Internal friction and transmission electron microscopy studies of magnesium—I. Internal friction

INTERNAL

FRICTION STUDIES

AND

OF

TRANSMISSION

MAGNESIUM-I.

R.

T.

C.

ELECTRON

INTERNAL

TSUIt$

and

H.

MICROSCOPY

FRICTION*

S. SACK?

The internal friction of Mg was studied at 4 kc/s and 40 kc/s as a function of temperature (4.2-3OO”K), plastic deformation, crystalline orientation, heat treatment and impurity content. In pre-strained polycrystalline samples a peak was observed at 20°K which annealed out readily at 300”K, and a very broad peak between 40 and 240°K which showed a complex dependence on deformation and annealing. In deformed single crystals only the broad peak was found. The variation with orientation indicates that the peak is caused by dislocation motion in the (0001) slip system. The annealing behavior points to a possible dissociation of non-basal dislocations into basal and non-basal components. Addition of I!/ Li suppressed both peaks, while Ni and Fe (which form precipitates) enhanced them. ETUDE

DU

MAGNESIUM

ELECTRONIQUE

PAR

EN

FROTTEMENT

INTERNE

TRANSMISSION-I.

ET

FROTTEMENT

MICROSCOPIE INTERNE

On a 8tudiB le frottement interne de Mg B 4 kc/s et 40 kc/s et 40 kc/s en fonction de la temperature (4,2-300”K), de la deformat,ion plastique, de l’orientation cristalline, du traitement thermique et de la teneur en impuretbs. Dans les &hantillons polycristallins ayant subi une pr&d8formation, on observe B 20°K un pit qui s’blimine facilement & 300”K, et entre 40” et 240”K, un t&s large pit qui d6pend de man&e complexe de la d&formation et du recuit Dans les monocristaux d8form&, on trouve seulement le large pit. Sa variation avec l’orientation indique que ce pit est provoqui: par le dbplacement des disLe recuit indique une dissociation possible des dislocations dans le systbme de glissement (0001). locations hors-base en composantes de base et hors-base. L’addition de 1% Li supprime les deux pits, tandis que Ni et Fe (qui donnent lieu it des pr&cipit&s) les accentuent. IXNERE

REIBUNG

Ulr’D VON

ELEKTRONENMIKROSKOPISCHE MAGNESIUI\I-I.

INNERE

DURCHSTRAHLUNG

REIBUNG

Die innere Reibung van Mg wurde bei 4 kc/s und 40 kc/s in Abhiingigkeit von der Temperatur (4,2der WBrmebehandlung und dem 300”K), der plaatischen Verformung, der Kristallorientierung, Verunreinigungsgehalt untersucht. In vorverformten polykristallinen Proben wurde bei 20°K ein Maximum beobachtet, welches bei 300°K schnell ausheilt; ferner ein sehr breites Maximum zwischen In verformten Einkristallen tritt 40 und 240°K mit komplexem Verformungs- und Anlassverhalten. nur das breite Maximum auf. Die Orientierungsabhhngigkeit l&St vermuten, dass das Maximum durch Versetzungswanderung im (OOOl)-Gleitsystem verursacht wird. Das Ausheilverhalten liisst auf eine miigliche Dissoziation von Nichtbasisversetzungen in Basis- und Nichtbasiskomponenten schliessen. Hinzufiigen von 1 y/o Li unterdriickt, beide Maxima, w&hrend Ni und Fe (die Ausscheidungen bilden) sie verstgrken.

1. INTRODUCTION

Since the discovery

of the Bordoni

a great deal of work has been concentrated and b.c.c. metals.(2) h.c.p.

metals;

{llzq, {1122}, (1 011 - } s1ip systems can be operative at or below room temperature.“) As will be shown in

peak in 1949,(l) on f.c.c.

this paper,

Very little has been reported on

Caswell(3) found

40 kc/s)

in heavily

cold-worked

Bordoni

et uZ.(~)observed

a peak at 21°K polycrystalline

the dislocations

only dominate

(at

background.

