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ANALYSIS OF THE DO3 SUPERLATTICE INORDERED IRON ALUMINIUM ALLOYS BY FIM

AND ATOM PROBEH.-J. Krause-Habrock, G. Frommeyer, J. Wittig, M. Kreuss

To cite this version:H.-J. Krause-Habrock, G. Frommeyer, J. Wittig, M. Kreuss. ANALYSIS OF THE DO3 SUPER-LATTICE IN ORDERED IRON ALUMINIUM ALLOYS BY FIM AND ATOM PROBE. Journal dePhysique Colloques, 1988, 49 (C6), pp.C6-365-C6-371. �10.1051/jphyscol:1988663�. �jpa-00228160�

JOURNAL DE PHYSIQUE Collogue C6, suppl6ment au noll, Tome 49, novembre 1988

ANALYSIS OF THE DO, SUPERLATTICE IN ORDERED IRON ALUMINIUM ALLOYS BY FIM AND ATOM PROBE

H.-J. KRAUSE-HABROCK, G. FROMMEYER, J.E. WITTIG* and M. KREUSS

Max-Planck-Institut fur Eisenforschung, Max-Planck-Strasse 1, 0-4000 Dusseldorf, F.R.G. *~anderbilt University, PO Box 6309, Station B, Nashville, TN 37235, U.S.A.

Abstract - The perfection of the ordered DO3 structure in Fe- (20/26/30 at%)Al alloys has been studied uslng field ion microscopy (FIM), atom probe (AP) measurements, and transmission electron microscopy (TEM) dark field (DF) imaging. FIM images provide a means to qualitatively characterize the degree of order in these materials. However, AP surveys of [Ill] poles supply additional informatkon concerning the local perfection of the DO3 superlattice. Although the microstructure can appear completely ordered in both FIM and TEM-DF images, the AP investigations have revealed that perfect DO3 order occurs in only limited regions (10 to 20 unit cells) while many imperfections are always present in the atomic arrangements of the DO3 structure.

Ordered Fe A1 alloys with A1 contents of 20 at% < CA < 30 at% exhibit excellent soft magnetic properties such as low coercive forces, minimum kysteresis losses and large relative permeability values expcially in the high frequency range [I]. In addition, the intermetallic carqxsurd, FeA1, has been considered as p~tential material for high tenperature application. The ordered B2 (CsCl type) lattice causes a marked increase of the flow stresses and creep strength at elevated temperature. Also, Fe-Al alloys exhibit sufficient hi& temperature oxidation resistance fran the preferential formation of stable protective alumina laye= [21.

The main disadvantage of these ordered allays is the lack of room temperature ductility. A basic understanding of the relationship between the microstructure ard the inherent brittleness requires a detailed lanmledge of the atomic distribution of the alloying elemts within the ordered superlattices. Important factors include the domain sizes a d the struc6 of the domain boundaries since these are considered to s t m l y influence the movement of dislocations.

Prior field ion microscopy (FIM) investigations of disorder - order phase transformations have predominantly studied reactions of disordered fcc lattice into the ordered L10 [3,41. L1 15.61 and other superlattices [7,8,9]. Cantrast interpretations of FIM images of binary alfoys is often m t straight forward because of invisibility of one of the atomic species. This can result f m preferential ionization, e.g. in Pt-Co allays where the smaller Co atoms will not be imaged [4,5], or preferential field evaporation of one substitutional atom type at a minimum evaporation field strength [10,11,12J.

For the case of Fe-Al alloys, a study by Paris et al. [I31 has concluded that Fe is the imaging species while the Al atans are invisible. Previous work by the current authors [91, which canbid FIM imagin~ with atan pmbe (A€') analysis of m3 ordered Fe3M type alloys. has unambiguously shcwn that this effect is caused by preferential field -ration of the Al at-. For a perfect m3 structure, there exists a geriodic arrangemmt in the 11111 direction, AAABAAAB..., mnsisting of three alternating 100% Fe (111) planes follawed by a pureAl (111) plane. Inthe [001] direction, the M) ordered structure has a ABAB type stacking of 100% Fe planes and a mixed layer of 5& Fe and 50% A1 at-. Atan probe surveys of the [Ill] pole in Fe3A1 have shown that when the last (111) Fe plane lyiw above the (111) Al plane begins to evaporate, the Al atoms belaw evaporate together with the Fe atoms as a double layer. This is consistent with the data from the [001] poles where pure Fe and mixed ??e-Al (001) planes are observed to field evaporate together.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988663

