Site selectivity in Fe doped β phase NiAlPatrick Chartier, Mahalingam Balasubramanian, Dale Brewe, Tariq Manzur, Douglas Pease, JosephBudnick, Li Huang, Chi Law, Scott Russell, and Clyde Kimball Citation: Journal of Applied Physics 75, 3842 (1994); doi: 10.1063/1.356063 View online: http://dx.doi.org/10.1063/1.356063 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/75/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Site-selective investigation of site symmetry and site occupation of iron in Fe-doped lithium niobate crystals J. Appl. Phys. 105, 013524 (2009); 10.1063/1.3056116 Structural characterization of Ni–Al (111) interface by surface x-ray absorption spectroscopy J. Appl. Phys. 92, 1862 (2002); 10.1063/1.1493652 Epitaxial growth of Fedoped sapphire thin films from amorphous Al oxide layers deposited on sapphiresubstrates Appl. Phys. Lett. 65, 180 (1994); 10.1063/1.112665 Solute pairing in solutionhardened CuNi, CuPd binary, and CuNiPd ternary fcc alloys J. Appl. Phys. 71, 150 (1992); 10.1063/1.350728 Phase formation in ionirradiated and annealed Nirich NiAl thin films J. Appl. Phys. 69, 2021 (1991); 10.1063/1.348726

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Site selectivity in Fe doped p phase NiAl Patrick Chartier Laboratoire de Metallurgic Physique (URA I31 CNRS), Universite de Poitiers, 40 Avenue de Recteur Pineau, 86022 Poitiers Cedex, France

Mahalingam Balasubramanian, Dale Brewe, Tariq Manzur, Douglas Pease, Joseph Budnick, and Li Huang Physics Department, The University of Connecticut, Storrs, Connecticut 06269-3046

Chi Law and Scott Russell United Technologies, Pratt & Whitney, East Hartford, Connecticut 06128

Clyde Kimball Physics Department, Northern Illinois University, Dekalb, Illinois 60115

(Received 27 August 1993; accepted for publication 17 December 1993)

We have investigated site selectivity in iron doped p phase ternary alloys of the composition Ni~e,,Al~lOO--x-y), where the Ni concentration is within a few at. % of stoichiometry. Experimental techniques used include extended x-ray absorption edge fine structure and Mijssbauer measurements. The transition metal atoms in excess of 50 at. %, which occupy sites with transition metal nearest neighbors, are not chosen randomly among Ni and Fe, but are exclusively Fe, to within the accuracy of the measurements. This result requires a re-analysis of previous magnetic susceptibility measurements on this system, which as reanalyzed here imply more reasonable moment values for the Fe dopants. Our findings regarding Fe site selectivity are of significance in regards to recent studies showing a ductilization effect of Fe dopants microalloyed into Ni&lse . t

I. INTRODUCTION II. EXPERIMENTAL DETAILS

Transition metal (TM) alloys based on the composi- tion Ni,,#& have high melting points and low densities, and show promise as high temperature structural materi- als; however, stoichiometric Ni,cAl,, is brittle, which lim- its the applicability of this material. Therefore, it was of interest when, recently, Darolia et al. found evidence for ductilization in p phase Ni,aFea,o-x or NiSOGaTe,,-, ternary alloys.’ Ductilization occurred for dopant concen- trations up to a few tenths of an at. %, but disappeared when x reached a value as great as 0.5. In light of this interesting result, it is of interest to explore the site selec- tivity of Fe in this system. For the binary alloy NiAl, ex- cess Ni atoms above the stoichiometric equiatomic compo- sition have long been known to occupy the Al type of site. For N&,,FeJ&-, the situation is not so clear without further experimental study. One point of view is to be found in the magnetic susceptibility studies of Ikeda on this system.2 He assumed that, for x excess transition metal (TM) atoms above 50 at. %, those x atoms going to Al type of sites would be chosen randomly among Ni and Fe. In such a model, for the alloy Ni,,FerAl,s, there would be fifty times the number of Ni atoms as Fe at sites with eight TM near neighbors. On the other hand, the heat of forma- tion of NiAl is much larger than that of FeAh3 thus, one might also guess that Ni atoms might therefore drive com- peting Fe atoms to the sites with TM near neighbors. We have performed experiments to show which of these points of view is correct.

