APPLICATIONS OF A.E.M. TO RAPIDLY SOLIDIFIED MATERIALS

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APPLICATIONS OF A.E.M. TO RAPIDLY SOLIDIFIED MATERIALS

J. Vander Sande, A. Garratt-Reed

To cite this version:

J. Vander Sande, A. Garratt-Reed. APPLICATIONS OF A.E.M. TO RAPIDLY SO- LIDIFIED MATERIALS. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-705-C2-708.

�10.1051/jphyscol:19842163�. �jpa-00223836�

Colloque C2, supplément au n°2, Tome 45, février 1984 page C2-705

A P P L I C A T I O N S OF A . E . M , TO R A P I D L Y S O L I D I F I E D M A T E R I A L S

J.B. Vander Sande and A.J. Garratt-Reed

Massachusetts Institute of Technology , Department of Materials Science and- Engineering, Cambridge, Massachusetts 02139, U.S.A.

Résumé - Nous présentons deux analyses faites par STEM: (a) analyse de poudres d'acier à haute teneur en phosphore atomisées par centrifu- gation; (b) étude du film d'oxyde formé sur aciers inoxydables pendant un traitement d'oxydation à haute température. Les aciers inoxydables étudiés étaient préalablement formés soit par un procédé conventionnel soit par solidification rapide.

Abstract - We describe (a) a STEM analysis of centrifugally-atomised powders of high-phosphorous steels, and (b) a study of the oxide scale formed on wrought or rapidly solidified stainless steels during high- temperature oxidation.

It is now well established that rapid solidification of alloys holds considerable advantage over conventional processing because of the fuller potential for micro- structural control realizable through rapid solidification. With this new emphasis on microstructural control via rapid solidification processing has come the need for detailed microstructural analysis. Also, with the reduction in size of characteristic microstructural entities that occurs on rapid solidification, there developed a need for new experimental techniques that could probe both struc- ture and composition with much higher resolution than was previously available.

Fortunately, the advent of scanning transmission electron microscopy (STEM) occurred virtually simultaneously with the developments of rapid solidification technologies. With STEM, the capability for compositional analysis by X-ray fluorescence spectroscopy with a spatial resolution on the order of 5 nm in electron transparent sections has been realized. In this paper we present two examples drawn from research involving microstructural characterization of rapidly solidified steels.

The field emission STEM used in this work is a V.G. Microscopes HB5 instrument fitted with an energy-dispersive X-ray analysis system and an electron energy loss spectrometer. The electron source is a cold tungsten field emitter. There is a two-lens electron probe forming system, by which typically 2 nA of 100 kV electrons can be formed into a probe approximately 1.6 nm diameter on the spec- imen. The vacuum in the specimen chamber is ^ 2 x 10"

torr, which minimizes any problems of specimen contamination. The X-ray detector is a Kevex Si (Li) detector in the ultra-thin window (UTW) configuration coupled to a Kevex 7000 multichannel analyser and microcomputer system for spectral manipulation.

Of particular importance in this research has been the construction of compo- sition profiles across solidification-related microstructural features. Where grain boundaries or cell walls have been analyzed the analysis was performed by orienting the grain boundaries parallel to the electron beam and then man- ually stepping the electron probe along a line perpendicular to the boundary.

After each point has been analyzed, the boundary was examined to ensure that no specimen drift had occurred before repositioning the probe at the next data point. Whenever possible, boundaries were selected that were perpendicular to the edge of the foil so that the step scan occurred in a region of uniform

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842163

C2-706 JOURNAL DE PHYSIQUE

thickness. Additionally, the l i n e of the step scan was oriented perpendicular t o the l i n e between the specimen and the X-ray detector.

STEM Analyses of High-Phosphorus Steel Powders

Steel powders with a composition of Fe-160r-1ONi-0.4P (wt. p c t . ) were produced via centrifugal atomization. The average cooling r a t e f o r these powders i s - 105"/sec.

