Interaction of Cavities and Dislocations in CrystalsBY A. J. FORTYH. H. Wills Physics Laboratory, University of BristolReceived 16th June 1964Dislocations may be expected to interact strongly with voids or cavities in crystals. The inter-action may be physical, through the stress fields of both the dislocation and the cavity, or chemical,by a transfer of point defects. The mechanisms for such interactions are discussed in detail, andillustrated by direct observations made by transmission electron microscopy on crystals of leadiodide. These show elastic interactions between mobile dislocations and cavities and the " climb "of sessile dislocations in the immediate vicinity of cavities. The paper discusses the possible effectson the mechanical and chemical processes associated with dislocations when cavities are present incrystals.The formation of cavities (or voids) in crystals and the interaction of these withdislocations can have a significant effect on both mechanical and chemical properties.For example, small voids, formed preferentially along dislocations or dispersedrandomly throughout a crystal, can impede the motion of dislocations and therebylead to hardening.Indeed, it is possible to account for some forms of radiation-hardening in this way ; the aggregation or condensation of lattice vacancies createdby irradiation produces voids and these interact with dislocations. A quantitativediscussion of radiation-hardening can be formulated along these lines.1Clearly it is important to understand the nature of interactions between voidsand dislocations.Whilst the cavities observed in electron microscope studies oflead iodide crystals are generally larger than those thought to be responsible for theradiation hardening of metals, and a precise quantitative treatment is therefore notrelevant in this case, a qualitative study of dislocationfcavity interactions in thismaterial does seem to be useful. Indeed, the special arrangements of dislocations andcavities in lead iodide offer important advantages to be gained from such a study.Specimens suitable for electron microscopy are readily prepared by growth fromaqueous solution in the form of platelets with well-developed (0001) habit planes.The crystallography ensures that both dislocations and the plate-like cavities formedby electron bombardment in the microscope lie in the plane of the specimen, an idealgeometrical situation for the direct observation of interactions.Interactions of cavities and dislocations can be broadly classified as physical orchemical ; the mutual interaction of stress fields gives rise to a mechanical or physicalforce, whilst the transfer of point defects between cavities and dislocations leads to achemical interaction. These two classes of interaction will be discussed in turn in thefollowing sections. The discussion will be in general terms but, wherever possible,it will be illustrated by examples of interactions observed in lead iodide.The specialcrystallographic orientation and the marked anisotropy of dislocation motion and themigration of point defects in lead iodide make it possible to separate the two forms ofinteraction with little ambiguity.GENERAL FEATURES OF DISLOCATIONS AND CAVITIES I N LEAD IODIDELead iodide crystallizes with a hexagonal layer structure, composed of close-packed layers of I- ions arranged in hexagonal close-packing, with the Pbz+ ions5A.J . FORTY 57placed in the octahedral interstices between alternate pairs of I- layers. Deformationoccurs most readily by slip in the basal plane (OOOl), presumably through the planesseparating the molecular sandwiches of PbI2. Dislocations introduced by deformationduring preparation of specimens for electron microscopy therefore lie in the basalplane and have their Burgers vectors in this plane.Crystals prepared by growthfrom aqueous solutions have the form of platelets in basal plane orientation. " Slipdislocations " in these appear under the electron microscope as sharply imaged linesextending across the specimen plane.The exposure of a crystal of lead iodide to the electron beam during examinationin the microscope leads to structural damage and, in some cases, to decomposition.2Under low intensities of illumination only structural damage occurs. This producessmall loops of dislocation which appear to lie in inclined planes, with Eloil) orienta-tions. The Burgers vectors of these have not been unambiguously determined but itis thought that there should be a large component in the [OOOl J direction ; for this isexpected if the loops are sessile dislocations formed by the collapse of {lOil) discs ofvacancies.Large, irregularly shaped loops of sessile dislocation appear in the crystals underhigh-intensity irradiation.These lie in the basal plane (0001) and have diametersvarying between one and a hundred microns. Again, the Burgers vector of theseloops has not been determined unambiguously but it is thought that they are formedby the aggregation and subsequent collapse of vacancies in plate-like clusters in thebasal plane. These loops appear under conditions of irradiation which lead simul-taneously to the formation of cavities. They grow into irregular shapes (see fig. 5,for example) by chemical interaction with cavities (see later). The collapse of asingle