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Faulted loops in neutron-irradiated zirconium
Journal of Nuclear Materials 68 (1977) 267-276 0 North-Holland Publishing Company
FAULTED LOOPS IN NEUTRON-IRRADIATED ZIRCONIUM A. JOSTSONS, R.G. BLAKE, J.G. NAPIER, P.M. KELLY and K. FARRELL Materials Division, AAEC Research
Establishment,
*
Lucas Heights, NSW, Australia
Received 9 June 1977
Faulted loops have been observed in high-purity zirconium irradiated at 723 K to 1.3 X 102’ neutrons/m2 (>O.i MeV). The transmission electron microscopy characterization of these l/6 (2023) faulted loops on (0001) is described in detail. It was found that the faulted loops were invariably vacancy in character although the coexisting population of perfect l/3 (11%) loops was of a mixed interstitial/vacancy nature. The faulted loops were observed in specimens of only two out of five batches of high-purity zirconium irradiated in this experiment. Even in these two specimens, the presence of faulted loops was restricted to the 723 K irradiation temperature; at 673 K only perfect l/3 (ll?O) loops were seen. Des boucles fautdes ont dte observdes dam du zirconium de haute pure& irradid a 723 K a des doses de 1,3 x 1O25 neutrons/m’ (>O,l MeV). La caractdrisation par microscopic electronique par transmission de ces boucles fautees l/6 (2023) sur les plans (0001) est d&rite en detail. I1 a etd observe que les boucles fautdes sont toujours du type lacune bien que la population coexistante des boucles parfaites l/3 (lI?O) dtait de type mixte interstitiel/lacune. Les boucles fauties ont dtd observies dans des echantillons appartenent a seulement deux sur cinq des fournitures de zirconium de haute purete irradies dans cette se’rie d’experience. Mime dans ces deux echantillons, la presence de boucles fautees etait limitee i la temperature d’irradiation de 723 K. A 673 K seules des boucles parfaites l/3 (lIZOr ont 6th observees. In hochreinem Zirconium, das bei 723 K mit Neutronen bis zu 1,3 . 1O25 rnF2 (E > 0,l MeV) bestrahlt worden war, wurden Versetzungsringe mit Stapelfehlern beobachtet. Die transmissionselektronenmikroskopische Charakterisierung dieser Versetzungsringe mit l/6 (2023) in (0001) wird eingehend beschrieben: Sie sind ausnahmslos vom Leerstellentyp, obwohl die koexistierende Population der fehlerlosen Versetzungsringe mit l/3 ( 1l?O) vom gemischten ZwischengitterLeerstellen-Typ ist. Die Versetzungsringe mit Stapelfehlern wurden in Proben aus nur zwei von insgesamt ftinf hochreinen Zr-Chargen, die in diesem Versuch bestrahlt worden waren, beobachtet. Auch in diesen zwei Proben war die Gegenwart von Versetzungsringen mit Stapelfehlern auf die Bestrahlung bei 723 K bt?schrCnkt, bei 673 K wurden nur fehlerlose Versetzungsringe mit l/3 ( 1120) festgestellt.
1. Introduction The nature of dislocation loops in neutron-irradiated zirconium has been investigated in considerable detail [l-4]. From an examination of zone-refined zirconi‘urn specimens irradiated in the temperature range 573 to 708 K to fast neutron fluences of 1.8 X 10M neutrons/ m2 (>l MeV), we concluded [l] that both vacancy and interstitial loops with perfect a/3 (1 120) Burgers vectors represent the sole form of irradiation-induced
* Metals & Ceramics Division, ORNL, Tennessee, USA 267
defect clustering. These loops do not possess the simple edge geometry since the normals are generally rotated towards one of the neighbouring ( IOiO) directions and, to a lesser degree, towards [OOOl]. This work reports the characterization of faulted loops which were observed in certain specimens of high-purity zirconium irradiated at 723 K in the Oak Ridge Research Reactor (ORR) to 1.3 X 102’ neutrons/m2 (x.1 MeV). Faulted loops were not observed in specimens of the same starting material irradiated at 673 K and below. Instead, the damage structure in these lower-temperature irradiations consisted entirely of the usual perfect a/3 (1120) loops.
268
A. Jostsons et al. /Loops
2. Experimental details The zirconium specimens studied form part of an experiment (ORR-228) to investigate the effects of specimen purity and irradiation temperature on the nature of the irradiation-induced damage structure. The zirconium was obtained from various sources and, with the exception of one lot of crystal bar material, was subsequently zone-refined in an electron-beam furnace at Oak Ridge National Laboratory (ORNL) for 2 to 20 passes. Specimens in the form of 0.5 mm thick X3.2 mm dia. discs were then annealed for &h at 983 K in a vacuum of 2 X 10M6 torr. The zirconium discs were stacked in stainless steel cans, evacuated and back-filled with helium before the cans were welded shut. The irradiations were carried out over two cycles of the Oak Ridge Research Reactor (ORR). The specimen fluences were derived from a map of measurements made on 14 dosimeter packages distributed throughout the experimental assembly. The data were consistent and it is estimated that the fluences are accurate to rtlO%. Temperature control was achieved by a combination of y-heating and wrap-around electric heaters balanced against a water-cooled aluminium heat sink. Two thermocouples were used to measure the temperature of each capsule and temperatures of 25°C around the nominal value were achieved. The electron microscopy was performed at the
in neutron-irradiated
zirconium
AAEC Research Establishment, Lucas Heights, using a JSEM-200 fitted with a 245” double-tilt goniometer stage modified to give k50° tilt about one axis. Specimens for electron microscopy were obtained by polishing on 600 mesh Sic paper to 250 pm thickness followed by electropolishing at 18 V and 223 K in a commercial automatic twin-jet unit. The electrolyte contained 6% perchloric acid and 32% butoxy ethanol in methanol. All electron micrographs have been printed with the emulsion side up and B, the beam direction, is defined to be the upward drawn normal to the diffraction pattern, i.e. it is anti-parallel to the direction of electron flow. Consistent indexing of zones nad reflections was obtained from Kikuchi patterns using the rules set out by Head et al. [5].
