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DISLOCATION PINNING AND DEPINNING IN ELECTRON IRRADIATED MOLYBDENUM
Jenifer Lomer, J. Richardson
To cite this version:
Jenifer Lomer, J. Richardson. DISLOCATION PINNING AND DEPINNING IN ELECTRON IR- RADIATED MOLYBDENUM. Journal de Physique Colloques, 1971, 32 (C2), pp.C2-169-C2-172.
�10.1051/jphyscol:1971238�. �jpa-00214564�
JOURNAL DE PHYSIQUE Colloque C2, supplkment au no 7 , tome 32, Juillet 1971, page C2-169
D1 SLOCATION PINNING AND DEPINNING IN ELECTRON IRRADIATED MOLYBDENUM
by Jenifer N. LOMER and J. F. R . RICHARDSON (*) J. J. Thomson Physical Laboratory, Whiteknights, Reading R G 6 2AF
R6sumC. - Les travaux prkkdents sur le polycristal de molybdkne irradie aux electrons ont demontrk que l'ancrage de dislocations a lieu entre 40 OK et 70 OK. Ceci est dans la region de l'ktage 14 de revenu de la resistivite et a 6tC interpret6 comme etant dfi a la migration des interstitiels vers les dislocations. Les mesures ont et6 prolongees jusqu'a 830 OK. Un autre etage de l'ancrage a eu lieu a 520 OK. Ceci peut &tre mis en correlation avec l'etage I11 de revenu de la r6sistivit6 et est cense se produire du fait de la migration d'un defaut du type de (c vacancy D.
Une region etendue de dksancrage a eu lieu entre 90-290 OK dans des Cchantillons polycrystallins et a des temperatures un peu plus basses dans des monocristaux. Le desancrage est censt se produire quand les dislocations de nature differente liberent des interstitiels. Ceci veut dire que la structure du rbseau de la dislocation doit Btre differente dans les monocristaux et dans Ies polycristaux.
Un desancrage partiel a eu lieu entre 550 OK et 650 OK dans tous les Cchantillons. I1 est cens6 relever du fait du groupement des dBfauts des types (c vacancy ) sur les lignes de la dislocation.
Abstract. - Previous work on polycrystalline electron irradiated molybdenum [l] shows that dislocation pinning occurred between 40 OK and 70 OK. This is the region of the resistivity annealing stage 1 4 and is interpreted as arising from the migration of interstitials to the dislocations. Measu- rements have now been extended to 830 OK. One further pinning stage occurred at 520 OK. This can be correlated with the resistivity annealing stage 111 and is thought to arise from the migration of a vacancy type defect.
A broad region of depinning occurred between 90 OK and 290 OK in polycrystalline samples, and at somewhat lower temperatures in single crystals. The depinning is thought to arise from the release of interstitials from dislocations of different character. This means that the structure of the dislocation network must be different in single and polycrystals. Partial depinning occurred bet- ween 550 OK and 650 OK in all the samples. It is thought to arise from clustering of the vacancy type defects on the dislocation lines.
1. Introduction. - In the kilocycle frequency range the internal friction in metals is predominantly due to the motion of dislocations. The damping will be redu- ced if the dislocations are pinned by point defects so that their motion is restricted. In this work the dislo- cation pinning and depinning in zone refined molyb- denum has been studied following electron irradia- tion and subsequent annealing. Lomer and Taylor [l]
showed that pinning occurred between 50 OK and 70 O K in samples irradiated near helium temperature. The pinning was followed by a broad region of depinning up to room temperature. The present work extends the measurements up t o 800 OK.
2. Experimental. - The samples of zone refined molybdenum, with an estimated purity of 99.99 % (table I) were prepared and hot rolled t o a thickness of 150 pm by Metals Research Ltd. (Cambridge). X-ray examination showed that two of the samples were polycrystalline while the remainder were single crys- tals. Before each experiment, the samples was annealed
(*) Now at : - Colgate, Palm-Olive Ltd. Manchester.
Impurity content of MO specimens Tmpurity Concentration
- -
0 2 < 0.000 5 %
N2 < 0.000 5 %
C < 0.005 %
Fe 0.005 %
for 18 hours at 1220 O K in a vacuum better than 1.310-3 Nm-2.
The sample was mounted to form a resonant canti- lever 10-14 mm long, and the internal friction was measured as the logarithmic decrement, A , of the free decay of the oscillations, as described by Lomer and Niblett [2]. The resonant frequency was between 400-800 Hz and the maximum strain amplitude was typically 3 X 10-6. This means that measurements were made in the strain amplitude dependent region ; however the decrements were always compared at a constant value of the strain amplitude.
