MARTENSITIC-TYPE TRANSFORMATION IN SINGLE CRYSTALS OF POTASSIUM HALIDES

Yu. F. Kozlov

Introduction

After irradiation in a reactor and some holding at room temperature, single-crystal specimens of potassium chloride and bromide often exhibit colorless recrystallized zones of considerable size. The process of successive regular restructuring of the crystal develops from nuclei on the surface of the specimens. In this process, grains of a new phase, of equilibrium form and oriented in a position close to a twin one, grow into the depth of the crystal; these grains have the chemical composition and structure of the parent crystal. In all cases, the growth of grains in the region where they emerge on the surface of the specimen is accompanied by the formation of a relief characteristic of martensitic transformations. According to the definitions given in [1, 2], the transformation under consideration may be classified as martensitic.

Usually, martensitic transformations are understood as self-sustaining diffusionless transformations in certain metals, similar in nature to first-order transitions. The relationship, noted in [3], between plastic deformation by twinning and martensitic transformations (m. t.) is based not only on the similarity of their external manifestations. The anomaly of the plastic properties of some metals at low temperatures is directly associated with low-temperature polymorphism [4]. In processes carried out by cooperative displacement of atoms, the orientational stability of the lattice is essential. This idea was consistently developed in the works of Stepanov [5]. In Scheil’s theory of m. t. [2], it is assumed that at the transformation temperature the initial lattice becomes unstable.

Alkali-halide crystals (a. h. c.) differ substantially from the known materials subject to m. t. in their type of bonding, bond energy, and absence of allotropic transitions under normal conditions over the entire temperature interval of their existence. Plastic deformation of these crystals by twinning is possible at relatively high temperatures [6, 7, 8]. In the range of ionic-radius ratios 0.73–1.0, proceeding only from electrostatic interaction, the more favorable structure for them could be the CsCl structure. The possibility of the existence of the NaCl structure in this range is explained by the increasing mutual polarization of the ions [9, 10]. According to [11], KCl and KBr at room temperature possess sharply pronounced elastic anisotropy, associated with the deviation of the symmetry of the charge distribution in the outer layers of the ions from spherical. Thus, at least

one could have expected anomalies in KCl associated with lattice instability.

The study of low-temperature self-sustaining twinning in f.c.c. itself is of considerable interest. In addition, the observed structural change indicates the possibility that the noted structural features influence the processes of disordering and recovery of the crystal lattice during irradiation and after it.

Brief description of the transformation and some observational results

The specimens under observation were single-crystal samples grown from melts of chemically pure and reagent-grade salts. The crystals were grown in porcelain crucibles in air and, after preparation, were not specially annealed. The surface of the samples after cleavage was not additionally treated. During the observations the samples were at a temperature of \(18 \pm 4^\circ\mathrm{C}\).

One of the important results of the observations may be considered the establishment of the fact that, among the series of crystals considered, only in KCl does the transformation take place without the use of special measures. In KBr crystals the transformation proceeds after they have been irradiated in a reactor. In KI the transformation was observed much more rarely, only after irradiation in deformed regions of the crystal. In other crystals the transformation was not observed.

The hexagonal grains of the martensitic phase in f.c.c. have a distinct pyramidal structure (Fig. 1). The bands by which the growth pyramids are distinguished are slip traces. From the geometry and sequence of appearance of the traces of plastic deformation it may be concluded that, during the transformation process, for some time there occurs basic deformation with accumulation of the elastic energy of lattice mismatch. After a certain critical value of it is reached in the martensitic grain, in the immediate vicinity of the phase boundary, slip develops. Impurities, to which an essential role is assigned in the manifestation of growth pyramids in the case of crystals grown from solution, here play a secondary role, insofar as they participate in the hardening and softening of the crystal. Irregularities in the structure of the growth pyramids are not connected with a nonuniform distribution of impurities in the initial crystal; rather, their dependence on the elastic resistance of the parent crystal is easily seen. In the region where the grain emerges onto the surface, the distance between the slip regions is consistent with the dimensions of the relief steps.

Fig. 1. Plate of a martensitic grain from an irradiated single crystal of potassium chloride

In unirradiated KCl crystals the facets of the grain have a convex, lens-like shape. The lens-like shape of the interface is ...

is characteristic of m.t. and indicates the existence of an elastic interaction between the martensitic and parent phases. In crystals hardened by irradiation, the grains have a more regular shape.

