Martensitic transformation of the Ni2Al phase in 63.1 at.% NiAl

Acta rnetall, mater. Vol. 41, No. 7, pp. 2135-2142, 1993 Printed in Great Britain. All rights reserved

0956-7151/93 $6.00 + 0.00 Copyright © 1993 Pergamon Press Ltd

MARTENSITIC T R A N S F O R M A T I O N OF THE Ni2A1 PHASE IN 63.1 at.% NiA1 A. S. M U R T H Y and E. G O O Department of Materials Science and Engineering, University of Southern California, Los Angeles, CA 90089-0241, U.S.A. (Received 22 June 1992; in revised form 14 December 1992)

Abstract--Electron diffraction and high resolution electron microscopy of 63.1 at.% NiAI have shown that the metastable hexagonal N i 2A1 precipitate, coherent in the B2 matrix phase, transforms concurrently with the martensitic transformation of the B2 NiAI matrix to the LI 0 phase. The Ni2A1 precipitate that is coherent with the LI 0 phase in monoclinic.

I. INTRODUCTION In the NiA1 phase diagram shown in Fig. 1 [1], B2 (or f12) NiAI phase is stable at high temperatures over a wide composition range of 45453 at.% Ni. The B2 phase can be retained by quenching for compositions up to 63 at. % Ni from an elevated temperature. At a higher nickel content, the B2 phase undergoes a martensitic transformation on quenching to a phase with a face centered tetragonal (Ll0) structure (c/a = 0.86). The M s temperature for the transformation varies sharply with Ni content [2], from - 2 5 0 ° C to 150°C over a composition range of 60-68 at.% Ni. The structural relationship between the parent and product phases can be described by Bain distortion with a {ll0} ( 1 ] 0 ) homogeneous shear in the B2 structure to result in the ( l l 0 ) B2 planes transforming to the (11 l) planes of the Ll 0 structure. Prior to the martensitic transformation, a number of anomalies including elastic softening [3], tweed microstructures, electron diffuse streaking and diffuse intensity maxima at ~(111) and ~(211) of the B2 phase have been noticed [3-9]. These diffuse spots were shown to be replaced by sharp superlattice reflections after ageing the Ni rich B2 phase at 300-500°C [5, 6, 8]. These extra reflections were explained by Reynaud [5] as due to the formation of the four possible variants of an ordered phase of Ni2AI composition. The crystal structure of this phase was described based on a hexagonal symmetry (space group: P~_ml, no. 164) with the unit cell p a r a m e t e r s , aNi2Al ~ X//2aB2 and CNi2Al~ N//3CB2• The formation of such an ordered phase requires that the excess nickel atoms in the B2 NiA1 occupy sites on the aluminium sublattice. Lasalmonie [6], however, interpreted the occurrence of the superlattice reflections at ~(111) and ~(211) B2 as due to the formation of disc-shaped ordered domains lying on the {112} planes. The structure of the ordered precipitates was shown to have hexagonal symmetry with a -~ (1 I~)B2 and c ~ - ( I I 1)a2. Arkhangel's kaya et al. [10] have AM41/7--N

characterized the intensity maxima as also due to an ordered phase, but give a modified atomic configuration of the Ni2A1 unit cell. It appears from these studies that the nature of the ordering in these precipitates is still in question. Ageing of the L10 phase at intermediate temperatures of 400-500°C has also been shown to result in the formation of the ordered phase with the NizAI composition. The occurrence of such an ordered phase in the L 10 phase was first investigated by Enami [11], and it was shown that in 63.8 NilCoAI, ageing

1800

I

1400

800

600

,o0 30

40

50

60 70 At. % NI

80

90

Ni

Fig. 1. The NiA1 phase diagram (from Singleton et al.

2135

[If).

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MURTHY and GOO: MARTENSITIC TRANSFORMATION OF Ni2A1

Fig. 3. The dark field image using g = ](022).

