- Email: [email protected]
Dislocations generated by zirconium hydride precipitates in zirconium and some of its alloys
JOURNAL OF NUCLEAR MATERIALS 48 (1973) 267-276.0
NORTH-HOLLAND PUBLISHING COMPANY
DISLOCATIONS GENERATED BY ZIRCONIUM HYDRIDE PRECIPITATES IN ZIRCONIUM AND SOME OF ITS ALLOYS G.J.C. CARPENTER,
J.F. WATTERS and R.W. GILBERT
Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories* Chalk River, Ontario, Canada
Received 28 June 1973 The character of dislocations emitted during the precipitation of rzirconium hydride in zirconium, Zircaloy-2, Zr-1% Al and Zr-1% Cr has been determined. Contrast experiments in the transmission electron microscope have shown that the dislocations possess Burgers vectors (b) of the type +a ( 1130). Of the three possible b’s only one or two are observed associated with~a given hydride needle, those giving a large component of b along a direction perpendicular to the needle. Dislocation generation is thought to result from the dilatational misfit associated with the hydride needles. The creation of dislocations with b’s along the c-axis is more difficult and the misfit strain along this direction is not relieved by plastic deformation. Le caractkre des dislocations imises au tours de la pricipitation des hydrures de zirconium y dans le zirconium, dans le Zircaloy 2, dans les alliages Zr-Al 1% et Zr-Cr l%a Ctt?dktermink. Les essais de contraste en microscopic dlectronique en transmission ont montre que les dislocations posskdent des vecteurs de Burgers (6) du type fa (1120). Des trois vecteurs b possibles un ou deux seulement ont 6th observ& associ& g une aiguille don&e d’hydrure, ceux donnant une forte composante de b le long d’une direction perpendiculaire g I’aiguille. La formation des dislocations resulterait du manque d’accommodation par dilatation associke aux aiguilles d’hydrures. La creation de dislocations avec des vecteurs b le long de l’axe c est plus difficile et la deformation par manque d’accommodation le long de cette direction n’est pas relaxee par deformation plastique. Es wurde die Art der Versetzungen untersucht, die bei der Ausscheidung von y-Zirkonhydrid in Zirkon, Zircaloy-2, Zr-1% Al und Zr-1 % Cr entstehen. Transmissionselektronenmikroskopische Untersuchungen nach dem Kontrastverfahren ergeben, dass die Versetzungen einen Burgers-Vektor der Form ia Cll?O) haben. Von den drei maglichen b-Vektoren werden nur einer oder zwei beobachtet, die zusammen mit dem vorleigenden nadelfilrmigen Hydrid auftreten und eine grosse Komponente von b senkrecht zu den Nadeln haben. Die Entstehung der Versetzungen beruht vermutlich auf der Dehnung, die mit dem nadelf&migen Hydrid auftritt. Die Bildung von Versetzungen mit b-Vektoren parallel zur c-Achse ist schwieriger, die Dehnung in dieser Richtung wird durch plastische Deformation nlcht erleichtert.
1. Introduction The importance of hydride formation in zirconium alloys used for structural and cladding applications in water-cooled reactors (such as the CANDU system) has led to numerous papers on the subject. Many of these papers are discussed in a review by Ells [ 11. More recently, results of a detailed study of the crystallography of hydride formation in zirconium and Zircaloy-2 have been published by Lorimer, Ridley and co-workers [2,3]. Their work complements, and in certain respects, (in particular the orientation relationship) contradicts earlier results obtained by Bailey 141. They show that fairly rapid cooling (7 lO”C/min) causes the formation of metastable y-zirconium hy-
dride in the form of needles lying along (1120) matrix directions. A feature of-this reaction is the generation of dislocations by the hydride precipitates. Bailey [4] suggested that the dislocations were probably in the form of vacancy loops produced by prismatic punching to relieve misfit stresses. The work reported here describes some contrast experiments designed to characterize these dislocations in more detail.
2. Experimental The composition and principal impurities of the materials examined are listed in table 1, together with the heat treatment.
268
G.J.C. Carpenter et al., Zirconium hydride precipitates in zirconium
Table 1 Composition
(by weight)
Material
and heat treatment Alloying
of the materials
used.
elements
Zr
Zircaloy-2
ZIP 1% Al
Sn-1.36% Fe-O. 12% Cr-0.09%
NiG0.04% O-0.12% Al-0.94%
Principal
Cr
Cr-0.87%
Sheet, 400 pm thick, water-quenched after 1 h at 840°C at 10m6 torr
C-80 ppm Hf-70 ppm H-17 ppm
Sheet, 400 pm thick, water-quenched after 1 h at 840°C at 10m6 torr
Fe-240 ppm O-100 ppm
O-975
ppm determined ppm
Fe-300 ppm H-24
Thin foils were made using a technique [5] which normally results in negligible surface hydride formation from the polishing solution. Specimens were examined at 100 kV in a Hitachi 11 A electron microscope fitted with an HK3A, f 30” double axis tilt stage that had been modified to reduce the contamination rate to a very low level [6]. Some specimens were also examined at AERE, Harwell, using an AEI EM7 microscope, operated at 1 MeV. The long arrows in the figures refer to the direction of the operating reflection, g.
Heat treatment
O-l 86 ppm Fe-165 ppm H-2 ppm
N-35 H-not Zr-1%
impurities
ppm
Sheet, 400 bumthick, air cooled following 1 h at 450°C at 10e6 torr Tube, 6.5 mm wall thickness, extruded at 65O”C, air cooled, specimens from near outer surface
nucleated homogeneously, some had precipitated at the spherical intermetallic particles normally present in the Zircaloys (fig. 1). This may be related to strains generated by the quench, due to differential thermal contraction of the particles. Dislocations created by the hydrides were in the form of loop segments attached to the ends of the needles. Where more than one needle has nucleated together, the dislocations had often interacted to form networks (fig. 2).
