The hardening of copper single crystals by electron irradiation

THE

HARDENING

OF COPPER

SINGLE

M. J. MAKINt

CRYSTALS

and

BY ELECTRON

IRRADIATION*

T. H. BLEWITT$

Irradiation with 4 Mev electrons has been found to increase the critical shear stress of copper single crystals and to raise the whole stress-strain curve to higher stresses. The effect is independent of the irradiation temperature below a dose of ~5 pA/hr cm+; above this dose the critical shear stress of specimens irradiated at 20°C increases to a saturation value of 0.5 kg. mm2 after 40 pA/hr cm-z. Specimens irradiated at - 195”C, however, continue to harden and after 100 ,uA/hr cm?’ have a critical shear stress of 1 kg/mm2. Crystals irradiated at - 78°C lie on an intermediate curve indicating that the rate of hardening is a continuous function of the temperature and is not due to irradiating above or below a particular recovery stage. Annealing at 20°C after irradiation at - 195°C produces only ~10 per cent recovery in shear stress. The annealing of the hardening after irradiation at 20°C commences at ~80°C and continues up to temperatures above 400°C. No discrete recovery stages are visible. Thin film transmission electron microscopy does not reveal any clustered damage. Electron hardening is compared in detail with that due to neutron hardening and it is concluded that the absence of the displacement spikes has a very profound effect. DURCISSEMENT

DE MONOCRISTAUX

DE CUIVRE

PAR

IRRADIATION

ELECTRONIQUE

On a observe qu’une irradiation par des electrons de 4 Mev aocroissait la contrainte tangentielle critique de monocristaux de cuivre, et deplapait toute la courbe contrainte-deformation vers des contraintes plus &levees. L’effet observe est independent de la temperature d’irradiation au-dessous d’une dose d’environ 5 pA/hr cm+; au-dessus de cette dose, la contrainte tangentielle critique des echantillons irradies a 20°C croit jusqu’a une valeur de saturation de 0,5 kg/mma aprBs 40 pA/hr CII-~. Les Bchantillons irradies a - 195”C, cependant, continuent a durcir et presentent une contrainte tangentielle critique de 1 kg/nun2 apres 100 yA/hr cm+. Les cristaux irradies a -78°C se situent sur une courbe intermediaire indiquant que la vitesse de durcissement est une fonction continue de la temperature et n’est pas due a ce que l’irradiation est faite au-dessus ou au-dessous d’une temperature de restauration particuliere. Un traitement thermique Q 20°C apres une irradiation L -195°C n’amene qu’une restauration de la contrainte tangentielle critique d’environ 10%. La disparition du durcissement dil a l’irradiation a 20°C debute a 80°C environ et se poursuit jusqu’a des temperatures superieures a 400°C. On n’observe pas de stades distincts dans la restauration des proprietes et l’examen de pellicules minces en microscopic Blectronique ne revele pas de deterioration du materiau. Les auters Btablissent une comparaison detail&e entre les durcissements par irradiation Qlectronique et par irradiation neutronique et concluent que I’absence de zones fortement perturbees dans l’irradiation 6lectronique a une t&s grande importance. VERFESTIGUNG

VON

KUPFEREINKRISTALLEN

DURCH

ELEKTRONENBESTRAHLUNG

Bestrahlung mit 4MeV-Elektronen erhoht die kritische Schubspannung von Kupfereinkristallen und verschiebt die ganze Spannungs-Dehnungskurve zu hoheren Spannungen. Unterhalb einer Dosis von 5 PAh cm-* ist der Effekt unabhangig von der Bestrahlungstemperatur. Oberhalb dieser Dosis steigt bei Proben, die bei 20°C bestrahlt wurden, die kritische Schubspannung auf einen Sattigungswert von 0.5 kp/ mm2 nach 40 ~Ah/cm*. Bei -195°C bestrahlte Proben verfestigen sich jedoch weiter und haben nach 100 @h/cm2 eine kritische Schubspannung von 1 kp/nun2. Bei -78°C bestrahlte Kristalle liegen auf einer mittleren Kurve; das deutet darauf hin, da8 die Verfestigungsgeschwindigkeit stetig von der Temperatur abhangt und nicht auf Bestrahlung oberhalb oder unterhalb einer besonderen Erholungsstufe beruht. Nach Bestrahlung bei -195°C fiihrt Anlassen bei 20°C nur zu ~10% Erholung der Schubspannung. Die Erholung der Verfestigung nach Bestrahlung bei 20°C beginnt bei ~80°C und dauert bis zu Temperaturen tiber 400°C an. Diskrete Erholungsstufen sind nicht sichtbar. Transmissionsaufnahmen diinner Filme im Elektronenmikroskop zeigen keine Schiidigung in Form von Clusters. Die Elektronenverfestigung wird im einzelnen mit der Verfestigung durch Neutronen verlichen; es wird geschlossen, da13 das Fehlen stark gestiirter Zonen (displacement spikes) einen sehr bedeutenden EinAuIj hat. INTRODUCTION

The hardening

2 mm in copper.

of metals by neutron

well known and the main characteristics have been established

irradiation

is

of the process

in a number of different metals

work on the mechanical

changes due to electron

limited.

