- Email: [email protected]
The influence of prior deformation on hydride precipitation in zircaloy
Acta metall, mater. Vol. 40, No. 2, pp. 363-372, 1992 Printed in Great Britain. All rights reserved
0956-7151/92$5.00+ 0.00 Copyright © 1992PergamonPress pie
THE I N F L U E N C E OF PRIOR D E F O R M A T I O N ON H Y D R I D E PRECIPITATION IN ZIRCALOY V. P E R O V I C 1, G. C. W E A T H E R L Y 2, S. R. MacEWEN3't and M. L E G E R I 'Metallurgical Research Department, Ontario Hydro Research Division, 800 Kipling Avenue, Toronto, Ontario, Canada M8Z 5S4, 2Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4M1 and 3Advanced Materials Research Branch, Chalk River Nuclear Research Laboratories, Atomic Energy of Canada Ltd, Chalk River, Ontario, Canada K0J lJO (Received 14 May 1991)
Abstract--Precipitation of hydrides has been studied in samples of Zircaloy subjected to prior tensile or compressive deformation before charging with hydrogen. The mean residual stress pattern in the alloys prior to charging was assessed by neutron diffraction techniques and provided a rough guide as to the preferred site of hydride nucleation. Heterogeneous hydride nucleation at grain boundaries or twin boundaries was commonly found in samples subjected to 4% prior deformation, while transgranular hydrides were most frequently observed after a prior 1/2% compressive deformation or an annealing. The local stress state at grain boundary facets or twins is thought to be the deciding factor in determining where hydrides nucleate and how hydride stacks form. R~sum~-On 6tudie la precipitation d'hydrures dans des 6chantillons de Zircaloy soumis fi une dfformation en traction ou en compression avant qu'on les charge en hydrog~ne. Le diagramme des contraintes r~siduelles moyennes dans les alliages avant la charge est obtenu par diffraction des neutrons et fournit u n guide grossier des sites prrfrrentiels de germination des hydrures. On trouve tr~s souvent une germination h~t~rog~ne des hydrures aux joints de grains ou de macle dans les ~chantillons soumis /t 4% de d~formation initiale, tandis que les hydrures intergranulaires sont plus fr~quemment observes apr~s une drformation initiale par compression de 0,5% ou un recuit. On pense que l'rtat de contrainte locale sur les facettes des joints de grains ou de macle est le facteur prrpond~rant qui drtermine o~ la germination des hydrures se produit et comment les empilements d'hydrure se forment.
Zusmmenfassung--Die Ausscheidung von Hydriden wird in Zirkaloy-Proben, die vor der WasserstoffBeladung einer Zug- oder Druckverformung unterworfen worden sind, untersucht. Die Verteilung der Restspannungen in den Legierungen vor der Wassserstoff-Beladung wird mittels Neutronenbeugung bestimmt und liefert einen groben Hinweis auf die bevorzugten Orte der Hydridbildung. Heterogene Hydridbildung an Korn- oder Zwillingsgrenzen wird iiblicherweise in Proben gefunden, die 4% vorverforint waren, wohingegen transgranulare Hydride am h/iufigsten nach einer Druckverformung yon 1/2% oder nach Ausheilen auftreten. Es wird angenommen, dab der lokale Spannungszustand an den Korngrenzfacetten oder Zwillingen entscheidend mitbestimmt, wo sich Hydride bilden und wie sie Hydridstapel formen.
1. INTRODUCTION When zirconium hydride precipitates in zirconium or its alloys at low temperatures, it does so by a displacive transformation, which involves both a shear and local volume change of the parent lattice [1-3]. A n u m b e r of interesting phenomena are associated with the transformation. Growth of large hydride plates appears to involve the repeated nucleation and growth of hydride sub units which eventually coalesce and grow into macroscopic hydride plates [4, 5]. A supersaturation of hydrogen (well above the terminal solid solubility measured in tPresent address: Alcan International Ltd, R&D Centre, PBO 8400, Kingston, Ontario, Canada K7L 5L9.
a dissolution experiment) is thus required to drive the growth of hydride plates [6]. The apparent habit plane of hydride plates is determined by the stacking of the smaller hydride plates, as first determined by Perovic et al. [7]. In this sense, the transformation bears a striking resemblance to the bainite transformation in steels, where similar morphological features have been reported [8]. In common with many martensitic or bainitic transformations, hydride precipitation can be controlled by a stress, either external or internal. A stress can help drive the shear component of the transformation or to accommodate the strains associated with the volume change of the hydride plate normal to its habit plane [3, 5]. The role of stress is complicated however, because even in well annealed zirconium alloys, large internal stresses can exist in polycrystals 363
364
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY
because of the pronounced anisotropy in the thermal expansivity and elastic properties of zirconium. Indeed it is unlikely that polycrystalline zirconium can be produced free of internal stresses. In a recent study of Zircaloy with a rod texture, MacEwen and Tome [9] have shown that samples annealed in the -phase at 600°C develop a residual stress ~ 100 MPa on the basal plane on cooling to room temperature. For the particular texture of the alloy they studied, this basal plane stress was effectively independent of grain orientation. As hydrides in zirconium alloys generally precipitate as plates with habit planes close to the basal plane, {10T7} [3,4, 10] this residual tensile stress should promote the formation of transgranular hydride plates lying near the trace of the basal plane. However, two additional factors further complicate the picture. In slowly cooled samples, grain boundaries are often observed to be the preferred site of nucleation [10-12]. Whether this is the result of a favourable residual stress at a particular grain boundary location, or heterogeneous nucleation associated with particular defects at an interface remains a matter of conjecture. The second factor which confuses the issue is the role of (I102) twins in nucleating hydrides [4, 10, 13, 14]. We shall see that both these sites, in addition to intragranular precipitation, play an important part in the experiments reported below. Most zirconium alloys used in the nuclear industry as pressure tubes enter service in the cold worked condition, so there is interest in determining the effects of cold work on hydride precipitation. In a previous publication [5], we have shown that the pattern of hydride precipitation in fine-grained cold worked Zr-Nb alloys is determined (in part) by the residual stresses left in the alloy. At the time of that study, there were no reliable methods to measure the residual stresses in fine grained alloys, although the first report of residual stress measurements in coarse-grained Zircaloy using neutron diffraction had appeared [15]. Since then the neutron diffraction method has been widely used to probe the relationship between the thermal/elastic/plastic anisotropy of Zr and texture in the development of internal stresses in cold-worked alloys [9, 16, 17]. The present paper explores the influence of residual stress on hydride formation in coarse grained zircaloy rods subjected to prior deformation. 2. EXPERIMENTAL
R
+ Maximum
3.7
T
Fig. 1. Basal pole figure for Zircaloy-2 rod; L is the rod axis. plane. The specimens were then deformed to 4% in tension or 1/2% or 4% in compression, prior to being charged with deuterium. A layer of deuteride (hereafter called hydride) was first deposited onto the surface of the samples by electrolysis and then the deuterium was diffused into the bulk of the samples by heating for 47 h at 300°C. This procedure leads to an equivalent hydrogen concentration of ~60 ppm (by wt). The charging procedure was done at a temperature well below the recovery temperature of Zircaloy so that on cooling hydride precipitation occurred in a cold-worked matrix. 2.2. Neutron diffraction All measurements were made on the C5 neutron spectrometer at the Chalk River Laboratory's N.R.U. reactor. The incident beam was monochromated using the (331) reflection of a germanium crystal, to give 2=1.76164A. The sample was mounted on a Kappa goniometer, which could be programmed to set any given orientation of the sample along the bisector of the incident and scattered beams. The complete sample (6.5 mm in diameter x 19mm) was irradiated by the beam. The residual strains in the samples were monitored by measuring the interplanar spacings of the (0002), (10T0), (1]10), (10It) and (1012) planes. Measurements were taken over an angular range (~) from 60 to 90 ° where ~ is defined as the angle between the longitudinal axis of the rod (i.e. parallel to the tensile
2.1. Sample preparation Tensile and compressive specimens, each 6.5 mm in diameter and with a 19.0mm gauge length, were machined from a Zircaloy-2 rod having a composition given in Table 1 and a mean grain size of 20/~m. The texture of the rod, shown in Fig. 1, was similar to that used in earlier studies [9, 15]; the majority of the basal plane normals lay perpendicular to the rod-axis, but were randomly distributed in the
Table 1. Composition of Zircaloy-2 (wt%) Cr Fe Ni Sn O Zr (all other elements < 200 ppm)
0.12 0.14 0.06 1.41 1045 ppm Balance
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY
365
2sl.
