Hardening of copper single crystals irradiated at — 195°C

LETTERS

TO THE EDITOR

Hardening of copper single crystals irradiated at - 195% Copper single crystals are known to harden rapidly

NE a_4 Y

during neutron irradiation at pile temperature (NSO”C)(~) and recovery of the irradiation-induced

f

hardness occurs only when the temperature

is raised

93 ,,,

to

and

$

300”C.(1)

vacancies mobile

Both

produced

the

interstitial

by irradiation

at 80°C in copper

atoms

are believed

and hence either or both

defects may be responsible for the hardening. information

the

to be S 3

To gain

1

irradiation experiments have been made at -195% where it is believed that the interstitial atoms are mobile but the vacancies are not. therefore,

the vacancies

I 0

scopically by

technique

seeding.

preparation

the

were

approximately

stress occurred

in the Harwell reactor BEPO for one week at -195°C.

specimen

The total dose of slow neutrons

-159”,

was 8 x 1017 n/cm2 to thermal

the

appropriate

The temperature

were

sealed in silica quills, and irradiated

and the ratio of fast (epithermal)

at which

was annealed -134”,

flux is

rise. shown

The stress-strain in curve

machine

permitting 1.

Substantial

in flow

by tests in which a

Annealing

at higher

no further effect.

suggest

that the reduction

in flow

stress on annealing is due to the migration of vacancies,

to

curve of one such specimen is

(a), Fig.

produced

These results

and tested

the temperature

by

for 30 min at respectively

remainder after 30 min at 20°C.

to decrease and were

to the tensile

given

-45”, 0” and 2O”C, between tests at -195°C. About 50 per cent of the reduction in flow stress occurred after 30 min at 0°C and the temperatures

without

value

-78”,

week to allow the radioactivity at -195°C

I 30

the reduction

was determined

about unity in the position used. After irradiation the specimens were stored in liquid nitrogen for a further then transferred

I 25

curve (b).

was ensured crystals

I 20

tion the flow stress was reduced by the annealing to

A standard

(yO = 40°, il, = 40’)

After

electropolished,

I 15

FIG. 1. Stress-strain curves of copper single crystals irradiated and tested at -195”C, (a) with no intermediate annealing and (b) after annealing at 20°C for 80 min.

from spectro-

pure (99.998 per cent) copper.

crystal orientation

I 10

% glide

by the irradiation

Single crystals of copper 2.3 mm in diameter by the Bridgman

I 5

At this temperature,

produced

will remain as isolated point defects. prepared

,

of the role played by each defect neutron

hardening

which

are believed

about

0°C.

Until

to become

mobile

this temperature

in copper is reached

as has been shown

vacancies

are present as isolated

previously. c2) The critical shear stress was raised from 0.11 to 2.64 kg/mm2 and after irradiation the stress-

reduction

in flow stress on annealing at 20°C may be

strain curve consisted

the dispersion

occurred on irradiation at -195”C,

due either to the removal

of an early region of very low

point defects.

at the The

of a strengt,hening effect of

of isolated vacancies

or to the annihi-

work hardening, up to 10 per cent glide, followed by a

lation by the condensation

sudden rapid rise in flow stress between 10 and 12 per

damage causing the large increase in flow stress. The nature of this damage is not known, but may be due

cent glide. The stress-strain at -195%

to annealed configurations

curve of a similar crystal irradiated

and annealed

of interstitial

of part of the

atoms, either

as aggregates in the lattice or as jogs on dislocations.

at 20°C for 80 min before

testing is shown in curve (b).

of vacancies

Quenching

The critical shear stress

experiments

on

aluminium’3)

and

has been reduced by 7 per cent and the flow stress remains lower than that of the unannealed specimen. Furthermore, the “bump” in the curve disappears entirely on annealing. Experiments have been made

topper,(4) however, indicate that quenched-in vacancies produce greater hardening after annealing than when present as a dispersion of isolated point defects. This observation suggests that the reduction

in which an irradiated and unannealed crystal has been tested to a given strain, unloaded, annealed at 20°C

in the flow stress of irradiated copper on annealing at 20°C is due to annihilation of part of the stable damage

for 80 min and retested.

In all stages of the deforma-

by vacancy

ACTA

VOL.

METALLURGICA,

6,

APRIL

1958

305

condensation.

If the effect were due to

ACTA

306

the removal vacancies expected

of the strengthening

then

quenched

METALLURGICA,

effect

copper

to soften on annealing

VOL.

6,

1958

of isolated

would

also

be

at 2O”C, whereas in

fact, hardening is observed. At the present time no explanation in the stress-strain curve “bump” and its removal on annealing. Further

experiments

is offered of the after irradiation

on these phenomena

progress. I should like to express my thanks Finniston

are in

to Dr. H. M.

and Dr. A. H. Cottrell for their interest and

encouragement. M.J.

MAKIN

Division Atomic Energy Research Establishment Metallurgy

Haruall, Didcot, Berks., England References 1. T. H. BLEWITT and R. R. COLTMAN Phys. Rev. 82, 769 (1951); M. J. MARIN Unpublished work. 2. A. W. MCREYNOLDS, W. AUOUSTYNIAK, M. MCKEOWN and D. B. ROSENBLATT Phys. Rev. 98, 418 (1955). 3. R. MADDIN and A. H. COTTRELL Phil. Mug. 40,735 (1955). 4. M. J. MAKIN Unpublished work. Received October 10, 1957.

FIG. 1. Cracks in Zn-ZnO film with thread-like formations.

Plastic deformation in zinc oxide films By means of electron-microscopic vacuum

deposited

zinc

layers,

examination some

of

interesting

observations were made concerning considerable plastic deformation of zinc oxide films formed on the zinc surface. The object of investigation a metallized-paper

was the zinc layer from

capacitor.

deposited on lacquer-coated

This layer is vacuum

paper and has a thickness

of about 0.1 ,u. The zinc layer was separated from the paper by dissolving the lacquer in acetone. The separated

zinc film was carefully

cleaned in trichlor-

ethylene, which dissolves the impregnant, specimen

holder

diffraction

and microscopy

microscope. The patterns

(copper

obtained

grid)

and

placed on a

examined

in a transmission by

electron

showed that the film was composed

diffraction

of zinc and zinc

oxide and that the crystals of the components, of which are hexagonal,

by

electron-

both

were oriented with the basal

planes parallel to the film surface. By electron microscopic examination we observed thin thread-like formations in some cracks in the sample film (Fig. 1). Most of these threads contained dark diffuse bands caused by Bragg reflections (Fig. 2). By altering the position of a thread relative to the electron beam, the dark bands could be moved

FIG. 2. Electron interference bands indicating that the threads are single crystals.