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Order-disorder phenomena in the platinum rich part of the PtV phase diagram
OOOl-6160/86 83.00 + 0.00 Pergamon Press Ltd
Acta metaN. Vol. 34, No. 1, pp. 43354, 1986 Printed in Great Britain
ORDER-DISORDER PHENOMENA IN THE PLATINUM RICH PART OF THE Pt-V PHASE DIAGRAM? D. SCHRYVERS’ ‘Rijksuniversitair
and S. AMELINCKX’J
Centrum Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, and %C.K./C.E.N. Mol., Boeretang 200, B-2400 Mol, Belgium
Belgium
(Received 30 May 1985) Abstract-The Pt-rich side of the Pt-V phase diagram is re-examined using electron diffraction and high resolution electron microscopy. An ordered Pt,V structure isomorphous with Pt,Ti is discovered and two long period antiphase boundary structures are found in Pt,V on annealing in the temperature interval between 900 and 1000°C. A formation mechanism for the stable DO,, phase is presented and some structural defects are discussed. Microdomains corresponding to the ordering relations extracted from the diffuse intensity distribution in reciprocal space are revealed using the high resolution technique. Resum&Nous avons etudie a nouveau la partie riche en platine du diagramme de phases Pt-V par diffraction Clectronique et microscopic electronique a haute resolution. Nous avons dicouvert une phase ordonnee Pt,V isomorphe a Pt,Ti et nous avons trouve deux structures antiphases a longue ptriode dans Pt,V recuit entre 900 et 1000°C. Nous presentons un mecanisme de formation pour la phase DO,, stable et nous discutons quelques defauts de structure. Nous avons observe par la technique de haute resolution des microdomaines correspondant aux relations d’ordre obtenues a partir de la repartition de l’intensite diffuse dans l’espace reciproque. Zusammenfassung-Die Pt-reiche Seite des Pt-V-Phasendiagrammes wurde mittels Elektronenbeugung und hochaufliisender Elektronenmikroskopie nochmals untersucht. Hierbei wurde eine geordnete Phase P&V, die mit Pt,Ti isomorph ist, entdeckt. Nach Anlassen im Temperaturintervall zwischen 900 und 1000°C finden sich in der Pt,V-Phase zwei langperiodische Antiphasenstrukturen. Ein Bildungsmechanismus wird fiir die stabile Don-Phase vorgeschlagen; aul3erdem werden einige Strukturfehler diskutiert. Aus der Verteilung der diffusen Intensitat im reziproken Raum wurden Ordnungszusammenhinge abgeleitet; die entsprechenden Mikrodomlnen wurden hochauflosend abgebildet.
1. INTRODUCTION
In the course of our systematic investigations on ordered phases and formation mechanisms in transition metal alloys by means of high resolution electron microscopy (HREM) and electron diffraction (ED) a study was performed on the Pt-rich side of the Pt-V phase diagram [l-4]. Although the phases in this part of the diagram are known to be based on an f.c.c. lattice up to the composition Pt,V, only three ordered structures were reported until now; the high temperature Ll, (Cu,Au) structure and the low temperature DO** (Al,Ti) structure with a 3: 1 bulk composition and the PtZV (N&MO) structure at the composition 2: 1 [5]. Since several new long period modulated phases were recently found with compositions intermediate between 8: 1 and 3: 1 in the Pt-Ti alloy system [6], it was not unreasonable to expect a similar behaviour in the Pt-V system. A reinvestigation of the Pt-V system was therefore considered worthwhile. In particular the long period antiphase boundary (LPAPB) structures of Pt-Ti are built with units of the Pt,Ti and the Pt,Ti (Ll,) TWork performed under the auspices of the association R.U.C.A.S.C.K. IIKW-NFWO,
with Brussels,
financial
support
of
the
43
structures and thus the existence of the DO,, structure at the Pt,V composition looked very promising for the occurrence of new ordered phases. Selected area electron diffraction and high resolution electron microscopy are very useful techniques for this kind of investigation. The small areas ( < 3 x lo4 nm’) giving rise to the diffraction pattern and the direct relationship between the high resolution image and the structure enables one to unravel complicated electron diffraction patterns rather easily. More specifically, many ordered structures based on the f.c.c. lattice are so-called column structures; when the structure is viewed along a particular orientation, mostly a cube zone, every column consists of one chemical species. Under the appropriate imaging conditions the high resolution image along such a zone shows a pattern of prominently bright dots corresponding to the geometrical configuration of the minority atom columns. These bright dots can then be used as an imaging code for the interpretation of the atomic structure at single defects in the ordering. The following chapters contain an overview of the results obtained in Pt-V alloys with compositions 8:1, 6:1, 5:1, 4:1, 3:l (=75:25), 71:29 and 2:l expressed in atomic ratios. The most important re-
SCHRYVERS
44
and AMELINCKX:
Pt-V PHASE DIAGRAM
suits will in this case be situated in the vicinity of the 3 : 1 composition. 2. THE
PtsV COMPOSITION
Pt,V samples homogenized for 1 day at llOO”C, quenched in water and annealed for 2 days at 700°C exhibit the cube zone electron diffraction pattern shown in Fig. l(a), where the indices refer to the basic f.c.c. lattice [l]. The distribution of superspots is identical with that of the [OOl] zone of the PtsTi superstructure while tilting experiments reveal that the entire reciprocal lattice is the same as that of Pt,Ti. In Fig. l(b) the projection of the Pt,V superstructure along its shortest axis is presented; large dots are vanadium atoms while small dots are platinum atoms. Open and solid dots belong to two different levels z = 0 and z = l/2 of the f.c.c. lattice, indicated in Fig. l(b) by solid lines. The dashed lines show the body centered tetragonal unit cell of the P&V phase. The new superstructure has a period of three f.c.c. unit cells in two mutually perpendicular [loo] directions, while the thickness remains one f.c.c. unit cell. The point group is 4/m 2/m 2/m. In the high resolution image shown in Fig. 2 the Pt,V structure is viewed along the same direction as the electron diffraction pattern and projected cell of Fig. 1. The circle in Fig. l(a) indicates the reflections selected to form this high resolution image; the basic (200) f.c.c. spots are included. For all the following high resolution images the same aperture will be used. The geometrical configuration of bright dots resembles that of the vanadium columns but, due to the projected nature of a high resolution image, no distinction can be made between columns of vanadium atoms at z = 0 or z = l/2. The drawn square represents the body centered unit cell. In the upper left corner a computer simulation (specimen thickness t = 8 nm, defocus Sf = -70 nm) made using the real-space method [7] is shown indicating that under
(4
‘\
FCC
.
(b)
i-i
.,/.
3
“q’ .
Y
.
Fig. 1. (a) Cube ED pattern of the Pt8V structure. The indices refer to the basic f.c.c. lattice and the circle indicates the selected diffracted beams to produce the HR image of Fig. 2. (b) Projected atomic model of Pt,V; the dashed lines represent the body centered tetragonal unit cell. these imaging conditions the white dots appear at the positions of projected vanadium columns. This result will be used as an imaging code; in all following high resolution images the white dots are interpreted as projected vanadium columns. Like in the Pt-Ti case no preferential orientation was found for the interfaces between two orientation variants of the ordered structure. 3. THE
Pt,V
COMPOSITION
At variance with the results on Pt-Ti, no new long range ordered phases could be determined in the large
Fig. 2. HR image of Pt,V viewed along the same direction as the ED pattern and model of Fig. 1. The inset shows a computer simulated image at t = 8 nm and Sf = -70 nm indicating that under these imaging conditions the white dots correspond with the V columns.
