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Comments on precipitate stability in neutron-irradiated Zircaloy-4
294
Received 3 April 1989: accepted 23 October 1989
Work on amorphous transformations and depletion of Fe. Cr or Ni from intermetallic particles in Zircaloy-2 and Zircaloy-4 during neutron irradiatjo~ was first published by Gilbert, Griffiths and Carpenter in 1985 [l]. At irradiation temperatures of about 570 K, it was reported that there was preferential depletion of Fe From Zr(Cr. Fe), intermetallic precipitates which corresponded with a concurrent crystalIine-to-arno~h~~us phase transformation. The depth of the Fe depletion (and the thickness of the amorphous layer) increased with increasing fluence. Zrz(Ni. Fe) particles did not show the same Fe depletion and amorphous phase transformation. It was also reported that hoth and Zr,(Ni, Fe) precipitates became Zr(Cr, Fe), amorphous (without Fe depletion) at low irradiation temperatures (about 350 K) and remained crystalline at high irradiation temperatures (about 673 K). These results were presented at a workshop in Eriangen that same year along with additional data on the phase stability and redistribution of the precipitates following irradiation at temperatures between 330 and 873 K [Z]. The effects of post”irradiation annealing for one hour at temperatures between 673-873 K were also reported. For Zircaloy-4 irradiated at about 560 K, it was shown that the Zr(Cr, Fe)z precipitates became amo~h~~us and that this was coincident with the depletion of Fe (figs. 1 and 2 [Z]). The amorphous phase recrystallised and new precipitates (containing Fe and Cr) were visible at grain boundaries and in the matrix following post-irradiation annealing for one hour at temperatures 2 773 K but not at 673 K. This was clearly illustrated after annealing at 873 K because of precipitate coarsening and the increased recovery of the radiation damage microstructure (figs. 3 and 4 121). These new precipitates were the result of the removal and redistribution of Fe and Cr from the parent intermetallic particle. In the proceedings of the Erlangen workshop, Yang et al. 131 also reported evidence of Zr(Fe, Cr), precipitates in Zircaloy-4 starting to become amorphous after irradia-
tion at 2M0c to fluenues of ahout 1 x 10” n/cm’. They reported that the Fe was depleted from the amorphous region of the precipitates and no amorphisation was observed after irradiation at 375°C. Similar data were again reported elsewhere [4]. It has been reported by Griffiths et al. (51 that for irradiation temperatures between about 350-700 K there is radiation-induced dissolution of the interm~talli~ particles in Zircaloy-2 and -4. The dissolution is apparent in the sense that there is a redistribution of material but it is not a thermal equilibrium process. It was concluded [S] that the redistribution occurred both as a result & sputtering (where atoms at the periphery of the precipitates are displaced into the matrix by energetic neutrons) and diffusion (especially at temperatures 2 550 K). The diffusion could be either substitutional (m the opposite direction to a vacancy flux) or interstitial (because of the creation of interstitial atoms by atomic displacements). In addition to a preferential depietion of Fe and concurrent crystalline-to-amorphous phase transformation. described previously, it was shown that Fe and Cr diffused away from ZrfCr, Fe), int~rmetalli~ particles in Zircaloy-4 preferentially along directions parallel to the basal plane during irradiation at temperatures between 5.50 and 600 K. fig. 5 [5]. The directionality of the migration away from the particles was attributed to some form of two dimensional interstitial diffus~~~~, presumably due to a coupling between a flux of Zr self-interstitial atoms with Fe and C‘r in the elements and the matrix. The depletion of “solute” amorphous transformation could not be reproduced h> thermal treatments alone and a mechanism was proposed [S] explaining the dissolution process in terms of interstitial point defect nligrati~)n. Preferential dispiatcment of substitutional Fe (and Cr) within the Zr(Cr. Fe), precipitates by neutrons and Zr self-mterstitial atoms (a mechanism proposed by Hood for enhanced recombination of point defects by Fe /6]) wah believed to be an important factor in determining the
M. GriJfirhs / Precipltare
295
srahiliry m Zircaloy-4
EnergyCevl
Energy [evl
8.5 x 10*%/m* Fig. 1. Crystalline-to-amorphous transformation of Zr(Cr. Fe), particles in Zircaloy-4 irradiated at 560 K. The amorphous layer is coincident with a depletion of Fe and Cr. The width is proportional to the neutron fluence and is approximately 10 nm per lo*’ n/m* (E > 1 MeV). From ref. [2].
