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INFLUENCE OF STRUCTURE DEFECTS ON B1 � B2
PHASE TRANSFORMATION IN ALKALI HALIDES
A. Rabinkin, E. Gutmanas
To cite this version:
C7-302 JOURNAL DE PHYSIQUE Colloque C7, suppldmeult au
no
12, Tome 37, Dkcembre 1976INFLUENCE OF STRUCTURE DEFECTS
ON B1
e
B2
PHASE TRANSFORMATION IN ALKALI HALIDES
A. RABINKIN and E. Y. GUTMANAS
Department of Materials Engineering, Technion, Haifa, Israel
RBsumB. - L'influence de differents defauts de structure (dislocations produites par deformation plastique, limites de grain, inclusion de composants durs et mous, cassures de separation, pas de separation) et de facteurs de gtometrie et de taille sur les processus de nucleation et de croissance des phases pendant la transformation de phase
A
haute pression B1 -+ B2 a Btk etudiQ dans des mono- et polycristaux de RbI, K1 et KBr. La methode de la cellale de Tkflon a ett utilisk pour creer une pression hydrostatique dans un appareil piston-cylindre. Une machine de verification Instvon a Btt utiliste pour le chargement des cellules. I1 a BtB possible d'obtenir la pressurisation avec un intervalle de vitesse large, de determiner la pression de depart requise pour la transforma- tion directe B1 + B2 (Pa) et pour I'inverse B2 -+ B1 (Pr), et d'interrompre & tout moment la tran-sition de phase. Des Pd de transformations BI + B2 plus bas ont kt6 obtenus pour : a ) des cristaux minces,
b) des poudres fines,
c ) des coins et des tords,
d) des surfaces de monocristaux en cassure fraiche, et e) des polycristaux avec inclusions de composants mous. L'influence des autres dkfauts de structure a Cte moins prononcb.
L'hystkrbis la plus forte (l'existence & des pressions plus faibles de phase B2 metastable formke a haute pression) a kt6 observke dans des polycristaux
B
inclusions de composants durs. La plus faible a Btt observee dans des poudres fines.Abstract. - Influence of various structure defects : dislocations introduced by plastic deforma- tion, grain boundaries, inclusion of hard and soft components, cleavage cracks, cleavage steps and of geometrical and size factors on nucleation and growth processes of phases during B1 + B2
high pressure phase transformations in RbI, KI and KBr single and polycrystals was studied. Teflon cell method was used to create hydrostatic pressure in piston-cylinder apparatus. Instvon testing machine has been used for cell loading. It was possible to obtain pressurization with a wide range of rates, to determine the starting pressure required for direct Bl -t B2 (Pd) and the reverse
R 2 -t B1 (Pr) transformations and to interrupt the phase transition in any of its stages. Lower Pd
for B1 + B2 transformations were obtained for : a) thin crystals,
b) fine powders, c) corners and edges,
d) fresh cleavage surfaces of single crystals and
e) polycrystals with soft component inclusions.
The influence of other structure defects was less pronounced. Highest hysteresis (existance of metastable high pressure phase B2 at lower pressures) has been observed in polycrystals with hard component inclusions, the lowest in fine powders.
As well known, when many alkali halides, for example RbI, K I or KCI, are subjected to high pressure phase transformation take place in them with a marked decrease in specific volume and reconstruc- tion of the crystal lattice from the NaC1-type to the CsC1-type structure (B1 -+ B2). Because their transi- tion pressures are comparatevely moderate, samples can be easy obtained in single or polycrystal shapes and different physical properties were well studied a numerous investigations of these transformations were done. I n the last years some works were published concerning more detailed study of the temperature
dependence of transition pressures, hysteresis and mechanisms of transformation [I-41 and influence o n it some experimental conditions [5-61. But the ques- tions concerning the influence of a crystal structure defects on nucleation and growth processes of phases having considerable interest for the theory cannot be regarded as completely solved.
1 . Experimental conditions. - The high purity grade materials were used. R b I powder was obtained from Merck, K I and KBr had purity better than 99.95
%.