or compressive axis) and the normal to the diffracting plane. As we are particularly interested in the strains of the basal plane, this angular range encompasses the majority of the grains in the rod (Fig. 1). For each Bragg plane, a Gaussian curve superimposed on a linearly varying background was used to fit the data. The mean position of the peak maximum was used to calculate the strains. The full width at half maximum can yield information on the distribution of strains within the volume of diffracting grains at a particular orientation. This distribution can arise from grain to grain variability or from strain gradients within a single grain in the sample volume being analyzed at a particular position [18].
0 0i0) 15 O ----..-~.
10 10il)
v
00
--"----49
-10
2.3. Electron microscopy
Three millimetre discs were removed from the samples, so that the disc normal lay parallel to the deformation axis. These discs were then electropolished at -55°C, 12V in a 10% perchloric acid, 90% methanol mixture and examined at 100 kV in a JEOL 100C electron microscope. The geometry of the samples was such that the relative orientation of the basal plane to the tensile axis was readily established from the tilt angle required to bring g = 0002 to the Bragg diffracting position in a given grain.
3. RESULTS 3.1. Tensile deformation (4- 4 % )
Figure 2 compares the values of the lattice strainst measured by neutron diffraction for the (0001), (1010), (10T1) and (10T2) planes after a prior tensile deformation of +4%. [The (1~10) planes showed almost identical strain values to the (10T0) planes over the range of ~t studied, and are therefore not included in Fig. 2]. The values of strain at ~t = 90 ° show the same trends as those previously reported by MacEwen et al. [15]. The strains on the (0002) and (10T0) planes are compressive and tensile respectively over the range of a studied, but in each case the magnitude of the strain decreases as ct decreases. The (10T7) planes (the hydride's habit plane) has not been directly measured, the probably behaviour can be inferred from the trends shown by the data. This estimate (the dotted line in Fig. 2) shows that over the range a = 90 to 60 ° (which accounts for about 90% of all the grains in the rod, when the texture is considered), the (1017) planes are in compression. The high residual strains found after tensile deformation are caused by the difficulty of pyramidal slip in Zr [15]. Compatibility of the strain tensor in each grain of a polycrystalline sample is dependent on a tThe strains were determined from the d values following the procedures described by MacEwen et al. [15].
.2sl,,,, 90
i,,,, 80
I , , , ,
70 Alpha (degrees)
I , , 60
Fig. 2. Zircaloy-2, deformed +4% showing the effect of plastic deformation and grain orientation on the residual lattice strains. The dotted line represent an estimated residual strain on the {101"7}plane (the hydride habit plane).
combination of plastic and elastic strain in each grain. As the stress for pyramidal {10I 1} (11 ~ ) slip (which gives a plastic strain in the ( c ) direction) is several times that required for prismatic (10r0} (11~0) slip (which only gives a plastic strain in the ( a ) direction), high residual stresses are found after unloading [15]. Evidence for {10T1} ( 1 1 ~ ) slip was obtained, but in all the examples studied the (c + a ) dislocations were very inhomogeneously distributed, in contrast to the more uniform distribution of ( a ) dislocations. The (c + a ) dislocations were edge in character and were frequently observed to form dipoles lying normal to the trace of the basal plane (Fig. 3). The slips bands (or more correctly their trace in the thin foil) lay either perpendicular to the at-~t grain boundaries (Fig. 3) or in a localized area near triple junctions (Fig. 4). Frequently grains were observed with no (c + a ) dislocations; conversely no grains were found with a uniform distribution of (c + a ) dislocations across the width of an entire grain. All of these observations suggest that there will be strain gradients within any grain, particularly at grain boundaries or triple points. (As noted earlier these gradients will contribute to the peak broadening observed in the neutron diffraction data [18].) The neutron diffraction results taken on their own point to a grain boundary rather than intergranular pattern of hydride precipitation as the {10T7} planes are in compression. The electron microscopy results suggest that this prediction might have to be modified if significant strain gradients exist within a single grain.
366
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY 25
2O
15
10
0o~'2)
o:
cooo
-10
(1010)
-15
-20 -25
f 9O
I
I
,
I , , , , I , 80
, 70
r III
I
60
Alpha (degrees)
Fig. 5. Zircaloy-2, deformed - 4 % showing the effect of plastic deformation and grain orientation on the residual lattice strains.
Fig. 3. Dislocation substructure of the Zirealoy-2 rod after + 4% deformation showing in (a) (a)-type dislocations and in (b) (a + c ) dislocations; the (a + c ) dislocations are edge in character and frequently form dipoles lying normal to the trace of the basal plane.
3.2. Compressive deformation ( - 4%) The strains measured by neutron diffraction after 4% compressive deformation are significantly smaller than those found after 4% tensile deformation. Over the same angular range 90 ~<~ ~<60°, the strains are
Fig. 4. Zircaloy-2, deformed +4% showing inhomogeneous distribution of (a + c) dislocations.