Dixon and Meechano)

ness of copper

of various crystal structures. In contrast, very little attention has so far been paid to hardening by irradia-

Previous

property

increased

from

irradiation

is very

report that the hard44.3 to 47.7 kg/mm2

after irradiation by 5 x lO1s electrons/cm2 at -20°C and that half this increase annealed after 8 hr at 170°C.

tion with charged particles and, in the case of heavy

Dieckampf2), in a study of 0.010 in. diam. large grained

charged particles,

polycrystalline

this is partly explained

by the very

short range of these particles making it impossible

to

irradiate conventional test specimens. In the case of electrons however this restriction need not apply, since the range of a 4 MeV electron is of the order of * Received August 1, 1961. t Metallurgy Division, A.E.R.E., $ Argonne National Laboratory, ACTA

METALLURGIA,

VOL.

Harwell, England. Illinois, U.S.A.

10, MARCH

1962

after 100 pA/hr cm-2 at

-195°C but no change in the yield stress. The effect of electron irradiation on the mechanical properties of metals is of considerable theoretical interest as a comparison to the effect of neutron irradiation because of the different distributions of damage

241

copper wires, reports an increase in the

flow stress at 1 y0 elongation

following

the

two

types

of

irradiation.

ACTA

242

Energetic

electrons,

because

METALLURGICA,

of their small mass, can

VOL.

cooling

10, 1962

by a liquid

is the only

transfer little energy to a metal atom and hence the

removing

the heat.

damage consists of relatively

specimen

and the accelerator

shielding

the specimen

of four defects distributed crystal.

isolated pairs or groups

at random

There is no possibility

throughout

of anything

the

like a

displacement spike being formed. Neutrons, however, because of their heavier mass and the absence of Rutherford

collisions, transfer on average a very much

flow down it. less

than

of a direct collision

difference

between

Primary

a 1 MeV neutron

knock-ons,

because

and a of their

charge and large mass, rapidly lose energy by collision with neighbouring to be produced disturbance. liberated

atoms and hence the damage tends in isolated

regions

A considerable

of very

quantity

during the formation

heavy

of heat is also

of these “zones”,

the

must be thin to avoid

from the beam.

The temperature

10°C

Calculations

atom).

way of

between

For irradi-

ations at 20°C the problem was solved by directing a jet of water onto the top of the specimen and letting it

greater energy per collision (up to 6,000 eV in the case copper

practicable

The film of liquid

in

the

rise in the water was

maximum

beam

showed that t’he maximum

current.

temperature

between the centre and the surface of the

specimen was also of the order of 10°C. Irradiations were also carried out at -20% and -78°C by this method using calcium chloride solution and

methyl

-195”C,

alcohol,

respectively.

To

the specimen was suspended

irradiate

at

in a target box

and

(Fig. 1) in a stream of liquid nitrogen blown through

exists of many atomic rearrangements during the cooling of the zone. Seegerc3) has estimated that approximately 20 per cent of the atoms within a

under pressure, the specimen being clamped to a thin copper window. To minimize the heating in the target

zone are ejected by a replacement

slit

the possibility

leaving an excess of vacancies The detailed

damage

neutron irradiation character,

collision mechanism

behind.

resulting

from

electron

and

of a metal is thus very different in

and it was felt that the mechanical

played

by the isolated

heavily damaged

point

and

icing

on

the

defects

zones in the hardening

properand the

mechanism.

Sindanyo showed

Single

crystals

of Johnson

maintained

the at

Above

250 pA/cm2. became both attributed

specimen

-195°C

DETAILS

up to this

Matthey

spectroscop-

accelerator

y0 = 2, = 40” was ensured

current

the

could

be

density

of

temperature and this was

All irradiations were therefore Since the

delivers a reproducible

copper calorimeter

standard

a beam

made with a beam current of 150 pA/cm2.

split

A

by

measurements

temperature

high and very unsteady

immediately

moulds.

prevented

to the failure of the liquid nitrogen to wet

ically pure copper, 1.7 mm in diameter, were grown in an argon atmosphere by the Bridgman technique using graphite

was

ring (Fig. 1). Thermocouple that

the specimen properly. EXPERIMENTAL

window

enclosing the space between the slit and the box in a

ties might reflect this difference and hence indicate the part

box, the beam was restricted by a wat,er cooled copper

orientation

of

by seeding to eliminate

density

was

measured after

was substituted

by

beam the current

a

calorimetric

method

a specimen

irradiation.