Mg;

in the basal plane not

the Bordoni Dislocations

type peak, but also the

in the prismatic

and pyra-

midal systems appear to have very litt,le effect on the

a peak in slightly deformed

damping except possibly in the case of a low tempera-

Zn ; Koda et al.c5) made measurements on Mg in the and Hart’man(6) ultrasonic region ; and Roberts

ture peak at approximately

studied

have been made of the effects of crystal orientation,

Mg at high strain amplitudes.

investigation extensive

is an attempt

In

The present

to fill this gap by an

amount

study of Mg at 4 kc/s and 40 kc/s.

Mg offers a special

interest

for the study

of the

{lOiO},

{llzO>

transmission taken

Received October 5, 1966. Excerpts of a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Cornell University. This work was mainly supported by the U.S. Atomic Energy Commission; additional support was received from the Advanced Research Projects Agency through t,he use of the technical facilities of the Cornell Materials Science Center. t Department of Engineering Physics, Cornell University, Ithaca, New York. $ Now at: IBM, T. J. Watson Research Center, Yorktown Heights, New York. ACTA

METALLURGICA,

VOL.

15, NOVEMBER

1967

deformation

number

systematic

studies

and temperature

of

doped

of

polycrystalline

In an attempt to clarify further the role of dislocations in the appearance of the Bordoni peak, a

and pyramidal

*

A

20°K at 40 kc/s.

investigation,

specimens were also measured.

making it easier to correlate the peak with specific dislocation motions. In addition to the easy glide slip, prismatic

of plastic

annealing.

Bordoni peak, since it has a very simple slip behavior,

basal

the present

friction

electron

microscopy

of some of the samples measurements.

study

was under-

used in the internal

The results of this investiga-

tion are reported in Part II of this paper.@) 2. EXPERIMENTAL

PROCEDURES

Polycrystalline and single crystal samples of 99.996% Mg, or doped Mg were measured.$ The single crystals were grown by the Bridgman method § The authors thank providing the material.

1715

the

Dow

Chemical

Company

for

lilt3

ACTA

in the form of rectangular 3$ in.

METALLURGICA,

bars about & in. x $ in. x

Oriented single crystals were prepared

same

method

using

a lit_in.

dia.

by the

spherical

seed.*

The orientation is defined by the angle 0 between the long axis of the specimen and the normal to the basal

VOL.

15,

(logarithmic) reproduced

1967

decrement.

by smooth curves for the sake of clarity. 3. RESULTS

plane which is perpendicular to one set of faces of the sample. One of the (1120) slip directions in the (0001)

3.1 Polycrystals

plane is also perpendicular

polycrystalline

5’ (see Fig. 5). it is sectioned saw.

to this set of faces within

After a single crystal has been grown off from the seed by a chemical

string

The surface of the cut is either mechanically

chemically

polished

are perpendicular

on a polishing

or

wheel ; the ends

to within 2” to the long axis of the

Two

types

tudinal or lateral compression. were sometimes cold-worked

by rolling.

Extreme care

was exercised in handling and cold-working mens.

A special jig was constructed

compression

exactly

For compression

doped

was by either longiPolycrystalline samples the speci-

to ensure uniform

along the length of the sample.

the amount

of deformation

is given

type peak extending

One is a

from about

structure and the (at 40 kc/s).

The

broad peak, which in many cases c,an be considered superposition

deformation

or Fe (0.011 at.%)

in deformed

mm).

other is a sharper peak at 20°K

Ni (0.03 and 0.41 at.%),

deformation

were observed

Mg (grain size 0.1-0.3

40” to 260°K showing considerable

practically

Plastic

of peaks

very broad Bordoni

sample. In a similar way the crystal is then cut to Polycrystalline samples were the right length. machined directly from pure Mg or Li (1 .l at. %), bulk Mg.

In the figures some data are

point by point, while others are reproduced

of two or more peaks, was observed all cold-worked

specimens.