Some nitrogen temperature irradiations were car-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971238
C2- 170 JENIFER N. LOMER AND J. F. R. RICHARDSON ried out in a cryostat similar to that described by Lomer
and Niblett [2]. Annealing between 90 OKand 330 OK was studied using a pulse technique [l], all measure- ments being taken at nitrogen temperature. A high temperature cell which incorporated a heater around the vacuum jacket was used for room temperature irradiations and subsequent annealing to 8 0 0 o K . Details of the cell are given in Richardson's thesis [3].
In this temperature region the annealing was studied by the three-warm-up technique [2]. Since the internal friction was found to be dependent on the measuring time at high temperatures, it was necessary to accu- rately control the warm-up rate of the cell, so that it was the same (1 OK per min.) for all three curves.
The sample was vibrated continuously during the anneal and G rested B for twelve hours between each anneal. This procedure gave reproducible results, as shown in figure 1.
I I l I
3 0 0 400 5 0 0 6 0 0 7 0 0 TEMPERATURE K
FIG. 2. - Logarithmic decrement as a function of temperature for a polycrystalline sample.
L 1 I I I I
300 400 500 6 0 0 7 0 0 TEMPERATURE O K
FIG. 1. - Logarithmic decrement as a function of temperature.
+ : following irradiation and anneal to 700 OK,
o : A repeat after resting the sample twelve hours at room temperature.
3. Results. - A typical set of warm-up curves for polycrystals irradiated at room temperature is shown in figure 2. Many experimental points were taken and averaged over a 20 OKrange, the average values being shown in the figure. The first curve shows the tempera- ture dependence of the internal friction prior to irra- diation. The sample was then irradiated to a dose of 1.2 X 1017 el cm-' with 2 MeV electrons at room temperature. The post irradiation curve shows the decrement on warming up immediately after the irra- diation, and the third curve shows the damping after the sample has been annealed t o 730 OK. It is seen that pinning occurs during the irradiation and is followed by almost complete depinning by 3300K.
+ : Pre-irradiation ; 4 : immediately following irradiation at room temperature ; X : after the anneal to 730 OK.
This is expected, since tEe work of Lomer and Tay- lor [l] showed that the depinning following the low temperature pinning stage was not complete until 330 OK. A further pinning stage is observed at 530 OK, followed by partial depinning between 5500K and 650 OK. This is indicated in the figure by the fact that the third curve lies above the post irradiation curve.
Figure 3 shows a typical set of curves for the single crystals. In this case, the sample was irradiated to a dose of 2.2 X 1017 el cm-' at room temperature and annealed to 820 OK. A single pinning stage is observed at 530 OK. Contrary to the results for the polycrystals, no appreciable pinning is seen at room temperature immediately after irradiation. In order to investigate this further, some single crystals were irradiated at nitrogen temperature and pulse annealed to room temperature. The results show that depinning began at the irradiation temperature and was complete by 230 OK. This is in contrast to the results of Lomer and Tayfor [ l ] for polycrystals where depinning occurred between l l0 OK and 330 OK. Depinning curves for the two types of sample are compared in figure 4.
4. Discussion. - 4.1 CORRELATION OF THE DISLOCA- TION PINNING WITH THE ELECTRICAL RESISTIVITY ANNE- ALING STAGES. - It has previously been shown [l) that dislocation pinning occurs in electron irradiated molybdenum in the same temperature range as the electrical resistivity annealing stage I, observed by Lucasson and Lucasson 141. Both are thought to arise
DISLOCATION PINNING AND DEPlNNING IN ELECTRON IRRADIATED C2-171
300 400 500 600 700 800 TEMPERATURE 0 K
FIG. 3. - Logarithmic decrement as a function of temperature for a single crystal sample.
$. : Pre-irradiation ; + : immediately following irradiation at room temperature ; X : after the anneal to 830 OK.
from the free migration of the interstitial. No further annealing of the electrical resistivity was observed below room temperature. De Jong and Afman [5]
studied the resistivity annealing following a room temperature electron irradiation and found a single recovery stage centred at 470 OK. By comparison with the resistivity annealing stages in other materials, it is labelled Stage 111. In the present work dislocation pinning was observed between 470 OK and 570 OK.
No further pinning occurred up to 830 OK.
