin to multiply /5,7/.

I V 1

-4

5. An i n c r e a s e of the strain rate

&

= 10 to 104 s-' d o e s not actually affect the c h a r a c t e r of the dislocation structure / l 6 / and i s , normally, foiiowed by a n increasing formation of v a c a n c i e s 117,181.

T h u s , 1 to 5 and, a l s o /1,12 to 15,19/ indicate similarity of the mechanisms controiiing V ( ~ , T ) a t al1

/Z

a d T in different crystals. T h e effect of impact s t r e s s e s on mean pathlength z ( ~a n d mean number ) ~ and, d s o d Z

/de(% ,T), rst(T),

gmax(c

,T)

and

CM(*)

of mobile s c r e w dislocations in pure

(c

= 0.5 M P ~ ) a n d impure

(/i.

= 2 M P ~ ) NaCl single

P P

c r y s t a l s h a s been studied a t T = 298,77 and 4.2 K ( ~ i g s 1 to 4 ) using the

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CIO-530 JOURNAL

DE PHYSIQUE

Fi0.1. Change of mean pathlength

e

(

)

,

the number of mobile dislocations n ( c

)

in pure NaCl at different fali heights H (mm) =

2 0 0 ( 2 ) , 5 0 0 ( 3 ) , 1 0 0 0 ( 5 ) , 2000(7).

Tp

i s the proportionality limit, T = 298 K.

techniques described in detail /6,7,13/. t

m z ~ - O ' l

( H i s the sample or the anvil fail height) and i s approximately constant along the length of the loaded samples/6,7/. At the loading end of the sample m - - ~ 0 ' 6

(+

20

%)

and i s decreasing linearly to zero a t the free end /6,7/. In pure NaCl

e s c r e w a t dl ?\ and

% ,

the s p r e a d of data points from sample to sample 121 increased with decreasing T . F i g s 1 to 2 present

e ( ~ )

,

n ( / t ) T of mobile dislocations a t

T = 29%, 4.2 K,

/C-

averaged a t H, t =

m

const up to the beginning of multiplication a t

Z N I ( ~ ) .

T h e data a t 77 K a r e the same a s for 298 K. T h e data treatment in

(

1

)

yields B

(

4.2 K ) 7 B ( 298

K)

7 B (77

K ) ,

like in 13 to 71, however, ordy for one of 11 samples tested at 4.2 K B (4.2 K ) ( ( B ( ~ ? K ) , like in /2,8/. At al1 T c s t ~ b ( Z N I (O(=const) and

&(

) T

gr0wS synchronously with

n

('ZJ

leTt

like it i s observed at

Z

( T )

113; Klsel, to b e published/. It i s ccara-P cteristîc that even if ( 1 ) i s fulfilled, there a r e usually revealed the regions of weak and strong sensitivi of

e

,

n to

2

a t aii 'T', charaîterined by two different dependences d c / d % ( r , 'T') a t aii T and in crystals of different impurity content T h e dislocation c r o s s slip i s often observed, a s well as the motion of dislocations in the direction opposite to the acting force i s manifested 1131. Under impact s t r e s s e s the crystals a r e frectured a t 4.2 K, sometimes without the observable motion of dislocations. T h e s t r e s s of the .crack formation

CF

i n c r e a s e s with a growth of the defor-

d C

ming s t r e s s rate

6 (

insert in ~ i . g . 2 ) . Inasmuch as B % the data of F i g s 1 to 4 demonstrate al1 the diversity of the values and dependences

B

(

T ) in different NaCL. With a n increase of H ( o r

6 )

dc/de

i n c r e a s e s monotonically with growing

2:

irrespective of T for the weak-sensitivity region of

f ( <

),

whereas for the strong-sensitivity region of t f ( 2

)

it initially increases, and then drops at all T. An increase of &max(c)'T' ( a n d of n) and increase

-

then d e c r e a s e of d c / d i with growing

.%

for individuai diçlo- cations ( ~ i g s 1,2) a r e anaiogous to crystal softening a t the 1 s t a g e of macro- deformation /15,20,21/ or to a negative rate sensitivity of the plastic flow at a n i n c r e a s e of

&

( a n d dl T). In macroscopic c a s e d Z / d i i s analogous to the hardening coefficient. S u c h discrimination between the types of dislocation drag a t ultrasound loading w a s performed in 181, and the data of /9,13,22/ confirm the similarity of the mechanisms controiiing the mobility of dislocations a t different types of loading.