The

a in

height

and peak temperature vary with the amount of deformation, Fig. 1. The height increases first with up to about

2-3%

and then decreases

upon further increase of cold-work. the

peak

shape

changes,

towards lower temperatures. for 11 y0 deformation 2-3%

deformation

At the same time

the maximum shifting The 20’ peak (shown only

in Fig. 1) appears after about

and increases with cold-work,

the peak temperature

remains

at 20°K.

nealing this peak decreases without

Upon

but an-

much change in

m “,b of reduction of the specimen length, width or thickness, while with cold-rolling the y0 reduction

its shape; in fact it anneals out at room temperature in a few weeks. The annealing behavior of the broad

of

peak is shown in Fig. 2. All the annealings were performed in an argon atmosphere in a horizontal

the

cross-sectional

Measurements longitudinal

area

is indicated.

were made in both transverse

modes

of vibration

~4 and 40 kc/s respectively, range of 4.2”-300°K. The

and

at frequencies

of

and in a temperature equipments of PaSg)

and Caswell(3) were used with only minor modifications. The * 1%”

internal

friction

is given

in terms

For details, see R. T. C. Tsui, Cornell University (MS.) and 1964 (Ph.D.).

of

the

theses,

furnace.

Curve A represents the decrement

(at 300°K)

after 2%

results immediately to A except

longitudinal after

(B)

the

peak

range.

increases

higher temperatures

The

were similar higher over

After a 150°C anneal

uniformly.

Annealing

at

results in an increase of the high

Annealed I 2 % 3.7 % 15

-a cl _ x

deformation

that the curve was ~30%

the entire temperature

5 months

compression.

10

; E m P 0 5

0

FIG. 1. Internal friction of polycrystals as a function of amount of deformation (cross-rolling). 40 kc/s.

TSUI

SACK:

AND

INTERNAL

FRICTION

sample

had

OF

Mg--I

1717

a homogeneous

appearance ;

both

Ni

and Fe doped materials had very rough surfaces and small cracks were found at some places.

The results

are shown in Fig. 3. No peak was observed even

20 h

after

plastic

deformation

The

is generally

very high in Ni or Fe doped

the broad peak remains even after 8-12 hr

x

materials;

anneal at 400°C (A and B).

5

the

peak

individual

10

with Li, F).

decrement

;

:: D

(C and

depend

The size and the shape of

critically

specimen.

on the

history

A small amount

of the

of cold-work

increases the peak height up to 2 orders of magnitude (D and E). small

after

Since in pure Mg the peak becomes large

doped polycrystal

0

100

200

300

TOK

side,

which

7 day stay at ~25°C

(D).

even

continues

of

deformation,

of Mg was cross-rolled by more than

cold-worked

pure polycrystalline

Mg.

Bt the same

time the 20°K peak appears and anneals out at 150°C (B) just as in pure Mg.

during

a

From 300°C on, the peak

3.2 Single crystals Single crystals with 8 = 90”, 75’, 60”, 30”, and 15”

height begins to decrease but the decrease is mostly

were prepared.

in the low temperature

crystals with 0 = 0” but with no success.

side (F).

The peak does not

disappear unless the annealing temperature is above 400°C (G). The annealing behavior is thus different from that in Cu where annealing raising

or lowering

the entire

has the effect

peak without

of

much

change in shape.

The background

room temperature

remains fairly constant throughout

the annealing

cycle.

It is about

as measured near a factor

than in single crystals of 90” orientation

very a Ni

10% (Fig. 4A). The peak is much lower but still in the 1O-3 range and its shape resembles that in heavily

FIG. 2. Successive annealing of a deformed polycrystal; A 5 mont,hs at room temperature after deformation; B 5 hr annealed at 15O’C; C 5 hr et 200°C; D 2 days F 5 hr at 300°C; at room temperature; E 5 hr at 250°C; G 5 hr at 400°C.

temperature

amounts

3 higher (see later).

Of three impurities studied [grain size 0.3-5 mm, concentration. see Section (2)], only the Li doped

Several attempts

that [OOOl] is a non-preferred basal plane is coincident

growth

in this particular direction of growth). condition was also found in Zn.(lO) The background

It appears

direction

with the solid-liquid

(the

interface A similar

internal friction is shown in Fig. 5.

All 4 specimens were very carefully annealed at 450°C for 12 hr and the decrement is very low, in the 10m5range.

The decrement for 8 = 45” is the highest

throughout the whole temperature value of 8 the resolved vibrational

50

40

w 0 = ;

30

; y : 0

20

IO

100

were made to grow

200

3

0

T OK

FIG. 3. Internal friction of doped polycrystals. Annealed A (Ni), B (Fe), C (Li); Deformed: D, 0.03% Ni, 1% deformation; E, 0.01% Fe, 4% deformation; F, 1.1% Li, 4% deformation.

range. For this shear stress is a

SCTA

1718

METALLURGICA,

VOL.

