MISORIENTATION DEPENDENCE OF DIFFUSION INDUCED GRAIN BOUNDARY MIGRATION (DIGM) IN ORIENTED Cu (Zn) BICRYSTALS

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MISORIENTATION DEPENDENCE OF DIFFUSION INDUCED GRAIN BOUNDARY MIGRATION (DIGM) IN ORIENTED Cu (Zn) BICRYSTALS

B. Giakupian, R. Schmelzle, W. Gust, R. Fournelle

To cite this version:

B. Giakupian, R. Schmelzle, W. Gust, R. Fournelle. MISORIENTATION DEPENDENCE OF DIF- FUSION INDUCED GRAIN BOUNDARY MIGRATION (DIGM) IN ORIENTED Cu (Zn) BICRYS- TALS. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-489-C1-494. �10.1051/jphyscol:1990176�.

�jpa-00230344�

COLLOQUE DE PHYSIQUE

Colloque Cl, supplement au nol, Tome 51, janvier 1990

MISORIENTATION DEPENDENCE OF DIFFUSION INDUCED GRAIN BOUNDARY MIGRATION (DIGM) IN ORIENTED Cu (Zn) BICRYSTALS

B. GIAKUPIAN, R. SCHMELZLE, W. GUST and R.A. FOURNELLE*

Max-Planck-Institut fur Metallforschung and Institut fur Metallkunde, Seestrasse 75, 0-7000 Stuttgart 1, F.R.G.

' Department of Mechanical ~ i i ~ i n e e r i n g , Marquette University, Milwaukee.

W I 53233, U.S.A.

Abstract

-

D i f f u s i o n induced g r a i n boundary m i g r a t i o n (DIGM) has been s t u d i e d i n t h e Cu(Zn) system u s i n g Cu b i c r y s t a l s w i t h i n i t i a l l y symmetrical t i l t g r a i n boundaries (GBs) o f t h e

<011> (011) type produced by d i f f u s i o n bonding. The mi s o r i e n t a t i o n has been v a r i e d syste- m a t i c a l l y over t h e range from 10.1 t o 171.9'. Measurements o f t h e GB v e l o c i t y showed i t t o have a s t r o n g dependence on t h e m i s o r i e n t a t i o n angle. The morphology o f DIGM has been s t u d i e d by o p t i c a l microscopy. As a s p e c i a l morphological phenomenon, f a c e t i n g has been i n - v e s t i g a t e d a t a symmetrical E 19a/26.52' t o l l > (011) GB.

1. I n t r o d u c t i o n

-

Under c e r t a i n c o n d i t i o n s G B r f i n a pure metal can be caused by t h e d i f f u s i o n o f a s o l u t e element i n t o t h e metal along t h e g r a i n boundary. As t h i s occurs an a l l o y e d zone (AZ) forms behind t h e m i g r a t i n g GB as t h e r e s u l t o f t h e s o l u t e atoms i n c o r p o r a t i n g them- selves i n t h e zone l e f t behind t h e m i g r a t i n g boundary [ l - 4 1 . These zones can develop i n t h e form o f bowed boundary segments d u r i n g DIGM a t various places along t h e GB. These bows i n t h e g r a i n boundary g e n e r a l l y d i s t r i b u t e themselves i n b o t h d i r e c t i o n s i n t h e g r a i n i n t e - r i o r s . Further, w i t h r e f e r e n c e t o t h e morphology, both an o r d i n a r y and an o s c i l l a t i n g GB m i g r a t i o n i n which t h e g r a i n boundary m i g r a t e s e i t h e r forwards o r back and f o r t h respec- t i v e l y , i s observed f o r DIGM. The t r a n s i t i o n from o r d i n a r y t o o s c i l l a t i n g GB m i g r a t i o n occurs a t T n 770 K i n t h e Cu(Zn) system.