Transition metal-rich samples were manufactured us- ing a vacuum arc melter and argon backfill, after pump down to the 10 Torr range. Initial starting material purities were three nines or better. By weighing before and after melt, recovery was found to be 99.9%. Aluminum-rich samples were given a homogenization anneal in evacuated quartz at 800 “C, after being first wrapped in Ta foil. Two types of annealed transition metal (TM) -rich samples were made. One batch was given a homogenization anneal at 800 “C in evacuated quartz after first being wrapped in Ta foil and the other samples were annealed at 1200 “C in evacuated quartz, without Ta wrapping. The extended x-ray absorption edge fine structure (EXAFS) measure- ments for TM-rich ternary alloys were carried out for as cast, as well as all types of annealed specimens. The Moss- bauer results were obtained only on TM-rich as cast and 1200 “C annealed specimens. The binary, stoichiometric N&Al,, samples used for comparison were made by arc melting, and wet chemical analysis-indicated that the de- sired stoichiometric composition was attained to within less than the experimental uncertainty of 0.8 at. %, and the atomic percentages of Ni and Al added to exceed 99.97% of the total. This sample was given a homogenization an- neal of 100 h at 1200 “C in argon in a controlled atmo- sphere vacuum furnace. X-ray diffraction analysis indi- cated only the expected peaks of the p phase, CsCl structure.

For fluorescence detection of EXAFS, where concen- trated absorbers are studied such as in stoichiometric

3842 J. Appl. Phys. 75 (8), 15 April 1994 0021-8979f 94/ 75(8)/3842/5/$6.00 @ 1994 American Institute of Physics

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Ni&&, the glancing emergent angle (GEA) fluorescence method was used:’ so as to largely eliminate fluorescence distortion. For these samples, the specimens were metallo- graphically polished flat and given a final electropolish to remove any cold working effects from the mechanical pol- ish.

One of the above specimens, the TM-rich ternary sam- ple annealed in quartz at 1200 “C, showed trace amounts of silicon ( - 1 at.%) contamination from the quartz, whereas this impurity was not detected in the 800 “C an- nealed specimens. EXAFS analysis showed only a slight enhancement of the Fourier transform in annealed versus corresponding as cast specimens, and no significant changes in the fitting. Therefore, for the TM-rich ternary alloys, we chose to discuss only as cast samples, since our only Miissbauer data for annealed specimens is for the 1200 “C! quartz anneal samples with slight silicon impurity, and our EXAFS results show no significant differences be- tween as cast and clean annealed ternary samples.

Extended x-ray absorption edge fine structure studies were carried out at the National Synchrotron Light Source on Beam Line X-l 1A using a Si[ 11 l] double crystal mono- chrometer. Data were taken using a standard fluorescence geometry for Fe dopant absorption edges and using GEA fluorescence methods for Ni K edges. For Fe K edges, a sample spinner was used so as to minimize diffraction ef- fects on the EXAFS. The Mijssbauer experiments were performed at room temperature by means of a constant acceleration spectrometer using a “Co in Rh source. The Ni,,,A14,Fel samples were powdered and encapsulated in acrylic. The line width is increased by.strain due to crush- ing. The Miissbauer absorption for these samples with nat- ural Fe substitution is very low ( < 0.4% ), even with thick absorbers, and in order to obtain reasonable computer fits, it was necessary to obtain total counts of order (lO’)/channel.

Ill. EXAFS DATA ANALYSIS

The background signal was subtracted from the spec- tra and the oscillatory portion of the data isolated by stan- dard methods. The origin of the energy scale was taken at the inflection point of the spectra, which was later adjusted during fitting. The Fourier transform was performed over approximately 3 to 14 inverse angstroms. Fitting was done using experimental amplitude and phases extracted from standard compounds, where CoSi, was used to extract Al backscattering parameters. These experimental amplitudes and phases have been corrected for the differences between Si and Al backscatterers, using a set of values computed using a spherical wave formalism.6 It is known that the number of neighbors in a shell N and the EXAFS Debye- Waller factor (T 2 can interact in fitting procedures so that one quantity can be enhanced at the expense of the other, relative to their true values. For NiAl, for which useful Ni K edge data can be obtained to somewhat higher k than for dilute species, we obtain excellent agreement with known shell distances for the lirst two shells about Ni atoms, but N values smaller than the correct values by about one atom for both first and second shells. [By fixing N at these values

