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ORDERING IN Ni4Mo : AN APFIM/TEM/HVEM STUDY
M. Miller, E. Kenik, T. Zagula
To cite this version:
M. Miller, E. Kenik, T. Zagula. ORDERING IN Ni4Mo : AN APFIM/TEM/HVEM STUDY. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-385-C6-390. �10.1051/jphyscol:1987663�. �jpa-00226871�
JOURNAL DE PHYSIQUE
Colloque C 6 , supplkrnent a u n O 1 l , Tome 48, novernbre 1987
ORDERING IN Ni,+Mo : AN APFIM/TEM/HVEM STUDY
M.K. Miller, E.A. Kenik and T.A. Zagula
Metals and C e r a m i c s Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376, U.S.A.
ABSTRACT - A combined atom probe field-ion microscopy, transmission electron microscopy
and high voltage electron microscopy study has been performed to investigate the various ordering phenomena in a high purity Ni,Mo alloy. A disordered material was produced by low temperature high voltage (1 MeV) electron irradiation to a dose of approximately 0.4 displacements per atom. No short range order was observed during the disordering of the long range ordered material. The disordered material was found to have a statistically random distribution of molybdenum. In-situ annealing experiments of the disordered material i n the temperature range 300 to 700°C, revealed that SRO occurred prior to the LRO state.
In short range ordered material produced by thermal annealing solution treated and quenched material, no statistical evidence for microdomains, clustering or a periodic composition fluctuation was observed with the atom probe.
INTRODUCTION
Ordering in Ni4Mo has been a topic of interest for a considerable period of time.l1] In this material both (Dl,) long range order (LRO) and (1fO) short range order (SRO) may be produced.
Several field-ion and atom probe investigations have been performed on the system in addition to studies with a variety of other t e ~ h n i ~ u e 3 . [ ~ - ~ ] It is generally agreed that the LRO phase has a Dl,- ordered structure where every fifth (420) plane of a face-centered-cubic (fcc) lattice is occupied by molybdenum atoms and the remaining fcc lattice sites by nickel atoms. This ordering results in a body- centered tetragonal structure. There is, however, no agreement on the atomic arrangement in the SRO structure. The SRO state is characterized by diffuse intensity maxima a t the (ItO) positions in reciprocal space. The SRO intensity maxima in diffraction patterns are distinct from the positions of the LRO superlattice reflections.
A wide variety of models have been proposed for the SRO ~ t a t e . [ ~ s ~ - ~ ] These models may be classified into three basic groups; 1) microdomains of defected Dl, or other related LRO structures (e.g.
DO,, or Ni,Mo), 2) non-random solutions involving clustering, and 3) periodic concentration fluctuations with wavelengths of several interplane spacings. The near atomic spatial resolution of the atom probe in both imaging and chemical analysis provides a technique that should be able to differentiate between these three groups of models.
Banerjee et aI. have reported that low-temperature high-voltage (1 MeV) electron irradiation produces a disordered structure in Ni4Mo which does not exhibit any SRO or LRO diffraction intensitie~.[~I This disordered material forms a reference state to which SRO material may be compared.
In this paper atom probe field-ion microscopy (APFIM), transmission electron microscopy (TEM), and high voltage electron microscopy (HVEM) have been applied to understand the ordering processes that occur i n Ni4Mo from the disordered state to the fully long range ordered state. A previous atom probe investigation by Yamamoto et al. suggested that small 5 to 20 atom clusters of NiMo were present in the SRO material. However, no statistical analysis was performed on the data to support this
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987663
JOURNAL DE PHYSIQUE
EXPERIMENTAL
The material used i n this study was a high purity Ni - 20 at. % Mo alloy. Both wire for field-ion microscopy and disks for transmission electron microscopy were given parallel annealing treatments.
Three sets of specimens were examined; 1) LRO material produced by annealing, 2) electron-irradiated disordered specimens of both LRO and SRO material, and 3) SRO material also produced by annealing solution treated and quenched material.