SCHRYVERS
and AMELINCKX:
Pt-V PHASE DIAGRAM
Fig. 3. HR image of small domains exhibiting the Pt,V structure found in samples with a 6: I nominal composition. The corresponding ED pattern is given as an inset, composition range between 8: 1 and 4: 1. In 6: 1 samples annealed for several days at 900 or 7OO”C, only small ordered domains exhibiting the Pt,V structure could be produced. High resolution images of these domains viewed along a cube zone are shown in Fig. 3, while the corresponding electron diffraction pattern is given as an inset. The areas showing a bright dot pattern are viewed along the same direction as the image of Fig. 2, i.e. along the a3 axis of Pt,V, while the line patterns arise from regions viewed along the a, + az direction. Due to the overlap of neighbouring orientation variants, many lines are seen to penetrate into some dotted zones. The elongation of the P&V superspots is due to the random distribution of domains belonging to one orientation variant. Beams diffracted from such domains interfere as if they were caused by one large ordered domain exhibiting a high density of aperiodically arranged antiphase boundaries (APB’s). Since many of these small domains are viewed along an a, + a* direction only producing superspots in the [lOO]* or [OlO]* directions these reflections will be more pronounced than those at (MO) positions. The difference between the stoichiometric 8: 1 composition inside the domains and the nominal 6: 1 composition of the bulk material can be accommodated at nonconservative APB’s in between different domains as well as by substitutional positioning of some vanadium atoms at sites belonging to the platinum sublattice in the stoichiometric Pt,V structure. As the samples are annealed much longer (1 week at 900°C) the Pt,V domains grow larger and become comparable to those found after 1 day at 900°C in the 8: 1 composition and the replacement of platinum atoms by vanadium atoms becomes the main manner for obtaining the bulk composition 6: 1. 4. THE P&V AND Pt,V COMPOSITIONS Homogenized diffuse intensity
samples of both compositions show contours like the ones encountered in
the 3 : 1 composition. In the next section an extensive discussion on the origin of such diffuse intensity will be given. Only after a long annealing treatment of 1 month at 700°C the 4: 1 material could partially be ordered in the Ll, structure. This result together with those presented in the previous chapter indicate that in the Pt-V alloy system no new long period ordered phases are produced in the composition region in between 8: 1 and 4: 1. If any ordering can be achieved, it consists of substitutionally disordered derivates of the simple Pt,V and Ll, structures. This means that on the average the same unit cell holds as in the stoichiometric situation, but some randomly chosen A sites will be occupied by B atoms to accommodate the bulk composition. 5. THE Pt3V COMPOSITION
The situation described by Waterstrat [5] indicated a clear cut transition between the high temperature (1OOOC) y -Pt phase and the low temperature (9OOC) DOZ2 phase at the 3 : 1 composition. A re-examination of this alloy however, using selected area electron diffraction and high resolution electron microscopy revealed the existence of a number of intermediate long period modulated structures in the temperature range between 1000 and 900°C. However, no long range ordered regions exhibiting the Ll, phase could be found at the nominal 3: 1 composition. In this chapter we will describe our results in a succession corresponding with decreasing temperature. 5.1. Samples quenched fLom 1050°C: microdomains When samples are water quenched from 105O”C, i.e. from above the ordering temperature, to room temperature the electron diffraction pattern shown in the inset of Fig. 4 is found. Apart from the sharp f.c.c. spots of the [OOl] zone the pattern exhibits streaks along the h = 2n + 1 and k = 2m + 1 lines. From tilting experiments and from the fact that the streaking vanishes in the outer parts of the electron
SCHRYVERS
46
and AMELINCKX:
Pt-V PHASE DIAGRAM
Fig. 4. HR image of the SRO transition state in Pt,V exhibiting; (A) Ll, squares, (B) DO,, triangles and (C) rotated small squares due to APB’s perpendicular to the electron beam. The corresponding diffuse intensity is shown in the inset. pattern it can be concluded that the diffuse intensity is confined to rather sharply defined lines in reciprocal space, their loci being defined by equations such as
diffraction
h=2n+l,
l=O.
Along these streaks reinforcements of diffuse intensity occur at the (loo), (010) and (110) positions. One way of extracting information from the pattern of diffuse intensity is based on the cluster theory. From the geometry of the locus of the diffuse intensity one can deduce the geometry of the prevailing clusters and the occupancy of the sites of these clusters. In the present case this theory predicts that these clusters should be tetrahedra of nearest neighbours on the f.c.c. lattice and having the macroscopic 3: 1 composition [8]. This cluster relation is satisfied in the Ll, structure as well as in the DO,, structure; it is thus also satisfied along the conservative APB’s in the LIZ structure, at which locally the DOZ2 structure is realized. In Fig. 5 this cluster is indicated in both structures, left Ll, and right DOZz. Since the reinforcements along the streaks are located at the potential sites of the superspots of a long range ordered LIZ domain, a predomination of unit cells of the Ll, phase is expected. In this material the predictions of the cluster theory could be checked by means of high resolution electron microscopy [4]. Since both Ll, and DO,* are column structures when viewed along a cubic direction, the geometrical configuration of the bright dots observed in Fig. 4 directly indicates the stacking of building units of these structures; again the pattern reveals the geometry of pure vanadium columns. It is clearly seen that the geometry of bright dots suggests the Ll, type of ordering (squares at A) fragmented by APB’s along which DOZ2 type configurations (triangles) occur. Locally small patches of the DO,, structure, viewed as centered rectangles are observed as well (B). The smaller square arrangement of dots at C rotated over 45” with respect to the Ll, square, is
consistent with the image of a microdomain of DOZ2 structure viewed along its c-direction. However this geometry is also consistent with a number of Ll, microdomains wrongly stacked along the viewing direction resulting in one or more APB’s perpendicular to this direction. The high contrast image of Fig. 4 disappears in thicker areas of the specimen since the number of APB’s perpendicular to the electron beam and thus disturbing the column structure arrangement will increase. In thinner areas the high resolution image reveals the basic f.c.c. lattice. Although some non-conservative APB’s are visible, violating the cluster relation and thus probably due to different nucleation sites in the matrix, the overall arrangement of bright dots strongly suggests the growing of microdomains following the cluster relation extracted form the geometry of the locus of diffuse intensity in the previous paragraph. These microdomains, not exceeding the size of 4 unit cells, are the only evidence for the Ll, phase in Pt,V. These results confirm the statement that the Ll, phase is only stable at higher Pt contents [5]. 5.2. Samples annealed LPAPB phases
between
900
and
1000°C:
In the course of the experiments it became clear that it was rather difficult to produce the stable DOZ2
Fig. 5. The tetrahedron of first nearest neighbours and having the stoichiometric composition 3 : 1 as a building unit for the Ll, structure (left) and the DO,, structure (right).
AMELINCKX:
Pt-V
PHASE
47
DIAGRAM
structure is a derivative of the DOZ2 structure. The (101) and (103) reflections of the DOZ2 structure are split into two satellites, their separation being equal to l/3 of the length of the diffraction vector 002. The splitting is in a cubic direction and is symmetric with respect to the DO2* spot positions, i.e. the fractional shifts are + l/2. Using the geometrical method of fractional shifts [9] one finds: (1) the basic structure is DOzz (from the positions of the most intense spots); (2) the APB’s are perpendicular to the [OOl] direction of DO,, (from the orientation of the satellite sequences); (3) the distance between APB’s is 1.5 times the DO,* c-axis (from the separation of the satellite spots) (4) the displacement vector R of the APB’s is (1/2)[ 1 lo] (DO,,) (from the fractional shifts).
206
200 (2?0)
(2G2)
100 . .Y.
(2%) 106 .
. .X.
(101)
(103) 006
(b)
OF0
(0:2)
(OF,+
. .
Fig. 6. (a) Cube ED pattern found in Pt,V samples annealed for 1 day at 960°C. (b) Schematic drawing of one quadrant of (a) (see text for legend of indices).
phase at e.g. 900°C as should be possible according to the phase diagram stated by Waterstrat [5]. Instead a number of different electron diffraction patterns were observed, indicating the existence of several new LPAPB structures in the temperature range between 900 and 1000°C. When a homogenized sample is annealed for 1 day at 960°C and then water quenched, the cube zone electron diffraction pattern shown in Fig. 6(a) is found [3]; one quadrant of the pattern is reproduced schematically in Fig. 6(b), the size of the dots being a rough measure for the relative intensities of the corresponding diffraction spots. The reflection at the (101) f.c.c. position is overpronounced due to the incorporation in the selected area of an orientation variant with the long period parallel with the electron beam. All reflections can be indexed on a primitive tetragonal unit cell with lattice parameters a = b = a, and c = 3a,, a, being the f.c.c. basic unit. These indices have been indicated in Fig. 6(a) without brackets. The crosses mark the positions of reflections due to the potential DO,, ordering; their indices are indicated between brackets. From (a) it is clear that the spots at or closest to the DO,, positions are the most intense ones, suggesting that the super-
Table
In Table 1 it is shown that the calculated fractional shifts using R are indeed consistent with the observed ones. In Fig. 7(a) a proposed model, based on these considerations is presented; only vanadium columns are given. In order to verify this structure further, high resolution electron microscopy images along the adirection [Fig. 7(b)] using the bright field mode including the first shell of f.c.c. reflections were made. Viewed along the u-direction the proposed structure
APE
APE
APE
G-4
I. R =(l/2)(llO]
g.R k+
Ohs.-(mod.
002 200 004 101 103
0 0 0 I 12 Ii.2
g.8 1)
Calc. (mod. 0 I 0 117 1:;
I)
Fig. 7. (a) Projection of the proposed atomic model for the LPAPB structure corresponding with the ED pattern of Fig. 6: only V atoms at two different levels are indicated. The building units DO,, and Ll, are also given. (b) HR image of the 960°C LPAPB structure viewed along the same direction as the ED pattern and the model; the Ll, squares and D022 triangles can clearly be distinguished.
48
SCHRYVERS
i
AMELINCKX:
Pt-V PHASE DIAGRAM APB
APB
R+I . t . . ._-_*__-_,__,____,__i =1&l t
l
.
APB
Aq[
.
pi
. ; ’ L---_~_---__O-_~--___~--_--l
.
.
.
.
.
.
.
1 . . b
(4 202 II:
200 0
101 .X.
@I 000 @
204 0
103 .X.
