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Remarks on the habit planes of zirconium hydride in Zircaloy-2
JOURNAL
OF NUCLEAR
REMARKS
28 (1968)
MATERIALS
ON THE
HABIT
336-338.
PLANES
0 NORTH-HOLLAND
OF ZIRCONIUM
PUBLISHINCf
HYDRIDE
CO.. AMSTERDAM
IN ZIRC!ALOY-2
V. S. ARUNACHALAM Metallurgy
Division,
Bhabha
G. Section
for Physical
Atomic
Research
Centre,
Trombay,
Bombay-74
India
ijSTBERG and B. LEHTINEN Metallurgy,
Received
AB
Atomenergi,
16 July
This rejoinderis concerned with the comments made recently by Westlake 1) on our previously published observations on the habit-planes of zirconium hydride 2) in Zircaloy-2. For a detailed review on the habit-planes of hydrides in zirconium and in Zircaloy-2, the reader is referred to a recent review by Ells 3). The main discrepancy between Westlake’s new observations and some earlier findings, including ours, is t’hat he report’s (lOi7) as the only habit-plane, while other workers have failed to observe this particular habit-plane. Instead, Louthan and Angerman 4) have found {lOiO}. (loil}, {lOis}, {lOi3}, (1121) and (1122) as the habit-planes for the precipitation of zirconium hydride while we have observed {loil}, (1013).poi.5) and (0002) as the habit-planes. Since our investigation appears to be the only one in which electron diffraction analysis was used for determining the habit-planes, it is worthwhile to consider the relative merits between electron microscopy techniques as used by us and the standard single-surface pole loci method as used by Westlake. By combining selected area diffraction patterns with the image of the planar feature, it is possible to determine the line of intersection of the planar feature with the foil surface. For a unique determination of the habit-plane, it is now necessary that either the foil thickness is accurately known or similar planar features are available in different orientations. We have 336
Stockholm-&,
Sweden
1968
used the latter method in our experimental observations. This involved an analysis of a large number of diffraction patterns and during our investigations, about 150 grains were analysed to determine the habit-planes. Limitations and errors arising in such analysis have been dealt elsewhere 5) ; suffice it to state. that accuracy better than a single-surface pole ana.lysis is hard to obtain in this method unless the orientation of the grains are determined from their Kikuchi patterns. In order to increase the accuracy of the orientation determination, we had coupled whenever possible, electron diffraction patterns with Kikuchi line patterns (maximum deviation & 2”). This small scatter enabled us to distinguish between different habit-planes making small acute angles with each other. In some cases, where Kikuchi lines were not observed, orientation determinations were made exclusively from the elect’ron diffraction patterns. Westlake’s experiments have raised the question of accurately determining the angles between traces of hydrides. He has shown that by increasing the magnification, clusters would be resolved into single platelets which would then provide a different habit plane other than the one calculated from the clusters at a lower magnification. This limitation however was not present in our investigations, as individual hydrides could always be resolved in an electron A further advantage in electron microscope.
THE
microscopy
techniques
HABIT
is that
PLANES
the
OF
specimen
ZIRCONIUM
337
HYDRIDE
hydrides ; the only exception
perhaps would be
{ lOi1). This is not to say that precipitation on only (1017)would explain the absence of
examined is free from surface hydrides. We shall next consider the problem of the basil plane as the habit plane for hydrides. Westlake has criticised this result of ours, saying
orientation.
that we have not produced the evidence in our paper. We feel the criticism is unjustified. For
terms of precipitation on {lOi3),(1015)and (lOi7) planes as all these planes make small
one thing, we have clearly stated in our paper that all habit-planes were determined from
angles with the basal plane. A more satisfactory
trace-analysis and so any confusion as to whether the precipitates are platelets on the basal plane, or needles whose directions lie in the basal plane, is uncalled for; for another thing, precipitation on the basal plane was found to occur exclusively on thin films while under observation inside the microscope, perhaps as a consequence of moderate heating. As the foil thickness could very well be the determining factor for the choice of this habit-plane, it is not surprising that precipitation on this plane has not been observed in bulk samples. Even if this precipitation were to occur in bulk samples, the extremely small sizes of these platelets (refer to figs. 3 and 4 in reference 2) would preclude any possibility of their being observed through optical microscopy. It is interesting to recall, that Westlake himself in an earlier publication referred to in our paper, has described the hydride precipitates as “needles on plates that are parallel to the basal plane” without naming the basal plane as the “habitIJane”. From our investigation it appears quite likely that the basal plane could very well have been the “habit-plane” in his investigations also. We now come to the consequence of a single habit-plane on the phenomenon of stressoriented hydrides. Westlake has drawn on the experimental results of Louthan and Angerman to show that the {lOi7} plane alone could satisfactorily account for the results. We are unable to accept this argument for the following reason: Even assuming Westlake’s model of randomlv oriented grains as described in his paper, it could be easily seen that only when the basal plane normal lies along the tensile axis, would a maximum number of habit planes be activated for the stress-orientation of
hydride
traces
on many
It could
grains
after
stress-
as well be explained
in
evidence should have come from hydride traces on samples hydrided under stress-free conditions. Such an analysis by Louthan and Angerman has yielded more than one habit plane. It is of interest to consider recent experiments by Marshall 6) on the stress-oriented hydrides in Zircaloy-2 tubing. Marshall’s experiments have shown that hydride platelets precipitated on different habit-planes, and the habit-planes in the same material change after stress-orientation treatment. In fairness, it must be admitted that Marshall has not determined specific habit planes; but from his experimental data on crystallographic plar,es most highly frequented by hydrides, and also by the change of these planes by stress-orientat’ion treatment, he is drawn to the conclusion that hydrides do not occupy a single invariant habit plane. This indeed has been borne out by our experimental resu1t.s. Further, recent experiments at one of our laboratories (BARC) 7) on the mechanical properties of stress-oriented hydrides in textured Zircaloy-2, also indirectly supFort ihe existence of many habit planes for the precipitation hydrides.
