Diffusion paths and structures in multiphase Cu− - Springer Link

Diffusion Paths and Structures in Multiphase Cu-Ni-Zn Couples at 775°C L. E. WIRTZ AND M. A. DAYANANDA Multiphase diffusion was investigated in the Cu-Ni-Zn system at 775°C for the development of diffusion structures involving two interfaces. Selected s e r i e s of diffusion couples characterized by a common 7 (cubic) terminal alloy joined to a set of ~ (fcc) alloys developed an intermediate/3 (bcc) phase with two interfaces, oj/3 and/3/7. The or//3 interface showed transitions from planar t o nonplanar and back to planar morphology, as the copper concentration of the ~ terminal alloy was decreased from 100 to about 30 a t . pct. Planar/3/7 interfaces were observed for all but two of the couples. The compositions on either side of planar ~//3 interfaces were consistent with those b a s e d on equilibrium tielines, while the compositions at nonplanar a///3 interfaces differed from those of equilib r i u m . Selected s e r i e s of couples assembled with 7 and/3 alloys were also investigated for the development of interface instability at the/3//7 interface. The diffusion paths of 7//3 couples were consistent with those of 7//~ couples.

DIFFUSION related problems are encountered by the materials engineer in many metallurgical proc e s s e s . The phenomenon of diffusion p l a y s a m a j o r role in processes dealing with cladding, galvanizing, thin films, phase transformation, oxidation and corrosion. When materials of different compositions a r e bonded together, diffusion can o c c u r at elevated temperatures and the product can undergo changes in internal structure as well as in mechanical p r o p e r t i e s . The structural stability of the bonded a s s e m bly governs the utility of a multiphase product or device. The g e n e r a l problem of the utility of bonded structures involves the stability of interphase bounda r i e s . Interracial instabilities can cause the f o r m a tion of nonplanar interfaces as well as two-phase l a y e r s which can affect the integrity of any multiphase product. Systematic diffusion studies to investigate the development of diffusion structures and t o characterize interfacial instabilities have been made1-4 in the Cu-NiZ n system with multiphase diffusion couples assembled with ot(fcc) and fl(bcc) alloys. T h e s e studies 2"4 included the development of both planar and nonplanar ~//3 interfaces with selected s e r i e s of ot v s /3 couples. Coates and Kirkaldy2 studied the interface development at 775°C with a s e r i e s of couples assembled with a binary Cu-Zn a alloy and/3 alloys of varying compositions. As the concentration of Ni was increased in the/3 terminal alloy, a transition from a stable planar ~//3 interface to a nonplanar ~/;3 interface was observed. Sisson and Dayananda3 investigated the development of diffusion structures in the Cu-Ni-Zn s y s tem at 775°C utilizing a s e r i e s of couples assembled with selected binary Cu-Ni ~ alloys and a ternary/3 alloy for diffusion t i m e s of 2 h t o 2 days. A t r a n s i tion from nonplanar to planar ~/f3 interfaces was obs e r v e d as the concentration of Cu in the ot terminal alloy was varied. In a more r e c e n t study4 Sisson inL. E. WIRTZ is with Lockheed Missiles & SpaceCo., Sunnyvale, CA 94088, and M.A. DAYANANDA is Professor of Materials Engineering, Purdue University, West Lafayette, IN 47907. Manuscript submitted April 20, 1976. METALLURGICAL TRANSACTIONSA

vestigated additional s e r i e s of multiphase couples characterized by a common/3 alloy and c~ t e r m i n a l alloys with compositions selected a l o n g a line of Ni/ Z n r a t i o of 0.4 and observed transitions from planar t o nonplanar a / ~ interface as the Cu concentration of the ot terminal alloy decreased. An SEM and e l e c tron microprobe analysis of these couples showed that the compositions of the ~ and fl phases at planar interfaces were in agreement with predictions b a s e d on equilibrium tie-line determinations. On the other hand, for couples developing nonplanar instabilities the concentrations at the interface were found t o be different from those predicted from the equilibrium tie- lines. It is the p u r p o s e of this paper t o present diffusion paths and diffusion structures for Cu-Ni-Zn multiphase couples involving two interfaces and diffusion path crossings a c r o s s two two-phase fields. S e v e r a l Cu-Ni-Zn alloys were investigated at 775°C and each s e r i e s was characterized by a common 7 t e r m i n a l alloy joined t o a set of ~ or/3 alloys. The compositions of the alloys employed are indicated on the Cu-Ni-Zn ternary isotherm at 775°C shown in Fig. 1. The 7 vs couples involved the formation of an intermediate/3 layer and the diffusion p a t h crossings a c r o s s b o t h (~ + {3) and (/3 + 7) phase fields of the ternary isot h e r m . This paper reports on the occurrence of instabilities at the ~/13 a n d / o r the/3/7 interface as obs e r v e d in t h e s e couples. Selected s e r i e s of 7 v s couples were also investigated for the development of interface instabilities at the 7//3 interface. These couples involved only the/3/7 interface and diffusion path crossings of the (/3 + 7) two-phase field. The diffusion structures of all the multiphase couples a s s e m bled in this study were examined metallographically. The couples were also investigated by electron microprobe analysis for the determination of concentration profiles and diffusion paths. EXPERIMENTAL

PROCEDURE

The ot(fcc), j3(bcc) and ¥(cubic) Cu-Ni-Zn alloys used in this investigation were prepared by induction VOLUME 8A, APRIL 1977 567

Zn

s "y

." r

?

o,

o,

o

7 ATOM % CO

Fig. 1--Alloys used in diffusion couples.

