In situ TEM study of the Ni5Al3 to B2 + L12 decomposition in Ni65Al35

April 1995

ELSEVIER

MaterialsLetters23 (1995) 105-I 11

In situ TEM study of the N&Al3 to B2 + L12 decomposition in Ni65A135 D. Schtyvers, Y.Ma EMAT, RUCA, University of Antwerp, Groenenborgerlaan

171, B-2020 Antwerp, Belgium

Received4 February 1995; accepted 9 February 1995

Abstract Homogenised and quenched Nie5Alj5 samples were heated and studied in situ in a CM20 electron microscope up to 900°C. The N&Al3 phase first forming around 550°C in the quenched Ll,, microtwinned martensite starts to decompose around 800°C yielding B2 precipitates in a twinned Liz matrix. The latter twinning is a remainder of the microtwinning in the original room temperature martensite. Also the crystallographic relations between precipitates and matrix can be traced back to the original formation of twinned martensite plates within the austenite. Some aspects of the dynamics of the process are discussed on the basis of snap shots and video recordings.

1. Introduction

Ni-rich Ni._Al,W-Xwith x > 63 at% transforms martensitically when quenching from a homogenisation temperature of 1250°C. The austenite phase has an ordered bee based B’2 structure (CsCl type) [ 11 in which the excess Ni atoms are randomly distributed over the Al sublattice [ 21. The martensitic transformation can be described by a reverse Bain distortion resulting in an fct lattice with L10 type ordering and a c/a ratio between 0.82 and 0.86 [ 3,4 1. Due to the small twin surface energy in this alloy [ 4 1, the stress at the habit plane is accommodated by very fine microtwinning on ( 111 }rct planes inside the martensite plates. When Ni65A135martensite samples are annealed at moderate temperatures, the randomly distributed Ni atoms order to form the NiSAl, ordered structure [ 51 (further on called 5:3) with an orthorhombic unit cell, but based on the same fct lattice. This phenomenon was studied before in situ as well as with pre-heated samples, from which it was concluded that the microtwin0167-577x/95/ $09.50 0 1995 Elsevier Science B.V. All rights reserved SSDlO167-577x(95)00030-5

ning remains unchanged and stacking faults on a second family of close packed planes not parallel with the twin planes are introduced to accommodate the slight ( 1%) increase in c/a [ 51. The formation of this 5:3 phase inhibits the material to show the reverse transformation to austenite, which is a major problem for potential applications of the shape memory behaviour of this alloy [ 61. In the present paper we describe the results of a continuing heating procedure up to 900°C during which the 5:3 phase decomposes into B2 grains distributed in an Liz matrix, the latter taking over the twinning of the 5:3 and thus of the original martensite.

2. Experimental procedure The material and sample preparation are described in detail elsewhere [ 51. The in situ experiment was performed in a CM20 Philips instrument equipped with a LaB6 filament and twin lens system. A double-tilt

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Gatan heating holder with tantalum cup and watercooling was used. The chemical analysis was performed with an atmospheric thin window link system and using the original N&Al,, composition as reference, yielding an averaged precision of about 2 at%.

3. Results As mentioned above, heating up to 550°C transforms the martensite into the stable 5:3 phase. Apart from the introduction of a few stacking faults, the crystallography and morphology of the plates remain unchanged [ 51. After a prolonged heating for about 1 h at this temperature, the edge of the thin foil deteriorates leaving small particles, as shown in Fig. la. At the same time a large amount of vague fringes parallel with the second family of close packed planes become visible in the broadest microtwins, i.e. those in diffraction condition. In view of the observation of stacking faults in shortly annealed samples, the present fringes could also be interpreted as due to overlapping stacking faults with increasing numbers. However, possible surface effects could not be ruled out at present. Further on, these defects will generally be referred to as planar defects. The corresponding new streaking, at 67” from the streaks due to the microtwinning already present in asquenched material, appears in the selected area electron diffraction (SAED) pattern of Fig. 1b. The latter reveals two < 101 > 5:3variants in twin relation with a {212},,3 close packed plane (i.e. a { 111 Jfctplane) as twin plane, the faint new streaking being only visible in the pattern of the broadest variant. The indicated angle of 86.5” between the 212 and 222 5:3 spots reflects the tetragonality of the basic fct lattice of the 5:3 structure (for a detailed description of the observed reflections we refer to Ref. [ 51). The indications in Fig. la of the close packed planes exhibiting the microtwins and the planar defects as (212) and (212), respectively, correspond with the broadest variant. Further annealing up to 780°C causes the rapid nucleation and growth of plate-like precipitates, tapered on all,edges, in which the twinning disappears. In the present experiments at least two orientations of these precipitates with respect to the twinning were observed. In the first case the main precipitate-matrix interface is parallel with the close packed planes on which the planar defects were observed at lower tem-