cavity into a series of concentric basal plane loops has been observedoccasionally.The most striking result of irradiation at high intensity is the appearance of electron-transparent regions, or bright patches.These patches have been interpreted 3 asthe images of plate-like cavities, lying in the basal plane, and formed by the aggrega-tion of vacancies created by irradiation. The cavities have diameters ranging from afew hundred A to several microns, and thicknesses between ten and a hundred A.The bright contrast due to enhanced transmission is predominant, but in some patchesthe brightness is considerably reduced and a narrow fringe of enhanced diffractionappears around them. Examples of the two kinds of contrast may be seen in fig.5.It is thought that the bright contrast arises from reduced absorption within a cavity,whilst the diffraction contrast reveals the existence of elastic strain of the crystalstructure surrounding it.There is some indirect evidence that cavities contain iodine gas and, during laterstages of the decomposition of a crystal, precipitates of metallic lead. Cavities clearlyplay an important part in the decomposition reaction, by providing sites within thecrystal for the precipitation of the products of decomposition. The relative amountsof absorption contrast and diffraction contrast probably depend on the pressure ofiodine gas in the cavities. For the mechanical stability of a thin plate-like cavitymust depend on the internal support provided by the gas.Strong diffraction contrastmight therefore be associated with a tendency towards collapse.Cavities are formed in the centre of decomposition (i.e., the centre of the irradiatedarea) and subsequently drift radially outwards with velocities of several microns persecond into the cooler parts of the crystal. It is impossible to account for this kindof movement simply but an explanation has been sought in terms of an electrolytictransfer of ions under the electric field set up by secondary emission from the crystalunder irradiation.4 An iodine atmosphere assists such a transfer58 CAVITIES AND DISLOCATIONSPHYSICAL INTERACTION BETWEEN CAVITIES AND DISLOCATIONSThe migration of a cavity through a crystal of lead iodide provides an ideal experi-mental situation for the study of interactions with dislocations. It is possible toobserve directly the movements of dislocations in close proximity to cavities, and,moreover, to investigate the variation of the strength of interaction with separation.By suitable adjustment of the electron beam it is possible to produce cavities anddirectthem towards dislocations in other parts of the crystal.A sequence of observationsmade as a cavity moves past a dislocation in this way provides a striking illustrationof mutual interaction.When a cavity in the basal plane approaches a ‘‘ slip dislocation ” (a dislocationlying in the basal plane and having its Burgers vector in that plane) the latter appearsto be drawn towards it and is eventually “ sucked ” into the cavity (see fig.1, forexample). The dislocation moves in its slip plane and the interaction must thereforebe physical rather than chemical; that is, the movement is caused by a mechanicalforce on the dislocation rather than by a transfer of point defects and climb of thedislocation.A dislocation in the neighbourhood of a cavity will experience an attractive force,either through the mutual interaction of the two stress fields or through the tendencyfor relaxation of the stress field around the dislocation to occur by deformationof the free surface at the wall of the cavity. The force due to mutual interaction ofstress fields is likely to be small in the present case because the long-range stress fieldof a plate-like cavity will have a negligible shear component in the basal plane (i.e., theplane in which the dislocation lies and moves).The situation is very similar to thatof two parallel dislocations with Burgers vectors perpendicular to one another. It ispossible to derive an attractive force due to the mutual relaxation of stress fields alongthe lines discussed by Newman and Bullough 5 for small voids or vacancies. How-ever, in the present situation, where the cavity provides a free surface close to thedislocation, the force can probably be accounted for almost entirely by a surfaceattraction. The analysis of surface image forces made by Head 6 might be appliedto this particular problem but it should be modified to take into account the limitedfree surface offered by a cavity.It is perhaps surprising that interaction occurs over such a long range (the dis-location starts to move when the separation is a few microns), especially since theobservations are made on a thin crystal.The stress field around an elastic singularitymight be expected to decay within a distance of the order of specimen thicknessbecause of relaxation of stresses at the crystal surfaces. However, a longer range ofinteraction might exist in this particular case because the slip dislocations lie in thebasal plane and have Burgers vector in that plane. The stress field is therefore notrelaxed so readily by the free surfaces, particularly if the dislocation is predominantlyin the screw orientation.The subsequent association of the cavity and dislocation after interaction is interest-ing.If the cavity is not forced to move further through the crystal it remains firmlyanchored to the dislocation. Dislocations can be “ decorated ” with cavities, asshown in fig. 2, and are thereby pinned inside the crystal. This provides an illustra-tion of the form of radiation-hardening discussed by Coulomb and Friedel.1 How-ever, if the cavity is forced to move beyond the dislocation (by increasing the beamintensity, for example) this remains firmly attached and extends or trails behind thecavity. Several examples of dislocations trailing behind migratory cavities are shownin fig. 3FIG. 1 .