C6-366 JOURNAL DE PHYSIQUE

Number of Fe- and At-Atom*

a Fe Fe Fe Fe Fe

b Fe Fe Fe Fe Fe

c A 1 A 1 A l A l A1

d Fe Fe Fe Fe Fe

a Fe Pe Fe Fe Fe

b Fe Fe Fe Fe Fe

c A 1 A1 A 1 A l A 1

d Fe F e F e F e F e

Number ot Fe- and Al-Atoms

Fig. la - Model ladder diagram from a [111] atom probe survey of a perfectly ordered m3 structure assrmring double evaporation of Fe ad Al layers. b - Experimental ladder di- frau an ordered m3 superlattice.

APB

Fig. 2a - Schaaatic di- of the PIM image of an anti-phase bounbry on a [001] pole. Only the plre Fe planes are -ed since the mixed Fe and Al layen have preferentially evaporated. b - FIM image of an AFB on a [001] pole from a DO3 mrperlattice.

The evaporation sequence of the periodicity of (111) plams allows the perfection of the DO superlattice to be ohsenfed in a ladder diagram as schematically depicted in fi- la ad experinentally in fiaure lb. First a plre Fe plane is detected, then waes a mixed Fe and Al double layer, and finally t v u ~ plre Fe planes are observed as a horizontal step in the di-. Preferential, empation of the Al at- has also explained the contrast pzduced by anti-phase boundaries (APB) in the DO superlattice. Fi 2a is a schematic -tation of an APE3 on a [001] pole. m y &e Eh layers are as semi-circular rings since the mixed Fe-Al layers will have preferentially m a t e d . These have been experiwntally observed only on the [OOl] pole as shm in f i m 2b.

In the current paper, the atan probe technique is used to study the perfection of W ordering in E'e-Al allays with Al concentration of 20, 26, and 30 at%. The results cannot yef express a quantitative determination of an orderirg parameter. However, the extraordinary spatial resolution of the F W A P pratides infomation unavailable fmn any other method about the local perfection of atanic ordering.

Fig. 3 - Remwentative FIM imeees of the f- alloys; a) Fe9dU10e b, F%$U20. C) %qA12e1 d) Fq$llgoe

Four Fe-Al alloys containing 10, 20, 26, and 30 at% aluminium have been prepared by W t i m meltirg m r vacuum. Samples from the ingot material were encapsulated in -t& quartz tubes, homogenized by heat treatnent at 1150 OC for 24 h atd furnace cooled (4 K/min) d&m to rwn temperature. After hanogenization sample blanks of the dimensions: 0.5 x 0.5 x 30 mn were cut for FIM investigations, and 2.3 mn discs, 0.4 ma thick, were taken for canprative

studies. These blanks were encapsulated in evacuated quartz tubes, bat treated at 1150 OC for 2 until 48 h a d by crushiw the tubes under water. The formation of th,e

The FIM specimens were electropolished at 20 OC initially in 18 ~ 1 % perchloric acid, 18 ~ 1 % 2-butorxyethanol, and 64 volf acetic acid at 30 V. Final polishing was perfod in a 1 ~ 1 % perchloric acid am3 99 ~ 1 % 2-htoxy ethaml at 20 V. FIM ard atom probe analysis were carried out in an ultra high vacuum chamber, basic pressure < lo-' mbar, using reon imaging gas of 5.10-~ mbar. The samples were helium cooled atd the tip surfaces of the specimens were cleaned arwl smoothed dam to the final shape by field evaporation of abut 100 atom layers.

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desorbed a t m s (FetAI)

6 - Represmtative section of a amcentration profile in (a) and the COmeqmdiW hMer diagram in (b) fmn the Fy&lgo alloy.

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Fig. 7 - Rqwesentative section of a ccmcentration profile in (a) and the correqmdiW 1- diagram in (b) fmn tk FeT4A& alloy.