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®

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at 400°C for 1-2 h resulted in the superlattice reflections of ~{012} and ~{022}L~0 in the diffraction patterns. Based on the SAD results, Enami [11] derived the crystal structure of the ordered phase to be monoclinic Ni2A1 belonging to space group C2/m with a"[0T2]Ll0, b~[100]Ll0 , C"[011]LI0 and fl ~ 80 °. In these studies, Enami [11] has reported that there occurred no changes in the microstructure observed both optically and by TEM upon ageing the LI 0 phase and the consequent formation of the monoclinic Ni2A1. Enami [1 l] has further indicated that there did not occur either the reverse transformation of the L10 phase to the B2 phase or the decomposition of the L10 phase on ageing even up to 500°C. The intent of the present studies has been to investigate the microstructural evolution of the Ni 2A1 ordered phase in both the B2 and Ll 0 phases as it forms during ageing or cooling. Our focus has also been to gain a better understanding of the possible transformation characteristics of the Ni2Ai phase, the two structures of which may postulated to be related by the same Bain strain responsible for the B2 to Ll 0 martensitic transformation. For crystal structure analyses, extensive electron microscopy investigations involving specimens heat treated in bulk are being made. High resolution lattice imaging technique has proved to be very useful in ascertaining the morphology and crystal structures of the ordered Ni2AI phase present in the B2 and LI 0 matrices.

2. E X P E R I M E N T A L

Fig. 2. SAD patterns with zone axes, (a) [011], (b) [i00] and (c) [11 I] of L10 phase from a specimen aged at 500°C for 3 h in bulk. The superlattice reflections may be indexed using the monoclinic Ni2A1 structure coexisting with the L10 phase.

In our present studies we chose NiAI with a composition of 63.1 at.% Ni which corresponds to a Ms of ~ 50°C. The alloy was fabricated for us by Raychem Corp. by plasma arc melting. The as-cast material was found to consist of predominantly the L10 phase indicating that the cooling rate during melt solidification was rapid enough to suppress the diffusional decomposition to B2 NiAI and L 12 Ni3 A1. The specimens were homogenized at 1200°C in argon

MURTHY and GOO: MARTENSITIC TRANSFORMATION OF Ni2AI atmosphere for 24 h in a horizontal tube furnace prior to quenching in ice-water. The ageing treatments of bulk speicmens were done at 500°C in argon atmosphere for varied periods of time, typically between 1-10 h. Specimens for the TEM analyses were made by first mechanical thinning to about 50ttm followed by twin-jet electropolishing using 30% nitric acid in methanol at -30°C. TEM was mostly done using a Philips 420 microscope, while HREM studies were made using an Akashi 002B microscope. Optical diffraction studies were made from the high resolution electron images using an optical bench equipped with a helium laser.

3. RESULTS AND DISCUSSION Room temperature TEM studies in 63.1at.% NaAI indicate that homogenization at 1200°C followed by ice-water quenching resulted in the complete transformation of the B2 to L10 phase. Ageing the room temperature L10 phase in bulk at 500°C for different periods of time resulted in different characteristics in the microstructure. Two regimes may be distinguished in the ageing behavior of the L10 phase: (a) ageing after 3 h and (b) ageing after 8-10 h.

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These characteristics are discussed separately in the following sections. 3.1. TEM results after ageing at 500°C for 3h The diffraction results of the specimens aged for 3 h indicated the presence of extra reflections besides the fundamental spots from L10 phase. Figure 2 shows various zone axes patterns of the L10 phase obtained from the same area of the specimen. In Fig. 2(a) is shown a [011] L10 SAD pattern; superlattice reflections at ~{022} and 3{311} are clearly observed. In Fig. 2(b) and (c) are shown the [100] and [111] L10 diffraction patterns. Besides the fundamental reflections from the Ll0 phase, superlattice reflections at 3{022} and ~{021} in [100] L10 and 3{022} in [111] L10 patterns are evident. These extra reflections in all these diffraction patterns may quite well be explained as due to an ordered phase with the Ni2A1 stoichiometry having C2/m monoclinic structure existing in coherence with the Ll0 phase. The dark field image from g =-~(0~2) reflection is shown in Fig. 3, where, globular precipitates of 1-5 nm in size are seen to be distributed homogeneously in the L10 matrix. In order to ascertain the crystal structure and the morphology of the ordered Ni2AI phase, high resolution lattice imaging was undertaken. Figure 4

Fig. 4. HREM lattice image showing regions of Ni2A1 phase in a matrix of L10 phase. The specimen orientation is along [l 11] L10.

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MURTHY and GOO: MARTENSITIC TRANSFORMATION OF Ni:AI

Fig. 6. The dark field image using g = ](12T) from [I 13] B2 pattern.

represents the HREM image where the zone axis was (111) L10. The lattice fringes with a spacing of 0.27 nm, which are observed over most of the micrograph, are due to the {110} planes of the L10 matrix phase. Domains of 1-10 nm in width exhibiting cross-fringes with a spacing of 0.36 nm are observed at different regions in the micrograph. The angle between these fringes and the fringes due to the {110} L10 planes is about 63 °. The cross-fringes may be characterized as due to (200) planes of the ordered Ni2AI phase with monoclinic symmetry.