everywhere less than 5 x 10 -4 (Fig. 5). The basal planes are left in residual tension for 90/> ~ i> 70 °, but for ~t ~<70° they are in compression. The (10T0) planes are under compression for the angular range studied. The strains on the hydride habit planes, (10T7), again were not measured, but they are probably not very different from zero, at least for the range 90° ~<~ ~<70 °, which as we have noted before encompasses the majority of the grains in the rod texture sample. The difference in behaviour of the tensile and compressively loaded samples extended to identical plastic strains is associated with {10T2} twinning, observed only under compression condition in our tests (Fig. 6). This twinning system provides relief of strain in the ( c ) direction, and is obviously more effective than the (c + a ) slip systems in reducing the mean residual stresses. (c + a ) dislocations were again observed in these samples (Fig. 7). They had a similar grain to grain variability in their occurrence and location, as already noted for the tensile loaded specimens, but they were much shorter and no longer lay on a single trace. Further proof that {1012} twinning is effective in reducing the residual stresses on compressive loading is provided by the experiments of MacEwen et al. [17], who reported on the grain interaction stresses in samples of Zircaloy compressively loaded from 0.3 to 8% plastic strain. For ~ = 90 °, the residual strains rise rapidly to a plastic strain of 1%, but decline on further deformation as {10T2} twinning becomes prevalent [17]. The strain on the basal planes reaches
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY
I •i
/./
367
A
V / /jo
ta---{xlNIMIlIl~xal,r
I ~'~;~°"
Fig. 6. Zircaloy-2, deformed -4%E showing pronounced twinning. Most of the observed twins are of (1012) type. a maximum value of 4.2 x 10 - 4 at 1% plastic strain, but declines to about 2 x 10 -4 at 4% and 10 -4 at 8% prior plastic strain. On the other hand, the strain on
the {10i0} planes reaches a maximum (compressive) strain of - 6 x 10 -4 at 1% plastic strain, decreasing to about 5 x 10 -4 at 4% plastic strain. The {11~0} planes also experience a maximum compressive strain of - 6 x 10 -4, but on continued plastic straining these planes actually go into tension, and at 8% plastic strain the residual strain is + 3 x 10 -4. 3.3. Hydride precipitation
Fig. 7. Zircaloy-2, deformed - 4 % showing (a + c) dislocations. The (a + c ) dislocations are much shorter and "kinked" in comparison to those in +4% sample (see Figs 3 and 4).
Hydride precipitation was observed at grain boundaries, twin boundaries or as transgranular plates, the frequency of occurrence of each mode of precipitation being dependent on the prior thermal/ mechanical treatment of the alloy. The results are summarized in Table 2. The relative frequency with which each nucleation site was found is indicated by the number of asterisks in the table. Grain boundary nucleation dominated in the tensile-loaded samples while in the compression-loaded samples, the pattern depended on the prior plastic strain, transgranular hydrides being favoured after 1/2% strain, twin boundary hydrides after 4% strain. After annealing
368
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY Table 2. Hydrideprecipitation as a functionof thermomcehaniealtreatment Nucleation site Treatment Transgranular Grainboundary Twinboundary 1. Tensile, +4% * *** Plastic strain 2. Compression,-1/2% .4 ** Plastic strain 3. Compression,-4% ** Plastic strain 4. (1, 2, or 3) followed *** * by 3 hrs at 650°C furnace cooled *Infrequentlyobserved--****most frequentlyobserved.
for 3 h/650°C (which will reduce the residual strains to the values determined by MacEwen and Tom6 [9]), transgranular hydrides were most frequently observed. Both ~ and ~-hydrides have been reported to form in Zircaloy. In this study, analysis by electron diffraction was consistent with the f.c.c, structure of the tS-hydride phase (ZrH].5) rather than the ordered f.c.t, structure of y-hydride (ZrH), in agreement with other studies of slowly cooled Zircaloy/l,19/. In all cases, the hydride had an (111)~ approx II(0001)z. [11016 II [1 l~0]zr orientation relationship, irrespective of the nucleation site (Fig. 8). The second example illustrated in Fig. 9 shows a twin boundary-nucleated hydride in the 4% compression samples, together with the diffraction pattern recorded at a [011]~ II[~-110] zone axis. The angle between the close packed (1T1)~ and (0002)z~ planes is ~1 °. The hydride-twin interface contains the [01 rl]z~ direction (which is the shear direction associated with the (0112) [01Tl] twin system in zirconium) and this is nearly parallel to the [200] direction in the tS-phase [Fig. 9(b)]. We have previously reported that hydrides form stacked arrays, both in well annealed and coldworked zirconium alloys [5, 20,21]. These observations were supported by the results of this study.
We now believe that all macroscopic hydride plates form by the coalescence of smaller hydride units, irrespective of whether they have nucleated at grain or twin boundaries or are intragranular hydrides. The apparent habit plane of a hydride plate is determined by the "stacking angle" of the original stack (see Refs [5] or [20]), and the growth processes which lead to plate coalescence. A few examples of his phenomenon, which shed new light on the process, follow. A typical example of a "S-shaped" hydride spanning a single grain is seen in Fig. 10. Some of the individual plates which have formed the hydride are still visible. These have nearly flat facet planes, close to the basal plane, and correspond to the expected {10T7} habit plane in Zircaloy [3, 4, 10]. In the centre of the grain, several plates have coalesced so that the apparent habit plane is closer to {1011} than (0001). The plates in this micrograph have an identical orientation relationship with the matrix grain, so that the "S-shape" arises solely from the stacking arrangement. The development of this particular morphology, which was frequently observed in the cold worked sample s , suggests to us that there can be appreciable strain gradients within a single grain. Similar morphologies are found at grain boundary nucleated hydrides (Fig. 11). In the dark field micrograph of Fig. 11, a series of grain boundary nucleated
'\ \
\×
/
/
o,i° o,,:,
qoo'/
\.
B = [211olalI[o1116 Fig. 8. Orientation relationship between the or-matrix and 6-hydride illustrating that ~(IT1)r[I (0001")z, with ~ [01116fl[2IT0]z,.
PEROVIC
et al."
HYDRIDE PRECIPITATION IN ZIRCALOY
369
Fig. 9. Zircaloy-2, deformed - 4 % showing (a) twin boundary nucleated hydride (b) orientation relationship.
hydrides have started to form a stack of hydride plates which are growing transgranularly. The apparent habit plane of the plate varies fron one lying close to the grain boundary plane to being inclined at approximately 30 ° to the g.b. plane, but again the individual hydride plates have (1017) habit planes. Figure 12 shows a later stage in the development of a grain boundary hydride, whose apparent habit plane is again parallel to the grain boundary plane. The individual hydride plates which once composed the stack have "collapsed" into a single plate--the only trace of this process are the arrays of dislocations which tend to align in the directions once occupied by the external facet planes of the original hydrides (Fig. 12). The degree to which the "apparent" habit plane varies from {10T7} depends on the degree of deformation in the sample. In cold worked samples, "S"shaped hydrides were commonly observed (Fig. 10), but in well annealed samples, the hydride plates tend to align with low stack angles, as first described by
Westlake [4]. With this morphology the microscopic and macroscopic habit planes nearly coincide [Fig. 13(a)]. However at higher magnification using dark field [see e.g. Fig. 13(b)] there is again clear evidence that a macroscopic hydride plate has formed by the nucleation, growth and coalescence of microscopic hydrides. 4. DISCUSSION Before considering the results of this study which relate to hydride precipitation, it will be helpful to review the information provided by the neutron diffraction experiments. The strain values determined for a particular set of planes (Figs 2, 5) are the mean values for that subset of grains in the polycrystalline array which contribute to the diffracted intensity at a given specimen-detector configuration. The line broadening on the other hand is determined by the strain distribution in the same set of grains and arises both from grain to grain variability in the strain and
370
PEROVIC et aL: HYDRIDE PRECIPITATION IN ZIRCALOY
Fig. 12. A later stage in the development of a grain boundary hydride. The individual small hydride plates have collapsed into a single plate and the only traces of this process are the dislocation arrays.