0:2
0
004
Fig. 8. (a) Cube ED pattern found in Pt,V samples annealed at 920°C. (b) Schematic drawing of one quadrant of (a) (see text for legend of indices). is a column structure. Using the same imaging code as for the P&V structure, one can conclude that the configuration of bright dots is the same as that of vanadium columns in the proposed model. The APB’s are visible as rows of squares, while the basic DO,, structure in between the APB’s is seen as a’ pattern of triangles. When a sample of the same bulk is annealed at a somewhat lower temperature, e.g. 920°C another LPAPB structure is formed [2]. The electron diffraction pattern revealing the satellite spots is shown in Fig. 8(a), while the DOzz spot positions again used as a reference are indicated by crosses at b. When the streaks are neglected, this diffraction pattern only differs form the previous one in the actual distance between two satellite spots of the same sequence; it is now nearly l/4 of the length of the diffraction vector 002. As a result the new superstructure will have the same orientation for the APB’s, the same displacement vector, but a different distance between two subsequent APB’s, now equal to 2 times the DO,, c-axis. The proposed model is drawn schematically in Fig. 9(a) and has lattice parameters a = b = a, and c = 8a,. A high resolution electron microscopy image viewed along an u-direction is shown in Fig. 9(b). Again the bright dots reveal the same structural configuration as the vanadium columns in the proposed model. The arrows point at stacking faults in the sequence of APB’s, resulting in extra atom layers in between the APB’s. The faint streaking in reciprocal space perpendicular to the APB’s is caused by these defects. When the same material is treated for several weeks below 900°C finally large areas of DO,, structure are
(b) Fig. 9. (a) Projection of the proposed atomic model for the 920°C structure. (b) Corresponding HR image. The arrows indicate faults against the stacking sequence of the APB’s
found occasionally revealing single APB’s of the type encountered in the long period structures. The ordering mechanism at these temperatures will be explained below. 5.3. Stiuctural considerations
In the high resolution image of Fig. IO(a) a set of ledges, indicated by arrows is visible in the 920°C phase. From the configuration of bright dots in the vicinity of such a ledge it can be concluded that the APB, revealed as a row of squares, is deplaced by one f.c.c. unit cell in the direction perpendicular to the plane of the APB. The low contrast area at the position of the ledge indicates a local disturbance of the ordered atom columns. From the enlargement of Fig. 10(b) it is seen that the ledges appear at subsequent APB’s and that, in between two ledges the distance of two APB’s is increased by half a cparameter of the DOz2 unit cell, i.e. one triangle more. It is believed that this structure leads to a more uniform distribution of the excess strain energy associated with a local fault in the ordering sequence produced during the growth of the ordered domain. The set of ledges can be regarded as a wavy APB running from the upper right corner to the lower left corner in the high resolution image. The displacement vector is (1/8)[441] in units of the long period structure, i.e. (1/2)[11 l] with respect to the DO,, structure. This displacement vector is a lattice vector
SCHRYVERS and AMELINCKX:
Pt-V PHASE DIAGRAM
Fig. 10. (a) HR image of a set of ledges and a misfit between two ordered domains with parallel long period. ib) Enlargement of three ledges: a DO,, triangle and Ll, square are indicated, of the I30,, structure, but not of the LI, structure; it closed dots indicate fuif pure vanadium columns. The therefore only influences the atomic configuration at gradual change from pure vanadium to pure piatthe periodic APB’s which locally have the L12 strum- inum columns is depicted by dots of decreasing size.
ture. At these sites a lower contrast is observed that can be explained by the overlap of the atomic ~O~~~~ra~~Q~s on both sides of the ~nterfa~~l~]. Such an overlap causes the gradual change of pure vanadium columns in pure platinum columns and can be accomplished by an inclination of the APB plane. In Fig. 11 the atomic configuration at two adjacent ledges is schematically presented; large open and
0000000000000000
Fig. i 1. Schematic drawing of two ledges indicating the gradual change from pure V to pure Pt columns depicted by dots (only V columns are shown) of decreasing size. 7” displacement vector is also indicated.
A description which is consistent with this feature Can be given in terms of the step function J&.X) introdu~ by Fujiwara in 1957 fll] to describe the sequences of antiphase domains in LPAPB structures (left part in Fig. 12). Suppose an increase in temperature rounds off the discontinuous steps as schematically drawn in the right part of Fig. 12, some APB’s will no longer be confined to one atom plane (for a more extensive discussion using this description see e.g. f123). The positions of the unit cells, located close to such a step become undetermined and hence also the positions of the minority atoms indicated by black dots; a given atom ma.y join either one or the other of the antiphase domains adjacent to the discontinuity. As a result the cohnnns at these sites will contain platinum as wefi as vanadium atoms causing a decrease in contrast under the current imaging _mditions. The periodicity of the basic step function will impose the same periodicity for the ledges, which is four Ll, unit cells in the case under consideration.
50
._____. .---__, ___.___ _--.___ ___ 5___.___ b____ SCHRYVERS
and AMELINCKX:
*_____.
._____.
___.___
___.___ .__-__.
.__-__.
c__-___L ._____,
?
_:
I___.___ I___.___
r-J
r-J
Fig. 12. Fujiwara’s step function and corresponding atomic positions at low (left) and high (right) temperatures.
In the image of Fig. 10 also a misfit between two ordered domains is seen. The white dots at the border planes on both sides of the misfit still mark the positions of vanadium columns having the DO,, ordering with respect to the adjacent cubic Pt-V plane away from the boundary. From the image however, it cannot be concluded unambiguously whether two or three cube planes parallel to the defect plane exist in between these two border planes. A measurement using the perfect long period structure away from the misfit as a measuring gauge is not useful, due to relaxation effects. In the case of three planes, they are probably locally ordered using the DOrr building unit (triangle) or the Ll, one (square). As a result, when looking perpendicular to the image, one sees a Pt-V plane with mixed columns in the center of the defect. From previous investigations it is already known that such columns can still be revealed as dots but with a much lower contrast than pure columns. New studies are now being performed to establish the influence of the actual distribution of atoms of different chemical nature along such columns on high resolution images. First results indicate that in columns where atoms of one type are grouped to some extent more contrast changes can be expected than in columns with randomly distributed atoms [ 131.If only two planes would be present in the defect region it would become difficult to fill them, even partially, with vanadium atoms since then first nearest neighbour pairs would exist between minority atoms. A detailed analysis of these defects is in progress. The high resolution image of Fig. 13 shows a contact region between two orientation variants of the 920°C phase, both having their c-axis in the plane of the image. The interface is primarily parallel to (110) f.c.c. planes; at the site where it changes its habit plane from (110) to (110) the interface is locally parallel to (100) f.c.c. plane being the APB plane of
Pt-V PHASE DIAGRAM
one of both orientation variants. From the enlargement and schematical representation of Fig. 14(a) and (b) the contact (110) plane is seen to be the coherent ordered plane along this direction. In this particular situation the APB’s, or Ll, rows, of both variants join at the interface, as depicted by dashed lines in Fig. 14(a). From the encircled part it is observed that the ordering is strongly preserved in the direct vicinity of the coherent interface, The configuration of bright dots in this region is clearly the same as the one indicated in the schematical representation. Since the mentioned (110) planes are parallel to the electron beam, no large overlapping regions showing coincidence patterns are found. When the models of the new long period structures are examined in more detail, it is noticed that the atom configuration along the APB’s corresponds to the square unit cell of the Ll, phase. The long period structures can thus be described by a mixing of motives of the Ll, and DOzz structures, already indicated in Fig. 7(a); moreover, as the temperature decreases, the amount of D02r motives increases at the expense of Ll, ones. This property is in good agreement with the fact that the microdomains found in samples quenched from above the ordering temperature merely consist of Liz unit cells, whereas at lower temperatures (< 9OOC) the DO,, phase eventually appears as the most stable structure. For the purpose of comparison of these results to similar ones in other alloy systems, the long period phases are more conveniently described with regard to the Ll, structure instead of the D0r2 one. This means that the APB’s are now chosen in between instead of at the Ll, motives, as indicated in Fig. 15 for the long period phase stable at 960°C. Using the Fujiwara notation the LPAPB structure of Fig. 7 (960°C) can then be denoted as 2121 whilst that of Fig. 9 (920°C) is 211211, each number (1 or 2) indicating the number of Ll, unit cells in between two subsequent APB’s The average distances between two subsequent APB’s given in number of f.c.c. unit cells, called the M-value, are 3/2 and 4/3 respectively. These values can be compared to M = 1 for DO,, and
Fig. 13. HR image of a contact region between two LP domains with perpendicular c-axes but both having their LP direction in the plane of the image.
SCHRYVERS
and
AMELINCKX:
Pt-V
PHASE
DIAGRAM
51
number of APB’s exhibiting locally the Ll, structure will be formed, even resulting in small patches of the long period structures stable at higher temperatures. In this section a description will be given of the ordering sequence that can be deduced from high resolution results in samples annealed for different periods at one constant temperature. The lower the temperature, the longer the annealing periods necessary to achieve a certain state will take. Since the samples are always homogenized above the ordering temperature and then quenched into water, the microdomain situation described in Section 5.1. will be the starting point. A sample annealed for a few (l-5) minutes at 900°C gives rise to the cube zone electron diffraction pattern shown in the inset of Fig. 16. The corresponding high resolution image reveals a juxtaposition of small ordered domains in different orientation variants due to many nucleation sites. A first observation indicates that the number of DOI motives has increased drastically when compared to the SRO state. The situation is now reversed; the Ll, motives are confined to some conservative APB’s inside DO,, domains, although these domains are in most cases too small to contain some unit cells of the LPAPB structures in the direction perpendicular to the APB’s No preferential orientation of the domain boundaries can be seen in this initial state. The domains exhibiting a small square pattern (region C) with a lower contrast have their DOI2 c-axis parallel to the electron beam (cf. 5.1). At the bottom
(4
(b) Fig. 14. (a) the direction
Enlargement of the interface of of the interface is given in f.c.c.
Fig.
13;
units. (b) Schematic presentation of the atomic arrangement at the coherent boundary. M = co for L12, indicating a sequence of decreasing M with decreasing temperature. 5.4. Dynamical ordering sequence at T < 900°C Until now only static results concerning the long period structures and their defects have been described. As a Pt,V sample is annealed at or below 900°C eventually the DO** structure will appear as the most stable one. Due to the apparently small difference between the free energy of the Ll, and DOZ2 configurations in this material, the atoms are not immediately ordered as the DO, structure. During the growth of the ordered domains a large
, I 10 I IO I I
,i,
2 0, 01
I I.1 t I
I IO IO I I
,i, 2
2 01 01
I I.1 I I
I IO IO I I
,i 01 01
I I .