Table I. Alloy Compositions Alloy

C o m p o s i t i o n , W t Pct

Designation

Cu

Ni

a2 a3 a4 as a6 /~1 ~2 ~3 ~4 71 72 73

61,6 59.4 46.1 30.9 31.9 30,6 20,5 54,3 38.3 15.8 -

38.4 12.3 16.4 40.4 68.1 19.2 22.7 46.2 14.8 21.3

C o m p o s i t i o n , At. Pct Zn

28.3 37.5 28.7 50.2 56.8 45.7 53.8 61.7 69.4 78.7

Cu

Ni

Zn

59.7 59.3 46.0 30.1 30.2 30.5 20.5 55.0 39.0 15.9

40,3 13.3 17.7 42.7 69.8 20.8 24.5 48.9 16.1 23.2

27.4 36.3 27.2 48.7 55.0 45.0 51.1 61.0 68.0 76.8

melting of OFHC Cu, electrolytic Ni (99.9 pct), and high p u r i t y Z n (99.999 pct) in alumina crucibles under a r g o n atmosphere. The liquid m e l t s were allowed to solidify and quenched in water. The alloy ingots were cut into diffusion disks approximately 1 cm in thickness and no g r o s s segregation could be detected in the alloys by microprobe analysis. The/3 and ~ all o y s were found t o be quite brittle. The compositions of the alloys used in this study are reported in Table I. The diffusion couples were assembled with disks of a y alloy sandwiched between disks of selected a or/3 alloys. The surfaces of the disks were polished metallographically through 0.05 p m alumina b e f o r e assembling the couples. I n e r t m a r k e r s in the form of 1.0 ~tm A12Oz particles were placed on the disk surf a c e s . The discs were held together with a Kovar s t e e l clamping device, and the assembled couple was placed inside a quartz capsule with V alloy chips added t o minimize Z n loss during the initial stage of the anneal. The capsule was flushed with hydrogen and evacuated to a p r e s s u r e less than 0.133 pascal and sealed. All couples were diffusion annealed at 775°C in a Lindberg heavy-duty three-zone tube furnace for 2 h. The temperature gradient a c r o s s 568-VOLUME

8A, A P R I L 1 9 7 7

the capsule was less than I°C and the temperature was controlled to ±0.5°C. After the anneal, the caps u l e s were broken and the couples quenched in icewater in o r d e r t o retain the high temperature structure of the diffusion zone. The diffused couples were mounted in quick-mount self-setting r e s i n and ground on a wet-belt g r i n d e r t o expose sections parallel to the direction of diffusion. The exposed section of each couple was metallographically polished through 0.05 /~m a l u m i n a and etched with a solution of 2 g K2Cr207, 8 ml H2SO4 (sp. gr. 1.84), 4 ml NaC1 (saturated solution) and 100 ml H20. 5 The revealed diffusion structures were then phot0micrographed at appropriate magnifications. The couples were repolished with 0.3 /~m alumina to r e m o v e the etched layer with care taken t o prevent s m e a r i n g a c r o s s interfaces. The couples were then analyzed for concentration profiles with an ARLAMX electron microprobe equipped with two wavelength dispersive spectrometers by a point-to-point counting technique. Analyses were made for Cu, Ni, and Z n by monitoring and measuring the intensities of CuK a (~ = 1.542,~), NiK a (X = 1.659.~) and ZnK~ (X = 1.436A) x-radiations. The operating conditions of the microprobe corresponded to 25 kV accelerating voltage and 16 nanoamperes sample current on pure copper. Standard ~,/3, and v alloys were used t o determine parameters for the conversion of measured X - r a y intensities t o composition on the b a s i s of the method of Ziebold and Ogilvie.6 T h e s e conversion parameters were 0.97, 0.73, and 1.09 for Cu, Ni, and Zn, respectively. EXPERIMENTAL RESULTS AND DISCUSSION Six s e r i e s of multiphase diffusion couples were investigated in this study with the a, /3, and y Cu-Ni-Zn alloys whose compositions a r e reported in Table I; these couples are listed by s e r i e s in Table II. Each of the couple s e r i e s is characterized by a common V terminal alloy joined t o a set of ~ or [3 alloys. All couples were diffusion annealed at 775°C for 2 h. The experimental observations on interface characteristics and locations of m a r k e r planes a r e also included in Table II. Diffusion Paths and Structures of