3. Observations A typical area of a foil of irradiated zirconium containing faulted loops is shown in fig. 1. The faulted loop concentration is low (-1018/m3), the loops are large and frequently intersect either one or both surfaces of the foil. In the following sections, the procedures used to determine the loop normal, its Burgers vector and interstitial or vacancy character will be described in some detail. 3.1. Loop normal Preliminary observations revealed that fault contrast was strong in reflections of the (2020) and { 1011) type. The behaviour of a number of loops imaged with g = (2020) whilst the foil was tilted from the vicinity of B = lf2i3] to [i2iO] is shown in fig. 2. The projected width of the faulted loops decreased as the foil was tilted toward B = [i2iO]. Fault fringes were not observed in g = (0002) images near B = [i2iO] but the associated partial dislocations were in strong contrast. This behaviour of the projected loop width together with that of the partial dislocations demonstrates unambiguously that the faults lie on the (0001) plane. 3.2. Burgers vector
Fig. 1. Faulted and perfect dislocation loops in three pass electron beam, zone-refined zirconium irradiated at 723 K to 1.3 x 1O25 neutrons/m* (>O.l MeV).
Faulted loops on (0001) of cph metals, in general, can have Burgers vectors of the type b = *i [OOOl] or
A. Jostsons et al. /Loops in neutron-iradiated zirconium
269
4X)nm
Fig. 2. Electron microscope tilt sequence illustrating the decrease in the projected width of the faulted loops in approaching B = [i2iO].All micrographs were recorded under strict two-beam conditions and the direction of g is indicated by the long arrows and described at the bottom right of each micrograph. The beam orientation B is given by the bracketed indices at bottom left. The larger fault is intersected by both top and bottom of the foil whereas the smaller loop is intersected by one foil surface only. The respective partial dislocations, a, b and c are indicated in each micrograph by the short arrows. Note the absence of fault contrast in the g = (0002) and (0004) images. The g = *(0002) images permit identification of surface hydrides picked up during electropolishing by anomalous strain contrast which changes from black to white on reversal of the sign ifg 111. Hydride contrast is minimized in g = (0004) and the partials a and c can be discerned clearly although the partial dislocation b is still masked by the overlapping contrast from artefact hydride.
270
A. Jostsons et al. /Loops in neutron-irradiatedzirconium
b = fi (2023). The former can arise from the condensation of a single layer of vacancies or interstitials with a resultant high energy fault because A-A stacking is involved. The latter is expected to have a lower fault energy because a violation of a single next-nearest neighbour stacking is involved. These lower energy faults are formed by shear according to the reaction &[OOOl] +$[lOiO]
-++[2023].
Fault contrast could arise from extended dislocations in (0001) with a fault vector R = 5 (lOi0). Such faults are unlikely in a high stacking fault energy metal like zirconium, nor are they likely to form by the condensation of irradiation-induced point defects. Moreover, the strong contrast of the bounding partials in g = (0002) in fig. 2 precludes the existence of R = 5 ClOiO> faults in these specimens. The strong fault contrast in g = (2020) in fig. 2 also suggests strongly that 53 [OOOl] is not the fault vector and thus it is most likely that b (2023) is the fault vector. Assuming that the partial dislocation has not interacted with any other dislocation during growth of the loop, it has a Burgers vector b = -R * and thus further information about the faulted loops can be obtained by observing the contrast of the partial dislocation bordeiing the fault. The contrast from faults and partial dislocations has been discussed in some detail by Head et al. [5], although much of their work is biased towards fee metals. The specific case of cph metals has been considered by a number of workers [6-81, but the most comprehensive discussion of faulted loops in cph metals was presented by Levy [9] in a study of quenched and irradiated magnesium. In order to assign a specific Burgers vector to a faulted loop, we have paid particular emphasis to the contrast of the partial dislocation defining the loop. The visibility of the partial dislocation is strongly influenced by the magnitude of g * b and, under some circumstances, by the sign of g * b. In practice, the interpretation of the contrast from partial dislocations is more favourable under conditions where the fault is out of contrast. * The negative sign arises because b is usually defined using the FS/RH convention and R is usually defied in the literature as being equal to the displacement of the crystal below the fault with respect to the top of the crystal.
Fig. 3 shows the images of two loops imaged near B= [OOOl] usingg= (1120) and (2020). First, the loops show fault contrast in all (2020) reflections. Secondly, the fault contrast is absent with g = (1120 } and the partial dislocation shows behaviour expected of g* b = 0 images in one of the three variants. The g* b = 0 images show strong “residual” contrast as a result of the large gab X u term, where u is a unit vector along the dislocation. These residual images also display invariant properties for kg which aids their identification [lo]. Thus, the strength of the contrast remains constant, for similar values of W, and the image appears as two arcs each consisting of a doublet which, for kg, remains symmetrically displaced about the dislocation core. The behaviour of the partial dislocations in { 1120 } reflections is not sufficient to assign a specific Burgers vector. As pointed out by Levy [9], the (1122) reflections near B = ( 11?33) can supply the required additional cases ofg * 6= 0. Fig. 4 shows 11122) and {lOil}images of the loops recorded near ( 1123 ) orientations. It is important to note again that strong residual contrast of the partial dislocations exists for (1122) reflections because of the strong g* b X u term. From figs. 3 and 4 it can be deduced that li = + i [2203] and b = +i [20?3] for loops 1 and 2, respectively, and the g. b values are summarized in table 1. At this stage no effort has been made to deduce the correct sign of B. Finally, it is worth noting that the partial contrast in the { 1011) images is difficult to interpret on the basis of the magnitude ofg. b alone. For example, the contrast is obviously strong for g* 6 = 2 and i as was expected, but the partial is also in strong contrast for g* b = t in some cases, e.g. for loop 1 in g = (1011) and weak in others, e.g. loops 1 and 2 in g = (Oil 1) (fig. 4). Applying the simple rule that the partial is invisible for ]g* b I< 3 would lead to problems in identification of b. 3.3. Loop nature The determination of the interstitial or vacancy character of a dislocation loop in irradiated metals is the final step in the characterization of the loop. Previously, we have established that the faulted loops in irradiated zirconium lie on (0001) and have i (2023) Burgers vectors. Consequently, these loops are not pure Frank loops but have a non-edge component as
271
Fig. 3. Contrast experiments in the vinicity of B = [OOOl]. Fault fringes are present with all g = (2020) reflections. Residual g. b = 0 andg *b X u # 0 contrast is shown by loops 1 and 2 ing = (ll?O) andg = (lzlO), respectively.
tally equivalent variants nor deviations from a single rational habit plane need be considered.
the result of the f (lOi ) shear in (0001). A number of methods have been proposed for the characterization of non-edge loops [ 1O-l 31. In the present case, the problem is considerably simplified because the fault plane is unique and, unlike the case in many other studied, neither the question of crystallographi-
Having obtained the direction of b in the previous section, its sign can be derived within the context of the FS/RH convention [ 141 by noting the image shifts in fg. For images printed with emulsion side up,
Table 1 Values of g. b for various b and g utilized in figs. 3,4 and 6 g-b 0220
2020
2200
+203]
*$
?$
k$
+,[20?3] +[0223]
+; *-43
+_43 *$
&llO]
0
+]i2io]
+2
1150
1210
2iio
1150
0
*1
*l
*l
+2 -3 *23
+-1 Al
0 21
+-1 0
2ii2
oil1
loil
ioll
0
0
*b
f-:
*-z
+-1 0
+-2
-6 +’
t;
*l
-6 +I.