1 I I 1 I I 1 1 I

4 5 6 7 8 9 1011

k (i-’ )

FIG. 1. K’ weighted X function for: (a) Fe K edge in N&,,Fe 01A149, (b) Fe K edge in Ni 46FeOzA152, (c) Ni K edge in Ni,&,, at room temper- ature, (d) Ni K edge in Ni,&, at 43 K.

and deducing ha 2 from temperature-dependent EXAFS, one of us (Brewe) obtains excellent agreement with ha 2 values calculated using known NiAl force constants’]. In the present study, the N values are fixed at those values giving good fits for stoichiometric NiAl. The number of floating parameters used are in all cases much less than the limit determined by the Shannon theorem.8

IV. RESULTS

A. EXAFS

Figure 1 shows the k vector weighted chi functions, or oscillatory portion of the raw data after background sub- traction, for Ni K edges in NissAISO at room temperature and 43 K, for the Fe K edge in N&Fee2Als2, and for the Fe K edge in Ni,,Fe&l,. We note that the quality of the data is good. Figure 2 shows the corresponding k cubed weighted Fourier transforms, labeled 2(a), Fe K edge in Nis,Fee,Al,,; 2(b), Fe K edge in Ni~,Fe&l,,; 2(c), Ni K edge in room temperature NissAlsO; and 2 (d) Ni K edge in N&Al,, at 43 K. The Ni K edges are obtained by the GEA technique. The Ni atom in this environment has eight Al near neighbors (NN) and six TM second neighbors. The first peak is nearly resolved into a doublet, corresponding to these two shells. We find similar results for the Fe ab- sorber in FeAl and the Co absorber in CoAl. By using the fitting procedures outlined above, we obtain good two shell fits to this data. The Fe K edge data were obtained at room temperature in standard fluorescence mode, but using sam- ple spinning to minimize effects of diffraction peaks on the data. The transforms of annealed NissAl,Fel are almost identical to those for the as cast material, but are slightly higher in amplitude. For the sample with Al concentration in excess of 50%, the excess Al atoms above 50 at.% result in TM vacancies, so that there are adequate sites, with 8 Al near neighbors (NN), to accommodate all TMs. On the

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012345678 R 66

FIG. 2. K3 weighted Fourier transforms for: (a) Fe K edge N&,&&l,, (b) Fe K edge in Ni46Fe02Als2, (c) Ni K edge in Ni,&O at room temperature, (d) Ni K edge in Ni,&l,, at 43 K.

other hand, for the TM-rich sample, there will be some of the excess TMs, past 50%, forced to sites with 8 TM NNs. There is a marked difference between the Fe FT for the Al-rich versus TM-rich cases. The Al-rich sample exhibits a FT with a resolved first and second shell doublet char- acteristic of NiAl or FeAl, and indicating that Fe in this system is in sites with 8 Al NNs. The Fe IT of Ni,,,Al,Fe,, on the other hand, does not show a resolved first and second shell doublet.

Initially, a two-shell fit of the Fe dopant EXAFS was carried out for both the Al-rich and TM-rich cases. For the TM-rich case, the Fe EXAFS was fit well by a model in which the NN shell was made up of TM atoms; the disor- der in the second shell was rather large and, therefore., a one-shell fit assuming Fe dopants with TM NNs was car- ried out. The fit was good, with a shell radius correspond- ing to that expected for TM atoms going into Al type sites in Nii, but with considerable second shell disorder. This fit is shown in Fig. 3 (b) . On the other hand, as will be seen, the oscillatory EXAFS function corresponding to Fe in a TM site does not agree at all with the shape of the curve in Fig. 3(b).

Turning now to the Ni46Al,,Fe, sample, we consider the fits to the Fe EXAFS in this specimen. Here, in con- trast to the case of Ni,,,A14sFe,, a good fit is obtained to the data by assuming Fe goes into sites such that the NN shell is made up of Al, rather than TM atoms. We find the disorder parameter in this case to be somewhat less than in the case of Ni EXAFS in NiAl. This fit is shown in Fig. 3(a). It is expected, in agreement with our findings, that Fe, for the Al rich alloys, will be forced to sites with Al NNs. Resistivity measurements show no Kondo minima for Fe in Al-rich alloys, indicating that Fe atoms sur-

I I I 1 I I I 1 ,

4 5 6 7 8 9 1011 k (i-l )

FIG. 3. Fits to Fe K edge assuming Fe goes exclusively to: (a) TM type site in Ni46FemA152, (b) Al type site iii Ni,,Fe&lJ9. Both fits compared to experimental data.

rounded by eight Al NNs do not develop a local moment.’ We sum up these results as follows: (1) For the Al-rich case the EXAFS indicates that Fe

goes to a site surrounded by Al NN atoms as expected. This type of site selection for the TM-rich case is in agree- ment with resistivity results showing no Kondo minimum for Al-rich, Fe doped Nil.

