Novel processes of dislocation multiplication observed in ice

Acta metall, mater. Vol. 41, No. 1, pp. 205-210, 1993

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NOVEL PROCESSES OF DISLOCATION M U L T I P L I C A T I O N OBSERVED IN ICE C. S H E A R W O O D t and R. W. WHITWORTH School of Physics and Space Research, University of Birmingham, Birmingham BI5 2TT, England (Received 8 June 1992)

A~traet--The processes by which dislocations in macroscopic-sized crystals of ice multiply under an applied stress have been revealed by synchrotron X-radiation topography. This paper describes two examples in which edge dislocation segments gliding rapidly on non-basal planes generate fresh dislocations on the basal primary slip system. The interpretation is based largely on the principles of cross-glide and the Frank-Read source, but in addition a new mechanism is proposed. This involves the cross-glide of a dislocation that is moving intermittently, and it could in principle apply to materials other than ice.

mation have the (11~0) Burgers vector lying in this plane. The unusual feature of ice is that edge dislocations with this Burgers vector are more mobile on non-basal planes such as (lI00), (lI01) or (1102) than on the basal plane (0001), but screw dislocations do not glide on non-basal planes [7, 9-13]. A step between segments on nearby basal planes is an edge segment on a non-basal plane. In ice it usually happens that, rather than acting as the fixed centre of a Frank-Read source, such steps glide away dragging screw segments behind them. This leads to the unusual mechanisms of multiplication described in [7] and [8], but the non-basal plane does not become a macroscopic slip plane because of the immobility of screw dislocations on it. Slip bands do not broaden by multiple cross-glide because screw segments cannot cross-slip off the basal plane. A novel process for dislocation multiplication, which occurs when a dislocation cuts a surface, has been reported by Ahmad and Whitworth [7]. The present paper describes in more detail two different examples of this kind. The process is specific to ice and could only occur in other materials with a similar high degree of plastic anisotropy, but one feature of the interpretation is essentially different from the F r a n k Read process and could be relevant more widely.

I. INTRODUCTION For macroscopic plastic deformation to occur in a crystalline material there has to be a process for dislocation multiplication. The generally accepted basis of all such processes was devised by Frank and Read [1]; it is described in detail by Read [2], who stresses its essentially three-dimensional character. For a general review see [3]. In the standard Frank-Read mechanism a segment of a dislocation line lying in its glide plane terminates at a node or a step taking it out of that plane. The forward motion of the segment on the glide plane causes the dislocation line to spiral around this point, resulting in slip on that plane by many Burgers vectors. Cross-glide of dislocation segments can generate Frank-Read sources on neighbouring glide planes, and the etching experiments of Johnston and Gilman [4] established "multiple cross glide" as the standard mechanism for slip band broadening. Although Frank-Read action has be observed in the electron microscope, e.g. [5], large scale features of the multiplication process can only be studied in much larger specimens. We have used synchrotron X-radiation topography to observe the motion of dislocations in high quality single crystals of ice, and have found the technique and the material well suited to observation of dislocation multiplication. The development of Frank-Read spirals has been observed on the (0001) glide plane [6], but dislocations in ice have special features which result in a range of multiplication processes which look very different from this [7, 8]. Ice has a hexagonal lattice and its macroscopic slip plane is well established as (0001). The only dislocations ever observed to participate in plastic defor-

2. EXPERIMENTAL

tPresent address: Microelectronics Research Centre, Cavendish Laboratory, Cambridge CB3 0HE, England.

The experiments were performed using the Synchrotron Radiation Source at the Science and Engineering Research Council Daresbury Laboratory as described previously [7, 13]. Single crystals of ice 20 mm high, 12 mm wide and 1.0-1.5 mm thick were oriented as shown in Fig. 1. The inclination of the (0001) plane to the vertical axis was about 21 °. White radiation topographs were obtained with diffraction vector 1]-00 and Bragg angle 6.5 °. They are projections of the dislocations onto the basal plane with

205

SHEARWOOD and WHITWORTH: DISLOCATION MULTIPLICATION IN .ICE

206

41-

0001

very little distortion, and as printed they correspond to viewing the crystal in the direction indicated. Stereo pairs were obtained by rotations of + 5° about [1T00]. Long sequences of topographs were obtained between successive applications of compressive stress ( ~ 1 MPa) and selected topographs are reproduced here. Example 1 was studied at - 15°C and example 2 at -20°C.

/ /

3. EXAMPLE 1

17oo

Q

Fig. 1. Diagram in (1120) plane showing the orientation of the specimens and the direction of viewing of the topographs.