3. Results Zirconium hydride was precipitated in the form of needles parallel to (1120) directions in Zr and Zircaloy-2, as observed by Bradbrook et al. [2]. The same morphology was adopted in the Zr-Cr alloy. Zr-1% Al was different, with hydride needles lying parallel to
to those hydrides which had apparently
Fig. 1. Zircaloy-2: hydride nucleation at intermetallic particles.
269
GJ.C. Carpenter et al., Zirconium hydride precipitates in zirconium
(a)
(b)
Fig. 2. Zircaloy-2: (a) g = 2fl0, all dislocations are in contrast, (b) g = 0220, dislocations at A vanish.
The Burgers vectors, b, were established by obtaining operating reflections, g, for which dislocation invisibility was obtained, satisfying the condition g-b = 0. An example is shown in fig. 2 and the detailed results are listed in table 2. All the hydrides in Zr-2 examined in this way yielded similar results, the dislocations having Burgers vectors of the type $I (1170). For any given hydride, only one or two of the three possible b’s were observed, those with a large component of b perpendicular to the needle direction. This is illustrated schematically in fig. 3 which shows a projection onto the (0001) plane. Dislocations generated on opposite sides of a hydride could have either the same b or different b’s, apparently at random. The sense of the dislocations was established by obtaining + g pairs where g had a large component parallel to b and using the same positive deviation parameter, sg (sg is positive when g lies inside the Ewald reflection sphere). No changes in the spacing of dislo-cations emitted at one side of the needles were observed,
Fig. 3. Illustrating the Burgers vectors of the dislocations generated on the basal plane by 7 hydride needles in Zircaloy-2.
proving them to have the same sign. In some cases there was a measurable change in spacing between dislocations on opposite sides of the hydrides indicating
Table 2 Burgers vectors of dislocations generated by hydride precipitates in Zircaloy-2. gab for g as specified Hydride direction
Set A
[ 12101
b
Dislocations
Set B
2iio
0220
0221
2Yzoo
2201
+o +o
0 #O
0 20
zo
+0 0
0
[jsllOl [11201
210
G.J. C. Carpenter et al., Zirconium hydride precipitates in zirconium
that they had opposite signs. It is difficult to be certain that this is always the case as the changes are small compared to the dislocation spacing. Stereo pairs were obtained at symmetrical orientations with respect to the c direction, using the same g and ss. It was concluded that the dislocation segments lie on or close to the basal plane. Evidence for large strain fields associated with the hydrides was seen in addition to the dislocations (fig. 4). The deviation parameter, sg, was zero in fig. 4a. The strain field is very strong and quite large values of sg were needed to reveal the inner dislocations clearly (fig. 4b). Approximately eight dislocations have been punched out to each side of a needle 2 500 A in diameter. A strain field is also seen along the c-axis direction when imaging with reflections having a large component along the c-axis (fig. 5). Images formed from the appropriate 1120 reflection showed no evidence for lattice strain along the direction of the needles. 3.2. Zirconium
Fig. 5. Zircaloy-2: z = [ 12101, g = lOi?, ,Yg = 0, showing tice strain due to the misfit along the c-axis.
The dislocations associated with hydride needles were generally less regular than in Zircaloy-2 (Fig. 6a). Some hydrides had rather few dislocations associated with them (Fig. 6b). Examination of large hydrides
contained entirely within thick regions of foil at 1 MeV showed that complex dislocation arrangements were possible with evidence of prismatic loop or dipole formation (fig. 7).
Fig. 4. Zircaloy-2: 5x10-3A-‘.
foil normal,
.z = [OOOl], g = 2fl0,
showing
the effect
of varying
the deviation
parameter:
lat-
(a) Sg = 0, (b) sg =
G.J.C. Carpenter et al., Zirconium hydride precipitates in zirconium
211
Fig. 6. Some dislocation configuratic Jns seen in zirconium at 100 kV.
Fig. 7. Dislocation contiguratil ons in zirconium at 1 MeV.
Contrast experiments at 100 kV showed that the dislocations were consistent with the results obtained for Zircaloy-2. Dislocation glide tended to occur readily in thin foils of zirconium and examples were seen where a dislocation from a hydride needle had crossslipped on intersecting the foil surface. 3.3. Zr-Cr
A typical micrograph is shown in fig. 8. The morphology of the hydrides and the punched out dislocations was very similar to that in Zircaloy-2. It was confirmed
that the dislocations possessed Burgers vectors of the type $a (1120) as illustrated in fig. 3, and were on the (0001) plane. Evidence for prismatic loop/dipole formation was sometimes seen (A in fig. 8). A large concentration of ZrCr, particles was present and these had sometimes restricted glide of the hydride dislocations (B in fig. 8). 3.4. Zr-Al This alloy was unusual in that the hydride needles were near (lOTO> directions. Fig. 9 shows an example of a foil in basal orientation. The arrowed dislocations
272
G.J.C. Carpenter et al., Zirconium hydride precipitates in zirconium
\ Fig. 8. Zr-1% Cr: typical microstructure and Z&r* precipitates.
showing hydrides
f b2
COITOI
Fig. 10. Illustrating the hydride orientation and dislocation Burgers vectors in Zr-1% Al.
(b)
(a)
Fig. 9. Zr-1% Al: z = [OOOl], hydrides parallel to (lOlO), (a)g = 2110, all dislocations in contrast. (b)g = 2200, some dislocations (arrowed) vanish.
disappeared using the 2300 reflection. The results of such experiments were consistent with most of the dislocations having b’s of the type $LI (1120) perpendicular to the needles as illustrated in fig. 10. The hydride needles appeared to be similar to those in the other materials but the structure and orientation relationship has not been examined.