diffusion of Fe (and Cr) out of the intermetallic partitles and, as a result, the stability of the amorphous phase. It was further concluded that the amorphous transformation occurred in the Fe-depleted region at the periphery of the precipitates because of the resultant hypostoichiometry in these regions [5]. The amorphous
phase stability during post-irradiation annealing was dependent on the Cr relative to Fe content. It was proposed that, during irradiation, diffusion of Fe (and Cr) out of the particles occurred because of a lower chemical potential for interstitial Fe (and Cr) in the matrix compared with the particle. The magnitude of
796
L
profile for a Zr(Cr, Fe),
Fe
.-__
C,
AMo*pHg,,
CRISTALLINE
T*:“l’,,
__.___-.___i
Fig. 2. Composition
-
..-
.A_
100
50
Distance
particle
200
from Centfe
irradiated
the effect was always larger for Fe and it was concluded that this was because the Fe was the faster (interstitial) diffuser [6]. A recent paper by Yang [7] described the crystalline-
ot Pattictelnm
at 560 K to a fluence of 3.5
x
10”
n/m’
(E > 1 MtV). From ref. [2].
to-amorphous transformation, dissolution and redistribution of intermetallic Zr(Cr. Fe), precipitates in Zircaloy-4 during irradiation at 561 K. In agreement with previous work [2,5] Yang also observed directional
Fig. 3. Precipitate morphology in Zircaloy-4 irradiated at 580 K to a fluence of 8 x 10” n/m’ for one hour. The grain boundary precipitates (A) and the rod-like precipitates (B) surrounding contain Fe and Cr. From ref. [2].
( E > 1 MeV) and annealed a recrystallised
Zr(Cr. Fe),
at 873 K particle
300
391
483
574
666
75-l
848
940
ENERGY bWl0" Fig. 4. Energy Dispersive X-ray Analysis of rod-shaped precipitates (upper trace) surrounding the Zr(Cr, Fe) z particies compared with the matrix (lower trace) for Zircaloy-4 irradiated at 580 K to a fluence of 8 x 102’ n/m2 (E > 1 MeV) and annealed at 873 K for one hour. From ref. [2].
dissolution of the precipitates and reprecipitation of Fe during post-irradiation annealing. Yang’s micrographs (fig. 2 in ref. [7]) are excellent illustrations of directional “dissolution” or redistribution of the Zr(Cr, Fe), precipitates in Zircaloy-4 after irradiation to fluences of about 15 x 1O25 n/m2 (E > 1 MeV). They show “scalloped” erosion of the precipitates along edges inclined to the basal plane. This feature was not obvious in the earlier studies because of the lower fluences involved (only up to about 8 x 10” n/m’). In all of the studies [l-5.7], there was no apparent relationship between the width of the amorphous zone and the crystallography of the matrix. This implies that diffusion of Fe within the precipitates was not determined by the external point defect fluxes. However, the two-dimensional redistribution or dissolution of the Fe and Cr [5,7] is clearly dependent on point defect fluxes within the matrix. Therefore, there appears to be two separate mechanisms governing the diffusion of Fe (and Cr) out of the intermetallic particles and the displacement and redistribution of Fe and Cr at the periphery of the particles. In both cases the magnitude of the effect is greatest for Fe and is consistent with Fe being the faster (interstitial) diffuser 161. Yang’s interpretations of the directional dissolution, together with the Fe depletion and crystalline-to-amorphous transformation, are not clear.