3.2. TEM results after ageing at 500°C for 8-10h

Fig. 5. SAD patterns with zone axes, (a) [I 13], (b) [110] and (¢) [111] of the B2 phase obtained from a specimen aged at 500°C for 10 h in bulk. The superlattice reflections at ~( I 11) and ~(112) B2 may be indexed using the hexagonal Ni2AI structure coexisting with the B2 phase.

Room temperature TEM studies on the specimens aged for extended periods of time (8-10 h) at 500°C indicate that the microstructure consists of the B2 matrix phase with a uniform distribution of ordered precipitates coherent with the B2 phase. Figure 5 shows [113], [110] and [111] B2 zone axes patterns obtained from the same area of the specimen. Superlattice reflections at ~(211) are evident in all these diffraction patterns. Extra reflections at ~(111) are also observed in the [110] B2 pattern. Precipitates extending over 50 nm are noticed in the dark field images (Fig. 6). High resolution electron microscopy studies on these precipitates responsible for the superlattice reflections indicate that the precipitates are coherent with the B2 matrix. A high resolution lattice image involving (1"i'0), (001), ~(1T1) and ~lT2) reflections is presented in Fig. 7. The fringes with spacings of 0.28 and 0.2 nm are due to {001} and {111} planes of the B2 phase. The lattice fringes with 0.35 and

MURTHY and GOO:

MARTENSITIC TRANSFORMATION OF Ni2A1

Fig. 7. The HREM image of B2 NiAI aged at 500°C for 10 h. The specimen orientation is along [1 lOl B2.

Fig. 8.(a) The optical diffractogram from regions showing only the B2 phase in the high resolution micrograph. (b) The diffractogram from a single variant of the Ni2A1 phase present in coherence with the B2 phase.

2139

2140

MURTHY and GOO: MARTENSITIC TRANSFORMATION OF Ni2AI respectively. This is also evidenced in the optical diffractogram shown in Fig. 8(b) which was obtained from a region showing the Ni2A1 phase in the HREM

0.5 nm spacings which are perpendicular to each other are due to ~{112} and ~{Tll} of B2 or equivalently, of { 100} and (001) hexagonal Ni2AI reflections

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Fig. 9.A schematic diagram of projections of [110] hexagonal and [010] monoclinic Ni 2A1 unit cells derived from the B2 and L10 structures. (a) [I'10] B2 projection, (b) [100] L10 projection and (c) superimposition of [1T0] B2 and [100] Ll 0 projections.

2141

MURTHY and GOO: MARTENSITIC TRANSFORMATION OF Ni2AI micrograph. The reflections observed at ~{T11} B2 in the diffractogram are due to one variant of the Ni 2A1 phase present in coherence with the B2 phase. Ageing the L10 phase in 63.1 at.% NiA1 for short periods of time resulted in a mixture of L10 and monoclinic Ni2A1 phases, while, ageing for prolonged periods of time resulted in a mixture of B2 and hexagonal Ni2AI phases. At the ageing temperature of 500°C ( > As), L10 phase would revert to the B2 phase and the ordering of the excess nickel atoms on the aluminium sublattice to result in the hexagonal Ni2AI superstructure may well thought to occur in this condition. Our observation that the room temperature phases consisted of L10 NiA1 and monoclinic Ni2A1 structures indicates that both B2 and hexagonal Ni2AI phases undergo the same Bain distortion during cooling. The martensitic nature of the transformation of the Ni2 A1 phase is further substantiated from our H R E M studies, where, the precipitates were observed so remain coherent with the L10 phase. During short periods of ageing, there indeed occurs some depletion of Ni content in the matrix due to the formation of Ni2A1 precipitates, but not as much to cause Ms to become lower than the room temperature. Consequently, the room temperature microstructure in specimens aged for short periods of time still consisted of the LI 0 phase. During extended periods of ageing, due to increased volume fraction of Ni2A1 phase forming, sufficient depletion of Ni in the B2 matrix occurs causing M s to become lower than the room temperature. Upon cooling, all the B2 NiAI phase gets retained along with the hexagonal Ni2A1 phase. The monoclinic structure of the Ni2AI phase may be derived by applying the same amount of Bain distortion as in deriving the L10 structure from the B2 NiA1 structure. The lattice parameters of the hexagonal structure in relation to B2 phase may be given as [5], a = ~'2aB2 and c = V/3aB2; fl = 60 °. The atom positions are as follows Ni: (0 0 0)..~[221~'3 3~" ( 0 0 1 ) ,

(1,~33312)

AI: (l!l 33~), d_25 ~)The lattice parameters and the atom positions corresponding to the monoclinic Ni2A1 structure are [11] __ 2 2 aNi2A 1 -- x / ( a L i o -~- 4CLIo) ,

bNi2AI = aLlo,

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2 2 = N/(aLlo + CLIo).