I
Fig. 15. The 960°C LPAPB structure as defined with respect to the Ll, structure; the dashed lines represent the APB’s in this reference system.
Fig. 16. HR image of a Pt,V sample annealed for a few (1 to 5) minutes at 900°C; A, B and C indicate the three different possible orientation variants. The upper right inset is the corresponding ED pattern while the three insets below are optical diffraction patterns each corresponding with one of the small domains. The circles indicate the position of f.c.c. basic reflections.
52
SCHRYVERS
and AMELINCKX:
Pt-V PHASE DIAGRAM
Fig. 17. HR image of Pt,V annealed for one hour at 900°C. The large domains are separated by f.c.c. (110) interfaces.
of Fig. different
16 optical diffractions taken from three domains in the high resolution negative are
reproduced, the used aperture placed in the laser beam has about the size of the small domains in the high resolution micrograph negative (5 mm). It is seen that, in a small ordered region, one single APB is sufficient to give rise to the splitting of the (101) D02, reflection, whereas in a larger domain a periodical arrangement of APB’s would be necessary [14]. Without the aid of a high resolution image the elongated spots in the electron diffraction pattern are difficult to interpret. When the sample is annealed for 1 h, a situation as seen in Fig. 17 is encountered. All three [loo] orientation variants of the D02r structure or its long period derivates are visible. As was the case in Fig. 13 of the previous chapter, the interfaces are all confined to (110) f.c.c. planes. Those indicated by arrows separate two domains with the long c-axis in the plane of the image and are viewed edge on. Since in the present case the stacking of APB’s on both sides of the interfaces changes, some misfits in atomic arrangement can be observed, especially at the rightmost arrow. The square domain at the left side of the picture (inside white dashed lines) exhibiting a square pattern of bright dots is a long period domain viewed
along its c-axis, i.e. perpendicular to the APB’s, The appearance of squares of bright dots in such an orientation variant is similar to these found at C in Fig. 16. Due to the periodic arrangement of APB’s perpendicular to the electron beam mixed columns will be projected along the viewing direction. An inspection of the unit cell indicates that these mixed columns (P&V or PtV,) indeed form a square co~guration. The boundaries of this domain seem to be parallel to (100) f.c.c. planes, but the dark contrast over about 10 nm perpendicular to the [loo] or [OlO] direction indicates that the interfaces are inclined, corresponding to the previous determination of (110) f.c.c. confined interfaces. At this low magnification it becomes clear that certain areas exhibit long period stackings of APB’s while others, in the same domain, do not. Some interesting features can be seen in enlargements made from different areas of Fig. 17. In Fig. 18 several contacts between two adjacent APB’s are visible; both APB’s join together by forming so-called “hairpins” (large arrows). The actual distance between the two joining APB’s varies from 1 DOzz unit cell to 3, as indicated at the corresponding arrow. At one side of the tip of the hairpin the APB’s have vanished, resulting in a larger area of DO,* phase inside the
Fig. 18. Enlargement of Fig. 17 revealing ledges at the small arrows and hairpins at the large ones.
SCHRYVERS
0
‘:s a2Y
---F
b
and AMELINCKX:
,’
4’ Y&
OEBiia
Fig. 19. Projected atomic model for the Pt,V structure (solid lines); the &axis is parallel with a f.c.c. fi IO]direction. The dashed lines represent the projection of the f.c.c. unit cell. domain, the unit cell being presented. Since the displacement vector of the conservative APB’s is l/2 of a lattice vector of the DOrr basic structure, two APB’s are sufficient to form a hairpin. Although this image is found in a static situation, it is believed that these hairpins play a role in the withdrawal of the APB’s from the structure, so as to eventually create large domains of the low temperature stable DO,, phase. From this point of view the hairpins are then frozen-in by the rapid quenching technique. In the same enlargement several ledges are seen, indicated by small arrows. In the assumption of an undercooled situation these ledges can be interpreted as a mechanism for bringing two subsequent APB’s close enough by sideway motion to form a hairpin and to be eliminated from the structure. Using these two mechanisms the material will eventually, after a sufficiently long annealing time, be ordered in the stable DO,, phase. 6. THE Pt,,V29 COMPOSITION When the amount of platinum is decreased it becomes much easier to order the material in large domains of the DOZ2structure, than in the case of the stoichiometric bulk composition 3: 1. At the 71:29 composition a two-phase region is found, consisting of the DO,, and the Pt2V phases, the latter being isostructural with Ni$lo. The unit cell of Pt,V is indicated in Fig. 19; this structure is a column structure when viewed along its a- or c-directions.
Fig. 20. HR image of the two-phase region found in shortly annealed Pt7,Vz9sampies; (A) = DO,,, (B) = Pt,V. The corresponding ED pattern is seen in the inset.
WV
PHASE DIAGRAM
53
Along both axes the high resolution image will consist of centered rectangles, differing in size and orientation from those of DO,, as can be observed in Fig. 20; areas (B) exhibit the PtJ ordering and are all viewed along their u-axis while at (A) the conventional image of the DOzz structure is observed. The faintest spots along the [l lo]* f.c.c. directions in the electron diffraction pattern of the inset of Fig. 20 are due to the Pt,V domains. From the same image it can be observed that all interphase interfaces are parallel to f.c.c. (120) planes; this is not surprising since these planes correspond to the (110) planes of the DOrr structure as well as the Pt,V structure. The contact planes thus exhibit the ordered arrangement of both structures without causing strain to the underlying f.c.c. lattice. The situation of Fig. 20 was found after a relatively short annealing treatment of 30min at 900°C. After longer annealing times the domains grow appreciably larger. 7. DISCUSSION
In the present study of the Pt-rich side of the Pt-V phase diagram a new superstructure PtsV isomorphous with Pt,Ti (Ni,Nb, Ni,Ta, etc.) was found while two metastable LPAPB phases were discovered at the 3: I composition. At variance with previous results in the Pt-Ti system no long period structures could be determined in the composition range between 8: 1 and 4: 1; only substitutionally disordered derivates of the PtrV and Ll, structures were found in this composition region. The most interesting structural features were encountered at elevated tem~ratures in the 3: 1 composition. In the vicinity of the order-disorder temperature a short range ordering was found following constraints that give rise to the predominance of the nearest neighbour tetrahedron with the bulk composition 3: 1 as a building unit. As a result the Ll, structure is found inside these microdomains, while the DOZ2 structure is mainly confined to conservative APB’s. In very thin parts of the specimen, yet thick enough to ensure a sufficient difference in projected potential between platinum and vanadium columns but where no defects perpendicular to the electron beam are present, the configuration of pure vanadium columns due to the stacking of such tetrahedra can be visualised by high resolution electron microscopy. At temperatures just below the order-disorder temperature a sequence of LPAPB structures presumably stable in narrow temperature ranges only can be inferred; two examples of such structures are resolved indicating a mixing of DO= and Ll, units; the amount of DO,, building units increases with decreasing temperature eventually leading to the stable DO,, phase itself. This result can be compared with recent investigations in the Al-Ti alloy where the existence of a simple and a so-called devils staircase was predicted from the occurrence of a large number of long period phases each stable in a small
54
SCHRYVERS
and AMELINCKX:
high temperature range. In the latter case however, temperature differences of 50” could be used since the alloy remains ordered until the melting temperature at 1340°C [12], while in the present investigation all long period phases occur in a temperature range of
IOO”C. Although no in situ experiments can be performed in the high resolution instruments now available, a detailed investigation of an as quenched state can provide useful information on the ordering and transformation mechanisms of ordered phases in alloys. In the present case the formation of the stable DO,, structure out of long period structures metastable at the annealing temperature is inferred from the presence of ledges and hairpin defects. REFERENCES 1. D. Schryvers, J. Van Landuyt and S. Amelinckx, Mater. Res. Bull. 18, 1369 (1983).
Pt-V PHASE DIAGRAM
2. D. Schryvers and S. Amelinckx, Mater. Rex Bull. 19, 979 (1984). 3. D. Schryvers, G. Van Tendeloo and S. Amelinckx, Mater. Res. Bull. 20, 367 (1985). 4 D. Schryvers and S. Amelinckx, Mater. Res. Bull. 20, ’ 361 (1985). 5. R. M. Waterstrat, MetaN. Trans. 4, 455 (1973). 6. D. Schryvers, J. Van Landuyt, G. Van Tendeloo and S. Amelinckx, Physica status. solidi (a) 75, 607 (1983). 7 D. Van Dyck and W. Coene, Ultramicroscopy 15, 29 ’ (1984). 8. R. De Ridder, G. Van Tendeloo and S. Amelinckx, Acta crystallogr. A32, 216 (1976). J. Van Landuyt, R. De Ridder, R. Gevers and S. 9. Amelinckx, Mater. Res. Bull. 5, 353 (1970). IO R. Portier. D. Gratias, M. Guymont and M. Stobbs, J. Microscl 119, 163 (I980). _ 11. K. Fujiwara, J. Phys. Sot. Japan 12, 7 (1957). 12. A. Loiseau, G. Van Tendeloo, R. Portier and F. Ducastelle, J. Physique 46, 595 (1985). 13. W. Coene, private communication. 14. D. Van Dyck, G. Van Tendeloo and S. Amelinckx, Ultramicroscopy 15, 357 (1984).