4. Discussion The dislocations generated by hydrides in the alloys were generally in the form of regular loop segments, while in zirconium irregular configurations were common. This may be due to the lower friction stress of dislocation glide in the unalloyed material, which
G.J.C. Carpenter et al., Zirconium hydn’de precipitates in zirconium
would allow rearrangement to occur more readily in the bulk material and in the thin foils under the action of surface image stresses. It is also possible that alloying lowers the stacking fault energy inhibiting rearrangements involving cross slip. There was no doubt that dislocations intersecting the foil surfaces could glide more easily in comparison with the alloys in which little evidence for reorientation was seen. The micrographs published by Bailey [4], who used lower purity sponge zirconium, are more similar to the structures we have observed in the alloys than to those in our iodide zirconium. Using the orientation relationship determined by Bradbrook et al. [2] for Zr and Zircaloy-2 the mismatch between precipitate and matrix planes along three orthogonal matrix directions has been calculated [7], and is shown schematically in fig. 11. From available lattice parameter data, the misfit should not be greatly changed by small alloying additions. As pointed out in previous work [2,4] the mismatch along the [ 11201, habit direction is very small and this is in agreement with the absence of an observable strain field in the matrix along this direction. The experiments show that the misfit strain along the [ liOO] direction is large enough to generate dislocations. It is of the same order as the upper bound calculated by Ashby and Johnson [8] for coherent spherical inclusions. The mismatch along the c-axis and [ liOO] direction is almost the same although little evidence has been observed for the generation of dislocations to relieve the strain in the c-direction. This is related to the high energy of dislocations with c Bur-
[ooorl + 5.1% t tllzol + 0.55% * tIToo1 + 5.64%
Fig. 11. The misfit parameters nium.
for y hydride
needles in zirco-
273
gers vectors. At room temperature, zirconium is only slightly anisotropic as evidenced by the small (10%) deviation of Zeners constant from unity: c44/c66
= 0.90.
Assuming we have isotropic elasticity, the energy of corresponding dislocations with different Burgers vectors will be proportional to b2. If the stacking fault energy is so high that no dissociation has occurred the appropriate Burgers vectors are c[OOOl] and $u [ 1 l!?O], with energies in the ratio: qoool]
/q
llZO]
= 2.5;
thus, there is a large difference in energy between the two. Relatively little evidence was obtained for the generation of prismatic loops as postulated by Bailey [4]. The mechanism for the formation of the shear dislocations observed is thought to be that suggested by Ashby and Johnson [8] for spherical misfitting particles. The application of this to needle-shaped particles is illustrated in fig. 12a. The maximum shear stress in the matrix will occur on a plane such as ABCD, somewhere between the midplane and the ‘top’ of the particle. A segment of shear loop is generated as shown, leaving a dislocation of opposite sign at the interface (fig. 12a-d). Growth of the precipitate is expected to result in generation of segments successively from the midplane (fig. 12b-d). The observation of dislocations on either side of the needle with opposite signs suggests that the dislocations are often generated as in fig. 12b-d. Either of the two possible b’s with a large component perpendicular to the needle seemed to be formed at random. This indicates that the dislocations punched out to one side of a needle did not influence the generation process on the other side. In the case of spherical misfitting inclusions, as the shear loops expand, the screw segments are believed to cross slip and pinch off to form prismatic loops [8]. The mechanism is shown as it could apply to y-hydride needles in fig. 12e-g. Regular rows of detached prismatic loops along the (OOOl)/(Ol~O) glide cylinder have rarely been observed experimentally. The dipole configurations, shown in figs. 6a, 7 and 8, indicate that a similar process may occur as the particles grow but rarely proceeds to completion, The implication is that one or more of the cross slip steps does not occur read-
274
G.J.C. Carpenter et al., Zirconium hydride precipitates in zirconium
INTERFACE LOOP /
(d
PRISMATIC LOOP /
GLIDE CVLINOER
Fig. 12. Illustrating dislocation generation on applying the Ashby-Johnson model [ 81 to y-hydride needles: (a) shear loop formation, (b)-(d) projection along [ 1 lzO] showing effect of growth, (e) and (f) cross slip, (g) prismatic loop formation.
ily. This could result from the stress being insufficient or the dislocations being widely extended on one of the planes. Both mechanisms may contribute. The hydrides were often tapered towards the ends, resulting in a reduced stress available at the ends to initiate the first cross slip step. There have been no direct measurements of the stacking fault energy on different planes in zirconium. Tyson [9] has argued on theoretical grounds that the stacking fault energy on the basal plane is high. A low value (56 ergs/cm2) has been deduced by Akhtar and Teghtsoonian [ 101 for the prism planes from mechanical property measurements. This would make the first cross slip easy but the second (from prism back to basal plane) would be more difficult as it requires recombination of a dislocation extended by about I8b. In some cases, incomplete prismatic loops have been observed with one end still attached to the
hydride particle (e.g. fig. 6a). This may be related to the asymmetry resulting from b not being perpendicular to the needles (fig. 13a, excluding Zr-Al), which could give rise to configurations as in fig. 13b. These arise because the screw segment is closer to the needle at one end (Y in fig. 13a) and therefore subject to a higher stress on the cross slip plane. Interaction of these partly formed loops with glide dislocations with the same b would produce dipole arrangements similar to those sometimes observed (fig. 7). The fact that the dislocation segments farthest from the hydride were smallest in fig, 7b can be rationalized by noting that they were formed first, when the particle was small, It has been assumed in the above discussion that the generation of dislocations is due primarily to the dilatational strains resulting from the phase transformation. At the low temperatures involved in hydride formation, we can safely ignore diffusional processes
G.J.C. Carpenter et al., Zirconium hydride precipitates in zirconium
215
Fig. 13. (a) asymmetry because b is not perpendicular to hydride, (b) partially formed loops pinned at one end of hydride.