15,

However,

1967

because

of the specific

orientation

of the

basal planes in these samples the amount of deformation achieved

in the first specimen was 7% but only

1.6% in the second. This is not surprising since both prismatic and pyramidal slips and {lOiZ}(lOi1) twinning

are possible if the compression

sample thickness while only pyramidal

is along the slip can occur

with compression in the direction of the sample width. Thus the second crystal is “harder” for such a compression.

No internal friction peaks were found

in either crystal (see Fig. 7A).

The decrement

values

were in the low 10” region from 4.2” to 300°K.

Curve

B shows for comparison in the same way. FIG. 4. Internal friction of 10% ~088 rolled Xi doped A 2 days at room temperature after polycrystal. deformation; B after 7 hr at 150°C.

maximum

in the basal plane ; it is zero for 19= 90”.

As 8 decreases,

the decrement

curves show some structure;

too.

The

it is not excluded

decreases

that

this is the result of a very slight amount of deformation that might have occurred

in spite of the care taken

in handling

There is no clear way in

the samples.

which the “true”

background

can be separated

the effects of very slight deformation. The effect of orientation in deformed

a 8 = 30” sample deformed

Since now there is a resolved shear

stress parallel to the basal plane for both deformation and vibration, a peak should be observed, as is indeed the case. Two freshly grown crystals with f3 = 45” and 75” were compressed along their thicknesses (i.e. along the basal plane) by about and dislocations

1%.

The slip

created by this kind of deformation

6

I

I

from

crystals can

be seen in Fig. 6. These samples were annealed at 450°C for 12 hr, and then compressed 0.5% at room temperature

along their length.

No increase of decre-

ment results in the 90” crystal. show

broad

decreasing

maxima with

at about

increasing

6.

crystals, one was compressed the

other

in the

along

same

applied

Of

curves

two

90”

along its thickness

and

They

were

machine

deformation

and

were

l ...*...

the heights other

its width.

compression

during

The other lSO”K,

deformed the forces

about

equal.

r .-1

l. . . . . . . . . . . . . . .

. . . . . (e. . . . . . .

u.

200

100

300

T lK

Fra. 6. Internal friction of 0.5% compressed crystals of different orientations.

should be the same for both ference in 19is equivalent the compression

crystals,

since the dif-

to a rotation

of 30” around

axis which should not alter the effect

of the deformation.

However,

the heights

of the

peaks differ markedly (Figs. 7C and D). With this kind of deformation (1072) twinning occurs easily as can be observed readily under a microscope appearance

by the

of slip lines and twin bands on the speci-

men surfaces. In order

to study

the dependence

height on the amount of cold-work,

of the peak

5 single crystals,

all with 8 = 45’, were compressed longitudinally by 0.2, 0.5, 1.0, 2.5, and 5% respectively (Fig. 8). The peak increases with increasing deformation up to 2.5% and decreases for still higher compression. The

L....

0

I

I

100

I

200

results are in very 300

TOK FIG.

5. Background

internal friction orientation.

as a function

of

good

agreement

with those

on

polycrystals, except for the absence of the 20” peak. The highest longitudinal compression that could be achieved without buckling or other more complicated

TSUI

INTERNAL

AND SACK:

0

FRICTIOS

100

OF

200

MB-1

1719

300

T ‘K

FIG. 7. Internal friction of crystals under different types of compression. A perpendicular to basal plane (1.6%); B with sheer component in basal plane (2%); C and D. along the basal plane (~1%).

at 4 kc/s in order to study No meaningful 'O-

However,

A .2% 0. .5%

the effect of frequency.

shift of the peak could be detected.

the peak is so broad

so sensitive

to the mechanical

and has a structure and thermal

of the sample that the absence shift

with

frequency

does

existence of a thermally

history

of a clearly defined

not

speak

activated

against

relaxation

the

process.

The comparison between curves for different frequencies is made even more diffcult by the fact that the peak height increases by a factor of about 5-10 by going to the lower frequency.