Slip traces in the martensitic phase are groups of angular, droplet-shaped pores (Fig. 2a and Fig. 2b). In external geometry, in the general case they resemble multiplied dislocation loops passing into parallel planes in the slip plane [110]. In some cases the groups constitute a continuous pore, passing continuously through [110] into a parallel plane, so that the layer of the growth pyramid is decorated by an almost continuous pyramidal pore, with its base facing in the direction of grain growth. For a complete similarity between the group of pores and dislocation loops, Fig. 2a should be compared with photographs 25 and 26 [12], which show dislocations in silicon decorated with copper. It may be assumed that the similarity here is not only external. The accumulation of dislocations in the slip region creates a stress field that intensifies the diffusion of vacancies toward the core of the accumulation. Pyramidal pores in the layers, and steps on the faces of a growing grain, may serve as an indication of layered growth of the phase.

Fig. 2a

Fig. 2b. Appearance of a group of pores in mutually perpendicular directions in a grain of irradiated potassium chloride. Magnification 300×.
a — emergence of pores onto the cleavage surface, a “rosette” of self-etching.
b — light is scattered by the combined group of pores.

Martensitic grains of irradiated KCl crystals, owing to slip traces decorated with vacancies, have a clearly expressed layered structure. Both the individual visible pores and their total volume are noticeably greater than in unirradiated crystals. Groups of pores in unirradiated crystals so often split and pass into other slip planes that the growth pyramids have a rather fibrous structure rather than a layered one. The change in the slip pattern after irradiation is analogous here to that observed in the plastic deformation of other materials after hardening or irradiation.

Kinetics of the Transformation and the Action of Various Factors

Visible nuclei of the phase arise more rapidly on fresh cleavages of relatively old crystals. The incubation period for KCl crystals is 4–6 months, i.e., it corresponds to the first stage of aging [13]. According to certain indications, the potential nuclei on which grains subsequently grow under favorable conditions are introduced during the growth of the single crystal. An increase in the concentration of nuclei can be achieved by irradiation.

Under unchanged external conditions and in the absence of any actions, the change in the growth rate of a grain in the direction normal to the phase-boundary surface has a periodic character. In the recorded cases, up to four periods were observed, including acceleration and deceleration of growth. In irradiated crystals of the same chemical composition, one period is observed. The moment of growth retardation always corresponds to a concentration of slip traces. The concentration of traces is replaced by their intense fragmentation. The fragmentation region scatters light intensely. The etch pattern of the surface in the region of slowing of the process resembles a pattern of change in slip planes decorated by screw dislocations (photo 56 [12]). In irradiated and unirradiated crystals the magnitude of the zones corresponding to the periods differs in size and is 1.0 and 0.3 cm, respectively. At the intersection of the concentration zones of adjacent growth pyramids, a change of the growth direction to the perpendicular is most often observed, together with the “capture” by the growing grain of regions of the untransformed parent crystal.

In one and the same specimen, in its adjacent regions, at one and the same moment in time, the rates of development of individual grains may differ substantially. They are small in absolute magnitude in comparison with the rate of formation of martensite in steel, but large enough to indicate a diffusionless mode of transformation. During the period of accelerated grain development, the absolute rate of normal displacement of the phase boundary may be characterized by a value of 0.1 cm/day. The rate of layer development may be estimated as a considerably larger value.

The boundaries of weakly misoriented blocks of the parent crystal are not an obstacle to grain development; the grain in a block crystal also grows as a block crystal. It is sufficient to draw a scratch on the surface of the specimen with a needle or an indenter in order subsequently to stop the transformation in a considerable region of the crystal. Scratches on the surface give rise to slip nuclei that readily develop into entire slip regions [14]. Thus, slip regions in the matrix are an obstacle difficult for the phase to overcome during its propagation.

In addition to accommodation stresses, periodically relieved by slip in the martensitic phase, one may assume the existence of stresses caused by the mismatch of the relative volumes of the phases. Accumulation of the energy of volume mismatch may lead to the development of plastic deformation of the matrix and to retardation of the process. The grain bypasses small plastically deformed zones in the matrix, surrounding it with a cloud of pores (Fig. 1). The explanation of the periodicity of the development of martensitic grains follows from what has been set forth.