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concentration profile . 4.6-

Fig. 9a - Concentration profile %4A126 b) correspording auto correlation curve.

C6-370 JOURNAL DE PHYSIQUE

Fiaures 4a-b are dark field (DF) ttrananisaion electron microscopy (TPM) i m q e s of the Fq,,A13 ordered alloy us* B2 ard DO3 superlattice reflections ree&mztively. In the m e ! d t i c m , the structure is ccmpletely D03 ordered with no evidence of APB's in the B2 DF image of fi- 4a. The APB's which appear in the DO3 DI? image in figure 4b as thick blurred lines ham also been detected in the FIM. Fiaures 5a-c are an evaporation sequeme of a [001] pole in the Fe70A130 alloy which reveals an Am. Because of the large size of the dasrains in this alloy, these features were rarely observed. Haever, in this particular case the overlapping rings of the Fe atoms (i .e. the Ybzgiqj species) suggests a possible segregation of Fe at- on this APB.

The [Ill] AP surveys in fiaures 6 - 8 are zqzesentative sections of the amcentration po.ofiles and ladder diagrams of the 30, 26, and 20 at% AZ allays which can be interpreted as a measurement of the perfection of LIO3 ordering. For the FeT4Al material, the fluctuations in the amcentration profiles and the CO- steps the ladder diagrams exhibit perfect DO3 ordering over -11 distances, two to three unit cells. But this perfect ordering vas never detected for aver more than a fewmit cells before the size of the st- in the ladder diagram indicated Al atoms were occupying sites on the Fe planes, see fiqure 7. This result was reproducibly observed with over 9362 atom (i.e. 312 atomic layers) meafllred for the F4r4A126 alloy. Possible explanations for the lack of perfection in the order- include mn-quilibrim conditions owing to insufficient annealing times (250 h at 400 OC) or perhaps at this temperature an entrupy effect pramtes imperfectian in the DO3 ordered stmture. Other possible causes which must be axsidered are the slightly m-stoichianetric canposition of 26 at% Al and the chance that preferential evaporation of Al atoas fmm neighbmring underlying planes could be influencing the ladder diagrams.

Far the Fe7$.130 alloy, the hyperstoichiometric conqosition produced a-derrease in the size of the pure Fe step in the [Ill] ladder diagrams, see f imre 6. As 1n the 26% A3 material, small regions of perfect Wg ordering were detected but they were fewer in number a d the regions were smaller in size than for the stoichiometric Fe3M canposition. Ficnw 8 depicts a m t a t i v e concentration profile and ladder diagram for the Fe8&12 material. These surveys were difficult to acquire c w i q to the poor -ing characterystics of the [Ill] poles. Hawever, the limited data that were obtained revealed only slight indications of the ordered structure corresponding to the distorted lattice which produced the inferior imagirg contrast.

Figure 9a is a complete composition profile for a F94Ala6 sample. !l'he total number of about 5000 atam c o w to 250 atanic planes or 42 nm. An interestirq observation is the rather large fluctuations in the ccmposition over long distances (8 to 10 nm). Apply- an auto correlation method W c h remaves statistical noise [14], figure 9b exhibits a long wavelength periodicity in the anposition. A larger data base is required in order to get more statistical information for a significant proof of these detected long range amcentration f luctuations.

From the investigated Fe Al alloys the nearly stoichiometric Fe74A126 alloy reveals the ~lst perfectly ordered atanic -t in the FIM imqes. The CO- atan probe surveys of the [111] poles show that this amposition is characterized by the regular ordered structure in the ladder diagrams.

Hawever, altholagh the orderirrg appears cotnplete in the FIM and W images, atom probe analysis reveals that the ordering is only locally perfect with marry imperfecticms present in the structure.

AP8's have d y been observed on the [001 I poles. FIM evaporation sequences suggest that segregation of Fe atoms may be present. Segregation of Fe in the APB's oould correspond to the thick blurred APBts detected in the TiM.

FIM-AP technique prwides a sensitive methcd to study the D03-ordering phase in Fe-A.l alloys.

The authors gratefully aclumvledge the financial sqprt from the Bundesministerium fXir Forschung uxi Technologic.

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