The structure of slip band extrusion revealed by transmission electron microscopy

LETTERS

difference

between adjacent

TO

readings is only about 10

THE

X11

EDITOR

This is the inverse of the effect noted by Jenkinst2) on

the most,(l) and becomes even less as the rate becomes

dropping the pressure. This effect can be accounted

slow at low pressures. There is a small error possible in the time readings as well, since the experimental method

gaseous diffusion model. The gas diffuses through small pores through the laminations. Large fissures

requires simultaneous

between the laminations

times the estimated

error of the Sieverts method

reading of two manometer

For this reason Am/Al/t

at

arms.

versusp is plotted rather than

in adjacent

for in terms of the

connect the ends of the pores

layers without introducing

any resistance

the more natural (Am)z/At versus p since squaring the

to flow.(l) This is the situation under normal conditions,

term containing both the possible errors would magnify the scatter. The possible error increases as the

when the pressure difference across any single lamination is small. However, if the pressure is suddenly raised on the outside of the scale, the outer laminations

pressure falls. The theory for the oxidation

process requires that

m2 = k’pt where k’ is a constant Am/A2/t

independent of pressure. Thus should vary asp l12. This relation is shown as

the full lines in Fig. 1. It can be seen that the agreement is extremely

good.

The effect of sharply varying the pressure studied by adding further amounts of oxygen measuring the change in rate. mg/cm2

of oxygen

absorbed

Fig. 2. The oxidation

was and

A rate curve plotted as versus time is shown in

was allowed to proceed until the

atmosphere was nearly exhausted, and a small amount of oxygen added, sufficient to alter the pressure to 550 mm Hg. The rate immediately increased, falling off as the atmosphere

was again exhausted,

retracing

the

last part of the original oxidation curve. At this point a large addition of oxygen was made, sufficient to raise the pressure nearly to atmospheric.

In this case

the rate rose sharply for a very short while, then fell off until the reaction slowly

increased,

almost stopped.

eventually

reaching

The rate then a normal rate.

will be compressed, thus reducing or removing the parallel fissures between the laminations which connect the pores.

The number of diffusion paths to the met&l

surface will therefore be much reduced, until the slow flow of gas through the remaining paths has equalised the pressure distribution The results obtained

The rate

in this series of experiments

confirm that the controlling bolic stage in the oxidation of gaseous oxygen particular

step in the second paraof titanium is the diffusion

through

the porous

they show that the parabolic

is linearly dependent by theory.

scale, and in rate constant

on oxygen pressure, as required

The materials for this investigation I.C.I. Metals Division.

were supplied by J. STRINGER

Department of Metallurgy University of Liverpool References 1.

J. STRINGER, Acta Met., 8, 558 (1960). 2. A. E. JENKINS, J. Inst. Met. 82, 213 (1953-54). * Received

34

through the scale.

will then increase to the steady state value.

February

26;

revised April 1, 1960.

32 30 28 26 "E

24

4

22

The structure of slip hand extrusion revealed transmission electron microscopy* Earlier

r20

observations(l)

on various

materials

shown that fatigue stress caused bhe extrusion

18 16

scrolls of debris from slip bands.

14

was very marked

in solution

by have

of thin

This phenomenon

treated aluminium-4°/,

copper alloy fatigued at room temperature, where it was observed to occur suddenly within a few stress

0

20

30 Time,

40

50

60

70

80

min

FIG. 2. The effect of abrupt pressure changes cm the oxidation 6-(80

py.)

rate of Ti-40/,

Fe at 910°C.

height of the extrusions cycles. (2) The maximum observed in this alloy was approximately 10 ,u, and interferometric measurements suggested that they were not thicker than 0.1 p. A light micrograph of a typical slip band extrusion is shown in Fig. 1. In an earlier study(2) the extrusions stripped from a specimen on a “Perspex”

replica were examined both by reflected

812

ACTA

METALLURGICA,

VOL.

8,

1960

and fatigued extruded

at room

material

surface embedded Judging

from

temperature.

was stripped

The slip band

from

the specimen

in either carbon or formvar replicas.

the

appearance

of extrusions

when

viewed in the electron microscope, they must all be remarkably similar in thickness, and the 0.1 ,U upper thickness limit originally

estimated

by interferometry

is consistent with their transparency back ends of the extrusions similar to fractures thin aluminium These

torn

we have produced

foils

edges

stripping

to electrons.

were serrated in

by straining

in the electson

must

operation,

have

and

microscope.‘3)

occurred

this

The

manner

a

during

the

that

the

suggests

extrusions were still coherent with t’he crystal material when FIG. 1. Optical micrograph extrusion.

stripped.

and crystal

of & typical slip band x 1500

examination where

and transmitted their opacity

light.

Their

high reflectivity

and

suggested that they were metallic.

In order to understand

the extrusion

it

is important to know if the extrusion has been enriched with or depleted writing

of solute atoms, but at the time of

no satisfactory

method

of analysis has been

evolved.

We have, however,

structure

of the slip band extrusion

been able to study the by transmission

electron microscopy. Specimens solution

of

copper

alloy

were

heat treated at 52O”C, cold water quenched

FIG. 2.

Transmission

transmission

electron

micrograph

of slip band

continuity

the

edges

of

electron

between fatigue

Figs.

replica

of

showing

typical

of

fracture

2 and

micrographs

substructure

extrusion

by “fractographic” the

still adhere.

on a formvar

elongated

3 are

extrusion

a striated all

or

extrusions

examined. This structure seemed to be composed

of elongated

crystallites lying in a direction parallel to the slip band from which the extrusion

came.