Various previous studies have shown that the fully disordered state cannot be achieved using conventional heat treatments. Therefore, high voltage electron irradiations were performed at 1 MeV in a Hitachi HU-1000 to produce the disordered state. During electron irradiation the specimen temperature was maintained at -85K. A dose rate of approximately 5 ~ 1 0 - ~ displacements per atom (dpa) per second, based on a threshold energy of 24eV, was used. Electron diffraction and in-situ annealing experiments of the disordered material were performed using Philips EM400T/FEG and EM430T microscopes. The annealing studies were complemented by conventional heat treatments in the temperature range 300 to 700°C.
All atom probe experiments were performed in the ORNL energy-compensated atom probe field-ion m i c r o s ~ o p e . ~ ~ ~ ] Field-ion images were recorded at a specimen temperature of 70K using neon as the imaging gas. Atom probe chemical analysis was performed using a pulse fraction of 20 to 25% and a specimen temperature of 30 to 40K to prevent the preferential evaporation of nickel. The nickel was found to evaporate almost exclusively as NiZ+, whereas, molybdenum evaporated as MoS+, Mo2+ and Mo4+. Therefore the , , ~ i ' + : 9 2 ~ 0 3 + and the 6 4 ~ i 2 f : 9 6 ~ 0 3 + isobars could not be distinguished. The ions corresponding to these two mass-to-charge ratios were taken as nickel and molybdenum, respectively, in the ion-by-ion determinations. This assumption will only produce a slight error in estimates from the ladder diagrams and composition profiles where ion-by-ion identification is required.
The overall composition of the alloy was calculated by deconvolution of the peaks in the mass spectrum using tables of natural abundances[ll] and was determined to be 20.4 at. OhMo.
Atom probe composition determinations were all performed using random area analysis. The resulting data were analyzed using both autocorrelation and power spectrum functions to detect either clustering or a periodic composition fluctuation. These statistical techniques were chosen because they provide a n estimate of the spatial extent of the cluster and periodicity. It should be noted that Markov chain analysis is more sensitive but only provides evidence as to whether or not clustering has occurred.
RESULTS AND DISCUSSION
The LRO material was found to exhibit very high quality field-ion micrographs, as shown in Fig. l(a). This image was obtained from material that was solution treated for 1 h a t 1000°C, water quenched, and then annealed for 8 h at 750°C, and 16 h at 700°C. An electron diffraction pattern of similarly prepared material indicating the characteristic superlattice reflections of the Dl, structure is shown in Fig. l(b). The ordering may also be observed in the field-ion images as shown& Fig. 2.
The bright rings i n this image.were found from atom probe selected area analysis to be the molybdenum planes and the dim rings the nickel planes. This material contained an inhomogeneously distributed network of antiphase boundaries. An example of a region with a small domain size is shown in Fig. 3.
The degree of long range ordering was not perfect i n this heat treatment since antisite defects and antiphase boundaries were observed in the field-ion micrographs, Fig. 4. No degree of ordering determination was performed with the atom probe in this material although the low number density of antisite defects i n the field-ion images would seem to indicate a value close to 1. It should be noted that antiphase boundaries do not interfere with order determination as they do in other techniques.
A field-ion micrograph and an electron diffraction pattern of the disordered material is shown in Figs. 5(a) and 5(b) respectively. The high image quality observed in Fig. l(a) is not present in this micrograph and the characteristic superlattice reflections due to ordering are absent in the diffraction pattern. The additional reflections are due to surface oxides and possibly radiation-induced defects.
Rel-rod streaks parallel to < I l l > projections were observed at the (001) zone for reflections away from the Ewald sphere (DO). These streaks are consistent with radiation-induced faulted dislocation loops.
Both LRO and SRO materials were electron irradiated in the HVEM and were both found to disorder after a dose of approximately 0.4 dpa. No SRO was observed in the HVEM during the disordering of the LRO material. No significant change was observed in the electron diffraction patterns over a 10 day period after the irradiation indicating that the disordered state was metastable at room
temperature. A portion of a ladder diagram of the disordered material is shown in Fig. 6. Some inhomogeneities are identified in this ladder diagram by the human eye. However, analysis of autocorrelation functions and power spectra obtained from extended composition profiles was unable to reveal any statistical deviations from a random solid solution. Representative examples of an autocorrelation function and a power spectrum taken from a composition profile with 20 ion blocks, are shown in Fig. 7 and Fig. 8, respectively.
