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Mechanical properties of fast neutron irradiated zinc at 77K
OOOI-6160/84 $3.00+ 0.00 Copyright lc 1984 Pergamon Press Lrd
Acre merall. Vol. 32, No. 3, pp. 389-395, 1984 Printed in Great Britain. All rights reserved
MECHANICAL PROPERTIES OF FAST NEUTRON IRRADIATED ZINC AT 77K C. J. IRIART, A. M. FORTIS and H. C. GONZALEZ ComisiCtn National de Energia Atbmica, Gcia. Desarroiio, Departamento Materiales, Av. de1 Libertador 8250, 1429 Buenos Aires, Argentina (Received
20 July 1983)
Abstract-Pure zinc single crystals were irradiated at liquid nitrogen temperature in the RA-I reactor of CNEA up to fast neutron doses of 3 x IO*’nm-*. Their plastic and fracture behaviour were studied at 77 K. Radiation hardening in zinc and copper was compared. It seems to be produced by the presence of similar irradiation-produced barriers to dislocation in both metals. For neutron doses greater than 4.5 x 10”nme2 the ductility in crystals with easy glide o~entations decreases rapidly by more than an order of magnitude. Fracture stress increased with dose, and it is dependent on crystallographic orientation. This fact shows that dislocation motion is necessary before fracture. Yield stress annealing experiments showed three important recovery stages located around I IO, 160 and 210 K. The last stage, which is the most pronounced one, is hardly seen by electrical resistivity recovery.
Rbun~&Nous
avons irradik aux neutrons rapides des monocristaux de zinc pur g la tempkature dei i’azote liquide, dans ie riacteur RA-I de la CNEA, avec des doses pouvant atteindre 3 x lo?’ nmm2. Nous avons CtudiC ieur comportement plastique et de rupture B 77 K. Nous avons cornpar& ies durcissements par irradiation dans ie zinc et dans le cuivre. Dans les deux m&aux, iis semblent provenir de la prksence d’obstacies semblabies, produits par l’irradiation, pour fe d~pla~rn~t des dislocations. Pour des dosess de neutrons superieures B 4,s x 1020nm- 2, la ductilite des cristaux orient&s pour le glissement facile diminue rapidement de plus d’un ordre de grandeur. La contrainte B la rupture augmente avec la dose et elle d&pend de l’orientation cristallographique. Ceci montre que le mouvement des dislocations est n&essaire pour la rupture. Des exptriences de recuit g la limite Clastique ont mis en ividence trois stades importants de ~stau~tion sit&s au voisinage de 110, 160 et 210 K. Ce dernier stade, qui est le plus pronon& est a peine observe dans la restauration de la risistivite tlectrique. Zinkeinkristalle wurden bei der Temperatur des fliissigen Stickstoffes im Reaktor U-1 der CNEA bis zu einer Dosis von 3 x lO*‘nm-* mit schnellen Neutronen bestrahlt. Plastizitiits- und Bruchverhalten wurden bei 77 K untersucht. Die ~trahlungsverfestigung von Zink und Kupfer wurden vergiichen. Es scheint in beiden Metallen von iihnlichen bestrahlungsinduzierten Hindernissen fiir die Versetzungsbewegung herzuriihren. Bei Neutronendosen iiber 4,5 x 1020nm-’ nimmt die Duktilitiit von Kristallen mit einer Orientierung fiir Einfachgleitung rasch urn mehr als eine GriiBenordnung ab. Die Bruchspannung nahm mit der Dosis zu und hing von der kristallografischen O~entierung ab. Das weist daiauf hi& daB Ver~t~ngs~wegung vor dem bruch niitig ist. Ausheilexperimente z&&ten, dal3 die FlieBspannung drei wichtige Erholungsstufen bei 110. 140 und 210 K durchliiuft. Die letzte Stufe, welche am ausgepriigtesten ist, IlDt sich in der Erholung des elektrischen -_ Widerstandes kaum entdecken. Zuwnmenfassung-Reine
1. INTRODUCTION
The effects of radiation on the mechanical properties of h.c.p. metals have not been as widely studied as in metals with f.c.c. and b.c.c. structures. The effects of neutron irradiation on the mechanical properties of zirconium and its alloys have received special attention in the past 20 years, as consequence of their use in nuclear technology [l, 21. To the authors’ knowledge, information about other h.c.p. metals is confined to recent works on magnesium [3-61 and zinc f6-81. Zinc single crystals showed important increments of the yield stress after fast-neutron irradiation at liquid nitrogen temperature. These increments are of the same order of magnitude as those measured in copper single crystals [8]. On the other hand, un389
irradiated zinc single crystals are not ductile at 77 K, fracturing by cleavage on basal and prismatic planes [9-l I]. This characteristic is still maintained in irradiated crystals, fracture stress increasing with neutron dose [6]. These precedents moved us to study the plasticity and fracture behaviour of fast neutron irradiated pure zinc single crystals at 77 K. Plasticity techniques were used for the determination of radiation damage recovery stages in zinc. These recovery stages have been studied before by electrical resistivity, which showed to be not very sensitive to the presence of irradiation-produced loops [12-141. Due to this fact, our main concern was the last recovery stage caused by loops annealing out, taking into account that plasticity is very sensitive to this kind of defects [6,8].
