S E G R E G A T I O N O F V A R I O U S E L E M E N T S I N B I N A R Y

A L L O Y S B A S E D O N C O B A L T

S . B . M a s l e n k o v , ]~. P . G r u z i n a , a n d N. N. B u r o v a

UDC 6 2 0 . 1 8 6 . 2 : 6 6 9 . 2 5 ' 2 9 5 ' 2 9 7 ' 2 9 4

I t was p rev ious ly shown [1] that the tendency to s eg rega t e is h ighest for e lements forming s table c h e m - ical compounds with the base meta l . In nickel al loys these a r e t i tanium, niobium, and tanta lum, fo r example . They have a s t rong affinity for nickel , which is man i fes t in the fo rmat ion of congruent compounds of the Ni3M type. Segregat ion is genera l ly negligible in the absence of chemica l in teract ions in al loys based on i ron and nickel .

This work was under taken to ve r i fy this assumpt ion in the case of coba l t -base al loys. We invest igated dendr i t ic segrega t ion of e lements fo rming subst i tut ional solid solutions with cobal t in the region of hypoeu- tec t ic a l loys . The al loys were mel ted f r o m pu re charge m a t e r i a l s in a vacuum induction furnace of the Bal t - s e r s type. The ca rbon concent ra t ion did not exceed 0.03%,* with no m o r e than 0.003% sul fur and phosphorus . The al loys solidified under s tandard conditions in 1-kg cruc ib les . Slow cooling ensured a c o a r s e c rys ta l l ine m a c r o s t r u c t u r e of the s a m e type fo r all heats that d i f fered somewhat in the extent of the zone of co lumnar c rys t a l s ; all cas t ings had a cen t ra l zone of equiaxed c r y s t a l s (Fig. l a ) . The samples we re taken f r o m the ax - ial zone of the ingots with the h ighes t dendri t ic segregat ion . The typica l s t ruc tu re of the coba l t -base al loys is shown in Fig. lb . In al loys with additions of Ti, Hf, Ta, and Mo the dendri t ic s t ruc tu re was revea led by chemica l etching in a reagen t consis t ing of 50 cm 3 HNO a + 45 c m 3 CH3COOH + 5 cm 3 H20. Alloys with z i r con - ium and a luminum were etched in V a s i l ' e v ' s reagent : 50 cm a HC1 + 25 cm3H2SO4 +10 cm 3 CuSO 4" 5H20 + 15 c m 3 H20. The dendri t ic s t ruc tu re of a l loys with i ron and nickel was r evea led by the au toradiographic method (Fig. 2).

Fo r quanti tat ive de te rmina t ion of seg rega t ion we used local m i c r o p r o b e analys is . The effect ive d i a m e - t e r of the p robe in the MS-46 ana lyze r did not exceed 1/~m. Cor rec t ions fo r absorp t ion and secondary r a d i a - t ion were made by the ca l ibra t ion method. Segregat ion of an e lement was judged f r o m the concentra t ional d i s - t r ibut ion and f r o m the resu l t s of m i c r o p r o b e analys is . The e r r o r did not exceed 5%. In t rac rys ta l l ine s e g r e - gation was de te rmined f r o m the segrega t ion coeff ic ient Ks , equal to the ra t io of the m a x i m u m concentra t ion of the e lement to the m i n i m um concent ra t ion in the bulk of the dendri t ic cel l (K s = Cmax/Cmin) .

* By weight, he re and below.

T A B L E 1

I

Element L a t t i c e

t y p e

]So luNl iv Slope o f a t e u t e c - Liquidus

ra" ar a, % t i c t e m p . Line f, o7o i e g / a t . % T y p e o f d i a g r a m

Hf p--Zr

Nb Ti Ta AI

H e x a g o n a l b e c

NI

W

1,36 1,60

cc

H e x a g o n a l b e e

1,46 ] ,47 1,46 1,43

27 28

1,26 1,26 t~27 1,37 I , ~

16 17 16 14

2,0 2.5

0,8 0,8 o,8

1t

4,0 12 3,0

16

13I 87

g

17,5

52 21 43 7

With limited solubility in solid solution and con- gment compounds

H

Mo bcc t,39 11 ~8,5 15 With limited solubility it. I f i V ~CC 1,34 7 35 6 sol id so lu t ion a n d i n e o n -

Cr n e e 1,27 2,4 41 20 gment compounds

IV IWith unlimited solubility

61,4 In solid and liquid states C.;

I. 1 ~. B a r d i n Cen t r a l S c i e n t i f i c - R e s e a r c h Ins t i tu te of F e r r o u s Meta l lu rgy . T r a n s l a t e d f r o m Meta l loved- enie i T e r r n i c h e s k a y a Obrabo tka Metal lov, No. 6, pp. 18-19 , June, 1979.