He appears to attribute the depletion of Fe (and an associated crystalline-to-amo~hous phase transformation) to the diffusion of vacancies from the matrix into the precipitates. Shrinkage of the particles was attributed to growth of the Zr matrix into the amorphous phase along a-directions. These explanations are not consistent with the conclusions drawn from earlier studies [1.2,5]. Because of the significance of Yang’s observations to theories of anisotropic diffusion during irradiation, it is important to analyse his results in relation to other data. Inconsistencies in the explanations for (1) the Fe depletion and crystalline-toamorphous phase transformation, and (2) the directional shrinkage of the precipitates. are discussed separately in the following paragraphs. (I) Yang concludes [7] that, the crystalline-toamorphous phase transformation at the periphery of the particles is assisted by an increased vacancy concentration in these regions due to diffusion of vacancies from the matrix. There is a corresponding depletion of Fe. In the absence of a suitable vacancy sink within the particles, any net flux of vacancies into the particles must presumably occur by a KirkendaIl type mechanism with Fe as the main element migrating in the opposite direction to the vacancy flux. There is no logical justification for this supposition because Fe is relatively insoluble in Zr [S] and there is no apparent reason why it should diffuse preferentially into the matrix as a substitutional element. In addition, an excess of vacancy loops have always been observed (along directions parallel with the basal plane) outside of these precipitates after irradiation to fluences where solute depletion and the amorphous transformation has occurred [9] This would indicate that. if anything. the precipitates were sinks for Zr self-interstitial atoms. (2) Yang has suggested that shrinkage of the Zr(Cr. Fe), precipitates occurs because of “exchanging of the matrix Zr atoms with Cr and Fe and with vacancies of the amorphous zone across the (precipitate) interface”. Although exchange of Zr with Fe and Cr atoms must be occurring, it is not clear why vacancies should first migrate into and then out of the particles. Yang also claims that “diffusion of substitutional atoms (Cr ?) [lo] and/or vacancies is energetically favourable along u-directions” and this accounts for the directional dependence of the dissolution of his particles. Because such a mechanism is linked with substitutional (vacancy) diffusion, the same energetic considerations must presumably affect thermally-activated diffusion. However. work by Hood and Schultz [S.lO.ll] has shown that anisotropy. associated with thermallyactivated diffusion of solute elements in a-Zr. is such
29X
Fig. 5 5istribution of precipitates (arrowed) around a Zr(Cr. Fe), particle in Ztrcaloy-4 irradiated 31 5X0 K to d fl uence of 8 x 10 .’ (E>l MeV: I and annealed at 873 K for one hour. There are more precipitates distributed along directions parallel with the b;asal plane corresponding with an increased solute concentration during Irradiation. From ref [S].
n/m2
that diffusion along the c-axis is faster than diffusion aIong the u-axis for all cases (including Cr and Fe). Yang’s observations [7] may be described by alternative interpretations which are consistent with the previously reported work [1,2,5]. (I) The first phenomenon, diffusion of Fe (and Cr) out of the intermetallic particles and the concurrent crystalline-to-amorphous transformation, has been attributed to the migration of interstitiai Fe (and Cr) [S]. The outward migration is easily explained by random diffusion of these elements as interstitial atoms assuming that the interface is not a barrier to interstitial diffusion (this would depend on a difference in chemical potential between Fe or Cr interstitial atoms in the particle compared with the matrix). Interstitial migra-
rion is believed to be responsible for the outward diffusion of Fe and Cr primarily because the observed effect ia radiation-induced and could not be reproduced by thermal means only [5]. Because the amorphous zone occurs at the periphery of the precipitates, is coincident with a depletion of Fe (and Cr), and advances inwards with increasing fluence [I]. it is logical that the crystalline-amorphous tranhformation occurs because of locat changes in the composition or defect concentration within the precipitate and not vice-versa. It is possible that the amorphous phase in the Fe-depleted region of the precipitates forms because of a higher vacancy concentration in these regions (as postulated by Yang [4,7]). however. the conclusions of other work [1.5,9] indicate that this would
M. Griffiths / Preeipitute st&lit.y