A thin

containing

a known mass of water

for the specimen and the temperature

the variation in tensile properties with crystal orienta-

rise measured after a short irradiation.

tion.

Tensile tests were carried out in a conventional hardbeam machine in which specimens could be irradiated

Specimens

with a gauge length

of 1 cm were

prepared by cutting the crystals into 2 in. lengths by warm dilute nitric acid and soldering loops to the ends to provide grips for tensile testing. the specimens were electropolished acid and the dimensions The irradiations

accurately

After preparation

an intermediate

rise in

in orthophosphoric EXPERIMENTAL

measured.

were carried out in the beam of a

4 MeV Metropolitan-Vickers linear accelerator. The diameter of the beam was ~11 mm and the penetration of a 4 MeV electron in copper is ~2

mm so that

the whole gauge length of the specimen was irradiated. The maximum energy density in the beam was quite high (1200 W/cm2) and considerable attention was devoted to securing adequate cooling of the specimens. In a beam of 300 PA/cm2 the heat generated in a specimen was ~225 W and calculations showed that cooling by a gas or by conduction and through a grip was completely

and tested at - 195°C without temperature.

along the specimen Direct inadequate.

RESULTS

Typical stress-strain curves showing the effect of electron irradiation at -195°C on the mechanical properties of copper single crystals are shown in Fig. 2, Following

irradiation

- 195°C without

the

crystals

intermediate

were

tested

at,

rise in temperature.

The effect of the irradiation was to increase the critical shear stress and generally to displace the stress-strain curve to higher stress levels. No yield point was introduced and the slope and magnitude of the “easy glide” region was unaltered. Similar results were obtained during tests at -195’C on crystals irradiated at 20°C.

MAKIN

AND

BLEWITT:

HARDENING

BY

ELECTRON

IRRADIATION

243

Accelerator

Liquid Nitrogen Inlet

I inch

FIG. 1. The target dose used for irradiating at - 195%.

The critical shear stress at -195°C has been determined as a function of dose during irradiations at -195, -78 and 20°C (Fig. 3). In all cases the specimens were stored at -195°C immediately after irradiation and specimens irradiated at low temperatures were not allowed to warm-up before testing. The critical shear-stress-dose curve is very sensitive to the irradiation temperature except at low doses. During i~adiation at - 195°C the critical shear stress continues to increase with electron dose, although at a decreasing rate, until after a dose of 100 ,uA/hr cm2 a critical shear stress of 1 kg/mm2 is reached, starting from the unirradiated value of 80 g/mm2. Irradiation at 2O*C,however, produces rapid hardening initially at the same rate as at -195°C up to a dose of 8 pA/hr cm2 after which the rate of hardening diminishes rapidly, the critical shear stress saturating at 500 g/mm-2 after 40 pA1h.r cm-2, remaining constant thereafter up to doses as high as 200 pA/hr cm-2. Irradiations at -78°C and -20°C produces curves intermediate between the 20°C and -195% results. Due to the difficulty of irradiating specimens at -20°C for long periods it was not possible to ascertain whether a true saturation occurred at this temperature. To determine whether the great difference in the hardening obtained during irradiation at - 195’C and 20% was due to irradiating on either side of a large recovery stage specimens were irradiated for 80 pA/hr cmm2at -195°C and were then annealed at 20°C for

several hours before testing at -- 195°C. This annealing decreased the critical shear stress by only 10 per cent (Fig. 4), i.e. from ~9OOg/mma to ~800 g/mm2, in comparison to the value of 500 gimm2 observed after irradiation at 20°C. Annealing for increased times at 20°C produced no further decrease. The results obtained at -78 and -20°C also suggest that the effect is not due to irradiating on either side of a recovery stage but is a ~ont~uous function of temperature. Annealing experiments to ascertain the temperature range in which recovery of the irradiation induced hardening occurred were carried out on a batch of

0’

5

IO

15

20

25

GLIDE, % Pm. 2. Stress-strain curves of electronirradiated copper

single crystals of identical orientation. y. = lo = 4V.

244

.4CTA

1

20

METALLURGICA,

10,

1962

I

/

100

80

60

40 ELECTRON

FIQ. 3.

