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

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

LETTERS

TO

THE

1403

EDITOR

elongation were 12-16 kg/mm2 and 0.5-l %, respectively. The work hardening rate, defined as the slope of the curve, is very high (1 - 4 x 103 kg/mm2 per unit elongation), and is large in the order of the tensile directions [210], [loo] and [IlO]. The deformation occurs most easily for the tensile direction of [llO]. Figure 2 shows, as an example, stress-strain curves for an as-deposited (lfi)film of 2.05 p thick. The tensile directions are inclined by 5”, 15” and 25” from [llO] towards 12111and they are denoted by A, B and C, respectively. Similar to the case of the as-deposited

2

b

Tensile

strain,

%

Fro. 2. Stress-strain curves of an as-deposited (1TT) gold film of 2.05 p in thickness. All the tensile specimens were taken from the same sheet. A, B and C indicate tensile directions inclined by 5”, 15” and 25” from [llO] towards 12111, respectively. Subscripts 1 and 2 refer to different tests. End points of the curves represent the fractured points.

1.0

0.5 Tensile

strain,

8-13 kg/mm2 and the elongation l-7%. Comparing these values with those of the as-deposited (001) films, it can be said that the annealing makes the elongation large and the tensile strength small appreciably. The shape of the curves reveals that there are two stages

%

FIG. 1. Stress-strain curves of an as-deposited (O#l) gold film of 1.77 p in thickness. All the tensile specimens were taken from the same sheet. Tensile directions are [llo], [210] and [loo], and they are denoted by A, B and C, respectively. Subscripts 1 and 2 refer to different tests. End points of the curves represent the fractured points.

(001) films, the curves are different with the tensile directions. For most of the specimens tested, the tensile strength was as high as 35-40 kg/mm2 but the elongation was as small as l-Z%. The work hardening rate is also very high up to high stress (3-5 x 163kg/mm2 per unit elongation). The specimen deformation occurs more easily for the B direction than for the A and C directions. Figure 3 shows, as an example, stress-strain curves obtained for an annealed (001) film of 2.60 p thick. The initial tensile directions are [llO], [210] and [loo], which are denoted by A, B and C, respectively. All of them give different stress-strain curves. For most of the specimens tested, the tensile strength was 10

B

2

3 4 Tensile strain,

5 %

6

Fm. 3. Stress-strain curves of an annealed (001) gold film of 2.60,~ in thickness. All the tensile specimens were taken from the same sheet and annealed simultaneously. Initial tensile directions are [llO], [210] and [loo], and they are denoted by A, B and C, respectively. Subscripts 1 and 2 refer to different tests. End points represent the fractured points.

8

1404

ACTA

METALLURGICA,

,.”

VOL.

15,

1967

._

Tensile strain,a/

FIG. 4. Stress-strain curves of &a annealed (lii) gold film of 2.14 ,u in thickness. All the tensile specimens were taken from the sa.me sheet and annealed simultaneously. A, B and C indicate initial tensile directions inclined by 3”, 12” and 27” from [IlO] towards [211], respectively. Subscripts 1. and 2 refer to different tests. End points represent the fractured points.

in the deformation process: an initial stage with a high work hardening rate and a latter stage with a low work hardening rate. At the initial stage the rate is in the same order of magnitude as that of the asdeposited films but its orientation dependence varies with the specimen films. Therefore, any clear orientation dependence could not be found at this stage. At the latter stage, the work hardening rate is relatively small (about 20 kg/mm2 per unit elongation for [210]), and its magnitude becomes large in the order of tensile directions [loo], [IlO] and [alO]. An X-ray examination of the specimen after fracture revealed that the tensile directions with respect to the crystallographic axis rotated towards [lOO] with rotation angles of 0” for the specimen Cl, 2’ for B, and about 2’ for A,. Figure 4 reproduces, as an example, stress-strain curves obtained for an annealed (Iii) film of 2.14 p thick. The initial tensile directions are inclined by 3”, 12” and 27’ from [IlO] towards [211] and they are denoted by A, B and C, respectively. The general trend of the curves is similar to the case of the annealed (001) films. For most of the specimens tested, the tensiiilestrength was 9-12 kg/mm2 and the elongation I&28%. Therefore, the elongation is much larger than that of the annealed (001) films but the tensile strength nearly equal. At the initial stage of deformation, orientation dependence of the work hardening rate was not clear. At the latter stage, the work hardening rate varies not only with initial tensile directions but also with strain, An X-ray examination of the specimen after fracture showed that the tensile directions rotated towards [211] with the angles of 7’ for the specimen of A,, 11” for B, and 3” for C,; Above results show clearly that the mechanical

