ORDERING IN METALLIC ALLOYS BY FIM-ATOM PROBE TECHNIQUES

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ORDERING IN METALLIC ALLOYS BY FIM-ATOM PROBE TECHNIQUES

R. Grüne, A. Hütten, L. V. Alvensleben

To cite this version:

R. Grüne, A. Hütten, L. V. Alvensleben. ORDERING IN METALLIC ALLOYS BY FIM- ATOM PROBE TECHNIQUES. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-295-C7-300.

�10.1051/jphyscol:1986751�. �jpa-00225946�

JOURNAL DE PHYSIQUE

Colloque C7, supplément au n o 11, Tome 47, Novembre 1986

ORDERZNG IN METALLIC ALLOYS BY FIM-ATOM PROBE TECHNIQUES

R. GRÜNE, A.

HÜTTEN

and L. v. ALVENSLEBEN

Institut für Metallphysik, Universitat Gottingen, 0-3400 Gottingen, F.R.G.

und Sonderforschungsbereich 126, 0-3392 Clausthal-zellerfeld, (Gottingen), F.R.G.

Abstract

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The ordering of Ni-12 at% Ti and AlNiCo 5 has been investigated us- ing Field Ion Microscopy (FIM) and Atom Probe (AP) techniques. Ni-12 at% Ti orders at 550 OC during the spinodal decomposition resulting in a LI2-structure for the Ti enriched areas. The homogenized state of AlNiCo 5 shows a state of order between Al and Ni before spinodal decomposition sets in at lower aging temperatures. The results are compared with those of a stoichiometric Ni3A1 specimen which was used as an ordered "mode1 alloy".

1

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INTRODUCTION

Originally the experimental information about ordered and partly ordered alloys is based on X-ray diffraction. Later, due to the development of the modern techni- ques of TEM the microstructural observations of the disorder-order transition have received increased attention.

The FIM in combination with a time of flight mass spectrometer provides a technique for direct observations of the ordering of alloys. In FIM images differences in the luminosity of subsequent field evaporated atomic layers are already a hint for ordering in the observed direction. Additionally the composition of successive planes in the ordered lattice could be determined by AP analysis.

It was the objective of the present study to characterize the ordering of Ni Al by using FIM and AP techniques and to compare the obtained results with those O? par- tial ordered Ni-12 at% Ti /1/ and ALNiCo 5 121. Ni3A1 and Ni Ti(equilibrium com- position of the decomposing phase of Ni-12 at% Ti) are ordering in the fcc LI 3.

structure. Here the Ni atoms occupy the face centered positions and the Al and Ti 2 atoms respectively nit at the corners of the unit cell. Therefore subsequent (200)-atomic layers are consisting of pure Ni and Ni-50 at% (~1,Ti) when the order- ing is complete.

II

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EXPERIMENTAL

The Ni Al material with a stoichiometric composition was obtained by melting its components in an arc furnaccj. FIM tips were made from spark eroded rods with the 3 dimensions of 0.3*0.3*15 nnn using standard thinning methods.

The Ni-12 atX Ti alloy was levitation melted and quenched into water; then wire drawing was performed (diameter 0.2 mm). After homogenisation for 90 minutes at 1200 OC in an argon atmosphere the material was quenched into iced brine and sub- sequently aged at 550 OC for various times up to 256 hours.

Finally the AlNiCo 5 alloy was provided as cast plates, annealed for 30 min at 1285 OC by Thyssen AG, Dortmund, with the following nominal composition (at%):

Fe 45.5, C o 23.7, Ni 12.2, Al 16.0, Cu 2.5. Spark eroded rods were homogenized for 15 hours at 900 OC in argon-filled quartz capsules. The FIM tips were produced by a two stage electropolishing technique, first in 25% perchloric acid and 75%

acetic acid, then in a solution of 10 g sodiumchromate in 100 ml acetic acid.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986751

C7-296 3OURNAL

DE

PHYSIQUE

For Ni-12 atX Ti and AlNiCo 5 the described heat treatments led to homogeneous specimens as the autocorrelation analysis of field evaporated atomic layers proved in both cases. Furthermore the compositions which were determined by AP analysis are in good agreement with the nominal compositions of both alloys.

