A FIM-AP investigation on the ordering process and antiphase domain boundaries in Ni3Fe alloy

Acta metall, mater. Vol. 42, No. 7, pp. 2499-2504, 1994

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Pergamon

0956-7151(93)E0103-A

Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0956-7151/94 $7.00 + 0.00

A FIM-AP INVESTIGATION ON THE ORDERING PROCESS A N D ANTIPHASE DOMAIN BOUNDARIES IN Ni3Fe ALLOY YANG LIN, FENGWUZHU and JIMEI XIAO Department of Material Physics, The University of Science and Technology, Beijing 100083, P. R. China

(Received 27 September 1993) Abstract--Field Ion Microscope (FIM) and Atom Probe (AP) have been used to investigate the ordering process and antiphase domain boundaries (APB) in the {200} atomic planes in Ni3Fe alloy. Although the ordering transition is of the continuous or homogeneous type, the ordering process is not homogeneous in various domains of the specimen. The conservative and non-conservative APBs in the {200} planes have been recorded by FIM micrographs and AP ladder diagrams from layer by layer analyses. The experimental results about APBs are discussed in terms of formation energy.

INTRODUCTION The transformation of ordering in Ni3Fe has been generally considered a thermodynamically second order one [1] without the phenomena of nucleation and growth. The process should be a continuous course of atomic interchange proceeding simultaneously throughout the crystal and the microstructure of the alloy should be homogeneous at all stage of the transition. But the F I M experimental results of Taunt and Ralph [2] exhibited that this transformation was not a classical continuous or homogeneous one. The measurements of the Plane Stability Ratio showed that the transition did not proceed homogeneously in the crystal though the perfectly ordered nucleations are not existent. Some domains with a higher degree of order grow to impingement into other domains which are ordering slower than the former. They proposed the "modular ordering" mechanism to describe this situation. The mechanism needs more experimental evidences. The structures of APBs between ordered domains have effects on the mechanical properties of an alloy, especially the movement and development of lattice defects. Present reported in this area experimentally as well as theoretically are the theses about the orientational distribution of APBs [3, 4], the mutual effects of APBs and dislocations [5, 6], and the reactions between different APBs [7]. Among the alloys with L12 superlattice, Ni3A1 and several other alloy systems are often investigated. However, there is only a few of the experimental data about APBs in Ni 3Fe available. One of the reasons is that the Electron Microscope is difficult to provide effective information of ordering and APBs in Ni 3Fe due to its magnetism. The application of F I M - A P is not the case.

The composition of subsequent {200} atomic planes in the ordered Ni3Fe with L12 superstructure is different from that in the disordered Ni3Fe [1]. If one of the planes is consisting of pure Ni when the ordering is complete, the next one of the planes is consisting of Ni-50%Fe. The composition difference which reports the atomic arrangement within and between the ordering domains, i.e. the ordering degree and APB structure, can be recorded with the F I M image [8] by the difference in contrast on the screen and with AP ladder diagram [9] from layer by layer analysis in the [200] direction. In this paper, we will present atom probe data to confirm the "modular ordering" mechanism and propose atomic structures of APBs in {200} planes of Ni 3Fe on the basis of the F I M micrographs and AP ladder diagrams. The discussion about the authenticity of these atomic structures is made in terms of the formation energy of APBs.

EXPERIMENTAL The composition of the material is stoichiometric Ni 3Fe. AP mass spectrum from random area of the matrix shows scarcely any adulterants. The samples were homogenized at 900°C for one hour before the anneal treatment. F I M images obtained from the homogenized specimen exhibited the irregularity expected from a disordered solid solution. Several visible rings surrounding the poles with low index indicate the existence of local order fluctuation with the amplitude of about 1 nm. Ladder diagram obtained from AP analysis is a straight line which shows the homogeneity in concentration of the specimen at the atomic scale. Five groups of the homogenized samples were isothermally annealed at 485°C. The annealing time

2499

2500

YANG LIN

et al.:

FIM-AP INVESTIGATION ON ORDERING PROCESS IN Ni3Fe

Fig. 1. FIM micrograph of Ni3Fe annealed at 485°C for 20 h after homogenization at 900°C.