IRIART er al.:
390
MECHANICAL
PROPERTIES
2. EXPERIMENTAL
Zinc single crystals tensile specimens were used in our experiments. The crystals were grown by a modified Bridgman technique [8], from high purity zinc rods (99.9993%), supplied by Johnson Matthey, and under a purified argon atmosphere. Two types of specimens were prepared: samples of square cross-section 3 mm wide and 30 mm gauge length, and cylindrical samples 2.5 mm in diameter and 27.5 mm gauge length. In both cases the samples were grown with spherical heads 6.35 and 5 mm respectively, in diameter. The crystals were grown starting from single seeds with different crystallographic orientations. These were selected taking into account the mechanical properties of unirradiated zinc single crystals [9-l 1, 15-l 61. The crystallographic orientations were in all cases determined by the Laue back-reflection technique. The critical resolved shear stress (CRSS) of the as-grown crystals at 77K was 0.2-0.4MPa. The irradiation was performed at liquid nitrogen temperature (78-81 K), in the cryogenic facility located in the core of CNEA’s lightwater reactor RA- 1 [17]. The fast-neutron flux was determined by the yield stress at 77 K of irradiated zinc single crystals. The yield stress, measured as a function of the effective bombardment time, was then compared with yield stress-dose curves previously published, in order to determine the flux [8]. The fast-neutron flux level of the reactor was lOI nm-* s-‘. The crystals were removed from the cryostat after irradiation, and stored in liquid nitrogen for a week or so, to allow activity decay. They were deformed in an Instron tensile machine (Model TTM) with a crosshead speed of 3.3 ,um s-i. The mounting and the Thermocouple
/
Llquld nitrogen
I -Thermal
Inlet
lnsulolor
Copper block
/ Dewor / /
Cavity for specimen contomer
Fig. I. Schematic diagram of the annealing system. The temperature of the copper block was regulated by the injection of liquid nitrogen.
OF IRRADIATED
ZINC
deformation were performed with the sample immersed in liquid nitrogen (77 K). Deformation markings were examined in a sample by photomicrography as the deformation proceeded. The sample was removed from the tensile machine and photographed while immersed in liquid nitrogen. Recovery experiments were performed using the annealing system of Fig. 1. In this system, the heat capacity of the copper block is very high in comparison to that of the specimen and hence the insertion of the specimen did not appreciably change the block temperature. The annealing temperature pulse showed a delay time before arriving to the annealing temperature. Annealing time was taken when the sample was at a temperature 2.5 K below the final annealing temperature. Following the annealing temperature pulse, the sample was remounted in the Instron machine while immersed in liquid nitrogen, and its effect on the yield stress was determined at 77 K. Since the same specimen was used throughout an annealing sequence, the plastic strain required to measure the yield stress was kept as small as possible (elongation _ lo-)). 3. RESULTS 3. I. PInstic behauiour
Single crystals presenting only basal slip were selected (see Figs 2 and 3). For such orientations, resolved shear deformation velocity on the basal planes was approximately constant during the test t111. It was found that CRSS at 77 K increased by 9.5 MPa for a dose of 4.5 x 102’nmm2 in a specimen with an initial CRSS of 0.20 MPa, and that slip actually took place on basal planes for crystals irradiated at different doses. Figure 2 shows some representative resolved shear stress vs resolved shear strain curves at 77 K for specimens tested up to fracture. It was assumed that throughout the crystals, rotation of basal planes during tests was homogeneous (see later photomicrographs). CRSS at 77 K and general shape of curves are not dependent upon the particular crystallographic orientation. It is apparent that work hardening after the yield point decreases with dose, showing some crystals softening at high doses. This behaviour is similar to copper single crystals, irradiated at similar doses [6, 181. The final portions of all the tensile curves show a constant work hardening of 3-4.5 MPa in unirradiated and 5.5-6.2 MPa in irradiated specimens. For doses lower than 4.5 x 10” nm-* ductility was approximately the same for the most part of the specimens tested (0.10
IRIART ef al.:
MECHANICAL
PROPERTIES
OF IRRADIATED
ZINC
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Resolved shear strain o Fig. 2. Shear stress vs shear strain curves at 77K of zinc single crystals irradiated at different neutron doses.
t
I 250~
b
0001
I oio
A IIZO
Fig. 3. Slip lines in a zinc single crystal irradiated to 3.7 x 1020nm-* in liquid nitrogen as a function of the strain. Deformation was performed at 77K. (a) u = 0.01, (b) a = 0.03, (c) n = 0.07, (d) II = 0.14.
391
392
IRIART er al.:
MECHANICAL
PROPERTIES OF IRRADIATED
tative zone of its body, as a function of resolved shear strain a on the basal planes. An homogeneous distribution of parallel slip bands throughout the deformation process can be seen. For values of a greater than 0.03 no substantial increase of the number of slip bands is observed; this shows that the consecutive deformation steps occur mainly in the previously activated zones, and only in minor way by the activation of new zones. This behaviour is similar to that observed in unirradiated zinc single crystals by Seeger [ 111. 3.2. Dislocation-barrier interaction In order to understand the nature of the irradiation-produced defects that are responsible of the radiation hardening in zinc we take the copper as reference. A zinc and a copper single crystals were irradiated simultaneously to a neutron dose of 1.5 x 1020nms2 in liquid nitrogen. The increments of the CRSS (Au) at 77K were 4.5 MPa in zinc and 7.1 MPa in copper. Then, we compare the radiation hardening in zinc and copper within a planar barrier interaction model with an expression of the form 18, 191 Aa = r3:2 Gb (dN)“* where G = shear modulus, b = Burgers vector, d = barrier diameter, N = barrier density and I = constant measure of the strength of obstacles in units of twice the dislocation line tension. Taking the reported values for G and b [19], a calculated value of N and our experimental data we obtain that the size and strength of obstacles in zinc and copper are related by 5(3:2d”2)CU _ 1. x3’*d’/2)Zn
basal plane of a twin nearly parallel to it, we call this type of fracture prismatic cleavage. In order to study the effect of neutron irradiation on the fracture process, we have used three kinds of crystallographic orientations, favouring a large basal slip or a quick fracture by either basal or prismatic cleavage. 3.3.1. Favourable basal slip. The crystal axis orientation for best favourable basal slip must be located in the meridian containing (0001) and (1120) poles in a (0001) stereographic projection. This orientation was calculated using the values of the stress normal to the cleavage plane at fracture (o,.,r) given by Barret [9] (for basal plane a s’ = 1.86 MPa and prismatic plane oFi = 17.65 MPa It is given by (0001) = 74”, (1 l%l> = 16”, where (0601) is the angle between the basal pole (0001) and the specimen axis, and (11%) is the angle between the pole (1120) and the crystal axis. Two sets of crystals with orientations near this one were used: the first with (00%) = 65”, (11%) = 25” and the second with (00%) = 70”, (1 1%) = 20”. All the specimens of the first set fractured by the basal cleavage of the original crystal. For doses lower than 2 x lO’“nm-* the crystals of the second set fractured by basal ‘cleavage. Above this value they showed a stepped cleavage surface, similar to that corresponding to described prismatic cleavage fracture, with two sets of facets. One set of facets was identified as basal planes of the original matrix. The other set of facets consisted of prismatic planes in the original matrix or basal planes in twins nearly parallel to them. The last set of facets was not differentiated by our optical method of identification. Figure 4 shows crr.,ras a function of neutron dose 32 t
This relation suggests that the radiation hardening at liquid nitrogen in zinc is a result of the presence of irradiation-produced barriers. The interaction with dislocations is the same as that operating in copper, being barriers most probably vacancy loops.