The background,

too, increases in the same way. The significance of this observation in the light of the Granato-Liicke 0

theory is discussed elsewhere.

100

200

300

All

T ‘K

deformation

measurements

reported

here

were

made

at

vibrational strain amplitudes of the order of 1O-slo-‘, well below the “break-away” value of l-2 x lo-’

FIG. 8. Internal friction of 45” crystal as a function of 96 deformation.

for annealed pure single crystals and 0.8-l

is about 5-8: b ; but no trace of the 20”

x lo--’ for

peak could be detected. Figure 9 shows the effects of annealing

of two 45’

crystals for 7 hr at 150°C (B and D) and for 6 weeks at 23°C (A and C). tudinal compression, the basal plane. nealing at compression

A and B represent a 2.5%

longi-

C and D 1% compression

along

It should

150°C along

be noted that upon an-

IO -

.

the peak height increases for the basal plane, in contrast to

0 I

longitudinal deformation where it decreases. Annealing at 300°C decreases the peak height in both cases (curve E).

These observations

find a parallel

in the behavior of the moduli (curves not reproduced)

:

for longitudinal compression the modulus increases after annealing at 150°, while for the other deformation it decreases. 3.3 Frequency and amplitude dependence The 40 kc/s.

majority

of

measurements

In addition a few experiments

were

made

at

were performed

FIG. 9. Annealing behavior of 45” crystals. A and B after longitudinal compression (2.5%); C and D compression along the basal plane (1%). A and C, 6 months at 23°C; B and D after additional 7 hr at 150°C; E 7 hr at 300°C.

ACTA

1720

polycrystals. somewhat

The addition while

of 0.01%

1 .l o/o Li moved

Plastic

deformation

strain;

for 11 o/o it is 1OW.

also

METALLURGICA,

Ni increased

it to above

increases

the

it

10-5.

breakaway

wide

preceding

variety

section

of

results

presented most

in easily

the by

first stating some general conclusions and then showing ments indicate the

broad

specific results.

that dislocations

peak,

but

that

All measure-

are responsible

they

contribute

for only

if they are in the basal plane and if there is a resolved vibrational

shear along that plane.

seems to be of different

the internal

along

the

friction

sample

axis

In fact, assuming that at low

stresses the dislocation

density is proportional

to the

resolved shear stress and that only basal dislocations density

out

by

the

orientation

and the peak height proportional

is qualitatively

should

electron

~opy@,‘~)),

resolved vibrational

be

micros-

independent

of

to the

stress, i.e. to sin 219. This relation

(within 50%)

additional

curves

not

practically

no internal

verified in Fig. 6 (and

shown

here).

friction

The fact

is found

that

in the 90”

crystal for all 3 modes of deformation shows that pyramidal and prismatic dislocations do not contribute to the clamping.

In compression

along the thickness in

compression

dislocations

are

been observed for small vibrational

the

created.

width Such

can be activated; only

These

where planes

twinning can

pyramidal

dislocations

in the electron microscope

deformations

predominant.

along the axis and

both systems

along

have

have

by Ueki(13) is not

yet

a resolved

shear stress and thus their dislocations

could contribute to the internal friction if they were sufficiently mobile in the temperature range under study.

the

difference

difference

in

orientation

for the 75’

crystal,

in the amount

of de-

which in this case is very difficult to achieve

shear stress.

with decreasing

resolved

(Fig. 5)

vibrational

It is thus logical to conclude

also caused by basal dislocations

that it is

moving in the basal

plane. 4.3 Dependence on amount of deformation deformations

smaller

increases slightly

than

2-3%

the

less than linearly

peak

with the

amount of deformation; since the dislocation density is also proportional to strain,(8s12) the increase in

should vary as sin 28.

(as borne

to

in a lower decrement

also decreases

For

deformation

produced

ascribed

precisely. It may be noted here that the background

height

On the basis of this picture

are

be

origin and will be discussed

4.2 Orientation dependence constant

can

The peak at 20”

separately.

for

The difference between curves C and D,

which are obtained with relatively small deformation,

formation

can be discussed

how they can explain

1967

and to a possible

General picture The

15,

to the peak.

resulting

4. DISCUSSION 4.1

VOL.

