Etch-pit observation of the dislocation motion in Cu-2at%Al single crystals

Scripta METALLURGICA

Vol. 6, pp. 1227-1230, 1972 Printed in the United States

ETCH-PIT

OBSERVATION

Pergamon

Press,

Inc.

OF THE DISLOCATION MOTION

IN Cu-2at%Al

SINGLE CRYSTALS

E. Kuramoto The Institute

for Solid State Physics,

of Tokyo, Roppongi,

Minato-ku,

(Received July 2, 1972;

The University

Tokyo, Japan

Revised October

30, 1972)

Introduction Many investigations hardening

have so far been performed

of face-centered

is important

to investigate

cubic alloys(l-16). dynamical

the present paper the motions

behaviors

of grown-in

on the solid solution

In the study of this problem it of individual

dislocations.

edge and 30 o dislocations

In

in Cu-2at%Al

alloy under a pulse stress were observed. Experimental

Procedure

Cu-AI single crystals were grown from a seed crystal by the Brid~eman method. Copper and aluminium have purities specimen has a rectangulsr Fig. i.

of 99.999% and 99.99%,

respectively.

cross section 4mmx2mm and has an orientation

The shown in

The wide side of the specimen is taken to be a (iii) plane so as to

o~serve the dislocations

by the etch-pit method.

The other three {Iii}

planes

A,B and C are also drawn and the three traces of these planes on t~e surface of the specimen are denoted by a,b and c, respectively. A stress pulse was applied on the specimen by the four-point using a magnetic

loading apparatus(17).

The effective

by 80% of the peak value and the effective interval

over which the stress

experiments apparatus,

were performed at 4.2K using a cryostat

most of the solid-solution cations at the absolute tensile or compressive

2.

hardening

pulse height.

connected

theories

predict

the behavior

In a four-point

bending

is that

of dislotest a uniform

is applied along the specimen axis and these

1227

The

to the loading

The reason for 4.2K to be adopted

zero temperature. stress

pulse height was defined

pulse length was defined by the time

is larger than the effective

which is shown in Fig.

bending method

1228

DISLOCATION

IN Cu-2at%Al

MOTION

SINGLE

CRYSTALS

Vol.

~6~.~

6, No.

12

STRESS ~ S ~ R

T~ME

(a)

FIG.

Stereographic tallographic

FIG.

i

projection orientation

2

of the crys-

A cryostat used for an

of the speci-

experiment

men and schematic representation the four-point

(b)

of

at 4.2K(schematic)

(a) and a stress pulse used

bending of the specimen.

stresses have the m a x i m a on the specimen

(b).

surfaces.

Schmid factors for nine

active slip systems are shown in Table I. Before testing the wide surfaces and electrical

polishing.

of the specimen were made flat by chemical

Then the specimens were annealed

for 48 hr in order to decrease the d i s l o c a t i o n lines almost perpendicular expected that a dislocation edge or 30 ° character shown in Table I. H20(125):Br2(0.5)

to the surface of the specimen. terminating

in vacuum

at I040°C

density and make the dislocation Consequently,

it is

at the surface of the specimen has an

in the region near the surface.

These characters

are also

The specimens were etched with an etchant HCI(30):CH3COOH(25): for 45 sec and the dislocation

density 8xlO4cm -2 was obtained.

In order to obtain the distance over which a dislocation moved the so-called double

etching method was used. Results and Discussion

Figure

3 shows an example of the dislocation motion in Cu-2at%Al

under a stress pulse of 0.5 msec width,

where arrows

large pit indicate the distance and the direction

connecting

of motion.

at 4.2K

a small and a

Such a corre-

spondence between a new and an old pit was obtained for about 70% of the total pits which moved.

In this experiment

on the surface of the specimen

the stress pulse was chosen so as to ~ive

the maximum

to 0.95~y to the B4 primary system.

tensile and compressive

~y is the resolved

critical

this alloy in a tensile test at 4.2K, which is 1.15 kg/mm 2.

stresses

equal

shear stress of

Vol.

6, No.

12

DISLOCATION MOTION

IN Cu-2at%Al

SINGLE CRYSTALS

1229

TABLE I Schmid factors and d i s l o c a t i o n characters for nine active slip systems Schmid factor

Character

B4

0.466

edge

B5

0.392

30 °

B2

0.086

30 o

A3

0.383

30 o

A6

0.280

edge

A2

0.iii

30 o

CI

0.187

edge

C5

0.107

30 °

C3

0.081

30 °

FIG. Dislocation motions

3

in Cu-2at%Al at 4.2K u n d e r a stress pulse

of 0.95~y and of 0.5 msec length.

Figure 4 shows the h i s t o g r a m r e p r e s e n t i n g the r e l a t i o n between the n u m b e r of d i s l o c a t i o n s and the distance of m o t i o n for the three directions a,b and c. distances are w i d e l y d i s t r i b u t e d in the range from 5~ to 180~.

The

It seems likely

that some of the d i s l o c a t i o r s moved over a l a r g e r distance than 180~ and lost the correspondences

to the initial sites.

v e l o c i t y is l a r g e r than 36cm/sec. along a and b d i r e c t i o n s are almost

This shows that the m a x i m u m

dislocation

The total numbers of d i s l o c a t i o n s w h i c h m o v e d equal, which seems c o n t r a d i c t o r y to the fact

that Schmi@ factors B4,B5 and B2 are c o n s i d e r a b l y l a r g e r than A3,A6 and A2 in Table I.

To explain this result it must be considered that edge dislocations are

difficult to move and only 30 o d i s l o c a t i o n s m o v e d u n d e r a pulse stress.

This

suggests that the size i n t e r a c t i o n between a d i s l o c a t i o n and solute atoms play

1230

IN Cu-2at%Al

DISLOCATION MOTION

t

Cu_2atoi, A !

L

o'=0.95~

B

Vol. 6, No. 12

an important role in the solidsolution hsrdenin~ of this alloy.

4.2 K

,-~

SINGLE CRYSTALS

A t =0.5xlO-~ec

Acknowledgement§

,fL .... ~

7:

.

.

.

.

.

.

.

The author wished to thank Professors T. Suzuki aPd S. Takeuchi for valuable discussions arld also thank Mr. T. Hash~moto and Miss M. Watanabe for their help in the experiment.

.

A

n=TI SO

tOO

150

A histogram representing the relation between the number of dislocations

n=8 DISTANCE OF MOVEMENT

(~)

FIG. 4

and the distance of motion for three directions a,b and c (4.2K). n is the total number.

References i)

H. Suzuki, Dislocations and M e c b a n i e ~ Properties of Crystals, p.361, J. C. Fisher et al. (eds.), John Wiley & Sons Inc., New York (1957).

2)

R. L. Fleischer,

Acta Met. ii, 203 (1963).

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4)

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5)

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ll)

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Jr.,