26 c
24 I-
3.3. Fracture behaviour Uniaxial tensile tests in unirradiated zinc single crystals produce easy fracture at 77 K by cleavage on basal and prismatic planes [9-l 11. According to the crystallographic orientation, the crystals fall into two main groups with different cleavage surfaces at fracture [lo, 151. In one case the crystals fractured by cleavage across the basal plane of the original matrix. We call this type of fracture basal cleavage. In the other case the crystals showed a stepped cleavage surface, with two sets of facets approximately at right angles to each other. The main set of facets consisted of prismatic planes of the original matrix and basal planes of the twins induced by the deformation. The other set of facets consisted of basal planes in the parent crystal. Assuming that initial fracture occurs in a prismatic plane or in a
ZINC
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4. Cleavage stress vs neutron dose at 77 K, in crystals with favourable basal slip orientations.
IRIART e/ al.:
MECHANICAL
PROPERTIES
OF IRRADIATED
ZINC
393
16 Pulses.
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Annealing temperoture & (K) Fig. 6. lsochronal annealing of the CRSS of a zinc single crystal, irradiated to 3.4 x 102”nm-’ in liquid nitrogen. o, = CRSS after irradiation before annealing. Reference temperature: 77 K.
3
dose ( 10” nm-” ) Fig. 5. Cleavage stress vs neutron dose at 77 K, in crystals with either favourable basal or prismatic cleavage orientations. Macroscopic deformation was only detected for the crystals marked by arrows. Neutron
It has been observed that all the crystals fractured by prismatic cleavage. From the family of available prismatic planes of the parent crystal, cleavage is presented on the prismatic planes with the greatest normal stress or on basal planes in twins nearly at 77 K. In the prismatic cleavage case the stress parallel to them, without any detected slip. cr$ normal to the apparent prismatic cleavage plane in increased with dose (Fig. 5). It changed from 12 MPa the original matrix at fracture (o$“) was plotted in an unirradiated sample to 32 MPa at 10” nmm2, in against the neutron dose. In both basal and prismatic crystals of the first set. These values of Q$!’ are cleavage, bNF increased with dose. Furthermore, crNF somewhat lower than those obtained in crystals of the shows a marked crystallographic orientation desecond set at the same doses (Fig. 5). Above 1021 nm-2 DloTo was 34 MPa in two crystals of the pendence. For doses lower than 4.5 x 10”’nm-2 ductility was third set IraNdated at different doses (Fig. 5). approximately the same for all the specimens 3.4. Yield stress recovery (0.10 < a c 0.30) in spite of the fact that at this dose the CRSS increased by nearly 45 times the uniThe isochronal annealing of the yield stress was t-radiated values. At higher doses, ductility decreased measured at 77 K in various zinc specimens irradiated by more of an order of magnitude (a < 0.01). We at liquid nitrogen at doses below 4.5 x 10Zonm-I, consider 4.5 x 1020nm-2 as a ductility transition performed with 10 min pulses between 80 and 300 K. dose. A monotonous decrease of the yield stress was 3.3.2. Favourable basal cleavage. Crystals with the obtained during the annealing experiments. Figure 6 orientation (do]) = 20” (11%) = 71’ were used. In shows a representative isochronal annealing curve, this case the crystal axis is located near the (0001) measured in a specimen irradiated to 3.4 x 102’nm-‘, pole. The ratio of the stress normal to the basal plane together with its differentiated curve. It can be estaband the resolved shear stress on the basal system, has lished three important recovery stages. They are increased considerably in these crystals. centered at 110, 160 and 210 K, with a yield stress All the specimens fractured by basal cleavage. recovery of about 20% in each one of the two lower Above 5 x 10’qnm-2, no deformation was detected. stages, and 45% in the last one. At room temperature High dependence of &! with dose was obtained the yield stress recovery was nearly 95%. (Fig. 5). It was more appreciable than that corresponding to crystals with favourable basal slip orien4. DISCUSSION tations (Fig. 4). At doses over 2 x 1020nme2 the values of 0:; were in the rank of 14-18 MPa. This Deformation markings showed that slip in zinc rank is approximately the same to that obtained by single crystals neutron irradiated at liquid nitrogen, Stafel and Wood for 0%’ at 195 K in zinc single with easy glide orientations, took place in the basal crystals, in the absence of basal slip [16]. system as in unirradiated specimens. The CRSS at 3.3.3. Favourable prismatic cleavage. Three sets of 77 K in the basal system in zinc showed to be a crystals with favourable prismatic cleavage orienfunction of neutron dose, but, as expected, no crystations were used: the first with (O&l) = !NY, (11%) tallographic orientation dependent. A substantial = 30”, the second with (Ot%l) = 88”, (1 l%J) = 6” and hardening was found by irradiation, in this case. The the third with (Ot%i) = 73”, (1 Tzb) = 28”. In the first relation of the CRSS increments of zinc and copper and the second set slip and cleavage on basal planes irradiated at the same doses is similar to the ratio of of the original crystal are geometrically inhibited. the shear moduli. This fact enables us to consider that
394
IRIART er al.:
MECHANICAL
PROPERTIES