*;
*i +1 -6
+-2
+2
il
+l
52
il
+-1
+2
0
51
?l
0
*2
*l
*2
kl
+l
+2
k1
*1
0
0 0
1212
0
272
A. Jostsons et al. / Loops in neutron-irradiatedzirconium
Fig. 4. The same loops as imaged in fig. 3 but now imaged near various (11231 orientations as shown. Each row of micrographs was recorded near the value of B shown at the left of the sequence. Note the strong residual contrast of foop 1 ing = +(Zik) and g = i(lZl2). Loop 2 is in residual contrast withg = t(llz?). Loop 1 is in outside contrast withg = (1122) and inside contrast in g= (1122).
outside contrast results when Cg*b)s> 0 and inside contrast results when Cg*b)s < 0, for s positive. Within thisconvention, the loop character is deduced using the relationships: n*b>O,
for interstitial loops,
n-b
for vacancy loops,
where n is the upward drawn loop normal. These rules
with some qualifications which have been discussed amply in the literature [ lo,1 1,131. Thus, for example, the orientation near B = [OOOl] where the loop is normal to the electron beam is “unsafe” since erroneous results may be obtained. On the other hand orientations such as B = (1123) are “safe”. Let us consider loop 1 in fig. 4. Outside contrast occurs withg = (1122) and thus b = [2?0?] which, apply
A. Jostsons et al. /Loops in neutron-irradiatedzirconium
213
for an upward pointing n = [OOOl] at B = [ 11231, gives the result that the loop is vacancy. A similar result is reached using the Foil and Wilkens [ 121 method. Although loop 2 is intercepted by the foil surface, it can be shown that it is in outside contrast with g = 0112) and inside contrast with g = (2fi2) in fig. 4. Consequently, b = $ [202?8] and again the loop is vacancy. The number of faulted loops characterized was small because of their low concentration but, invariably, they were found to have vacancy character.
it is in residual contrast; hence it can be shown that the inner partial is in outside contrast with g = (T2n). Consequently, the Burgers vector of the inner partial is b = i [2023] which, using our convention, is again vacancy. The sign of the outer partial cannot be deduced from the contrast behaviour of the partials in fig. 6. Nevertheless, consider the following possible reactions:
3.4. Multiple loops
a[20231 t 90223]=
In addition to the simple i (2023) faulted loops described in the previous sections, double and more complex loops were also observed as shown in fig. 5. Such multiple loops were also observed and discussed by Hales et al. [7] in a study of quenched magnesium. Here we shall analyse the contrast of an annular loop which has been intercepted by one foil surface (fig. 6). Fault fringes are seen in the outer ring of the concentric loop whereas the inner ring never exhibits fringe contrast. An analysis of the contrast behaviour in the various reflections used shows that the outer partial has b =,$, [0223] whereas the inner partial has b = fi [2023] (see table 1 for theg * b values). Since the full concentric loop cannot be seen, the outside/inside contrast cannot be deduced in a straightforward manner. Nevertheless, consider the behaviour of the par-tials in g = ?(1212) in fig. 6. The dislocation core position for the outer partial can be readily deduced since
i [TO231 •t ‘6[02?3] = + fl 1001, or
$[ii23].
If the first reaction occurred then we would expect to see fault fringes in the central ring for g = (2020) and g = (1Oi l}, whereas the second reaction leads to a fault vector which is equal to a lattice vector. The observations support the second reaction and hence the double loop configuration is due to vacancy loops on adjacent (0001) planes, as discussed in some detail by Hales et al. [7]. The dislocation configurations in fig. 5 reveal that more complex multiple faults can form than the case analysed. Many of the possible reactions have been discussed in some detail by Price [ 151 and Berghezan et al. [8]. Another simple configuration analysed was consistent with the collapse and shear of a vacancy annulus on one plane, in which case the two partials are parallel. 3.5. Perfect loops In addition to the faulted t(2023) loops, these specimens contained the usual 4 (1120) perfect loops. Two such loops are labelled 1 and 2 in fig. 6. Their Burgers vectors are f$ [2 1 lo] and +; fl2iO], respectively, as can be deduced with the aid of table 1. Following the rules laid down elsewhere [ 1] for the characterization of these loops, loop 1 is interstitial whereas loop 2 is vacancy. Analyses of other perfect loops confirmed the existence of a population of mixed vacancy and interstitial types.
4. Discussion Fig. 5. Complex multiple loops in zirconium neutron-irradiated at 723 K and imaged withg = (2020) near B = [OOOl].
Faulted loop images in neutron-irradiated zirconium have been analysed rigorously, and it has been estab-
A. Jostsons et al. /Loops in neutron-irradiatedzirconium
Fig. 6. Contrast sequence of an annular double loop intersected by one foil surface and two perfect l/3 (1120) loops marked 1 and 2 in the g = (1120) image. Note the presence of fault fringes in the outer annulus of the double loop in g = {20?0} and g = {lOi 1) images. The partial dislocation shows strong residual contrast in both g = {I 1%?I and g = {I 152) images because of the l~geg*~X~termwheng.~=O.