Fig. 10. Array of small hydride plates forming an "S"shaped large hydride. The flat facet planes of small hydrides are close to the basal plane and correspond to the expected {1017} habit plane. from strain gradients within a single grain. The good agreement which MacEwen et al. [15] found between their strain measurements and plasticity theory was based on a comparison of the mean strain values (after converting to stresses) and the average stress state a,jr calculated from the known yield stress surface of Zircaloy and the texture of the alloy. These mean strain values can only be used as a rough guide to decide whether transgranular hydride formation is favoured or not. If a tensile strain on the basal plane favours the formation of near-basal plane hydrides, one would expect transgranular hydrides to form in the stress relieved and - 0 . 5 % deformed samples, but not in the + 4 % deformed sample. In fact transgranular hydrides are found in the latter
Fig. 11. Array of small hydride plates forming a "large" grain boundary hydride. Note that the stacking of small hydride plates forming "large" grain boundary hydrides and "large" transgranular hydrides are identical (see Fig. 13).
sample (see Table 2), although the dominant precipitation mode is intergranular. An argument based solely on mean strain values ignores the potent role of heterogeneous nucleation sites, as well as strain gradients and grain to grain variability. We believe that it is probably a combination of favourable heterogeneous nucleation site and a local tensile stress which determined the nucleation and subsequent growth pattern of hydrides. The importance of stress (strain) variability in a single grain was recognized by MacEwen et al. [18]. The maximum deviation in the stress (Aa~) from the mean value (,~) is expected to occur at grain boundary facets where the facet normal is parallel to a "hard" direction in one grain and a "soft" direction in the neighbouring grain (within a given grain, Aa~ can be + r e or - v e , but S(tr~ + Aa~) dv = 0 must integrate to zero over the cross-section in any volume of the body in the absence of external stresses). Similar ideas where developed by Jayaram and co-workers [22, 23] to explain the crystallographic orientation dependence of cracking in Co-WC cermets. The slip system of WC is {1100} (a + c ) so no plastic deformation is possible in response to a tensile stress parallel to the c-axis [22]. In the Co-WC system, the presence of cracked WC boundaries was found to correlate well with particular grain facet orientations where the c-axis in one or other grain was oriented at a large angle to the facet plane [22]. For rather similar reasons, hydrides would be expected to precipitate at grain boundary facets in Zircaloy where tensile stresses can develop. The pattern of (a + c ) slip observed in deformed samples (Figs 3 and 4) and the formation of "Sshaped" hydrides (Fig. 10) also point to the importance of the local stress state rather than the mean stress in biasing where hydrides nucleate and how they grow. The geometry of the stacking of small hydride plates is determined by the interaction of the self-stress associated with the parent and daughter
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY
371
predominantly tensile in nature, which is in agreement with the mean strain being slightly + v e for ~t i> 70 ° for a compressive prestrain of 0.5%. At the periphery of the grain, the residual pattern changes to one which (in this particular grain) favours a much lower stack angle. The role of twin boundaries in nucleating hydrides in samples compressively strained 4% must also be determined by the local residual stress state. After annealing at 650°C for 3 h, followed by furnace cooling, a treatment which leaves the twin boundaries intact but recovers the structure, hydride nucleation at twin boundaries is suppressed (see Table 2). The twin boundary itself does not appear to be an effective nucleating site in the annealed state, only after cold working. The reasons for this behaviour are unclear. 5. CONCLUSIONS Hydride nucleation and growth in cold-worked zircolay is determined by (a) the availability of suitable heterogeneous nucleation sites, and (b) the local stress state at grain boundary facets, twin boundaries and grain interiors. The mean internal stress state, as measured by neutron diffraction, correlated reasonably well with the observation of the formation of transgranular hydrides in annealed samples and samples compressed by 1/2% prior to charging with hydrogen. The formation and coalescence of hydride "stacks" leads to macroscopic hydride plates whose apparent habit plane can deviate significantly from the microscopic habit plane, and show a marked variation within a single grain. This variability is believed to reflect the rapid variation in internal stress state from one location to another within a single grain and from one grain facet to the next. Acknowledgements--The research was funded under the
CANDU Owners Group (COG), COG 31 (DHC and Fracture), WP 3510. G. C. Weatherly is grateful to the Natural Sciences and Engineering Research Council of Canada for support. The authors are also grateful to Mr R. Jarochowicz for technical assistance and Mr H. Seahra for charging the specimens with deuterium. Fig. 13. Zircaloy-2, well annealed showing (a) a large hydride with the "apparent" habit plane close to the {10T7}, (b) small hydride plates aligned with low stack angle and a habit plane close to the basal plane.
hydride phases and an external or internal stress distribution. High stack angles would be favoured by a tensile stress acting normal to the habit plane, while lower stack angles would be favoured by a shear stress acting in the same sense as the shear displacement associated with the hydride shape change [2, 3, 7, 20]. If this viewpoint is correct, the morphology of the hydride in Fig. 10 could be explained by assuming that in the centre of the grain the residual stresses (acting on the habit plane) are
REFERENCES
1. J. S. Bradbrook, G. W. Lorimer and N. Ridley, J. nucl. Mater. 42, 142 (1972). 2. G. J. C. Carpenter, Acta metall. 26, 1225 (1978). 3. G. C. Weatherly, Acta metall. 29, 501 (1981). 4. D. G. Westlake, J. nucl. Mater. 26, 208 (1968). 5. V. Perovic, G. C. Weatherly and C. J. Simpson, Acta metall. 31, 1381 (1983). 6. M. Jovanovic, A. Stern, H. Kneis, G. C. Weatherly and M. Leger, Can. Metall. Q. 27, 323 (1988). 7. v. Perovic, G. R. Purdy and L. M. Brown, Scripta metall. 15, 217 (1981). 8. H. K. D. Bhadeshia and D. V. Edmonds, Acta metall. 28, 1265 (1980). 9. S. R. MacEwen and C. Tome, ASTM STP 939, 631 (1987). 10. C. E. Elis, J. nucl. Mater. 28, 129 (1968).
372
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY
11. J. E. Bailey, Acta metall. 11, 267 (1963). 12. V. S. Arunachalam, B. Lihtinin and G. Ostberg, J. nucl. Mater. 21, 241 (1967). 13. D. G. Westlake and E. S. Fisher, Trans. T.M.S.A.LM.E. 224, 254 (1962). 14. C. Ray, Atomic Energy of Canada Limited Report, AECL 2095 (1964). 15. S. R. MacEwen, J. Faber and A. P. L. Turner, Acta metall. 31, 657 (1983). 16. S. R. MacEwen, N. Christodoulou, C. Tome, J. Jackman, T. M. Holden, J. Faber and R. L. Hitterman, Proc. 8th Int. Conf. on Texture of Materials (edited by J. S. KaUend and G. Gottstein), p. 825. Metall Soc. Warrendale, Pa (1988).
17. S. R. MacEwen, N. Christodoulou and A. SalinasRodrigez, Metall. Trans. 21A, 1083 (1990). 18. S. R. MacEwen, J. Faber and A. P. L. Turner, Scripta metall. 18, 629 (1984). 19. B. Nath, G. W. Lorimer and N. Ridley, J. nuel. Mater. 49, 262 (1973). 20. V. Perovic, G. C. Weatherly and C. J. Simpson, Seripta metall. 16, 409 (1982). 21. V. Perovic and (3. C. Weatherly, J. nucl. Mater. 126, 160 (1984). 22. V. Jayaram, R. Sinclair and D. J. Rowclife, Acta metall. 31, 373 (1983). 23. V. Jayaram, Acta metall. 35, 1307 (1987).