hardening in zinc is a result of similar irradiationproduced barriers. It has been proposed that in low temperature irradiated copper these barriers are vacancy loops [20]. We can expect that also in zinc they are vacancy loops. The observed decrease of the work hardening with dose after the yield point in zinc, is also present in magnesium [2 1,221 and copper [6, 181 single crystals neutron irradiated at similar doses. All of these metals show work softening at doses over 4.5 x 1020nm12. This behaviour could be associated with the defect-free channels formation by dislocation passage or dislocation channeling phenomenon [23], which, in our opinion, begins to be significant above the mentioned dose. It is apparent that the increase of the fracture stress (basal or prismatic cleavage) with neutron dose is related to the presence of irradiation-produced barriers which hinder dislocation motion. This fact shows that dislocation motion is necessary before fracture. Our results show that the amount of strain at fracture must be related with the stress normal to the cleavage plane when dislocation motion occurs. Values of about 18 and 34 MPa seem to be the maximum limits of the fracture stress normal to the basal and prismatic planes at fracture, respectively, in irradiated zinc. It has been observed in copper that the dislocation channeling phenomenon is accompanied by an increase in the strain for each active slip plane [23]. In a ductile metal like magnesium, this phenomenon produces a sharp yield point drop followed by successive stress drops [21,22]. But in a brittle metal like zinc, it contributes to lower the macroscopic strain at fracture; as it was observed in our crystals with favorable basal slip orientations at doses over 4.5 x 1020nm-2. We can conclude that the stress and strain at fracture are affected by the irradiation-produced defects and their instability, in a tensile test in zinc single crystals. But, in our opinion, the fracture mechanisms are the same in irradiated and unirradiated specimens. Using plasticity techniques, three recovery stages were located in fast-neutron irradiated zinc, around 110, 160 and 210 K. The temperature rank, measured in units of the melting temperature, and the fraction of yield stress annealing out in the stage around 110 K in zinc, are nearly the same to that corresponding to stage III in neutron irradiated copper single crystals at 4.2 K. In this case the main yield stress recovery occurs in the next two stages as in zinc [24]. The located recovery stages have been previously studied by electrical resistivity and c-axis spacing in neutron irradiated zinc (12, 141. The recovery stage around 110K could be associated with either the migration of dumbell configuration interstitials [ 121 or a change in the nature of clusters or dislocation loops [14]. The next recovery stage centered around 160 K could be associated with the vacancy migration
OF IRRADIATED
ZINC
to annihilate interstitial dislocation loops [12, 141. The recovery around 210K, clearly seen in our plasticity determinations, may be associated with the annealing of vacancy loops. Recovery behaviour of this kind of defects has not been studied extensively in zinc, due to the fact that electrical resistivity showed to be not very sensitive to their presence [12-141. The amount of yield stress recovery measured in each stage in zinc showed that plasticity techniques are apt to study defects mobility, in particular at the last high temperature-recovery stage. In view of this fact, plasticity techniques will be used in the future in order to obtain the activation energies associated to the located recovery stages. 5. CONCLUSIONS From the results of these experiments, concluded that:
it can be
(1) At liquid nitrogen the irradiation-produced barriers that hinder the dislocation motion are similar in zinc and copper. (2) Dislocation motion is necessary before fracture. The increase of fracture stress with dose is an evidence of this conclusion. (3) For neutron doses lower than 4.5 x 10” nme2, the barriers are stable. Ductility is the same as the unirradiated case. This fact suggests that fracture mechanisms are the same. (4) Above 4.5 x 1020nm-2, the dislocation channeling phenomenon enhances the inhomogeneous deformation and produced a fast fracture. (5) Plasticity techniques showed to be apt to study defects mobility in neutron irradiated zinc below 4.5 x 1020nme2. In this case the sensitivity is similar to the resistivity techniques in the recovery stages around 110 and 160 K. Better sensitivity of plasticity techniques for the recovery stage at 210K is thus apparent. Acknowledgemenrs-The authors would like to thank the members of the Departamento Materiales and Departamento Reactores of the CNEA, for their valuable help.
REFERENCES 1. D. L. Douglass, The Metallurgy of Zirconium, IAEA, SUPPL. Vienna (1971). 2. Zirconium in the Nuclear Industry, ASTM STP 633 (1977). 3. J. Hillairet, L. Trabut and R. Le Diouron, CEA-R4033 (1970). 4. J. C. Nicoud, CEA-R-4134 (1970). 5. C. Pomar, T. H. Blewitt and E.. A. Bisogni, Physica Status Solidi (a) 10. 519 (1972). 6. H. C. Gonzalez; Tesis de Doctorado en Fisica, Instituto de Fisica J. Balseiro, Universidad National de cuyo, Argentina (1973). 7. F. W. Kunz and A. N. Holden, Acta merall. 2, 816
(1954). 8. H. C. Gonzalez and E. A. Bisogni, Physic0 status solidi (a) 62, 351 (1980). 9. C. Barret, Srrucrure of Metals, p. 386. McGraw-Hill, New York (1952).
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MECHANICAL
PROPERTIES
10. A. Deruyttere and G. B. Greenough, J. Insr. Mcrals 84, 337 (1955-56). 11. A. Seeger and H. Traiible, Z. Merallk. 51, 435 (1960). 12. J. Nihoul, Physica srarus solidi 3, 2061 (1963). 13. J. Roggen, J. Nihoul, J. Cornelis and L. Stals. J. Nucl. Mater. 69-70, 700 (1978). 14. H. Maeta, S. Takamura and S. Okuda. Ph,rs. Letr. 75A,
131 (1979). 15. L. C. Weiner and M. Gensamer, J. Im. Metals 85, 441 (1956-57). 16. E. J. Stafel and D. S. Wood, Fracture of Solids,Metall. Sot. Conf., Maple Valley, Washington, 1962. p. 521. Interscience, New York (1963). 17. H. C. Gonzalez, T. H. Blewitt and E. A. Bisogni,
OF IRRADIATED
ZINC
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Micromaquina de Traction y Criostato que operan en et reactor RA-I. CNEA, Nt 29,76. 18. T. H. Blewitt and T. J. Koppenaal. Radiarion Efecrs in Merals. AIME SEM1.V.. Ashville. NC (1965). 19. J. Friedel. Les Disolarions. Gauthier-Villars, Pergamon Press. Paris (1964) (English editions). 20. A. Seeger. Proc. Second Inr. Co@ on Peacqful Uses of Atomic EnerR),. New York, p, 250 (1958). 21. H. Gonzalez~‘To be published. 22. C. J. Iriart and H. C. Gonzllez. To be oubhshed 23. J. V. Sharp. Phil. Msg. 16, 77 (1967). ’ 24. J. Diehl. Chr. Leitz and W. Schilling. Ph.vs. Lerr. 4, 236 (1963).