result from the outward migration of interstitial Fe as opposed to the inward migration of vacancies [4,7]. An alternative explanation for the amorphous transformation is that the crystal is hypostoichiometric and more susceptible to a radjation-induced crystalline-toamorphous transformation in the Fe-depleted regions. (2) The second phenomenon, the displacement and redistribution of Fe and Cr at the periphery of the particles, can be explained by an external flux of Zr self-interstitial atoms. Recent theoretical 112,131 and experimental evidence [14,15] for preferential Zr self-interstitial atom (SIA) migration in the basal plane indicates that the particle erosion and directional redistrihution of Fe or Cr in the Zr matrix could occur because of a coupling between the Fe (and Cr) and Zr SIAs. Ultimately, the particles will be replaced by the matrix and therefore they must be net sinks for Zr atoms. One possible explanation for the precipitate redistribution is that a flux of Zr SIAs, diffusing preferentially in the basal plane, impinges on the intermetallic particles and displaces the Fe and Cr atoms (by a replacement mechanism [6]) into interstitial states which are then free to recombine with vacancies or to form precipitate clusters. Further replacement sequences produce a net flow of Fe and Cr in the opposite direction to the self-interstitial flux. The effect would lead to increased Fe and Cr around the precipitates in directions parallel with the basal plane. This is what is observed during irradiation at 573 K 15). The effect appears to be less pronounced at higher temperatures (2 673 K) presumably because thermally activated diffusion (leading to more diffusion of Fe and Ct perpendicular to the basal plane [lo]) increases in significance compared with the radiationinduced diffusion at the higher irradiation temperatures. The periodicity of the erosion shown in fig. 2 of ref. [7] corresponds approxjmately with the spacing of the rows of dislocation loops (visible as “corduroy” contrast) also shown in the same figure. The periodicity is then consistent with a higher flux of SIAs between the layers of loops (vacancy dislocation loops in this case) that are parallel with the basal plane 112,131. It is clear from micrographs b, c and d of fig. 2 [7] that preferential redistribution of the precipitates is occurring parallel with the basal planes. This effect is most obvious for figure 2b of ref. [7], showing tunneiiing in directions parallel to the basal plane, and fig. 2d of ref. f7], showing erosion of the precipitate at surfaces which are perpendicular to the basal plane and no erosion for surfaces parallel to the basal plane. The long thin precipitate shown in fig. 2d of ref. [7] has been described in ref. [7] as the remains of a much larger one that has been dissolved. The orientation is such that dissolution
in Zirecrloy’-4
299
presumably occurred faster along directions perpendicular to the basal plane which is contrary to the evidence from the other precipitates showing preferential dissoiution along directions parallel to the basal plane. An alternative explanation for fig. 2d of [7], which is consistent with anisotropic self-interstitial diffusion, is that an elongated precipitate (occasionally observed in the Zircaloys) is aligned such that its long axis is nearly parallel with the basal plane. The cross-section for interaction with a flux of Zr SIAs in the basal plane is therefore minimised for this orientation and the precipitate surface remains relatively smooth. Increased removal of Fe and Cr is observed at the ends of this and other precipitates shown in fig. 2 where the cross-section for interaction with point defect fluxes diffusing in the basal plane is highest. Finally, although Yang claims that there will be a single phase solid solution of Fe and Cr following irradiation, in practice it is impossible to determine the state of these elements (that is, solute or precipitate) after irradiation at temperatures I 580 K because of the difficulties in detecting small precipitates or solute clusters in an as-irradiated foil. Whereas some degree of solution must exist during irradiation to account for the transfer of material, there are no measurements to show how much of the redistributed material is in solution at any one time. The precipitates visible after post-irradiation annealing could form from coarsening of smaller precipitates. Yang implies that the “dissolved’ Fe and Cr could enhance vacancy migration and this “can have a major impact on the growth property of the material”. it is not clear how this would affect irradiation growth unless he is referring to the enhancement of recombination previously postulated by Hood [6]. This would be contrary to previously reported work [9] showing that there is a correlation between precipitate “dissolution”, c-component vacancy loop stability and accelerated irradiation growth.
Conclusion The directional dissolution of intermetallic precipitates in Zircaioy-4 irradiated with neutrons at 561 K reported by Yang 171, can be interpreted in terms of the interaction between the precipitates and a flux of Zr self-interstitial atoms diffusing parallel with the basal plane. This interpretation is then consistent with the conciusions from previously reported work and with theories of irradiation damage in Zr-alloys based on anisotropic interstitial diffusion.
Acknowledgements
The author would like to thank G.M. Hood, L.M. Howe, S.R. MacEwen and B.A. Cheadle of CRNL for their comments on the manuscript.
References [I] R.W. Gilbert. M. Griffiths and G.J.C. Carpenter, J. Nucl. Mater. 135 (1985) 265. [Z] M. Griffiths. R.W. Gilbert and B.A. Cheadle. 1”: Proc. Workshop on Second Phase Particles in Zircaloys. Erlangen, 1985, issued as AECL 8852. (31 W.J.S. Yang, R.P. Tucker, B. Cheng and R.B. Adamson. ibid. ref. [2]. [4] W.J.S. Yang. R.P. Tucker. B. Cheng and R.B. Adamson. J. Nucl. Mater. 138 (1986) 185.