A. Jostsons et al. /Loops in neutron-irradiatedzirconium
lished that the loops lie on (0001) and their Burgers vector is of the type b = 2 (2023). The loop nature has been demonstrated to be vacancy and, despite the analysis of over 20 loops, no faulted interstitial loops were observed. The Burgers vector analysis and loop nature determination were based essentially on images for which g . b = 0, 1 and 2, for which well-established visibility criteria exist in the literature. Loop images recorded under non-integer values of g * b were not relied upon since there are no theoretical calculations for the specific case of i (2023) faults in cph metals. Almost exclusively, all theoretical analyses of images of partial dislocations bordering stacking faults are based on the fee case where g * b can take the values of 0, *f,*$ +l, +$, etc. In cph metals, faults with b = $2023) can give rise to different non-integer values ofg. b; for example w-ithg= {IOil} g. b can take values of f$, f$, +z. It is sometimes assumed that the visibility of the partial dislocation is influenced largely by the magnitude of g - b and hence, from the fee case, it could be argued that partials withg- 6 = 5: would be invisible. Such intuitive arguments are obviously false as shown by the visibility of partials with g - b = fi in figs. 4 and 6. Preliminary calculations using the computer programs for theoretical image simulation developed by Head, Humble and coworkers [5] have shown that there are situations where the partial dislocation is expected to be visible when g. b = +b. A more detailed account of the theoretical images will be published elsewhere. Nevertheless, it is emphasize,d that the analysis performed in this study is valid and based on well-established rules for dislocation contrast under integer values of g * b. The occurrence of faulted loops in neutron-irradiated zirconium is a comparatively rare phenomenon. Thus, in the Ave batches of high-purity Zr irradiated in this experiment, perfect 4 (1 120) loops represent the sole form of loops for irradiation temperatures up to and including 673 K. Faulted loops were only observed in specimens from two of the five batches irradiated at 723 K. Even in these specimens, the faulted 6 (2023) loops coexisted with perfect 4 (1120) loops. Furthermore, faulted loops were not present in specimens made from zone-refined zirconium and irradiated at Lucas Heights at 723 K. These observations suggest that both the irradiation temperature and specimen purity may influence the loop habit plane and its Burgers vector.
275
It is possible that faulted loops on (0001) may become energetically favoured with respect to perfect 4 (1120) loops near the prism planes as the result of changes in dislocation stability resulting from changes in elastic constants of Zr with temperature. No such trends are discernible from the measured elastic constants of Zr [ 161. An effect of specimen purity has been noted in quenched magnesium by Hillairet et al. [ 17 1. In high purity Mg (1 ppm impurities), perfect vacancy loops with b = 5 (1 120) on prism planes were the sole form of vacancy clusters, whereas in Mg with 30 ppm impurities, Hillairet et al. noted the presence of faulted 5 (2023) loops on (0001) in addition to the perfect loops on prism planes. This behaviour of loop nature in quenched magnesium is somewhat analogous to the present case for irradiated zirconium. Nevertheless, mass-spectrographic analyses of specimens from the various batches of zirconium used in this study did not indicate significant differences in substitutional element levels. No results are available concerning the level of interstitial impurities, but attempts are being made to determine these; the detailed analytical results will be published later together with quantitative measurements of loop dependence on irradiation temperature and specimen purity. The mechanism by which impurities influence the loop morphology is not understood. Nevertheless, in addition to the case of quench loops in Mg, evidence is now appearing in the literature that solute segregation can occur at point defect agglomerates. Thus, Okamoto and Wiedersich [ 181 have discussed the segregation of alloying elements to voids and free surfaces during irradiation, and Yoshida et al. [ 193 have observed enrichment of Cu near stacking fault tetrahedra in quenched o-brass. It is also interesting that our previous work on irradiated Zr [ 1 ] has shown that for f (1120 ) loops, the vacancy loops are markedly elliptical whereas the interstitial loops are almost circular. If the faulted 2 (2023) loops and the elliptical $ (1120) vacancy loops represent the departure from the norm, solute segregation to vacancies may be responsible for both phenomena and irradiation temperature may be the factor which determined whether condensation of vacancies takes place on (0001) or near prism planes.
216
A. Jostsons et al. /Loops in neutron-irradiated zirconium
Acknowledgement The authors gratefully acknowledge Oak Ridge National Laboratory for the supply of the irradiated zirconium specimens which were produced under a project sponsored by the US Energy Research Development Administration.
Note added in proof Subsequent to the submission of this paper, specimens irradiated at 773 K and 823 K have been examined. The faulted loops are still present at 773 K in one of the batches of zirconium. No evidence of displacement damage was observed in any of the zirconium specimens after the 823 K irradiation.
References [1] A. Jostsons, P.M. Kelly and R.G. Blake, J. Nucl. Mater., 66 (1977) 236. [2] D.O. Northwood and R.W. Gilbert, J. Nucl. Mater. 41 (1974) 271. [ 31 R.G. Blake, A. Jostsons and P.M. Kelly, 8th Int. Congress on Electron Microscopy, Canberra, Austr. Acad. of Science, Vol. 1 (1974) p. 610.
[4] P.M. Kelly, R.G. Blake and A. Jostsons, AAEC Report E404 (1977). [S] A.K. Head, P. Humble, L.M. Clareborough, A.J. Morton and C.T. Forwood, Computed Electron Micrographs and Defect Identification (North-Holland, Amsterdam, 1973) p. 39. [6] J.E. Harris and B.C. Masters, Proc. Roy. Sot. A292 (1966) 240. [ 71 R. Hales, R.E. Smallman and P.S. Dobson, Proc. Roy. Sot. A307 (1968) 71. [ 81 A. Berghezan, A. Fourdeux and S. Amelinckx, Acta Met. 9 (1961) 464. [9] V. Levy, J. de Microsc. 19 (1974) 1. [lo] D.M. Maher and B.L. Eyre, Phil. Haag.23 (1971) 409. [ll] P.M. Kelly and R.G. Blake, Phil. Mag. 28 (1973) 415. [ 121 H. Flill and M. Wilken, Phys. Status SoIidi A31 (1975) 519. [ 131 G.J.C. Carpenter, Phys. Status Solidi A37 (1976) K61. [ 141 M.H. Loretto and R.E. SmalIman, Defect Analysis in Electron Microscopy (Chapman & Hall, London, 1975) p. 17. [ 151 P.B. Price, Electron Microscopy and Strength of Crystals, G. Thomas and J. Washburn, eds. (Interscience, New York, 1963) p. 41. [ 161 E.S. Fischer and L.C.R. Alfred, Trans. Met. Sot. AIME 242 (1968) 1275. [17] J. Hillairet, C. Mairy, J. Espinasse and V. Levy, Acta Met. 18 (1970) 1285. [ 181 P.R. Okamoto and H. Wiedersich, J. Nucl. Mater. 53 (1974) 336. [ 191 S. Yoshida, 0. Haruyama, H. Yamaguchi and I. Hashimoto, Jap. J. Appl. Phys. 16 (1977) 5.