0956-7151/92$5.00+ 0.00 Copyright © 1992PergamonPress pie
THE I N F L U E N C E OF PRIOR D E F O R M A T I O N ON H Y D R I D E PRECIPITATION IN ZIRCALOY V. P E R O V I C 1, G. C. W E A T H E R L Y 2, S. R. MacEWEN3't and M. L E G E R I 'Metallurgical Research Department, Ontario Hydro Research Division, 800 Kipling Avenue, Toronto, Ontario, Canada M8Z 5S4, 2Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4M1 and 3Advanced Materials Research Branch, Chalk River Nuclear Research Laboratories, Atomic Energy of Canada Ltd, Chalk River, Ontario, Canada K0J lJO (Received 14 May 1991)
Abstract--Precipitation of hydrides has been studied in samples of Zircaloy subjected to prior tensile or compressive deformation before charging with hydrogen. The mean residual stress pattern in the alloys prior to charging was assessed by neutron diffraction techniques and provided a rough guide as to the preferred site of hydride nucleation. Heterogeneous hydride nucleation at grain boundaries or twin boundaries was commonly found in samples subjected to 4% prior deformation, while transgranular hydrides were most frequently observed after a prior 1/2% compressive deformation or an annealing. The local stress state at grain boundary facets or twins is thought to be the deciding factor in determining where hydrides nucleate and how hydride stacks form. R~sum~-On 6tudie la precipitation d'hydrures dans des 6chantillons de Zircaloy soumis fi une dfformation en traction ou en compression avant qu'on les charge en hydrog~ne. Le diagramme des contraintes r~siduelles moyennes dans les alliages avant la charge est obtenu par diffraction des neutrons et fournit u n guide grossier des sites prrfrrentiels de germination des hydrures. On trouve tr~s souvent une germination h~t~rog~ne des hydrures aux joints de grains ou de macle dans les ~chantillons soumis /t 4% de d~formation initiale, tandis que les hydrures intergranulaires sont plus fr~quemment observes apr~s une drformation initiale par compression de 0,5% ou un recuit. On pense que l'rtat de contrainte locale sur les facettes des joints de grains ou de macle est le facteur prrpond~rant qui drtermine o~ la germination des hydrures se produit et comment les empilements d'hydrure se forment.
Zusmmenfassung--Die Ausscheidung von Hydriden wird in Zirkaloy-Proben, die vor der WasserstoffBeladung einer Zug- oder Druckverformung unterworfen worden sind, untersucht. Die Verteilung der Restspannungen in den Legierungen vor der Wassserstoff-Beladung wird mittels Neutronenbeugung bestimmt und liefert einen groben Hinweis auf die bevorzugten Orte der Hydridbildung. Heterogene Hydridbildung an Korn- oder Zwillingsgrenzen wird iiblicherweise in Proben gefunden, die 4% vorverforint waren, wohingegen transgranulare Hydride am h/iufigsten nach einer Druckverformung yon 1/2% oder nach Ausheilen auftreten. Es wird angenommen, dab der lokale Spannungszustand an den Korngrenzfacetten oder Zwillingen entscheidend mitbestimmt, wo sich Hydride bilden und wie sie Hydridstapel formen.
1. INTRODUCTION When zirconium hydride precipitates in zirconium or its alloys at low temperatures, it does so by a displacive transformation, which involves both a shear and local volume change of the parent lattice [1-3]. A n u m b e r of interesting phenomena are associated with the transformation. Growth of large hydride plates appears to involve the repeated nucleation and growth of hydride sub units which eventually coalesce and grow into macroscopic hydride plates [4, 5]. A supersaturation of hydrogen (well above the terminal solid solubility measured in tPresent address: Alcan International Ltd, R&D Centre, PBO 8400, Kingston, Ontario, Canada K7L 5L9.
a dissolution experiment) is thus required to drive the growth of hydride plates [6]. The apparent habit plane of hydride plates is determined by the stacking of the smaller hydride plates, as first determined by Perovic et al. [7]. In this sense, the transformation bears a striking resemblance to the bainite transformation in steels, where similar morphological features have been reported [8]. In common with many martensitic or bainitic transformations, hydride precipitation can be controlled by a stress, either external or internal. A stress can help drive the shear component of the transformation or to accommodate the strains associated with the volume change of the hydride plate normal to its habit plane [3, 5]. The role of stress is complicated however, because even in well annealed zirconium alloys, large internal stresses can exist in polycrystals 363
364
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY
because of the pronounced anisotropy in the thermal expansivity and elastic properties of zirconium. Indeed it is unlikely that polycrystalline zirconium can be produced free of internal stresses. In a recent study of Zircaloy with a rod texture, MacEwen and Tome [9] have shown that samples annealed in the -phase at 600°C develop a residual stress ~ 100 MPa on the basal plane on cooling to room temperature. For the particular texture of the alloy they studied, this basal plane stress was effectively independent of grain orientation. As hydrides in zirconium alloys generally precipitate as plates with habit planes close to the basal plane, {10T7} [3,4, 10] this residual tensile stress should promote the formation of transgranular hydride plates lying near the trace of the basal plane. However, two additional factors further complicate the picture. In slowly cooled samples, grain boundaries are often observed to be the preferred site of nucleation [10-12]. Whether this is the result of a favourable residual stress at a particular grain boundary location, or heterogeneous nucleation associated with particular defects at an interface remains a matter of conjecture. The second factor which confuses the issue is the role of (I102) twins in nucleating hydrides [4, 10, 13, 14]. We shall see that both these sites, in addition to intragranular precipitation, play an important part in the experiments reported below. Most zirconium alloys used in the nuclear industry as pressure tubes enter service in the cold worked condition, so there is interest in determining the effects of cold work on hydride precipitation. In a previous publication [5], we have shown that the pattern of hydride precipitation in fine-grained cold worked Zr-Nb alloys is determined (in part) by the residual stresses left in the alloy. At the time of that study, there were no reliable methods to measure the residual stresses in fine grained alloys, although the first report of residual stress measurements in coarse-grained Zircaloy using neutron diffraction had appeared [15]. Since then the neutron diffraction method has been widely used to probe the relationship between the thermal/elastic/plastic anisotropy of Zr and texture in the development of internal stresses in cold-worked alloys [9, 16, 17]. The present paper explores the influence of residual stress on hydride formation in coarse grained zircaloy rods subjected to prior deformation. 2. EXPERIMENTAL
R
+ Maximum
3.7
T
Fig. 1. Basal pole figure for Zircaloy-2 rod; L is the rod axis. plane. The specimens were then deformed to 4% in tension or 1/2% or 4% in compression, prior to being charged with deuterium. A layer of deuteride (hereafter called hydride) was first deposited onto the surface of the samples by electrolysis and then the deuterium was diffused into the bulk of the samples by heating for 47 h at 300°C. This procedure leads to an equivalent hydrogen concentration of ~60 ppm (by wt). The charging procedure was done at a temperature well below the recovery temperature of Zircaloy so that on cooling hydride precipitation occurred in a cold-worked matrix. 2.2. Neutron diffraction All measurements were made on the C5 neutron spectrometer at the Chalk River Laboratory's N.R.U. reactor. The incident beam was monochromated using the (331) reflection of a germanium crystal, to give 2=1.76164A. The sample was mounted on a Kappa goniometer, which could be programmed to set any given orientation of the sample along the bisector of the incident and scattered beams. The complete sample (6.5 mm in diameter x 19mm) was irradiated by the beam. The residual strains in the samples were monitored by measuring the interplanar spacings of the (0002), (10T0), (1]10), (10It) and (1012) planes. Measurements were taken over an angular range (~) from 60 to 90 ° where ~ is defined as the angle between the longitudinal axis of the rod (i.e. parallel to the tensile
2.1. Sample preparation Tensile and compressive specimens, each 6.5 mm in diameter and with a 19.0mm gauge length, were machined from a Zircaloy-2 rod having a composition given in Table 1 and a mean grain size of 20/~m. The texture of the rod, shown in Fig. 1, was similar to that used in earlier studies [9, 15]; the majority of the basal plane normals lay perpendicular to the rod-axis, but were randomly distributed in the
Table 1. Composition of Zircaloy-2 (wt%) Cr Fe Ni Sn O Zr (all other elements < 200 ppm)
0.12 0.14 0.06 1.41 1045 ppm Balance
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY
365
2sl.