We consider a single dislocation in the sequence (a)-(h) of topographs shown in Fig. 2. Labelled tracings of the dislocation from these topographs are reproduced in Fig. 3, in which the horizontal and

Fig. 2. Sequence of topographs (a) to (h) showing the dislocation described as example 1.

\b \0 \d \ . C

C FD

C

\,

C

11~0

A

\__

\

-\

~

~

\

Fig. 3. Labelled tracings of the dislocation from Fig. 2. Small open circles mark intersections with the front face and small solid circles with the back face. The front and back faces are not parallel to the plane of the figure.

SHEARWOOD and WHITWORTH: DISLOCATION MULTIPLICATION IN ICE diagonal reference lines lie at the same absolute positions on the specimen. Topographs obtained with different diffraction vectors show that the Burgers vector of the dislocation is in the [21"T0] direction indicated and that there are no interactions with other dislocations out of contrast in Fig. 2. Stereo views reveal that in topograph (a) the dislocation runs from the front face at A to the back face at D. Reconstructions of Fig. 3 (a, c, g) projected on the vertical (11 ~0) plane perpendicular to Figs 2 and 3 are shown in Fig. 4, though distances through the thickness of the specimen are not actually measurable. There are initially three segments AB, BC and CD, of which CD is the most interesting, but it is simplest to describe AB first. AB lies on a basal (0001) plane, which is almost parallel to the plane of Figs 2 and 3 and is seen edge-on in Fig. 4. The segment glides slowly upwards throughout the sequence, taking up the usual hexagonal form with screw and 60 ° segments [7, 13]. [The more rounded shape in (g) resulted from some recovery due to delay in recording this topograph while stereo-images were being taken.] The segment BC runs back away from the plane of AB, and under the stress an edge component E glides rapidly downwards on a non-basal plane. Initially this plane is seen to be edge-on in Fig. 3(c), so it must be approximately (0110). The edge segment trails screw segments behind it, and these subsequently glide on (0001) planes, giving the appearance of a loop which gets wider in the topographs. For some reason the segment BC is held back at F, and in the topographs another loop appears to grow from this point. The generation of dislocations in the basal plane from such obstacles to non-basal slip is common in ice (see example in [8]). The segment CD cuts the back surface at D, and this point of emergence glides very rapidly down the back face along a line that corresponds approxi-

¢ ~D

/

g

IB

I /

A'

A'

/

v

/

E/

///

/

//

mately to the trace of the (01T 1) plane. It trails a long screw segment CG which itself glides to the left on a (0001) plane. The point G finally reaches the front surface between topographs (f) and (g). An edge segment ID running from the front to the back of the specimen then glides off to the bottom right in (g, h). It is marked by arrows in Fig. 2, but because it is seen end-on it has low contrast and may not be clear in the printed form of (h); it is definitely present in the original topographs. The emergence point H glides upward from the point where the screw dislocation CG originally cut the surface. This requires glide on a non-basal plane as shown in Fig. 4. At the same time the screw component glides to the left on its (0001) plane leaving the hook-like termination at H in Fig. 2(h). The final configuration at H is equivalent to that at D in (a), and the sequence could be repeated indefinitely. At first sight we have a single-ended F r a n k - R e a d source on a (0111) plane sweeping around the point C, but the situation is not so simple because the screw dislocation CG does not glide on the non-basal plane of the source. Instead it glides out of the plane of the source and would not then be annihilated when point H sweeps upward along the front face. Each full cycle of the source on the (01TI) plane will produce not only a pair of non-basal edge segments like ID, but also a pair of basal screw dislocations like CH. This is a new process by which fast non-basal edge dislocations can generate dislocations on the basal plane. A cusp which subsequently turns into a loop can be seen at J on the screw segment CG. Such features are common but their origin is not clearly established. They do not have the right properties to be due to cross-glide of the screw dislocation off the basal plane, and it seems more plausible that they arise from the loss or accumulation of interstitials [14]. On a longer time scale we have often observed that screw dislocations in ice become helical by this process at these temperatures. 4. EXAMPLE 2

t~

C/

207

H,

0001

_/ 1100

Fig. 4. Reconstruction for cases (a), (c) and (g) of example 1 showing the dislocation in projection on (1120). The broken lines are traces of (0001) glide planes.

