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The effect of alpha-particle bombardment on the creep of cadmium single crystals
. J. Nuclear Energy, 1955. Vol. 1, pp. 181-193.
Pergan~on Press Ltd., London
THE EFFECT OF ALPHA-PARTICLE BOMBARDMENT ON THE CREEP OF CADMIUM SINGLE CRYSTALS By M. J. MAKIN Atomic Energy Research Establishment, Harwell, Berks (Received 3 August
1954:)
Abstract-The effect of alpha-particle bombardment on the creep of cadmium single crystals at room temperature has been measured in an attempt to repeat the observations of ANDRADE. No significant change in creep rate on initiation of the bombardment has been detected, although experiments were made on crystals differing in purity, surface condition, and orrentation. It is considered that the increase in creep rate observed by ANDRADE was either due to geometrical softening or to the disruption of a surface film of somewhat critical thickness.
I. INTRODUCTION The published literature on the effect of particle bombardment on the creep of metals is very scanty and to some extent contradictory. The earliest contribution was from ANDRADE (1945), who reported an effect due to the bombardment of cadmium single crystals by alpha particles from a polonium source. ANDRADE found that a marked increase in the rate of glide occurred if the bombardment was started during the transient stage of the creep-curve. Crystals 0.040-in. diameter stressed so as to produce a creep-rate of about 0.05 per cent per minute showed an acceleration in rate of up to five times on bringing up the alpha sources, although the crystals were bombarded over only one-third of their length. ‘The magnitude of the effect was found to decrease the greater the extension of the crystal previous to the initiation of the bombardment, until after 12 per cent extension no effect was found. The strength of the source used by ANDRADE* was approximately 28 millicuries, and under the conditions of his experiment the flux at the surface of the crystal was about 1.5 x 10s particles crne2 see-l Since the range of the particles in the cadmium was only about 0*0002 in. the effect of the bombardment may be regarded as a surface efFect. ANDRADE attributed his results to the initiation of new glide-planes at the surface by the disturbance produced during bombardment. Once initiated, glide on a particular plane is independent of surface condition. The decrease in the size of the effect after large strains was interpreted as implying that only during transient creep were new slip-p!snes becoming active and that secondary creep was due to continued slip on already developed planes. Hence initiation of new glide-planes by alpha-particle bombardment could only occur during transient tree-p. KITTEL (1947) has reported the results of four tests on the creep of polycrystalline aluminium wire under alpha-particle bombardment from an 85-millicurie polonium source. He found that the minimum creep-rate was slightly lowered by the bombardment. Experiments have been reported by WITZIG (1952) on the creep of polycrystalline copper under bombardment with 16-MeV deuterons from a cyclotron. Copper wires 0.020-in. in diameter tested under 10,000 lb.-in.-2 at 260°C showed no significant * Private
communication. 181
M. J.
182
MAKIN
change in rate when a deuteron beam of 1012 particles cmP2 set-l was switched on or The experiment was not analogous to that of ANDRADE, however, as the range of a 16-MeV deuteron in copper is about 0.0128 in. so the particles penetrated over half the diameter of the specimens. Since the accuracy of the experiment was not very high, about *20 per cent, it is not certain that no effect occurred. JONES, MUNRO, and HANCOCK (1954) have reported an experiment on the creep of aluminium under neutron bombardment in a pile. An aluminium tube, two inches in diameter and of 0.062-in. wall thickness, tested under a hoop stress of 3,500 lb-in.? at 5O”C, showed no significant change in steady-state creep-rate on Under these coninsertion into a fast-neutron flux of about 1 x 10%. cm_2 set-l. ditions the damage caused by the flux was probably distributed fairly uniformly throughout the specimen. The accuracy of the experiment was about &lo per cent. SLATER (1951) indicates that the creep-rate under particle bombardment may be either increased or decreased depending upon the relative influence of interstitials, vacancies, and “thermal spikes” produced by the passage of a particle through the lattice. In the absence of annealing, interstitials and vacancies may be expected to result in a hardening of the metal, an erect which has been observed with both single crystals and polycrystals irradiated at a low temperature (CEIB and GRACE, 1952; BLEWITT and COLTMAN, 1951.) Thermal spikes are regions locally raised to a high temperature due to atomic collisions with an incident particle. In a material under stress, relaxation will occur in these regions, resulting in a more rapid creep-rate than would occur without the radiation. In view of the lack of information on this subject and the contradictory results of the published experiments, the effect found by ANDRADE was studied more closely. This experiment has a higher precision than the others, and it is the only one in which an increase in creep-rate has been observed. It is of particular interest to know whether the effect is a general one which occurs with all cadmium crystals or if it is only observable with crystals having a particular surface condition. off.
2. EXPERIMENTAL
DETAILS
(i) Preparation of Specimens Single crystals of cadmium one millimetre in diameter were prepared by the method of ANDRADE and ROSCOE(1937) from metal supplied by Johnson Matthey & Co. Ltd. Two purities of cadmium were used, which will be referred to as spectroscopic and standard purity throughout the paper. Spectroscopic examination of both samples was carried out by Johnson Matthey Ltd., with the following results:-
Standard purity sample %
’ Spectroscopic purity sample %
Cadmium 99.96 (by difference) Lead 0.02 Thallium 0.01 Copper OWO5 Silver 0401 Zinc OGO2 No bismuth
Cadmium 99.99 (by difference) Silver < 0401 j Copper faintly visible Calcium in spectrograph Iron
!
The effect of alpha-particle
bombardment
on the creep of cadmium single crystals
183
The cadmium wires were inserted into quill tubes of internal diameter slightly greater than the diameter of the wire. The tubes were evacuated, filled with argon, and sealed. By this method the wire was grown into a crystal in only a small volume --Pulley
--Perspex dry box
Concrete
block
FIG. I.--The creep machine.
of argon, and the risk of contamination with nitrogen was minimized. The surface of the crystals “as grown” was very bright, and previous tests have shown them to be practically free from oxide films. The orientation of the crystals was determined from back-reflection Laue X-ray photographs by the method due to GRENINCER (1935). The crystals were then cut into three-centimetre lengths and stored in vacua before use. (ii) Apparatus Creep tests on single crystals of cadmium were carried out under constant load at room temperature. The general arrangement of the apparatus is shown in Fig. 1. The load was applied to the specimen through wire loops welded to the ends of the