V/ot Couples S e r i e s I: Y1 vs Cu, a3, c~2, ~5, ~6, Ni The diffusion paths determined from the experimental concentration profiles for all the couples in S e r i e s I are shown in Fig. 2. In this figure the dotted l i n e s drawn within the two-phase (cv +/3) r e g i o n of the ternary isotherm correspond t o the equilibrium tie-lines determined by Sisson4 from two-phase alloys equilibrated at 775°C. For all the couples, the path crossings of the two-phase regions, (~ +/3) and (/3 + 7), are shown by dashed lines. The diffusion structures developed in the various couples of S e r i e s I are shown in F i g s . 3, 4, and 5 and were characterized by the development of the/3 phase d u r i n gdiffusion. A planar ~3/v interface was observed for couples vff/Cu, y J a 3 , and yu/a2 shown in F i g s . 3(a), 3(b), and 4(a). In addition t o a planar /3/~ interMETALLURGICAL

TRANSACTIONS A

an

Table II. Development o f Interfaces in Cu-Ni-Zn Multiphase Couples Diffused for 2 h at 775°C

Couple Series

Couple Designation

I

7,/Cu "~1/(~

(7 vs ~)

1I

(7 vs ~)

Ill

Characteristics* and Direction~ ofMotion of Interface a/3

P; (+)

3/7

P; ( ) P; ( )

Marker Location

7Ja= 7Jas

71/a6 7~/Ni

P; (+) T;(+)

7z/Cu 7=/a,* 7Jas 7rjNi

P; (+)

P; ( )

/3layer

NP; (+) NP; (+)

P; ( ) P;( )

3 layer 3 layer

T;(+)

P; (-)

3 layer

7a/Cu

P; (+)

P;( )

7Ja4

NP; (+) NP; (+)

3 layer 3 layer 3'phase 7 phase

(7 vs a)

P; (-)

L;(-) L; (-) P; (-)

P; (-)

73]~s 7JNi

P; (+)

P; (-) P; ( )

IV (7 vs3)

7J/~*

-

L; (-)

V (7 vs 3)

Yd~3

-

P; (-)

Yz/3~ 7~J3~

-

P; (-) NP; (+)

72/34

-

P; (+)

7~/3~

-

P; (-)

7~/3~

-

P; (-)

7~/34

VI (7 vs 3)

P;(+)

7/ IO

'- '

!

~ 6 0

~

/o/ 20

30

X

40 50 60 ATOM % Cu

70

80

90

F i g . 2--Diffusiori p a t h s for couple S e r i e s I at 775°C.

/3phase 13phase 3"phase 7 phase

b e at a n a n g l e to the t i e - l i n e s ; s i m i l a r o b s e r v a t i o n s h a v e b e e n m a d e by S i s s o n4 with f l / a c o u p l e s . C o u p l e s y , / a 3 a n d v ~ a 2 with p l a n a r [ 3 / y i n t e r f a c e s h a v e path c r o s s i n g s of the t w o - p h a s e (fi + V) r e g i o n w h i c h a r e n e a r l y i d e n t i c a l . D a s h e d c u r v e s d r a w n w i t h i n the (B + ~) t w o - p h a s e r e g i o n a s p a r t of the d i f f u s i o n p a t h s f o r t h e c o u p l e s V l / a s a n d ~ / ~ i n d i c a t e the (/3 + v) t w o - p h a s e l a y e r s f o r m e d in t h e i rd i f f u s i o n z o n e s s h o w n in F i g s . 4(b) and 5 ( a ) ; a l s o , t h e i r d i f f u s i o n path s e g m e n t s e x i t i n g into the ~ p h a s e f i e l d a n d e n t e r i n g b a c k into the (/3 + y) r e g i o n r e p r e s e n t the y p h a s e l a y e r f o r m e d b e t w e e n the (fl + V) t w o - p h a s e and the fi l a y e r s o b s e r v e d in t h e i r d i f f u s i o n s t r u c t u r e s . T h e ~//3 i n t e r f a c e s that a p p e a r to be p l a n a r in F i g s . 4 ( b ) , 5 ( a ) , and 5(b) a r e r e p r e s e n t e d by the s t r a i g h t d a s h e d l i n e s in the (~ + ~) r e g i o n of the i s o t h e r m in F i g . 2; t h e s e l i n e s m a y b e c o n s i d e r e d to i n d i c a t e the o r i e n t a t i o n s of the t i e - l i n e s in the (~ + V) r e g i o n , s i n c e no s e p a r a t e t i e line d e t e r m i n a t i o n s w e r e m a d e f o r this r e g i o n .

