Dislocation densities in tungsten

LETTERS

TO

THE

1399

EDITOR

The significance of this picture is not only that it shows the shortest period ever resolved for alloys with “long period stacking order”, but that the stripe patterns can be observed in martensites containing high densities of imperfections. It thus definitely proves the view that the long period stacking order which appears in martensites of non-ferrous alloys is that of a genuine c~stallographic nature and not that caused by a quasi-periodic assembly of ordinary stacking faults. The authors would like to thank L. Cicotte for preparing the alloys and R. Benoit and J. Sprys for their assistance in the electron microscopy.

about 35” of (001) planes responded to etch pitting by this reagent. Etch pits were symmetrical on (001) planes and became asymmetric as the etching plane varied from (001). All observations in this study refer to {OOl) type planes. The etch pit density as a function of strain at 295°K is shown in Fig. 1. Figure 2 illustrates the appearance of the etched surfaces at different strain levels. The dislocation density after varying amounts of strain at 295°K as measured in the electron microscope from thin films of the specimens is shown in Fig. 3. Figure 4 illustrates typical dislocation structures. The dislocation density was determined by counting the number of dislocation intersections with the foil ~c~e~t~~c~oruto~ R. S. TOTH surface, i.e. counting the number of dislocation “ends”. Ford Motor Co. H. SATO Since it was not always possible to distinguish interDearborn, Michiyan 48121 sections with each surface, the total number of “ends” was counted and this number divided by two to yield References 1. For the definitionof terms deding with stackingvariants, the dislocation density. Dislocation densities measured refer to anv of the arevious publications of the 1 uresent by this technique are directly comparable to measureauthors which appearbelow. * 2. R. S. TOTH and H. SATO, Structure of martensites in p ments by the etch pit technique. The more general Au-Cd alloys, to be published. method of measuring dislocation densities in thin 3. IX. SATO, R. S. TOTR and G. HONJO, Remarks on the structure of martensites in &-Al allovs. Acta Met.. 15. 1381 films is to determine the total dislocation line length (1967). [Preceding paper.] in a unit volume of the material. The dislocation 4. P. R. SWANN and H. WARLIMONT,Acta Met. 11,511(1963). 5. Z. NISHIYAMA and S. KAJIWARA, Jcq. J&. oppl. Php. 2, density measured by the surface intersection technique 475 (1963). is approximately one-half the density measured by the 6. M. WILKENS and H. WARLIDPONT. Acta Met. 11.1099 (1963) and H. WARLIMQNTand M. W&KENS, 2. Me&k. Sb,38i line length technique.(5*6) (1964); 56, 850 (1965). The results show that the dislocation multiplication 7. Il. SATO, R. S. TOTR and G. HONJO,J. Phys. Chem. Xolids, 28, 137 (1967). rate determined by the two methods, as well as the 8. R. S. TOTH end Il. SATO, A&. Phys. Lett. 9, 101 (1966). actual dislocation density as a function of strain at 9. H. SATOand R. 8. TOTH, J. a&. P&s. 21,3367 (1966). 295”K, depends on the technique used (see Figs. 1 * Received Januery 20, 1967. and 3). It should also be noted that different etch pit solutions can give different results.(r) In the present work the dislocation density N as measured by etch Dislocation densities in tungsten* pits is approximately linearly dependent on strain E (Fig. 1) and the multiplication rate 8N/& w 5 x lO8/ As part of an investigation of the deformation and unit strain. From thin films, the dislocation density fracture of tungsten single crystals, dislocation is a linear function of strain up to a strain of about densities have been measured by both etch pitting 7% (Fig. 3), and in this region aN/& M 2.5 x lO’i/ and electron microscopy. The inter-relation between the two different measurements has recently been a unit strain. The dislocation densities determined by the two methods differ by more than two orders of matter for discussion.(1*2) Sheet tensile specimens of orientation [OlO] were magnitude. Thus, the dislocation density for the low strain values can be expressed as: prepared with the surface parallel to an {OOl) plane. After electropolishing in a 2% NaOH solution at an applied voltage of 10 V, the specimens were deformed N =N 0 E a/ 2 various amounts at 295°K. A solution of CuSO, in ammonia@**) was used for theetch pitting experiments. where No and N, are the dislocation densities at zero Etching was always carried out on a freshly eIe&rostrain and a strain e respectively. A direct relation polished surface. Thin films were produced by between the dislocation density measured by electron electropolishing and examined in a 100 kV AEI EM6 microscopy N,, and by etch pits N,, can be derived electron microsoope . using this equation and yields: A preliminary experiment contirmed the result of Wolff@) a,nd Sohadler(4) that only planes within N,, = (5 x lo2 N,,) + (1.5 x IO*) 1

”

6

+az

1400

ACTA

METALLURGICA,

m f

a

VOL.

15,

1967

5

RASTIC STRAIN ‘

%.

G-J

(b)

FIG. 4. Electron micrographs of the dislocation structure at strains of (a) 1.9% (b) 6.5% (c) 16%. Plane of foil (001).

(4

FIU. 3. The dislocation density 8s measured by trctnsmission electron microscopy a65k3function of strain at 295°K.

0

.