M. J.
184
MAKIN
crystal. The lower end of the specimen was attached to a rigid support and the upper end connected to the scale-pan by a thin copper wire. The device shown at A was used to apply the load to the crystal. It consisted of a stop attached to a carriage, the vertical position of which could be controlled by rotation of the knurled ring M. Slow ascent of the carriage enabled the load to be transferred smoothly from the stop to the specimen. The extension of the crystal was measured on a scale after magnification by an optical lever. The aluminium pulley L was encircled by a fine tungsten wire, the ends of which were connected to the rod attached to the upper end of the specimen so that extension of the specimen caused a rotation of the pulley. Mounted on the pulley shaft was a small concave mirror which produced an image of a crosswire on the scale. The magnification achieved was about two hundred. It was necessary to perform the experiment inside a drybox to minimize the danger of contamination from the polonium alpha sources. A slightly reduced pressure inside the box was maintained by an extractor fan to prevent the egress of polonium through the hole surrounding the thin rod carrying the load to the upper end of the specimen. The polonium alpha sources of initial strength 140 millicuries were deposited on the inside surface of a split silver cylinder. The dimensions of the sources were much the same as those used by ANDRADE,i.e., about one cm long and eight mm in diameter, and they were supported on two movable stands so that they could be brought up to the crystal during a creep test. The flux produced as the surface of the crystal was about 5 x 10s particles cm-2 se&. The whole apparatus was mounted on a steel framework resting on wooden blocks, and the system was made independent of external vibration by mounting the blocks on thick foam rubber. Great care was taken to avoid any accidental straining of the fragile specimens, and special techniques were adopted to prevent this; in particular the crystals were never permitted to rest in the horizontal position with any part of their length unsupported. The load to produce the required creep-rate was estimated from the orientation of the crystal and its surface condition. Initially a load slightly less than this was applied, and if this proved insufficient the specimen was unloaded before adding more weights to the scale-pan. The preliminary extension of the crystal during this procedure was less than 0.03 per cent. After the correct load was found, the specimen was allowed to creep and a curve of the extension against time plotted. No precautions to control the temperature of the specimen were necessary, as the duration of the test was usually less than one hour. The accuracy of the experiment is estimated as &5 per cent. 3. EXPERIMENTAL (i) “As-grown”
RESULTS
Crystals
The results of seven tests on spectroscopic purity (a) Spectroscopic purity. specimens in the “as-grown” condition are listed in Table 1, and a typical extension time curve is shown in Fig. 2. All the specimens showed smooth creep-curves and a complete absence of any yield-point phenomena. In every case the bombardment was started within that range of extension, i.e., up to 3 per cent, in which an effect was observed by ANDRADE No change in creep-rate on either bringing up or removing the alpha-particle sources
The effect of alpha-particle
bombardment
185
on the creep of cadmium single crystals
TABLE 1 .---SPECTROSCOPIC PURITY “AS-GROWN” CRYSTALS
No.
Yo
Initial extension Resolved shear stress ~before bombardment gm/mm* %
10
I
1
28 29 30 31 32 33
45 10 18 36 14 40 30
45 13 22 40 26 40 36
I L
0.61 261 0.42 0.14 0.31 0.09 0.17
16.7 18.8 22.0 22.9 19.4 20.1 18.1
I
I
Rate of extension immediately before bombardment %/min 0.041 0.182 0.095 OGO8 0.072 0.013 0.018
Ratio of creep-rates
I
0.93 0.96 1.00 1.02 0.98 1.00 1.Ol
Note: 1y,, = angle between specimen axis and glide-plane. 1, = angle between specimen axis and glide direction.
0.06 E f .k 0.04 VI 5 f; W
0.02
15
10 Time FIG. 2.-Spectroscopic
20
In mtnutes
purity crystal No. 30. Initial length 24.0 mm, load 40 g.
was detected in any test despite a wide range of initial orientations and creep-rates. Movement of the sources, however, sometimes produced a small elongation of the specimen. This was observed on both bringing up and removing the sources, and was produced by vibration in the apparatus when the hands were inserted into the rubber gloves of the dry-box. This extension, never greater than 0.025 per cent, could be almost eliminated by very careful handling. Microscopic examination of the specimens after test revealed an almost uniform distribution of glide bands. (b) Standard Puritan. Experiments were also made on standard cadmium crystals in the “as grown” condition. It was found that the creep of specimens tested only a few hours after growth was not as smooth as in the spectroscopic purity specimens
M. J.
186
MAKIN
but took place in a series of small jerks, the size of which varied from specimen to specimen. The creep-curves were irregular and unsuitable for testing with the alphaparticle sources. Crystals tested ten or twenty hours after growth, however, showed much smoother creep-curves, and after leaving for ninety hours at room temperature in
Time FIG. 3.-Standard
in minutes
purity crystal No. 22. Initial length 30.8 mm, load 130 g.
vacua the curves were as smooth as those given by spectroscopic purity crystals. This unusual phenomenon is discussed more fully later in the paper. Details of four experiments on standard purity crystals in this condition are given
in Table
2. TABLE
No.
10 20 22 23
~0
17 12 12.5 36
Resolved shear stress gm/mm2
&I
18 20.5 23 38
2.--STANDARD
i , I
35.4 34.5 33.0 34.0
PURITy “AS-GROWN"
Rate of extension
Initial extension before bombardment
I I
% 0.47 0.32 0.44 0.57
CRYSTALS
im~~~~~~m~~re i
~ Ratio Of ~ creep-rates
%/min 0.091 0.060 0.052 0.104
I ~
0.99 0.99 0.98 1.00
No change in creep rate on either bringing up or removing the alpha sources was detected. A typical curve of extension against time is shown in Fig. 3. Microscopic examination of the specimens after test indicated a fairly uniform distribution of glide-bands.
The effect of alpha-particle
bombardment
on the creep of cadmium single crystals
187
(ii) Annealed Standard Cadmium Crystals It was suspected that the jerky flow observed in standard cadmium crystals tested soon after growth was due to nitrogen in the metal. To check this, a few specimens were annealed in vacua for one hour at 200°C. The extension of a specimen tested immediately after annealing was so irregular that it was almost impossible to take creep measurements. No extension. was observed until a large load was applied, and then yield occurred suddenly with a marked reduction in resistance to flow and an acceleration in creep-rate. This increase in rate was so rapid that unless the load was quickly removed the elongation could reach 10 per cent in a few seconds. If the extension was stopped after the first yield, however, a very much reduced load was sufficient to produce a creep-rate of about 0.05 per cent per minute. The yield-point was followed by a period of fairly smooth flow, after which creep became increasingly jerky until the majority of the extension was occurring in the jerks with periods of Microscopic examination of the crystals recomplete cessation of flow between. vealed that the glide was concentrated into one or two regions within which heavy deformation had occurred. Re-annealing of the specimens caused the return of the yield-point. Specimens tested some hours after annealing hacl lower values of the upper yield stress and less jerky creep than those tested immediately. After ninety hours at room temperature in vacua the yield-point had completely disappeared and a smooth creep curve was obtained. The stress required to produce a given creep-rate was found to be increased, however, and microscopic examination revealed large glide in localized regions. With the exception of the gradual disappearance of the yield-point on ageing at room temperature the behaviour of these specimens is consistent with results obtained by other workers on crystals containing dissolved nitrogen. The specimens exhibit the phenomena of upper and lower yield-point described by COTTRELL and GIBBONS (1948). GIBBONS (1952) has reported that localized deformation is observed with crystals containing nitrogen and the creep behaviour is generally very similar to that described by DUMBLETON (1954) for zinc crystals at 100°C. In view of the close similarity between the observed phenomena and those described in the literature it was concluded that the crystals contained nitrogen. It was thought possible that the effect obtained by ANDRADE may be observable with annealed and aged standard cadmium crystals, and five tests were performed to check this. The results are shown in Table 3. Due to the localized nature of the deformation, these crystals showed a much greater tendency to geometrical softening than unannealed specimens of similar orientation where the glide was distributed fairly uniformly. This resulted in creepcurves in which the rate of creep accelerated continuously during the test. It was observed that the extension produced by the vibration on moving the sources sometimes initiated geometrical softening and hence an i-ncrease in creep-rate. No effect directly attributable to the alpha-particle bombardment was found. (iii) Oxide-coated (a)
coated
Crystal.7
Spectroscopic
Puritan.
spectroscopic
purity
Nine irradiation experiments were made on oxidespecimens. Crystals were oxidized by heating in the
M. J.
188 TABLE
No.
