experimental evidence for pretransitional fluctuations and domain movements

Experimental Evidence for Pretransitional Fluctuations and Domain Movements

J. VAN LANDUYT, G. VAN TENDELOO, M. VAN SANDE, L. DELAEY, A N D S. AMELINCKX

A review is given of the observation techniques by electron microscopy and electron diffraction which allow information to be obtained on phase transitions and pretransitional phenomena in solids. The review is illustrated with a few examples of different types of transitions. The a ~ fi phase transition in quartz is discussed in some more detail. It is shown that in this case defect mobility is responsible for the observed pretransitional phenomena. Observations are reported of shimmering phenomena in two alloy systems: CuZnA1 and a ~,-brass (Cu4A19). The experimental conditions of appearance of the phenomena in these different systems are compared and related with possible physical origins.

1. I N T R O D U C T I O N

E L E C T R O N diffraction and electron microscopy are very powerful tools for the study of phase transitions and pretransitional phenomena. Due to the high scatter- ing power of the electrons the response is very fast and information can be obtained from very small trans- forming areas, whereby the diffraction data as well as the diffraction contrast image can be quickly obtained only instants apart and in the same instrument.

The kind of evidence that is to be expected from transition and pretransitional phenomena can be sub- divided in direct and indirect evidence. In this intro- ductory part we shall illustrate the kind of observables which can teach us something about phase transitions whereby one study will be developed in more detail, namely: the a ~ fi phase transition in quartz. In the final part we shall present evidence for shimmering effects observed in alloy systems: in a y-brass Cu9A14 and in fi-CuZnA1.

2. G E N E R A L ASPECTS OF THE OBSERVATION OF TRANSITIONS

BY ELECTRON MICROSCOPY

2.1. Direct Evidence

Thanks to the availability for modern electron mi- croscopes of heating and cooling stages, direct observations of dynamical phenomena is possible.

2.2. Diffraction Evidence

The symmetry changes accompanying phase transitions are directly revealed in the diffraction pattern either by superlattice reflections or by spot splitting

J. VAN LANDUYT, Assistant Professor, G. VAN TENDELOO and M. VAN SANDE, Research Assistants, are with the University of Antwerp, RUCA; L. DELAEY is Professor with the University of Louvain; and S. AMELINCKX is Professor with the University of Antwerp, RUCA and Director General at the Natxonal Nuclear Center, SCK Mol, Belgium.

This paper is based on a presentation made at a symposium on "Pretransformation Phenomena, Fluctuations and Related Effects" held at the annual meeting of The Metallurgical Society of AIME, New Orleans, Louisiana, February 18-22, 1979, under the sponsor- ship of the Structures Activity, Materials Science Division, ASM.

METALLURGICAL TRANSACTIONS A

associated with lattice deformations. Also atom vibrations wilt be revealed in diffraction patterns by diffuse streaks related to the vibrating mode.

The possibility to observe these changes while occurring allows a determination of the transition temperature and also certain aspects of the nature of the transition such as the appearance of a pretransitional state or intermediate phases.

2.3. Direct Image

By keeping the specimen in the heating holder of the microscope close to the transition temperature it is often possible by making use of the slight heating caused by the electron beam, to create a gradient across the transition temperature over the observed area. In this way any physical phenomenon occurring at the transition can directly be observed. In more sluggish transformations direct observations of successive stages of the process can be observed.

Depending on the type of transition the transformation mechanism can directly be visualized either as:

- -growing nuclei or plates - - a moving discontinuous front - -as a complex configuration of defects forming a

phase "boundary" --dislocation movement associated with a shear

transformation. Examples of each as illustrated in Figs. 1, 2, 3 and 4

will now be discussed. These are transmission electron micrographs, either

taken at different stages of the transition as in Figs. 1 and 2 or "at temperature" in Figs. 3 and 4.

Ordering of interstitial impurities in niobium (colum- bium) was observed to proceed by a nucleation and growth process of nuclei increasing in number and size until impinging in Figs. l(a) to (c). Each nucleus is observed to consist of microtwins growing in two mutually orthogonal directions. The resulting phase has a cross hatched morphology of microtwins of the ordered phase. ~

Another example of successive stages of a transition is represented in Fig. 2. The phase transition between y and fi-phase of 1 T-TaS 2 is observed to occur by the passage of discontinuous front which by temperature

ISSN 0360-213318110511-0715500.7510 �9 1981 AMERICAN SOCIETY FOR METALS AND VOLUME 12A, MAY 1981--715

THE METALLURGICAL SOCIETY OF AIME

Fig. i--Ordering reaction in niobium containing interstitial gaseous impurities. Nucleation and growth of suboxide nuclei gives rise to a coalescence structure of domains.

control can be shifted reversibly across the field of view. This transition corresponds with a semiconductor to metal transition between two incommensurate su- perstructures of the 1 T-phase3

Figure 3 illustrates the phase transition in quartz. The crystal is heated in the heating holder of the electron microscope, to a temperature close to the transition temperature which is 573 ~ The slight beam heating is then used to produce a temperature gradient across the observed area.