In order to prevent preferential field evaporation of one component, al1 AP studies were performed at a sufficient high ratio of the pulse voltage to the dc level. For al1 alloys VpIVdc was above 15%.

The FIM and AP investigations were performed on the Gottingen instrument described by Piller 131 and Wagner 141.

III

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^{RESULTS AND} DISCUSSION

FIM observations showed that the superlattice planes (002) appeared during field evaporation as alternating dim and brinht rings (fig. 1 ) . This behaviour was already reported by Taunt et al. 5 Horton and Miller 161 and Brenner et a1.171.

Prior to the AP analysis the specimen has to be manipulated correctly, i.e., the probe hole must cover the (002)-pole and the magnification must be high enough or the diameter of the projection of the probe hole small enough that only atnms of one atomic layer are recorded. When the number of pulses increases as no atom reached the detector, one atomic layer was field evaporated and the field evaporated atoms of the next plane still lay outside the projection of the probe hole. It is clear that this method leads to certain inaccuracies.

The ladder diagram which is shown in fig. 2 was obtained after al1 experimental parameters were set at its best. The horizontal sections are pure Ni-layers whereas the sections with a 45 O slope are filled half with Ni and half with Al atoms. The ladder diagram shows already clearly the ordering of this alloy. The different Ni or Al content of subsequent (002)-planes follows more or less the theoretical values of the LI2 structure. The differences to the theoretical values may be due either to bad manipulation or may be attributed to the fact that the in- vestigated alloy still contains Al atoms on Ni sites and vice versa. Fig. 3 shows the related concentration profile.

In fig. 4 the ut oc or relation function R(k) of the Ni-atoms is displayed. A clustering of Ni atoms without ordering would only give a decrease of ~ ( k ) , whereas the strict periodicity of this function is proof of the ordering. The k -value gives the average number of Ni-atoms in the Ni-layere. The analysis of tgese data in terms of Narkow chains gives only poor information whether the alloy is ordered or not. Autocorrelation analysis atom by atom or layer by layer is cer- tainly a better method for the distinction between ordered and non or partly or- dered alloys.

From the AP analysis one also can get the concentration of the bright and dim rings which were Ni and Al mixed and pure Ni respectively. Hence the differences in the

luminosity between alternating layers is due to their different compositions.

Ni-12 at% Ti:

The field ion miciograph (fig. 5) shows that the annealing at 550 OC of the super- saturated Ni-12 a*% Ti alloy leads to an ordering in the (002)-planes in the Ti-enriched areas. Again bright and dim imaging layers were subsequently field evaporated. In comparison to Ni Al the differences in the luminosity are related to differences in the chemical composition of these layers. This different in im- 3 aging is not clearly understood up to now. The compositions of the different layers were determined from a ladder diagramm (fig. 6 ) . The brightly imaging (002)-layers were enriched in Ti whereas the dim ones were pure Ni layers. If the decomposition and hence the ordering, which in this case are both of spinodal type, would be complete the expected LI2-structure would have been obtained /8/.

The AP analysis of specimens annealed up to 256 hours at 550 OC shows infact that the alloy decomposes spinodally, i . , the Ti composition of the enriched areas reaches the equilibrium concentration of 25 at% Ti during the reaction, and the ordering is complete as far as the local concentration allows it to.

ALso in this alloy differences in the luminosity of subsequent field evaporated (002)-layers are clearly visible (fig. 7). The ordering is characterized by the diagram shown in fig. 8, where the various atom species versus the total number of evaporated ions were obtained in an AP study. The vertical divisions separate regions in which Al atoms arrive frequently at the detector £rom those where they do not, but Ni ions do. This correlation is not visible between the other com- ponents. The concentration profile (fig. 9 ) shows even more clearly the occurence of ordering. In order to quantify the observed ordering between Ni and Al atoms the average composition of the bright and dim layers were determined after field evaporation of 1.000 atoms and listed in table 1.

Table

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¹

Average Compositions (at%) for both Types of Rings bright rings dim rings

According to the distribution of the alloy components in bright and dim rings, or- dering occurs only between Ni and Al, whereas the other components are nearly equally distributed.

The degree of ordering is calculated to be about 80% 121.