for the relief of misfit strains. Bradbrook et al. [2] have noted the possibility that stresses generated by differential thermal contraction may be important. This is impossible to assess accurately, as no data exists for the metastable -y-phase. An attempt to gain an order of magnitude estimate for the misfit resulting from this has been made using the thermal expansion coefficient for the polycrystalline &phase, K = 2.9X lo-‘j/deg C [ 111. The thermal expansion coefficients for zirconium, perpendicular to and parallel to the c-axis are as follows [ 121: K,=5.1X10-6/degC
K,, =9.2X10e6/degC.
Defining the corresponding misfit strains as 6 relative to the matrix, we obtain for a 300 deg C temperature drop: 6, = 0.06%
6,, = 0.18%.
This suggests that the contribution from differential thermal contraction strains is likely to be small compared to that from the phase transformation. Since hydride precipitates are formed rapidly at quite low temperatures (< 400°C) [2], it is unlikely that self-diffusion plays a significant role in the reaction. Precise figures are not available, but an estimate based in recent single crystal measurements by Hood [ 131 suggests that the random self-diffusion distance (roughly x/Dt where D = diffusion coefficient, t = time) is less than 1 atom jump in 1 set at 400°C. Hydrogen, on the other hand, is mobile at very low temperatures: for example, the diffusion distance in 1 set at room temperature is about 300 A, from an extrapolation of high temperature data [ 141. This indicates that the transformation is indeed similar to a bainitic reaction as suggested by Bradbrook et al. [2]. Since the hydrides form with a close packed plane parallel to the matrix basal plane, a simple picture for the transforma-
tion could be given by having hydrogen agglomerate to form a h.c.p. nucleus with shears to give the change in crystal structure. Passage of an appropriate sequence of partial dislocations of the type $0 ClOiO) could give the required change in stacking sequence. We cannot rule out the possibility that the shear events required to complete the phase transformation contribute to the generation of dislocations by the hydrides. However, we believe they are due to the dilatational misfit because the shears within the hydrides do not relieve the misfit strains in the matrix. Since a large proportion of the (11 l& close-packed planes must be sheared in order to effect the transformation, there is clearly not a 1: 1 correspondence with the dislocations punched out into the matrix. We can compare the experimental number of dislocations punched out in Zircaloy-2 with a crude estimate of the number required to completely remove the strain field. This is given by:
n=-.
6[llzo]DP b
. ’
where D, is the particle diameter, by assuming prismatic loops are punched out and glide away from the particle. Using D, = 2500A and b = 3.16 A we obtain n = 44. This should be compared with the experimental value of half the total number of shear loops generated, viz. 8. Thus, only a small proportion of the misfit stress is relieved plastically as also evidenced by the large elastic strain fields observed. The opposite conclusion was reached by Bailey [4], probably because he used an incorrect orientation relationship in calculating the misfit. His relationship predicted a negative misfit which is out of line with the expansion that accompanies the phase transformation. When the complete prismatic loop generation sequence occurs we expect interstitial loops to be produced, not vacancy loops as
216
G.J. C. Carpenter et al., Zirconium hydride precipitates in zirconium
suggested by Bailey [4]. The misfit could only be totally relieved if the dislocations broke away completely from the hydride, for example by prismatic loop formation. In the case of the small hydrides, the shear segments are pinned at the ends of the needles and an equilibrium is established between the stress field and the line tension. It is therefore not possible to asses whether the reason we do not observe more loop segments is due to difficulty of loop nucleation or to pinning. The fact that no dislocations were observed with c-type Burgers vectors, despite a large misfit, indicates that the hydride behaves like a coherent precipitate [8] and dislocation nucleation is difficult. The observation of large hydride precipitates in conventional electron microscopes is difficult due to dislocation interactions with the foil surfaces and difficulties in foil preparation [2]. It is anticipated that further progress in understanding the precipitation of zirconium hydrides, particularly the b-phase, will come from studies using high voltage microscopy.
Acknowledgement The authors wish to thank Dr. S.R. MacEwen for useful discussions and Dr. B. Cox for his comments on the manuscript.
References 111 C.E. Ells, J. Nucl. Mater. 28 (1968) 129. 121 J.S. Bradbrook, G.W. Lorimer and N. Ridley, J. Nucl. Mater. 42 (1972) 142. [31 N. Ridley, K.G. Gaulkin and G.W. Lorimer, Symposium on L’Hydrogene darts les Mktaux, Paris (29 May 1972) p. 484. I41 J.E. Bailey, Acta Met. 11 (1963) 267. [51 G.J.C. Carpenter and J.F. Watters, Vacancy Precipitation in Zirconium Alloys, to be published in Acta Met. [61 M.J. Ward and J. Morralee, J. of Phys. E, 6 (1973) 123. [71 G.J.C. Carpenter, J. Nucl. Mater. 48 (1973) 264. 181 M.F. Ashby and L. Johnson, Phil. Mag. 20 (1969) 1009. [91 W.R. Tyson, Acta Met. 15 (1967) 574. [lOI A. Akhtar and E. Teghtsoonian, Acta Met. 19 (1971) 655. C.P. Kempter, R.O. Elliot and K.A. Gscheidner, J. Chem. Phys. 33 (1960) 837. L.T. Lloyd, Argonne National Laboratory Report ANL6591 (1963). G.M. Hood, to be published. M.W. Mallett and W.M. AJbrecht, J. Electrochem. Sot. 104 (1957) 142.