I51 M. Grlfflths. R.W. Gilbert and G.1.C;. Carpenter. J Nucl. Mater. 150 (1987) 53. 161 G.M. Hood. AECL 5692 (1977). [‘I W.J.S. Yang, J. Nucl. Mater. 15X (19X8) 71. I81 G.M. Hood, J. Nucl. Mater. 159 (1988) 149. [91 M. Griffiths and R.W. Gilbert. J. Nucl. Mater. 150 (lY87) 16’). [lOI G.M. Hood and R.J. Schultz. Acta Metall. 22 (1074) 45Y. 1111G.M. Hood and R.J. Schultz_ III: Proc. Eigth 1111.Symp. on Zirconium I” the Nuclear Industry, San Diego. 19X8 (ASTM. to be published). C.H. Woo. J. Nucl. Mater. 159 (198X) 237. J.H. Evnns. J. Nucl. Mater. 132 (19X5) 147. M. Griffitha, R.W. Gtlbert and (‘.F.. Coleman. J Nuci. Mater. 159 (198X)405 .I. Nucl. Mater. 15Y (I9XX) 190. [I51 M.Grlffiths.
Received 3 April 1989: accepted 23 October 1989
Work on amorphous transformations and depletion of Fe. Cr or Ni from intermetallic particles in Zircaloy-2 and Zircaloy-4 during neutron irradiatjo~ was first published by Gilbert, Griffiths and Carpenter in 1985 [l]. At irradiation temperatures of about 570 K, it was reported that there was preferential depletion of Fe From Zr(Cr. Fe), intermetallic precipitates which corresponded with a concurrent crystalIine-to-arno~h~~us phase transformation. The depth of the Fe depletion (and the thickness of the amorphous layer) increased with increasing fluence. Zrz(Ni. Fe) particles did not show the same Fe depletion and amorphous phase transformation. It was also reported that hoth and Zr,(Ni, Fe) precipitates became Zr(Cr, Fe), amorphous (without Fe depletion) at low irradiation temperatures (about 350 K) and remained crystalline at high irradiation temperatures (about 673 K). These results were presented at a workshop in Eriangen that same year along with additional data on the phase stability and redistribution of the precipitates following irradiation at temperatures between 330 and 873 K [Z]. The effects of post”irradiation annealing for one hour at temperatures between 673-873 K were also reported. For Zircaloy-4 irradiated at about 560 K, it was shown that the Zr(Cr, Fe)z precipitates became amo~h~~us and that this was coincident with the depletion of Fe (figs. 1 and 2 [Z]). The amorphous phase recrystallised and new precipitates (containing Fe and Cr) were visible at grain boundaries and in the matrix following post-irradiation annealing for one hour at temperatures 2 773 K but not at 673 K. This was clearly illustrated after annealing at 873 K because of precipitate coarsening and the increased recovery of the radiation damage microstructure (figs. 3 and 4 121). These new precipitates were the result of the removal and redistribution of Fe and Cr from the parent intermetallic particle. In the proceedings of the Erlangen workshop, Yang et al. 131 also reported evidence of Zr(Fe, Cr), precipitates in Zircaloy-4 starting to become amorphous after irradia-
tion at 2M0c to fluenues of ahout 1 x 10” n/cm’. They reported that the Fe was depleted from the amorphous region of the precipitates and no amorphisation was observed after irradiation at 375°C. Similar data were again reported elsewhere [4]. It has been reported by Griffiths et al. (51 that for irradiation temperatures between about 350-700 K there is radiation-induced dissolution of the interm~talli~ particles in Zircaloy-2 and -4. The dissolution is apparent in the sense that there is a redistribution of material but it is not a thermal equilibrium process. It was concluded [S] that the redistribution occurred both as a result & sputtering (where atoms at the periphery of the precipitates are displaced into the matrix by energetic neutrons) and diffusion (especially at temperatures 2 550 K). The diffusion could be either substitutional (m the opposite direction to a vacancy flux) or interstitial (because of the creation of interstitial atoms by atomic displacements). In addition to a preferential depietion of Fe and concurrent crystalline-to-amorphous phase transformation. described previously, it was shown that Fe and Cr diffused away from ZrfCr, Fe), int~rmetalli~ particles in Zircaloy-4 preferentially along directions parallel to the basal plane during irradiation at temperatures between 5.50 and 600 K. fig. 5 [5]. The directionality of the migration away from the particles was attributed to some form of two dimensional interstitial diffus~~~~, presumably due to a coupling between a flux of Zr self-interstitial atoms with Fe and C‘r in the elements and the matrix. The depletion of “solute” amorphous transformation could not be reproduced h> thermal treatments alone and a mechanism was proposed [S] explaining the dissolution process in terms of interstitial point defect nligrati~)n. Preferential dispiatcment of substitutional Fe (and Cr) within the Zr(Cr. Fe), precipitates by neutrons and Zr self-mterstitial atoms (a mechanism proposed by Hood for enhanced recombination of point defects by Fe /6]) wah believed to be an important factor in determining the
M. GriJfirhs / Precipltare
295
srahiliry m Zircaloy-4
EnergyCevl
Energy [evl
8.5 x 10*%/m* Fig. 1. Crystalline-to-amorphous transformation of Zr(Cr. Fe), particles in Zircaloy-4 irradiated at 560 K. The amorphous layer is coincident with a depletion of Fe and Cr. The width is proportional to the neutron fluence and is approximately 10 nm per lo*’ n/m* (E > 1 MeV). From ref. [2].