Closely r e l a t e d t o DIGM i s another metal l u r g i c a l process, d i f f u s i o n induced r e c r y s t a l l i z a - t i o n (DIR), a r e a c t i o n i n which f i n e g r a i n s w i t h increased f o r e i g n atom c o n c e n t r a t i o n nucleate on t h e specimen s u r f a c e and grow according t o t h e DIGM mechanism. The growth o f t h e DIR g r a i n s i n t h e z d i r e c t i o n perpendicular t o t h e specimen s u r f a c e i s e s s e n t i a l l y d e t e r - mined by t h e annealing t i m e t. I n a d d i t i o n t o DIR, f a c e t f o r m a t i o n a t v a r i o u s GBs i n t h e surface r e g i o n (z < 5 p) a l s o occurs. For t h i s phenomen t h e segments which have migrated appear t o a l i g n themselves on c e r t a i n c r y s t a l l o g r a p h i c planes.

2. E x ~ e r i m e n t a l Methods - The m a t e r i a l s used t o make t h e s y n t h e t i c a l l y prepared b i c r y s t a l s were s i n g l e c r y s t a l s o f h i g h p u r i t y Cu (5N8), which were grown by t h e Bridgman technique.

The s i n g l e c r y s t a l s were s t r e s s r e l i e f annealed f o r 15 hours a t 1273 K a f t e r being grown from t h e m e l t . Subsequently, they were o r i e n t e d by t h e Laue back r e f l e c t i o n technique and c u t i n t o 3 mm t h i c k s l i c e s by spark erosion. A f t e r a p p r o p r i a t e p r e p a r a t i o n t h e s l i c e s were d i f f u s i o n bonded a t 1323 K f o r 15 hours. Rectangular samples w i t h t h e dimensions 4.5 X 3.0 X 2.5 mm3 were then c u t o u t p a r a l l e l t o t h e t o l l > d i f f u s i o n d i r e c t i o n . With t h i s procedure w e l l d e f i n e d b i c r y s t a l s o f t h e <011> (011) t y p e were prepared, where <011> i s t h e t i l t a x i s and (011) a r e t h e planes between which the t i l t angle 0 i s measured. The angle 0 was s y s t e m a t i c a l l y v a r i e d over t h e e n t i r e range from 10.1 t o 171.9'. Before being g i v e n t h e DIGM anneal, t h e b i c r y s t a l s were again s t r e s s r e l i e f annealed a t 1223 K i n o r d e r t o avoid GB m i g r a t i o n due t o i n t e r n a l stresses. For t h e DIGM anneal t h e samples along w i t h t h e Zn source, which was i n t h e form o f t u r n i n g s o f a Cu-30 wt.% Zn a l l o y were placed i n an evacua- t e d q u a r t z capsule. An undesired uptake o f 0, d u r i n g t h e experiment was avoided by p l a c i n g a Ta f o i l , h i g h p u r i t y g r a p h i t e and an Ar-20% H, m i x t u r e i n t h e r e a c t i o n capsule. A f t e r t h e DIGM anneal t h e samples were quenched i n water.

Metal l o g r a p h i c specimen p r e p a r a t i o n proved t o be d i f f i c u l t , on t h e one hand, because t h e s i n g l e c r y s t a l Cu m a t e r i a l was v e r y s o f t and, on t h e o t h e r hand, because o f t h e r e l a t i v e l y s l i g h t p e n e t r a t i o n o f DIGM i n t o t h e sample i n t e r i o r (p range). The f o l l o w i n g metallographic specimen p r e p a r a t i o n procedure proved i t s e l f t o be t h e most s u i t a b l e . F i r s t , t h e specimens were ground w i t h 4000 g r i t S i c paper i n such a way t h a t no g r e a t pressure was a p p l i e d t o t h e sample. P o l i s h i n g was c a r r i e d o u t f i r s t w i t h 3 and then w i t h 1 pn diamond paste under l i g h t pressure u s i n g a v e l v e t c l o t h . Subsequently t h e samples were etched w i t h a s o l u t i o n c o n t a i n i n g 1.5 m1 d i s t i l l e d water, 2 m1 conc. NH OH and 0.25 m1 aqueous H,O, s o l u t i o n f o r about 5