3 phase 3 phase

f a c e s e p a r a t i n g t h e / 3 and ~ p h a s e r e g i o n s , a twop h a s e (/3 + ~) l a y e r s a n d w i c h e d by V p h a s e l a y e r s w a s a l s o o b s e r v e d in c o u p l e s 7 J c e s a n d ~ c ~ 6 , a s c a n b e s e e n in F i g s . 4(b) a n d 5 ( a ) . C o u p l e y J N i s h o w n in F i g . 5(b) d e v e l o p e d a p l a n a r ~ / ~ i n t e r f a c e . With r e g a r d to the o~/~ i n t e r f a c e the c o u p l e s in S e r i e s I s h o w e d a t r a n s i t i o n f r o m p l a n a r to n o n p l a n a r and then b a c k to p l a n a r m o r p h o l o g y , a s the c o p p e r c o n t e n t of the a t e r m i n a l a l l o y w a s g r a d u a l l y d e c r e a s e d f r o m 100 to a b o u t 30 a t . pct. A p l a n a r a / ~ w a s o b s e r v e d a s e x p e c t e d f o r the b i n a r y c o u p l e 7 ~ / C u s h o w n in F i g . 3 ( a ) . F o r c o u p l e s y~/O~a, y , / a 2 , and y ~ / a 5 w a v e l i k e , n o n p l a n a r a / ~ interfaces were o b s e r v e d as can b e s e e n in F i g s . 3 ( c ) , 4 ( a ) , a n d 4 ( c ) . A r e t u r n to the p l a n a r m o r p h o l o g y w a s o b s e r v e d f o r the a/[~ i n t e r f a c e in t h e c o u p l e y l / / a 6 s h o w n in F i g . 5 ( a ) . T h e diff u s i o n zone of the c o u p l e y ~ N i d e v e l o p e d two twop h a s e l a y e r s of ( a + / 3 ' ) and ( a +/3) b e t w e e n the ~ and s i n g l e p h a s e r e g i o n s a s s h o w n in F i g . 5 ( c ) . T h e m a r k e r p l a n e ( X m ) w a s l o c a t e d w i t h i n the fl p h a s e f o r all S e r i e s I c o u p l e s with the e x c e p t i o n of the c o u p l e y J N i w h o s e m a r k e r p l a n e was f o u n d w i t h i n the y p h a s e . F o r c o u p l e s ~%/O~a a n d ~ / / a s s h o w n in F i g s . 3(b) and 4 ( b ) , p o r o s i t y w a s c o n c e n t r a t e d in t h e r e g i o n of the m a r k e r p l a n e to s u c h a n e x t e n t that s u b s e q u e n t e t c h i n g a f t e r p o l i s h i n g left t h e a p p e a r a n c e of a gap. F o r the c o u p l e s y ~ / a 3 , VJo~2, and yx/o~s e x h i b i t i n g n o n p l a n a r o J f l i n t e r f a c e s , the d i f f u s i o n path c r o s s i n g s of the t w o - p h a s e ( a + ~) r e g i o n in F i g . 2 a r e f o u n d to TRANSACTIONS A

40~-

y phase y phase

*P= planar;NP = nonplanar; L = layered structure with (~ + '~) and 7 layersin the diffusion zone; T= two phase (a + 3') and (a + 3) layers between a and 3 phases. tThe direction from 7 ~ ~ -~ a is consideredpositive; the reverse direction is negative.

METALLURGICAL

~

/3layer 3 layer 3 layer 3 layer 3 layer 7 layer

NP; (+) NP; (+) NP; (+)

S e r i e s II: Y2 vs C u , or4, ors, Ni T h e d i f f u s i o n p a t h s f o r the c o u p l e s in S e r i e s II a r e p r e s e n t e d in F i g . 6 . All c o u p l e s in this s e r i e s d e v e l o p e d p l a n a r ~ / y i n t e r f a c e s a n d h e n c e the path c r o s s i n g s of t h e (~ + ~) r e g i o n m a y be a s s u m e d p a r a l lel to t i e - l i n e s . T h e c o u p l e y2//Cu d e v e l o p e d a p l a n a r c¢//3 i n t e r f a c e . T h e c o u p l e s 7 J ~ 4 and ~ J a s d e v e l o p e d w a v e l i k e , n o n p l a n a r a / ~ i n t e r f a c e s s i m i l a r to t h o s e o b s e r v e d f o r c o u p l e s y1//a3 a n d a f t / a s in F i g s . 3(c) a n d 4 ( c ) . C o r r e s p o n d i n g l y , the path c r o s s i n g s of the ( a + fl) t w o - p h a s e f i e l d f o r t h e 7 J a 4 a n d 7 J a 5 c o u p l e s a r e f o u n d to cut a c r o s s t i e - l i n e s . T h e y 2 / N i c o u p l e d e v e l o p e d a s t r u c t u r e s i m i l a r to that of c o u p l e y J N i ; two t w o - p h a s e ( a + fl') and ( a + ~) l a y e r s s i m i l a r to t h o s e s h o w n in F i g . 5(c) w e r e o b s e r v e d in the d i f f u s i o n zone. T h e m a r k e r p l a n e s w e r e f o u n d in t h e / 3 p h a s e r e gion f o r all c o u p l e s in S e r i e s II. T h e d e v e l o p m e n t of p o r o s i t y in the r e g i o n of the m a r k e r s w a s s e r i o u s f o r c o u p l e s y J o t 4 and ~2//a~ and w a s s i m i l a r to the obs e r v a t i o n s m a d e f o r c o u p l e s Y1/~3 and yl/Ots. VOLUME 8A, APRIL 1977-569

Y

"y !

