Precipitation in an Fe-40%Ni-6%Nb alloy

Seripta M E T A L L U R G I C A

Vol, 3, pp. 267-270, 1969 Printed in the United States

PRECIPITATION

IN A N

Fe-40%Ni-6%Nb

Pergamon

Press,

Inc

ALLOY

K. A. Peard and D. W. Borland Department of Metallurgy, University of Melbourne, Melbourne, Australia.

(Received

February

17,

1969)

Recent investigations (I, 2) of austenitic alloys containing Ni and N b have revealed the formation of metastable precipitates in the form of platelets on {I00~ .

Manenc

et al. , (I) using X-ray diffraction, have reported the presence of such a precipitate in an Fe-30%Ni-5%Nb

alloy aged at temperatures up to 650°C and have deduced a face-centred

tetragonal structure having lattice parameters a = 3.599 ~

and c = 3.574 ~.

Kirman

and Warrington (2) have detected the formation of body-centred tetragonal Ni3Nb platelets on {100}• in an electron-microscope ~ 5 % F e and 5%Nb.

study of an alloy containing

55%Ni,

15%Cr,

In thin foils it is difficult to distinguish this structure from that

of yi (ordered f. c. c. ), which is the strengthening phase in a n u m b e r of precipitationhardened austenitic alloys; K i r m a n and Warrington have shown that only diffraction patterns from certain matrix zones are capable of resolving the ambiguity. The preliminary results reported in this note have been obtained from carbon extraction replicas of an F e - 4 0 % N i - 6 % N b 775°C.

alloy, solution treated at 1200°C and aged at

The matrix of this alloy was austenitic and showed no tendency to transform to

martensite at r o o m temperature,

either before or after ageing.

The hardness curve obtained (Fig. I) shows features which suggest a two-stage ageing process.

The first increase in hardness is associated with the precipitation of a

uniformly distributed phase in the form of thin plates which have attained a size 0.2~ square after 4 hours.

267

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PRECIPITATION

IN F e - 4 0 % N i - 6 % N b

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VPN

240

220

200

180

I

I

Io

Ioo

Ageing

Hardness

Electron

diffraction

and hence with the electron unambiguously pattern

as arising

Time.

(Hours.)

FIG. 1. v s A g e i n g T i m e a t 775°C.

patterns

obtained from the platelets

beam normal

(lying flat on the replicas,

t o t h e p l a n e of t h e p l a t e ) c o u l d b e i n d e x e d

from a body-centred

tetragonal

and the indexing adopted is shown in Fig.

structure.

A s k e t c h of a t y p i c a l

2.

110

2Q0

00~

110

(001)

F I G . 2. Sketch of a typical electron diffraction pattern from precipitate, i n d e x e d o n t h e b a s i s of a b. c. t e t r a g o n a l

The pattern of t h e 100 r e f l e c t i o n .

can be distinguished

with those predicted

f r o m t h a t t o b e e x p e c t e d f r o m y~ b y t h e a b s e n c e

A careful inspection

any evidence of this reflection,

of a large number

and the relative

by an application

structure.

intensities

of the structure

of p l a t e s f a i l e d t o r e v e a l

of t h e s p o t s w e r e

consistent

factor equation to the body-centred

tetragonal structure postulated by K i r m a n & Warrington (2). The "a" parameter calculated from the diffraction patternR is 3.7 ~; it is not possible to determine the

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PRECIPITATION

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269

"c" parameter since diffraction patterns were available only f r o m (001) zones. The evidence is consistent with the report of K i r m a n and Warrington, but not, apparently, with the f. c.t. structure reported by IVlanenc et al. in their study of an alloy of quite similar composition to that used in the present work. In view of the easy nucleation which would result if a high degree of atomic matching exists between the particle and matrix across the habit plane, the following orientation relationship is predicted:

{100}y

{100}b.c.t.

II

<100>b c . t .

II <100>

•

The p a r a l l e l i s m of {100} p l a n e s is s u p p o r t e d by the t r a c e a n a l y s i s of a m i c r o g r a p h i n which { 111 }¥ p l a n e s w e r e a l s o d e l i n e a t e d by the p r e s e n c e of a n o t h e r p r e c i p i t a t e d p h a s e (Laves phase). M a n e n c et al. (1) r e p o r t that in t h e i r a l l o y the p l a t e l e t s w e r e n o t f o r m e d at 700°C or 800°C, w h e r e a s i n t h i s w o r k t h e y w e r e o b s e r v e d to f o r m at 775°C. a g e i n g , a n d show a t e n d e n c y to l i n k up c o r n e r - t o - c o r n e r

T h e y grow d u r i n g

to f o r m " s t r i n g e r s " .

The p r e c i p i t a t i o n b e h a v i o u r in this a l l o y is a l s o c o m p l i c a t e d by the a p p e a r a n c e of two o t h e r p h a s e s : (a)

A n o r t h o r h o m b i c p h a s e , p r o b a b l y the e q u i l i b r i u m f o r m of (FeNi)BNb.

T h i s p h a s e was f i r s t o b s e r v e d a f t e r a g e i n g for

5 h o u r s at 775°C a n d a p p e a r s as l a r g e i r r e g u l a r p l a t e s .

It

does n o t p r e c i p i t a t e c o p i o u s l y and its m e c h a n i s m of f o r m a t i o n is u n c l e a r . (b)

L a v e s p h a s e , F e z N b , f i r s t f o r m e d a f t e r 10 h o u r s a g e i n g a t 775°C a n d g r o w i n g r a p i d l y as p l a t e s on {111}¥.

The p e a k

i n the h a r d n e s s c u r v e is a s s o c i a t e d with the f o r m a t i o n of this p h a s e . T h i s w o r k is b e i n g e x t e n d e d , u s i n g t h i n - f o i l e l e c t r o n m i c r o s c o p y , to o b t a i n a m o r e c o m p l e t e u n d e r s t a n d i n g of the p r e c i p i t a t i o n b e h a v i o u r of this alloy. REFERENCES I.

J. Manenc,

J. Bourgeot and H. de Boer, Scripta Met., _Z, 453 (1968).

2.

I. K i r m a n and D. H. Warrington, J. Iron Steel Inst. , 205, 1264 (1967).