of
Finally, we would like to emphasise two features in our experiment which should not be overlooked while discussing the results. The habit planes reported by us were found from both inter- and intra-granular hydrides. In spite of rather a high concentration of hydrogen in our experiments, we had observed hydrides during earlier stages of precipitation and hence on a much finer scale. The validity of our results on macroscopic course hydrides has not yet been investigated. For these many reasons, it appears that a single habit plane as suggested by Westlake is not adequate to explain all the experimental
338
V.
S.
ARUNACHA.LAM
observations in hydrided Zircaloy-2. (1077) may be one of the habit planes, but the reasons so far advanced to support the position that it is the only plane are not adequate and in many case contrary to experimental results.
ET
AL.
2) V. 8. Arunachalam, ibid. 21 (1967)
3) C. Ells, ibid. 28 (1968) 4)
M. R. Louthan Sot. AIME
1) D. G. Westlake,
J. Nucl.
Mat.
26 (1968)
208
129
and C. L. Angerman,
236 (1966)
Trans. Met.
221
5) P. B. Hirsch et'al.,in Electron microscopy thin crystals
References
B. Lehtinen and G. o&berg,
241
(Butterworths,
6)
R. P. Marshall,
7)
T.
K.
Atomic
Sinha
J. Nucl.
and V.
of
1965) p. 313
Mat.
24 (1967)
S. Arunachalam,
Research Centre Report,
Bombay
34 Bhabha (1968)
OF NUCLEAR
REMARKS
28 (1968)
MATERIALS
ON THE
HABIT
336-338.
PLANES
0 NORTH-HOLLAND
OF ZIRCONIUM
PUBLISHINCf
HYDRIDE
CO.. AMSTERDAM
IN ZIRC!ALOY-2
V. S. ARUNACHALAM Metallurgy
Division,
Bhabha
G. Section
for Physical
Atomic
Research
Centre,
Trombay,
Bombay-74
India
ijSTBERG and B. LEHTINEN Metallurgy,
Received
AB
Atomenergi,
16 July
This rejoinderis concerned with the comments made recently by Westlake 1) on our previously published observations on the habit-planes of zirconium hydride 2) in Zircaloy-2. For a detailed review on the habit-planes of hydrides in zirconium and in Zircaloy-2, the reader is referred to a recent review by Ells 3). The main discrepancy between Westlake’s new observations and some earlier findings, including ours, is t’hat he report’s (lOi7) as the only habit-plane, while other workers have failed to observe this particular habit-plane. Instead, Louthan and Angerman 4) have found {lOiO}. (loil}, {lOis}, {lOi3}, (1121) and (1122) as the habit-planes for the precipitation of zirconium hydride while we have observed {loil}, (1013).poi.5) and (0002) as the habit-planes. Since our investigation appears to be the only one in which electron diffraction analysis was used for determining the habit-planes, it is worthwhile to consider the relative merits between electron microscopy techniques as used by us and the standard single-surface pole loci method as used by Westlake. By combining selected area diffraction patterns with the image of the planar feature, it is possible to determine the line of intersection of the planar feature with the foil surface. For a unique determination of the habit-plane, it is now necessary that either the foil thickness is accurately known or similar planar features are available in different orientations. We have 336
Stockholm-&,
Sweden
1968
used the latter method in our experimental observations. This involved an analysis of a large number of diffraction patterns and during our investigations, about 150 grains were analysed to determine the habit-planes. Limitations and errors arising in such analysis have been dealt elsewhere 5) ; suffice it to state. that accuracy better than a single-surface pole ana.lysis is hard to obtain in this method unless the orientation of the grains are determined from their Kikuchi patterns. In order to increase the accuracy of the orientation determination, we had coupled whenever possible, electron diffraction patterns with Kikuchi line patterns (maximum deviation & 2”). This small scatter enabled us to distinguish between different habit-planes making small acute angles with each other. In some cases, where Kikuchi lines were not observed, orientation determinations were made exclusively from the elect’ron diffraction patterns. Westlake’s experiments have raised the question of accurately determining the angles between traces of hydrides. He has shown that by increasing the magnification, clusters would be resolved into single platelets which would then provide a different habit plane other than the one calculated from the clusters at a lower magnification. This limitation however was not present in our investigations, as individual hydrides could always be resolved in an electron A further advantage in electron microscope.