is 20, 52, 98, 180 and 250 h respectively. Tips of all the groups were investigated by F I M image and AP layer per layer microanalysis on the {200} poles. The tips of Ni 3Fe for F I M - A P investigation were obtained by the standard electropolishing technique in the solution of 10% Na2CrO 3 and 90% acetic acid in the voltage range of 10-35 V d.c. Most of the photographs of the F I M images were taken at the temperature between 30 K and 70 K. Atom-probe layer by layer microanalyses were conducted mainly at {200} poles. During the procedure of AP experiments, the pulse fraction was maintained close to 0.2 and the temperature higher than 60 K. These conditions were chosen to reduce the risk of tip failure during the field evaporation. An atom probe FIM, imported from Germany and described in detail by Mertens [10], has been used for the experiments. In the F I M images of ordered

Fig. 3. FIM micrograph of Ni3Fe annealed at 485°C for 250 h after homogenization at 900°C.

Ni 3Fe, the bright ring alternates with the dim ring on the {200} poles and vice versa. The dim ring is field-evaporated more rapidly than the bright ring. The ring with different luminosity corresponds to the atomic plane with different composition. Unfortunately, it has not been determined whether the bright ring corresponds to the pure Ni layer or to the N i - 5 0 % F e layer. RESULTS After annealing, the overall regularity of the F I M images is improved with an increase in the number and regularity of the high index poles and an increase in the number and clarity of the rings surrounding the low index poles. The longer the annealing time, the more regularity of the images is observed, indicating a higher degree of order. The result is exhibited in Figs l, 2 and 3 which are the F I M micrographs of the specimen with the annealing time of 20, 98 and 250 h

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Fig. 2. FIM micrograph of Ni 3Fe annealed at 485°C for 98 h after homogenization at 900°C.

Fig. 4. AP ladder diagram of Ni3Fe annealed at 485°C for 20 h after homogenization at 900°C.

YANG LIN et al.:

FIM-AP INVESTIGATION ON ORDERING PROCESS IN Ni3Fe

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Fig. 7. The APB on the (100) pole of a Ni3Fe specimen annealed at 485°C for 250 h.

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Fig. 5. AP Ladder diagram of NiaFe annealed at 485°C for 98 h after homogenization at 900°C. respectively. No contrast is observed which could be ascribed to be a well ordered particle or "nuclei" embedded in a disordered matrix. Ladder diagrams are obtained through AP layer by layer microanalyses on the {100} poles of the specimens of all the five groups. Figures 4, 5 and 6 are three of them which correspond to the specimen annealed for 20, 98 and 250 h respectively. The perpendicular section of the curve is pure Ni layer while the section with a slope is the layer filled half with Ni and half with Fe. Comparison of those diagrams exhibits that if the specimen is annealed for a longer time, more successive ladders and more improved ladder integrity are observed. These indicate that the ordering domain increases its degree of order and expends its size with the lengthening of annealing time. In the ladder diagrams of the partially ordered specimen, there is not any section containing continuous integrate ladders corresponding to perfect ordered domain or any section containing no form of ladder at all corresponding to completely disordered domain. It demonstrates that the ordering transition NO. OF N I

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is not a nucleation and growth process. On the other hand, the ladders are not distributed evenly along the curve. Somewhere, the ladders are integral and the segment with successive ladders is long. Elsewhere, the ladders are not integral and the segment with successive ladders is short. The observation demonstrates that the ordering transition is not classically homogeneous or continuous as well. APBs are observed in specimens of the last two groups annealed for 180 and 250 h. Figure 7 is an example of the FIM micrographs of the {100} poles of the specimen. There is a straight line running across the set of concentric rings. Two parts of the same atomic plane on the two sides of the line have different contrast. One is bright half-ring when the other is dim half-ring. That is, one half of the same plane is consisting of pure Ni when the other half is consisting of Ni-50% Fe. The straight line is the very APB which we are searching for. It is perpendicular to (200) atomic planes, the profile of which is schematized in Figure 8. Since the APB is only one or two space of atomic plane in width, three kinds of atomic stacking sequence of the boundary are proposed, as shown in Fig. 9(a-c). Figure 9(a) shows a FeNi nonconservative APB (ncAPB) with a shear vector of 1/2[10i]. There is an additional Ni-50%Fe atomic plane on the boundary compared with the stacking sequence in the ordered domain. Figure 9(b) shows a NiNi ncAPB with a shear vector of 1/21101]. There is an additional pure Ni atomic plane on the boundary. Figure 9(c) is a conservative APB (cAPB). There is not any change in composition near the boundary