Xm -

-× Ill \

(2

(a)

(b) f3

(c) Fig. 3--Diffusion zones of TI/Cu and ~'1/~3 couples in Series I annealed for 2 h at 775°C; (a) yl/Cu couple, magnification 34 times, (b) "y t/c~ couple, magnification 34 times, and (c)~/1/c~3 couple, magnification 217.6 times.

S e r i e s III: T3 v s Cu, ~4, ~5, Ni The diffusion paths for the couples in this s e r i e s are shown in Fig. 7. All couples developed planar /3/~ interfaces. However, nonplanar or//3 interfaces were observed for couples ~3/~4 and y~/c~5, while a planar ~//3 interface was seen for the couple y3/Cu. These observations a r e s i m i l a r to those made for couples in S e r i e s II. The m a r k e r plane was found within the/3 phase layer for the couples 7a/Cu and ),~/ot4, while it was located in the r r e g i o n for the couples ~3/~5 and r3/Ni. The diffusion zones of the couples y3//ot4 and T3/Ni are presented in Fig. 8. The path crossings of the two-phase (~ +/3) r e g i o n for couples ~/c~4 and ~a/~5 exhibiting nonplanar ~//3 interfaces are at an a n g l e t o the tie-lines; this implies nonequilibrium compositions on either side of the ~//3 interface. The p a t h crossings a c r o s s the (/3 + y) r e g i o n correspond to planar /3/r interfaces for all the couples in this s e r i e s . Diffusion Paths and Structures of y//3 Couples Unlike T/or couples, the ~//3 couples do not involve the development of a new phase; hence, t h e i r diffusion structures reflect only the diffusion path crossing 570--VOLUME 8A, APRIL 1977

of the two-phase r e g i o n (V +/3) of the ternary isotherm. The diffusion paths for the couple Series IV, V, and VI are presented in F i g s . 9, 10, and 11, respectively. In Fig. 9 the diffusion path for the couple ~ / 3 , d i p s from the ~, composition point into the two-phase (13 + ~) r e g i o n but e x i t s back into the ~ phase field giving rise to a two-phase (/3 + ~) l a y e r within the ~ phase layer; it then crosses the (/3 + ~) r e g i o n corresponding t o a planar/3/~ interface. The diffusion s t r u c ture of ~,//3~ couple is shown in Fig. 12(a). The r,//34 couple showed a planar ~//3 interface and its path could be consistent with a tie-line crossing a c r o s s the (/3 + ~) two-phase field. Both the couples in S e r i e s IV developed appreciable porosity in the r e g i o n of the m a r k e r planes found within the ~ layers. Couples ~2//33, ~2//3,, and y2//34 in S e r i e s V developed planar f3/y interfaces, while the ~'J/32 couple showed evidence of s m a l l perturbations at the/3/)~ interface° The diffusion zone of the couple ~2//3z is seen in Fig. 12(b). A transition from planar to nonplanar morphology and then a r e t u r n to planar morphology were exhibited by the 7///3 interface in this couple s e r i e s , as the composition of/3 terminal alloy was v a r i e d from the Cu-Zn binary side to the Ni-Zn binary side of the ternary isotherm. The m a r k e r planes were located within the/3 phase r e g i o n for the couples ~2//33 and ~'~///31 METALLURGICAL TRANSACTIONS A

¥

B+~f :il'~ . . . . . . : it.

/

,

_

¥

X m

-

×

m

Ca)

~b)

(c) Fig. 4--Diffusion zones of yl/ce2 and T1/c~5 couples in Series I annealed for 2 h at 775°C; (a) yl/o~e couple, magaaifieation 34 times, (b) ? 1/c~5 couple, magnification 68 times, and (c) 2/t/c~5 couple, magnification 217.6 times. and were found in the 7 phase for the couples 72/132 and Vz//34. /3 phase grew at the expense of -/phase in the couples -/2/fl3 and 7J/31, but was consumed by 7 in the couples -/2//32 and Y2//34. The couples y3///33 and Y3//3~ in S e r i e s VI developed planar/3/7 interfaces. For both couples the m a r k e r plane was located in the /3 phase, which grew at the expense of the y phase. Since equilibrium tie-lines were not determined in the (/3 + y) r e g i o n of the isotherm, the path c r o s s ings of the (fl + 7) r e g i o n for the various couples shown in F i g s . 9 through 11 cannot be compared directly with tie-lines. However, if l o c a l equilibrium is considered valid for planar interfaces, the path crossings of couples with planar / 3 / ' / i n t e r f a c e s would indicate the tie-line orientations within the (/3 + y) region. Comments on Interface Morphologies, Diffusion Paths and Marker Planes The -///or and ~,///3 couples investigated in this study exhibit certain trends in the development of diffusion METALLURGICAL TRANSACTIONSA