No discrete disloca-

tions could be detected in these long parallel boundaries (indicated

aluminium44/,

of

extrusions

stripped

mechanism

This

has been confirmed

by arrow A, Fig. 3) although

boundaries resolved

extrusion

(indicated as

small

showing

cross linking

by arrow angle

elongated

H) were sometimes The dislocation arrays.

substructure.

x 12,000

LETTERS

FIG.

electron

TO THE

3. Transmission electron micrograph of slip band

diffraction

pattern

shown in Fig. 4 confirms

the presence of this substructure.

The long boundaries

813

EDITOR

extrusion

showing

caused complete

elongated

polygonization

It is known from observation in a few stress cycles

x 62,000

substructure.

(~10

to an equiaxed

form.

that extrusion occurs

cycles)

and as there are

were unusually straight and were not jogged by the cross linking boundaries. They were in all respects

about the same number of striations in the extrusions

similar in appearance

it is reasonable

but

prolonged

to large angle grain boundaries,

heating

in the

electron

microscope

to suppose that each striation

corre-

sponds to, and is the result of, one cycle of stress. is, however. the

difficult

boundaries

to ascertain

between

the elongated

They may be arrays of edge dislocations to the extrusion the extrusion polygonization process,

crystallites. lying parallel

surface with their Burger’s vector in

direction.

These could result from the

of buckles

or directly

illustrated

It

the true nature of

in Fig. 5.

from

formed

in the extrusion

sharply

kinked

bands

as

It can be seen that a kink band

would naturally form where the extrusion emerged from the crystal surface, during that part of the cycle

FIG. 4.

Electron

diffraction pattern from slip band extrusion (III).

Flc.

-5. Diagrammatic

representation process.

of extrusion

ACTA

814

METALLURGICA,

VOL.

8,

1960

Any mechanism

would have to take into account

the new observation

that

dislocation

arrays,

most

probably edge dislocations predominantly of one sign, exist in the extrusion. This is most likely to be the result of changing

frictional

forces

under extension

and closure of the surrounding crystal. It may be that reverse sliding occurs on the plane a-b and only in one direction

on the plane c-d.

in the reverse direction

across a-d acts as an obstacle. cohesion

is soon

lost

between

the

faces

two

Slip cannot occur

d-c because along

a-b,

will

the kink band

It is likely that true but

the

be greater

friction

under

the

closure half of the cycle than on the extending half. There is evidence that, as with silver chloride, extrusion

in aluminium-4%

leave behind a void.

copper

alloy

does not

It is likely that the extrusion-

crystal interface a-b nearest the specimen surface, i.e. the interface that forms an acute angle with the surFIG. 6. Transmission electron micrograph of slip band extrusion showing precipitation produced by heating x 8,000 in the electron beam.

face, loses cohesion and becomes a crack. This loss of cohesion could occur before the extrusion and be the cause rather than the effect. Prolonged

when two parts of the crystal

on each side of the

precipitation

heating

in the electron

of 6’ in about

caused and

extrusion were moving together in the directions x and y. We visualize that during this part of the cycle

form

sliding will occur along CA as indicated by the arrows,

manner.(‘)

but (1-h will be stuck.

the distribution

of solute had occurred

with fatigue

When the cycle reverses sliding will occur along n-h. and c-d will be stuck. If a kink band forms

stress it does not seem to have affected

the material

extruded.

during

changes

this half of the cycle,

i.e. when the crystal

surface is extended. it must be less heavily bent than its predecessor because a rolling up of the extrusion is always observed

as indicated

that edge dislocations

in Fig. 5. This suggests

of one sign predominate

in the

as appeared

beam

t,he same quantity

aluminium-4

rather

in thin foils of the homogeneous

“1; copper

on

the

Ministry

extrusions

was

remarkably

the

thickness

constant,

and

similar

structural interface

is eventually

ex-

very much

P. J. E. FORSPTH C. A. STUBBINGTON

of Aviation

Farnhorough, Hants.

of these in this

respect they are dissimilar to those observed in silver chloride(4) and in many metals. It seems that in copper-and we suspect in other aluminium-ii?;, aluminium alloys-the sliding involved in the extrusion process occurs on a relatively small number of slip planes. In many other cases slip is occurring on a more widely distributed series of planes. It is clear that reverse glide is the mechanism of extrusion, but it is diffirult to ascribe a particular dislocation It is certain that a gyrating screw mechanism. dislocation mechanism’@ is untenable in this case, nor can the mechanism of interfering planes be applicable.@)

a

more difficult, and illustrates in a striking manner how sharply confined to a few crystal planes the true

exist there are more closely spaced, and this results in the curling observed at the tips. that

that

If this is so it will make detection

fatigue damage may be.

stated

in

extrusion--crystal

than in the material

truded.

heated

It may be that the important occur

extrusion. This predomination of one sign becomes greater near the tip because the sub-boundaries that

It has been

alloy

This is shown in Fig. 6. If any change in

slip on different

References

1. P. J. E. FORSVTH and C. A. STUBBINGTON, Xature,

Land.

1’95. 767 (1955).

2. P. j. E. @OR&H and C. A. RTUBBIXW~ON, J. Inst. Met. 83, 395 (1954-.5.51. 3. R. N. WILSON and P. J. E. FORSYTH, J. Nci. Instmm., in press. 4. P. J. E. FORSYTH, Bull. Amer. Sot. Test. Mat. No. (June 1958). :5. N. F. MOTT, Actn Met. 6 (1958). 6. 9. H. COTTRELL and D. HULL, Proc. Roy. Sm. A242, (1957). 7. c. REAUVAIS, Mfaaus et Corros.406, 247 (1959). * Rereived

Febzwwy

237

2 11

26, 1960.

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