diffusion of Fe (and Cr) out of the intermetallic partitles and, as a result, the stability of the amorphous phase. It was further concluded that the amorphous transformation occurred in the Fe-depleted region at the periphery of the precipitates because of the resultant hypostoichiometry in these regions [5]. The amorphous
phase stability during post-irradiation annealing was dependent on the Cr relative to Fe content. It was proposed that, during irradiation, diffusion of Fe (and Cr) out of the particles occurred because of a lower chemical potential for interstitial Fe (and Cr) in the matrix compared with the particle. The magnitude of
796
L
profile for a Zr(Cr, Fe),
Fe
.-__
C,
AMo*pHg,,
CRISTALLINE
T*:“l’,,
__.___-.___i
Fig. 2. Composition
-
..-
.A_
100
50
Distance
particle
200
from Centfe
irradiated
the effect was always larger for Fe and it was concluded that this was because the Fe was the faster (interstitial) diffuser [6]. A recent paper by Yang [7] described the crystalline-
ot Pattictelnm
at 560 K to a fluence of 3.5
x
10”
n/m’
(E > 1 MtV). From ref. [2].
to-amorphous transformation, dissolution and redistribution of intermetallic Zr(Cr. Fe), precipitates in Zircaloy-4 during irradiation at 561 K. In agreement with previous work [2,5] Yang also observed directional
Fig. 3. Precipitate morphology in Zircaloy-4 irradiated at 580 K to a fluence of 8 x 10” n/m’ for one hour. The grain boundary precipitates (A) and the rod-like precipitates (B) surrounding contain Fe and Cr. From ref. [2].
( E > 1 MeV) and annealed a recrystallised
Zr(Cr. Fe),
at 873 K particle
300
391
483
574
666
75-l
848
940
ENERGY bWl0" Fig. 4. Energy Dispersive X-ray Analysis of rod-shaped precipitates (upper trace) surrounding the Zr(Cr, Fe) z particies compared with the matrix (lower trace) for Zircaloy-4 irradiated at 580 K to a fluence of 8 x 102’ n/m2 (E > 1 MeV) and annealed at 873 K for one hour. From ref. [2].
dissolution of the precipitates and reprecipitation of Fe during post-irradiation annealing. Yang’s micrographs (fig. 2 in ref. [7]) are excellent illustrations of directional “dissolution” or redistribution of the Zr(Cr, Fe), precipitates in Zircaloy-4 after irradiation to fluences of about 15 x 1O25 n/m2 (E > 1 MeV). They show “scalloped” erosion of the precipitates along edges inclined to the basal plane. This feature was not obvious in the earlier studies because of the lower fluences involved (only up to about 8 x 10” n/m’). In all of the studies [l-5.7], there was no apparent relationship between the width of the amorphous zone and the crystallography of the matrix. This implies that diffusion of Fe within the precipitates was not determined by the external point defect fluxes. However, the two-dimensional redistribution or dissolution of the Fe and Cr [5,7] is clearly dependent on point defect fluxes within the matrix. Therefore, there appears to be two separate mechanisms governing the diffusion of Fe (and Cr) out of the intermetallic particles and the displacement and redistribution of Fe and Cr at the periphery of the particles. In both cases the magnitude of the effect is greatest for Fe and is consistent with Fe being the faster (interstitial) diffuser 161. Yang’s interpretations of the directional dissolution, together with the Fe depletion and crystalline-to-amorphous transformation, are not clear.