-

10 S. Here t h e etchant always had t o be f r e s h l y prepared.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990176

cl-490 COLLOQUE DE PHYSIQUE

Since the dependence of GB migration on the depth z below the specimen surface is of special interest, an exact determination of the depth was carried out. This was successfully done with the aid o f a Vickers hardness impression, which was placed in the center of the speci- men before the p01 ishing procedure. From the difference between the diagonals o f the hard- ness impressions before and after the polishing procedure (d, , ^d,) the depth z could be exactly determined using the re1 ation,

z =

0.143 (d, - ^d,),

which is based on the geometry o f the Vickers 136' pyramidal indentor. Light microscopy (LM) proved to be more than adequate for the study of the morphology of DIGM and DIR as well as for the measurement o f GB migration distances.

3. Exoerimental Results - For the study of the morphology and kinetics of DIGM metallo- graphic specimens of surfaces parallel and perpendicular to the toll> diffusion direction were prepared. On all specimens studied a fine grained DIR layer interdispersed with twins was observable after the DIGM anneal at T

=

693 K for various annealing times t. This layer (Fig.1) covered the entire specimen surface. Electron microprobe studies showed the DIR grains on the specimen surface to have a Zn concentration of about 22 wt.% Zn. For further investigation this fine grained layer had to be removed by p01 ishing. When this was done the surface layer and thus the number o f DIR grains decreased with increasing depth below the specimen surface until finally only the DIGM reaction region was observable. In the region lying between the surface with 100% DIR and the depth at which no DIR is observed DIR grains as well as DIGM reaction seams exhibited approximately the same Zn concentration.

The reaction fronts (RFs) migrated randomly into both crystal halves, which indicates in general that the GBs were highly symmetric (Fig.2). Metallographic specimens perpendicular to the toll> directions exhibited depth profiles o f GBs, which had migrated, having three different forms (Figs.3a-c).

Fig.1 DIR. Entire sample surface covered Fig.2 DIGM. GB segments migrate randomly with fine grained alloy layer. into both crystallites. 244 h at

1030 h at 693 K. 693 K, z

= 35 m.

Fig.3 Three different shapes o f DIGM depth profiles: concave (a), straight (b) and

convex (c). RF

-

reaction front,

AZ

- alloyed zone. 503 h at 693 K.

For the determination o f the GB migration rate v, W-t diagrams with

W

as the average seam width along the GB for constant z were drawn. The slopes of the resulting straight lines corresponds to the steady state migration rate v for a given depth z (Fig.4). In addition, for the determination of the depth dependence of v, W-z diagrams for constant annealing times t were drawn (Fig.5). The W-t curves obtained with their aid (see Fig.4) show that v can be viewed as constant over the entire depth range studied. The dependence of the v values determined in this manner on the misorientation angle

B

is presented in Fig.6.

Additionally, concentration profiles were determined which gave the Zn concentration as a function o f the distance to the specimen surface. With the aid o f wave length dispersive X-ray analysis the Zn content in the reaction regions was determined for the various depths.

The concentration profiles show an exponential decrease with increasing depth (Fig. 7).

Within a seam no regularity in the concentration distribution was observed. The grain bound- ary diffusivity was determined using the mass balance equation [5] which yields the follow- ing analytical relationship:

log

X

(z)

=

log

xo

- ¹ p z.

mb ^lnlO

In this equation x(z) is the solute atom concentration immediately behind the GB at a depth z,

X

is the solute atom concentration in the seam at the surface, v is the migration rate,

S

is ?he segregation factor,

6

is the width of the GB and D is the grain boundary diffusion coefficient. The dependence of the s6D, values determined Prom Eq.(l) on the misorientation angle

B

is also given in Fig.6.