FAULTED LOOPS IN NEUTRON-IRRADIATED ZIRCONIUM A. JOSTSONS, R.G. BLAKE, J.G. NAPIER, P.M. KELLY and K. FARRELL Materials Division, AAEC Research
Establishment,
*
Lucas Heights, NSW, Australia
Received 9 June 1977
Faulted loops have been observed in high-purity zirconium irradiated at 723 K to 1.3 X 102’ neutrons/m2 (>O.i MeV). The transmission electron microscopy characterization of these l/6 (2023) faulted loops on (0001) is described in detail. It was found that the faulted loops were invariably vacancy in character although the coexisting population of perfect l/3 (11%) loops was of a mixed interstitial/vacancy nature. The faulted loops were observed in specimens of only two out of five batches of high-purity zirconium irradiated in this experiment. Even in these two specimens, the presence of faulted loops was restricted to the 723 K irradiation temperature; at 673 K only perfect l/3 (ll?O) loops were seen. Des boucles fautdes ont dte observdes dam du zirconium de haute pure& irradid a 723 K a des doses de 1,3 x 1O25 neutrons/m’ (>O,l MeV). La caractdrisation par microscopic electronique par transmission de ces boucles fautees l/6 (2023) sur les plans (0001) est d&rite en detail. I1 a etd observe que les boucles fautdes sont toujours du type lacune bien que la population coexistante des boucles parfaites l/3 (lI?O) dtait de type mixte interstitiel/lacune. Les boucles fauties ont dtd observies dans des echantillons appartenent a seulement deux sur cinq des fournitures de zirconium de haute purete irradies dans cette se’rie d’experience. Mime dans ces deux echantillons, la presence de boucles fautees etait limitee i la temperature d’irradiation de 723 K. A 673 K seules des boucles parfaites l/3 (lIZOr ont 6th observees. In hochreinem Zirconium, das bei 723 K mit Neutronen bis zu 1,3 . 1O25 rnF2 (E > 0,l MeV) bestrahlt worden war, wurden Versetzungsringe mit Stapelfehlern beobachtet. Die transmissionselektronenmikroskopische Charakterisierung dieser Versetzungsringe mit l/6 (2023) in (0001) wird eingehend beschrieben: Sie sind ausnahmslos vom Leerstellentyp, obwohl die koexistierende Population der fehlerlosen Versetzungsringe mit l/3 ( 1l?O) vom gemischten ZwischengitterLeerstellen-Typ ist. Die Versetzungsringe mit Stapelfehlern wurden in Proben aus nur zwei von insgesamt ftinf hochreinen Zr-Chargen, die in diesem Versuch bestrahlt worden waren, beobachtet. Auch in diesen zwei Proben war die Gegenwart von Versetzungsringen mit Stapelfehlern auf die Bestrahlung bei 723 K bt?schrCnkt, bei 673 K wurden nur fehlerlose Versetzungsringe mit l/3 ( 1120) festgestellt.
1. Introduction The nature of dislocation loops in neutron-irradiated zirconium has been investigated in considerable detail [l-4]. From an examination of zone-refined zirconi‘urn specimens irradiated in the temperature range 573 to 708 K to fast neutron fluences of 1.8 X 10M neutrons/ m2 (>l MeV), we concluded [l] that both vacancy and interstitial loops with perfect a/3 (1 120) Burgers vectors represent the sole form of irradiation-induced
* Metals & Ceramics Division, ORNL, Tennessee, USA 267
defect clustering. These loops do not possess the simple edge geometry since the normals are generally rotated towards one of the neighbouring ( IOiO) directions and, to a lesser degree, towards [OOOl]. This work reports the characterization of faulted loops which were observed in certain specimens of high-purity zirconium irradiated at 723 K in the Oak Ridge Research Reactor (ORR) to 1.3 X 102’ neutrons/m2 (x.1 MeV). Faulted loops were not observed in specimens of the same starting material irradiated at 673 K and below. Instead, the damage structure in these lower-temperature irradiations consisted entirely of the usual perfect a/3 (1120) loops.
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A. Jostsons et al. /Loops
2. Experimental details The zirconium specimens studied form part of an experiment (ORR-228) to investigate the effects of specimen purity and irradiation temperature on the nature of the irradiation-induced damage structure. The zirconium was obtained from various sources and, with the exception of one lot of crystal bar material, was subsequently zone-refined in an electron-beam furnace at Oak Ridge National Laboratory (ORNL) for 2 to 20 passes. Specimens in the form of 0.5 mm thick X3.2 mm dia. discs were then annealed for &h at 983 K in a vacuum of 2 X 10M6 torr. The zirconium discs were stacked in stainless steel cans, evacuated and back-filled with helium before the cans were welded shut. The irradiations were carried out over two cycles of the Oak Ridge Research Reactor (ORR). The specimen fluences were derived from a map of measurements made on 14 dosimeter packages distributed throughout the experimental assembly. The data were consistent and it is estimated that the fluences are accurate to rtlO%. Temperature control was achieved by a combination of y-heating and wrap-around electric heaters balanced against a water-cooled aluminium heat sink. Two thermocouples were used to measure the temperature of each capsule and temperatures of 25°C around the nominal value were achieved. The electron microscopy was performed at the
in neutron-irradiated
zirconium
AAEC Research Establishment, Lucas Heights, using a JSEM-200 fitted with a 245” double-tilt goniometer stage modified to give k50° tilt about one axis. Specimens for electron microscopy were obtained by polishing on 600 mesh Sic paper to 250 pm thickness followed by electropolishing at 18 V and 223 K in a commercial automatic twin-jet unit. The electrolyte contained 6% perchloric acid and 32% butoxy ethanol in methanol. All electron micrographs have been printed with the emulsion side up and B, the beam direction, is defined to be the upward drawn normal to the diffraction pattern, i.e. it is anti-parallel to the direction of electron flow. Consistent indexing of zones nad reflections was obtained from Kikuchi patterns using the rules set out by Head et al. [5].