Acta metaN. Vol. 34, No. 1, pp. 43354, 1986 Printed in Great Britain
ORDER-DISORDER PHENOMENA IN THE PLATINUM RICH PART OF THE Pt-V PHASE DIAGRAM? D. SCHRYVERS’ ‘Rijksuniversitair
and S. AMELINCKX’J
Centrum Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, and %C.K./C.E.N. Mol., Boeretang 200, B-2400 Mol, Belgium
Belgium
(Received 30 May 1985) Abstract-The Pt-rich side of the Pt-V phase diagram is re-examined using electron diffraction and high resolution electron microscopy. An ordered Pt,V structure isomorphous with Pt,Ti is discovered and two long period antiphase boundary structures are found in Pt,V on annealing in the temperature interval between 900 and 1000°C. A formation mechanism for the stable DO,, phase is presented and some structural defects are discussed. Microdomains corresponding to the ordering relations extracted from the diffuse intensity distribution in reciprocal space are revealed using the high resolution technique. Resum&Nous avons etudie a nouveau la partie riche en platine du diagramme de phases Pt-V par diffraction Clectronique et microscopic electronique a haute resolution. Nous avons dicouvert une phase ordonnee Pt,V isomorphe a Pt,Ti et nous avons trouve deux structures antiphases a longue ptriode dans Pt,V recuit entre 900 et 1000°C. Nous presentons un mecanisme de formation pour la phase DO,, stable et nous discutons quelques defauts de structure. Nous avons observe par la technique de haute resolution des microdomaines correspondant aux relations d’ordre obtenues a partir de la repartition de l’intensite diffuse dans l’espace reciproque. Zusammenfassung-Die Pt-reiche Seite des Pt-V-Phasendiagrammes wurde mittels Elektronenbeugung und hochaufliisender Elektronenmikroskopie nochmals untersucht. Hierbei wurde eine geordnete Phase P&V, die mit Pt,Ti isomorph ist, entdeckt. Nach Anlassen im Temperaturintervall zwischen 900 und 1000°C finden sich in der Pt,V-Phase zwei langperiodische Antiphasenstrukturen. Ein Bildungsmechanismus wird fiir die stabile Don-Phase vorgeschlagen; aul3erdem werden einige Strukturfehler diskutiert. Aus der Verteilung der diffusen Intensitat im reziproken Raum wurden Ordnungszusammenhinge abgeleitet; die entsprechenden Mikrodomlnen wurden hochauflosend abgebildet.
1. INTRODUCTION
In the course of our systematic investigations on ordered phases and formation mechanisms in transition metal alloys by means of high resolution electron microscopy (HREM) and electron diffraction (ED) a study was performed on the Pt-rich side of the Pt-V phase diagram [l-4]. Although the phases in this part of the diagram are known to be based on an f.c.c. lattice up to the composition Pt,V, only three ordered structures were reported until now; the high temperature Ll, (Cu,Au) structure and the low temperature DO** (Al,Ti) structure with a 3: 1 bulk composition and the PtZV (N&MO) structure at the composition 2: 1 [5]. Since several new long period modulated phases were recently found with compositions intermediate between 8: 1 and 3: 1 in the Pt-Ti alloy system [6], it was not unreasonable to expect a similar behaviour in the Pt-V system. A reinvestigation of the Pt-V system was therefore considered worthwhile. In particular the long period antiphase boundary (LPAPB) structures of Pt-Ti are built with units of the Pt,Ti and the Pt,Ti (Ll,) TWork performed under the auspices of the association R.U.C.A.S.C.K. IIKW-NFWO,
with Brussels,
financial
support
of
the
43
structures and thus the existence of the DO,, structure at the Pt,V composition looked very promising for the occurrence of new ordered phases. Selected area electron diffraction and high resolution electron microscopy are very useful techniques for this kind of investigation. The small areas ( < 3 x lo4 nm’) giving rise to the diffraction pattern and the direct relationship between the high resolution image and the structure enables one to unravel complicated electron diffraction patterns rather easily. More specifically, many ordered structures based on the f.c.c. lattice are so-called column structures; when the structure is viewed along a particular orientation, mostly a cube zone, every column consists of one chemical species. Under the appropriate imaging conditions the high resolution image along such a zone shows a pattern of prominently bright dots corresponding to the geometrical configuration of the minority atom columns. These bright dots can then be used as an imaging code for the interpretation of the atomic structure at single defects in the ordering. The following chapters contain an overview of the results obtained in Pt-V alloys with compositions 8:1, 6:1, 5:1, 4:1, 3:l (=75:25), 71:29 and 2:l expressed in atomic ratios. The most important re-
SCHRYVERS
44
and AMELINCKX:
Pt-V PHASE DIAGRAM
suits will in this case be situated in the vicinity of the 3 : 1 composition. 2. THE
PtsV COMPOSITION
Pt,V samples homogenized for 1 day at llOO”C, quenched in water and annealed for 2 days at 700°C exhibit the cube zone electron diffraction pattern shown in Fig. l(a), where the indices refer to the basic f.c.c. lattice [l]. The distribution of superspots is identical with that of the [OOl] zone of the PtsTi superstructure while tilting experiments reveal that the entire reciprocal lattice is the same as that of Pt,Ti. In Fig. l(b) the projection of the Pt,V superstructure along its shortest axis is presented; large dots are vanadium atoms while small dots are platinum atoms. Open and solid dots belong to two different levels z = 0 and z = l/2 of the f.c.c. lattice, indicated in Fig. l(b) by solid lines. The dashed lines show the body centered tetragonal unit cell of the P&V phase. The new superstructure has a period of three f.c.c. unit cells in two mutually perpendicular [loo] directions, while the thickness remains one f.c.c. unit cell. The point group is 4/m 2/m 2/m. In the high resolution image shown in Fig. 2 the Pt,V structure is viewed along the same direction as the electron diffraction pattern and projected cell of Fig. 1. The circle in Fig. l(a) indicates the reflections selected to form this high resolution image; the basic (200) f.c.c. spots are included. For all the following high resolution images the same aperture will be used. The geometrical configuration of bright dots resembles that of the vanadium columns but, due to the projected nature of a high resolution image, no distinction can be made between columns of vanadium atoms at z = 0 or z = l/2. The drawn square represents the body centered unit cell. In the upper left corner a computer simulation (specimen thickness t = 8 nm, defocus Sf = -70 nm) made using the real-space method [7] is shown indicating that under
(4
‘\
FCC
.
(b)
i-i
.,/.
3
“q’ .
Y
.
Fig. 1. (a) Cube ED pattern of the Pt8V structure. The indices refer to the basic f.c.c. lattice and the circle indicates the selected diffracted beams to produce the HR image of Fig. 2. (b) Projected atomic model of Pt,V; the dashed lines represent the body centered tetragonal unit cell. these imaging conditions the white dots appear at the positions of projected vanadium columns. This result will be used as an imaging code; in all following high resolution images the white dots are interpreted as projected vanadium columns. Like in the Pt-Ti case no preferential orientation was found for the interfaces between two orientation variants of the ordered structure. 3. THE
Pt,V
COMPOSITION
At variance with the results on Pt-Ti, no new long range ordered phases could be determined in the large
Fig. 2. HR image of Pt,V viewed along the same direction as the ED pattern and model of Fig. 1. The inset shows a computer simulated image at t = 8 nm and Sf = -70 nm indicating that under these imaging conditions the white dots correspond with the V columns.