The temperature at which an annealing stage is observed will depend on the defect concentration and on the annealing kinetics. These can be very different in resistivity and internal friction experiments. In the F. C . C. metals where the annealing stages are close together, the correlation between the pinning and resis- tivity annealing stages is difficult. However in elec- tron irradiated molybdenum only one annealing stage is observed between 80 OKand 800 OK. Therefore there seems little doubt that the observed pinning stage can be correlated with the resistivity stage 111. The fact that the stages appear at very similar temperatures in molybdenum, adds weight to the correlations assumed for copper and gold by Lomer and Niblett [2] and Jordan and Lomer [5].
line molybdenum [l]. It was attributed to the release of interstitials from the dislocations. The width of the stage showed that depinning occurred with a wide range of activation energies. It was suggested that the binding energy of the interstitial to the dislocation depends on the character of the dislocation. The results from the single crystal specimens show that the depin- ning is complete by a lower temperature, figure 4.
This could result from a difference in the form of the dislocation network in the single crystals.
1 *+
".' 160
I I I I
1 5 0
2,OO 2 5 0 3 0 0 TEMPERATURE K
Pro irradiation value
''4
r0.41 I I I I I
l 0 0 150 200, 2 5 0 3 0 0
TEMPERATURE K 0.8
m'
FIG. 4. -Variation of the logarithmic decrement upon irra- diating at liquid nitrogen temperature and then pulse annealing.
A : Single crystal ; B : Polycrystal (after Lomer and Taylor [l]).
Friction drop upon irradiation
However, since the single crystals and the polycrys- tals were obtained in different batches, it is possible that the differences in substructure arise from the exact treatment they received during preparation. For ins- tance, Key and Weissman [7] found that dislocations in polycrystalline molybdenum formed regular hexa- gonal networks, if the material was prestrained 5 %
and then heated to 1200 O K . It is therefore possible that the distinction between single and polycrystals is unimportant.
The high temperature depinning was incomplete in all the samples examined and occurred over a rela- tively narrow temperature range. It is therefore unlikely that the depinning arises from defects leaving the dis- 4 . 2 DEPINNING. - A broad depinning stage was locations. In this case it seems more probabli that it is observed between 130 OK and 330 OK in polycrystal- due to clustering of the defects on the dislocation lines.
12
C2- 1 72 JENIFER N. LOMER AND J. F. R. RICHARDSON 4 . 3 INTERPRETATION OF STAGE 111. - The contro-
versey over which defect is mobile in stage I11 in the F. C. C . metals is well known. It is apparent from the literature that the same difficulty exists in the interpre- tation of the data for molybdenum. In the case of an electron irradiation the basic defects will be single vacancies, interstitials, and a small percentage of di- vacancies. Theoretical calculations on a-iron [g] show that the stable configuration of interstitial is the
< 110 > split interstitial and that the crowdion configuration is meta stable. Exponents of the two- interstitial model suggest that the crowdion is mobile in stage I and the split interstitial is mobile in stage 111.
In the present work, the depinning data show that the stage I defect cannot pin above room temperature.
Therefore a mechanism which involves the release of the stage I defect from impurity traps during the stage IIX anneal cannot explain the observed pinning. It also seems unlikely that the configuration of a second
type of interstitial would be retained at the dislocation so that it could act as an effective pinning point above room temperature. It is therefore concluded that the pinning during stage I11 is due to a vacancy type
defect.
In the present work, where the electron energy was 2 MeV, up to 5 % of the total concentration of defects could occur as divacancies. This concentration would be sufficient to account for the stage I11 pinning. Further experiments carried out at energies less than 1.5 MeV will be necessary to determine whether the vacancy or the divacancy is responsible for stage 111.
Acknowledgements. - We would like to thank Dr. R. J. Taylor for useful discussions and many other members of the J. J. Thomson Physical Laboratory who have helped in various ways. One of us (J. F. R. R.) is grateful to the S. R. C. for a research studentship.
References
[l] LOMER (JENIFER N.) and TAYLOR (R. J.), Phil. Mug., [5] DE JONG (M.) and AFMAN (H. B.), Acta. Met., 1967,
1969, 19, 437. 15, 1.
[2] LOMER ( J E ~ F E R N.) and NIBLETT (D. H.), Phil. &fag., 161 JORDAN (M. R.) and L ~ M E R (JEN1FER N.), Phil- Mug.,
1962, 7, 1211. 1968, 18, 1161.
[7] KEH (A. S.) and WEISSMANN (S.), Electron Microscopy [3] RICHARDSON (J. F. R.), Ph. D. Thesis, Reading, 1970. and Strength of Crystals. 1963. , v. 265. Ed. Thomas [4] LUCASSON (A.), and LUCASSON (P.), J. Phys. Chem. Sol., and ~ a s c b u r n .
1966, 27, 1423. [8] JOHNSON (R. A.), Phys. Rev., 1964, 134A, 1329.