(2) For the TM-rich case, our fitting results indicate that the excess TM atoms above 50 at.% go to sites with eight TM NNs and that these atoms are within experimen- tal error exclusively Fe, rather than being chosen randomly among Ni and Fe. This finding disagrees with the site se- lection assumptions of Ikeda and indicates that if Ni and Fe TMs compete for insufficient TM type sites, the Ni atoms drive the Fe atoms to the Al-type location.

We also tested whether our Fe edge EXAFS data in Ni50FelA149 could be fit by a model in which the Fe dop- ants were forming small clusters of a! iron. The agreement is poor, indicating that (r iron clusters are not forming, a conclusion which is in agreement with the Mijssbauer re- sults.

B. MBssbauer

Figure 4 shows the Mossbauer absorption at 295 K for “Fe in the as-prepared NiScA149Fel alloys and a-Fe. A single line, broadened by strain due to crushing the alloy, is observed for the alloy. The average Mijssbauer shift is 0.21 kO.04 mm/s with respect to o-Fe for the as-prepared sam- ple and for the annealed sample.

Frankel et al. examined NiAl and FeAl, both with CsCl ordering, by annealing 57Co (concentration - lo-‘) into each sample and performing a Mossbauer source ex- periment. A single line was observed in both cases with a shift of 0.10 mm/s (no error quoted) .I0 Here, it is inferred that the 57Co (Fe) occupies the transition element site; that is, the Fe atom has only Al near-neighbors. Further,

3844 J. Appl. Phys., Vol. 75, No. 8, 15 April 1994 Chartier et al.

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VELOCITY (mm/r)

FIG. 4. Mijssbauer spectra for a iron and as cast Ni5&14,Fel.

by performing Miissbauer experiments in a magnetic field, Frankel et al. lo show that the Fe atom carries no moment in either the ordered NiAl or FeAl alloy when the Fe atom occupies the transition element site.

The Miissbauer absorption spectrum shown in Fig. 4 is for Fe in the as prepared N&Al,Fei alloys and for a 6-pm-thick natural Fe standard absorber. The same mate- rial is used in the Mossbauer experiments as in the other measurements described in this work which characterize the material. A single line, broadened by strain due to crushing the alloy, is observed. The average Mossbauer shift is 0.21 *to.04 mm/s with respect to a-Fe for the as- prepared sample and for the annealed sample. Because of the low Fe concentration in the alloy and consequent low absorption, a large total count (-8X 106/velocity chan- nel) was taken to obtain statistically relevant fits. There is no systematic correlation in the deviations of the data from the analyzed fit which would indicate either magnetic or nonmagnetic clusters of an a-Fe. However, to obtain an estimate of the maximum Fe content in the form of Fe clustering, hyperiine parameters for a-Fe were fixed and fitted to the data summed so as to provide a low pass filter; that is, summed to emphasize the presence of the most intense lines of the magnetic a-Fe spectrum; also, possible asymmetries in the observed line from nonmagnetic clus- tering were examined using this technique. The result was that 6% Fe might exist in a form other than NiS,+&,Feel. It would appear that the Fe atoms are, to a major extent, homogeneously distributed in the alloy and occupy a single site.

C. Re-analysis of magnetic susceptibility data of lkeda

Ikeda measured temperature-dependent magnetic sus- ceptibilities of (Ni, i -,Fe,) 1.04Ala96 alloys. ’ If X0 is the

susceptibility of stoichiometric Ni50A150, Ikeda plotted l/X--X, versus temperature and fit the data to a Curie- Weiss law relation. He also investigated Nifio+fi150-X al- loys and concluded that Ni atoms, in excess of 50 at.%, that went to Al types of sites, gave a negligible contribution to the susceptibility compared to Fe atoms on Al types of sites in the ternary system. The Curie constant is given by

C=NM2/3k, (1)

where M is the magnetic moment, k is Boltzmann’s ‘con- stant, and N is the density of magnetic atoms. Ikeda de- duced a moment per magnetic Fe atom in the ternary al- loys of about 24 Bohr magnetons.