structures, interface morphologies and locations of m a r k e r planes. Regarding the development of the/3 phase layer between the terminal ~ and a alloys, planar/3/V and a//3 interfaces were favored for V/a couples set up with C u - r i c h ot alloys as observed in 71/Cu, y J C u , ~3/Cu couples. In these couples,/3 grew at the expense of b o t h o~ and ~ phases with the ~ phase b e i n g consumed f a s t e r than the ot phase. Also, zinc diffused from 7 to through the intermediate/3 layer, while copper diffused in the opposite direction. T h e r e was no nickel flux in the binary 7l//Cu couple but nickel diffused from to ot in the same direction as that of zinc in couples yJCu and v J C u . With the variation of the ~ terminal alloy from y~ t o y~, the growth of the/3 phase towards the V side was found t o d e c r e a s e by more than a factor of 2, while the corresponding/3 growth towards the ot side v a r i e d little. On the other hand, the development of a/3 phase layer with planar /3//T but nonplanar a//3 interfaces

was observed in couples r/

3,

72/ 4,

Jot4, and ~3//c~5. T h e s e couples were characterized by o~ t e r m i n a l alloys with compositions in the m i d r e VOLUME 8A, APRIL 1977 571

¥

8+'y

Y X m

(b)

(a)

~+~

ct it) Fig. 5--Diffusion zones of Yl/a~ and Yl/Ni couples in Series I anr,ealed for 2 h at 775°C; (a) y1/a~ couple, magnification 68 times, (b) Yl/Ni couple, magnification 217.6 times, and (c) y t / N i couple, magnification 1088 times. gion of the a phase field away from the Cu c o r n e r of the ternary isotherm. In t h e s e couples the Z n flow was from 7 t o a , while the Cu and Ni flows were from to 7 through the fl l a y e r . The Ni flow from a t o 7 observed in these couples exhibiting nonplanar a / ~ interfaces was in the direction o p p o s i t e t o that obs e r v e d in the 7z/Cu and va/Cu couples developing planar a/fl interfaces. The approximate ratios of t r a c e r diffusivities at 900°C for the various components as m e a s u r e d by Anusavice and DeHoffv in a Cu-Zn-Ni alloys are: D*Zn/D?Lu/D*Ni = 9 / 3 / 1 . T h e s e ratios are comparable t o the corresponding r a t i o s of atomic mobilities determined by Sisson4 in a as well as in/3 Cu-Zn-Ni alloys at 775°C with s i n g l e phase ternary diffusion experiments. S i m i l a r ratios of mobilities have been observed 8 for the components in 7 Cu-Zn-Ni alloys. Hence, in all the t h r e e phases, a , /3 and 7, nickel is considered to be the slowest diffusing component while zinc diffuses the fastest. Also, the mobilities of the components for ~ alloys4 are found to be two to t h r e e o r d e r s of magnitude l a r g e r than those for a alloys7 and about an o r d e r of magnitude l a r g e r than those~ for y alloys. The compositions of the a and/3 phases at planar 572-VOLUME 8A, APRIL 1977

r~/~ interfaces a g r e e d with those expected from equilib r i u m tie-lines within the (a + ~) two-phase region. On the o t h e r hand, the compositions at nonplanar interfaces differed from those of equilibrium and correspondingly, the diffusion path crossings of the (a + fl) region were at an a n g l e t o tie-lines for couples developing nonplanar a/fl interfaces. These observations can be appreciated qualitatively in t e r m s of the relative magnitudes of fluxes of the various components on either side of the a / ~ interface. On the b a s i s of mass balance for each component, the interface velocity Ca/fl is given by Zn

C zn-C n

Cu UH

-Cet J U

JflNi

Ni

C0.-C. 1'41 IN1

[11

where J ~ and ~a r e f e r t o the interdiffusion fluxes and C~ and Ca r e f e r t o the concentrations of each component in the ~ and ~ phases, respectively, at the a/fl interface. If the direction from ~ to a is defined positive, the experimental ¢a//~ is found t o be positive for all the ~ / a couples investigated in this study (Table II). As Ni diffuses considerably slower than either Zn or Cu in the a phase, Ca/~ b a s e d on fluxes METALLURGICAL TRANSACTIONS A

Zn

Y m

Q

,o ATOM % Cu

F i g . 6--Diffusion p a t h s for couple S e r i e s II at 775°C.

(a)

Zn

Y -

~ , ~ " ~ , _ ~ %

.~. /

\

/ Nit Fig.