or compressive axis) and the normal to the diffracting plane. As we are particularly interested in the strains of the basal plane, this angular range encompasses the majority of the grains in the rod (Fig. 1). For each Bragg plane, a Gaussian curve superimposed on a linearly varying background was used to fit the data. The mean position of the peak maximum was used to calculate the strains. The full width at half maximum can yield information on the distribution of strains within the volume of diffracting grains at a particular orientation. This distribution can arise from grain to grain variability or from strain gradients within a single grain in the sample volume being analyzed at a particular position [18].
0 0i0) 15 O ----..-~.
10 10il)
v
00
--"----49
-10
2.3. Electron microscopy
Three millimetre discs were removed from the samples, so that the disc normal lay parallel to the deformation axis. These discs were then electropolished at -55°C, 12V in a 10% perchloric acid, 90% methanol mixture and examined at 100 kV in a JEOL 100C electron microscope. The geometry of the samples was such that the relative orientation of the basal plane to the tensile axis was readily established from the tilt angle required to bring g = 0002 to the Bragg diffracting position in a given grain.
3. RESULTS 3.1. Tensile deformation (4- 4 % )
Figure 2 compares the values of the lattice strainst measured by neutron diffraction for the (0001), (1010), (10T1) and (10T2) planes after a prior tensile deformation of +4%. [The (1~10) planes showed almost identical strain values to the (10T0) planes over the range of ~t studied, and are therefore not included in Fig. 2]. The values of strain at ~t = 90 ° show the same trends as those previously reported by MacEwen et al. [15]. The strains on the (0002) and (10T0) planes are compressive and tensile respectively over the range of a studied, but in each case the magnitude of the strain decreases as ct decreases. The (10T7) planes (the hydride's habit plane) has not been directly measured, the probably behaviour can be inferred from the trends shown by the data. This estimate (the dotted line in Fig. 2) shows that over the range a = 90 to 60 ° (which accounts for about 90% of all the grains in the rod, when the texture is considered), the (1017) planes are in compression. The high residual strains found after tensile deformation are caused by the difficulty of pyramidal slip in Zr [15]. Compatibility of the strain tensor in each grain of a polycrystalline sample is dependent on a tThe strains were determined from the d values following the procedures described by MacEwen et al. [15].
.2sl,,,, 90
i,,,, 80
I , , , ,
70 Alpha (degrees)
I , , 60
Fig. 2. Zircaloy-2, deformed +4% showing the effect of plastic deformation and grain orientation on the residual lattice strains. The dotted line represent an estimated residual strain on the {101"7}plane (the hydride habit plane).
combination of plastic and elastic strain in each grain. As the stress for pyramidal {10I 1} (11 ~ ) slip (which gives a plastic strain in the ( c ) direction) is several times that required for prismatic (10r0} (11~0) slip (which only gives a plastic strain in the ( a ) direction), high residual stresses are found after unloading [15]. Evidence for {10T1} ( 1 1 ~ ) slip was obtained, but in all the examples studied the (c + a ) dislocations were very inhomogeneously distributed, in contrast to the more uniform distribution of ( a ) dislocations. The (c + a ) dislocations were edge in character and were frequently observed to form dipoles lying normal to the trace of the basal plane (Fig. 3). The slips bands (or more correctly their trace in the thin foil) lay either perpendicular to the at-~t grain boundaries (Fig. 3) or in a localized area near triple junctions (Fig. 4). Frequently grains were observed with no (c + a ) dislocations; conversely no grains were found with a uniform distribution of (c + a ) dislocations across the width of an entire grain. All of these observations suggest that there will be strain gradients within any grain, particularly at grain boundaries or triple points. (As noted earlier these gradients will contribute to the peak broadening observed in the neutron diffraction data [18].) The neutron diffraction results taken on their own point to a grain boundary rather than intergranular pattern of hydride precipitation as the {10T7} planes are in compression. The electron microscopy results suggest that this prediction might have to be modified if significant strain gradients exist within a single grain.
366
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY 25
2O
15
10
0o~'2)
o:
cooo
-10
(1010)
-15
-20 -25
f 9O
I
I
,
I , , , , I , 80
, 70
r III
I
60
Alpha (degrees)
Fig. 5. Zircaloy-2, deformed - 4 % showing the effect of plastic deformation and grain orientation on the residual lattice strains.
Fig. 3. Dislocation substructure of the Zirealoy-2 rod after + 4% deformation showing in (a) (a)-type dislocations and in (b) (a + c ) dislocations; the (a + c ) dislocations are edge in character and frequently form dipoles lying normal to the trace of the basal plane.
3.2. Compressive deformation ( - 4%) The strains measured by neutron diffraction after 4% compressive deformation are significantly smaller than those found after 4% tensile deformation. Over the same angular range 90 ~<~ ~<60°, the strains are
Fig. 4. Zircaloy-2, deformed +4% showing inhomogeneous distribution of (a + c) dislocations.
everywhere less than 5 x 10 -4 (Fig. 5). The basal planes are left in residual tension for 90/> ~ i> 70 °, but for ~t ~<70° they are in compression. The (10T0) planes are under compression for the angular range studied. The strains on the hydride habit planes, (10T7), again were not measured, but they are probably not very different from zero, at least for the range 90° ~<~ ~<70 °, which as we have noted before encompasses the majority of the grains in the rod texture sample. The difference in behaviour of the tensile and compressively loaded samples extended to identical plastic strains is associated with {10T2} twinning, observed only under compression condition in our tests (Fig. 6). This twinning system provides relief of strain in the ( c ) direction, and is obviously more effective than the (c + a ) slip systems in reducing the mean residual stresses. (c + a ) dislocations were again observed in these samples (Fig. 7). They had a similar grain to grain variability in their occurrence and location, as already noted for the tensile loaded specimens, but they were much shorter and no longer lay on a single trace. Further proof that {1012} twinning is effective in reducing the residual stresses on compressive loading is provided by the experiments of MacEwen et al. [17], who reported on the grain interaction stresses in samples of Zircaloy compressively loaded from 0.3 to 8% plastic strain. For ~ = 90 °, the residual strains rise rapidly to a plastic strain of 1%, but decline on further deformation as {10T2} twinning becomes prevalent [17]. The strain on the basal planes reaches
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY
I •i
/./
367
A
V / /jo
ta---{xlNIMIlIl~xal,r
I ~'~;~°"
Fig. 6. Zircaloy-2, deformed -4%E showing pronounced twinning. Most of the observed twins are of (1012) type. a maximum value of 4.2 x 10 - 4 at 1% plastic strain, but declines to about 2 x 10 -4 at 4% and 10 -4 at 8% prior plastic strain. On the other hand, the strain on
the {10i0} planes reaches a maximum (compressive) strain of - 6 x 10 -4 at 1% plastic strain, decreasing to about 5 x 10 -4 at 4% plastic strain. The {11~0} planes also experience a maximum compressive strain of - 6 x 10 -4, but on continued plastic straining these planes actually go into tension, and at 8% plastic strain the residual strain is + 3 x 10 -4. 3.3. Hydride precipitation
Fig. 7. Zircaloy-2, deformed - 4 % showing (a + c) dislocations. The (a + c ) dislocations are much shorter and "kinked" in comparison to those in +4% sample (see Figs 3 and 4).
