A FIM/AP INVESTIGATION OF A RAPIDLY SOLIDIFIED ALUMINIUM-CHROMIUM ALLOY

HAL Id: jpa-00226852

https://hal.archives-ouvertes.fr/jpa-00226852

Submitted on 1 Jan 1987

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

A FIM/AP INVESTIGATION OF A RAPIDLY SOLIDIFIED ALUMINIUM-CHROMIUM ALLOY

A. Cerezo, B. Shollock, G. Smith

To cite this version:

A. Cerezo, B. Shollock, G. Smith. A FIM/AP INVESTIGATION OF A RAPIDLY SOLIDIFIED ALUMINIUM-CHROMIUM ALLOY. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-287-C6- 292. �10.1051/jphyscol:1987647�. �jpa-00226852�

Colloque C6, suppl6ment au nO1l, Tome 48, novembre 1987

A FIMIAP INVESTIGATION OF A RAPIDLY SOLIDIFIED ALUMINIUM-CHROMIUM ALLOY

A. Cerezo, B.A. Shollock and G.D.W. Smith

Department of Metallurgy and Science of Materials, Oxford University, Parks Road, Oxford OX1 3PH, U.K.

Abstract A rapidly solidified A1

-

I

-

The hardening of conventional A1 alloys by the formation of GP zones and fine-scale precipitates is well known. However, these alloys often exhibit a rapid loss of hardness on precipitate coarsening at elevated temperatures. In aluminium/transition-metal alloys formed by rapid solidification, the metastable solid solution decomposes to form low solute diffusivity, low solubility transition- metal aluminide precipitates, exhibiting increased thermal stability [I].

The material examined in the present work was produced by vapour deposition onto a substrate held at a temperature of 360-370°C [2]. This technique has the advantage of producing significant quantities of bulk material. Studies carried out on this material so far include transmission electron microscopy (TEM) and X-ray diffraction 121. Whilst these techniques give a general indication of precipitate distributions, detailed information on the nanometre scale has been lacking.

I1

-

Field-ion specimens were prepared from bulk material by the standard two-stage electropolishing technique in 25% perchloric acid in glacial acetic acid at O°C and 20V dc followed by 15-25V dc in 2% perchloric acid-butoxyethanol mixture at room temperature. The latter solution was also used to re-sharpen specimens by the drop polishing technique [3]. FIM images and atom probe analyses were obtained on the FIMlOO system at Oxford [4]. Initial imaging was carried out in 10-~mbar of Ar with a specimen temperature of gOK, allowing the surface oxide to be easily removed.

Subsequent imaging was performed at 60-75K in 5x10-5mbar Ne. Analyses were obtained in UHV (<10-~Ombar) with a pulse ratio of 20-25%. Additional analysis was performed in an older instrument without energy compensation [5] at a specimen temperature of 90K and a base pressure of 10-I Ombar.

I11

-

The general microstructure of the as-deposited material is shown in the TEM micrograph of figure la. Vapour deposition results in columnar grains with some tendency to form intermetallic precipitates (CrA1,) along grain boundaries as shown

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987647

C6-288 JOURNAL DE PHYSIQUE

at 1 in figure la. Some precipitation of intermetallics, both spherical (2) and rod- or plate-shaped (3) also occurs within the grains. The field-ion micrograph in figure lb shows the ordered image obtained from a large intermetallic while figure lc shows an image of a small rod-shaped precipitate of the type seen in figure la.

However, the majority of the volume within the grains is free from precipitates on this scale and reveals only a slight mottling in the TEM.

Figure 1: a) TEM micrograph showing the general microstructure of the A1-Cr vapour deposited material. b) FIM micrograph of a large intermetallic precipitate. c) Rod- or plate-shaped precipitate in the FIM.

observed i n t h e TEM. This can be compared with t h e FIM micrograph of an Al-4wt%Cr w i r e formed by in-rotating-water quenching [63 where no c l u s t e r i n g i s observed, a s shown i n f i g u r e 2b. The s i z e of t h e s e p a r t i c l e s i n t h e vapour-deposited m a t e r i a l i s exaggerated by t h e l o c a l magnification e f f e c t , but an e s t i m a t e based on t h e number of atoms imaged i n d i c a t e s t h a t they a r e 0.5-lnm i n diameter. I n o r d e r t o r e v e a l t h e p a r t i c l e shapes and d i s t r i b u t i o n , t h e images were photographed u s i n g exposures of 30s-2mins during slow f i e l d evaporation. The r e s u l t i n g micrographs, such a s f i g u r e 2a, r e p r e s e n t t h e p r o j e c t i o n of t h e p a r t i c l e s through 1-5 atomic planes of m a t e r i a l . From t h e s e , t h e number d e n s i t y of t h e p a r t i c l e s is estimated t o be around 102'm-3.

It was a l s o found t h a t c e r t a i n p a r t i c l e s appear elongated, seen i n f i g u r e 2a, suggesting t h a t some a r e rod-shaped, r a t h e r than s p h e r i c a l , with l e n g t h s up t o 5nm having been observed.

Figure 2: Field-ion micrographs of t h e vapour deposited a l l o y ( a ) and t h e m a t e r i a l formed by in-rotating-water quenching ( b ) . Note t h e Cr-rich p a r t i c l e s imaging b r i g h t l y i n a ) .