He appears to attribute the depletion of Fe (and an associated crystalline-to-amo~hous phase transformation) to the diffusion of vacancies from the matrix into the precipitates. Shrinkage of the particles was attributed to growth of the Zr matrix into the amorphous phase along a-directions. These explanations are not consistent with the conclusions drawn from earlier studies [1.2,5]. Because of the significance of Yang’s observations to theories of anisotropic diffusion during irradiation, it is important to analyse his results in relation to other data. Inconsistencies in the explanations for (1) the Fe depletion and crystalline-toamorphous phase transformation, and (2) the directional shrinkage of the precipitates. are discussed separately in the following paragraphs. (I) Yang concludes [7] that, the crystalline-toamorphous phase transformation at the periphery of the particles is assisted by an increased vacancy concentration in these regions due to diffusion of vacancies from the matrix. There is a corresponding depletion of Fe. In the absence of a suitable vacancy sink within the particles, any net flux of vacancies into the particles must presumably occur by a KirkendaIl type mechanism with Fe as the main element migrating in the opposite direction to the vacancy flux. There is no logical justification for this supposition because Fe is relatively insoluble in Zr [S] and there is no apparent reason why it should diffuse preferentially into the matrix as a substitutional element. In addition, an excess of vacancy loops have always been observed (along directions parallel with the basal plane) outside of these precipitates after irradiation to fluences where solute depletion and the amorphous transformation has occurred [9] This would indicate that. if anything. the precipitates were sinks for Zr self-interstitial atoms. (2) Yang has suggested that shrinkage of the Zr(Cr. Fe), precipitates occurs because of “exchanging of the matrix Zr atoms with Cr and Fe and with vacancies of the amorphous zone across the (precipitate) interface”. Although exchange of Zr with Fe and Cr atoms must be occurring, it is not clear why vacancies should first migrate into and then out of the particles. Yang also claims that “diffusion of substitutional atoms (Cr ?) [lo] and/or vacancies is energetically favourable along u-directions” and this accounts for the directional dependence of the dissolution of his particles. Because such a mechanism is linked with substitutional (vacancy) diffusion, the same energetic considerations must presumably affect thermally-activated diffusion. However. work by Hood and Schultz [S.lO.ll] has shown that anisotropy. associated with thermallyactivated diffusion of solute elements in a-Zr. is such
29X
Fig. 5 5istribution of precipitates (arrowed) around a Zr(Cr. Fe), particle in Ztrcaloy-4 irradiated 31 5X0 K to d fl uence of 8 x 10 .’ (E>l MeV: I and annealed at 873 K for one hour. There are more precipitates distributed along directions parallel with the b;asal plane corresponding with an increased solute concentration during Irradiation. From ref [S].
n/m2
that diffusion along the c-axis is faster than diffusion aIong the u-axis for all cases (including Cr and Fe). Yang’s observations [7] may be described by alternative interpretations which are consistent with the previously reported work [1,2,5]. (I) The first phenomenon, diffusion of Fe (and Cr) out of the intermetallic particles and the concurrent crystalline-to-amorphous transformation, has been attributed to the migration of interstitiai Fe (and Cr) [S]. The outward migration is easily explained by random diffusion of these elements as interstitial atoms assuming that the interface is not a barrier to interstitial diffusion (this would depend on a difference in chemical potential between Fe or Cr interstitial atoms in the particle compared with the matrix). Interstitial migra-
rion is believed to be responsible for the outward diffusion of Fe and Cr primarily because the observed effect ia radiation-induced and could not be reproduced by thermal means only [5]. Because the amorphous zone occurs at the periphery of the precipitates, is coincident with a depletion of Fe (and Cr), and advances inwards with increasing fluence [I]. it is logical that the crystalline-amorphous tranhformation occurs because of locat changes in the composition or defect concentration within the precipitate and not vice-versa. It is possible that the amorphous phase in the Fe-depleted region of the precipitates forms because of a higher vacancy concentration in these regions (as postulated by Yang [4,7]). however. the conclusions of other work [1.5,9] indicate that this would
M. Griffiths / Preeipitute st&lit.y