3. Observations A typical area of a foil of irradiated zirconium containing faulted loops is shown in fig. 1. The faulted loop concentration is low (-1018/m3), the loops are large and frequently intersect either one or both surfaces of the foil. In the following sections, the procedures used to determine the loop normal, its Burgers vector and interstitial or vacancy character will be described in some detail. 3.1. Loop normal Preliminary observations revealed that fault contrast was strong in reflections of the (2020) and { 1011) type. The behaviour of a number of loops imaged with g = (2020) whilst the foil was tilted from the vicinity of B = lf2i3] to [i2iO] is shown in fig. 2. The projected width of the faulted loops decreased as the foil was tilted toward B = [i2iO]. Fault fringes were not observed in g = (0002) images near B = [i2iO] but the associated partial dislocations were in strong contrast. This behaviour of the projected loop width together with that of the partial dislocations demonstrates unambiguously that the faults lie on the (0001) plane. 3.2. Burgers vector
Fig. 1. Faulted and perfect dislocation loops in three pass electron beam, zone-refined zirconium irradiated at 723 K to 1.3 x 1O25 neutrons/m* (>O.l MeV).
Faulted loops on (0001) of cph metals, in general, can have Burgers vectors of the type b = *i [OOOl] or
A. Jostsons et al. /Loops in neutron-iradiated zirconium
269
4X)nm
Fig. 2. Electron microscope tilt sequence illustrating the decrease in the projected width of the faulted loops in approaching B = [i2iO].All micrographs were recorded under strict two-beam conditions and the direction of g is indicated by the long arrows and described at the bottom right of each micrograph. The beam orientation B is given by the bracketed indices at bottom left. The larger fault is intersected by both top and bottom of the foil whereas the smaller loop is intersected by one foil surface only. The respective partial dislocations, a, b and c are indicated in each micrograph by the short arrows. Note the absence of fault contrast in the g = (0002) and (0004) images. The g = *(0002) images permit identification of surface hydrides picked up during electropolishing by anomalous strain contrast which changes from black to white on reversal of the sign ifg 111. Hydride contrast is minimized in g = (0004) and the partials a and c can be discerned clearly although the partial dislocation b is still masked by the overlapping contrast from artefact hydride.
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A. Jostsons et al. /Loops in neutron-irradiatedzirconium
b = fi (2023). The former can arise from the condensation of a single layer of vacancies or interstitials with a resultant high energy fault because A-A stacking is involved. The latter is expected to have a lower fault energy because a violation of a single next-nearest neighbour stacking is involved. These lower energy faults are formed by shear according to the reaction &[OOOl] +$[lOiO]
-++[2023].
Fault contrast could arise from extended dislocations in (0001) with a fault vector R = 5 (lOi0). Such faults are unlikely in a high stacking fault energy metal like zirconium, nor are they likely to form by the condensation of irradiation-induced point defects. Moreover, the strong contrast of the bounding partials in g = (0002) in fig. 2 precludes the existence of R = 5 ClOiO> faults in these specimens. The strong fault contrast in g = (2020) in fig. 2 also suggests strongly that 53 [OOOl] is not the fault vector and thus it is most likely that b (2023) is the fault vector. Assuming that the partial dislocation has not interacted with any other dislocation during growth of the loop, it has a Burgers vector b = -R * and thus further information about the faulted loops can be obtained by observing the contrast of the partial dislocation bordeiing the fault. The contrast from faults and partial dislocations has been discussed in some detail by Head et al. [5], although much of their work is biased towards fee metals. The specific case of cph metals has been considered by a number of workers [6-81, but the most comprehensive discussion of faulted loops in cph metals was presented by Levy [9] in a study of quenched and irradiated magnesium. In order to assign a specific Burgers vector to a faulted loop, we have paid particular emphasis to the contrast of the partial dislocation defining the loop. The visibility of the partial dislocation is strongly influenced by the magnitude of g * b and, under some circumstances, by the sign of g * b. In practice, the interpretation of the contrast from partial dislocations is more favourable under conditions where the fault is out of contrast. * The negative sign arises because b is usually defined using the FS/RH convention and R is usually defied in the literature as being equal to the displacement of the crystal below the fault with respect to the top of the crystal.
Fig. 3 shows the images of two loops imaged near B= [OOOl] usingg= (1120) and (2020). First, the loops show fault contrast in all (2020) reflections. Secondly, the fault contrast is absent with g = (1120 } and the partial dislocation shows behaviour expected of g* b = 0 images in one of the three variants. The g* b = 0 images show strong “residual” contrast as a result of the large gab X u term, where u is a unit vector along the dislocation. These residual images also display invariant properties for kg which aids their identification [lo]. Thus, the strength of the contrast remains constant, for similar values of W, and the image appears as two arcs each consisting of a doublet which, for kg, remains symmetrically displaced about the dislocation core. The behaviour of the partial dislocations in { 1120 } reflections is not sufficient to assign a specific Burgers vector. As pointed out by Levy [9], the (1122) reflections near B = ( 11?33) can supply the required additional cases ofg * 6= 0. Fig. 4 shows 11122) and {lOil}images of the loops recorded near ( 1123 ) orientations. It is important to note again that strong residual contrast of the partial dislocations exists for (1122) reflections because of the strong g* b X u term. From figs. 3 and 4 it can be deduced that li = + i [2203] and b = +i [20?3] for loops 1 and 2, respectively, and the g. b values are summarized in table 1. At this stage no effort has been made to deduce the correct sign of B. Finally, it is worth noting that the partial contrast in the { 1011) images is difficult to interpret on the basis of the magnitude ofg. b alone. For example, the contrast is obviously strong for g* 6 = 2 and i as was expected, but the partial is also in strong contrast for g* b = t in some cases, e.g. for loop 1 in g = (1011) and weak in others, e.g. loops 1 and 2 in g = (Oil 1) (fig. 4). Applying the simple rule that the partial is invisible for ]g* b I< 3 would lead to problems in identification of b. 3.3. Loop nature The determination of the interstitial or vacancy character of a dislocation loop in irradiated metals is the final step in the characterization of the loop. Previously, we have established that the faulted loops in irradiated zirconium lie on (0001) and have i (2023) Burgers vectors. Consequently, these loops are not pure Frank loops but have a non-edge component as
271
Fig. 3. Contrast experiments in the vinicity of B = [OOOl]. Fault fringes are present with all g = (2020) reflections. Residual g. b = 0 andg *b X u # 0 contrast is shown by loops 1 and 2 ing = (ll?O) andg = (lzlO), respectively.
tally equivalent variants nor deviations from a single rational habit plane need be considered.
the result of the f (lOi ) shear in (0001). A number of methods have been proposed for the characterization of non-edge loops [ 1O-l 31. In the present case, the problem is considerably simplified because the fault plane is unique and, unlike the case in many other studied, neither the question of crystallographi-
Having obtained the direction of b in the previous section, its sign can be derived within the context of the FS/RH convention [ 141 by noting the image shifts in fg. For images printed with emulsion side up,
Table 1 Values of g. b for various b and g utilized in figs. 3,4 and 6 g-b 0220
2020
2200
+203]
*$
?$
k$
+,[20?3] +[0223]
+; *-43
+_43 *$
&llO]
0
+]i2io]
+2
1150
1210
2iio
1150
0
*1
*l
*l
+2 -3 *23
+-1 Al
0 21
+-1 0
2ii2
oil1
loil
ioll
0
0
*b
f-:
*-z
+-1 0
+-2
-6 +’
t;
*l
-6 +I.