VOL.

hr cme2

DOSE,

The critical shear stress of copper single crystals as & function of eleotron dose at various temperatures.

specimens

irradiated

for

31 pA1h.r cm-2

at

20°C.

The critical shear stress of these specimens at - 195% was

490 g/mm2, almost at the saturation value. Each specimen was annealed for 30 min at a particular temperature way.

and then tested at -195%

in the usual

The results shown in Fig. 5 reveal that recovery

commences

at

temperatures

increases

steadily

Recovery

was not

30 min at 420°C.

with

as low

as 80%

increasing

complete,

however,

No particular

and

temperature.

annealing

even

after

stages are

recognisable.

ELECTRON

In view

MICROSCOPE

of the direct

OBSERVATIONS

observation

of damage

in

copper after neutron(4y5) and alpha particle(6) irradiation by thin film transmission attempt

was made to observe

electron irradiation.

electron microscopy, the defects

an

following

Annealed copper foils, 0.0005 and

0.002 in. thick, were irradiated at 20°C for doses up to 1500 pA/hr cm-2 and at -195°C for doses up to 200 ,uA/hr cm- 2. Electropolishing was carried out in an orthophosphoric acid-water bath at 20°C and the

GLIDE, ‘lo FIG. 4. The decrease in flow stress which occurs annealing at 20°C crystals irradirtted at -195’C.

on

specimen examined in a Siemens Elmiskop I microscope. No defects attributable to the electron irradiation were observed. A further experiment was carried out in which defects were introduced by other methods, such as neutron irradiation or quenching, and then observing whether these defects were altered by subsequent electron irradiation (1500 PAlhr cm-2) at 20°C.

MAKIN

AND

BLEWITT:

HARDENIXG

No significant effect of the irradiation was observed in either case. DISCUSSION

Bombarding electrons lose energy in a solid by electrostatic collision with both the electrons and the nuclei in the material and because of the much greater equality in the masses, most of the energy is lost in electron-electron collisions. In a metal these produce, however, no permanent damage, only electron-nucleus collisions result in dispIaced atoms. The cross-section for such a collision, in which more than 25 eV is transferred to the copper atom, is quite large and has been calculated(7) to be ~30 barns for a I-MeV electron increasing to ~75 barns for a 4-MeV electron. The maximum energy transferred is then 68 and 272 eV and the mean energy 44 and 66 eV respectively. Hence, the knocked-on atoms can themselves produce o&y a very small number of further dispIacements. The total damage rate is nevertheless quite rapid. For example, taking average values, the concentration of defects produced in a copper single crystal 1.7 mm diameter will be ~3.4 x 1O-6 per pA/hr cm-2. In neutron irradiation of a hollow uranium cylinder in the BEPO reactor at Harwell, where the fast &IX is 4 x 1O1rneutrons/cm2 see-l, the concentration of displacements produced per week should be 18 x lo-“, assuming a collision cross-section of 3 X 1O-24cm2 and thirty defect pairs per primary knock-on. Hence, an irradiation of 5.3 ,uA/hr cm-2 is equivalent to 1 week in the reactor and with the accelerator at full beam power the specimen can be given this dose in

BY

ELECTRON

IRRADIATION

only 2.55 min. The damage rate is therefore very rapid compared with neutron irradiation. The damage differs from that produced by reactor neutron irradiation, however, in that it consists of very widely dispersed pairs of interstitial atoms and vacancies instea.d of heavily clustered groups of defects. This difference in the spatial distribution of the damage after electron and neutron irradiations has a very profound effect on the radiation hardening and this is apparent both in the ma,gnitude and the nature of the hardening. Perhaps the most striking difference is in the sensitivity of electron hardening to the irradiation temperature. This does not occur during neutron irradiation, where the two hardening curves obtained at -195°C and 20°C differ by only the ~10O/~ recovery which occurs at about 0°CY8). This latter recovery effect, incidentally, is one of the very few points of similarity between the two types of hardening. The high temperature recovery of the hardening is very different after electron and neutron irradiation.@) Instead of the steady decrease in critical shear stress with annealing temperature observed after electron irradiation, Fig. 5, recovery after neutron irradiation occurs in a fairly well defined stage beginning (for a 30-min anneal) at about 300°C and extending up to ~380°C. No recovery is observed at temperatu~s as low as 100°C. EIectron hardening is much closer in magnitude to quench hardeningus~rlJbut there are very important differences, particularly in the annealing behaviour. Quenched gold recovers between 500 and 700% and @=3fp

-I -a

A. hr. cm

Anneding Tim@ 3Omin Testing Tempemture - I~S”C.

aim loo

Annealing

245

400 Temperature, “C

Fta. 5. The deeream in critic& shear stress on annealing of crystals irradiated at 20°C.