The formation of the facets presents a special case of morphological development in which the final position o f grain boundaries, which migrated during the formation of DIGM seams, corresponds to special crystallographic planes. This phenomenon is especially striking for the

C

19a/26.5Z0 toll> (011) boundary, for which facets can even be observed on the specimen surface. Special studies of this phenomenon have shown that the marked triangular shape of the facets quickly is lost through a rounding-off of the apex of the triangles with increas- ing depth z

(0 <

z < 2

pm)

below the surface. Below z

S

6

pm

facets cannot be distinguished from ordinary DIGM seams (Fig.8).

Fig.4 Seam widths as a function of annealing time at 693 K for various depths below the specimen surface.

C

33a/20.05' <011>

(011) GB.

Fig.5 Seam widths as a function of the depth z below the specimen surface at 693 K.

C

33a/20.05' toll> (011) GB.

_{6 0}

CI-492 COLLOQUE DE PHYSIQUE

8

,693 K D i f f

.

U < O l l >

2

6

0 0 30 60 90 120 150 18 O [ d e g l

Fig.6 Migration velocity as well as grain boundary diffusivity as a function of misorienta- tion angle at 693

K.

The diffusion takes place parallel to the toll> tilt axis.

2 0 . 0 5 ~ < 0 1 1 > {OII} -

X o = 2 2 . 4 w t . % Zn

-

A A

e x p

- c a l c

-

0 10 20 3 0 40 50

z [ p m 1

Fig.7 Zn concentration profile. C 33a/20.05" toll> (011)

GB,

240 h .at 693

K.

Fig.8 Facets. Change of the morphological appearance of facets with depth below the speci- men surface. C 19a/26.52" <011> COll) GB, 3 h at 693 K.

4. Discussion - Because the DIR layer penetration into the specimen interior to some depth z does not allow the observation of the DIGM reaction region and thus presents a complicating factor for DIGM measurements, an attempt was made to suppress DIR. After the sample prepara- tion described in Section 2 the bicrystals were additionally chemically polished for various lengths of time (t

=

5 - 20 min) with a solution of equal volume percentages of H,PO,, HNO, and CH3COOH before the DIGM anneal. As a result it was found that each GB type exhibited very different etching behavior. In contrast to

C

19a-GBs grooves could be observed to form quickly (t

=

5 min) at C 9-GBs. This made further use of these boundaries impossible. For boundaries which showed no groove format.ion after 20 min, it was possible in our studies to suppress the DIR reaction up to an annealing time of 24 h maximum whereas Chen and King

[6]

did not observe any DIR even after annealing for

48

h.

In Fig.5 the measured W-z values were described by a power function because nearly all pro-

files exhibited the shape presented in Fig.3a. For the evaluation of the w-t curves (Fig.4)

the W-z data for t

=

51 h were disregarded because the determination of the W-t dependence

over a larger temperature range could not be carried out with them. The velocities v ob-

tained from the slopes were constant considering the measurement uncertainty for the depths

z studied (about

t

0.2

pm).

The dependence of v on

B

presented in Fig.6 shows clearly that

GBs in the small angle region

( B 1 10')

did not exhibit any migration until after annealing

for times of 5000 h. In the high angle region the C 3 twin boundary likewise shows no migra-

tion tendency after 5000 h. This is probably due to the short periodicity distance of its

structural units and the low energy associated with it. This structural influence of the C 3

Cl-494 COLLOQUE DE PHYSIQUE

s t r u c t u r a l u n i t s determines l i k e w i s e t h e m i g r a t i o n behavior o f GBs whose m i s o r i e n t a t i o n angle B d e v i a t e s up t o about 5 10' from t h a t o f t h e C 3 GB. Analogous t o those i n t h e small angle r e g i o n s GBs w i t h B 1 171' a l s o do n o t e x h i b i t any tendency t o m i g r a t e a f t e r t = 5000 h.