The second example is presented in the form of a series of stereo-pairs in Fig. 5. These can be conveniently viewed directly through two 10 cm converging lenses held one in front of each eye. The cross lines are far in front of the specimen and cannot easily be brought into stereo-vision; the horizontal cross lines should appear coincident and the vertical ones should be ignored. In the pair (a) the dislocation of interest PS lies approximately on the basal plane, which is the plane of projection, and it cuts the front face at P. The dislocation LM lies well behind it. The Burgers vector is in the [2TT0] direction marked b. At the point P where the dislocation emerges it must be assumed to have curved forward from its basal plane by climb so that it does not cut the surface too obliquely. This will produce a segment on a

208

SHEARWOOD and WHITWORTH: DISLOCATION MULTIPLICATION IN ICE screw segment QR which then glides slowly to the right on its basal plane. The track of the emergence points P, P' and P", together with the appearance of topographs taken at a more oblique angle, show that

non-basal plane, and under stress this segment will glide rapidly so that the point of emergence moves to P' in (b) and P" in (c). The non-basal segment P'Q which is seen almost end-on in (b) trails behind it a

(a)

(b)

(c)

(d) Fig. 5. Stereo pairs of topographs (a)-(d) showing the motion of the dislocation described as example 2.

SHEARWOOD and WHITWORTH:

DISLOCATION MULTIPLICATION IN ICE

209

at L, but has the opposite Burgers vector to PS. It therefore glides downwards and will run out of the specimen, so that it does not act as a source. It is interesting to note that both segments RS and LM take up edge orientations rather than the more usual 60° or screw orientations reported in [7] and [13]. This is to be expected where a curved loop is becoming straightened rather than expanding.

:z'~To

5. D I S C U S S I O N a

Fig. 6. Perspective view of the glide planes and positions of the dislocations in the cases of Fig. 5 (a~:).

the glide plane of P'Q is approximately (0111); this is a plane for which the resolved force on dislocations with this Burgers vector is close to its maximum value. Figure 6 gives a perspective view of the (0001) and (0111) glide planes and shows the dislocation lines corresponding to topographs (a) to (c). The motion of P'Q up to image (b) corresponds closely to that of GD in example 1. However just beyond (b) the segment starts to trail a narrow loop in screw orientation, and this develops to the state seen in (c) and drawn in Fig. 6. The loops so formed widen by glide on the basal plane and are formed in increasing numbers, so that by the time of image (d) there are many long loops lying on a set of basal planes between that of QR and the point of emergence. The way in which these loops are nucleated is not resolved in the topographs, but possible mechanisms will be considered in Section 5. In topograph (d) a loop is clearly seen at R. The formation of this loop shows that when P'Q glides up the (0111) plane the screw dislocation trailing behind it lies on a (0001) plane that is in front of that of RS. There must be a linking edge segment on a non-basal plane, which will lie closer to (0001) than (01ill. If this glides more slowly than RS a loop will be left behind. Initially this loop may not be resolved in the topographs, but as the screw segments separate on their (0001) planes the loop will become visible. The form of dislocation multiplication seen in this example occurs quite commonly in ice. In Fig. 5 a similar source operating on the back face is seen at K. Another is just off the figure to the upper left and sends down loops which are first seen at F in (b); in (d) they overlap with the loops generated by the source we have been considering. The dislocation LM has a non-basal segment which cuts the front surface

Example 1 shows how in the presence of free surfaces Frank-Read action can occur even on a plane where screw dislocations are immobile, but in the process dislocations can be generated on an intersecting plane. In that example the non-basal edge segment moved smoothly across its glide plane, but in example 2 the non-basal segment nucleated loops on a whole set of intersecting basal planes. Such action is a very effective source of slip on new basal planes in ice, and when the dislocations so produced cut the surface some of them can start the process all over again. The theoretical question, which cannot be answered directly by the study of our topographs, is how these trailing loops are formed. In the textbooks (such as [2]) an edge dislocation should glide across its glide plane as a straight line without leaving any disruption of the lattice behind it, and the introduction of jogs or kinks does not alter this fact. However, if some portions of the dislocation line encounter obstacles which hold them back temporarily it is possible under conditions applicable to ice for loops to be nucleated on an oblique plane. One possible process is illustrated in Fig. 7. ABCD is a non-basal plane on which the edge segment JK glides under stress to the right, and EFGH is a basal plane on which there is also a resolved stress. It intersects ABCD along a line parallel to the Burgers vector. Screw dislocations are mobile on EFGH but not on ABCD. The diagram shows four successive configurations displaced from one another. Suppose that as the dislocation glides the portion OP lags behind MN, and the linking segment NO gets into the right-handed screw orientation shown. Remember that NO cannot glide past E

D

/ J

F

M

M

M

C

H

Fig. 7. Proposed mechanism for the nucleation of a dislocation loop NORQ on the plane EFGH arising from the interrupted motion of an edge dislocation JK on the plane ABCD.