Acre merall. Vol. 32, No. 3, pp. 389-395, 1984 Printed in Great Britain. All rights reserved
MECHANICAL PROPERTIES OF FAST NEUTRON IRRADIATED ZINC AT 77K C. J. IRIART, A. M. FORTIS and H. C. GONZALEZ ComisiCtn National de Energia Atbmica, Gcia. Desarroiio, Departamento Materiales, Av. de1 Libertador 8250, 1429 Buenos Aires, Argentina (Received
20 July 1983)
Abstract-Pure zinc single crystals were irradiated at liquid nitrogen temperature in the RA-I reactor of CNEA up to fast neutron doses of 3 x IO*’nm-*. Their plastic and fracture behaviour were studied at 77 K. Radiation hardening in zinc and copper was compared. It seems to be produced by the presence of similar irradiation-produced barriers to dislocation in both metals. For neutron doses greater than 4.5 x 10”nme2 the ductility in crystals with easy glide o~entations decreases rapidly by more than an order of magnitude. Fracture stress increased with dose, and it is dependent on crystallographic orientation. This fact shows that dislocation motion is necessary before fracture. Yield stress annealing experiments showed three important recovery stages located around I IO, 160 and 210 K. The last stage, which is the most pronounced one, is hardly seen by electrical resistivity recovery.
Rbun~&Nous
avons irradik aux neutrons rapides des monocristaux de zinc pur g la tempkature dei i’azote liquide, dans ie riacteur RA-I de la CNEA, avec des doses pouvant atteindre 3 x lo?’ nmm2. Nous avons CtudiC ieur comportement plastique et de rupture B 77 K. Nous avons cornpar& ies durcissements par irradiation dans ie zinc et dans le cuivre. Dans les deux m&aux, iis semblent provenir de la prksence d’obstacies semblabies, produits par l’irradiation, pour fe d~pla~rn~t des dislocations. Pour des dosess de neutrons superieures B 4,s x 1020nm- 2, la ductilite des cristaux orient&s pour le glissement facile diminue rapidement de plus d’un ordre de grandeur. La contrainte B la rupture augmente avec la dose et elle d&pend de l’orientation cristallographique. Ceci montre que le mouvement des dislocations est n&essaire pour la rupture. Des exptriences de recuit g la limite Clastique ont mis en ividence trois stades importants de ~stau~tion sit&s au voisinage de 110, 160 et 210 K. Ce dernier stade, qui est le plus pronon& est a peine observe dans la restauration de la risistivite tlectrique. Zinkeinkristalle wurden bei der Temperatur des fliissigen Stickstoffes im Reaktor U-1 der CNEA bis zu einer Dosis von 3 x lO*‘nm-* mit schnellen Neutronen bestrahlt. Plastizitiits- und Bruchverhalten wurden bei 77 K untersucht. Die ~trahlungsverfestigung von Zink und Kupfer wurden vergiichen. Es scheint in beiden Metallen von iihnlichen bestrahlungsinduzierten Hindernissen fiir die Versetzungsbewegung herzuriihren. Bei Neutronendosen iiber 4,5 x 1020nm-’ nimmt die Duktilitiit von Kristallen mit einer Orientierung fiir Einfachgleitung rasch urn mehr als eine GriiBenordnung ab. Die Bruchspannung nahm mit der Dosis zu und hing von der kristallografischen O~entierung ab. Das weist daiauf hi& daB Ver~t~ngs~wegung vor dem bruch niitig ist. Ausheilexperimente z&&ten, dal3 die FlieBspannung drei wichtige Erholungsstufen bei 110. 140 und 210 K durchliiuft. Die letzte Stufe, welche am ausgepriigtesten ist, IlDt sich in der Erholung des elektrischen -_ Widerstandes kaum entdecken. Zuwnmenfassung-Reine
1. INTRODUCTION
The effects of radiation on the mechanical properties of h.c.p. metals have not been as widely studied as in metals with f.c.c. and b.c.c. structures. The effects of neutron irradiation on the mechanical properties of zirconium and its alloys have received special attention in the past 20 years, as consequence of their use in nuclear technology [l, 21. To the authors’ knowledge, information about other h.c.p. metals is confined to recent works on magnesium [3-61 and zinc f6-81. Zinc single crystals showed important increments of the yield stress after fast-neutron irradiation at liquid nitrogen temperature. These increments are of the same order of magnitude as those measured in copper single crystals [8]. On the other hand, un389
irradiated zinc single crystals are not ductile at 77 K, fracturing by cleavage on basal and prismatic planes [9-l I]. This characteristic is still maintained in irradiated crystals, fracture stress increasing with neutron dose [6]. These precedents moved us to study the plasticity and fracture behaviour of fast neutron irradiated pure zinc single crystals at 77 K. Plasticity techniques were used for the determination of radiation damage recovery stages in zinc. These recovery stages have been studied before by electrical resistivity, which showed to be not very sensitive to the presence of irradiation-produced loops [12-141. Due to this fact, our main concern was the last recovery stage caused by loops annealing out, taking into account that plasticity is very sensitive to this kind of defects [6,8].