10 18 20 23 26 -
All
Resolved shear stress gm/mm2
18 10 20.5 38 15
68.0 61.3 77.0 66.4 76.8
wo
17 10 12 36 11
!_
3.---ANNEALED
MAKIN
STANDARD
CADMIUM
CRYSTALS
Rate of extension Initial extension immediately before before bombardment bombardment %/min % ~~ ~_ __._ 0.016 0.042 O@l4 0.092 0.109
0.079 0.37 0.22 0.41 0.50
Ratio of creep-rates
1.01 0.98 1.31 1.04 1.16
atmosphere to 240°C for various times; after half an hour at temperature a faint brown coloration was visible on the surface of the crystal and a dark brown coat was produced after fifteen hours. As it was anticipated that a change in creep-rate on bombardment might occur only with specimens covered with a very thin oxide film, tests were also made on crystals exposed to the atmosphere at room temperature. The results are summarized in Table 4. TABLE
No:
a.--OXIDE-COATED
SPECTROSCOPIC
PURITY
CRYSTALS
-
-
Yo
1,
Resolved shear stress gm/mm2
~
Initial extension before bombar>ment ”
/ EL% / immediately
j / Ratlo of
before bombardment %/min
creep-rates
%
Oxidation Treatment
, A A 29 29 30 31 31 31 31
17 17 18 18 36 14 14 14 14
21 21 22 22 40 26 26 26 26
19.8 24.4 23.8 22.0 22.9 26.0 25.0 30.5 37.5
-
~ 1 ~ ~ I
0.57 0.72 0.13 0.20 0.15 0.10 0.33 0.22 0.39
0,069 0.136 0.019 0.041 0,018 0.022 0.045 0.038 0.065
1.01 1.00 1.00 1.02 0.94 0.98 1.01
1.03 0.97
4tmos.for 4 weeks 4tmos.for 4 weeks + hr at 240°C 4 hr at 240°C 3 hr at 240°C Q hr at 240°C 2 hr at 24O’C 2$ hr at 240’C 51 hr at 24O’C
No change in creep-rate on bombardment was observed in any of these tests, and microscopic examination revealed a uniform distribution of slip lines. The effect of the oxide film in hardening the crystal is apparent. (b) Standard Purity Ten experiments were made on standard cadmium crystals oxidized for various times at 240°C. Since the oxidation treatment corresponded to an annealing. yieldpoint phenomena were found if the specimens were tested too soon after oxidation. After ninety hours at room temperature, however, the yield-point had disappeared and the following results were obtained:
The effect of alpha-particle
bombardment
TABLE ~-OXIDE-COATED
on the creep of cadmium single crystals
189
STANDARD PURITY CRYSTALS
Initial extension/ Rate of extension
~ Oxidation treatment
No.
10 14 17 18 18 18 20 20 26 26
17 26.5 22 10 10 10 12 12 11 11
18 41 24 10 10 10 20.5 20.5 15 15
90.3
77.3 107.0 65.3 70.5 68.5 80.8 84.5 81.8 86.6
0.50 0.77 0.36 0.26 0.81 0.20 0.29 0.36 0.20 0.59
0.170 0.258 0.019 0.121 0.118 0.034 0.123 0.145 0.047 0.051
15 hr at 240°C 15hrat24OT Grown in air 1 hr at 240°C 3 hr at 240°C 3 hr at 240°C 1 hr at 240°C 3 brat 240°C & hr at 240°C $ hr at 240°C
1.02 0.98 1.01 1.00 1XKl 1.17 1.01 1.17 1.83 103
L
No change in creep-rate attributable to the bombardment occurred, but it was sometimes found’ that there was an acceleration in rate due to geometrical softening initiated by vibration during movement of the sources. A curve showing this behaviour in a specimen oxidized for three hours is shown in Fig. 4. Other
5 --
o-
.5-
b 0
10
20
Time N-Iminutes FIG.
4.-Standard
cadmium crystal No. 20. Oxidized three hours at 240°C. Initial length 30.8 mm, load 340 g.
specimens cut from the same crystal and oxidized for the same time showed very similar behaviour when the actions required to move up the sources were performed without initiating the bombardment. 2
M. J. MAKIN
190
(iv)
Hardening of Crystals by Bombardment It was observed
condition
that
spectroscopic
showed a small hardening
The load required to produce a creep-rate mined
and then the specimen
purity
crystals
after prolonged
tested
in the “as-grown”
alpha-particle
bombardment.
of about 0.05 per cent per minute was deter-
was unloaded
and the bombardment
commenced.
After the desired time the sources were removed and the specimen re-tested under the same load.
The results of three such experiments
are given in Table
6.
TABLE 6.-HARDENING OF CRYSTALSBY PROLONGED BOMBARDMENT bombardment
No.
Creep-rate after bombardment %/min
30 30 30
hrs
0.04 0.02 0.05
The magnitude bombarded
1 2 1
of the effect has no special
over only one-third
of its total
on starting
significance,
as the specimen
Microscopic
was re-
of creep tests in which no change in creep-rate
has
after test.
OF RESULTS
it is concluded that the by ANDRADE are not general for cadmium crystals. It is still possible,
effects reported
or stopping
length.
of the specimens
4. DISCUSSION In view of the large number
20.7 20.0 21.2
examination
vealed nothing unusual in the appearance
been detected
Resolved shear stress gm/mma
Duration of
of course,
that ANDRADE obtained
possessed
some
particular
surface
the bombardment,
a true radiation condition,
effect, since his crystals may have
purity,
or orientation
for which
an
effect occurs but the present work indicates that if this is the case then the conditions necessary (i)
are somewhat
critical.
EfSect of Alpha-particle Bombardment To place the matter in its proper perspective
alpha-particle
flux under which the experiments
parison with those normally
used in radiation
a flux of only 1.5 X lo8 particles crnd2 set-‘, was only about three times greater.
it is necessary
to emphasize
that the
were done was very low in com-
experiments
on metals.