NORTH-HOLLAND PUBLISHING COMPANY
DISLOCATIONS GENERATED BY ZIRCONIUM HYDRIDE PRECIPITATES IN ZIRCONIUM AND SOME OF ITS ALLOYS G.J.C. CARPENTER,
J.F. WATTERS and R.W. GILBERT
Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories* Chalk River, Ontario, Canada
Received 28 June 1973 The character of dislocations emitted during the precipitation of rzirconium hydride in zirconium, Zircaloy-2, Zr-1% Al and Zr-1% Cr has been determined. Contrast experiments in the transmission electron microscope have shown that the dislocations possess Burgers vectors (b) of the type +a ( 1130). Of the three possible b’s only one or two are observed associated with~a given hydride needle, those giving a large component of b along a direction perpendicular to the needle. Dislocation generation is thought to result from the dilatational misfit associated with the hydride needles. The creation of dislocations with b’s along the c-axis is more difficult and the misfit strain along this direction is not relieved by plastic deformation. Le caractkre des dislocations imises au tours de la pricipitation des hydrures de zirconium y dans le zirconium, dans le Zircaloy 2, dans les alliages Zr-Al 1% et Zr-Cr l%a Ctt?dktermink. Les essais de contraste en microscopic dlectronique en transmission ont montre que les dislocations posskdent des vecteurs de Burgers (6) du type fa (1120). Des trois vecteurs b possibles un ou deux seulement ont 6th observ& associ& g une aiguille don&e d’hydrure, ceux donnant une forte composante de b le long d’une direction perpendiculaire g I’aiguille. La formation des dislocations resulterait du manque d’accommodation par dilatation associke aux aiguilles d’hydrures. La creation de dislocations avec des vecteurs b le long de l’axe c est plus difficile et la deformation par manque d’accommodation le long de cette direction n’est pas relaxee par deformation plastique. Es wurde die Art der Versetzungen untersucht, die bei der Ausscheidung von y-Zirkonhydrid in Zirkon, Zircaloy-2, Zr-1% Al und Zr-1 % Cr entstehen. Transmissionselektronenmikroskopische Untersuchungen nach dem Kontrastverfahren ergeben, dass die Versetzungen einen Burgers-Vektor der Form ia Cll?O) haben. Von den drei maglichen b-Vektoren werden nur einer oder zwei beobachtet, die zusammen mit dem vorleigenden nadelfilrmigen Hydrid auftreten und eine grosse Komponente von b senkrecht zu den Nadeln haben. Die Entstehung der Versetzungen beruht vermutlich auf der Dehnung, die mit dem nadelf&migen Hydrid auftritt. Die Bildung von Versetzungen mit b-Vektoren parallel zur c-Achse ist schwieriger, die Dehnung in dieser Richtung wird durch plastische Deformation nlcht erleichtert.
1. Introduction The importance of hydride formation in zirconium alloys used for structural and cladding applications in water-cooled reactors (such as the CANDU system) has led to numerous papers on the subject. Many of these papers are discussed in a review by Ells [ 11. More recently, results of a detailed study of the crystallography of hydride formation in zirconium and Zircaloy-2 have been published by Lorimer, Ridley and co-workers [2,3]. Their work complements, and in certain respects, (in particular the orientation relationship) contradicts earlier results obtained by Bailey 141. They show that fairly rapid cooling (7 lO”C/min) causes the formation of metastable y-zirconium hy-
dride in the form of needles lying along (1120) matrix directions. A feature of-this reaction is the generation of dislocations by the hydride precipitates. Bailey [4] suggested that the dislocations were probably in the form of vacancy loops produced by prismatic punching to relieve misfit stresses. The work reported here describes some contrast experiments designed to characterize these dislocations in more detail.
2. Experimental The composition and principal impurities of the materials examined are listed in table 1, together with the heat treatment.
268
G.J.C. Carpenter et al., Zirconium hydride precipitates in zirconium
Table 1 Composition
(by weight)
Material
and heat treatment Alloying
of the materials
used.
elements
Zr
Zircaloy-2
ZIP 1% Al
Sn-1.36% Fe-O. 12% Cr-0.09%
NiG0.04% O-0.12% Al-0.94%
Principal
Cr
Cr-0.87%
Sheet, 400 pm thick, water-quenched after 1 h at 840°C at 10m6 torr
C-80 ppm Hf-70 ppm H-17 ppm
Sheet, 400 pm thick, water-quenched after 1 h at 840°C at 10m6 torr
Fe-240 ppm O-100 ppm
O-975
ppm determined ppm
Fe-300 ppm H-24
Thin foils were made using a technique [5] which normally results in negligible surface hydride formation from the polishing solution. Specimens were examined at 100 kV in a Hitachi 11 A electron microscope fitted with an HK3A, f 30” double axis tilt stage that had been modified to reduce the contamination rate to a very low level [6]. Some specimens were also examined at AERE, Harwell, using an AEI EM7 microscope, operated at 1 MeV. The long arrows in the figures refer to the direction of the operating reflection, g.
Heat treatment
O-l 86 ppm Fe-165 ppm H-2 ppm
N-35 H-not Zr-1%
impurities
ppm
Sheet, 400 bumthick, air cooled following 1 h at 450°C at 10e6 torr Tube, 6.5 mm wall thickness, extruded at 65O”C, air cooled, specimens from near outer surface
nucleated homogeneously, some had precipitated at the spherical intermetallic particles normally present in the Zircaloys (fig. 1). This may be related to strains generated by the quench, due to differential thermal contraction of the particles. Dislocations created by the hydrides were in the form of loop segments attached to the ends of the needles. Where more than one needle has nucleated together, the dislocations had often interacted to form networks (fig. 2).