SCHRYVERS
and AMELINCKX:
Pt-V PHASE DIAGRAM
Fig. 3. HR image of small domains exhibiting the Pt,V structure found in samples with a 6: I nominal composition. The corresponding ED pattern is given as an inset, composition range between 8: 1 and 4: 1. In 6: 1 samples annealed for several days at 900 or 7OO”C, only small ordered domains exhibiting the Pt,V structure could be produced. High resolution images of these domains viewed along a cube zone are shown in Fig. 3, while the corresponding electron diffraction pattern is given as an inset. The areas showing a bright dot pattern are viewed along the same direction as the image of Fig. 2, i.e. along the a3 axis of Pt,V, while the line patterns arise from regions viewed along the a, + az direction. Due to the overlap of neighbouring orientation variants, many lines are seen to penetrate into some dotted zones. The elongation of the P&V superspots is due to the random distribution of domains belonging to one orientation variant. Beams diffracted from such domains interfere as if they were caused by one large ordered domain exhibiting a high density of aperiodically arranged antiphase boundaries (APB’s). Since many of these small domains are viewed along an a, + a* direction only producing superspots in the [lOO]* or [OlO]* directions these reflections will be more pronounced than those at (MO) positions. The difference between the stoichiometric 8: 1 composition inside the domains and the nominal 6: 1 composition of the bulk material can be accommodated at nonconservative APB’s in between different domains as well as by substitutional positioning of some vanadium atoms at sites belonging to the platinum sublattice in the stoichiometric Pt,V structure. As the samples are annealed much longer (1 week at 900°C) the Pt,V domains grow larger and become comparable to those found after 1 day at 900°C in the 8: 1 composition and the replacement of platinum atoms by vanadium atoms becomes the main manner for obtaining the bulk composition 6: 1. 4. THE P&V AND Pt,V COMPOSITIONS Homogenized diffuse intensity
samples of both compositions show contours like the ones encountered in
the 3 : 1 composition. In the next section an extensive discussion on the origin of such diffuse intensity will be given. Only after a long annealing treatment of 1 month at 700°C the 4: 1 material could partially be ordered in the Ll, structure. This result together with those presented in the previous chapter indicate that in the Pt-V alloy system no new long period ordered phases are produced in the composition region in between 8: 1 and 4: 1. If any ordering can be achieved, it consists of substitutionally disordered derivates of the simple Pt,V and Ll, structures. This means that on the average the same unit cell holds as in the stoichiometric situation, but some randomly chosen A sites will be occupied by B atoms to accommodate the bulk composition. 5. THE Pt3V COMPOSITION
The situation described by Waterstrat [5] indicated a clear cut transition between the high temperature (1OOOC) y -Pt phase and the low temperature (9OOC) DOZ2 phase at the 3 : 1 composition. A re-examination of this alloy however, using selected area electron diffraction and high resolution electron microscopy revealed the existence of a number of intermediate long period modulated structures in the temperature range between 1000 and 900°C. However, no long range ordered regions exhibiting the Ll, phase could be found at the nominal 3: 1 composition. In this chapter we will describe our results in a succession corresponding with decreasing temperature. 5.1. Samples quenched fLom 1050°C: microdomains When samples are water quenched from 105O”C, i.e. from above the ordering temperature, to room temperature the electron diffraction pattern shown in the inset of Fig. 4 is found. Apart from the sharp f.c.c. spots of the [OOl] zone the pattern exhibits streaks along the h = 2n + 1 and k = 2m + 1 lines. From tilting experiments and from the fact that the streaking vanishes in the outer parts of the electron
SCHRYVERS
46
and AMELINCKX:
Pt-V PHASE DIAGRAM
Fig. 4. HR image of the SRO transition state in Pt,V exhibiting; (A) Ll, squares, (B) DO,, triangles and (C) rotated small squares due to APB’s perpendicular to the electron beam. The corresponding diffuse intensity is shown in the inset. pattern it can be concluded that the diffuse intensity is confined to rather sharply defined lines in reciprocal space, their loci being defined by equations such as
diffraction
h=2n+l,
l=O.
Along these streaks reinforcements of diffuse intensity occur at the (loo), (010) and (110) positions. One way of extracting information from the pattern of diffuse intensity is based on the cluster theory. From the geometry of the locus of the diffuse intensity one can deduce the geometry of the prevailing clusters and the occupancy of the sites of these clusters. In the present case this theory predicts that these clusters should be tetrahedra of nearest neighbours on the f.c.c. lattice and having the macroscopic 3: 1 composition [8]. This cluster relation is satisfied in the Ll, structure as well as in the DO,, structure; it is thus also satisfied along the conservative APB’s in the LIZ structure, at which locally the DOZ2 structure is realized. In Fig. 5 this cluster is indicated in both structures, left Ll, and right DOZz. Since the reinforcements along the streaks are located at the potential sites of the superspots of a long range ordered LIZ domain, a predomination of unit cells of the Ll, phase is expected. In this material the predictions of the cluster theory could be checked by means of high resolution electron microscopy [4]. Since both Ll, and DO,* are column structures when viewed along a cubic direction, the geometrical configuration of the bright dots observed in Fig. 4 directly indicates the stacking of building units of these structures; again the pattern reveals the geometry of pure vanadium columns. It is clearly seen that the geometry of bright dots suggests the Ll, type of ordering (squares at A) fragmented by APB’s along which DOZ2 type configurations (triangles) occur. Locally small patches of the DO,, structure, viewed as centered rectangles are observed as well (B). The smaller square arrangement of dots at C rotated over 45” with respect to the Ll, square, is
consistent with the image of a microdomain of DOZ2 structure viewed along its c-direction. However this geometry is also consistent with a number of Ll, microdomains wrongly stacked along the viewing direction resulting in one or more APB’s perpendicular to this direction. The high contrast image of Fig. 4 disappears in thicker areas of the specimen since the number of APB’s perpendicular to the electron beam and thus disturbing the column structure arrangement will increase. In thinner areas the high resolution image reveals the basic f.c.c. lattice. Although some non-conservative APB’s are visible, violating the cluster relation and thus probably due to different nucleation sites in the matrix, the overall arrangement of bright dots strongly suggests the growing of microdomains following the cluster relation extracted form the geometry of the locus of diffuse intensity in the previous paragraph. These microdomains, not exceeding the size of 4 unit cells, are the only evidence for the Ll, phase in Pt,V. These results confirm the statement that the Ll, phase is only stable at higher Pt contents [5]. 5.2. Samples annealed LPAPB phases
between
900
and
1000°C:
In the course of the experiments it became clear that it was rather difficult to produce the stable DOZ2
Fig. 5. The tetrahedron of first nearest neighbours and having the stoichiometric composition 3 : 1 as a building unit for the Ll, structure (left) and the DO,, structure (right).
AMELINCKX:
Pt-V
PHASE
47
DIAGRAM
structure is a derivative of the DOZ2 structure. The (101) and (103) reflections of the DOZ2 structure are split into two satellites, their separation being equal to l/3 of the length of the diffraction vector 002. The splitting is in a cubic direction and is symmetric with respect to the DO2* spot positions, i.e. the fractional shifts are + l/2. Using the geometrical method of fractional shifts [9] one finds: (1) the basic structure is DOzz (from the positions of the most intense spots); (2) the APB’s are perpendicular to the [OOl] direction of DO,, (from the orientation of the satellite sequences); (3) the distance between APB’s is 1.5 times the DO,* c-axis (from the separation of the satellite spots) (4) the displacement vector R of the APB’s is (1/2)[ 1 lo] (DO,,) (from the fractional shifts).
206
200 (2?0)
(2G2)
100 . .Y.
(2%) 106 .
. .X.
(101)
(103) 006
(b)
OF0
(0:2)
(OF,+
. .
Fig. 6. (a) Cube ED pattern found in Pt,V samples annealed for 1 day at 960°C. (b) Schematic drawing of one quadrant of (a) (see text for legend of indices).
phase at e.g. 900°C as should be possible according to the phase diagram stated by Waterstrat [5]. Instead a number of different electron diffraction patterns were observed, indicating the existence of several new LPAPB structures in the temperature range between 900 and 1000°C. When a homogenized sample is annealed for 1 day at 960°C and then water quenched, the cube zone electron diffraction pattern shown in Fig. 6(a) is found [3]; one quadrant of the pattern is reproduced schematically in Fig. 6(b), the size of the dots being a rough measure for the relative intensities of the corresponding diffraction spots. The reflection at the (101) f.c.c. position is overpronounced due to the incorporation in the selected area of an orientation variant with the long period parallel with the electron beam. All reflections can be indexed on a primitive tetragonal unit cell with lattice parameters a = b = a, and c = 3a,, a, being the f.c.c. basic unit. These indices have been indicated in Fig. 6(a) without brackets. The crosses mark the positions of reflections due to the potential DO,, ordering; their indices are indicated between brackets. From (a) it is clear that the spots at or closest to the DO,, positions are the most intense ones, suggesting that the super-
Table
In Table 1 it is shown that the calculated fractional shifts using R are indeed consistent with the observed ones. In Fig. 7(a) a proposed model, based on these considerations is presented; only vanadium columns are given. In order to verify this structure further, high resolution electron microscopy images along the adirection [Fig. 7(b)] using the bright field mode including the first shell of f.c.c. reflections were made. Viewed along the u-direction the proposed structure
APE
APE
APE
G-4
I. R =(l/2)(llO]
g.R k+
Ohs.-(mod.
002 200 004 101 103
0 0 0 I 12 Ii.2
g.8 1)
Calc. (mod. 0 I 0 117 1:;
I)
Fig. 7. (a) Projection of the proposed atomic model for the LPAPB structure corresponding with the ED pattern of Fig. 6: only V atoms at two different levels are indicated. The building units DO,, and Ll, are also given. (b) HR image of the 960°C LPAPB structure viewed along the same direction as the ED pattern and the model; the Ll, squares and D022 triangles can clearly be distinguished.
48
SCHRYVERS
i
AMELINCKX:
Pt-V PHASE DIAGRAM APB
APB
R+I . t . . ._-_*__-_,__,____,__i =1&l t
l
.
APB
Aq[
.
pi
. ; ’ L---_~_---__O-_~--___~--_--l
.
.
.
.
.
.
.
1 . . b
(4 202 II:
200 0
101 .X.
@I 000 @
204 0
103 .X.