For larger deformation

along the basal plane

(Figs. 7C, D), when twinning becomes noticeable, t*he internal friction increases very rapidly.* In this case not only are there dislocations

produced

in the

original basal plane by indirect processes, but the newly created basal planes are now activated by bhe compression and can also have a resolved vibrational shear stress. Both systems will thus contribute * A similar result was found by Ueki”3’ for lightly doped Mg crystals (90”); for an increase of longitudinal compression from 1 to 296 the decrement increases by a factor of -10.

internal

friction

in dislocation

seems to be caused by an increase

density.

In this range of deformation

the electron

micrograph

and smooth

dislocations

shows predominantly in the dosed

packed

long direc-

tions which could cause the peak in accordance

with

Seeger’s kink model. (14) With increasing deformation an increasing needed

internal

according

stress

to PadQ)

develops(12)

However, it is doubtful that these dislocations the main contribution

which

is

for a peak to appear.

to the peak.

provide

For instance in

the low stress region dipoles are observed which could contribute according to the theory of Gilman.(l5) But there are also patches

of networks

and become more prominent They could also contribute well

in a way that is not yet

In fact in Park’s theory

understood.

necessary

that appear

as the strain increases.?

to have

long

already

“bowed

out”

internal

stress.

Thus,

dislocations under

the

in principle,

of a few times the kink width,

it is not

since they influence

of

are the

short segments,

i.e., -200

8,

could

contribute, while 6he long dislocation map contribute less if the internal stress is too weak. Some support for such an interpretation can perhaps be found in the fact that high peaks are observed in samples that show few long dislocations deal of networks for deformations basal

plane,

but a great

and entanglements, as for instance parallel or perpendicular to the

or where precipitates

are present.

A

recent study of Al also seems to support such a view.(lS) t Hirsch and Lall~“~’ have pointed out that low angle sub-boundaries may exist even in single crystals under certain growth conditions. Small amounts of deformation from such crystals will give rise to dislocation networks and twinning. The 45” crystals used in this investigation were grown at 14 in/hr. According to Hirsch and Lally, they may contain a subboundary running along their length axis, although this cannot be verified from our Lam back reflection pictures

TSUI

Some other observations with this hypothesis

AND

INTERNAL

SACK:

(~400”)

that seem to be in agreement

is exceeded, of the long

could be caused by the dislocations, or by the

networks and tangles becoming too tight. But it could also be explained by recrystallization: at higher

strains

subgrain

recrystallization small

amount

is retarded

by

appear ;

to

when

t,he addition

of non-precipitating

decrement continues compression.(13) Much

boundaries

increase

of

impurities, at

least

to

a

structure

thin film sample

discussion

is based

n-ill be modified

preparation; responsible

are

seen

longer

in

bhe

electron

(~30O/~).

behavior;

this

speaks

rather

for

ments several hours delay occurred between the times of deformation

and measurements.

exist a component

Thus there could

of the peak that anneals out during

big

has

been

found

by

Routborto8)

on a

friction

doping, annealing, etc.

shape of peak

to Cu and Al. and sensitivity

Bs mentioned

again in

earlier

to bhe preparation

at different

frequencies

a relaxation

phenomenon.

whether

the

of the

one is dealing with

The shape shows distinct

changes with t’ype of deformation

4.5 Effect of impurities 1.1 at.% Li, which is soluble in Mg, pin the dislocations : the background is very low, no peak appears after deformation

and annealing which

(Figs. 3C, F), and the break-away

strain is very high.

With

very low solubility,

precipitates

small

amount

in solution

the dislocations

seem to form

After light deformation

Neither

the pin

during the cooling

process,

the peak becomes very large,

but is smaller for high deformation.

It thus behaves

as in pure Mg.

Again it is difficult to perceive

long

mobile

and

very

dislocations

are the

that major

agents. 4.6 Peak at 20°K This peak

appears

doped with Ni or Fe;

affected

the amount

For

form.

nor the precipitates

(see Part

suggests that the peak is made up of several peaks by the various parameters.