3. Results Zirconium hydride was precipitated in the form of needles parallel to (1120) directions in Zr and Zircaloy-2, as observed by Bradbrook et al. [2]. The same morphology was adopted in the Zr-Cr alloy. Zr-1% Al was different, with hydride needles lying parallel to
to those hydrides which had apparently
Fig. 1. Zircaloy-2: hydride nucleation at intermetallic particles.
269
GJ.C. Carpenter et al., Zirconium hydride precipitates in zirconium
(a)
(b)
Fig. 2. Zircaloy-2: (a) g = 2fl0, all dislocations are in contrast, (b) g = 0220, dislocations at A vanish.
The Burgers vectors, b, were established by obtaining operating reflections, g, for which dislocation invisibility was obtained, satisfying the condition g-b = 0. An example is shown in fig. 2 and the detailed results are listed in table 2. All the hydrides in Zr-2 examined in this way yielded similar results, the dislocations having Burgers vectors of the type $I (1170). For any given hydride, only one or two of the three possible b’s were observed, those with a large component of b perpendicular to the needle direction. This is illustrated schematically in fig. 3 which shows a projection onto the (0001) plane. Dislocations generated on opposite sides of a hydride could have either the same b or different b’s, apparently at random. The sense of the dislocations was established by obtaining + g pairs where g had a large component parallel to b and using the same positive deviation parameter, sg (sg is positive when g lies inside the Ewald reflection sphere). No changes in the spacing of dislo-cations emitted at one side of the needles were observed,
Fig. 3. Illustrating the Burgers vectors of the dislocations generated on the basal plane by 7 hydride needles in Zircaloy-2.
proving them to have the same sign. In some cases there was a measurable change in spacing between dislocations on opposite sides of the hydrides indicating
Table 2 Burgers vectors of dislocations generated by hydride precipitates in Zircaloy-2. gab for g as specified Hydride direction
Set A
[ 12101
b
Dislocations
Set B
2iio
0220
0221
2Yzoo
2201
+o +o
0 #O
0 20
zo
+0 0
0
[jsllOl [11201
210
G.J. C. Carpenter et al., Zirconium hydride precipitates in zirconium
that they had opposite signs. It is difficult to be certain that this is always the case as the changes are small compared to the dislocation spacing. Stereo pairs were obtained at symmetrical orientations with respect to the c direction, using the same g and ss. It was concluded that the dislocation segments lie on or close to the basal plane. Evidence for large strain fields associated with the hydrides was seen in addition to the dislocations (fig. 4). The deviation parameter, sg, was zero in fig. 4a. The strain field is very strong and quite large values of sg were needed to reveal the inner dislocations clearly (fig. 4b). Approximately eight dislocations have been punched out to each side of a needle 2 500 A in diameter. A strain field is also seen along the c-axis direction when imaging with reflections having a large component along the c-axis (fig. 5). Images formed from the appropriate 1120 reflection showed no evidence for lattice strain along the direction of the needles. 3.2. Zirconium
Fig. 5. Zircaloy-2: z = [ 12101, g = lOi?, ,Yg = 0, showing tice strain due to the misfit along the c-axis.
The dislocations associated with hydride needles were generally less regular than in Zircaloy-2 (Fig. 6a). Some hydrides had rather few dislocations associated with them (Fig. 6b). Examination of large hydrides
contained entirely within thick regions of foil at 1 MeV showed that complex dislocation arrangements were possible with evidence of prismatic loop or dipole formation (fig. 7).
Fig. 4. Zircaloy-2: 5x10-3A-‘.
foil normal,
.z = [OOOl], g = 2fl0,
showing
the effect
of varying
the deviation
parameter:
lat-
(a) Sg = 0, (b) sg =
G.J.C. Carpenter et al., Zirconium hydride precipitates in zirconium
211
Fig. 6. Some dislocation configuratic Jns seen in zirconium at 100 kV.
Fig. 7. Dislocation contiguratil ons in zirconium at 1 MeV.
Contrast experiments at 100 kV showed that the dislocations were consistent with the results obtained for Zircaloy-2. Dislocation glide tended to occur readily in thin foils of zirconium and examples were seen where a dislocation from a hydride needle had crossslipped on intersecting the foil surface. 3.3. Zr-Cr
A typical micrograph is shown in fig. 8. The morphology of the hydrides and the punched out dislocations was very similar to that in Zircaloy-2. It was confirmed
that the dislocations possessed Burgers vectors of the type $a (1120) as illustrated in fig. 3, and were on the (0001) plane. Evidence for prismatic loop/dipole formation was sometimes seen (A in fig. 8). A large concentration of ZrCr, particles was present and these had sometimes restricted glide of the hydride dislocations (B in fig. 8). 3.4. Zr-Al This alloy was unusual in that the hydride needles were near (lOTO> directions. Fig. 9 shows an example of a foil in basal orientation. The arrowed dislocations
272
G.J.C. Carpenter et al., Zirconium hydride precipitates in zirconium
\ Fig. 8. Zr-1% Cr: typical microstructure and Z&r* precipitates.
showing hydrides
f b2
COITOI
Fig. 10. Illustrating the hydride orientation and dislocation Burgers vectors in Zr-1% Al.
(b)
(a)
Fig. 9. Zr-1% Al: z = [OOOl], hydrides parallel to (lOlO), (a)g = 2110, all dislocations in contrast. (b)g = 2200, some dislocations (arrowed) vanish.
disappeared using the 2300 reflection. The results of such experiments were consistent with most of the dislocations having b’s of the type $LI (1120) perpendicular to the needles as illustrated in fig. 10. The hydride needles appeared to be similar to those in the other materials but the structure and orientation relationship has not been examined.