IRIART er al.:
390
MECHANICAL
PROPERTIES
2. EXPERIMENTAL
Zinc single crystals tensile specimens were used in our experiments. The crystals were grown by a modified Bridgman technique [8], from high purity zinc rods (99.9993%), supplied by Johnson Matthey, and under a purified argon atmosphere. Two types of specimens were prepared: samples of square cross-section 3 mm wide and 30 mm gauge length, and cylindrical samples 2.5 mm in diameter and 27.5 mm gauge length. In both cases the samples were grown with spherical heads 6.35 and 5 mm respectively, in diameter. The crystals were grown starting from single seeds with different crystallographic orientations. These were selected taking into account the mechanical properties of unirradiated zinc single crystals [9-l 1, 15-l 61. The crystallographic orientations were in all cases determined by the Laue back-reflection technique. The critical resolved shear stress (CRSS) of the as-grown crystals at 77K was 0.2-0.4MPa. The irradiation was performed at liquid nitrogen temperature (78-81 K), in the cryogenic facility located in the core of CNEA’s lightwater reactor RA- 1 [17]. The fast-neutron flux was determined by the yield stress at 77 K of irradiated zinc single crystals. The yield stress, measured as a function of the effective bombardment time, was then compared with yield stress-dose curves previously published, in order to determine the flux [8]. The fast-neutron flux level of the reactor was lOI nm-* s-‘. The crystals were removed from the cryostat after irradiation, and stored in liquid nitrogen for a week or so, to allow activity decay. They were deformed in an Instron tensile machine (Model TTM) with a crosshead speed of 3.3 ,um s-i. The mounting and the Thermocouple
/
Llquld nitrogen
I -Thermal
Inlet
lnsulolor
Copper block
/ Dewor / /
Cavity for specimen contomer
Fig. I. Schematic diagram of the annealing system. The temperature of the copper block was regulated by the injection of liquid nitrogen.
OF IRRADIATED
ZINC
deformation were performed with the sample immersed in liquid nitrogen (77 K). Deformation markings were examined in a sample by photomicrography as the deformation proceeded. The sample was removed from the tensile machine and photographed while immersed in liquid nitrogen. Recovery experiments were performed using the annealing system of Fig. 1. In this system, the heat capacity of the copper block is very high in comparison to that of the specimen and hence the insertion of the specimen did not appreciably change the block temperature. The annealing temperature pulse showed a delay time before arriving to the annealing temperature. Annealing time was taken when the sample was at a temperature 2.5 K below the final annealing temperature. Following the annealing temperature pulse, the sample was remounted in the Instron machine while immersed in liquid nitrogen, and its effect on the yield stress was determined at 77 K. Since the same specimen was used throughout an annealing sequence, the plastic strain required to measure the yield stress was kept as small as possible (elongation _ lo-)). 3. RESULTS 3. I. PInstic behauiour
Single crystals presenting only basal slip were selected (see Figs 2 and 3). For such orientations, resolved shear deformation velocity on the basal planes was approximately constant during the test t111. It was found that CRSS at 77 K increased by 9.5 MPa for a dose of 4.5 x 102’nmm2 in a specimen with an initial CRSS of 0.20 MPa, and that slip actually took place on basal planes for crystals irradiated at different doses. Figure 2 shows some representative resolved shear stress vs resolved shear strain curves at 77 K for specimens tested up to fracture. It was assumed that throughout the crystals, rotation of basal planes during tests was homogeneous (see later photomicrographs). CRSS at 77 K and general shape of curves are not dependent upon the particular crystallographic orientation. It is apparent that work hardening after the yield point decreases with dose, showing some crystals softening at high doses. This behaviour is similar to copper single crystals, irradiated at similar doses [6, 181. The final portions of all the tensile curves show a constant work hardening of 3-4.5 MPa in unirradiated and 5.5-6.2 MPa in irradiated specimens. For doses lower than 4.5 x 10” nm-* ductility was approximately the same for the most part of the specimens tested (0.10
IRIART ef al.:
MECHANICAL
PROPERTIES
OF IRRADIATED
ZINC
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t
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Fig. 3. Slip lines in a zinc single crystal irradiated to 3.7 x 1020nm-* in liquid nitrogen as a function of the strain. Deformation was performed at 77K. (a) u = 0.01, (b) a = 0.03, (c) n = 0.07, (d) II = 0.14.
391
392
IRIART er al.:
MECHANICAL
PROPERTIES OF IRRADIATED
tative zone of its body, as a function of resolved shear strain a on the basal planes. An homogeneous distribution of parallel slip bands throughout the deformation process can be seen. For values of a greater than 0.03 no substantial increase of the number of slip bands is observed; this shows that the consecutive deformation steps occur mainly in the previously activated zones, and only in minor way by the activation of new zones. This behaviour is similar to that observed in unirradiated zinc single crystals by Seeger [ 111. 3.2. Dislocation-barrier interaction In order to understand the nature of the irradiation-produced defects that are responsible of the radiation hardening in zinc we take the copper as reference. A zinc and a copper single crystals were irradiated simultaneously to a neutron dose of 1.5 x 1020nms2 in liquid nitrogen. The increments of the CRSS (Au) at 77K were 4.5 MPa in zinc and 7.1 MPa in copper. Then, we compare the radiation hardening in zinc and copper within a planar barrier interaction model with an expression of the form 18, 191 Aa = r3:2 Gb (dN)“* where G = shear modulus, b = Burgers vector, d = barrier diameter, N = barrier density and I = constant measure of the strength of obstacles in units of twice the dislocation line tension. Taking the reported values for G and b [19], a calculated value of N and our experimental data we obtain that the size and strength of obstacles in zinc and copper are related by 5(3:2d”2)CU _ 1. x3’*d’/2)Zn
basal plane of a twin nearly parallel to it, we call this type of fracture prismatic cleavage. In order to study the effect of neutron irradiation on the fracture process, we have used three kinds of crystallographic orientations, favouring a large basal slip or a quick fracture by either basal or prismatic cleavage. 3.3.1. Favourable basal slip. The crystal axis orientation for best favourable basal slip must be located in the meridian containing (0001) and (1120) poles in a (0001) stereographic projection. This orientation was calculated using the values of the stress normal to the cleavage plane at fracture (o,.,r) given by Barret [9] (for basal plane a s’ = 1.86 MPa and prismatic plane oFi = 17.65 MPa It is given by (0001) = 74”, (1 l%l> = 16”, where (0601) is the angle between the basal pole (0001) and the specimen axis, and (11%) is the angle between the pole (1120) and the crystal axis. Two sets of crystals with orientations near this one were used: the first with (00%) = 65”, (11%) = 25” and the second with (00%) = 70”, (1 1%) = 20”. All the specimens of the first set fractured by the basal cleavage of the original crystal. For doses lower than 2 x lO’“nm-* the crystals of the second set fractured by basal ‘cleavage. Above this value they showed a stepped cleavage surface, similar to that corresponding to described prismatic cleavage fracture, with two sets of facets. One set of facets was identified as basal planes of the original matrix. The other set of facets consisted of prismatic planes in the original matrix or basal planes in twins nearly parallel to them. The last set of facets was not differentiated by our optical method of identification. Figure 4 shows crr.,ras a function of neutron dose 32 t
This relation suggests that the radiation hardening at liquid nitrogen in zinc is a result of the presence of irradiation-produced barriers. The interaction with dislocations is the same as that operating in copper, being barriers most probably vacancy loops.