THE
microscopy
techniques
HABIT
is that
PLANES
the
OF
specimen
ZIRCONIUM
337
HYDRIDE
hydrides ; the only exception
perhaps would be
{ lOi1). This is not to say that precipitation on only (1017)would explain the absence of
examined is free from surface hydrides. We shall next consider the problem of the basil plane as the habit plane for hydrides. Westlake has criticised this result of ours, saying
orientation.
that we have not produced the evidence in our paper. We feel the criticism is unjustified. For
terms of precipitation on {lOi3),(1015)and (lOi7) planes as all these planes make small
one thing, we have clearly stated in our paper that all habit-planes were determined from
angles with the basal plane. A more satisfactory
trace-analysis and so any confusion as to whether the precipitates are platelets on the basal plane, or needles whose directions lie in the basal plane, is uncalled for; for another thing, precipitation on the basal plane was found to occur exclusively on thin films while under observation inside the microscope, perhaps as a consequence of moderate heating. As the foil thickness could very well be the determining factor for the choice of this habit-plane, it is not surprising that precipitation on this plane has not been observed in bulk samples. Even if this precipitation were to occur in bulk samples, the extremely small sizes of these platelets (refer to figs. 3 and 4 in reference 2) would preclude any possibility of their being observed through optical microscopy. It is interesting to recall, that Westlake himself in an earlier publication referred to in our paper, has described the hydride precipitates as “needles on plates that are parallel to the basal plane” without naming the basal plane as the “habitIJane”. From our investigation it appears quite likely that the basal plane could very well have been the “habit-plane” in his investigations also. We now come to the consequence of a single habit-plane on the phenomenon of stressoriented hydrides. Westlake has drawn on the experimental results of Louthan and Angerman to show that the {lOi7} plane alone could satisfactorily account for the results. We are unable to accept this argument for the following reason: Even assuming Westlake’s model of randomlv oriented grains as described in his paper, it could be easily seen that only when the basal plane normal lies along the tensile axis, would a maximum number of habit planes be activated for the stress-orientation of
hydride
traces
on many
It could
grains
after
stress-
as well be explained
in
evidence should have come from hydride traces on samples hydrided under stress-free conditions. Such an analysis by Louthan and Angerman has yielded more than one habit plane. It is of interest to consider recent experiments by Marshall 6) on the stress-oriented hydrides in Zircaloy-2 tubing. Marshall’s experiments have shown that hydride platelets precipitated on different habit-planes, and the habit-planes in the same material change after stress-orientation treatment. In fairness, it must be admitted that Marshall has not determined specific habit planes; but from his experimental data on crystallographic plar,es most highly frequented by hydrides, and also by the change of these planes by stress-orientat’ion treatment, he is drawn to the conclusion that hydrides do not occupy a single invariant habit plane. This indeed has been borne out by our experimental resu1t.s. Further, recent experiments at one of our laboratories (BARC) 7) on the mechanical properties of stress-oriented hydrides in textured Zircaloy-2, also indirectly supFort ihe existence of many habit planes for the precipitation hydrides.
of
Finally, we would like to emphasise two features in our experiment which should not be overlooked while discussing the results. The habit planes reported by us were found from both inter- and intra-granular hydrides. In spite of rather a high concentration of hydrogen in our experiments, we had observed hydrides during earlier stages of precipitation and hence on a much finer scale. The validity of our results on macroscopic course hydrides has not yet been investigated. For these many reasons, it appears that a single habit plane as suggested by Westlake is not adequate to explain all the experimental
338
V.
S.
ARUNACHA.LAM
observations in hydrided Zircaloy-2. (1077) may be one of the habit planes, but the reasons so far advanced to support the position that it is the only plane are not adequate and in many case contrary to experimental results.
ET
AL.
2) V. 8. Arunachalam, ibid. 21 (1967)
3) C. Ells, ibid. 28 (1968) 4)
M. R. Louthan Sot. AIME
1) D. G. Westlake,
J. Nucl.
Mat.
26 (1968)
208
129
and C. L. Angerman,
236 (1966)
Trans. Met.
221
5) P. B. Hirsch et'al.,in Electron microscopy thin crystals
References
B. Lehtinen and G. o&berg,
241
(Butterworths,
6)
R. P. Marshall,
7)
T.
K.
Atomic
Sinha
J. Nucl.
and V.
of
1965) p. 313
Mat.
24 (1967)
S. Arunachalam,
Research Centre Report,
Bombay
34 Bhabha (1968)