SRO was observed during in-situ annealing of the electron-irradiated disordered material in the temperature range 400 to 600°C. During in-situ annealing of the disordered material at 700°C, SRO was observed as a precursor to LRO.
A field-ion micrograph of the SRO state produced by annealing the quenched material at 650°C for 5 min. is shown in Fig. 9(a). This a ing treatment has been reported to produce the maximum SRO intensity in the diffraction patterns.fzl No substructure is evident in this micrograph and it is distinctly different to the LRO material. This implies that if microdomains are present their size is probably at the unit cell level and SRO would be better described in other terms. An electron diffraction pattern of the same material reveals the characteristic SRO (1i0) intensity maxiina, Fig. 9(b).
A ladder diagram of the SRO ordered material again visually reveals some small inhomogeneities, Fig.
10. Both high or even pure nickel regions and high molybdenum regions appear in this ladder diagram.
However, it is very easy for the eye to distinguish inhomogeneities even in a random distribution. The autocorrelation analysis of an extended composition profile based on a 20 ion block size is shown in Fig. 11. The positive value of the autocorrelation function coefficient, R(k), at lag k=l is not significant at the 95% confidence level. Autocorrelation analysis based on composition profiles with block sizes ranging from 1 atom to 50 atoms revealed a similar behavior. No periodic composition fluctuations were statistically detected from these autocorrelation function analyses. No periodic compositional fluctuations were distinguished by a power spectrum analysis of this random area composition profile, Fig. 12. However, it should be noted that careful experimental design for a specific orientation of the predicted periodicity might be helpful in increasing any effective compositional differences.
CONCLUSIONS
Disordered Ni,Mo material was produced by low temperature high voltage (1 MeV) electron irradiation at a dose of approximately 0.4 displacements per atom. No short range order was observed during the disordering of the long range ordered material. The disordered material was found to have a random distribution of molybdenum. In-situ annealing experiments of the disordered material in the temperature range 300 to 700°C, revealed that SRO occurred prior to the LRO state. Visual observation of ladder diagrams incorrectly appeared to provide evidence for clustering. Also, no statistical evidence for microdomains, clustering or a periodic composition fluctuation has been observed with the atom probe in the short range ordered alloy.
Acknowledgment
The authors would like to thank K.F. Russell for her technical assistance. Research sponsored by the Division of Materials Sciences, U.S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
REFERENCES
1. C.R. Brooks, J.E. Spruiell, and E.E. Stansbury, Inter. Metals Rev., 29 (1984) 210.
2. P.R. Okamoto and G. Thomas, Acta Metall., 19 (1971) 825.
3. B.G. LeFevre, H. Grenga, and B. Ralph, Phil. Mag., 18 (1968) 1127.
4. M. Yamamoto, T. Hashizume, and T. Sakurai, Scripta Metall., 19 (1985) 357.
5. G. Thomas and R. Sinclair, Acta Metall., 25 (1977) 231.
6. B. Chakravarti, E.A. Starke, Jr., C.J. Sparks, and R.O. Williams, J. Phys. Chem. Solids, 35 (1974) 1317.
7. R. De Ridder, G. Van Tendeloo, and S. Amelinckx, Acta Cryst., A32 (1976) 216.
8. W.M. Stobbs and J-P.A.A. Chevalier, Acta Metall., 26 (1978) 233.
9. S. Banerjee, K. Urban, and M. Wilkens, Acta Metall., 32 (1984) 299.
10. M.K. Miller, J. de Physique, 47-C2 (1986) 493.
11. M.K. Miller and G.D.W. Smith, Surface and Interface Analysis, 1 (1979) 149
C6-388 JOURNAL DE PHYSIQUE
Fig. 1. I.RO materia! solution treated 1 h at 1000°C, water quenched, and annealed 8 h at 750°C and 16 h a t 700°C, (a) FIM micrograph and (b) [112] electron diffraction pattern.
Fig. 2. The long range ordering is evident i n Fig. 3. The LRO material contained the bright molybdenum planes and the dim nickel antiphase boundaries.
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Fig. 4. The degree of order was not perfect due to antisite defects.
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