From the observations and their analysis it follows that at the transition a high density of defects is formed following a regular network with a mesh size that becomes smaller until unresolvable in the fl-phase (right part of Fig. 3).

These defects are the well known Dauphin~ twins which relate two crystallographically equivalent orientations ct. and 0/2 of the a-phase. The observations have led to a novel interpretation of the a --~ fl transition in quartz whereby the fl-phase can be considered as a time average of the a~ and c~ 2 orientations of the a-phase?

A shear transition at work can also be observed directly by the passage of the transformation dislocations across the field of view. Figure 4 is an "instant shot" of such an observation where the transformation partials are observed as nearly straight dislocation lines

at angles of 60 deg. The passage of these dislocations reshuffles the layers into the polytypic succession 2H.4

2.4. Indirect Information from Structure Defect Analysis

The direct information obtained from transmitted images as described in the previous section is often limited by a reduced resolution due to the experimental circumstances, i.e. heating, cooling. However, a lot of information down to the atomic scale can also be obtained from post t ransformation observations at room temperature. The diffraction data can of course also be used here to detect and analyze phases either in an equilibrium or a quenched-in state.

In particular, information can be obtained from the observation and analysis of lattice defects associated with the transformations. Often these defects play an important role in the transition itself such as the dislocations in a shear transformation or sometimes they are secondary consequences of it such as twins to accommodate strain, or antiphase boundaries originat- ing from the coalescence of domains.

Phase transformations are almost invariably accom- panied by changes in symmetry. A reduction in symmetry occurs in going from the high temperature to the low temperature phase or from the disordered to the ordered phase. This reduction in symmetry results in the formation of a domain structure because the lower symmetry structure can be formed from the higher one in a number of crystallographically equivalent ways giving rise to orientation and translation variants. A typical example of a domain structure resulting from the reduction of symmetry at a transition is shown in Fig. 5 for niobium ditelluride. Niobium ditelluride has a monoclinic structure (pointgroup 2/m) that is derived from the C d / O H / 2 structure (pointgroup 6/m2 m2/m) by a displacive transition?

3. OBSERVATIONS OF P R E T R A N S I T I O N A L EFFECTS

IN VARIOUS M A T E R I A L S

3.1. The a ~ fl Phase Transition in Quartz: a Displacive Transition 3

Quartz is well known to undergo a phase transition at 573 ~ from low quartz a-phase to high quartz fl-phase. Although this transition has been studied often and by many different techniques the actual physical process was not unambiguously characterized. We therefore hoped that electron microscopy could shed some light on the process by direct observation at the transition temperature. The specimens were heated in the heating specimen holder and a gradient was produced by local heating with the electron beam.

3.1.1. Structural Data. a-quartz is hexagonal and belongs to the pointgroup 32 whereas fl-quartz belongs to pointgroup 62. The loss is symmetry is clear from the projection along the c-axis in Figs. 6(a) to (c). Both the alpha and the beta structures are represented, and the alpha phase is represented in its two variants cq and ~x 2.

716--VOLUME 12A, MAY 198 ! METALLURGICAL TRANSACTIONS A

Fig. 2--Successive stages of a phase front between -~- and fl-TaS 2.

Fig. 3 - -The a- to fl-phase transition in quartz. A large number of Dauphin6 twin domains is observed to reduce size until below resolution.

Fig. 4- -Par t ia l dislocations required for the shear t ransformation from the 1T to 2H polytype in TaS 2.

M E T A L L U R G I C A L T R A N S A C T I O N S A V O L U M E 12A, MAY 1981--717

Fig. 5 - -Example of a defect structure con- sisting of orientation and translation variants caused by an ordering transition in NbTe: . The orientation varmnts exhibit background intensity differences. Annphase boundaries (a few are marked A) delimit the translation variants.

The structure consists of interlinked SiO4 tetrahedra which are projected as trapezia in the alpha phase and as squares in the beta phase. The relationship between the a~ and a 2 variants is that of a rotation twin with the c-axis as twin axis: the Dauphin6 relationship.