On the surface of KCl specimens irradiated with large integral neutron fluxes, polycrystalline crusts are often observed; recrystallization in these cases does not extend farther than 0.01 cm. In control specimens, at the same time, individual grains with dimensions of \(0.2 \times 0.2 \times 0.3\) cm grow. Thus, irradiation has a dual action: a stimulating one, expressed in an increase in the concentration of potential nuclei, and a suppressing one—in relation to...

…with a substantial decrease in grain size. Neutron irradiation and additional mechanical stresses can both stimulate and inhibit the martensitic transformation in steel [15]. In our case, a similar action of the indicated factors can be explained by their ability to create both stressed regions of the crystal, which promote nucleation of the phase, and slip regions, which hinder its development.

The difference in relative volumes in our case may be due to a difference in the defectiveness of the phase lattices. Then the increase in the distance between the striations in irradiated crystals may indicate comparable defectiveness of the transformed and untransformed regions of the crystal.

Surface phenomena

In addition to the foregoing, one should dwell on a feature of the behavior of the surface of cleavage faces of a martensitic grain. This feature consists in its capacity for self-etching. The surface of ordinary control specimens and of cleavage faces of the untransformed region of a KCl crystal, owing to their moderate hygroscopicity, remains clean for a long time. The surfaces of cleavage faces shown in Figs. 3a and 3b were obtained

Fig. 3a. Patterns of self-etching and the result of brief action of acetic acid on the cleavage surface of a martensitic grain from an unirradiated crystal.
a — system of large and small self-etching pits

several minutes after the preparation of the cleavages, without the use of any etchants. Large self-etching pits fit into the known schemes for the arrangement of exits of dislocation half-loops in the system of slip planes. The loop exits in Fig. 3a are associated with the edges of the split layer of pores. Often the dislocation exits decorated by self-etching are a continuation of a group of pores in the plane [100].

“Rosettes” of small etch pits are easily removed by the action of an etchant, but at the end of their natural development they represent formations of a regular skeletal shape. The bottoms of the formations are inclined to the cleavage surface; the edges form considerable ledges (Fig. 3b). If one represents

...the action of the “etchant” constant and take into account the shape of the etch pits, the rosettes are obtained during the intermittent motion of the unsecured edges of screw dislocations in the slip planes of the grain. The observed phenomenon demonstrates a possible path of rapid restructuring of the crystal at low temperatures. Independently of the mechanism by which the etch pits are formed, the phenomenon confirms the existence of significant stress in the martensitic grain. The etchability of the cleavage surface in the unirradiated and irradiated crystal under the action of ordinary etchants is comparable with the etchability of the surface of crystals irradiated in a reactor (Fig. 3c).

b — one of the final stages in the development of the “rosettes.” The photographs were taken several minutes after the formation of the surface.

c — acid etch pits in the region of the layer of pores (dark band). Magnification \(300\times\).

Conclusion

The fact that a transformation possessing all the necessary features of a martensitic transformation is observed precisely in those crystals that differ from related ones by the substantial features indicated is a confirmation of the correctness of the classification made and of the correctness of the developing conception of the internal nature of the transformation. This fact also makes it possible to conclude that a martensitic transformation is not a particular phenomenon characteristic of a narrow group of materials, but a more general physical phenomenon inherent in the crystalline state.

In the example considered, the idea [16] is confirmed that twinning should be understood not as an accidental process developing on the faces of a crystal, but as a manifestation of stages in the development of the crystalline state. It is probable that the structural features of the outer electron shells of ions, and their variability depending on the environment, play an essential role not only in the processes of crystal formation from different media, but also in the processes of restructuring of the crystal under the action of internal and external forces.

The study of spontaneous, heterogeneously nucleated, self-sustaining twinning raises the question of the causes that give rise to the phenomenon and of the mechanism of autocatalysis. The process is initiated by sufficiently strong lattice irregularities. It may be assumed that the possibility of continuation of the process is determined primarily by the orientational instability of the lattice. The process is controlled by diffusion in the fields of the stresses that arise. The transformation mechanism is dislocational. In the course of the process, part of the bond energy is expended which, owing to the defectiveness of real crystals, is usually not utilized.

The author expresses gratitude to Professor A. A. Vorob’ev for his constant attention to the work and for discussion of the results.

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