Inspection of the Ikeda data analysis procedure shows that he assumed that for TM atoms in excess of 50 at.%, Fe atoms went into Al sites in a ratio randomly chosen between Ni and Fe. Since there is much more Ni than Fe in this system, such an assumption results in a small value assumed for N and a consequently large value for M. We have reworked Ikeda’s results, using his susceptibility curves, but with an assumption about site selection consis- tent with the present experimental results. There is not enough Ni in Ikeda’s alloys to reach 50% TM based on Ni concentrations alone. We therefore assume:

( 1) all Ni goes to TM type sites, (2) Fe atoms go to TM type sites until a total of 50%

TM atoms is reached, (3) all TM atoms in excess of 50%, the remaining Fe

atoms, go to the Al type of sites. We assume only these to be magnetic.

Re-analyzed in this way, both of Ikeda’s susceptibility plots give self-consistent moments (the moments are not consistent between the plots if the moment is deduced as done by Ikeda. The moments we obtain are 7.8 and 7.3 Bohr magnetons per Al substituted Fe, for Ikeda compo- sitions with z=O.l and z=O.25, respectively. If one as- sumes a central Fe atom with a moment of 2.2,~~ sur- rounded by eight Ni atoms with moments of 0.6~~ (the saturation moments of the pure elements) one obtains a moment of 7,~~. Caskey et aZ. find Fe in Al type of sites in Fe50+&0-, to be associated with moments of 7.8.” Kondo effect impurity resistivity studies on Fe doped Ni+llOO--a, carried out by Ochia and Brittain,’ should also be interpreted in terms of the site selectivity findings re- ported here.

V. CONCLUSIONS

By EXAFS and re-analyzed magnetic susceptibility re- sults, Fe dopants in NiAl, for TM concentrations in excess of 50%, go exclusively to Al-type sites. In particular, Fe atoms in Ni.&l,,Fei go exclusively to Al-type sites, rather than the Al-type sites being randomly occupied between Ni and Fe. The Miissbauer results show that magnetic a-Fe clusters do not form and the spectra are consistent with the above model.

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ACKNOWLEDGMENTS

We are grateful to Richard Wexler of Pratt and Whit- ney for his assistance with sample preparation and to We- imin Peng and Xia Lei for assistance in performing the Miissbauer experiments. This work was supported under DOE contract Nos. DE-AS05-80ER10742 and DE-F602- 90ER45424.

’ R. Darolia, D. Lahrman, and R. Field, Scripta Metall. 26, 1007 ( 1992). *K. Ikeda, J. Phys. Sot. Jpn. 42,91 (1977). 3M. Ettenberg, K. L. Komarek, and E. Miller, in Ordered Alloys, Pro-

ceediigs of the Third Bolton Landing Conference on Ordered Alloys; Structural Application and Physical Metallurgy, Sept. 1969, edited by B. H. Kear, C. T. Sims, N. S. Stoloff, and J. H. Westbrook (Calitor’s

Publishing Division, Baton Rouge, LA, 1970), p. 49. 4D. M. Pease, D. L. Brewe, Z. Tan, J. I. Budnick, and C. C. Law, Phys.

Lett. A 138, 230 (1989). sD. L. Brewe, E. C. Bouldin, D. M. Pease, J. I. Budnick, and 2. Tan,

Rev. Sci. Instrum. 63, 3298 (1992). ‘Obtained from A. McKale (private communication). ‘D. L. Brewe, D. M. Pease, J. I. Bucnick, Z. Tan, C. C. Law, S. M.

Russell, and M. H. Choi (unpublished). Also, D. L. Brewe, Ph. D. dissertation, University of Connecticut, 1993.

8J. Max, Methodes et Techniques de Traitment du Signal, Vol. 1, (Mas- son, Paris, 1989), p. 78.

‘Y. Ochial and J. 0. Brittain, Solid State Commun. 22, 443 (1977). ‘OR. B. Frankel, D. J. Sellmeyer, and N. A. Blum, Phys. Lett. 33A, 13

(1970). “G R Caskey, J. M. Franz, and D. J. Sellmeyer, J. Phys. Chem. Solids

36, Ii79 (1973).

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