Fe and Ni which have a

II(s)). On the contrary, they enhance the peak (Fig. 3). Even in the annealed state peaks are present: single dislocations and

networks

sample make it difficult t’o ascertain by measurements

differently

Again

An indicat’ion of such an effect but not

The peak is very broad, broader than the Bordoni

breadth

in height

networks than for long smooth dislocation. It must be mentioned, however, t,hat in the present experi-

one,

microscope.

peaks in Cu or Al, and has little structure, contrast

the

that period.

during the

for t,he internal

of deformations,

4.4 Annealing

temperature;

a very

However this does not seem to be very probable under all conditions

at room

the

in particular, highly Thus it could be that

may be lost.

t’he dislocations no

high stability

peak remains for months with only a slight decrease

working at 1 c/s.

of the preceding

mobile dislocations

1721

Mg-I

6%

comparison of decrement measurements with electronAs pointed out in Part II of this study@) micrographs. the dislocation

OF

needed to anneal the peak out completely,

and the

will be discussed later.

The decrease of the decrement once a certain degree of deformation disappearance

FRICTIOK

only

in polycrystals,

pure or

it increases monotonically

of deformation

and anneals

with

out rather

instance deformation along the basal plane (Figs. 7C, 9C, vs. 9A) and annealing (Fig. 2D, E) seem to give

rapidly even at room bemperature. This peak was not studied extensively and it is not possible, at

prominence

present, to state what processes are responsible for its appearance. Clearly grain boundaries or other

t,o the high temperature

Perhaps this part reflect,8 “harder” not anneal

part of the peak. networks which do

out so easily and are more abundantly

produced by the harder deformation process. Annealing experiments show another interesting deformation of single feature : for longit’udinal crystals, annealing reduces t,he peak from the beginning as one would normally expect. For other deformations and for polycrystals t,he peak first increases and then Apparently dislocat’ions produced in decreases. non-basal planes can dissociate at higher t)emperatures creat,ing basal dislocations that can contribute to the peak. One such reaction iso’) &[1123] Another

remarkable

g[llzo]

+ [OOOl].

result is t,he high temperature

properties

related to the polycrystalline

character

of

the specimen must play a decisive role. 4.7 Conclusion Summarizing,

it can be said that all the results

support the view that only dislocations in the basal plane which can move under bhe vibrational stress (shear stress along basal plane) contribute to the broad peak in deformed Mg. No definite conclusions can be drawn as to the validity of the different theories for the Bordoni peak, but-in combination with elecbromicrographs presented in Part II-the observations suggest that entangled dislocations or networks can contribut,e as well as smooth dislocations.

1722

ACTA

METALLURGICA,

REFERENCES 1. P. G. BORDONI, Ric. Scient.

19,856(1959);

19,851(1949);

J. acoust. Sot. Am. 28, 495 (1954). 2. See, e.g. D. H. SIBLETT and J. WILKS, Adv. Phys. 9, 1 (1960); H. SACK, Acta Met. 3,455 (1962). 3. H. C. CASWELL, Thesis, Cornell University (1957); J. appl. Phys. 29, 1210 (1958). 4. P. G. BORDONI, M. Nuoro and L. VERDINI, Nuovo Cim. Ser. 10 19,373 (1960). 5. S. KODA, K. KAMIC~KI and H. KAYANO, J. phys. Sot. Japan 18,Suppl.1, 195 (1963). 6. J. M. ROBERTS and D. E. HARTXAS, J. phys. Sot. Japan 18, Suppl. 1, 119 (1963). 7. See, e.g. M. H. ROBLIX, Thesis, RPI (1962). 8. R. T. C. TSUI, Acta Met. 15,1723 (1967). [This issue.] 9 <. V. K. PARER,Thesis. Cornell rniversity (1958); J. appl. Phys. 32, 895 (1961).

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10. T. VREELAND, JR., D. S. WOOD and D. S. CLARK, O.S.R. Tech. Note, No. TN-55-65. 11. J. ROUTBORT and H. (1966).

S. SACK, J. app2.

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to be

Faraday

15. J. J. GILMAN, J. phys. Sot. Japan, 18,SuppZ. 1,172 (1963). 16. F. MAYADAS, Thesis, Cornell University (1966); to be published. 17. A. SEEQER, Theo& der Gitterfehlstellen, Enc. Phys. VI-l, p. 485. Springer (1955). 18. J. ROUTBORT, Cornell University Thesis (1965); Phys. Status Solidi, in press.