3.1.2 Observations. Close to the transition a large number of defects is observed. These defects are very mobile at temperatures just below the transition. In bright field images these defects are hardly visible, whereas a pronounced dark-light contrast is observed in the dark field images of particular reflections such as e.g. the (3051) reflection in Fig. 7. These features together with the observation of only two variants allow us to identify these defects as the Dauphin6 twins described above, and the contrast as structure factor contrast. Indeed the extinction distances (structure factors) for some reflections which are simultaneously excited in the two crystal parts are drastically different e.g. for 303-1 t~. = 6049k t~2 = 1437k. In Fig. 3 a typical

configuration of defects is shown in a specimen where a temperature gradient of a few degrees across the transition is produced by the electron beam. It was furthermore observed that the walls of the columnar domains of Dauphin6 twins are constantly vibrating (Fig. 7). The closer to the transition the smaller the mesh size of the regular network of domains until they reach sizes below the resolution limit of the microscope which is about 20k under these experimental conditions. Recalling the relation between the a~ and a 2 variant, it is clear that the vibration of a Dauphin6 twin boundary corresponds on an atomic scale with the libration of the interlinked SiO 4 tetrahedra. Noting from Fig. 6(c) that the beta-position is the mean position between cq and a2, and in view of our observations about the mobility and the size of the Dauphin6 domains we conclude that the beta-phase has to be interpreted as a time average of the alpha-phase variants.

Just prior to the transition, as is evident from the cin6

(a)

0 0

, - . . . . . . . - ( D - . . . . . . 7

i / '

i i I / O

O~ 0 o ,' 0 /

i I 0 I I ' - . . . . . . . { 3 - . . . . . . . ~ ' 0~1

0 0

Q

(c)

fb)

/s

,'- i 7

" . . . . I . . . . 1 . . . . . "

Q Q

, . . . . . . . . -<3- . . . . . . . 7 III ~ i II

d ' o o i I

I I Q i I

/ - - f

c . . . . . . . 0 . . . . . . . . ~ 2

Q Q

Fig. 6- -St ruc ture model for quartz as pre- sented along the c-ax~s. (a) a-quartz, one variant e.g. at, (b) the other variant of a- quartz, a_,, (c) a t, a , and the projection of the fl-phase (tetrahedron projected as a sphere).

7 1 8 - - V O L U M E 12A, MAY 1981 M E T A L L U R G I C A L TRANSACTIONS A

Fig. 7 - - D a u p h i n e twin boundaries as imaged in dark fieM by structure factor contrast. Notice also the area in grey shade in a different configuration m the two photo- graphs. The grey area is due to high frequency vibrations of Dauphine twin boundaries.

film that was produced on this transition, a shimmering is observed which in the present situation has to be associated with the mobility and high frequency vibration of the Dauphin6 twin boundaries.

3.2. Shimmering Effects in the beta-CuZnA1 Alloys

At the Boston conference a more extensive account will be given on the shimmering effects in beta-CuZnA1 alloys. 6 1 want to stress in particular some observational aspects of the shimmering of this system, where our attention was focused on the microstructural aspects.

At room temperature this alloy exhibits a quite complicated microstructure. A [110]~ diffraction pattern like shown in Fig. 8 contains apart from the fundamental and B 2 and D O 3 superlattice bcc reflections supplementary streaked spots to be attributed to a 9R martensite structure. All these different reflections have been indicated on Fig. 8.

Under bright field conditions or using a low index bcc fundamental reflection, strong intensity fluctuations are observed; most clearly at the edge of the low index bend contours. The amplitude of these fluctuations is moreover a function of the beam current (temperature) since increasing the size of the 2nd condensor aperture also increases the amplitude. After repeated cycling the response of the alloy is lower and after a number of times the shimmering is hardly observable.

Making dark field micrographs using a B 2 or a DO 3 superstructure reflection an antiphase boundary structure is observed but in both cases the boundaries are perfectly stable. On the contrary dark fields in a 9R reflection reveal a small scale domain fragmentation with one variant lighting up. At higher magnifications these domains are seen to contain a large number of two dimensional defects, most probably stacking faults along the close packed planes associated with the

formation of the 9R phase. When viewing along [I 10]B these faults are seen edge on and are observed to vibrate continuously.

They sometimes change position or even disappear. At this magnification they resemble very strongly the twin mobility observed near to the a ~ fl transition in quartz? Moreover within a few minutes the configuration of the orientation variants sometimes changes completely (Fig. 9). Also these dark field observations

Fig. 8- -Dif f rac t ion pattern of CuZnA1 where the relevant spots have been marked,

M E T A L L U RG1CAL T R A N S A C T I O N S A VOLUME 12A, MAY 1981--719

Fxg. 9--Dark field images as recorded in the 9R-spots at high magnification. Notice the changes in configuration of the orientation as well as translation variants. Arrows mark reference points for comparison of defect configurations.

are time dependent and after repeated cycling the fluctuations becomes less important and finally vanish almost completely.