Powders were "sandwiched1' in a Ni e l e c t r o p l a t e and prepared a s thin sections f o r electron microscopy by standard techniques. Observations of these powders revealed t h a t t h e i r microstructure was composed of c r y s t a l l i n e dendrites or c e l l s surrounded by an i n t e r c r y s t a l l ine ( o r intercel l u l a r ) amorphous phase. By means of quantitative X-ray analysis the composition of the amorphous phase was found t o be Fe-36Cr-7Ni-8P ( a t . pct. ).

In Figure 1 i s presented the data which supports the observation and analysis of the amorphous phase in t h i s alloy. The convergent beam patterns across the dendrites i n the image show a d e f i n i t e amorphous character, through the diffuse halo, f o r the i n t e r c e l l u l a r phase. The composition p r o f i l e substantiates the segregation of Cr and P t o t h i s phase. In addition t o these observations i t has been found t h a t c 5% of the powder i s ferromagnetic ( i .e., bcc) while the re- mainder of the powder i s the expected f c c phase. The bcc powders predominate a t the smaller s i z e s and a r e present a s a r e s u l t of the nucleation dependent phenom- enon of a l t e r n a t i v e phase formation i n isolated liquid droplets. Additional observations of t h i s type have a l s o been made i n 303 s t a i n l e s s s t e e l . Analysis of the Oxide Layer Formed on Stainless Steel

In s t a i n l e s s s t e e l s , a layer of s i l i c a i s formed between the metal and the chromium oxide during high-temperature oxidation. This s i l i c a can grow down the grain bound- a r i e s of t h e underfyinq metal, possibly providing a "pegging" e f f e c t (1). In some cases, i t has been reported t h a t the s i l i c a can form an e s s e n t i a l l y continuous layer (2,3). W a r e investigating the e f f e c t s of rapid s o l i d i f i c a t i o n processing e upon the oxidation c h a r a c t e r i s t i c s a s we would expect ( a ) an increase in the chromium transport to the surface by grain boundary migration, and ( b ) possibly an increase i n s i l i c o n migration by the same mechanism, both of which should improve the oxidation performance of the material.

W e studied a wrought (W) 310 s t a i n l e s s s t e e l , and a rapidly s o l i d i f i e d (RS) 303 s t e e l prepared from centrifugally atomized powder. The compositions a r e given in Table 1. The s i l i c o n content i s somewhat above half of the optimum content found by Evans e t . a l ( 2 ) t o minimize the parabolic r a t e constant f o r oxidation.

Average grain s i z e s were 4 5 ~ and 5 . 6 ~ f o r the W-310 and RS-303 respectively.

I t has been shown by Yurek e t . a l . (1) t h a t i n cyclic oxidation experiments per- formed a t 900°C the RS-303 performed very much b e t t e r than t h e W-310, t h e weight gain of the RS 303 beinq lower, and spallation on t h i s s t e e l being v i r t u a l l y non-exi s t e n t .

W e prepared transverse s l i c e s of the oxide layer by a process e s s e n t i a l l y as developed by M. Tinker (private communication). Briefly, the process was a s follows: Specimens were prepared f o r oxidation in the form of small bars, 2.0mm x 5mm x c 15 mm. After oxidation, a thin nickel layer was deposited by vacuum evaporation on each face of the bar. Approximately 0.5mm of nickel was electrodeposited on the sample, the cross-section now being 3.0mm x 1.5mm.

Slices approx. 0.5mm thick were cut from the bar with a diamond s l i t t i n g wheel.

The s l i c e s were thinned on s i l i c o n carbide paper followed by diamond polish to a thickness of c 5 0 ~ . The f i n a l thinning was performed by ion-milling.