ANDRADE used
and in the present experiments
the flux
The flux available in existing reactors and nuclear
machines is greater than this by a factor of lo4 or more. It is possible to consider the effect of radiation on a crystal in two different ways. In the first the bombardment is considered as ejecting atoms from their normal lattice positions, thereby forming vacancies and interstitials. It has been calculated (SEITZ, 1949) that only a very small fraction of the energy of a high-energy particle is used in this way, the rest being dissipated as heat. In the absence of annealing, however, the atomic displacements so formed are frozen in, and by prolonging the irradiation it is possible to build up large amounts of damage. If annealing occurs, then the concentration of displacements will not rise in proportion to the radiation dose,
but
may
become
approximately
constant
after
a time,
although
here
the
The effect of alpha-particle bombardment on the creep of cadmium single crystals
191
situation is complex as there is evidence which indicates that interstitials and vacancies anneal at different rates. The energy of the alpha particles emitted from the polonium is 5.30 MeV and they have a range in air of 3.9 cm. The number ofdisplacements produced by a 5-MeV alpha particle has been calculated by SEITZ (1949) for beryllium, aluminium, and germanium, and if the figure for cadmium is not very different then it can be assumed that about fifty displacements are produced by each particle. Since the flux at the surface of the crystal was only about 5 . IO8 particles crne2 set-I., the number of displacements produced per minute in the absence of annealing was about 1.5 x 1Ol2 per IX-~. If all these displacements occurred in a layer 0.0005-cm thick on the surface of the crystal, the concentration produced per minute was about 6.5 x 10e6 per cent. ANDRADE observed the increase in creep-rate to occur practically instantaneously on initiating the bombardment and cert.ainly within one minute, so the concentration of displacements at which the effect first occurred was certainly less than 6.5 x 1O--6 per cent. It is extremely unlikely that so low a concentration could affect the creeprate appreciably in a material of ordinary purity. The second way in which the bombardment might affect the crystal is associated with the energy transferred to the crystal during the impact of a particle. This energy is initially concentrated in a very small region, which is consequently raised to a high temperature. In the interior of the metal the duration of this “thermal spike’ will be very short and it is probable that recrystallization will occur on to the parent lattice. If a thermal spike occurs near the surface of the metal, however, the conduction of heat will be less rapid and the restraint offered by the surrounding material less severe, so that the possibility that an observable effect will occur is greatly increased. This is confirmed* by the rapid disintegration of metal foils under radiation where it appears that atoms of the metal are vaporized from the surface of the foil. BRINKMAN (1952) has estimated that about 1.0 eV per atom is required to melt a region inside a crystal lattice, and SEITZ has calculated that temperatures of the order of 10%“K persist for lo-l1 set in a thermal spike. Thus, a 5-MeV particle possesses suficient energy to effectively melt a total of 5 x lo6 atoms for a period of lo-l1 sec. It is unlikely, of course, that this melting would occur in the form of a sphere; according to BRJNKMANthe energy is distributed along the tracks of the particle and the knockedon metal atoms in regions where the temperature does not reach melting and which culminate in melted regions about 2 x lo4 atoms in size near the end of the tracks. The diameter of these melted regions, if they arc assumed to be spherical, is about 75 11 and this sets an upper limit to the size of any dislocation loops which may be formed, Since the stress necessary to expand loops of this size is very large, it is extremely unlikely that they could contribute to the creep of the crystal. It thus appears that the mechanism proposed by ANDRADE, i.e.. the initiation of dislocations by the bombardment, is unlikely to operate. Another mechanism exists, however, by which an acceleration in creep-rate on bombardment could occur. The very great strengthening effect of a thin oxide or metal film on the surface of a single crystal is well established (ROWOF, 1936), and there is evidence (BARRETT, 1953) to suggest that the effect is due to the film acting as a barrier which prevents dislocations from passing out of the crystal. During the increase in local temperature followin g the impact of an alpha particle, the film may * Dr. A.
Private
communication.
192
M. J. MAKIN
be weakened sufficiently to be fractured locally by the pressure of piled-up dislocations. This effect might be important with quite low fluxes, as under critical conditions a single particle could initiate a new glide-band. The time available for the initial fracture of the film is rather short, however, at most about lo-lo set, and this renders the process unlikely. This difficulty could be overcome by postulating that the film was left disrupted by the impact of the particle and that fracture occurred later. If this were so, prolonged bombardment of the crystal would result in a cumulative softening, which is not observed. Whatever the exact mechanism taking place, a consideration of the small amount of damage likely to occur on bombardment by the low flux used by ANDRADEsuggests that if the effect was not due to a spurious cause, then some magnification factor was in operation. It is suggested that a surface film of critical dimensions may be such a factor. It is possible, however, that the effect obtained by ANDRADEwas due to some cause other than the alpha-particle bombardment. It has been observed in the present work that slight vibration caused by moving the sources will sometimes initiate geometrical softening and hence an increased creep-rate. High-orientation spectroscopic purity and medium-orientation annealed and aged standard purity specimens showed this behaviour. The orientation and the purity of specimens was not quoted by ANDRADE, but the author has been privileged to inspect the original notebooks and it is considered that this may account for at least some of ANDRADE’S results. It is significant that an effect was reported only in the early stages of creep; this is what one would expect if geometrical softening were the cause of the increase in creep-rate. (ii) Yield-point Phenomena While it has not been established that the effects observed in the standard cadmium after annealing are necessarily due to nitrogen, the general similarity of behaviour is regarded as reasonable grounds for the assumption that the effects are due to nitrogen, although the behaviour is modified by the other impurities present. The disappearance of the yield-point with time at room temperature is not easy to explain and seems to be associated with the other impurities in the standard cadmium, as it does not take place in the pure metal (COTTRELL and GIBBONS, 1948). One possible explanation is that an equilibrium distribution of nitrogen is set up At high temperatures the nitrogen tends between the dislocations and the impurities. to associate itself with the dislocations, and at room temperature and below the system has a lower free energy when the nitrogen is associated with the impurities. It is significant, however, that although the yield-point disappears and glide occurs without jerks, some of the phenomena associated with the yield-point remain. The slip tends to be concentrated into localized regions, and the stress necessary to extend the crystal is much higher than before annealing. It is intended to investigate these phenomena more fully at a later date. (iii) Hardening of Crystals by Bombardment The slight hardening of spectroscopic purity crystals after prolonged bombardment may be due to oxidation of the specimen from the ions or ozone produced in the air by the passage of the alpha particles. It has been calculated that the total number of ion pairs produced inside the sources in an hour is about 9 x 1016, and there is
. The effect of alpha-particle bombardment
on the creep of cadmium single crystals
193
to show (LIND, 1928) that these would produce at least an equal number of ozone molecules. On these figures there are about 300 ozone molecules produced per hour for each cadmium atom in the surface of the crystal, and under these conditions some oxidation is anticipated. It has been observed that all metal objects inside the dry-box tarnished in a few days. 5. CONCLUSIONS No change in the creep-rate of cadmium single crystals has been detected on the initiation of alpha-particle bombardment, although experiments were made on crystals of various purity, surface condition, and orientation. It is concluded that the results obtained by ANDRADEwere due either to a spurious effect, such as geometrical softening initiated by vibration, or, less likely, to the disruption by the particle bombardment of a surface film of somewhat critical thickness. A slow hardening of the crystal under prolonged alpha-particle bombardment was observed, and it is considered that this may have been due to oxidation by ozone produced during the passage of the particles through the air.
evidence
6. ACKNOWLEDGMENTS The author is indebted to Dr. J. W. GLEN, who designed the apparatus and performed preliminary tests. My thanks are also due to Dr. H. M. FINNISTONand Mr. S. F. PUGH for much helpful advice and encouragement throughout the work. REFERENCES ANDRADE,E. N. DA C. (1945) Nature, 156, 113. ANDRADE, E. N. DA C., and ROSCOE,R. (1937) Proc. Phys. Sot., 49, 152. BARRETT, C. S. (1953) J. Metals, 5 (12), 1652. BLEWITT, T. H., and COLTMAN,R. R. (1951), Phys. Reo., 82, 769. BRINKMAN,J. A. (1952) NAA-SR-198. C~ITRELL, A. H., and GIBBONS,D. F. (1948) Nature, 162,488. DUMBLETON,M. J. (1954) Proc. Phys. Sot. B., 67,98. GEIB, I. G., and GRACE, R. E. (1952) AECU-2225. GIBBONS,D. F. (1952) Proc. Phys. Sot. B., 65, 193. GRENINGER,A. B. (1935) 2. Krist, 91, 434. JONES, E. R. W., MUNRO, W., and HANCOCK,N. H. (1954) J. Nuclear Energy, 1,76. KIITEL, J. H. (1947) Nat. Advis. Cttee. Aero. RES. Memo. (E7E13). LIND(1928)Chemical Eficts of Alpha Particles and Electrons. Chemical Catalogue Co. Incorp. ROSCOE,R. (1936) Phil. Mug., 21, 399. SEITZ, F. (1949) Discussions of Faraday Society, 5, 271. SLATER, J. C. (1951) J. Appl. Phys., 22, 237. WITZIG, W. F. (1952) J. Appl. Phys., 23, 1263.