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Fig. 6. AP Ladder diagram of Ni3Fe annealed at 485°C for 250 h after homogenization at 900°C. AM 42,7

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Fig. 8. The schematic longitudinal section of the APB in Fig. 7: [] indicate the Ni-50%Fe composition; [] indicate the pure Ni composition.

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FIM-AP INVESTIGATION O N O R D E R I N G

PROCESS IN Ni3Fe

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Fig. 9. Three atomic structures of APBs in {200} planes of Ni 3Fe: • Fe; O Ni. (a) FeNi non-conservative APB. (b) NiNi non-conservative APB. (c) Conservative APB.

in the [001] direction. The AP layer per layer analysis provided useful information for answering the question "Which of the three possible APB structures is more probable?" Figure 10(a-c) are obtained from the AP layer by layer analyses on the {100} poles of the specimen annealed for 180 h. In Fig. 10(a), the part of A and B sections with the composition of Ni-50%Fe, are inserted in successive integral ladders which exhibit domains with high degree of order. It is the record of FeNi ncAPB. Section A and section B correspond to the concentration of the atomic planes of A ' and B' in Fig. 9(a) respectively. (The direction of AP analysis is consistent with [001] direction in Fig. 9(a).) Ladder diagrams like Fig. 10(a) indicate the existence of FeNi type ncAPB. In Fig. 10(b), the part of C and D sections, a platform with the composition of pure Ni, is the record of ncAPB with two adjacent layers of

pure Ni. (Two Fe atoms on the platform are antisite atoms which occupy only 4%) Section C and section D correspond to the concentration of atomic planes of C' and D' in Fig. 9(b) respectively. Ladder diagrams like Fig. 10(b) indicate the existence of NiNi type ncAPB. Non-conservative APBs can be recorded on the ladder diagram when the direction of AP analysis is perpendicular to the APB because of the concentration variation on the boundary. On the contrary, conservative APB can not be recorded as above because there is not such a concentration variation at the boundary. However, it does not mean that the conservative APB can not be recorded by Atom Probe. When the plane of a conservative APB is situated in the exact center of the probe hole and parallel to the direction of AP detection as shown in Fig. 11, half layer atoms

YANG LIN

et al.:

FIM-AP INVESTIGATION ON ORDERING PROCESS IN Ni3Fe

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of pure Ni and half layer atoms of N i - 5 0 % F e in the same atomic plane will reach the AP detector simultaneously, and the ladders on the ladder diagram will be replaced by a segment close to a straight line with a slope of 1 on 3. Figure 10(c) is a example of ladder diagrams with such a segment. The segment from I to J consists of about 10 atomic layers indicating the length of the APB in the direction of AP analysis. In short, three probable atomic stacking sequences near APBs in {200} planes of Ni 3Fe are all recorded by AP layer per layer microanalyses. There are some other beneficial results about the three types of APBs on the FIM images. If you look along [111] direction at the atomic structure of a non-conservative APB showed in Fig. 9(a) and (b), the boundary can be seen not on the surface of {002} and {020} atomic planes but on the {200} atomic-planes with different composition on two sides of the boundary. It means that in the F I M image with {111 } pole in the center, two of the three {100} poles remain intact while the rings surrounding the third pole are disintegrated, if the recorded APB parallel to {200} planes is non-conservatwe. If you look in the same direction at the atomic structure of a conservative APB as shown in Fig. 9(c), the boundary can be seen on the surface of {020} and {200} atomic planes. It means that the F I M image will have only one of the {100} poles intact, and the other two poles are disintegrated by the APB. Two types of F I M images as described above are observed in the experiments.