428 0026 -0673 /79 /0506 -0428507 .50 © 1979 P lenum Publ ish ing C o r p o r a t i o n

Fig. 1. Characteristic macrostruc- ture of ingots (a) and microstructure of dendritic segregation area (b) of binary cobalt alloys.

Fig. 2. Autoradiograph of the dendritic structure of the cobalt alloy with 5% Fe.

TABLE 2

<~ '~ Concn. of element, wt.% [

.i av. min. max. KS

1,0 O.4 213 2.0 0.90 7,0 4.0 2,t 10,O 8,0 4.3 18,7

$,4 0,02.5 13,7

6,0 0,2 20,0 12,9 level 20,0

4,7 4,4 ] 6,2

1,7 0.03 6,0 4,2 0,9 7,0 8,6 1,8 19,S

17,2 6,1 22,0

3.4 2.6 24,0 8,4 4,7 24,0

18,3 t 17,7 20,5

1 3,2 t~9 3,7 6,4 3,9 7,5

12,5 10,0 21,0

12,4 I 12,7 II,5 24~8 I 26,2 23,5

4,3 I 4,1 8,0 L

2,8 1 2,7 0,9 7,~ [ 7,5 1,7

3,0 t 5,2 5,7 F

5,0 ] 5,9 6,5 B

2,0 ] !,6 2,8 6,0 ] 1,6 4,4

12,9 6,5 17,2

5 7

TI 4,7 4,1

Zr 400

H! 100

Y 1,4--1,~

20 7.

Nb I0

9 Ta 5

Cr t,1--I,2

1,9 MO 1,9

2,1

--1,13 W --1,12

Mn 2,0

--2,6 Re --4,7

Fe 1,1

N! 1,1

AI 1.7 9,7 2.6

All the alloys investigated were divided into four groups (Table

I) very limited solubility;

If) high solubility and congruent (stable) compounds;

III) great solubility and incongruent (unstable) compounds;

1) in terms of systems with:

IV) unlimited or extremely large solubility.

429

The chemical composition of the alloys and the results of determining the segregation of elements are given in Table 2, while the physicochemical character is t ics of the systems are given in Table 1.

Segregation was highest (Table 1) for two groups of elements - with low solubility (Hf, Zr) and with high solubility but forming congruent compounds (Nb, Ti, Ta, A1). For elements forming incongruent compounds or continuous solid solutions the values of the segregation coefficients are much lower. The atomic radius of elements with high tendency to segregate differs substantially (> 12%) from the atomic radius of cobalt (1.25 ~), which agrees with one ofthe i Hume--Rothery solubility cr i ter ia . It should be noted that elements highly segregated in cobalt are character ized by a phase diagram with a steep slope f of the liquidus line. For most elements it amounts to N 20 dog/at.%, although this character is t ic is evidently of no independent s ig- nificance. For example, f N 7 deg/at.% for cobalt with aluminum, while f = 20 deg/at.% for cobalt with chro- mium. If this were of independent significance,the segregation of chromium would exceed that of aluminum, although this is not observed. The most probable reason for the high segregation of aluminum is its substan- tial chemical interaction with cobalt, which is manifest in the formation of congruently melting Co~A1.

The results obtained for binary cobalt alloys confirm the assumption that chemical interaction is the most important physicochemical factor determining the tendency to segregate during crystallization of t rans- sition elements.

L I T E R A T U R E C I T E D

1. S. B. Maslenkov, " Relationship between dendritic segregation and the type of phase diagram," in: Spe- cial Steels and Alloys [in Russian] , Tr. TsNIIChM, No. 77, Metallurgiya, Moscow (1970), p. 7.

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