26 c
24 I-
3.3. Fracture behaviour Uniaxial tensile tests in unirradiated zinc single crystals produce easy fracture at 77 K by cleavage on basal and prismatic planes [9-l 11. According to the crystallographic orientation, the crystals fall into two main groups with different cleavage surfaces at fracture [lo, 151. In one case the crystals fractured by cleavage across the basal plane of the original matrix. We call this type of fracture basal cleavage. In the other case the crystals showed a stepped cleavage surface, with two sets of facets approximately at right angles to each other. The main set of facets consisted of prismatic planes of the original matrix and basal planes of the twins induced by the deformation. The other set of facets consisted of basal planes in the parent crystal. Assuming that initial fracture occurs in a prismatic plane or in a
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4. Cleavage stress vs neutron dose at 77 K, in crystals with favourable basal slip orientations.
IRIART e/ al.:
MECHANICAL
PROPERTIES
OF IRRADIATED
ZINC
393
16 Pulses.
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Annealing temperoture & (K) Fig. 6. lsochronal annealing of the CRSS of a zinc single crystal, irradiated to 3.4 x 102”nm-’ in liquid nitrogen. o, = CRSS after irradiation before annealing. Reference temperature: 77 K.
3
dose ( 10” nm-” ) Fig. 5. Cleavage stress vs neutron dose at 77 K, in crystals with either favourable basal or prismatic cleavage orientations. Macroscopic deformation was only detected for the crystals marked by arrows. Neutron
It has been observed that all the crystals fractured by prismatic cleavage. From the family of available prismatic planes of the parent crystal, cleavage is presented on the prismatic planes with the greatest normal stress or on basal planes in twins nearly at 77 K. In the prismatic cleavage case the stress parallel to them, without any detected slip. cr$ normal to the apparent prismatic cleavage plane in increased with dose (Fig. 5). It changed from 12 MPa the original matrix at fracture (o$“) was plotted in an unirradiated sample to 32 MPa at 10” nmm2, in against the neutron dose. In both basal and prismatic crystals of the first set. These values of Q$!’ are cleavage, bNF increased with dose. Furthermore, crNF somewhat lower than those obtained in crystals of the shows a marked crystallographic orientation desecond set at the same doses (Fig. 5). Above 1021 nm-2 DloTo was 34 MPa in two crystals of the pendence. For doses lower than 4.5 x 10”’nm-2 ductility was third set IraNdated at different doses (Fig. 5). approximately the same for all the specimens 3.4. Yield stress recovery (0.10 < a c 0.30) in spite of the fact that at this dose the CRSS increased by nearly 45 times the uniThe isochronal annealing of the yield stress was t-radiated values. At higher doses, ductility decreased measured at 77 K in various zinc specimens irradiated by more of an order of magnitude (a < 0.01). We at liquid nitrogen at doses below 4.5 x 10Zonm-I, consider 4.5 x 1020nm-2 as a ductility transition performed with 10 min pulses between 80 and 300 K. dose. A monotonous decrease of the yield stress was 3.3.2. Favourable basal cleavage. Crystals with the obtained during the annealing experiments. Figure 6 orientation (do]) = 20” (11%) = 71’ were used. In shows a representative isochronal annealing curve, this case the crystal axis is located near the (0001) measured in a specimen irradiated to 3.4 x 102’nm-‘, pole. The ratio of the stress normal to the basal plane together with its differentiated curve. It can be estaband the resolved shear stress on the basal system, has lished three important recovery stages. They are increased considerably in these crystals. centered at 110, 160 and 210 K, with a yield stress All the specimens fractured by basal cleavage. recovery of about 20% in each one of the two lower Above 5 x 10’qnm-2, no deformation was detected. stages, and 45% in the last one. At room temperature High dependence of &! with dose was obtained the yield stress recovery was nearly 95%. (Fig. 5). It was more appreciable than that corresponding to crystals with favourable basal slip orien4. DISCUSSION tations (Fig. 4). At doses over 2 x 1020nme2 the values of 0:; were in the rank of 14-18 MPa. This Deformation markings showed that slip in zinc rank is approximately the same to that obtained by single crystals neutron irradiated at liquid nitrogen, Stafel and Wood for 0%’ at 195 K in zinc single with easy glide orientations, took place in the basal crystals, in the absence of basal slip [16]. system as in unirradiated specimens. The CRSS at 3.3.3. Favourable prismatic cleavage. Three sets of 77 K in the basal system in zinc showed to be a crystals with favourable prismatic cleavage orienfunction of neutron dose, but, as expected, no crystations were used: the first with (O&l) = !NY, (11%) tallographic orientation dependent. A substantial = 30”, the second with (Ot%l) = 88”, (1 l%J) = 6” and hardening was found by irradiation, in this case. The the third with (Ot%i) = 73”, (1 Tzb) = 28”. In the first relation of the CRSS increments of zinc and copper and the second set slip and cleavage on basal planes irradiated at the same doses is similar to the ratio of of the original crystal are geometrically inhibited. the shear moduli. This fact enables us to consider that
394
IRIART er al.:
MECHANICAL
PROPERTIES