Single crystals were grown by Bridgeman method in
INFLUENCE OF STRUCTURE
quartz crucibles. A te$on cell method was used to create hydrostatic pressure in a piston-cylinder appa- ratus. The decanol alcohol was emploied as a pressure medium instead of usual 50 : 50-mixture of isoamyl alcohol with n-pentan because the last one is strongly interacted with RbI. The single crystal samples of RbI, KI and KBr having rectangular bar shapes with ( 100 ) faces and sizes approximately 4 x 4 x 8 mm were obtained by cleavage shaping. The polycrystalline samples having cylindrical shape including samples
DEFECTS ON B1 Z B2 PHASE C7-303
with various quantity and sizes of second component were prepared by pressing powders of alkali halides or mixture of it with second component in piston- cylinder apparatus. An Instron testing machine was used for the loading which made it possible to obtain a wide range of pressurization rates. Also dilatometric data could be obtained for the starting pressures required for the direct B1 + B2(P,) and reverse B2 4 B1 (P,) transformations as well as the associated
hysteresis (is P = P,
-
P,). In single crystals, hydro-C7-304 A. RABINKIN AND E. Y . GUTMANAS statically pressed at 300 K the B1 + B2 transformation
might be interrupted by stopping the loading. Due to big stresses induced by volume changes during the B1
+
B2 transformations the transformed regions were broken up into fine polycrystals and lost their transmissivity. By optical examination of the crystals following loading the location of the nucleated trans- formed regions was easy determined.2. Experimental results. - 2.1 SINGLE CRYSTALS.
- 2.1.1 RbI single crystals in following states were studied.
-
1. After crystal growth and cleavage shap- ing. 2. Containing terminated cracks. 3. After y-irra- diation, with internal<
110>
cracks. 4. After annealing at 800 K. 5. After plastic deformation (10%
compression along [IOO]). The B2 phase first appears when Pd-
0.4 CPa (4.0 kbar) at the crystal corners (fig. la) at cleavage steps (fig. lb) and between cracks for case 3 (fig. lc). The extent of transformed regions is usually a few tenths of mm in size. Due to tensile strains the transformation steps and starts again when P 1.0.5 GPa at locations where B2-phasewas previously present as well as on crystal edges and tip of cracks. In deformed crystals the B2-phase arises only on corners and edges of bars when P, = O.5GPa. Appreciable volume effects were observed in all speci- mens by dilatometry only when P exceeded 0.5 GPa. For chemically polished single crystals Pd was also approximately 0.4 GPa but transformation occured without delay.
2.1 . 2 Single crystals KI and KBr was studied.
-
1. After crystal growth and cleavage shaping. 2. Contain- ing terminated cracks. 3. After annealing at 900 K. 4. With the scraths and pricks on the surfaces. For both substances the same results were obtained for all types of the samples. Up to 2.0 GPa no steps of transformation was observed. The first little kinks on loading diagrams appears at 2.08-2.1 GPa in the case of KI and 2.13-2.15 GPa for KBr. These kinks always were accompanied by noticeable sound clicks. Just after these kinks were appeared the loading was stopped. Optical examination of extracted samples was shown that all the surfaces of the samples were lost its transmissivity. After longitudinal or cross cleavage of these samples one can see the new B2-phase was arisen more or less uniform on all surfaces of the single crystals (fig. 2a, b).These results can be qualitatively explained using the well-known theoretical criteria of 3. Cahn theory f7, 81 for nucleation and growth processes during the phase transformation. According his theory nucleation occurs more readily at corners, edges or surfaces of the crystals or at dislocations thai by homogeneous nucleation when the surface energy or dislocation strain energy are noticeable cornpare with activation
energy for homogenious nucleation. The type of FIG. 2. - Single crystals a) KI and b) KBr after pressurization up to 2.1 GPa and 2.15 GPa, respectively under 300 K x 10.
INFLUENCE OF STRUCTURE DEFECTS ON B1 B2 PHASE C7-305
state o, and energy of boundaries between new and old phases 0,-
,,.
When this ratio exceeds 0.9 the grain boundary energy is high enough to make corner nucleation preferable. Probably the RbI single crystals are good examples of such behaviour. In the range o,/o,-,, = 0.3 - 0.6 nucleation at grain faces or boundaries (in the cases of polycrystalline samples) is favored. And a t lower values of the ratio, homoge- nious nucleation predominates. Unfortunately, there are not any data about 0,-,, in alkali halides. Theabsence of nucleation on slip bands or isolated dis- locations in studied crystals shows that dislocation strain energy is not enough for the promoting nuclea- tion. The geometrical factors and surface energy probably play main role in nucleation of high pressure phases in alkali halides.