Hydride precipitation was observed at grain boundaries, twin boundaries or as transgranular plates, the frequency of occurrence of each mode of precipitation being dependent on the prior thermal/ mechanical treatment of the alloy. The results are summarized in Table 2. The relative frequency with which each nucleation site was found is indicated by the number of asterisks in the table. Grain boundary nucleation dominated in the tensile-loaded samples while in the compression-loaded samples, the pattern depended on the prior plastic strain, transgranular hydrides being favoured after 1/2% strain, twin boundary hydrides after 4% strain. After annealing
368
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY Table 2. Hydrideprecipitation as a functionof thermomcehaniealtreatment Nucleation site Treatment Transgranular Grainboundary Twinboundary 1. Tensile, +4% * *** Plastic strain 2. Compression,-1/2% .4 ** Plastic strain 3. Compression,-4% ** Plastic strain 4. (1, 2, or 3) followed *** * by 3 hrs at 650°C furnace cooled *Infrequentlyobserved--****most frequentlyobserved.
for 3 h/650°C (which will reduce the residual strains to the values determined by MacEwen and Tom6 [9]), transgranular hydrides were most frequently observed. Both ~ and ~-hydrides have been reported to form in Zircaloy. In this study, analysis by electron diffraction was consistent with the f.c.c, structure of the tS-hydride phase (ZrH].5) rather than the ordered f.c.t, structure of y-hydride (ZrH), in agreement with other studies of slowly cooled Zircaloy/l,19/. In all cases, the hydride had an (111)~ approx II(0001)z. [11016 II [1 l~0]zr orientation relationship, irrespective of the nucleation site (Fig. 8). The second example illustrated in Fig. 9 shows a twin boundary-nucleated hydride in the 4% compression samples, together with the diffraction pattern recorded at a [011]~ II[~-110] zone axis. The angle between the close packed (1T1)~ and (0002)z~ planes is ~1 °. The hydride-twin interface contains the [01 rl]z~ direction (which is the shear direction associated with the (0112) [01Tl] twin system in zirconium) and this is nearly parallel to the [200] direction in the tS-phase [Fig. 9(b)]. We have previously reported that hydrides form stacked arrays, both in well annealed and coldworked zirconium alloys [5, 20,21]. These observations were supported by the results of this study.
We now believe that all macroscopic hydride plates form by the coalescence of smaller hydride units, irrespective of whether they have nucleated at grain or twin boundaries or are intragranular hydrides. The apparent habit plane of a hydride plate is determined by the "stacking angle" of the original stack (see Refs [5] or [20]), and the growth processes which lead to plate coalescence. A few examples of his phenomenon, which shed new light on the process, follow. A typical example of a "S-shaped" hydride spanning a single grain is seen in Fig. 10. Some of the individual plates which have formed the hydride are still visible. These have nearly flat facet planes, close to the basal plane, and correspond to the expected {10T7} habit plane in Zircaloy [3, 4, 10]. In the centre of the grain, several plates have coalesced so that the apparent habit plane is closer to {1011} than (0001). The plates in this micrograph have an identical orientation relationship with the matrix grain, so that the "S-shape" arises solely from the stacking arrangement. The development of this particular morphology, which was frequently observed in the cold worked sample s , suggests to us that there can be appreciable strain gradients within a single grain. Similar morphologies are found at grain boundary nucleated hydrides (Fig. 11). In the dark field micrograph of Fig. 11, a series of grain boundary nucleated
'\ \
\×
/
/
o,i° o,,:,
qoo'/
\.
B = [211olalI[o1116 Fig. 8. Orientation relationship between the or-matrix and 6-hydride illustrating that ~(IT1)r[I (0001")z, with ~ [01116fl[2IT0]z,.
PEROVIC
et al."
HYDRIDE PRECIPITATION IN ZIRCALOY
369
Fig. 9. Zircaloy-2, deformed - 4 % showing (a) twin boundary nucleated hydride (b) orientation relationship.
hydrides have started to form a stack of hydride plates which are growing transgranularly. The apparent habit plane of the plate varies fron one lying close to the grain boundary plane to being inclined at approximately 30 ° to the g.b. plane, but again the individual hydride plates have (1017) habit planes. Figure 12 shows a later stage in the development of a grain boundary hydride, whose apparent habit plane is again parallel to the grain boundary plane. The individual hydride plates which once composed the stack have "collapsed" into a single plate--the only trace of this process are the arrays of dislocations which tend to align in the directions once occupied by the external facet planes of the original hydrides (Fig. 12). The degree to which the "apparent" habit plane varies from {10T7} depends on the degree of deformation in the sample. In cold worked samples, "S"shaped hydrides were commonly observed (Fig. 10), but in well annealed samples, the hydride plates tend to align with low stack angles, as first described by
Westlake [4]. With this morphology the microscopic and macroscopic habit planes nearly coincide [Fig. 13(a)]. However at higher magnification using dark field [see e.g. Fig. 13(b)] there is again clear evidence that a macroscopic hydride plate has formed by the nucleation, growth and coalescence of microscopic hydrides. 4. DISCUSSION Before considering the results of this study which relate to hydride precipitation, it will be helpful to review the information provided by the neutron diffraction experiments. The strain values determined for a particular set of planes (Figs 2, 5) are the mean values for that subset of grains in the polycrystalline array which contribute to the diffracted intensity at a given specimen-detector configuration. The line broadening on the other hand is determined by the strain distribution in the same set of grains and arises both from grain to grain variability in the strain and
370
PEROVIC et aL: HYDRIDE PRECIPITATION IN ZIRCALOY
Fig. 12. A later stage in the development of a grain boundary hydride. The individual small hydride plates have collapsed into a single plate and the only traces of this process are the dislocation arrays.