properties of the vacuum deposited single crystal gold films depend on the tensile direction. A reason why the previous works have failed to detect the orientation dependence seems to be in that stress -strain curves for various tensile directions were not compared one another for specimen films prepared simult&neously. In fact, crystal size, amount of twins, impurities and other lattice defects are hard to be controlled at the time of deposition and annealing, and they are very sensitive to the mechanical properties. Crystallites in the as-deposited films contain many lattice defects and their size is, according to our observation, as small as several hundred ~gstroms. The movement of dislocations is so severely interrupted by the crystal and twin boundaries as well as other lattice defects, that the work hardening rate is very high until fracture occurs. In spite of the strong interruption against dislocation movement, the single crystalline a,ggregate showed the deformation depending on the orientation. This is interesting but the deformation mechanism is not clear at present time. In the annealed films, crystal size is of the order of film thickness and the degree of crystallite orientation is greatly improved. The lattice defects are also reduced consider&bly. These facts result in the small work hardening rate as observed at the latter stage of deformation, where the interaction between moving dislocations with others is responsible for the orientation dependence of deformation. In the annealed (001) films, the tensile directions rotated towards [lOO] but remained on the (001) plane. This suggests that the primary and the critical slip systems ((lli)-[loll or (ill)-[IOi], end (lll)[lOi] or (1 Ii)-[loll) operate simultaneously. This is different from the case of bulk f.c.c. metals, where a.

LETTERS

TO

tensile direction, initially on the (001) plane, leaves the original plane to rotate towards more general direction with the progress of deformation as the result of only one of the two possible slip systems operating. The characteristic behavior of the film deformation is considered to arise from the extremely small dimension in one direction, but its clear explanation is hard to be given at this moment. In the case of annealed (Iii) films, the tensile direction rotates on the (Iii) plane towards [211] during deformation. The operating slip system is considered to be mainly the primary one (( 1Ii)-[loll) when the tensile direction is close to [llO] and the center of the projection triangle, and to be both the primary and the conjugate ones ((lli)-[loll and (lil)-[IlO]) when the tensile direction is close to [211]. The change of work hardening rate with strain observed in Fig. 4 is considered to be due to the rotation of the tensile direction, which is accompanied by the change of the resolved shear stresses on the secondary slip systems relative to that of the primary system. Thus, the almost constant work hardening rate up to fracture for the initial tensile direction of C may be due to that the tensile direction does not rotate further after it coincides with [211], and the rate increase observed in the region of large strain for the initial tensile direction of B may be caused by the tensile direction coming close to [211], while the gradual decrease of the rate with strain for the initial tensile direction of A may be due to the tensile direction rotating towards fo the center of projection triangle. In fact, the calculated rotation angles of tensile directions based on the above slip systems and the observed elongations coincide with the observed rotation angles. Plastic deformation through twinning was reported for the (001) gold films of 0.1-0.5 p in thickness.(*s5) In the present study the as-deposited films contained so many twins that the above statement could not be confirmed. However, in the case of the annealed films free from twins, the twin spots could nof be observed in the X-ray diffraction patterns of the specimens after fracture. Therefore, it can be said that the annealed films are deformed not by twinning but by slip. M. FUKAMACHI S. NAGAKURA S. OKETANI Department of Metallurgy

THE

EDITOR

1406

References 1. J. W. MENTER and

D. W. PASHLEY, Structureand Prop-

erties of Thin Ji%ns,p. 111. Wiley (1959). 2. D. W. PASHLEY, Proc. R.Soc. A255, 218 (1960). 3. C. A. NEUGEBAUER, J. appl.Phya. 31, 1096 (1960). 4. A.CALTIN,W.P. WALKER~~~K.R.LAWLESS, ActaMet.

8, 734 (1960). 5. A. CALTIN and W. P. WALKER, J. appl. Phys. 31, 2135 I Iclfwl. 6. J. M. BLACKELY, J. appl. Phys. 35, 1756 (1964). 7. M. FUKAMACHI,S.NAGAKURA~~~S.OKETANI. ActaMet. 14, 62 (1966). \----I-

* Received July 28, 1966; revised February 15, 1967.

Electrotransport

of bismuth

tin*

This note describes an electrotransport experiment to determine the effective diffusion coefficient of Bi in liquid tin (1 at. ‘A Bi) as a function of current density. A vertical capillary-reservoir method was used in this experiment. To avoid free convection,(l) thin-walled Pyrex capillary tubing of 0.9 mm i.d., 34 mm in length was chosen and a temperature gradient was imposed in the furnace such fhaf the top of the capillary was 9-10°C above that of the bottom. The Bi and Sn were of 99.999% purity. The electrotransport capillary construction and similar experimental procedure have been described elsewhere.t2s3) Initially the capillary was filled with the alloy of the same composition as the reservoir and held at 430°C. A constant current was passed through the capillary until a steady state was established. A steady state is attained when the resistivity is constant with respect to time. The average concentration of the alloy in the capillary during the electrotransport process was determined by measuring the resistivity of the liquid column. To convert the resistivity value to concentration, the results of Ref. 2 have been used. Corrections were made to account for the current and temperature fluctuation to obtain a smooth concentration-time curve. At steady state, the flux of solute ion (Bi) due to electrotransport is necessarily balanced by a flux of solute ion due to back diffusion.By

performing a mass balance and making use of Fick’s first law in one-dimension, one can write

Tokyo Institute of Technology Oh-okayama, Meguro-ku, Tokyo, Japan

in liquid

+ C,U,E = DeR.z