Pergan~on Press Ltd., London
THE EFFECT OF ALPHA-PARTICLE BOMBARDMENT ON THE CREEP OF CADMIUM SINGLE CRYSTALS By M. J. MAKIN Atomic Energy Research Establishment, Harwell, Berks (Received 3 August
1954:)
Abstract-The effect of alpha-particle bombardment on the creep of cadmium single crystals at room temperature has been measured in an attempt to repeat the observations of ANDRADE. No significant change in creep rate on initiation of the bombardment has been detected, although experiments were made on crystals differing in purity, surface condition, and orrentation. It is considered that the increase in creep rate observed by ANDRADE was either due to geometrical softening or to the disruption of a surface film of somewhat critical thickness.
I. INTRODUCTION The published literature on the effect of particle bombardment on the creep of metals is very scanty and to some extent contradictory. The earliest contribution was from ANDRADE (1945), who reported an effect due to the bombardment of cadmium single crystals by alpha particles from a polonium source. ANDRADE found that a marked increase in the rate of glide occurred if the bombardment was started during the transient stage of the creep-curve. Crystals 0.040-in. diameter stressed so as to produce a creep-rate of about 0.05 per cent per minute showed an acceleration in rate of up to five times on bringing up the alpha sources, although the crystals were bombarded over only one-third of their length. ‘The magnitude of the effect was found to decrease the greater the extension of the crystal previous to the initiation of the bombardment, until after 12 per cent extension no effect was found. The strength of the source used by ANDRADE* was approximately 28 millicuries, and under the conditions of his experiment the flux at the surface of the crystal was about 1.5 x 10s particles crne2 see-l Since the range of the particles in the cadmium was only about 0*0002 in. the effect of the bombardment may be regarded as a surface efFect. ANDRADE attributed his results to the initiation of new glide-planes at the surface by the disturbance produced during bombardment. Once initiated, glide on a particular plane is independent of surface condition. The decrease in the size of the effect after large strains was interpreted as implying that only during transient creep were new slip-p!snes becoming active and that secondary creep was due to continued slip on already developed planes. Hence initiation of new glide-planes by alpha-particle bombardment could only occur during transient tree-p. KITTEL (1947) has reported the results of four tests on the creep of polycrystalline aluminium wire under alpha-particle bombardment from an 85-millicurie polonium source. He found that the minimum creep-rate was slightly lowered by the bombardment. Experiments have been reported by WITZIG (1952) on the creep of polycrystalline copper under bombardment with 16-MeV deuterons from a cyclotron. Copper wires 0.020-in. in diameter tested under 10,000 lb.-in.-2 at 260°C showed no significant * Private
communication. 181
M. J.
182
MAKIN
change in rate when a deuteron beam of 1012 particles cmP2 set-l was switched on or The experiment was not analogous to that of ANDRADE, however, as the range of a 16-MeV deuteron in copper is about 0.0128 in. so the particles penetrated over half the diameter of the specimens. Since the accuracy of the experiment was not very high, about *20 per cent, it is not certain that no effect occurred. JONES, MUNRO, and HANCOCK (1954) have reported an experiment on the creep of aluminium under neutron bombardment in a pile. An aluminium tube, two inches in diameter and of 0.062-in. wall thickness, tested under a hoop stress of 3,500 lb-in.? at 5O”C, showed no significant change in steady-state creep-rate on Under these coninsertion into a fast-neutron flux of about 1 x 10%. cm_2 set-l. ditions the damage caused by the flux was probably distributed fairly uniformly throughout the specimen. The accuracy of the experiment was about &lo per cent. SLATER (1951) indicates that the creep-rate under particle bombardment may be either increased or decreased depending upon the relative influence of interstitials, vacancies, and “thermal spikes” produced by the passage of a particle through the lattice. In the absence of annealing, interstitials and vacancies may be expected to result in a hardening of the metal, an erect which has been observed with both single crystals and polycrystals irradiated at a low temperature (CEIB and GRACE, 1952; BLEWITT and COLTMAN, 1951.) Thermal spikes are regions locally raised to a high temperature due to atomic collisions with an incident particle. In a material under stress, relaxation will occur in these regions, resulting in a more rapid creep-rate than would occur without the radiation. In view of the lack of information on this subject and the contradictory results of the published experiments, the effect found by ANDRADE was studied more closely. This experiment has a higher precision than the others, and it is the only one in which an increase in creep-rate has been observed. It is of particular interest to know whether the effect is a general one which occurs with all cadmium crystals or if it is only observable with crystals having a particular surface condition. off.
2. EXPERIMENTAL
DETAILS
(i) Preparation of Specimens Single crystals of cadmium one millimetre in diameter were prepared by the method of ANDRADE and ROSCOE(1937) from metal supplied by Johnson Matthey & Co. Ltd. Two purities of cadmium were used, which will be referred to as spectroscopic and standard purity throughout the paper. Spectroscopic examination of both samples was carried out by Johnson Matthey Ltd., with the following results:-
Standard purity sample %
’ Spectroscopic purity sample %
Cadmium 99.96 (by difference) Lead 0.02 Thallium 0.01 Copper OWO5 Silver 0401 Zinc OGO2 No bismuth
Cadmium 99.99 (by difference) Silver < 0401 j Copper faintly visible Calcium in spectrograph Iron
!
The effect of alpha-particle
bombardment
on the creep of cadmium single crystals
183
The cadmium wires were inserted into quill tubes of internal diameter slightly greater than the diameter of the wire. The tubes were evacuated, filled with argon, and sealed. By this method the wire was grown into a crystal in only a small volume --Pulley
--Perspex dry box
Concrete
block
FIG. I.--The creep machine.
of argon, and the risk of contamination with nitrogen was minimized. The surface of the crystals “as grown” was very bright, and previous tests have shown them to be practically free from oxide films. The orientation of the crystals was determined from back-reflection Laue X-ray photographs by the method due to GRENINCER (1935). The crystals were then cut into three-centimetre lengths and stored in vacua before use. (ii) Apparatus Creep tests on single crystals of cadmium were carried out under constant load at room temperature. The general arrangement of the apparatus is shown in Fig. 1. The load was applied to the specimen through wire loops welded to the ends of the