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3 2 8 0 ( 9 ) . Insert: the s t r e s s

TF

of

s

ming s t r e s s rate

6 .

c r a c k formation asafunction of defor-

Fi 3 Ultimate v a l u e s of the mean

*

_{dislocation pathlengths}

_&

_max

_{( b e f o r e}

multiplication) a s a function of s t r e s s ,

.

temperature, strain r a t e s a n d impurity

content in different NaCl. c ~ 0 . 1 5 M P a

a

-6 1W - points 1,3

-

tm- ( 2 0

2

5 ) 1 0 s , 2- t _m2 5 * 1 0 - ~ s 171;

.Zp-

0.4 MPa: 4,5

-

g t i 9 . 2 - 1 0 ' ~ s 171; ~ 0 . 5 M P a

:

m 6,8,10

-

t m ~ ( 2 5 ~ 7 ) 1 F 6 s ; c P z 2 MPa: -5

1 2

-

tm& ( 6 + 1 ) 1 0 s

-

this work. 1 ' ( ~ ) ~ 3 0 0 ( 1 , 2 , 4 , 6 , 1 2 ) , 77 (3,5,8) a n d

4.2 ( 1 0 ) .

r r

of the introduced dislocations a t

L

%

L

_M

113; Kisel, to b e publishedl, a n d c s t r k

&

M, this means that unpinning,

motion, d r a g a n d multiplication of dislocations, determining the plastic fiow d o n g the hardening curve, like in the region of smailer S a n d /12,13,15 to 211, a r e closely interconnected a n d a r e controlled by the s a m e mechaniçms. T h i s conclusion a g r e e s with

I l l /

too y h e r e it w a s shown that e v e n a t small S dislocations move a n d multiply along the deformation c u r v e a t times compa- rable with t

(

H

)

in this work. Only conset%ative a n d non-conservative motion of jogs 1161 formed a t c r o s s s l i p s of s c r e w components 119,231 c a n

Z,M~CI

b e the mechanismî providing hardening and softening of a wide c l a ç s of

the

materials a t ail temperatures, s t r e s s e s , impurity concentrations as a function of

5

1131. Intensification of the c r o s s s l i p with growing impurity content o r with d e c r e a s i n g T /13,19/ a g r e e s with a n appropriate i n c r e a s e of the dislocation d r a g from the data of Fig.3.

Softening ( o r facilitation of the dislocation motion) c a n o c c u r d u e to a change of the height a n d concentration of jogs 1131 on dislocations with a change of

?'

-

Fig.3 ( 7 7 K ) a n d 113,231, deforming r a t e

-

Fig.1, /15,20,21,23/, impurity concentration /13,15,20/ a n d crystal type 119,241.

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CIO-532

JOURNAL

DE

PHYSIQUE

with the conduction electrons a n d external magnetic field in this mode1 h a s to b e sensitive to the field magnitude a n d i t s orientation with r e s p e c t to the

+

vector b 191. T h e dislocation motion a n d multiplication in normal a n d super- conducting c r y s t a l s of Zn ( a n d ~ b confirms this 1261. )

REFERENCES

/1/ Kumar, A., High S p e e d Testing, N.-Y., Interscience _? (1969 /2/ Vreeland, S. et al, Scr. M e t

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(

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2

( 1 9 8 2 ) 940; Ibid.

24 ( 1 9 8 2 ) 1564; Ibid.

a

( 1 9 8 3 )

-

1892; Ibid. 25 ( 2 9 8 3 ) 3636.

/8/ ~ o b e l e v y ~ . ~ . , Soifer, Ya.M., Altçhits, V.I., Fiz. Tverd. T e l a

2

( 1 9 7 9 ) 1172.

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18

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26 (

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/II/

Golovin, Yu.1. e t al, Fiz. ~ T v e r d . 'Pela

21

( 1 9 8 5 ) 555. 1121 Klyavin, O.V. e t al, Fiz. Tverd. T e l a 1 4 (1972) 163.

1131 Kisel, V.P., Phys. Stat. Sol. ( a ) 36 (1976) 297; p r e s e n t Conference Proceedings.

1141 Nikitenko, V.I. e t al, Zh. Eksp. Teor. Fiz. ( 1 9 7 1 ) 1780; Phys. Stat.

S O L

( a ) 7 3 ( 1 9 8 2 ) K 141.

1151 ~ a v i d s o n , D.L., Lindholm, U.S., Mater. Sci. Eng.

13

( 1 9 7 4 ) 29. /16/ Edington, J.W., Phil. Mag.

19

( 1 9 6 9 ) 1189.

1171 Klein, M.J., G a g e r , W.B., J. Appl. Phys.

37

( 1 9 6 6 ) 4112. 1181 s o b , M., Buchar, J., Bucki, M., Mater. S c i Eng.

65

( 1 9 8 4 ) L9.

1191 Smirnov, B.I., Dislokatçionnaya struktura i uprochenie kristallov, Lenin- grad, Nauka

(

1981).

1201 H e s s e , J., Phys. Stat. Sol.

9

( 1 9 6 5 ) 209; Ibid.

21

(1967) 495. /21/ Argon, A.S. e t al, Phil. Mag.

25

( 1 9 7 2 ) 1095.

1221 K a r d a s h e v , B.K., Lebedev, A.B., Nikanorov, S.P., Cryst. Res. a n d

0

+

*'

@

=

Reciprocai value of mean pathlength sensitivity to s t r e s s

d T / d

e

_-

as a function of mean

--.

_-.

-.

s t r e s s

%

a n d i t s duration, T in

a

--.

--

-

different NaCI. Designations of

2 ---.

-.

Fig.3 a n d additional points 7, 1 3 ( ~ o o K ) , 9 ( 7 7 ~ ) a n d 11 ( 4 . 2 ~ ) belong to the g r o u p s 6 to 11 and 12,13. C u w e 1 c o r r e ponds to the

weak-sensitivity of

/ ( c

)

T,

_ii"

curve 2

-

to the strong one.