EFFECT OF THE TRIPLE JUNCTION ON GRAIN-BOUNDARY SLIDING IN ALUMINUM TRICRYSTALS

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EFFECT OF THE TRIPLE JUNCTION ON GRAIN-BOUNDARY SLIDING IN ALUMINUM

TRICRYSTALS

S. Miura, S. Hashimoto, T. Fujii

To cite this version:

S. Miura, S. Hashimoto, T. Fujii. EFFECT OF THE TRIPLE JUNCTION ON GRAIN-BOUNDARY

SLIDING IN ALUMINUM TRICRYSTALS. Journal de Physique Colloques, 1988, 49 (C5), pp.C5-

599-C5-604. �10.1051/jphyscol:1988575�. �jpa-00228072�

EFFECT OF THE TRIPLE JUNCTION ON GRAIN-BOUNDARY SLIDING IN ALUMINUM TRICRYSTALS

S. MIURA, S. HASHIMOTO and T.K. FUJI1

Department of Engineering Science, Faculty of Engineering, Kyoto University, Kyoto 606, Japan

Abstract - Aluminum tricrystals were crept at 773K under a constant stress in order to investigate the effect of the triple junction on high temperature de- formation. It has been demonstratedthat grain-boundary sliding (GBS) can be accommodated by sliding and/or migration along other boundaries, but non-uni- form sliding due to the existence of the triple junction gives rise to the localized deformation zone along the boundary. In certain cases, this could be accommodated by the so-called triple-point fold without significant plastic deformation interior the grain. The origin of the fold was found to be a lat- tice slip, whose activated system has to contain the triple junction, but not a deformation band.

Sliding at grain boundaries is important as a mode of deformation of metals at high temperatures. The sliding is generally restricted at the concurring node of three grain boundaries, (i.e. the triple junction) as well as at grain-boundary ledges. This gives rise to a constraint of plastic compatibility around the triple junction. In certain cases, the plastic incompatibility at the triple point due to grain-boundary sliding can be accommodated by a localized sheared deformation zone, the so-called fold in ductile materials /1-5/, which is in contrast with a wedge- shape crack in brittle materials /6-8/.

We have recently reported that the fold is well characterized as slip band, whose activated systems of types {111]<110> or {100)<110> containing the triple jun- ction /9,10/. In this paper we consider the conditions required to the fold for- mation and equilibrium among grain-boundary slidings at the triple junction, by presenting experimentally observed evidence in 2 randomly oriented rricrystal and

<llO>-tilt tricrystals containing two X=3(111) and X=Y(411) CSL boundaries.

2.-EXPERIMENTAL PROCEDURE

The material used was aluminum of 99.999% purity. Tricrystals composed of columnar grains were vertically grown by the Bridgman method. The precise descrip- tion about the preparation of tricrystal specimen and the creep experiment were re- ported elsewhere /9,10/. In the present experiment two types of tricrystals were examined. The first tricrystal is a so- called randomly oriented tricrystal, whose orientation relationships between grains and grain boundary planes will be shown later in Fig.2(a). All of the boundary could not be specified as CSL boundaries with X<101 using the deviation-angle cri- terion by Brandon /11/.

The second type of tricrystal con- tains Xs=3, 3, and 9 CSL boundaries,

(Type C/9,10/), as schematically shown in Fig.1. Deviation angles of boundaries a, b and c from the ideally oriented CSL models were estimated to be 6, 5.5 and 0.5 degrees, respectively. Inclinations from the symmetrical position of these boundaries were 4, 0.3 and 8 degrees, respectively.

Fig.1 Schematic representation of a tricrystal (Type C ) contain- ing <llO>-tilt two X=3{lll} and X=9{411} boundaries. Dotted lines show possible fold planes of types {Ill} and {loo}. "ow

and

"+"

points of the abab.. stacking layer.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988575

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3.--RESULTS AND DISCUSSION 3.1. Rand.om Tricrystal

The displacement along grain bounda- ry, b was transmitted to the other two boundaries. Neither the fold nor the prominent intragranular plastic defor- mation zone were found out in the whole grains judging from the linearity of scribed markers as shown in Fig.2. Al- though possible (100) and (111) planes containing the triple junction exist in grain 1, no triple-point fold was formed.

