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.