210

SHEARWOOD and WHITWORTH: DISLOCATION MULTIPLICATION IN ICE E

F

A

K

K

N

////- / '7/

D

/ J

J

M

M

C

H

Fig. 8. Alternative mechanism to that in Fig. 7 for the generation of dislocation segments XW and YZ on planes of the type EFGH due to a super-jog ST on an edge dislocation JK which glides to the right on the plane ABCD.

this position because screw segments are assumed to be immobile on ABCD. NO will then be subject to the resolved stress on the plane EFGH which will cause it to glide to QR, leaving opposite edge segments NQ and RO. If OP now glides forward again to form the straight dislocation MNS, a loop NORQ is left behind on the plane EFGH. This loop can subsequently expand on that plane. For this process to operate, OP has to be held back long enough for NO to glide sufficiently far off EFGH that the final loop NORQ is stable against collapse. It is not clear why OP should be held back. Pinning followed by thermally activated release is a possibility, but it must be remembered that in our experiments the stress was not applied continuously. It had to be removed every time a topograph was taken, and there could be some backward and forward motion associated with this. Long loops such as those observed could also be formed if the fast gliding edge dislocation carried a relatively immobile super-jog. This is illustrated in Fig. 8, which shows the same planes as Fig. 7. If the edge dislocation JK acquires an immobile super-jog ST and then advances to MN, the screw segments SU and TV will trail behind it. These can subsequently glide to XW and YZ on planes parallel to EFGH, and they can then act as sources on these planes. For this to happen the super-jog has to be big enough for the stress on EFGH to separate the segments SU and TV against their mutual attraction. At the stresses used here ST would have to be of the order of 1000 lattice parameters high. It is difficult to understand why such a large super-jog should appear on an isolated glissile dislocation. It cannot be formed by pure glide processes, and would have to be the result of climb, such as was assumed to account for the loop J in Fig. 3(h). 6. CONCLUSIONS The technique of synchrotron X-radiation topography has revealed large scale features of dislocation

multiplication processes in ice at a level of detail not previously achieved in this or any material. The observations depend on the special property of ice that edge, but not screw, dislocation segments are more mobile on non-basal planes than on the basal primary slip plane. A non-basal edge segment that cuts the surface can operate as a Frank-Read source generating dislocations not only on its glide plane but also on an intersecting basal plane. Non-basal edge dislocations have also been observed to nucleate long loops in screw orientation on a whole set of basal planes. The mechanism by which this happens is not clearly established, but one proposal involves a new process which is totally independent of that of Frank and Read. This process is certainly a geometrical possibility, and even if not happening here it could operate in other materials in which dislocation motion is intermittent due, for example, to thermal unpinning. So little has been directly observed about multiplication in other materials that we should keep an open mind about unexpected possibilities. Acknowledgements--This work was supported by a research grant from the Science and Engineering Research Council, and we acknowledge the use of the synchrotron radiation facilities at the SERC Daresbury Laboratory.

REFERENCES

1. F. C. Frank and W. T. Read, Phys. Rev. 79, 722 (1950). 2. W. T. Read, Dislocations in Crystals. McGraw-Hill, New York (1953). 3. H. Neuh~iuser, Dislocations in Solids (edited by F. R. N. Nabarro), Vol. 6, Chap. 31. North-Holland, Amsterdam (1983). 4. W. G. Johnston and J. J. Gilman, J. appl. Phys. 31, 632 (1960). 5. F. Louchet, J. Phys. C 13, L847 (1980). 6. S. Ahmad, M. Ohtomo and R. W. Whitworth, Nature Lond. 319, 659 (1986). 7. S. Ahmad and R. W. Whitworth, Phil. Mag. A 57, 749 (1988). 8. S. Ahmad, C. Shearwood and R. W. Whitwortb, Physics and Chemistry of Ice (edited by N. Maeno and T. Hondoh), p. 492. Hokkaido Univ. Press, Sapporo (1992). 9. S. Ahmad, M. Ohtomo and R. W. Whitworth, J. Physique 48, C1-175 (1987). 10. A. Fukuda, T. Hondoh and A. Higashi, J. Physique 48, C1-163 (1987). 11. C. Shearwood and R. W. Whitworth, J. Glaciol. 35, 281 (1989). 12. T. Hondoh, H. Iwamatsu and S. Mac, Phil. Mag. A 62, 89 (1990). 13. C. Shearwood and R. W. Whitworth, Phil. Mag. A 64, 289 (1991). 14. K. Goto, T. Hondoh and A. Higashi, Jap. J. appl. Phys. 25, 351 (1986).