0:2
0
004
Fig. 8. (a) Cube ED pattern found in Pt,V samples annealed at 920°C. (b) Schematic drawing of one quadrant of (a) (see text for legend of indices). is a column structure. Using the same imaging code as for the P&V structure, one can conclude that the configuration of bright dots is the same as that of vanadium columns in the proposed model. The APB’s are visible as rows of squares, while the basic DO,, structure in between the APB’s is seen as a’ pattern of triangles. When a sample of the same bulk is annealed at a somewhat lower temperature, e.g. 920°C another LPAPB structure is formed [2]. The electron diffraction pattern revealing the satellite spots is shown in Fig. 8(a), while the DOzz spot positions again used as a reference are indicated by crosses at b. When the streaks are neglected, this diffraction pattern only differs form the previous one in the actual distance between two satellite spots of the same sequence; it is now nearly l/4 of the length of the diffraction vector 002. As a result the new superstructure will have the same orientation for the APB’s, the same displacement vector, but a different distance between two subsequent APB’s, now equal to 2 times the DO,, c-axis. The proposed model is drawn schematically in Fig. 9(a) and has lattice parameters a = b = a, and c = 8a,. A high resolution electron microscopy image viewed along an u-direction is shown in Fig. 9(b). Again the bright dots reveal the same structural configuration as the vanadium columns in the proposed model. The arrows point at stacking faults in the sequence of APB’s, resulting in extra atom layers in between the APB’s. The faint streaking in reciprocal space perpendicular to the APB’s is caused by these defects. When the same material is treated for several weeks below 900°C finally large areas of DO,, structure are
(b) Fig. 9. (a) Projection of the proposed atomic model for the 920°C structure. (b) Corresponding HR image. The arrows indicate faults against the stacking sequence of the APB’s
found occasionally revealing single APB’s of the type encountered in the long period structures. The ordering mechanism at these temperatures will be explained below. 5.3. Stiuctural considerations
In the high resolution image of Fig. IO(a) a set of ledges, indicated by arrows is visible in the 920°C phase. From the configuration of bright dots in the vicinity of such a ledge it can be concluded that the APB, revealed as a row of squares, is deplaced by one f.c.c. unit cell in the direction perpendicular to the plane of the APB. The low contrast area at the position of the ledge indicates a local disturbance of the ordered atom columns. From the enlargement of Fig. 10(b) it is seen that the ledges appear at subsequent APB’s and that, in between two ledges the distance of two APB’s is increased by half a cparameter of the DOz2 unit cell, i.e. one triangle more. It is believed that this structure leads to a more uniform distribution of the excess strain energy associated with a local fault in the ordering sequence produced during the growth of the ordered domain. The set of ledges can be regarded as a wavy APB running from the upper right corner to the lower left corner in the high resolution image. The displacement vector is (1/8)[441] in units of the long period structure, i.e. (1/2)[11 l] with respect to the DO,, structure. This displacement vector is a lattice vector
SCHRYVERS and AMELINCKX:
Pt-V PHASE DIAGRAM
Fig. 10. (a) HR image of a set of ledges and a misfit between two ordered domains with parallel long period. ib) Enlargement of three ledges: a DO,, triangle and Ll, square are indicated, of the I30,, structure, but not of the LI, structure; it closed dots indicate fuif pure vanadium columns. The therefore only influences the atomic configuration at gradual change from pure vanadium to pure piatthe periodic APB’s which locally have the L12 strum- inum columns is depicted by dots of decreasing size.
ture. At these sites a lower contrast is observed that can be explained by the overlap of the atomic ~O~~~~ra~~Q~s on both sides of the ~nterfa~~l~]. Such an overlap causes the gradual change of pure vanadium columns in pure platinum columns and can be accomplished by an inclination of the APB plane. In Fig. 11 the atomic configuration at two adjacent ledges is schematically presented; large open and
0000000000000000
Fig. i 1. Schematic drawing of two ledges indicating the gradual change from pure V to pure Pt columns depicted by dots (only V columns are shown) of decreasing size. 7” displacement vector is also indicated.
A description which is consistent with this feature Can be given in terms of the step function J&.X) introdu~ by Fujiwara in 1957 fll] to describe the sequences of antiphase domains in LPAPB structures (left part in Fig. 12). Suppose an increase in temperature rounds off the discontinuous steps as schematically drawn in the right part of Fig. 12, some APB’s will no longer be confined to one atom plane (for a more extensive discussion using this description see e.g. f123). The positions of the unit cells, located close to such a step become undetermined and hence also the positions of the minority atoms indicated by black dots; a given atom ma.y join either one or the other of the antiphase domains adjacent to the discontinuity. As a result the cohnnns at these sites will contain platinum as wefi as vanadium atoms causing a decrease in contrast under the current imaging _mditions. The periodicity of the basic step function will impose the same periodicity for the ledges, which is four Ll, unit cells in the case under consideration.
50
._____. .---__, ___.___ _--.___ ___ 5___.___ b____ SCHRYVERS
and AMELINCKX:
*_____.
._____.
___.___
___.___ .__-__.
.__-__.
c__-___L ._____,
?
_:
I___.___ I___.___
r-J
r-J
Fig. 12. Fujiwara’s step function and corresponding atomic positions at low (left) and high (right) temperatures.
In the image of Fig. 10 also a misfit between two ordered domains is seen. The white dots at the border planes on both sides of the misfit still mark the positions of vanadium columns having the DO,, ordering with respect to the adjacent cubic Pt-V plane away from the boundary. From the image however, it cannot be concluded unambiguously whether two or three cube planes parallel to the defect plane exist in between these two border planes. A measurement using the perfect long period structure away from the misfit as a measuring gauge is not useful, due to relaxation effects. In the case of three planes, they are probably locally ordered using the DOrr building unit (triangle) or the Ll, one (square). As a result, when looking perpendicular to the image, one sees a Pt-V plane with mixed columns in the center of the defect. From previous investigations it is already known that such columns can still be revealed as dots but with a much lower contrast than pure columns. New studies are now being performed to establish the influence of the actual distribution of atoms of different chemical nature along such columns on high resolution images. First results indicate that in columns where atoms of one type are grouped to some extent more contrast changes can be expected than in columns with randomly distributed atoms [ 131.If only two planes would be present in the defect region it would become difficult to fill them, even partially, with vanadium atoms since then first nearest neighbour pairs would exist between minority atoms. A detailed analysis of these defects is in progress. The high resolution image of Fig. 13 shows a contact region between two orientation variants of the 920°C phase, both having their c-axis in the plane of the image. The interface is primarily parallel to (110) f.c.c. planes; at the site where it changes its habit plane from (110) to (110) the interface is locally parallel to (100) f.c.c. plane being the APB plane of
Pt-V PHASE DIAGRAM
one of both orientation variants. From the enlargement and schematical representation of Fig. 14(a) and (b) the contact (110) plane is seen to be the coherent ordered plane along this direction. In this particular situation the APB’s, or Ll, rows, of both variants join at the interface, as depicted by dashed lines in Fig. 14(a). From the encircled part it is observed that the ordering is strongly preserved in the direct vicinity of the coherent interface, The configuration of bright dots in this region is clearly the same as the one indicated in the schematical representation. Since the mentioned (110) planes are parallel to the electron beam, no large overlapping regions showing coincidence patterns are found. When the models of the new long period structures are examined in more detail, it is noticed that the atom configuration along the APB’s corresponds to the square unit cell of the Ll, phase. The long period structures can thus be described by a mixing of motives of the Ll, and DOzz structures, already indicated in Fig. 7(a); moreover, as the temperature decreases, the amount of D02r motives increases at the expense of Ll, ones. This property is in good agreement with the fact that the microdomains found in samples quenched from above the ordering temperature merely consist of Liz unit cells, whereas at lower temperatures (< 9OOC) the DO,, phase eventually appears as the most stable structure. For the purpose of comparison of these results to similar ones in other alloy systems, the long period phases are more conveniently described with regard to the Ll, structure instead of the D0r2 one. This means that the APB’s are now chosen in between instead of at the Ll, motives, as indicated in Fig. 15 for the long period phase stable at 960°C. Using the Fujiwara notation the LPAPB structure of Fig. 7 (960°C) can then be denoted as 2121 whilst that of Fig. 9 (920°C) is 211211, each number (1 or 2) indicating the number of Ll, unit cells in between two subsequent APB’s The average distances between two subsequent APB’s given in number of f.c.c. unit cells, called the M-value, are 3/2 and 4/3 respectively. These values can be compared to M = 1 for DO,, and
Fig. 13. HR image of a contact region between two LP domains with perpendicular c-axes but both having their LP direction in the plane of the image.
SCHRYVERS
and
AMELINCKX:
Pt-V
PHASE
DIAGRAM
51
number of APB’s exhibiting locally the Ll, structure will be formed, even resulting in small patches of the long period structures stable at higher temperatures. In this section a description will be given of the ordering sequence that can be deduced from high resolution results in samples annealed for different periods at one constant temperature. The lower the temperature, the longer the annealing periods necessary to achieve a certain state will take. Since the samples are always homogenized above the ordering temperature and then quenched into water, the microdomain situation described in Section 5.1. will be the starting point. A sample annealed for a few (l-5) minutes at 900°C gives rise to the cube zone electron diffraction pattern shown in the inset of Fig. 16. The corresponding high resolution image reveals a juxtaposition of small ordered domains in different orientation variants due to many nucleation sites. A first observation indicates that the number of DOI motives has increased drastically when compared to the SRO state. The situation is now reversed; the Ll, motives are confined to some conservative APB’s inside DO,, domains, although these domains are in most cases too small to contain some unit cells of the LPAPB structures in the direction perpendicular to the APB’s No preferential orientation of the domain boundaries can be seen in this initial state. The domains exhibiting a small square pattern (region C) with a lower contrast have their DOI2 c-axis parallel to the electron beam (cf. 5.1). At the bottom
(4
(b) Fig. 14. (a) the direction
Enlargement of the interface of of the interface is given in f.c.c.
Fig.
13;
units. (b) Schematic presentation of the atomic arrangement at the coherent boundary. M = co for L12, indicating a sequence of decreasing M with decreasing temperature. 5.4. Dynamical ordering sequence at T < 900°C Until now only static results concerning the long period structures and their defects have been described. As a Pt,V sample is annealed at or below 900°C eventually the DO** structure will appear as the most stable one. Due to the apparently small difference between the free energy of the Ll, and DOZ2 configurations in this material, the atoms are not immediately ordered as the DO, structure. During the growth of the ordered domains a large
, I 10 I IO I I
,i,
2 0, 01
I I.1 t I
I IO IO I I
,i, 2
2 01 01
I I.1 I I
I IO IO I I
,i 01 01
I I .