Letters 23 (I995) 105-111

peratures. In the other case, this interface is parallel with the twin planes. Examples of both cases are shown in the bright field (BF) images of Figs. 2a and 2b, respectively. Some of these precipitates are still confined within the thin foil, as seen from the overlapping of twin contrast bands in precipitates labelled A in Fig. 2a. A typical width of a precipitate is about 50 nm while their lengths can extend to a few microns. Others clearly extend through the entire foil as for precipitate B. The latter one only shows a few remaining defects on the tips of the precipitate (upper left corner). On the other hand, the Ll, matrix shows many defects as short white or black lines confined within a given twin variant (see arrows). In Fig. 2b these contrast lines are again parallel with the second family of close packed planes, but in Fig. 2a they appear in different orientations. When such a heated sample is allowed to cool down slowly inside the microscope, no phase transformations or changes in morphology are observed. The crystal structures and their crystallographic relation of the precipitates and the twinned matrix can be obtained from diffraction patterns as shown in Fig. 3a, with a schematic in Fig. 3b. From this it can be concluded that the sample now consists of B2 precipitates observed along a ,, zone embedded in a twinned LIZ matrix viewed along a < 101 > ret zone. In the schematic, reflections belonging to the former are indicated with circles, while those for both variants of the latter are given by up and down pointing triangles. The main differences between the 5:3 and Ll, twinned patterns of Figs. 1b and 3a, respectively, are the positions of the ordering reflections and the cla ratio which increases from 0.89 to 1. The lattice parameters measured at room temperature for the B2 and L 1Zstructures are 0.268 nm and 0.338 nm, respectively. EDX microanalysis yields Ni60,5A139,5 for the B2 and Ni7c,Al30for the Lla phases. The BF image of Fig. 4 shows the main feature of the growth process of the B2 precipitates through the twinned Ll, matrix. At a given temperature, the precipitate-matrix volume ratio stabilises after a few minutes. When the temperature is slightly increased (e.g. by 10”) the smaller precipitates will grow by first forming a small extension (indicated X in Fig. 4) along their long axis. This extension can grow to about lOO200 nm in a few seconds thus consuming a few subsequent twin variants of the LIZ matrix. After this time, the growth in length is stopped and the extension will broaden to the original width of the precipitate, mainly

D. Schryvers, Y. Ma /Materials Letters 23 (199.5) 105-111

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Fig. 1. (a) Small particles appearing at the edge of a thinned sample after annealing at 550°C for about 1 h. The twin variants in diffraction condition show fringes parallel with the second family of close packed planes. (b) < 101 > 5:3SAED patterns showing streaking due to the microtwinning (p.T) and The new planar defects (PD). Note the indicated angle of 86.5”. reflecting the tetragonality of the basic 5:3 lattice.

determined by the exi.stence of nearby precipitates. The latter process occurs at least twice as slow as the former. The dislocations at the growth interface are clearly visible in Fig. 4 and move along with the interface.

4. Discussion According to the phase diagram, a two-phase region consisting of the bee based B2 phase and the fee Ll, y phase exists above 700°C at a nominal composition of NisSAlS5 [ 71. Here the compositions of both phases are %A139 and Ni72.5A127.5rrespectively. Below this tem-

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Letters 23 (1995) 105-l I I

Fig. 2. Plate-like B2 precipitates in an Lll twinned matrix with their primary interface parallel with (a) planar defects from Fig. la and (b) twin planes. Some defects are observed inside the Lll matrix (see arrows).

perature an ordered N&Al3 phase is known to be stable, as proven by the precipitation of this phase in the B2 matrix in Ni62.5A137.5samples [ 81. However, quenching from a homogenisation temperature of 125O”C, i.e. above the two-phase region, results in a martensitic transition of the B2 phase without the precipitation of the Liz phase. As a result, the room temperature mate-

rial has an fct based Llo structure with multiple microtwinning. As found before, the 5:3 phase can be obtained by annealing the martensite at 550°C [ 51. Since this 5:3 structure is also based on an fct lattice, this Llo to 5:3 transition primarily involves reordering besides a slight increase of 1% of the c/a ratio. As a result of the latter, a few stacking faults are introduced

D. Schryvers, Y. Ma/Materials

v

Letters 23 (1995) 105-111

V

A

A 11082

0 A

V

109

&a

qa

iih,,

A

V

n

Fig. 3. (a) SAED pattern revealing the orientation relation between the B2 aad Liz lattices, seen along [ 1101 and [ 1011 zones, respectively. The 90“ angle corresponds with the cubic symmetry of the Ll, lattice. (b) Schematic of (a) : B2 as open circles and Lla as up and down pointing triangles (diffemnt variants), small triangles being ordering reflections.