2 . 2 POLYCRYSTALS OF RbI. - Powder of RbI (seizes
-
a few pm was compressed in hydrostatic conditions. The three types of curves of B1 + B2- transition can be easy observed changing the rate of pressurization Vp (fig. 3). Curve 1 obtains when V p- A V , Relaitive units
FIG. 3 .
-
The curves of B1z
B2-
transformations in alkali halides obtained under pressurization with the different rates.(See text.)
is low ( < 1 MPa/s) and B2-phase have enough time for nucleation and growth in equilibrium conditions. In this case Pd = 0.4 GPa and AP = 0.076 GPa the P, value is practically the same as for P, of a single crystals. When the pressure transmitting liquid compresses with the rate which is exactly equal to rate of volume changes of a sample one can obtain curve 3. Knowing the sizes of particles and trying to find such rate of loading one can make rough esti- mates of average rate of transformation. Such esti- mates show that the last oneis equal
-
1 pm/s at 300 K.When V p is so high ox the crystals so big that the trans- formation does not have enough time to develop, P, and AP begin to depend on Vp (curve 2). For example, under V , = 0.1 GPa/s (1 000 bar/s) these powders have P, = 0.53 GPa and AP = 0.36 GPa. These data sufficiently less than analogous one obtained in [5] for RbCl and under more moderate
(only up to
--
13 bar/s)rates of pressurization. Polycrystalline bulk specimens were prepared from powders by pressing at 300 K and these hydrostatically pressed. The pertinent values for P, and AP were 0.54 GPa and 0.17 GPa. After 700 K annealing the Pd was only 0.42 and AP was 0.06 GPa.
When polycrystalline specimens were pressed direct- ly inside of the channel of piston-cylinders apparatus P, and AP begin strongly enhance with the number of pressurization cyles, going up from 0.42 GPa and 0.15 GPa for lth cycle to 0.64 GPa and 0.5 GPa during 8th cycle.
Polycrystalline mixture of RbI with various quantity and sizes of second component (soft, CsI, KI or graphite powder and hard, S i c or NaF) were prepared
and studied for simulating high pressure transforma- tions in two phase structures. In the presence of NaF and especially S i c Pd was less than in the case of pure RbI and hysteresis was considerably bigger. The high pressure B2-phase was more stable under unloading. For example, the pressed mixture of RbI with 20 vol.
%
of 120 grade S i c has P, = 0.50 GPa and AP = 0.26 GPa. In the same conditions pure RbI has P, = 0.53 GPa and AP = 0.15 GPa.In the specimens containing 90 vol.
%
CSI or K I rubidium iodide was transformed unusually :P, <P, and negative hysteresis (fig. 4) was observed. This effect can be qualitatively explained by the taking in account the differencies between compressi-
C7-306 A. RABINKIN AND E. Y. GUTMANAS
bilities of involved phases and elastic interaction on the original crystal structure on the value of a pressure the phase boundaries during a loading. More detailed at which the transformations start and on kinetics studies of high pressure behaviour of alkali halides of this transformations and b) the factors which can can be used for the better understanding of such help to stabilize in metastable state at atmospheric phenomena in solids as : a) influence of the states of conditions new high pressure phases.
References
[I] PETRUNINA, T. I. and ESTRIN, E. I., SOV. Phys.-Dokl. 13 [4] LAUKHIN, V. N., RABINKIN, A. G., ESTRIN, E. I., SOV. Phys.-
(1969) 1243. JETP 37 (1973) 1150.
[2] PETRUNMA, T. I., SOSHNIKOV, V. I. and ~ T R I N , E. I., sOV. 151 LACAM, A. and PEYRONNEAU, J., J. physique 34 (1973) 1047. Phys.-Cvystalfogr. 17 ( 1 972) 367. [6] LACAM, A. and PEYRONNEAU, J., Revue Phys. Appl. 10
(1975) 295.
[31 LAUKHIN, V. N., UBINKIN, A. G . and ESTRIN, E. I., SOY. [7] CAHN, J. W., Acta Met. 4 (1956) 449.
Phys.-Sol. State 14 (1972) 1551. [8] CAHN, J. W., Acta Met. 5 (1957) 169.
DISCUSSION
Ch. L. MARQUARDT. -YOU have shown data on gamma-irradiated samples. What is the role played by th radiation-induced point defects in the nucleation of the high pressure phase ?
A. RABINKIN.