Fig.2 Grain.-boundary sliding around the triple-point in a ,'random" tricrystal.

(a) Stereographic projections of <loo>-poles of each grain. (b) Triple- point before a creep deformation. (c) Grain-boundary sliding and migra- tion after 17.6Ks under a constant stress of O.6MPa at 773K.

Fig.3 Restriction of grain-boundary sliding S(b) near the triple point and projected am-ount of S1(a), S'(c) onto the extended line from bo in a "random" tricrystal.

The amount of GBS along boundary b, S(b) was restricted near the triple junc- tion inside a 0.5mm area, as shown in Fig.3. The amount of GBS along boundaries, a and c, S(a) and S(c) decreased with increasing the distance from the triple junc- tion. If a crack is not formed at the junction, the equilibrium of displacement is satisfied. The relation among them can be written as,

lim S(b) = lirn S' (a)

+

,

Dto P O W O

where S'(a) = S(a)cosOa and S'(c)=S(c)cosOc, and D is distance from the triple junc- tion, and 8, and Oc angles between extended lines from % and the corresponding boundaries. At the triple junction, the S ( b ) is almost equal to the sum of S'(a) and S'(C), as can be seen in Fig.3. It is confined that the equilibrium of displace- inent was satisfied at the junction.

3.2 Zs=3, 3, 9 (Type C ) Tricrystal

Creep tests were conducted on the specimens cut from a Type C tricrystal in or- der to examine the influence of the tensile axis, grain-boundary respective ori- entations and crystallographic orientations of grains on the deformation behaviour.

The prominent triple-line folds were formed in the grain 2 and grain 3 which are situated against the slid boundaries (a) and (c) of specimens n0C1-1 and n°C1-2.

The activated fold planes were identified as the (111)-type planes as indicated by dotted line in Fig.1. It has been reported by the present authors /9,10/ that the origin of fold is a lattice slip but not a deformation band as suggested by Gifkins /3/ and Friedel /13/. Activated fold planes are found to be {111} or {loo} contain- ing the <llO>-triple junction, at least, in two types of tricrystals having Xs=3(111), 3(111) and 9(122) or (114) in our experiments.

The fold in grain 2 of tricrystal n°C1-1 was found to diffuse toward the grain boundary, b. This may be attributable to the climb and glide motions of dislocations emitted from the triple junction. The shearing displacement of folds, S(fl), S(f2) decreased with increasing the distance from the junction, as will be shown in Fig.5 quantitatively, and finally the folds faded out.

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F i g . 4 Dependance of l o a d i n g a x i s ( p a r a l l e l t o t h e v e r t i c a l s c r i b e d markers) on grain-boundary s l i d i n g behaviours i n t h e Cs=3, 3, 9 t r i c r y s t a l s .

( a ) and ( b ) show t r i p l e - p o i n t f o l d formations i n g r a i n s G2 and G3 i n t r i c r y s t a l s n°C1-1 and n°C1-2, r e s p e c t i v e l y . ( c ) shows no f o l d formation i n g r a i n G 1 due t o t h e absence of a p o s s i b l e system a s t h e f o l d , i n n°C1-3. Note t h a t no displacement of t h e t r i p l e p o i n t and t h e remarkable p l a s t i c zone along boundaries were observed i n c o n t r a s t w i t h t h e former specimens.

grain boundary, b slides predominantly as the result of a high shear stress on the boundary, because the possible slip systems to produce folds do not exist in the grain. The hypothesis is found to be valid from the ex- periment of tricrystal n°C1-3 in which the folds are not observed under the stress level of O.GMPa, as shown in Fig.4(c). In this specimen the intra- granular plastic deformations were remarkable in the vicinity of boundary, a in grain 2 and in the vicinity of boundary, c in grain 3. This is due to the complete restriction of GBS judging from the no offset of a scribed marker across the boundary just near the triple junction.