I
Fig. 15. The 960°C LPAPB structure as defined with respect to the Ll, structure; the dashed lines represent the APB’s in this reference system.
Fig. 16. HR image of a Pt,V sample annealed for a few (1 to 5) minutes at 900°C; A, B and C indicate the three different possible orientation variants. The upper right inset is the corresponding ED pattern while the three insets below are optical diffraction patterns each corresponding with one of the small domains. The circles indicate the position of f.c.c. basic reflections.
52
SCHRYVERS
and AMELINCKX:
Pt-V PHASE DIAGRAM
Fig. 17. HR image of Pt,V annealed for one hour at 900°C. The large domains are separated by f.c.c. (110) interfaces.
of Fig. different
16 optical diffractions taken from three domains in the high resolution negative are
reproduced, the used aperture placed in the laser beam has about the size of the small domains in the high resolution micrograph negative (5 mm). It is seen that, in a small ordered region, one single APB is sufficient to give rise to the splitting of the (101) D02, reflection, whereas in a larger domain a periodical arrangement of APB’s would be necessary [14]. Without the aid of a high resolution image the elongated spots in the electron diffraction pattern are difficult to interpret. When the sample is annealed for 1 h, a situation as seen in Fig. 17 is encountered. All three [loo] orientation variants of the D02r structure or its long period derivates are visible. As was the case in Fig. 13 of the previous chapter, the interfaces are all confined to (110) f.c.c. planes. Those indicated by arrows separate two domains with the long c-axis in the plane of the image and are viewed edge on. Since in the present case the stacking of APB’s on both sides of the interfaces changes, some misfits in atomic arrangement can be observed, especially at the rightmost arrow. The square domain at the left side of the picture (inside white dashed lines) exhibiting a square pattern of bright dots is a long period domain viewed
along its c-axis, i.e. perpendicular to the APB’s, The appearance of squares of bright dots in such an orientation variant is similar to these found at C in Fig. 16. Due to the periodic arrangement of APB’s perpendicular to the electron beam mixed columns will be projected along the viewing direction. An inspection of the unit cell indicates that these mixed columns (P&V or PtV,) indeed form a square co~guration. The boundaries of this domain seem to be parallel to (100) f.c.c. planes, but the dark contrast over about 10 nm perpendicular to the [loo] or [OlO] direction indicates that the interfaces are inclined, corresponding to the previous determination of (110) f.c.c. confined interfaces. At this low magnification it becomes clear that certain areas exhibit long period stackings of APB’s while others, in the same domain, do not. Some interesting features can be seen in enlargements made from different areas of Fig. 17. In Fig. 18 several contacts between two adjacent APB’s are visible; both APB’s join together by forming so-called “hairpins” (large arrows). The actual distance between the two joining APB’s varies from 1 DOzz unit cell to 3, as indicated at the corresponding arrow. At one side of the tip of the hairpin the APB’s have vanished, resulting in a larger area of DO,* phase inside the
Fig. 18. Enlargement of Fig. 17 revealing ledges at the small arrows and hairpins at the large ones.
SCHRYVERS
0
‘:s a2Y
---F
b
and AMELINCKX:
,’
4’ Y&
OEBiia
Fig. 19. Projected atomic model for the Pt,V structure (solid lines); the &axis is parallel with a f.c.c. fi IO]direction. The dashed lines represent the projection of the f.c.c. unit cell. domain, the unit cell being presented. Since the displacement vector of the conservative APB’s is l/2 of a lattice vector of the DOrr basic structure, two APB’s are sufficient to form a hairpin. Although this image is found in a static situation, it is believed that these hairpins play a role in the withdrawal of the APB’s from the structure, so as to eventually create large domains of the low temperature stable DO,, phase. From this point of view the hairpins are then frozen-in by the rapid quenching technique. In the same enlargement several ledges are seen, indicated by small arrows. In the assumption of an undercooled situation these ledges can be interpreted as a mechanism for bringing two subsequent APB’s close enough by sideway motion to form a hairpin and to be eliminated from the structure. Using these two mechanisms the material will eventually, after a sufficiently long annealing time, be ordered in the stable DO,, phase. 6. THE Pt,,V29 COMPOSITION When the amount of platinum is decreased it becomes much easier to order the material in large domains of the DOZ2structure, than in the case of the stoichiometric bulk composition 3: 1. At the 71:29 composition a two-phase region is found, consisting of the DO,, and the Pt2V phases, the latter being isostructural with Ni$lo. The unit cell of Pt,V is indicated in Fig. 19; this structure is a column structure when viewed along its a- or c-directions.
Fig. 20. HR image of the two-phase region found in shortly annealed Pt7,Vz9sampies; (A) = DO,,, (B) = Pt,V. The corresponding ED pattern is seen in the inset.
WV
PHASE DIAGRAM
53
Along both axes the high resolution image will consist of centered rectangles, differing in size and orientation from those of DO,, as can be observed in Fig. 20; areas (B) exhibit the PtJ ordering and are all viewed along their u-axis while at (A) the conventional image of the DOzz structure is observed. The faintest spots along the [l lo]* f.c.c. directions in the electron diffraction pattern of the inset of Fig. 20 are due to the Pt,V domains. From the same image it can be observed that all interphase interfaces are parallel to f.c.c. (120) planes; this is not surprising since these planes correspond to the (110) planes of the DOrr structure as well as the Pt,V structure. The contact planes thus exhibit the ordered arrangement of both structures without causing strain to the underlying f.c.c. lattice. The situation of Fig. 20 was found after a relatively short annealing treatment of 30min at 900°C. After longer annealing times the domains grow appreciably larger. 7. DISCUSSION
In the present study of the Pt-rich side of the Pt-V phase diagram a new superstructure PtsV isomorphous with Pt,Ti (Ni,Nb, Ni,Ta, etc.) was found while two metastable LPAPB phases were discovered at the 3: I composition. At variance with previous results in the Pt-Ti system no long period structures could be determined in the composition range between 8: 1 and 4: 1; only substitutionally disordered derivates of the PtrV and Ll, structures were found in this composition region. The most interesting structural features were encountered at elevated tem~ratures in the 3: 1 composition. In the vicinity of the order-disorder temperature a short range ordering was found following constraints that give rise to the predominance of the nearest neighbour tetrahedron with the bulk composition 3: 1 as a building unit. As a result the Ll, structure is found inside these microdomains, while the DOZ2 structure is mainly confined to conservative APB’s. In very thin parts of the specimen, yet thick enough to ensure a sufficient difference in projected potential between platinum and vanadium columns but where no defects perpendicular to the electron beam are present, the configuration of pure vanadium columns due to the stacking of such tetrahedra can be visualised by high resolution electron microscopy. At temperatures just below the order-disorder temperature a sequence of LPAPB structures presumably stable in narrow temperature ranges only can be inferred; two examples of such structures are resolved indicating a mixing of DO= and Ll, units; the amount of DO,, building units increases with decreasing temperature eventually leading to the stable DO,, phase itself. This result can be compared with recent investigations in the Al-Ti alloy where the existence of a simple and a so-called devils staircase was predicted from the occurrence of a large number of long period phases each stable in a small
54
SCHRYVERS
and AMELINCKX:
high temperature range. In the latter case however, temperature differences of 50” could be used since the alloy remains ordered until the melting temperature at 1340°C [12], while in the present investigation all long period phases occur in a temperature range of
IOO”C. Although no in situ experiments can be performed in the high resolution instruments now available, a detailed investigation of an as quenched state can provide useful information on the ordering and transformation mechanisms of ordered phases in alloys. In the present case the formation of the stable DO,, structure out of long period structures metastable at the annealing temperature is inferred from the presence of ledges and hairpin defects. REFERENCES 1. D. Schryvers, J. Van Landuyt and S. Amelinckx, Mater. Res. Bull. 18, 1369 (1983).
Pt-V PHASE DIAGRAM
2. D. Schryvers and S. Amelinckx, Mater. Rex Bull. 19, 979 (1984). 3. D. Schryvers, G. Van Tendeloo and S. Amelinckx, Mater. Res. Bull. 20, 367 (1985). 4 D. Schryvers and S. Amelinckx, Mater. Res. Bull. 20, ’ 361 (1985). 5. R. M. Waterstrat, MetaN. Trans. 4, 455 (1973). 6. D. Schryvers, J. Van Landuyt, G. Van Tendeloo and S. Amelinckx, Physica status. solidi (a) 75, 607 (1983). 7 D. Van Dyck and W. Coene, Ultramicroscopy 15, 29 ’ (1984). 8. R. De Ridder, G. Van Tendeloo and S. Amelinckx, Acta crystallogr. A32, 216 (1976). J. Van Landuyt, R. De Ridder, R. Gevers and S. 9. Amelinckx, Mater. Res. Bull. 5, 353 (1970). IO R. Portier. D. Gratias, M. Guymont and M. Stobbs, J. Microscl 119, 163 (I980). _ 11. K. Fujiwara, J. Phys. Sot. Japan 12, 7 (1957). 12. A. Loiseau, G. Van Tendeloo, R. Portier and F. Ducastelle, J. Physique 46, 595 (1985). 13. W. Coene, private communication. 14. D. Van Dyck, G. Van Tendeloo and S. Amelinckx, Ultramicroscopy 15, 357 (1984).