hardening in zinc is a result of similar irradiationproduced barriers. It has been proposed that in low temperature irradiated copper these barriers are vacancy loops [20]. We can expect that also in zinc they are vacancy loops. The observed decrease of the work hardening with dose after the yield point in zinc, is also present in magnesium [2 1,221 and copper [6, 181 single crystals neutron irradiated at similar doses. All of these metals show work softening at doses over 4.5 x 1020nm12. This behaviour could be associated with the defect-free channels formation by dislocation passage or dislocation channeling phenomenon [23], which, in our opinion, begins to be significant above the mentioned dose. It is apparent that the increase of the fracture stress (basal or prismatic cleavage) with neutron dose is related to the presence of irradiation-produced barriers which hinder dislocation motion. This fact shows that dislocation motion is necessary before fracture. Our results show that the amount of strain at fracture must be related with the stress normal to the cleavage plane when dislocation motion occurs. Values of about 18 and 34 MPa seem to be the maximum limits of the fracture stress normal to the basal and prismatic planes at fracture, respectively, in irradiated zinc. It has been observed in copper that the dislocation channeling phenomenon is accompanied by an increase in the strain for each active slip plane [23]. In a ductile metal like magnesium, this phenomenon produces a sharp yield point drop followed by successive stress drops [21,22]. But in a brittle metal like zinc, it contributes to lower the macroscopic strain at fracture; as it was observed in our crystals with favorable basal slip orientations at doses over 4.5 x 1020nm-2. We can conclude that the stress and strain at fracture are affected by the irradiation-produced defects and their instability, in a tensile test in zinc single crystals. But, in our opinion, the fracture mechanisms are the same in irradiated and unirradiated specimens. Using plasticity techniques, three recovery stages were located in fast-neutron irradiated zinc, around 110, 160 and 210 K. The temperature rank, measured in units of the melting temperature, and the fraction of yield stress annealing out in the stage around 110 K in zinc, are nearly the same to that corresponding to stage III in neutron irradiated copper single crystals at 4.2 K. In this case the main yield stress recovery occurs in the next two stages as in zinc [24]. The located recovery stages have been previously studied by electrical resistivity and c-axis spacing in neutron irradiated zinc (12, 141. The recovery stage around 110K could be associated with either the migration of dumbell configuration interstitials [ 121 or a change in the nature of clusters or dislocation loops [14]. The next recovery stage centered around 160 K could be associated with the vacancy migration
OF IRRADIATED
ZINC
to annihilate interstitial dislocation loops [12, 141. The recovery around 210K, clearly seen in our plasticity determinations, may be associated with the annealing of vacancy loops. Recovery behaviour of this kind of defects has not been studied extensively in zinc, due to the fact that electrical resistivity showed to be not very sensitive to their presence [12-141. The amount of yield stress recovery measured in each stage in zinc showed that plasticity techniques are apt to study defects mobility, in particular at the last high temperature-recovery stage. In view of this fact, plasticity techniques will be used in the future in order to obtain the activation energies associated to the located recovery stages. 5. CONCLUSIONS From the results of these experiments, concluded that:
it can be
(1) At liquid nitrogen the irradiation-produced barriers that hinder the dislocation motion are similar in zinc and copper. (2) Dislocation motion is necessary before fracture. The increase of fracture stress with dose is an evidence of this conclusion. (3) For neutron doses lower than 4.5 x 10” nme2, the barriers are stable. Ductility is the same as the unirradiated case. This fact suggests that fracture mechanisms are the same. (4) Above 4.5 x 1020nm-2, the dislocation channeling phenomenon enhances the inhomogeneous deformation and produced a fast fracture. (5) Plasticity techniques showed to be apt to study defects mobility in neutron irradiated zinc below 4.5 x 1020nme2. In this case the sensitivity is similar to the resistivity techniques in the recovery stages around 110 and 160 K. Better sensitivity of plasticity techniques for the recovery stage at 210K is thus apparent. Acknowledgemenrs-The authors would like to thank the members of the Departamento Materiales and Departamento Reactores of the CNEA, for their valuable help.
REFERENCES 1. D. L. Douglass, The Metallurgy of Zirconium, IAEA, SUPPL. Vienna (1971). 2. Zirconium in the Nuclear Industry, ASTM STP 633 (1977). 3. J. Hillairet, L. Trabut and R. Le Diouron, CEA-R4033 (1970). 4. J. C. Nicoud, CEA-R-4134 (1970). 5. C. Pomar, T. H. Blewitt and E.. A. Bisogni, Physica Status Solidi (a) 10. 519 (1972). 6. H. C. Gonzalez; Tesis de Doctorado en Fisica, Instituto de Fisica J. Balseiro, Universidad National de cuyo, Argentina (1973). 7. F. W. Kunz and A. N. Holden, Acta merall. 2, 816
(1954). 8. H. C. Gonzalez and E. A. Bisogni, Physic0 status solidi (a) 62, 351 (1980). 9. C. Barret, Srrucrure of Metals, p. 386. McGraw-Hill, New York (1952).
IRIART er al.:
MECHANICAL
PROPERTIES
10. A. Deruyttere and G. B. Greenough, J. Insr. Mcrals 84, 337 (1955-56). 11. A. Seeger and H. Traiible, Z. Merallk. 51, 435 (1960). 12. J. Nihoul, Physica srarus solidi 3, 2061 (1963). 13. J. Roggen, J. Nihoul, J. Cornelis and L. Stals. J. Nucl. Mater. 69-70, 700 (1978). 14. H. Maeta, S. Takamura and S. Okuda. Ph,rs. Letr. 75A,
131 (1979). 15. L. C. Weiner and M. Gensamer, J. Im. Metals 85, 441 (1956-57). 16. E. J. Stafel and D. S. Wood, Fracture of Solids,Metall. Sot. Conf., Maple Valley, Washington, 1962. p. 521. Interscience, New York (1963). 17. H. C. Gonzalez, T. H. Blewitt and E. A. Bisogni,
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ZINC
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Micromaquina de Traction y Criostato que operan en et reactor RA-I. CNEA, Nt 29,76. 18. T. H. Blewitt and T. J. Koppenaal. Radiarion Efecrs in Merals. AIME SEM1.V.. Ashville. NC (1965). 19. J. Friedel. Les Disolarions. Gauthier-Villars, Pergamon Press. Paris (1964) (English editions). 20. A. Seeger. Proc. Second Inr. Co@ on Peacqful Uses of Atomic EnerR),. New York, p, 250 (1958). 21. H. Gonzalez~‘To be published. 22. C. J. Iriart and H. C. Gonzllez. To be oubhshed 23. J. V. Sharp. Phil. Msg. 16, 77 (1967). ’ 24. J. Diehl. Chr. Leitz and W. Schilling. Ph.vs. Lerr. 4, 236 (1963).