M. J.
184
MAKIN
crystal. The lower end of the specimen was attached to a rigid support and the upper end connected to the scale-pan by a thin copper wire. The device shown at A was used to apply the load to the crystal. It consisted of a stop attached to a carriage, the vertical position of which could be controlled by rotation of the knurled ring M. Slow ascent of the carriage enabled the load to be transferred smoothly from the stop to the specimen. The extension of the crystal was measured on a scale after magnification by an optical lever. The aluminium pulley L was encircled by a fine tungsten wire, the ends of which were connected to the rod attached to the upper end of the specimen so that extension of the specimen caused a rotation of the pulley. Mounted on the pulley shaft was a small concave mirror which produced an image of a crosswire on the scale. The magnification achieved was about two hundred. It was necessary to perform the experiment inside a drybox to minimize the danger of contamination from the polonium alpha sources. A slightly reduced pressure inside the box was maintained by an extractor fan to prevent the egress of polonium through the hole surrounding the thin rod carrying the load to the upper end of the specimen. The polonium alpha sources of initial strength 140 millicuries were deposited on the inside surface of a split silver cylinder. The dimensions of the sources were much the same as those used by ANDRADE,i.e., about one cm long and eight mm in diameter, and they were supported on two movable stands so that they could be brought up to the crystal during a creep test. The flux produced as the surface of the crystal was about 5 x 10s particles cm-2 se&. The whole apparatus was mounted on a steel framework resting on wooden blocks, and the system was made independent of external vibration by mounting the blocks on thick foam rubber. Great care was taken to avoid any accidental straining of the fragile specimens, and special techniques were adopted to prevent this; in particular the crystals were never permitted to rest in the horizontal position with any part of their length unsupported. The load to produce the required creep-rate was estimated from the orientation of the crystal and its surface condition. Initially a load slightly less than this was applied, and if this proved insufficient the specimen was unloaded before adding more weights to the scale-pan. The preliminary extension of the crystal during this procedure was less than 0.03 per cent. After the correct load was found, the specimen was allowed to creep and a curve of the extension against time plotted. No precautions to control the temperature of the specimen were necessary, as the duration of the test was usually less than one hour. The accuracy of the experiment is estimated as &5 per cent. 3. EXPERIMENTAL (i) “As-grown”
RESULTS
Crystals
The results of seven tests on spectroscopic purity (a) Spectroscopic purity. specimens in the “as-grown” condition are listed in Table 1, and a typical extension time curve is shown in Fig. 2. All the specimens showed smooth creep-curves and a complete absence of any yield-point phenomena. In every case the bombardment was started within that range of extension, i.e., up to 3 per cent, in which an effect was observed by ANDRADE No change in creep-rate on either bringing up or removing the alpha-particle sources
The effect of alpha-particle
bombardment
185
on the creep of cadmium single crystals
TABLE 1 .---SPECTROSCOPIC PURITY “AS-GROWN” CRYSTALS
No.
Yo
Initial extension Resolved shear stress ~before bombardment gm/mm* %
10
I
1
28 29 30 31 32 33
45 10 18 36 14 40 30
45 13 22 40 26 40 36
I L
0.61 261 0.42 0.14 0.31 0.09 0.17
16.7 18.8 22.0 22.9 19.4 20.1 18.1
I
I
Rate of extension immediately before bombardment %/min 0.041 0.182 0.095 OGO8 0.072 0.013 0.018
Ratio of creep-rates
I
0.93 0.96 1.00 1.02 0.98 1.00 1.Ol
Note: 1y,, = angle between specimen axis and glide-plane. 1, = angle between specimen axis and glide direction.
0.06 E f .k 0.04 VI 5 f; W
0.02
15
10 Time FIG. 2.-Spectroscopic
20
In mtnutes
purity crystal No. 30. Initial length 24.0 mm, load 40 g.
was detected in any test despite a wide range of initial orientations and creep-rates. Movement of the sources, however, sometimes produced a small elongation of the specimen. This was observed on both bringing up and removing the sources, and was produced by vibration in the apparatus when the hands were inserted into the rubber gloves of the dry-box. This extension, never greater than 0.025 per cent, could be almost eliminated by very careful handling. Microscopic examination of the specimens after test revealed an almost uniform distribution of glide bands. (b) Standard Puritan. Experiments were also made on standard cadmium crystals in the “as grown” condition. It was found that the creep of specimens tested only a few hours after growth was not as smooth as in the spectroscopic purity specimens
M. J.
186
MAKIN
but took place in a series of small jerks, the size of which varied from specimen to specimen. The creep-curves were irregular and unsuitable for testing with the alphaparticle sources. Crystals tested ten or twenty hours after growth, however, showed much smoother creep-curves, and after leaving for ninety hours at room temperature in
Time FIG. 3.-Standard
in minutes
purity crystal No. 22. Initial length 30.8 mm, load 130 g.
vacua the curves were as smooth as those given by spectroscopic purity crystals. This unusual phenomenon is discussed more fully later in the paper. Details of four experiments on standard purity crystals in this condition are given
in Table
2. TABLE
No.
10 20 22 23
~0
17 12 12.5 36
Resolved shear stress gm/mm2
&I
18 20.5 23 38
2.--STANDARD
i , I
35.4 34.5 33.0 34.0
PURITy “AS-GROWN"
Rate of extension
Initial extension before bombardment
I I
% 0.47 0.32 0.44 0.57
CRYSTALS
im~~~~~~m~~re i
~ Ratio Of ~ creep-rates
%/min 0.091 0.060 0.052 0.104
I ~
0.99 0.99 0.98 1.00
No change in creep rate on either bringing up or removing the alpha sources was detected. A typical curve of extension against time is shown in Fig. 3. Microscopic examination of the specimens after test indicated a fairly uniform distribution of glide-bands.
The effect of alpha-particle
bombardment
on the creep of cadmium single crystals
187
(ii) Annealed Standard Cadmium Crystals It was suspected that the jerky flow observed in standard cadmium crystals tested soon after growth was due to nitrogen in the metal. To check this, a few specimens were annealed in vacua for one hour at 200°C. The extension of a specimen tested immediately after annealing was so irregular that it was almost impossible to take creep measurements. No extension. was observed until a large load was applied, and then yield occurred suddenly with a marked reduction in resistance to flow and an acceleration in creep-rate. This increase in rate was so rapid that unless the load was quickly removed the elongation could reach 10 per cent in a few seconds. If the extension was stopped after the first yield, however, a very much reduced load was sufficient to produce a creep-rate of about 0.05 per cent per minute. The yield-point was followed by a period of fairly smooth flow, after which creep became increasingly jerky until the majority of the extension was occurring in the jerks with periods of Microscopic examination of the crystals recomplete cessation of flow between. vealed that the glide was concentrated into one or two regions within which heavy deformation had occurred. Re-annealing of the specimens caused the return of the yield-point. Specimens tested some hours after annealing hacl lower values of the upper yield stress and less jerky creep than those tested immediately. After ninety hours at room temperature in vacua the yield-point had completely disappeared and a smooth creep curve was obtained. The stress required to produce a given creep-rate was found to be increased, however, and microscopic examination revealed large glide in localized regions. With the exception of the gradual disappearance of the yield-point on ageing at room temperature the behaviour of these specimens is consistent with results obtained by other workers on crystals containing dissolved nitrogen. The specimens exhibit the phenomena of upper and lower yield-point described by COTTRELL and GIBBONS (1948). GIBBONS (1952) has reported that localized deformation is observed with crystals containing nitrogen and the creep behaviour is generally very similar to that described by DUMBLETON (1954) for zinc crystals at 100°C. In view of the close similarity between the observed phenomena and those described in the literature it was concluded that the crystals contained nitrogen. It was thought possible that the effect obtained by ANDRADE may be observable with annealed and aged standard cadmium crystals, and five tests were performed to check this. The results are shown in Table 3. Due to the localized nature of the deformation, these crystals showed a much greater tendency to geometrical softening than unannealed specimens of similar orientation where the glide was distributed fairly uniformly. This resulted in creepcurves in which the rate of creep accelerated continuously during the test. It was observed that the extension produced by the vibration on moving the sources sometimes initiated geometrical softening and hence an i-ncrease in creep-rate. No effect directly attributable to the alpha-particle bombardment was found. (iii) Oxide-coated (a)
coated
Crystal.7
Spectroscopic
Puritan.
spectroscopic
purity
Nine irradiation experiments were made on oxidespecimens. Crystals were oxidized by heating in the
M. J.
188 TABLE
No.