ACTA

246

METALLURGICA,

annealing for as long as 6 hours at 250°C produced detectable

softening,

behaviour

in

marked

of electron irradiated

duced tion

by electron

dislocations

obstacles

unequivocably

from pro-

is due to the forma-

resisting

suggests that most probably obstacles.

very small, as they

the motion

of

the hardening

is due to

that

the

type

at different

neutron

and

hence

there

must

of irradiation

suggests

collision

nucleation mutual

during

that

this process

nucleation

point

of intermingled irradiation

will

occur

in the hardening.

will

defects.

ture at which recovery

between - 195°C and 2073, and, thirdly,

These differences the difference the

leading

to

early of the

by the low tempera-

commences.

this hardening The

complete electron

hypothesis

absence

associated

irradiation.

that the “zones”

and principally

for the majority

mental evidence for this hypothesis by comparing

spikes

This supports the formed in the displace-

hardening produced by neutron irradiation.c3) obtained

with

of the damage pro-

of displacement

ment spikes are responsible

of the Experi-

has recently been

the mechanical

properties

of

neutron irradiated copper with the defects observed by thin film electron microscopy.(5) due

interstitial-vacancy produced

to

the

The hardening pro-

is therefore the type of

introduction

of

pairs and is different

by either zones or quenched

isolated from that

in vacancies.

principal

ACKNOWLEDGMENTS

The authors would like to acknowledge assistance

and that

due to

differences

are,

the valuable

given by the operating staff of the electron

tory.

The main feature of this work has been to determine

between

stages.

are undoubtedly

in the distribution

linear accelerator

in

some of the basic characteristics of hardening due to electron irradiation and to demonstrate the great irradiation.

on the irradiation

and the absence of discrete recovery

result

CONCLUSIONS

difference

the great

temperature

vacancies

The instability

resulting damage is demonstrated

neutron

secondly,

the very low temperature at which recovery commences

may

of clusters

on a very fine scale and a large amount of

recombination

saturation

defects,

after

produce the same

hardening

hardening

of the

electron

upon the tempera-

of migrating

The high rate of formation interstitials

of point

be some

other

the

of electron

duced by electron irradiation

The early saturation

depend upon the homogeneous

and

concentration

these zones are

clusters of point defects that can

give rise to hardening.

by the random

with

for the hardening.

however,

effect at 20°C and the dependence ture

the damage identical

and this is so if the zones pos-

During electron irradiation, process of forming

of

temperatures.

irradiation

tulated by Seegerc3) are responsible formed

or amount

must be virtually

that formed at 20°C

not

of

with

be seen in the electron

of the hardening on the irradiation

during

magnitude

with that due to neutrons

duced by the two types of irradiation

implies

at -195°C

small

cannot

stored is different

Conversely, formed

very

compared

dependence

during

The dependence damage

the

hardening

1962

If this is so then these must be

microscope. temperature

firstly,

10,

irradiation doses which theoretically

or to the difficulty of initiating dislocation The absence of yield points, however,

movement. dispersed

the

whether the hardening

irradiation

of dispersed

no

to

copper.

It is not possible to determine the present experiments

contrast

VOL.

at the Wantage

Research

Labora-

REFERENCES 1. C. E. DIXON and C. J. MEECHAN, P&s. Rev. 91,237 (1953). 2. H. DIECIUMP, NAA-SR-1452 (1955). 3. A. SEEGER, PTOC. 2nd Int. Conf. Peaceful Uses Atomic Energy, Geneva 6, 250 (1958). 4. J. SILCOX end P. B. HIRSCH, Phil. hfag. 4, 1356 (1959). 5. M. J. MAIUN, A. D. WHAPHAM and F. J. MINTER, Phil. Mag. 6, 465 (1961). 6. R. S. BARNES and D. J. MAZEY, Phil. Mag. 5,1247 (1960). 7. R. S. PEASE and G. H. KINCHIN, Rep. Progr. Phys. 18, 1 (1955). 8. M. J. MAKIN, Acta. Met. 6, 305 (1958). 9. T. H. BLEWITT, R. R. COLTMAN,R. E. JAMISONand J. K. REDMAN, J. Nucl. Mat. 2, 277 (1960). 10. H. KIMURA, R. MADDIN and D. KUHLMAN-WILSDORF, Acta. Met. 7, 154 (1959). 11. M. MESHII and J. W. KAUFFMAN, A&c. Met. 7, 180 (1959).