After aging of homogenized specimen at 680 OC again successions of bright and dim rings of atoms around the (002) pole are visible. Therefore we expect the bcc a -phase enriched in Ni and Al again to be ordered in agreement with Pfeiffer /9/

and Bronner et al. 2 /IO/. Hetherington et al. /Il/ state that there is order also in the bcc al-phase between Fe and Co. According to the results on homogenized specimens, the ordering of the a phase starts before decomposition takes place, whereas the ordering of the al-phase could only develop during or after decomposi- 2- tion occurs.

Conclusions

We have shown by experiments on three alloys ( N i 3 ~ 1 , Ni-12 at% Ti, AlNiCo 5) that the field ion microscope in combination with an atom probe is well suited for the analysis of ordering in metallic alloys. .Even if the alloy is only partly ordered or if only a precipitated phase is ordered the detection of the ordering is well possible.

Acknowledgements

The authors wish to thank Dr. K. Kuntze (Th~ssen AG, ~ortmund) for the provision of the AlNiCo 5 material and Prof. P. Haasen, Dr. R. Wagner and M. Oehring for helpful discussions.

References

/1/ R.Grüne, P-Haasen, J. de physique, colloque C2, 4 7 (?986), 259.

/2/ A.Hütten, R-Grüne, Scripta Met. 20 (19861, 551.

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131 J.Piller, Diploma thesis, ~ n i v e r s r t ~ of Gottingen 1977.

/ 4 / R-Wagner, Field ion microscopy in material science, "Crystals", Vol.

6 , Springer Verlag, Berlin 1982.

C7-298 JOURNAL DE PHYSIQUE

/ 5 / R.J.Taunt, R.Sinclair and B-Ralph, Phys. stat. sol (a) 16 (1973),

-

469.

/6/ J.A.Horton, W.K.Miller, J. de physique, colloque C2, 4 7 (1986), 209.

/ ? / S.S.Brenner, D.Sieloff, PI.G.Buri<e, J. de physique, c~lloque C2, (1986). 215.

/8/ A.J.Watts and B.Ralph, Acta met., 25 (19771, 1013 191 I.Pfeiffer, Cobalt%, (1969), I I ~

/IO/ C.Bronner

,

3-Sauze, E.Planchard, J.M.Drapier, D.Coutsouradis and L-Habraken, Cobalt

2,

^{(1967), 123.}

/ I l / Hetherington, M.G., A.Cerez.0, J.P.Jakubovics and G.W.D. Smith, 3 . de physique, colloque Cg,

45

^{(1984), 429.}

300

VI

-

O

<

L 200

<

'.-

_O

L w .a

1

i o o

O

200 400 600

Number o f Ni - A t oms

Fig.2

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Ladder diagram of Al atoms versus Ni atoms. Results in <002> direction of Ni3A1.

Fig.1

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Contrast variation of successive rings of the (002)-pole of Ni3A1. ¹⁰⁰

-

⁸⁰

2

-

.

-

60

-

m

-

_C L

8 ⁴⁰

C O

Y =

²⁰

O

10 20 30 40

Desorbed (002)-Layers

Fig.3

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Ni-concentration profile in <002>

direction of Ni Al. The marked position shows probably an 3 anti-phase-boundary.

100 200 3 0 0 400 5 0 0

Correlat ion Length

k

iatomsl

Fig.4

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Autocorrelation of Ni atoms detected in <002> direction of Ni Al. 3

Fig.6

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Ladder diagram, Ni-12 at% Ti:

measured directly on pole in <002>, in a region with mean Ti concentration of 20 at%.

3ig.5

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^{Contrast variation in a}

neon-field-ion image of successive rings of the (002)-pole of Ni-12 at% Ti, aged for 8 h 25 rnin at 700 OC.

JOURNAL DE PHYSIQUE

Fig.7

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^Neon-f ield-ion images of AlNiCo 5 show the alternating brightly and dimly imaging (002)-layers.

0

56

^{160 150} 200

total number of ions

8 16 24 32 40

Desorbed (002)-Layers

Fig.9

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Concentration profiles of detected Al- and Mi- atoms in the <002> direction of homogenized AlNiCo 5 specimen.

Fig.8

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Diagrams of the atom species Fe, Co, Ni and Al versus the total number of evaporated ions. Results in <002> direc- tion of a AlNiCo 5 specimen.