-

,

\o

V

V

v

V

I0

20

50

40 50 60 ATOM % Cu

V

v

v

v

v

70

80

90

(b) Fig. 8--Diffusion zones of y~/a4 and 7jNi couples in Series III annealed for 2 h at 775°C; (a) ya/a4 couple, magnification 68 times, and (b) yJNi, magnification 217.6 times. \C

u

7--Diffusion p a t h s for couple S e r i e s III at 775°C.

of Zn or Cu tends t o be generally higher than that b a s e d on Ni fluxes. However, Eq. [1] needs t o be satisfied for all components by an appropriate choice~'1° of the diffusion path crossing a c r o s s the (a + 8) twophase field. Since Ni diffuses in the negative d i r e c tion (a ~/3) against the motion of the a/fl interface for the couples developing nonplanar morphology, a Ni build-up can be expected a h e a d of the moving interface in the a phase. For tie-lines in the r e g i o n of path crossings of the (a + fl) phase field in F i g s . 2, 6 and 7, it can be seen that C~i > C~i and for @~/~ t o be positive, IJ~, I has t o be g r e a t e r than IJ~. I. Since a Ni build-up on'the a side of the interface ~ s u l t s in an i n c r e a s e in C~i and a corresponding d e c r e a s e in IJ~i I, it is possible that for some couples no tie-line would be appropriate in satisfying Eq. [1] for all components including Ni. On the other hand, if the equilibrium constraint is r e l a x e d at the interface, a buildup of Ni in ot can make C~i g r e a t e r than CflNi and [JflNi [ METALLURGICAL

Xm

TRANSACTIONS A

can b e c o m e g r e a t e r than I J ~ i l . For the couples, ~1/ Or3, y J O t 2 , y2//Ot4, y2//tXs, y3//tX4 and y~/a~ that developed nonplanar a / ~ interfaces, it can be seen from F i g s . 2, 6 a n d 7 , C~n > C z n , Ccu > andC~qi >Cgi for the path crossings a c r o s s the ~- ~) two-phase region. Since Z n diffuses in the positive direction (i.e. a) and Cu and Ni m i g r a t e in the negative direction (i.e. ~ ~ 8.), a positive ~e/~ requires that I J ~ I > I J ~ - I , IJ~ I > IJ~ I and 170..I > I J ~ . l . T ~ s e i n equal~i~ies a ~ consistUent with t~e fact ~ a t the diffusivities of the components in the 8 phase are two to t h r e e o r d e r s of magnitude l a r g e r than those in the phase. For the couples v J C u and v J C u w i t h planar ~ / ~ interfaces, the path crossings of the (a + 8) r e g i o n (Figs. 6 a n d 7 ) s h o w C ~ >C~ , C~ >Cfl- a n d C ~ > C~i and t h e s e inequalities are consistent with those observed for tie-lines. On the b a s i s of Eq. [1] a positive ~a/R observed experimentally for t h e s e couples requires~that 15~ I >'15~ I, IJ~ I > IJ~ land 15~ I ~ n z C Cu Ni > [J~i [. It is interesting ~o note t~at these inequalities relating the magnitudes of fluxes are exactly the VOLUME 8A, APR1L 1977-573

Zfl

Zn

~. 40~

+- 47+ .,,' , - , ' , *\° /-k--' o@ so,~p,

o p

-

iZ3.

~

°\o

~. kso ÷

5oA

7 Ni

%

Cu

P

t

,

IB

~

0

U

",-,,,,~o ÷

Zn

5o-

,.z 20

5o

v 30

.

v 40 ATOM

v u 50 60 % Cu

v 70

V 80

v 90

\ Cu

Fig. 10--Diffusion paths for couple Series V at 775°C. same as those observed for couples that developed nonplanar ~//13 interfaces. Regarding the morphology of the 13//7 interface in 7 vs ct couples, it was found t o be always planar, if the diffusion structure showed the development of only the 13 phase layer within the diffusion zone. Instabilities at the 13/7 interface s e e m e d to develop only for couples with s m a l l differences in the copper concentration of the terminal 7 and ot alloys as in the case of couples ~ ' J ~ s and ~'J~6. The diffusion structures developed in these couples exhibited t h r e e adjacent layers of (13 + 7), 7 and/3 phases between the terminal phases (Figs. 4(b) and 5(a)). The formation of these layers can be appreciated in view of the fact that the Z n depletion of 71 alloy during diffusion can result in its enrichment in Cu as to make its composition fall within the (13 + 7) two-phase r e g i o n of the isot h e r m . This enrichment has t o be balanced out by a Cu drop in another section of the couple. Such d r o p s 574-VOLUME 8A, A P R I L 1977

/ t

v I0

v 20

v 30

v 40

v. v 50 60 ATOM % C u

v 70

v 80

v 90

"~Cu

Fig. ll--Diffusion paths for couple Series VI at 775°C.

Fig. 9--Diffusion paths for couple Series IV at 775°C.

" iO

/-J')"