We have reported that the pref- erential slip planes as fold may be predicted by the concept of the stress transmission factor, proposed by Livingston and Chalmers /14/ for inter- preting the induced slips at the grain

the possible slip systems to produce the folds. The asterisks indicate observed systems.

T~sf=("-"f,

(gs'gf)+($s.g7) (if -9S)

where 5 ^and

g

and "f" refer to GBS and lattice slip to produce a,fold, respectively.

Grain

G 2

G 3

Fig. 5 Restriction and continuity of grain boundary sliding at the triple point in tricrystal n°C1-1.

Slip System [I101

*

(171) [ ~ I I [loll

*

[oi11 (loo) [Oil]

I

7101 *

( u i ) [OIII 11011

*

toi11 (loo) [Oll]

~ s f l 0.23 0.01 0.24 0.80 0.03 0.19 0.04 0.21 0.84 0.02

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boundary in a deformed bicrystal. Although the calculated Nsf values for the {111)- slips are lower thanthosefor the {loo}-slips, only the {llll-fold could be observed in the specimens n°C1-1 and n°C1-2, as listed in Table. The Nsf value ratio of the 1100)-slips to the (111)-slips was found to be 3.5, which may glve the degree of dif- ficulty in activating slips on the {loo}-plane.

GBS along boundaries a and c decreased with decreasing distance from the triple point with comparable gradients to that measured in the random tricrystal (see Fig.5).

It is worth noting that the junction moved (about 12um) from the initial position in the specimen n°C1-1, as has been also observed in the random tricrystal. This is in contrast with no displacement of the junction in the same tricrystal (n°C1-3) crept by a mode giving rise to no fold formation. The equilibrium and continuity of displacements at the junction are valid taking the contribution of shearing by the folds into account, as can be seen in Fig.5.

It is shown that GBS can be accommodated by sliding and/or migration along other boundaries without a significant plastic deformation, but a strongly restrict- ed sliding at the triple junction gives rise to a remarkable deformation zone. The plastic compatibility requirement can be satisfied by the formation of fold if the possible slip systems to produce folds containing the triple junction exist in the grain interior against the slid grain boundary.

The autho:rs wish to thank the Sumitomo Chemical Industries Ltd. for supplying materials. The financial supports given by the Light Metal Educational Foundation of Japan and the Ministry of Education, Science and Culture of Japan, as the Grant- in-Aid for Scientific Research, are also acknowledged.

REFERENCES

W. Betttsridge and S.W. Franklin, J. Inst. Metals 80, 147 (1951-52).

D. McLetnn, Ibid.

81,

R.C. Gifkins, Ibid.

82,

R.C. Pond, D.A. Smith and P.W.J. Southerden, Phil. Mag.

2,

T.G. Langdon and R.C. Gifkins, Acta Met.

21,

H.C. Chiing and N.G. Grant, Trans. AIME

206,

J.A. Williams, Acta Met.

15,

H.E. Evans, Mechanisms of Creep Fracture, p.163, Elsevier Applied Science Pub., London (1984).

S. Hash:imoto, T.K. Fujii, H. Fujii and S. Miura, Grain Boundary Structure and Reltrted Phenomena, Proc. JIMIS-4, p.921, Suppl. Trans. JIM (1986).

S. Hashimoto, T.K. Fujii and S. Miura, Scripta Met.

21,

D.G. Brandon, Acta Met.

14,

P. Lagarde and M. Biscondi, Can. Met. Quart.

13,

J. Friedel, Dislocations, p.319, Pergamon Press, Oxford (1964).

J.D. Livingston and B. Chalmers, Acta Met.

5 ,