These two kinds of instabilities i.e. the defect mobility observed in dark field at high magnification and the

shimmering effect observed at lower magnifications most probably have the same physical origin, although this is difficult to prove unambiguously by electron diffraction techniques only.

3.3. Shimmering Effects in T-Brass

In a T-brass of composition A14Cu 9 a typical shimmering is observed by heating the specimen with the electron beam. The effect observed here appears at low magnification (20.000 times) very similar to the two previous cases; we therefore envisaged observations at very high magnification and we tried to correlate the shimmering phenomenon with the structural particu- larities of this phase.

The general aspects of the phenomena observed can be summarized as follows:

1) The effect is observed above room temperature upon heating the specimen with the electron beam.

2) Some aging seems to reduce the magnitude of the phenomenon after repeated cycling.

The structure of this alloy phase is usually described as a slightly rhombohedral ly deformed cesium chloride structure composed of 26-atom clusters of two slightly different compositions. In a study on structure imaging of these T-brasses 7 it was shown that the cluster configuration could be clearly imaged as shown in Fig. l0 as a centered square array of two kinds of dots with different intensity corresponding with the two compositionally different clusters. These observations were also performed on "shimmering" specimen areas, and particular attention was paid to the effect of the shimmering on the structure imaging and vice versa. A video recording was made where the cluster images are clearly distinguished and repeated viewing enabled us to conclude that the cluster images remain stationary on a fluctuating background. No defect mobility was observed of the kind reported in the two previous cases.

It should also be mentioned that in the present case

Fig. 10~High resolution image of the rhombohedrally deformed T-brass (CsCl)- structure. The two types of clusters are imaged in different intensity. The high resolution imaging is not hampered by the shimmering.

720--VOLUME 12A, MAY 1981 METALLURGICAL TRANSACTIONS A

no phase transition is known to occur for this alloy phase in the temperature range under consideration. To further ensure this we have heated the specimen in the microscope to temperatures way above the shimmering conditions and no transition was observed.

4. G E N E R A L R E M A R K S C O N C E R N I N G S H I M M E R I N G EFFECTS A N D

P R E T R A N S I T I O N A L P H E N O M E N A

The three reported cases in the present paper and the other literature available on this subject clearly show that all shimmering observations do not have the same origin.

In some cases e.g. quartz, CuZnA1, defect mobility in a pretransitional state can be called responsible for the observations. In quartz this is very clearly the Dauphin6 twin boundaries which vibrate at a very high frequency. In CuZnA1, orientation and translation variants are mobile in the 9R surface martensite.

The type of shimmering reported here for the "g-brass and observed by us and others in various alloy systems must be due to a (or) different kind(s) of physical phenomena. In many cases it could be shown that no transition was expected in the temperature range under consideration, a fact which is furthermore substantiated

in a striking way by the observations reported at th~s conference by Wayman et al s on shimmering effects in pure elements such as Cu and graphite.

Although somewhat in conflict with the case of graphite it is the opinion of some of the authors that in these cases the physical phenomenon responsible or the visual effects may occur only in a surface layer, which may be due to the polishing technique or to an adsorbed layer. It is clear however, that further experiments are required to elucidate this fascinating phenomenon.

R E F E R E N C E S

1. J. Van Landuyt: Phys. Status Solidi, 1964, vol. 6, p. 957. 2. J. Van Landuyt, G. Van Tendeloo, and S. Amelinckx: Phys.

Status Solidi (a), 1976, vol. 36, p. 767. 3. G. Van Tendeloo, J. Van Landuyt, and S. Amelinckx: Phys.

Status Solidi (a), 1976, vol. 33, p. 732. 4. J. Van Landuyt, G. Van Tendeloo, and S. Amelinckx: Phys.

Status Solidi (a), 1974, vol. 26, p. 585. 5. J. Van Landuyt, G. Remaut, and S. Amelinckx: Phys. Status

Solidi, 1970, vol. 41, p. 271. 6. L. Delaey, G. Van Tendeloo, J. Van Landuyt, and Y. Murakami:

Proc. of lnt. Conf. on Mart. Tf., p. 520-25, ICOMAT, Cambridge, MA, 1979.

7. M. Van Sande, J. Van Landuyt, and S. Amelinckx: Phys, Status Solidi (a), 1979, vol. 55, p. 41.

8. K. Otsuka, H. Kubo, and C. M. Wayman: Metall. Trans. A, IS81, vol. 12A, p. 595.

METALLURGICAL TRANSACTIONS A VOLUME 12A, MAY 1981--721

experimental evidence for pretransitional fluctuations and domain movements

Documents