4. Discussion The dislocations generated by hydrides in the alloys were generally in the form of regular loop segments, while in zirconium irregular configurations were common. This may be due to the lower friction stress of dislocation glide in the unalloyed material, which
G.J.C. Carpenter et al., Zirconium hydn’de precipitates in zirconium
would allow rearrangement to occur more readily in the bulk material and in the thin foils under the action of surface image stresses. It is also possible that alloying lowers the stacking fault energy inhibiting rearrangements involving cross slip. There was no doubt that dislocations intersecting the foil surfaces could glide more easily in comparison with the alloys in which little evidence for reorientation was seen. The micrographs published by Bailey [4], who used lower purity sponge zirconium, are more similar to the structures we have observed in the alloys than to those in our iodide zirconium. Using the orientation relationship determined by Bradbrook et al. [2] for Zr and Zircaloy-2 the mismatch between precipitate and matrix planes along three orthogonal matrix directions has been calculated [7], and is shown schematically in fig. 11. From available lattice parameter data, the misfit should not be greatly changed by small alloying additions. As pointed out in previous work [2,4] the mismatch along the [ 11201, habit direction is very small and this is in agreement with the absence of an observable strain field in the matrix along this direction. The experiments show that the misfit strain along the [ liOO] direction is large enough to generate dislocations. It is of the same order as the upper bound calculated by Ashby and Johnson [8] for coherent spherical inclusions. The mismatch along the c-axis and [ liOO] direction is almost the same although little evidence has been observed for the generation of dislocations to relieve the strain in the c-direction. This is related to the high energy of dislocations with c Bur-
[ooorl + 5.1% t tllzol + 0.55% * tIToo1 + 5.64%
Fig. 11. The misfit parameters nium.
for y hydride
needles in zirco-
273
gers vectors. At room temperature, zirconium is only slightly anisotropic as evidenced by the small (10%) deviation of Zeners constant from unity: c44/c66
= 0.90.
Assuming we have isotropic elasticity, the energy of corresponding dislocations with different Burgers vectors will be proportional to b2. If the stacking fault energy is so high that no dissociation has occurred the appropriate Burgers vectors are c[OOOl] and $u [ 1 l!?O], with energies in the ratio: qoool]
/q
llZO]
= 2.5;
thus, there is a large difference in energy between the two. Relatively little evidence was obtained for the generation of prismatic loops as postulated by Bailey [4]. The mechanism for the formation of the shear dislocations observed is thought to be that suggested by Ashby and Johnson [8] for spherical misfitting particles. The application of this to needle-shaped particles is illustrated in fig. 12a. The maximum shear stress in the matrix will occur on a plane such as ABCD, somewhere between the midplane and the ‘top’ of the particle. A segment of shear loop is generated as shown, leaving a dislocation of opposite sign at the interface (fig. 12a-d). Growth of the precipitate is expected to result in generation of segments successively from the midplane (fig. 12b-d). The observation of dislocations on either side of the needle with opposite signs suggests that the dislocations are often generated as in fig. 12b-d. Either of the two possible b’s with a large component perpendicular to the needle seemed to be formed at random. This indicates that the dislocations punched out to one side of a needle did not influence the generation process on the other side. In the case of spherical misfitting inclusions, as the shear loops expand, the screw segments are believed to cross slip and pinch off to form prismatic loops [8]. The mechanism is shown as it could apply to y-hydride needles in fig. 12e-g. Regular rows of detached prismatic loops along the (OOOl)/(Ol~O) glide cylinder have rarely been observed experimentally. The dipole configurations, shown in figs. 6a, 7 and 8, indicate that a similar process may occur as the particles grow but rarely proceeds to completion, The implication is that one or more of the cross slip steps does not occur read-
274
G.J.C. Carpenter et al., Zirconium hydride precipitates in zirconium
INTERFACE LOOP /
(d
PRISMATIC LOOP /
GLIDE CVLINOER
Fig. 12. Illustrating dislocation generation on applying the Ashby-Johnson model [ 81 to y-hydride needles: (a) shear loop formation, (b)-(d) projection along [ 1 lzO] showing effect of growth, (e) and (f) cross slip, (g) prismatic loop formation.
ily. This could result from the stress being insufficient or the dislocations being widely extended on one of the planes. Both mechanisms may contribute. The hydrides were often tapered towards the ends, resulting in a reduced stress available at the ends to initiate the first cross slip step. There have been no direct measurements of the stacking fault energy on different planes in zirconium. Tyson [9] has argued on theoretical grounds that the stacking fault energy on the basal plane is high. A low value (56 ergs/cm2) has been deduced by Akhtar and Teghtsoonian [ 101 for the prism planes from mechanical property measurements. This would make the first cross slip easy but the second (from prism back to basal plane) would be more difficult as it requires recombination of a dislocation extended by about I8b. In some cases, incomplete prismatic loops have been observed with one end still attached to the
hydride particle (e.g. fig. 6a). This may be related to the asymmetry resulting from b not being perpendicular to the needles (fig. 13a, excluding Zr-Al), which could give rise to configurations as in fig. 13b. These arise because the screw segment is closer to the needle at one end (Y in fig. 13a) and therefore subject to a higher stress on the cross slip plane. Interaction of these partly formed loops with glide dislocations with the same b would produce dipole arrangements similar to those sometimes observed (fig. 7). The fact that the dislocation segments farthest from the hydride were smallest in fig, 7b can be rationalized by noting that they were formed first, when the particle was small, It has been assumed in the above discussion that the generation of dislocations is due primarily to the dilatational strains resulting from the phase transformation. At the low temperatures involved in hydride formation, we can safely ignore diffusional processes
G.J.C. Carpenter et al., Zirconium hydride precipitates in zirconium
215
Fig. 13. (a) asymmetry because b is not perpendicular to hydride, (b) partially formed loops pinned at one end of hydride.
for the relief of misfit strains. Bradbrook et al. [2] have noted the possibility that stresses generated by differential thermal contraction may be important. This is impossible to assess accurately, as no data exists for the metastable -y-phase. An attempt to gain an order of magnitude estimate for the misfit resulting from this has been made using the thermal expansion coefficient for the polycrystalline &phase, K = 2.9X lo-‘j/deg C [ 111. The thermal expansion coefficients for zirconium, perpendicular to and parallel to the c-axis are as follows [ 121: K,=5.1X10-6/degC
K,, =9.2X10e6/degC.