-Electron micrograph showing the physical interaction betweencavities and “ slip dislocations ” in a crystal of lead iodide.x 20,000.FIG. 2.-The “ decoration ” ofFIG. 3.-The trailing of dislocations behind migratory cavities. FIG. 4.-The ‘‘ decoration ” ofx 10,000. migratorFIG. 5a, b, c.-Sequence of electronment of a large sessile dislocationwith cavitiesA. J . FORTY 59The elongation of pre-existing dislocations in this manner is interesting. Theperturbation of the dislocation line is not smoothed out by line tension effects and thismight be taken to indicate a hardening of the structure in the wake of a cavity. Thereare no visible defects here, but it would be surprising if the " re-crystallized " materialin the wake of a cavity migrating at a velocity of several microns per second werehighly imperfect. The aggregation or condensation of point defects in the initialstages of annealing of the imperfect structure might well preserve the elongatedconfiguration of the trailing dislocation.This pinning effect is shown on a moremacroscopic scale in fig. 4 where small cavities have appeared along the trailingdislocations in the wake of large cavities.CHEMICAL INTERACTIONS OF CAVITIES AND DISLOCATIONSThe shapes and relative positions of cavities and dislocations change markedlyif they are sufficiently close for a transfer of point defects and, subsequently, climb ofthe dislocations to take place. The rate of transfer must depend primarily on thetemperature of the crystal (about 200°C in those crystals which undergo rapid de-composition), but the direction of flow of material will be determined by the strainfields of the dislocations and cavities, andwill depend also on the nature of the diffusingspecies of point defect.The exchange of vacancies or interstitials between neigh-bouring sessile dislocation loops, and the importance of this in the annealing behaviourof loops in irradiated or quenched metals, has been discussed already by Kroupa,Silcox and Whelan.7 The corresponding general problem of the transfer of defectsbetween sessile dislocation loops and cavities could be discussed in a similar manner.However, the particular case of chemical interaction in crystals of lead iodide underirradiation with an electron beam is more difficult to analyze; both the anisotropyof the strain fields and of the diffusion of defects in this material and also the large,undefined temperature gradients that are established by the non-uniform exposureto the electron beam must be taken into account.Therefore the observationspresented below, although of considerable general interest, cannot be analyzedsatisfactorily at present.Fig. 5 shows a sequence of electron micrographs of a part of a crystal containinga large sessile dislocation loop (i.e., a dislocation loop lying in the basal plane andhaving its Burgers vector perpendicular to that plane). At a number of places theirregularities in the shape of the loop can be associated with the presence of cavities,either in contact with the loop or close to it. The shape and size of the loop changesmarkedly in this sequence. A sessile loop can move in this fashion only by climb inthe basal plane.This requires a transfer of point defects, most probably molecularvacancies or associated Pb2f and I- vacancies, either along the dislocation line itselfor through the crystal structure between the dislocation and cavities. The apparentcorrelation between irregularities in the dislocation and the proximity of cavitiessuggests that the latter process, involving a chemical interaction, is most likely. Thecavities will exert a physical force on the dislocation because of the mutual interactionof the stress ficlds (this situation is similar to that of parallel edge dislocations inparallel slip planes with similar Burgers vectors) but the loop cannot glide under thisforce. However, the existence of the force will affect the climb of the dislocation.The direction of the force will depend on whether a cavity lies inside or outsidethe sessile loop and, by a transfer of vacancies, the loop can climb away fromsome cavities and towards others under the influence of this force. This accountsfor the irregular behaviour that is commonly observed when chemical interactiontakes place60 CAVITIES AND DISLOCATIONS1 Coulomb and Friedel, Dislocations and Mechanical Properties of C r y & f s (ed. Fisher, Johnson,2 Forty, Disc. Faraday SOC., 1961, 31,247.3 Forty, Phil Mag., 1960,5,787.4 Forty, Phil. Mag., 1961, 6, 895.5 Bulloiigh and Newman, Phil. Mug., 1962,7, 529.6 Head, Phil. Mag., 1953,43,433.7 Kroupa, Silcox and Whelan, Phil. Mag., 1961, 6, 971.Thomson and Vreeland), Wiley (New York), 1956, p. 555