result from the outward migration of interstitial Fe as opposed to the inward migration of vacancies [4,7]. An alternative explanation for the amorphous transformation is that the crystal is hypostoichiometric and more susceptible to a radjation-induced crystalline-toamorphous transformation in the Fe-depleted regions. (2) The second phenomenon, the displacement and redistribution of Fe and Cr at the periphery of the particles, can be explained by an external flux of Zr self-interstitial atoms. Recent theoretical 112,131 and experimental evidence [14,15] for preferential Zr self-interstitial atom (SIA) migration in the basal plane indicates that the particle erosion and directional redistrihution of Fe or Cr in the Zr matrix could occur because of a coupling between the Fe (and Cr) and Zr SIAs. Ultimately, the particles will be replaced by the matrix and therefore they must be net sinks for Zr atoms. One possible explanation for the precipitate redistribution is that a flux of Zr SIAs, diffusing preferentially in the basal plane, impinges on the intermetallic particles and displaces the Fe and Cr atoms (by a replacement mechanism [6]) into interstitial states which are then free to recombine with vacancies or to form precipitate clusters. Further replacement sequences produce a net flow of Fe and Cr in the opposite direction to the self-interstitial flux. The effect would lead to increased Fe and Cr around the precipitates in directions parallel with the basal plane. This is what is observed during irradiation at 573 K 15). The effect appears to be less pronounced at higher temperatures (2 673 K) presumably because thermally activated diffusion (leading to more diffusion of Fe and Ct perpendicular to the basal plane [lo]) increases in significance compared with the radiationinduced diffusion at the higher irradiation temperatures. The periodicity of the erosion shown in fig. 2 of ref. [7] corresponds approxjmately with the spacing of the rows of dislocation loops (visible as “corduroy” contrast) also shown in the same figure. The periodicity is then consistent with a higher flux of SIAs between the layers of loops (vacancy dislocation loops in this case) that are parallel with the basal plane 112,131. It is clear from micrographs b, c and d of fig. 2 [7] that preferential redistribution of the precipitates is occurring parallel with the basal planes. This effect is most obvious for figure 2b of ref. [7], showing tunneiiing in directions parallel to the basal plane, and fig. 2d of ref. f7], showing erosion of the precipitate at surfaces which are perpendicular to the basal plane and no erosion for surfaces parallel to the basal plane. The long thin precipitate shown in fig. 2d of ref. [7] has been described in ref. [7] as the remains of a much larger one that has been dissolved. The orientation is such that dissolution
in Zirecrloy’-4
299
presumably occurred faster along directions perpendicular to the basal plane which is contrary to the evidence from the other precipitates showing preferential dissoiution along directions parallel to the basal plane. An alternative explanation for fig. 2d of [7], which is consistent with anisotropic self-interstitial diffusion, is that an elongated precipitate (occasionally observed in the Zircaloys) is aligned such that its long axis is nearly parallel with the basal plane. The cross-section for interaction with a flux of Zr SIAs in the basal plane is therefore minimised for this orientation and the precipitate surface remains relatively smooth. Increased removal of Fe and Cr is observed at the ends of this and other precipitates shown in fig. 2 where the cross-section for interaction with point defect fluxes diffusing in the basal plane is highest. Finally, although Yang claims that there will be a single phase solid solution of Fe and Cr following irradiation, in practice it is impossible to determine the state of these elements (that is, solute or precipitate) after irradiation at temperatures I 580 K because of the difficulties in detecting small precipitates or solute clusters in an as-irradiated foil. Whereas some degree of solution must exist during irradiation to account for the transfer of material, there are no measurements to show how much of the redistributed material is in solution at any one time. The precipitates visible after post-irradiation annealing could form from coarsening of smaller precipitates. Yang implies that the “dissolved’ Fe and Cr could enhance vacancy migration and this “can have a major impact on the growth property of the material”. it is not clear how this would affect irradiation growth unless he is referring to the enhancement of recombination previously postulated by Hood [6]. This would be contrary to previously reported work [9] showing that there is a correlation between precipitate “dissolution”, c-component vacancy loop stability and accelerated irradiation growth.
Conclusion The directional dissolution of intermetallic precipitates in Zircaioy-4 irradiated with neutrons at 561 K reported by Yang 171, can be interpreted in terms of the interaction between the precipitates and a flux of Zr self-interstitial atoms diffusing parallel with the basal plane. This interpretation is then consistent with the conciusions from previously reported work and with theories of irradiation damage in Zr-alloys based on anisotropic interstitial diffusion.
Acknowledgements
The author would like to thank G.M. Hood, L.M. Howe, S.R. MacEwen and B.A. Cheadle of CRNL for their comments on the manuscript.
References [I] R.W. Gilbert. M. Griffiths and G.J.C. Carpenter, J. Nucl. Mater. 135 (1985) 265. [Z] M. Griffiths. R.W. Gilbert and B.A. Cheadle. 1”: Proc. Workshop on Second Phase Particles in Zircaloys. Erlangen, 1985, issued as AECL 8852. (31 W.J.S. Yang, R.P. Tucker, B. Cheng and R.B. Adamson. ibid. ref. [2]. [4] W.J.S. Yang. R.P. Tucker. B. Cheng and R.B. Adamson. J. Nucl. Mater. 138 (1986) 185.
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