10 18 20 23 26 -
All
Resolved shear stress gm/mm2
18 10 20.5 38 15
68.0 61.3 77.0 66.4 76.8
wo
17 10 12 36 11
!_
3.---ANNEALED
MAKIN
STANDARD
CADMIUM
CRYSTALS
Rate of extension Initial extension immediately before before bombardment bombardment %/min % ~~ ~_ __._ 0.016 0.042 O@l4 0.092 0.109
0.079 0.37 0.22 0.41 0.50
Ratio of creep-rates
1.01 0.98 1.31 1.04 1.16
atmosphere to 240°C for various times; after half an hour at temperature a faint brown coloration was visible on the surface of the crystal and a dark brown coat was produced after fifteen hours. As it was anticipated that a change in creep-rate on bombardment might occur only with specimens covered with a very thin oxide film, tests were also made on crystals exposed to the atmosphere at room temperature. The results are summarized in Table 4. TABLE
No:
a.--OXIDE-COATED
SPECTROSCOPIC
PURITY
CRYSTALS
-
-
Yo
1,
Resolved shear stress gm/mm2
~
Initial extension before bombar>ment ”
/ EL% / immediately
j / Ratlo of
before bombardment %/min
creep-rates
%
Oxidation Treatment
, A A 29 29 30 31 31 31 31
17 17 18 18 36 14 14 14 14
21 21 22 22 40 26 26 26 26
19.8 24.4 23.8 22.0 22.9 26.0 25.0 30.5 37.5
-
~ 1 ~ ~ I
0.57 0.72 0.13 0.20 0.15 0.10 0.33 0.22 0.39
0,069 0.136 0.019 0.041 0,018 0.022 0.045 0.038 0.065
1.01 1.00 1.00 1.02 0.94 0.98 1.01
1.03 0.97
4tmos.for 4 weeks 4tmos.for 4 weeks + hr at 240°C 4 hr at 240°C 3 hr at 240°C Q hr at 240°C 2 hr at 24O’C 2$ hr at 240’C 51 hr at 24O’C
No change in creep-rate on bombardment was observed in any of these tests, and microscopic examination revealed a uniform distribution of slip lines. The effect of the oxide film in hardening the crystal is apparent. (b) Standard Purity Ten experiments were made on standard cadmium crystals oxidized for various times at 240°C. Since the oxidation treatment corresponded to an annealing. yieldpoint phenomena were found if the specimens were tested too soon after oxidation. After ninety hours at room temperature, however, the yield-point had disappeared and the following results were obtained:
The effect of alpha-particle
bombardment
TABLE ~-OXIDE-COATED
on the creep of cadmium single crystals
189
STANDARD PURITY CRYSTALS
Initial extension/ Rate of extension
~ Oxidation treatment
No.
10 14 17 18 18 18 20 20 26 26
17 26.5 22 10 10 10 12 12 11 11
18 41 24 10 10 10 20.5 20.5 15 15
90.3
77.3 107.0 65.3 70.5 68.5 80.8 84.5 81.8 86.6
0.50 0.77 0.36 0.26 0.81 0.20 0.29 0.36 0.20 0.59
0.170 0.258 0.019 0.121 0.118 0.034 0.123 0.145 0.047 0.051
15 hr at 240°C 15hrat24OT Grown in air 1 hr at 240°C 3 hr at 240°C 3 hr at 240°C 1 hr at 240°C 3 brat 240°C & hr at 240°C $ hr at 240°C
1.02 0.98 1.01 1.00 1XKl 1.17 1.01 1.17 1.83 103
L
No change in creep-rate attributable to the bombardment occurred, but it was sometimes found’ that there was an acceleration in rate due to geometrical softening initiated by vibration during movement of the sources. A curve showing this behaviour in a specimen oxidized for three hours is shown in Fig. 4. Other
5 --
o-
.5-
b 0
10
20
Time N-Iminutes FIG.
4.-Standard
cadmium crystal No. 20. Oxidized three hours at 240°C. Initial length 30.8 mm, load 340 g.
specimens cut from the same crystal and oxidized for the same time showed very similar behaviour when the actions required to move up the sources were performed without initiating the bombardment. 2
M. J. MAKIN
190
(iv)
Hardening of Crystals by Bombardment It was observed
condition
that
spectroscopic
showed a small hardening
The load required to produce a creep-rate mined
and then the specimen
purity
crystals
after prolonged
tested
in the “as-grown”
alpha-particle
bombardment.
of about 0.05 per cent per minute was deter-
was unloaded
and the bombardment
commenced.
After the desired time the sources were removed and the specimen re-tested under the same load.
The results of three such experiments
are given in Table
6.
TABLE 6.-HARDENING OF CRYSTALSBY PROLONGED BOMBARDMENT bombardment
No.
Creep-rate after bombardment %/min
30 30 30
hrs
0.04 0.02 0.05
The magnitude bombarded
1 2 1
of the effect has no special
over only one-third
of its total
on starting
significance,
as the specimen
Microscopic
was re-
of creep tests in which no change in creep-rate
has
after test.
OF RESULTS
it is concluded that the by ANDRADE are not general for cadmium crystals. It is still possible,
effects reported
or stopping
length.
of the specimens
4. DISCUSSION In view of the large number
20.7 20.0 21.2
examination
vealed nothing unusual in the appearance
been detected
Resolved shear stress gm/mma
Duration of
of course,
that ANDRADE obtained
possessed
some
particular
surface
the bombardment,
a true radiation condition,
effect, since his crystals may have
purity,
or orientation
for which
an
effect occurs but the present work indicates that if this is the case then the conditions necessary (i)
are somewhat
critical.
EfSect of Alpha-particle Bombardment To place the matter in its proper perspective
alpha-particle
flux under which the experiments
parison with those normally
used in radiation
a flux of only 1.5 X lo8 particles crnd2 set-‘, was only about three times greater.
it is necessary
to emphasize
that the
were done was very low in com-
experiments
on metals.