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In Fig. 9 are shown the schematic diagrams of the projections of the hexagonal and monoelinic Ni2AI derived from the B2 and LI 0 NiAI structures respectively. The ordering in the B2 structure may be

considered to occur such that every third [11~] row of AI atoms in the B2 unit cell is substituted by the excess Ni atoms. Fig. 9(a) shows [1-i'0] B2 projection with such an ordering to result in the hexagonal Ni2A1 unit cell. The open circles represent the Ni atoms and the filled circles, the A1 atoms. On Bain distortion the hexagonal Ni2A1 structure changes to the monoclinic Ni2A1 structure. The projection of [100]Ll0 (the direction corresponding to [IT0] B2) with the ordering inherited from hexagonal Ni2A1 is shown in Fig. 9(b). The martensitic transformation characteristic to the two structures of the Ni2A1 phase may be described by the following lattice correspondence relationship for planes and the orientation

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4. CONCLUSIONS The ageing of the LI 0 phase in 63.1 at.% NiAI at 500°C (>As) for 3 h resulted in the formation of ordered coherent precipitates of monoclinic Ni2A1 phase in a matrix of L10. The Ni2AI precipitates were spherical and 1-5 nm in diameter. The room temperature phases in specimens aged for 8-10h consisted of hexagonal Ni2AI precipitates extending up to 50 nm in size and distributed homogeneously in a matrix of B2 phase. Ordering of Ni atoms to result in the hexagonal Ni2AI phase is thought to occur in the B2 phase during ageing. During cooling from the ageing temperature, both the B2 and hexagonal Ni2A1 phases undergo a martensitic transformation in specimens aged for short periods of time to result in the LI 0 and monoclinic Ni2A1 phases. In specimens aged for longer periods of time, the B2 and hexagonal Ni2AI phases get retained during cooling due to the lowering of M s caused by the formation of increased amounts of Ni2AI phase and the consequent depletion of Ni content in the B2 matrix. The HREM study on the specimens aged under the two conditions indicates that the precipitates remained highly coherent with the respective B2 and L10 matrices.

Acknowledgements--The authors wish to thank Dr Alan Pelton at Raychem Corporation for providing the NiAI alloy, the National Center for the Integrated Photonic Technology and the center for Electron Microscopy and Microanalysis at the Univeristy of Southern California for the use of the electron microscopes. The authors also wish to thank Dr Channing Ahn at California Institute of Technology for the use of their facilities for optical diffraction studies. This work was carried out under the auspices of the U.S. Department of Energy, contract No. DE-FG0388ER45346.

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MARTENSITIC TRANSFORMATION OF Ni2AI

REFERENCES

I. M. F. Singleton, J. L. Murray and P. Nash. Binary Alloy Phase Diagrams, p. 140. Am. So¢. Metals, Metals Park, Ohio (1986). 2. A. Au and C. M. Wayman, Scripta metall. 6, 1209 (1972). 3. K. Enami, J. Hasunuma, A. Nagasawa and S. Nenno, Scripta metall. 10, 879 (1976). 4. A. Lasalmonie, Scripta metall. 11, 527 (1978). 5. F. Reynaud, J. appl. Phys. 9, 263 (1976).

6. Alain Lasalmonie, in 6th European Congress on Electron Microscopy, pp. 573-5, September (1976). 7. I. M. Robertson and C. M. Wayman, Phil. Mag. A 48, 443 (1983). 8. I. M. Robertson and C. M. Wayman, Phil. Mag. A 48, 443 (1983). 9. I. M. Robertson and C. M. Wayman, Phil. Mag. A 48, 629 (1983). 10. A. A. Arkhangerskaya, V. S. Litinov and V. V. Poleva, Phys. Metall. 48, 115 (1981). 11. K. Enami, J. Physique C4, 727 (1982).