Figure 2 shows an annular dark f i e l d image of the W-310, oxidized a t 1000°C f o r

100 hours i n dry oxygen, with a sketch indicating the composition of the

d i f f e r e n t regions. Figure 3 i s another dark f i e l d region of t h e same specimen,

w i t h the corresponding s i l i c o n pulse map, showing three areas of s i l i c a , two a t

the oxidelmetal i n t e r f a c e , one within the metal.

undergone a s i m i l a r o x i d a t i o n treatment. The metal/oxide i n t e r f a c e i s seen t o be very convoluted, and s t r u c t u r e s a r e seen growing back i n t o t h e metal. Energy- d i s p e r s i v e X-ray a n a l y s i s shows these t o be composed o n l y o f s i l i c o n and oxygen, and m i c r o d i f f r a c t i o n shows them t o be v i t r e o u s . A t h i g h m a g n i f i c a t i o n s , a s i l i c a l a y e r can sometimes be seen between t h e oxide and t h e metal, ( ~ i g . 5), and sometimes i t can be seen t h a t no such l a y e r e x i s t s .

No s i g n i f i c a n t d i f f e r e n c e s were n o t i c e d i n t h e o u t e r oxide l a y e r s o f t h e two s t e e l s . Thus i t appears t h a t the improvement i n o x i d a t i o n r e s i s t a n c e o f t h e r a p i d l y s o l i d i f i e d s t e e l i s due a t l e a s t i n p a r t t o t h e promotion o f t h e forma- t i o n o f a v i t r e o u s i n t e r g r a n u l a r f i l m o f s i l i c a a t and near t h e metal/oxide i n t e r f a c e i n t h i s s t e e l . It i s p o s s i b l e t h a t f u r t h e r improvements i n performance c o u l d be obtained if t h i s l a y e r became continuous, although i t has been shown (2) t h a t increases i n s i l i c o n content above about 0.9% cause degradation i n t h e o x i d a t i o n c h a r a c t e r i s t i c s .

Fig. 1 - B r i g h t f i e l d STEM image from a 90 um diameter high- phosphorous s t e e l powder.

m i c r o d i f f r a c t i o n p a t t e r n s pe

composition p r o f i l e t r a v e se

t h e i n t e r c e l l u l a r r e g i o n between

two c r y s t a l l i n e d e n d r i t e s as

i n d i c a t e d .

C2-708 JOURNAL DE _PHYSIQUE

References

1. G.J. Yurek, D. Eisen and A.J. Garratt-Reed, Met. Trans. A 13A (1982) 473.

2. H. E . Evans, D.A. Hilton, R . A . Holm and S. J . Webster, Oxid. Met. 19 (1983) 1 . 3. M.J. Bennett, J.A. Desport and P.A. Labun, Proc. 4 1 s t Ann. Mtg. EMSA,

Ed. G.W. Bailey, San Francisco Press (1983) 214.

TABLE I - Compositions of S t e e l s (wt. %)

Steel Cr Ni Mn Si Co S P Mo Cu C Fe

W-310 24.1 19.5 1.6 .53 -- .008 .002 .2 -- .033 bal .

RS-303 17.31 8.68 1.60 .62 .15 .34 .D28 .37 .78 .059 bal .

Fig. 2. Annular d a r k - f i e l d (ADF) image of s c a l e forhed on W-310. In t h e sketch ( r i g h t ) regions marked 1 a r e spinel (Cr2Mn04), t h e region marked 2 i s a fine-grained C r p O j with Fe and M n dissolved i n varying amounts, regions m a r k e d 3 a r e complex regions including s i l i c a . Fig. 3. ADF image ( l e f t ) and corresponding s i l i c o n map

( r i g h t ) of scale/metal region on t h e W-310. Three a r e a s of s i l i c a a r e seen.

Fig. 4. ADF image of scale/metal i n t e r f a c e region of RS 303 s t e e l , showing s t r u c - t u r e s (determined t o be s i l i c a ) growing along t h e g r a i n boundaries of t h e metal.

Fig. 5. ADF image of s i l i c a l a y e r ( b r i g h t band) growing between Cr2O3 (above) and

metal (below) i n RS-303.