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Fig. 10. (a) the FeNi non-conservative APB in a AP ladder diagram on the {100} pole of Ni 3Fe annealed for 180 h. (b) The NiNi non-conservative APB in a AP ladder diagram on the {100} pole of Ni3Fe annealed for 180 h. (c) Conservative APB in a AP ladder diagram on the {I00} pole of Ni3Fe annealed for 180 h.

The experimental results indicate that the ordering transition in Ni3Fe alloy is of neither the nucleation and growth type nor the classical homogeneous or continuous type. In the process, some domains are ordered rapidly, reaching a higher degree of order;

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YANG LIN et al.: FIM-AP INVESTIGATION ON ORDERING PROCESS IN Ni3Fe

other domains are ordered slowly, having a lower degree of order. Domains with a higher velocity of ordering gradually increase in size to consume the domains with a lower velocity of ordering. The results are consistent with the "modular ordering" mechanism [2]. The facts that narrow APBs exist in partially ordered specimens (180 h annealing, for example) exhibit that ordering domains can meet together in a specimen far from perfectly ordered, and the diffuse interface between domains, a deduction presented by Taunt and Ralph from "modular ordering" mechanism, is not always the case. Starostenkov [7] calculated the formation energy of conservative and non-conservative APBs on various crystal orientation in Ni3Fe in an approximation which allows for atomic bonding in seven co-ordination spheres and allows for atomic interaction by sets of Morse Potentials. From his data, the formation energy of cAPB on the {200} atomic planes is about 88 mJ/m 2 and the absolute of the formation energy of NiNi ncAPB and FeNi ncAPB are close to each other and both higher than 500 mJ/m 2. The signs of the formation energies of the two kinds of ncAPBs are opposite. One is positive and the other is negative. The average which corresponds to the energy of a couple of ncAPBs, so called "dual complex of APBs" which are parallel to each other and have opposite local change in composition, is merely 10mJ/m 2. Therefore, the energy of dual complex of APBs is much lower than that of the cAPB. When the atomic concentration in the quenched specimen is homogeneous without any considerable segregation in atomic scale, the ncAPB can not arise individually in the process of ordering. For example, in order to compensate for the local change of composition resulted from the presence of NiNi ncAPB, a FeNi ncAPB must arise nearby. Therefore when the formation energy is discussed, not the individual ncAPB but the dual complex of ncAPBs

should be taken into account. Since the formation energy of dual complex of ncAPBs is much lower than that of cAPB, the calculation confirmed the authenticity of the F I M - A P results of the existence of ncAPB. Meanwhile, the calculation does not reject the existence of cAPB, because the formation energy calculated by Starostenkov does not include the part corresponding to the atomic diffusion during the ordering process, and this part of energy consumed by the formation of a cAPB is much lower than that consumed by a ncAPB. The balance between the "real" formation energy of the cAPB and that of the dual complex of ncAPBs is lower than the balance between the "not-real" ones. The existence of cAPB is still reasonable. CONCLUSION 1. F I M - A P can be used to investigate the ordering process and APB structure of alloys. 2. The ordering process in Ni3Fe follows the "modular ordering" mechanism. 3. Conservative, NiNi type non-conservative and FeNi type non-conservative APBs are all existent in the {200} planes of the well-ordered Ni 3Fe. REFERENCES

1. H. Warlimont (editor), Order-Disorder Transformation in Alloys. Springer, Berlin (1974). 2. R. J. Taunt and B. Ralph, Physica status solidi (a) 24, 207 (1974). 3. V. Paidar, Acta metall. 33, 1803 (1985). 4. M. D. Starostenkov, Fizika Metall 66, 1103 (1988). 5. J. Douin, Phil. Mag. 58, 923 (1988). 6. P. Veyssiere, Phil. Mag. Lett. 59, 61 (1989). 7. M. D. Starostenkov, Phys. Met. Metall, 72, 47 (1991). 8. R. Wagner, Crystals-Growth and Applications, Vol. 6. Springer, Berlin (1983). 9. M. K. Miller and G. D. W. Smith, Atom Probe Microanalysis. MRS, Pittsburgh, Pa (1989). 10. P. Mertens, V. Vidic and H. Becker, Hahn-MeitnerInstitut Berlin Report (1985).