.

x 60,000

ACTA

1402

Consequently, the

provided

dislocation

deformed

METALLURGICA,

only low strains are involved,

density

for [OlO] tungsten

from

the etch pit density on (001) faces. These results support

the contention

determine

the

significant

error

dislocation

dislocation

properties.

if the dislocation microscopy allow

It is interesting

densities)

change of resistivity resistivity about

data

by electron

are used to calculate

of Shukovsky

0.75 x lo-l2 @&cm3

of

to note that by 2 to

of describing

with dislocation

a

values

in the present work (multiplied methods

to

can produce

absolute

densities as determined

for the different

location

density

in determining

dis-

the rate of

density from the the value

et d.(l)

differs

from

that

are grateful

work

Materials Division, through (O.A.R.),

has

been

Laboratory, A.F.S.C.,

supported Research

under

the European

by and

contract

Air

Since it was considered to be unusual that

the gold and silver films had such different deformation characters, vacuum

careful tensile tests were undertaken

deposited

the previous

single crystal

results,

show orientation silver films.

characters

for

Force

Technology Research,

United States Air Force.

References B. SHUKOVSEY, R. M. ROSE and J. WULFF, Acta Met. 14, 821(1966); ibid. 15, 391 (1967). A. S. WRONSKI and A. A. JOHNSON, Acla Met. 15,389 (1967). U. E. WOLFF, Acta Met. 6, 559 (1958). H. W. SOHADLER, ONR Contract Report No. Nom--2614(00) (April 1962). J. D. LIVINGSTON, Acta Met. 10, 229 (1962). P. B. HIRSCH. A. HOWIE, R. B. NICHOLSON. D. W. PASHLEY and M. J. W&ELAN, Elekm Microscopy df Thin Cy&zl~, p. 422. Butterworths (1965). Z. S. BASINSKI, J. S. DUGDALE and A. HOWIE, Phil. Mag. 8, 1989 (1963).

* t $ P.O.

Received February 24, 1967. Now at: Stuttgart, Germany. Now at: Ford Motor Company, 20000 Box 2053, Dearborn, Michigan, U.S.A.

Rotunda

films, (001) and

while

Drive,

2,~.

the (111)

(iii),on

mica-silver

deposition,

the substrates

evaporation

(Iii)

of gold

films, their

the deposited

being

During

the

By chemical

microanalysis

analysis

the purity of

films were found to be 99.7 wt. % and

impurities

to be zinc and silicon.

was measured

in the

contained twins.

The film

with a multiple-beam

examinations

ferometer. produced films had a well developed texture

surface

were kept at 300°C in a

of 2 x low5 mm Hg.

vacuum

major

The (001) films, their

substrates.(2)

and electron probe X-ray

thickness

1.H.

7.

below together with

by vacuum

of about

to

of

BARBARA WARLIMONT-MEIER~ P. BEARDMORES D. HULL Deportment of Metallurgy University of Livercpool

5. 6.

films were found

observed.

Two kinds of single crystal films, were prepared

for

Unlike

similar to that of the

These are described

other deformation

X-ray

2. 3. 4.

the gold

dependence

gold films.

to a thickness

AF 61(052)-689

Office of Aerospace

single

surface being (OOl), were formed on cleaved rock salt

assistance with the electron microscopy. This

On the

deposited

crystal silver films of O.P8 ,u in thickness showed a clear orientation dependence in their stress-strain

surfaces,

to Mr. D. C. Wynne

has not been found.

of

Baskinski et ~1.“) by a factor of only 2.5. The authors

properties

other hand, tensile tests of vacuum

curves.(7)

of Wronski

and Johnson(2) that the use of etch pit techniques

1967

15,

mechanical

crystals

in tension at 295°K can be determined

VOL.

as-deposited

showed

inter-

that

the

single crystalline

state,

although

they

The amount of twins was reduced by

annealing at about 1000°C for 2 hr in nitrogen stream and,

sometimes,

the films became

free from

twins.

Therefore,

such twin free films were used as tensile

specimens

for the annealed

done in several directions size of 0.3-1.5 length

by

Tensile tests were

mm wide and of 24

using

previously.“)

films.

with specimens having the

the

same

In this device,

and the corresponding

mm long in gauge

testing

device

as used

the load was prescribed

strain

measured.

For

each

kind of film, several sheets were prepared by separate evaporations tensile

and six tensile specimens

direction)

(two for one

were cut out of each sheet.

The

films prepared separately were found to give systematically different stress-strain curves. Therefore, deformation characters were compared among stressstrain curves in various tensile directions for the specimens subjected

cut out

of the same

to simultaneous

sheet

of film

and

annealing.

Several researches have been carried out on the mechanical properties of vacuum deposited single

Figure 1 reproduces, as an example, stress-strain curves for an as-deposited (001) film of 1.77 ,u thick elongated along [llO], [210] and [loo] directions, which are denoted in the figure by A, B and C, respectively. The subscripts 1 and 2 attached to

crystal 10 I

the letters refer to two different tests. For most of the specimen films tested, the tensile strength and the

Orientation dependence of stress-strain curves in vacuum deposited single crystal gold films*

gold films in a thickness range from 0.1 to but the orientation dependence of their