Fig. 10. Array of small hydride plates forming an "S"shaped large hydride. The flat facet planes of small hydrides are close to the basal plane and correspond to the expected {1017} habit plane. from strain gradients within a single grain. The good agreement which MacEwen et al. [15] found between their strain measurements and plasticity theory was based on a comparison of the mean strain values (after converting to stresses) and the average stress state a,jr calculated from the known yield stress surface of Zircaloy and the texture of the alloy. These mean strain values can only be used as a rough guide to decide whether transgranular hydride formation is favoured or not. If a tensile strain on the basal plane favours the formation of near-basal plane hydrides, one would expect transgranular hydrides to form in the stress relieved and - 0 . 5 % deformed samples, but not in the + 4 % deformed sample. In fact transgranular hydrides are found in the latter
Fig. 11. Array of small hydride plates forming a "large" grain boundary hydride. Note that the stacking of small hydride plates forming "large" grain boundary hydrides and "large" transgranular hydrides are identical (see Fig. 13).
sample (see Table 2), although the dominant precipitation mode is intergranular. An argument based solely on mean strain values ignores the potent role of heterogeneous nucleation sites, as well as strain gradients and grain to grain variability. We believe that it is probably a combination of favourable heterogeneous nucleation site and a local tensile stress which determined the nucleation and subsequent growth pattern of hydrides. The importance of stress (strain) variability in a single grain was recognized by MacEwen et al. [18]. The maximum deviation in the stress (Aa~) from the mean value (,~) is expected to occur at grain boundary facets where the facet normal is parallel to a "hard" direction in one grain and a "soft" direction in the neighbouring grain (within a given grain, Aa~ can be + r e or - v e , but S(tr~ + Aa~) dv = 0 must integrate to zero over the cross-section in any volume of the body in the absence of external stresses). Similar ideas where developed by Jayaram and co-workers [22, 23] to explain the crystallographic orientation dependence of cracking in Co-WC cermets. The slip system of WC is {1100} (a + c ) so no plastic deformation is possible in response to a tensile stress parallel to the c-axis [22]. In the Co-WC system, the presence of cracked WC boundaries was found to correlate well with particular grain facet orientations where the c-axis in one or other grain was oriented at a large angle to the facet plane [22]. For rather similar reasons, hydrides would be expected to precipitate at grain boundary facets in Zircaloy where tensile stresses can develop. The pattern of (a + c ) slip observed in deformed samples (Figs 3 and 4) and the formation of "Sshaped" hydrides (Fig. 10) also point to the importance of the local stress state rather than the mean stress in biasing where hydrides nucleate and how they grow. The geometry of the stacking of small hydride plates is determined by the interaction of the self-stress associated with the parent and daughter
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY
371
predominantly tensile in nature, which is in agreement with the mean strain being slightly + v e for ~t i> 70 ° for a compressive prestrain of 0.5%. At the periphery of the grain, the residual pattern changes to one which (in this particular grain) favours a much lower stack angle. The role of twin boundaries in nucleating hydrides in samples compressively strained 4% must also be determined by the local residual stress state. After annealing at 650°C for 3 h, followed by furnace cooling, a treatment which leaves the twin boundaries intact but recovers the structure, hydride nucleation at twin boundaries is suppressed (see Table 2). The twin boundary itself does not appear to be an effective nucleating site in the annealed state, only after cold working. The reasons for this behaviour are unclear. 5. CONCLUSIONS Hydride nucleation and growth in cold-worked zircolay is determined by (a) the availability of suitable heterogeneous nucleation sites, and (b) the local stress state at grain boundary facets, twin boundaries and grain interiors. The mean internal stress state, as measured by neutron diffraction, correlated reasonably well with the observation of the formation of transgranular hydrides in annealed samples and samples compressed by 1/2% prior to charging with hydrogen. The formation and coalescence of hydride "stacks" leads to macroscopic hydride plates whose apparent habit plane can deviate significantly from the microscopic habit plane, and show a marked variation within a single grain. This variability is believed to reflect the rapid variation in internal stress state from one location to another within a single grain and from one grain facet to the next. Acknowledgements--The research was funded under the
CANDU Owners Group (COG), COG 31 (DHC and Fracture), WP 3510. G. C. Weatherly is grateful to the Natural Sciences and Engineering Research Council of Canada for support. The authors are also grateful to Mr R. Jarochowicz for technical assistance and Mr H. Seahra for charging the specimens with deuterium. Fig. 13. Zircaloy-2, well annealed showing (a) a large hydride with the "apparent" habit plane close to the {10T7}, (b) small hydride plates aligned with low stack angle and a habit plane close to the basal plane.
hydride phases and an external or internal stress distribution. High stack angles would be favoured by a tensile stress acting normal to the habit plane, while lower stack angles would be favoured by a shear stress acting in the same sense as the shear displacement associated with the hydride shape change [2, 3, 7, 20]. If this viewpoint is correct, the morphology of the hydride in Fig. 10 could be explained by assuming that in the centre of the grain the residual stresses (acting on the habit plane) are
REFERENCES
1. J. S. Bradbrook, G. W. Lorimer and N. Ridley, J. nucl. Mater. 42, 142 (1972). 2. G. J. C. Carpenter, Acta metall. 26, 1225 (1978). 3. G. C. Weatherly, Acta metall. 29, 501 (1981). 4. D. G. Westlake, J. nucl. Mater. 26, 208 (1968). 5. V. Perovic, G. C. Weatherly and C. J. Simpson, Acta metall. 31, 1381 (1983). 6. M. Jovanovic, A. Stern, H. Kneis, G. C. Weatherly and M. Leger, Can. Metall. Q. 27, 323 (1988). 7. v. Perovic, G. R. Purdy and L. M. Brown, Scripta metall. 15, 217 (1981). 8. H. K. D. Bhadeshia and D. V. Edmonds, Acta metall. 28, 1265 (1980). 9. S. R. MacEwen and C. Tome, ASTM STP 939, 631 (1987). 10. C. E. Elis, J. nucl. Mater. 28, 129 (1968).
372
PEROVIC et al.: HYDRIDE PRECIPITATION IN ZIRCALOY
11. J. E. Bailey, Acta metall. 11, 267 (1963). 12. V. S. Arunachalam, B. Lihtinin and G. Ostberg, J. nucl. Mater. 21, 241 (1967). 13. D. G. Westlake and E. S. Fisher, Trans. T.M.S.A.LM.E. 224, 254 (1962). 14. C. Ray, Atomic Energy of Canada Limited Report, AECL 2095 (1964). 15. S. R. MacEwen, J. Faber and A. P. L. Turner, Acta metall. 31, 657 (1983). 16. S. R. MacEwen, N. Christodoulou, C. Tome, J. Jackman, T. M. Holden, J. Faber and R. L. Hitterman, Proc. 8th Int. Conf. on Texture of Materials (edited by J. S. KaUend and G. Gottstein), p. 825. Metall Soc. Warrendale, Pa (1988).
17. S. R. MacEwen, N. Christodoulou and A. SalinasRodrigez, Metall. Trans. 21A, 1083 (1990). 18. S. R. MacEwen, J. Faber and A. P. L. Turner, Scripta metall. 18, 629 (1984). 19. B. Nath, G. W. Lorimer and N. Ridley, J. nuel. Mater. 49, 262 (1973). 20. V. Perovic, G. C. Weatherly and C. J. Simpson, Seripta metall. 16, 409 (1982). 21. V. Perovic and (3. C. Weatherly, J. nucl. Mater. 126, 160 (1984). 22. V. Jayaram, R. Sinclair and D. J. Rowclife, Acta metall. 31, 373 (1983). 23. V. Jayaram, Acta metall. 35, 1307 (1987).