Polarity of work-hardening

LETTERS

anelastic

loops.

The lower

the true lattice frictional

value

TO

is believed

THE

EDITOR

849

to be

stress, and the higher value,

the sum of lattice frictional

stress and the drag due to

jogs.

The extrapolated value for 7F at zero strain is ~0.8 kg/mm2, which is about 50 percent of the initial flow stress. The interrupted stress/strain curves of the crystals

show transient

effects

that are not

found in crystals pulled continuously. on reloading

the

ordered

crystal,

commences

in a discontinuous

propagation

of a new Liiders band;

is ~5

percent.

diffusion

controlled

responsible It

The evidence locking to

plastic

manner

flow

re-

and by the

the Liiders strain

points to some kind of mechanism

for the anomalous

is a pleasure

In particular,

reloading

as

being

FIG. 1

effect.

acknowledge

the

valuable

which a dislocation

came before it was stored.

assistance given by Mr. J. Zeiger, and the support of --

this is not an unreasonable

the Office if Naval Research.

incorporated

_

B. H. KEAR

Pratt & Whitney Aircraft

A result of special work-hardening be

References 1. B. H. KEAR, Acla Met. 12, 555 (1964). 2. A. H. COTTRELL,Dislocations and Plastic Flow of Crystals D. 111. Clarendon Press. Oxford (19531. 3. 3. M. ROBERTS and N. BROWN, ‘TY&. Met. Sot. AIME 218, 454 (1960). 4. P. R. STRUTT and B. H. KEAR (to be published). 5. J. C. FISHER, Acta Met. 2, 9 (19.54). 6. A. E. VIDOZ and L. M. BROWN, Phil. Mug. 7, 1167 (1962).

* Received Januctry 7, 1964.

repeatedly

hardening

progress

in each secondary

deformation

out of a previously these.

All experiments rate

previous

latent

slip system after plastic slices, at various angles,

compressed

cube, and then com-

The orientation

axis is kept constant, strain

the

of the compression

10” from

(110) towards

(211).

are done at room temperature,

of 2 % per minute.

observations

at a

As deduced

from

by different experimental

tech-

niques, (l) the flow stress in the new system is between 1.1 and 1.3 times the flow stress of the old system (1 kg/mm2).

The

slip systems

will be reported

results

have,

details

however,

for

work-hardening

later.

emerged

new light on the mechanism It was observed

all specific

secondary

Some

which

general

may

throw

of work-hardening.

that the flow stress and the initial

slope in the secondary

slip system

depend

on the direction

in which this secondary

system

is strained.

must

formation

is retained

a work-hardened

It

be

concluded

in the dislocation

specimen

about

slip

that

in-

structure

of

the direction

from

has

been

mechanism

is that the initial

system,

of secondary

tests

test is in the A direction,

hardening

slope is negative,

Since why

(but

Figure 1 shows the orientation

secondary

test (0) and (A, B):

if the

the initial work-

if it is in the B direction,

as usual.

behavior

may

be at the root

dependence

cross-slip

important

may

only

and only if

axis in the primary

in the two types

system

observed

so) in the cross-slip

strained in one sense.

orientation

by single slip into stage III of aluminum.

This is done by spark-cutting pressing

of

This

of the compression

This

in

significance

slope in the secondary

negative.

it is positive

Polarity of work-hardening* are

in any of the work-hardening

currently discussed.

North Haven, Connecticut

Determinations

While

result, it is a feature not

is

of the

of work-hardening

generally

strong

in stage III.

assumed

to

be

very

at these large strains, it has not been clear

orientations

with

zero

external

resolved

shear

stress on the cross-slip plane did not form an extreme of the band of stress-strain tions. ence

curves for various orienta-

The observed trend in the orientation is consistent

with

the

new

particular, the extremely low work-hardening crystals

follows

in a straightforward

the mutual work-softening slip systems.

In

of (100)

manner

of the primary

The question

depend-

observation.

from

and cross-

remains why one combi-

nation of signs of primary and cross-slip leads to mutual work-softening while the other leads to mutual hardening. U. F. KOCKS Division

of Engineering

and Applied Physics Harvard University Cambridge, Massachusetts Reference 1. U. F. KOCKS, Trans. Met. Sot. AIME, * Received January 22, 1964.

in press.