7 ATOM

N~ ~

/

in Cu concentrations can be seen in the diffusion paths of the 7Jot5 and 7Jot6 and give rise to the formation of ~, layers within t h e i r diffusion zones. In the case of ~, v s /3 couples, all couples except ~,j13~ and 7 J f 1 2 developed a planar 13/7 interface and the path crossings of the (13 + 7) r e g i o n in F i g s . 9, 10 and 11, appear quite s i m i l a r for couples with the same fl terminal alloy. Most of these crossings seem close t o lines of constant Ni/Cu ratios; this is consistent with the fact that the diffusivity of Z n is considerably l a r g e r than those of Ni and Cu in both/3 and ¥ phases. A slightly perturbed f3//y interface was obs e r v e d only for the couple 72/t32 but no definite comments can be made about possible deviations of interface compositions from those of equilibrium, as no tie-line determinations were made within (13 + 7) r e g i o n of the isotherm. The couple 7j131 developed a two-phase (13 + 7) layer sandwiched by 7 layers on either side; this diffusion structure is s i m i l a r to the one observed for the 71/~s and 7u/or6 couples. It may be noted that the couples yt//13~, 7,/~5, and 7J~6 have s i m i l a r Cu concentrations in t h e i r t e r m i n a l alloys; t h e i r path crossings of the (13 + 7) region are also quite s i m i l a r . As indicated in Table II, m a r k e r planes were found in the 13 layer for most of the 7 vs ot couples; in such couples the velocity of 13//7 interface was appreciably l a r g e r than that of the m a r k e r plane, although both the interface and the m a r k e r plane moved in the n e g a tive (~ ~ 7) direction. In the case of ), v s 13 couples, the motion of the 13//7 interface was either in the positive or negative direction depending on the terminal alloys of the couples, while the m a r k e r planes were located either in the ~, or 13 phase. For the couples 7~//Ni and 7Jf34, the diffusion path segments within the 7 phase r e g i o n (Figs. 2 and 9) are s i m i l a r and indicate that the flow of Z n is uphill against its own concentration gradient on the 7 side of the couples. Hence, it is apparent that the thermodynamic activity of Z n within the ~, phase field appears to d e c r e a s e with an i n c r e a s e in Ni and a d e c r e a s e in Cu. In conclusion this study of multiphase diffusion with METALLURGICAL

TRANSACTIONSA

Y

C u - N i - Z n couples i n v o l v i n g two interfaces, a//3 a n d f3/~, h a s s h o w n t h a t b o t h i n t e r f a c e s c a n be p l a n a r , o r interracial instabilities can develop at either o r both o f t h e m d e p e n d i n g o n t h e t e r m i n a l alloys. T h e diffus i o n path c r o s s i n g s f o r p l a n a r a / / 3 i n t e r f a c e s a r e consistent with tie-line c r o s s i n g s , w h i l e those for n o n p l a n a r interfaces d e v i a t e f r o m tie-lines. Such d e v i a t i o n s m a y d e p e n d o n t h e d i r e c t i o n s of f l o w of t h e individual c o m p o n e n t s , t h e relative magnitudes o f t h e f l u x e s on e i t h e r side o f t h e i n t e r f a c e a n d t h e d i r e c t i o n o f m o t i o n of t h e i n t e r f a c e . D i f f u s i o n p a t h s a n d s t r u c t u r e s o f ~ vs c~ c o u p l e s a r e c o n s i s t e n t w i t h t h o s e o b s e r v e d f o r y v s {3 c o u p l e s .

-. B. 7 IIII

_

III

I1!

III

× In

ACKNOWLEDGMENT T h i s p a p e r i s b a s e d on a d i s s e r t a t i o n s u b m i t t e d by L . E . W i r t z t o P u r d u e U n i v e r s i t y in p a r t i a l fulfillm e n t of t h e r e q u i r e m e n t s f o r t h e d e g r e e o f M . S . i n Metallurgical E n g i n e e r i n g , T h e r e s e a r c h was supp o r t e d by c o n t r a c t E ( 1 1 - 1 ) - 1 4 3 6 w i t h t h e U.S. E n e r g y R e s e a r c h and Development Administration,

(a)

REFERENCES

B (b]

F i g . 12--Diffusion z o n e s of 71//31 and 7~//3 z c o u p l e s annealed for 2 h at 775°C; (a) 7t//31 couple, m a g n i fication 68 t i m e s , and (b) "Y~/132 couple, magnification 217.6 t i m e s .

METALLURGICAL

TRANSACTIONSA

1. C. W.Taylor, Jr., M. A. Dayananda, and R. E. Grace: Met. Trans., 1970, vol. 1, pp. 127-31. 2 . D. E. Coates andJ. S. Kirkaldy: Met. Trans., 1971, vol. 2, pp. 3467-77. 3 . R. D. Sisson, Jr. andM. A. Dayananda: Met. Trans., 1972, voL3 , pp. 647-52. 4 . R. D, Sisson, Jr.: Ph.D. Thesis, Purdue University, 1975. 5. G. L. Kehl: The Principles o f Metallographic Laboratory Practice, p . 420, McGraw-Hill, NewYork, N.Y., 1949. 6 . T. O. Ziebold and R. E. Ogilvie: Anal. Chem., 1964, vol. 36, pp. 322-27. 7. K J. Anusavice andR. T. DeHoff: Met. Trans., 1972, vol. 3 , pp. 1279-98. 8 . L. E. Wirtz: MS. Dissertation, Purdue University, 1975. 9. J. S. Kirkaldy: Can. J. Phys., 1958, vol. 36, pp. 907-16. 10. D. E. Coates: MeL Trans., 1972, vol. 3 , pp. 1203-12.

VOLUME 8A,APRIL 1977-575