*;
*i +1 -6
+-2
+2
il
+l
52
il
+-1
+2
0
51
?l
0
*2
*l
*2
kl
+l
+2
k1
*1
0
0 0
1212
0
272
A. Jostsons et al. / Loops in neutron-irradiatedzirconium
Fig. 4. The same loops as imaged in fig. 3 but now imaged near various (11231 orientations as shown. Each row of micrographs was recorded near the value of B shown at the left of the sequence. Note the strong residual contrast of foop 1 ing = +(Zik) and g = i(lZl2). Loop 2 is in residual contrast withg = t(llz?). Loop 1 is in outside contrast withg = (1122) and inside contrast in g= (1122).
outside contrast results when Cg*b)s> 0 and inside contrast results when Cg*b)s < 0, for s positive. Within thisconvention, the loop character is deduced using the relationships: n*b>O,
for interstitial loops,
n-b
for vacancy loops,
where n is the upward drawn loop normal. These rules
with some qualifications which have been discussed amply in the literature [ lo,1 1,131. Thus, for example, the orientation near B = [OOOl] where the loop is normal to the electron beam is “unsafe” since erroneous results may be obtained. On the other hand orientations such as B = (1123) are “safe”. Let us consider loop 1 in fig. 4. Outside contrast occurs withg = (1122) and thus b = [2?0?] which, apply
A. Jostsons et al. /Loops in neutron-irradiatedzirconium
213
for an upward pointing n = [OOOl] at B = [ 11231, gives the result that the loop is vacancy. A similar result is reached using the Foil and Wilkens [ 121 method. Although loop 2 is intercepted by the foil surface, it can be shown that it is in outside contrast with g = 0112) and inside contrast with g = (2fi2) in fig. 4. Consequently, b = $ [202?8] and again the loop is vacancy. The number of faulted loops characterized was small because of their low concentration but, invariably, they were found to have vacancy character.
it is in residual contrast; hence it can be shown that the inner partial is in outside contrast with g = (T2n). Consequently, the Burgers vector of the inner partial is b = i [2023] which, using our convention, is again vacancy. The sign of the outer partial cannot be deduced from the contrast behaviour of the partials in fig. 6. Nevertheless, consider the following possible reactions:
3.4. Multiple loops
a[20231 t 90223]=
In addition to the simple i (2023) faulted loops described in the previous sections, double and more complex loops were also observed as shown in fig. 5. Such multiple loops were also observed and discussed by Hales et al. [7] in a study of quenched magnesium. Here we shall analyse the contrast of an annular loop which has been intercepted by one foil surface (fig. 6). Fault fringes are seen in the outer ring of the concentric loop whereas the inner ring never exhibits fringe contrast. An analysis of the contrast behaviour in the various reflections used shows that the outer partial has b =,$, [0223] whereas the inner partial has b = fi [2023] (see table 1 for theg * b values). Since the full concentric loop cannot be seen, the outside/inside contrast cannot be deduced in a straightforward manner. Nevertheless, consider the behaviour of the par-tials in g = ?(1212) in fig. 6. The dislocation core position for the outer partial can be readily deduced since
i [TO231 •t ‘6[02?3] = + fl 1001, or
$[ii23].
If the first reaction occurred then we would expect to see fault fringes in the central ring for g = (2020) and g = (1Oi l}, whereas the second reaction leads to a fault vector which is equal to a lattice vector. The observations support the second reaction and hence the double loop configuration is due to vacancy loops on adjacent (0001) planes, as discussed in some detail by Hales et al. [7]. The dislocation configurations in fig. 5 reveal that more complex multiple faults can form than the case analysed. Many of the possible reactions have been discussed in some detail by Price [ 151 and Berghezan et al. [8]. Another simple configuration analysed was consistent with the collapse and shear of a vacancy annulus on one plane, in which case the two partials are parallel. 3.5. Perfect loops In addition to the faulted t(2023) loops, these specimens contained the usual 4 (1120) perfect loops. Two such loops are labelled 1 and 2 in fig. 6. Their Burgers vectors are f$ [2 1 lo] and +; fl2iO], respectively, as can be deduced with the aid of table 1. Following the rules laid down elsewhere [ 1] for the characterization of these loops, loop 1 is interstitial whereas loop 2 is vacancy. Analyses of other perfect loops confirmed the existence of a population of mixed vacancy and interstitial types.
4. Discussion Fig. 5. Complex multiple loops in zirconium neutron-irradiated at 723 K and imaged withg = (2020) near B = [OOOl].
Faulted loop images in neutron-irradiated zirconium have been analysed rigorously, and it has been estab-
A. Jostsons et al. /Loops in neutron-irradiatedzirconium
Fig. 6. Contrast sequence of an annular double loop intersected by one foil surface and two perfect l/3 (1120) loops marked 1 and 2 in the g = (1120) image. Note the presence of fault fringes in the outer annulus of the double loop in g = {20?0} and g = {lOi 1) images. The partial dislocation shows strong residual contrast in both g = {I 1%?I and g = {I 152) images because of the l~geg*~X~termwheng.~=O.