Defining the corresponding misfit strains as 6 relative to the matrix, we obtain for a 300 deg C temperature drop: 6, = 0.06%
6,, = 0.18%.
This suggests that the contribution from differential thermal contraction strains is likely to be small compared to that from the phase transformation. Since hydride precipitates are formed rapidly at quite low temperatures (< 400°C) [2], it is unlikely that self-diffusion plays a significant role in the reaction. Precise figures are not available, but an estimate based in recent single crystal measurements by Hood [ 131 suggests that the random self-diffusion distance (roughly x/Dt where D = diffusion coefficient, t = time) is less than 1 atom jump in 1 set at 400°C. Hydrogen, on the other hand, is mobile at very low temperatures: for example, the diffusion distance in 1 set at room temperature is about 300 A, from an extrapolation of high temperature data [ 141. This indicates that the transformation is indeed similar to a bainitic reaction as suggested by Bradbrook et al. [2]. Since the hydrides form with a close packed plane parallel to the matrix basal plane, a simple picture for the transforma-
tion could be given by having hydrogen agglomerate to form a h.c.p. nucleus with shears to give the change in crystal structure. Passage of an appropriate sequence of partial dislocations of the type $0 ClOiO) could give the required change in stacking sequence. We cannot rule out the possibility that the shear events required to complete the phase transformation contribute to the generation of dislocations by the hydrides. However, we believe they are due to the dilatational misfit because the shears within the hydrides do not relieve the misfit strains in the matrix. Since a large proportion of the (11 l& close-packed planes must be sheared in order to effect the transformation, there is clearly not a 1: 1 correspondence with the dislocations punched out into the matrix. We can compare the experimental number of dislocations punched out in Zircaloy-2 with a crude estimate of the number required to completely remove the strain field. This is given by:
n=-.
6[llzo]DP b
. ’
where D, is the particle diameter, by assuming prismatic loops are punched out and glide away from the particle. Using D, = 2500A and b = 3.16 A we obtain n = 44. This should be compared with the experimental value of half the total number of shear loops generated, viz. 8. Thus, only a small proportion of the misfit stress is relieved plastically as also evidenced by the large elastic strain fields observed. The opposite conclusion was reached by Bailey [4], probably because he used an incorrect orientation relationship in calculating the misfit. His relationship predicted a negative misfit which is out of line with the expansion that accompanies the phase transformation. When the complete prismatic loop generation sequence occurs we expect interstitial loops to be produced, not vacancy loops as
216
G.J. C. Carpenter et al., Zirconium hydride precipitates in zirconium
suggested by Bailey [4]. The misfit could only be totally relieved if the dislocations broke away completely from the hydride, for example by prismatic loop formation. In the case of the small hydrides, the shear segments are pinned at the ends of the needles and an equilibrium is established between the stress field and the line tension. It is therefore not possible to asses whether the reason we do not observe more loop segments is due to difficulty of loop nucleation or to pinning. The fact that no dislocations were observed with c-type Burgers vectors, despite a large misfit, indicates that the hydride behaves like a coherent precipitate [8] and dislocation nucleation is difficult. The observation of large hydride precipitates in conventional electron microscopes is difficult due to dislocation interactions with the foil surfaces and difficulties in foil preparation [2]. It is anticipated that further progress in understanding the precipitation of zirconium hydrides, particularly the b-phase, will come from studies using high voltage microscopy.
Acknowledgement The authors wish to thank Dr. S.R. MacEwen for useful discussions and Dr. B. Cox for his comments on the manuscript.
References 111 C.E. Ells, J. Nucl. Mater. 28 (1968) 129. 121 J.S. Bradbrook, G.W. Lorimer and N. Ridley, J. Nucl. Mater. 42 (1972) 142. [31 N. Ridley, K.G. Gaulkin and G.W. Lorimer, Symposium on L’Hydrogene darts les Mktaux, Paris (29 May 1972) p. 484. I41 J.E. Bailey, Acta Met. 11 (1963) 267. [51 G.J.C. Carpenter and J.F. Watters, Vacancy Precipitation in Zirconium Alloys, to be published in Acta Met. [61 M.J. Ward and J. Morralee, J. of Phys. E, 6 (1973) 123. [71 G.J.C. Carpenter, J. Nucl. Mater. 48 (1973) 264. 181 M.F. Ashby and L. Johnson, Phil. Mag. 20 (1969) 1009. [91 W.R. Tyson, Acta Met. 15 (1967) 574. [lOI A. Akhtar and E. Teghtsoonian, Acta Met. 19 (1971) 655. C.P. Kempter, R.O. Elliot and K.A. Gscheidner, J. Chem. Phys. 33 (1960) 837. L.T. Lloyd, Argonne National Laboratory Report ANL6591 (1963). G.M. Hood, to be published. M.W. Mallett and W.M. AJbrecht, J. Electrochem. Sot. 104 (1957) 142.