The m a t e r i a l was aged a t 350°C f o r up t o 20 hours without showing s i g n i f i c a n t growth o f t h e s e p a r t i c l e s a l t h o u g h t h e TEM s t u d i e s showed f u r t h e r l a r g e r - s c a l e p r e c i p i t a t i o n occuring i n some g r a i n s a t t h e l o n g e s t aging times. Figure 3 shows both TEM and FIM micrographs f o r t h e m a t e r i a l a t v a r i o u s aging times, showing the constancy of t h e p a r t i c l e d i s t r i b u t i o n . No change i n t h e mottled c o n t r a s t i n t h e TEM has been observed f o r aging up t o 200 hours. Since peak hardness occurs near 15 minutes a t 350°C. t h e s e specimens a r e h e a v i l y over-aged although, a s t h e l a c k of coarsening s u g g e s t s , t h e reduction i n hardness is l i m i t e d .

Since i t was found t h a t atom probe analyses obtained with imaging gas present gave a g r e a t l y exaggerated C r c o n c e n t r a t i o n , perhaps due t o build-up of r e s i d u a l gases, a l l analyses were c a r r i e d o u t i n UHV. Figure 4 shows a s e c t i o n of a C r composition-

C6-290 JOURNAL DE PHYSIQUE

TEM FIM

as-deposited a

Figure 3: Comparison of TEM and FIM micrographs of the vapour-deposited material for a range of aging treatments.

was found that a significantly non-random distribution was obtained only for sample sizes of around 10, reflecting the small size of the particles. This can be compared with a sample distribution from the in-rotating-water quenched specimen, which is very close to a binomial.

Number of Ions

Figure 4: Chromium concentration profile for the vapour deposited material (as-deposited). See sample distribution, figure 5a).

Cr Ions in Sample

b)

Figure 5: Sample distributions (sample size 10) for vapour deposited (a) and water quenched (b) materials. The filled blocks represent the observed distribution while the expected (binomial) is shown by open blocks. It is seen that a) is non-random while b) does not differ significantly from the expected distribution.

C6-292 JOURNAL DE PHYSIQUE

Using t h e sample d i s t r i b u t i o n s , t h e composition of t h e Cr-enriched r e g i o n s can be e s t i m a t e d a t around 4Oat%Cr. Figure 6 shows p o r t i o n s of t h e l a d d e r diagrams obtained from v a r i o u s specimens, showing t h e a n a l y s i s of t h e s e C r - r i c h p a r t i c l e s . A s i m i l a r e s t i m a t e of t h e C r c o n c e n t r a t i o n i s obtained from t h e s e diagrams. These measurements must be t r e a t e d with some c a u t i o n , s i n c e t h e y w i l l b e complicated both by a c o n t r i b u t i o n from t h e matrix and by t h e tendency of C r atoms t o be r e t a i n e d on t h e s u r f a c e . However, t h e s e f a c t o r s a r e i n d i c a t i v e of t h e g e n e r a l d i f f i c u l t i e s involved i n measuring t h e composition of sub-nanometre p a r t i c l e s i n t h e atom probe.

F i g u r e 6: Ladder diagrams o f Cr-rich regions i n d i f f e r e n t specimens of t h e vapour d e p o s i t e d m a t e r i a l ( v a r i o u s aging t r e a t m e n t s ) . These a r e p o r t i o n s of a n a l y s e s o b t a i n e d by random probing.

ACKNOWLEDGEMENTS

The m a t e r i a l s used i n t h e Dresent study were k i n d l y SuDDlied t o us by RAE - -

Farnborough (vapour-deposited) and U n i t i k a Ltd. (water quenched w i r e ) . The a u t h o r s would l i k e t o thank P r o f e s s o r S i r P e t e r Hirsch FRS f o r t h e p r o v i s i o n o f l a b o r a t o r y f a c i l i t i e s . Helpful d i s c u s s i o n s with D r . B. Cantor, D r . E.D. Boyes, D r . A. Bowen, R. Gardiner and C. Gilmore a r e g r a t e f u l l y acknowledged. The FIM/AP f a c i l i t y a t Oxford was developed with t h e a i d of a g r a n t from t h e Paul Instrument Fund of t h e Royal S o c i e t y and i s supported by t h e SERC.

REFERENCES

[I] Jones, H., J. Mat. S c i .

9

[2] Bickerdike, R.L., Clarke, D.. Eastabrook. J.N., Hughes, G . , Mair, W.N., P a r t r i d g e , P.G. and Ranson, H.C.. I n t . J. Rapid S o l i d i f i c a t i o n 2, (1986) 1.

[3] Almed, A . J . and C a r r o l l , J.J., J. Vac. S c i . Technol. A2, (1984) 1388.

[4] Cerezo, A . , Smith, G.D.W. and Waugh, A.R., J. Phys. ( P a r i s ) (1984) C9-329.

[5] M i l l e r , M.K.M., Beaven, P.A.. Smith, G.D.W., S u r f . I n t e r f a c e Anal. 1 _- (1979) 149.

[6] Ohnaka,I., Fukusako, T . , Ohmichi, T., Masumoto, T.. Inoue, A . , Hagiwara. M., i n Proc. 4 t h I n t . Conf. on Rapidly Quenched Metals Sendai (1981) 31.