As can be seen i n Fig.6 t h e c a l c u l a t e d s6Db values, w i t h t h e exception o f t h e value f o r t h e 45" <011> (011) GB, behave analogously t o t h e v values i n t h e i r v a r i a t i o n w i t h t h e t i l t angle B. However, w h i l e t h e expected cusp [l] i n t h e v-B curve a t B = 38.94' ( C 9) c o u l d n o t be seen, i t i s c l e a r l y v i s i b l e i n t h e s6Db-B curve. Further, t h e v e r y h i g h values o f s6Db and v f o r t h e 141.06' (C 9) t o l l > (011) GB a r e v e r y n o t i c a b l e .

The measured Zn concentrations, (Fig.7), which are

<

1.5 wt.% Zn, a r e on t h e same order o f magnitude as those g i v e n i n Ref.[8]. They are, however, small compared t o those i n Refs.[9,10]. On t h e one hand, t h e d i f f e r e n t t i l t axes and, on t h e o t h e r hand, t h e d i f f e r e n c e i n b i c r y s t a l f a b r i c a t i o n techniques may have c o n t r i b u t e d t o t h e d i f f e r e n t observations.

The o r i g i n o f f a c e t s (Fig.8) presents an i n t e r e s t i n g morphological phenomenon, whose o r i g i n s and causes cannot y e t be completely explained. Short t i m e experiments (t = 30 min) have shown t h a t t h e f a c e t s already possess t h e i r c h a r a c t e r i s t i c t r i a n g u l a r shape i n t h e very e a r l y stage o f annealing. M e t a l l o g r a p h i c s t u d i e s on t h e same g r a i n boundary have shown t h a t t h e r e i s a unique angle a t t h e apex o f t h e t r i a n g l e (a

=

^85'). Nevertheless i t has n o t y e t been e x p l a i n e d i f d i f f e r e n t h a b i t planes o r i f planes o f t h e same t y p e c u t t h e t o l l > speci- men surface.

5. Conclusions

-

The r e s u l t s can be summarized as f o l l o w s : (1) DIR c o u l d n o t be supressed a t t h e l o n g e r annealing times.

(2) The GB m i g r a t i o n r a t e v as w e l l as t h e GB d i f f u s i v i t y s6Db a r e s t r o n g l y dependent on t h e m i s o r i e n t a t i o n angle B and v a r y analogously w i t h B .

(3) The Zn c o n c e n t r a t i o n i n t h e r e a c t i o n r e g i o n s decreases exponenti a l l y w i t h i n c r e a s i n g depth.

(4) Strong t r i a n g u l a r f a c e t i n g occurs f o r t h e C l9a/26.5Ze t o l l > (011) GB.

References

F.S. Chen and A.H. King, S c r i p t a Met. 20 (1986) 1401.

L i Chongmo and M. H i l l e r t , Acta Met. 30 (1982) 1133.

J.W. Cahn, J.D. Pan and R.W. B a l l u f f i , S c r i p t a Met. 13 (1979) 503.

F.J.A. den Broeder, T h i n S o l i d Films 124 (1985) 135.

R.A. Fournelle,

B.

Giakupian, W. Gust and B. Predel, J. Physique C5 49 (1988) 593.

F.S. Chen and A.H. King, S c r i p t a Met. 21 (1987) 649.

P.H. Pumphrey, i n : Grain Boundary S t r u c t u r e and Properties, G.A. Chadwick and D.A.

Smith (eds.), Academic Press, London (1976) 139.

[8] Z.M. Guan, G.X. L i u , D.B. W i l l i a m s and M.R. N o t i s , Acta Met. 37 (1989) 519.

[g] A.H. King, F.S. Chen, G. D i x i t and A.J.Jr. Aldykiewicz, Mater. Sci. Forum (1989), i n press.

[ l 0 1 F.S. Chen and A.H. King, Acta Met. 36 (1988) 2827.

Acknowl edqements

One o f t h e authors would l i k e t o acknowledge t h e support o f N a t i o n a l Science Foundation Grant DMR-8508736 f o r support d u r i n g t h i s study.