A. Jostsons et al. /Loops in neutron-irradiatedzirconium
lished that the loops lie on (0001) and their Burgers vector is of the type b = 2 (2023). The loop nature has been demonstrated to be vacancy and, despite the analysis of over 20 loops, no faulted interstitial loops were observed. The Burgers vector analysis and loop nature determination were based essentially on images for which g . b = 0, 1 and 2, for which well-established visibility criteria exist in the literature. Loop images recorded under non-integer values of g * b were not relied upon since there are no theoretical calculations for the specific case of i (2023) faults in cph metals. Almost exclusively, all theoretical analyses of images of partial dislocations bordering stacking faults are based on the fee case where g * b can take the values of 0, *f,*$ +l, +$, etc. In cph metals, faults with b = $2023) can give rise to different non-integer values ofg. b; for example w-ithg= {IOil} g. b can take values of f$, f$, +z. It is sometimes assumed that the visibility of the partial dislocation is influenced largely by the magnitude of g - b and hence, from the fee case, it could be argued that partials withg- 6 = 5: would be invisible. Such intuitive arguments are obviously false as shown by the visibility of partials with g - b = fi in figs. 4 and 6. Preliminary calculations using the computer programs for theoretical image simulation developed by Head, Humble and coworkers [5] have shown that there are situations where the partial dislocation is expected to be visible when g. b = +b. A more detailed account of the theoretical images will be published elsewhere. Nevertheless, it is emphasize,d that the analysis performed in this study is valid and based on well-established rules for dislocation contrast under integer values of g * b. The occurrence of faulted loops in neutron-irradiated zirconium is a comparatively rare phenomenon. Thus, in the Ave batches of high-purity Zr irradiated in this experiment, perfect 4 (1 120) loops represent the sole form of loops for irradiation temperatures up to and including 673 K. Faulted loops were only observed in specimens from two of the five batches irradiated at 723 K. Even in these specimens, the faulted 6 (2023) loops coexisted with perfect 4 (1120) loops. Furthermore, faulted loops were not present in specimens made from zone-refined zirconium and irradiated at Lucas Heights at 723 K. These observations suggest that both the irradiation temperature and specimen purity may influence the loop habit plane and its Burgers vector.
275
It is possible that faulted loops on (0001) may become energetically favoured with respect to perfect 4 (1120) loops near the prism planes as the result of changes in dislocation stability resulting from changes in elastic constants of Zr with temperature. No such trends are discernible from the measured elastic constants of Zr [ 161. An effect of specimen purity has been noted in quenched magnesium by Hillairet et al. [ 17 1. In high purity Mg (1 ppm impurities), perfect vacancy loops with b = 5 (1 120) on prism planes were the sole form of vacancy clusters, whereas in Mg with 30 ppm impurities, Hillairet et al. noted the presence of faulted 5 (2023) loops on (0001) in addition to the perfect loops on prism planes. This behaviour of loop nature in quenched magnesium is somewhat analogous to the present case for irradiated zirconium. Nevertheless, mass-spectrographic analyses of specimens from the various batches of zirconium used in this study did not indicate significant differences in substitutional element levels. No results are available concerning the level of interstitial impurities, but attempts are being made to determine these; the detailed analytical results will be published later together with quantitative measurements of loop dependence on irradiation temperature and specimen purity. The mechanism by which impurities influence the loop morphology is not understood. Nevertheless, in addition to the case of quench loops in Mg, evidence is now appearing in the literature that solute segregation can occur at point defect agglomerates. Thus, Okamoto and Wiedersich [ 181 have discussed the segregation of alloying elements to voids and free surfaces during irradiation, and Yoshida et al. [ 193 have observed enrichment of Cu near stacking fault tetrahedra in quenched o-brass. It is also interesting that our previous work on irradiated Zr [ 1 ] has shown that for f (1120 ) loops, the vacancy loops are markedly elliptical whereas the interstitial loops are almost circular. If the faulted 2 (2023) loops and the elliptical $ (1120) vacancy loops represent the departure from the norm, solute segregation to vacancies may be responsible for both phenomena and irradiation temperature may be the factor which determined whether condensation of vacancies takes place on (0001) or near prism planes.
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A. Jostsons et al. /Loops in neutron-irradiated zirconium
Acknowledgement The authors gratefully acknowledge Oak Ridge National Laboratory for the supply of the irradiated zirconium specimens which were produced under a project sponsored by the US Energy Research Development Administration.
Note added in proof Subsequent to the submission of this paper, specimens irradiated at 773 K and 823 K have been examined. The faulted loops are still present at 773 K in one of the batches of zirconium. No evidence of displacement damage was observed in any of the zirconium specimens after the 823 K irradiation.
References [1] A. Jostsons, P.M. Kelly and R.G. Blake, J. Nucl. Mater., 66 (1977) 236. [2] D.O. Northwood and R.W. Gilbert, J. Nucl. Mater. 41 (1974) 271. [ 31 R.G. Blake, A. Jostsons and P.M. Kelly, 8th Int. Congress on Electron Microscopy, Canberra, Austr. Acad. of Science, Vol. 1 (1974) p. 610.
[4] P.M. Kelly, R.G. Blake and A. Jostsons, AAEC Report E404 (1977). [S] A.K. Head, P. Humble, L.M. Clareborough, A.J. Morton and C.T. Forwood, Computed Electron Micrographs and Defect Identification (North-Holland, Amsterdam, 1973) p. 39. [6] J.E. Harris and B.C. Masters, Proc. Roy. Sot. A292 (1966) 240. [ 71 R. Hales, R.E. Smallman and P.S. Dobson, Proc. Roy. Sot. A307 (1968) 71. [ 81 A. Berghezan, A. Fourdeux and S. Amelinckx, Acta Met. 9 (1961) 464. [9] V. Levy, J. de Microsc. 19 (1974) 1. [lo] D.M. Maher and B.L. Eyre, Phil. Haag.23 (1971) 409. [ll] P.M. Kelly and R.G. Blake, Phil. Mag. 28 (1973) 415. [ 121 H. Flill and M. Wilken, Phys. Status SoIidi A31 (1975) 519. [ 131 G.J.C. Carpenter, Phys. Status Solidi A37 (1976) K61. [ 141 M.H. Loretto and R.E. SmalIman, Defect Analysis in Electron Microscopy (Chapman & Hall, London, 1975) p. 17. [ 151 P.B. Price, Electron Microscopy and Strength of Crystals, G. Thomas and J. Washburn, eds. (Interscience, New York, 1963) p. 41. [ 161 E.S. Fischer and L.C.R. Alfred, Trans. Met. Sot. AIME 242 (1968) 1275. [17] J. Hillairet, C. Mairy, J. Espinasse and V. Levy, Acta Met. 18 (1970) 1285. [ 181 P.R. Okamoto and H. Wiedersich, J. Nucl. Mater. 53 (1974) 336. [ 191 S. Yoshida, 0. Haruyama, H. Yamaguchi and I. Hashimoto, Jap. J. Appl. Phys. 16 (1977) 5.