in the otherwise unchanged microtwins. The structural relation existing between the Llo martensite and B2 austenite lattices and originating from the purely displacive character of the martensitic transition is thus retained after the reordering to the 53 structure. When this 5:3 structure, based on a tetragonal lattice, is then transformed into a cubic LIZ structure, again a reordering plus a small increase in c/a to 1 takes place. The reordering is now part of a decomposition increasing the Ni content in the twinned regions. Meanwhile the remaining parts are depleted from Ni and these will

transform back to the B2 structure. As a result the crystallographic relation between the B2 and the LIZ lattices is expected to be related to that between the austenite B2 and martensite Llo lattices. Also, the Lla matrix is not expected to show strong differences with the former 53 microstructure and orientations. These considerations are confirmed by the SAED pattern in Fig. 3a, where sections of both the B2 and LIZ structures are observed. Indeed, the < 111 > zone of the B2 phase consists of three < 110 > directions. During the martensitic transition, the families of planes

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Letters 23 (1995) 105-111

Fig .4. Snap-shot of a growing precipitate. The small region X extends into the L12 matrix, consuming a few twin variants before it broadens to the width of the rest of the precipitate. The interface dislocations are clearly visible.

corresponding with two of these directions become close packed planes of a given variant of the Ll, structure. These planes remain close packed when the martensite becomes 5:3 and eventually Ll,. One of these families will further act as microtwin planes with a second variant, a construction which remains again unchanged during the entire ensuing transformation path. The only effect of the slight increase in c/a ratio is the introduction of stacking faults on the second family of edge on close packed planes. When part of the material returns to the B2 structure, these close packed planes again become ( 1IO} planes returning the zone to the original < 1 1 1 > . A small rotation of the B2 lattice inside the precipitates and with respect to the L l2 lattice of the twinned matrix can be measured from the SAED pattern of Fig. 3a. An angle of 3” exists between the iii,,, and 110B2reflections which is comparable to the opposite rotation accompanying the martensitic transition from B2 to Lie. The former < 101 > zone of the 5:3 structure (cf. Fig. lb) is inherited by the L12 matrix, now indeed showing the proper 90” angle between the 12i and ii1 fee reflections corresponding with the cubic symmetry of its basic lattice. Thus

[loll,:,+

[loll,,,

@i2),:,+(iil),,,

and [liol,,, and (llO)uZ.

As for the directions of growth, in both cases shown in Fig. 2 the main interface occurs parallel with close packed planes of the twinned matrix. In the first case these planes are the ones exhibiting the planar defects in the early stages of the heating, while in the second case these are the actual twinning planes. The growth process described above belongs to the first case and indicates that the close packed planes exhibiting the planar defects around 550°C act as good glide planes for the interface dislocations. Apparently the existence of these planar defects facilitates the movement of the dislocations. Close packed planes parallel with the twin planes and not exhibiting the planar defects only act as glide planes much later in the growth process thus broadening the precipitate. As for the B2 lattice, the main interface is close to parallel with one of the ( 1lO)u* planes. Due to the high density of precipitates and their specific orientation, the thickness growth of smaller ones is limited by the existence of larger ones grown during an earlier stage.

D. Schryvers, Y. Ma /Materials Letters 23 (199s) 105-l I1

The defects observed in Fig. 2a as short white lines parallel to the second family of close packed planes in the LIZ matrix are presumably again stacking faults accommodating the increase in c/a. The defects observed in Fig. 2b are not confined to these planes and are expected to be anti-phase boundaries (APBs) resulting from the reordering from 5:3 to LIZ accompanying the decomposition. As already indicated in the literature [ 7,9] , this reordering is expected to be limited to the mixed Ni-Al planes leaving the main Ni lattice unchanged, in its turn limiting the number and type of APBs.

5. Conclusions

The present paper describes the results of an in situ heating experiment inducing a decomposition from N&Al3twinned material into a twinned LIZ matrix with embedded plate-like B2 precipitates. The crystallog-

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raphy and morphology are consistent with the expectations in view of the history of a martensitic transition which happened before the present experiment. Details of the interface structure and stacking or ordering defects will be obtained by high resolution electron microscopy in the near future. References [ 1] K. Becker and F. Ebert, Z. Physik 16 (1923) 165. [2] A.J. Bradley and A. Taylor, Proc. Roy. Sot. A 159 (1937) 56. 131 S. Chakravorty and CM. Wayman, Metall. Trans. A 7 (1976) 555,569. [4] D. Schryvers, Phil. Mag. A 68 ( 1993) 1017. [5] D. Schryvers and Y. Ma, J. Alloys Compounds, in press. 161 E.P. George, C.T. Liu, J.A. Horton, C.J. Sparks, M. Kao, H. Kunsmann and T. King, Mater. Char. 32 ( 1994) 139. [ 71 P.S. Khadkikar, I.E. Locci, K. Vedula and G.M. Michal, Metall. Trans. A 24 (1993) 83. [8] D. Schryvers, Y. Ma, L. Toth and L.E. Tanner, Acta Metall. Mater., accepted for publication. [9] I.M. Robertson and CM. Wayman, Phil. Mag. 48 (1983) 443.