ANDRADE used
and in the present experiments
the flux
The flux available in existing reactors and nuclear
machines is greater than this by a factor of lo4 or more. It is possible to consider the effect of radiation on a crystal in two different ways. In the first the bombardment is considered as ejecting atoms from their normal lattice positions, thereby forming vacancies and interstitials. It has been calculated (SEITZ, 1949) that only a very small fraction of the energy of a high-energy particle is used in this way, the rest being dissipated as heat. In the absence of annealing, however, the atomic displacements so formed are frozen in, and by prolonging the irradiation it is possible to build up large amounts of damage. If annealing occurs, then the concentration of displacements will not rise in proportion to the radiation dose,
but
may
become
approximately
constant
after
a time,
although
here
the
The effect of alpha-particle bombardment on the creep of cadmium single crystals
191
situation is complex as there is evidence which indicates that interstitials and vacancies anneal at different rates. The energy of the alpha particles emitted from the polonium is 5.30 MeV and they have a range in air of 3.9 cm. The number ofdisplacements produced by a 5-MeV alpha particle has been calculated by SEITZ (1949) for beryllium, aluminium, and germanium, and if the figure for cadmium is not very different then it can be assumed that about fifty displacements are produced by each particle. Since the flux at the surface of the crystal was only about 5 . IO8 particles crne2 set-I., the number of displacements produced per minute in the absence of annealing was about 1.5 x 1Ol2 per IX-~. If all these displacements occurred in a layer 0.0005-cm thick on the surface of the crystal, the concentration produced per minute was about 6.5 x 10e6 per cent. ANDRADE observed the increase in creep-rate to occur practically instantaneously on initiating the bombardment and cert.ainly within one minute, so the concentration of displacements at which the effect first occurred was certainly less than 6.5 x 1O--6 per cent. It is extremely unlikely that so low a concentration could affect the creeprate appreciably in a material of ordinary purity. The second way in which the bombardment might affect the crystal is associated with the energy transferred to the crystal during the impact of a particle. This energy is initially concentrated in a very small region, which is consequently raised to a high temperature. In the interior of the metal the duration of this “thermal spike’ will be very short and it is probable that recrystallization will occur on to the parent lattice. If a thermal spike occurs near the surface of the metal, however, the conduction of heat will be less rapid and the restraint offered by the surrounding material less severe, so that the possibility that an observable effect will occur is greatly increased. This is confirmed* by the rapid disintegration of metal foils under radiation where it appears that atoms of the metal are vaporized from the surface of the foil. BRINKMAN (1952) has estimated that about 1.0 eV per atom is required to melt a region inside a crystal lattice, and SEITZ has calculated that temperatures of the order of 10%“K persist for lo-l1 set in a thermal spike. Thus, a 5-MeV particle possesses suficient energy to effectively melt a total of 5 x lo6 atoms for a period of lo-l1 sec. It is unlikely, of course, that this melting would occur in the form of a sphere; according to BRJNKMANthe energy is distributed along the tracks of the particle and the knockedon metal atoms in regions where the temperature does not reach melting and which culminate in melted regions about 2 x lo4 atoms in size near the end of the tracks. The diameter of these melted regions, if they arc assumed to be spherical, is about 75 11 and this sets an upper limit to the size of any dislocation loops which may be formed, Since the stress necessary to expand loops of this size is very large, it is extremely unlikely that they could contribute to the creep of the crystal. It thus appears that the mechanism proposed by ANDRADE, i.e.. the initiation of dislocations by the bombardment, is unlikely to operate. Another mechanism exists, however, by which an acceleration in creep-rate on bombardment could occur. The very great strengthening effect of a thin oxide or metal film on the surface of a single crystal is well established (ROWOF, 1936), and there is evidence (BARRETT, 1953) to suggest that the effect is due to the film acting as a barrier which prevents dislocations from passing out of the crystal. During the increase in local temperature followin g the impact of an alpha particle, the film may * Dr. A.
Private
communication.
192
M. J. MAKIN
be weakened sufficiently to be fractured locally by the pressure of piled-up dislocations. This effect might be important with quite low fluxes, as under critical conditions a single particle could initiate a new glide-band. The time available for the initial fracture of the film is rather short, however, at most about lo-lo set, and this renders the process unlikely. This difficulty could be overcome by postulating that the film was left disrupted by the impact of the particle and that fracture occurred later. If this were so, prolonged bombardment of the crystal would result in a cumulative softening, which is not observed. Whatever the exact mechanism taking place, a consideration of the small amount of damage likely to occur on bombardment by the low flux used by ANDRADEsuggests that if the effect was not due to a spurious cause, then some magnification factor was in operation. It is suggested that a surface film of critical dimensions may be such a factor. It is possible, however, that the effect obtained by ANDRADEwas due to some cause other than the alpha-particle bombardment. It has been observed in the present work that slight vibration caused by moving the sources will sometimes initiate geometrical softening and hence an increased creep-rate. High-orientation spectroscopic purity and medium-orientation annealed and aged standard purity specimens showed this behaviour. The orientation and the purity of specimens was not quoted by ANDRADE, but the author has been privileged to inspect the original notebooks and it is considered that this may account for at least some of ANDRADE’S results. It is significant that an effect was reported only in the early stages of creep; this is what one would expect if geometrical softening were the cause of the increase in creep-rate. (ii) Yield-point Phenomena While it has not been established that the effects observed in the standard cadmium after annealing are necessarily due to nitrogen, the general similarity of behaviour is regarded as reasonable grounds for the assumption that the effects are due to nitrogen, although the behaviour is modified by the other impurities present. The disappearance of the yield-point with time at room temperature is not easy to explain and seems to be associated with the other impurities in the standard cadmium, as it does not take place in the pure metal (COTTRELL and GIBBONS, 1948). One possible explanation is that an equilibrium distribution of nitrogen is set up At high temperatures the nitrogen tends between the dislocations and the impurities. to associate itself with the dislocations, and at room temperature and below the system has a lower free energy when the nitrogen is associated with the impurities. It is significant, however, that although the yield-point disappears and glide occurs without jerks, some of the phenomena associated with the yield-point remain. The slip tends to be concentrated into localized regions, and the stress necessary to extend the crystal is much higher than before annealing. It is intended to investigate these phenomena more fully at a later date. (iii) Hardening of Crystals by Bombardment The slight hardening of spectroscopic purity crystals after prolonged bombardment may be due to oxidation of the specimen from the ions or ozone produced in the air by the passage of the alpha particles. It has been calculated that the total number of ion pairs produced inside the sources in an hour is about 9 x 1016, and there is
. The effect of alpha-particle bombardment
on the creep of cadmium single crystals
193
to show (LIND, 1928) that these would produce at least an equal number of ozone molecules. On these figures there are about 300 ozone molecules produced per hour for each cadmium atom in the surface of the crystal, and under these conditions some oxidation is anticipated. It has been observed that all metal objects inside the dry-box tarnished in a few days. 5. CONCLUSIONS No change in the creep-rate of cadmium single crystals has been detected on the initiation of alpha-particle bombardment, although experiments were made on crystals of various purity, surface condition, and orientation. It is concluded that the results obtained by ANDRADEwere due either to a spurious effect, such as geometrical softening initiated by vibration, or, less likely, to the disruption by the particle bombardment of a surface film of somewhat critical thickness. A slow hardening of the crystal under prolonged alpha-particle bombardment was observed, and it is considered that this may have been due to oxidation by ozone produced during the passage of the particles through the air.
evidence
6. ACKNOWLEDGMENTS The author is indebted to Dr. J. W. GLEN, who designed the apparatus and performed preliminary tests. My thanks are also due to Dr. H. M. FINNISTONand Mr. S. F. PUGH for much helpful advice and encouragement throughout the work. REFERENCES ANDRADE,E. N. DA C. (1945) Nature, 156, 113. ANDRADE, E. N. DA C., and ROSCOE,R. (1937) Proc. Phys. Sot., 49, 152. BARRETT, C. S. (1953) J. Metals, 5 (12), 1652. BLEWITT, T. H., and COLTMAN,R. R. (1951), Phys. Reo., 82, 769. BRINKMAN,J. A. (1952) NAA-SR-198. C~ITRELL, A. H., and GIBBONS,D. F. (1948) Nature, 162,488. DUMBLETON,M. J. (1954) Proc. Phys. Sot. B., 67,98. GEIB, I. G., and GRACE, R. E. (1952) AECU-2225. GIBBONS,D. F. (1952) Proc. Phys. Sot. B., 65, 193. GRENINGER,A. B. (1935) 2. Krist, 91, 434. JONES, E. R. W., MUNRO, W., and HANCOCK,N. H. (1954) J. Nuclear Energy, 1,76. KIITEL, J. H. (1947) Nat. Advis. Cttee. Aero. RES. Memo. (E7E13). LIND(1928)Chemical Eficts of Alpha Particles and Electrons. Chemical Catalogue Co. Incorp. ROSCOE,R. (1936) Phil. Mug., 21, 399. SEITZ, F. (1949) Discussions of Faraday Society, 5, 271. SLATER, J. C. (1951) J. Appl. Phys., 22, 237. WITZIG, W. F. (1952) J. Appl. Phys., 23, 1263.