HIGH RESOLUTION ELECTRON MICROSCOPY OF GRAIN BOUNDARIES IN fcc AND bcc METALS

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GRAIN BOUNDARIES IN fcc AND bcc METALS

H. Ichinose, Y. Ishida

To cite this version:

H. Ichinose, Y. Ishida. HIGH RESOLUTION ELECTRON MICROSCOPY OF GRAIN BOUND- ARIES IN fcc AND bcc METALS. Journal de Physique Colloques, 1985, 46 (C4), pp.C4-39-C4-49.

�10.1051/jphyscol:1985403�. �jpa-00224646�

JOURNAL DE PHYSIQUE

Colloque C4, suppldment au n04, Tome 46, avril 1985 page C4-39

HIGH RESOLUTION ELECTRON MICROSCOPY OF GRAIN BOUNDARIES IN f c c AND bcc METALS

H. Ichinose and Y. Ishida

InsCitute of Industria2 Science, University of Tokyo, 7-22-1 Roppongi Minato-ku, Tokyo, Japan

ABSTRACT

HREM observation of small and high angle boundaries was performed in gold and iron in order to assess the influence of the crystal structure. A small angle boundary consisting of dislocation arrays was usually inclined with respect to'the foil surface. High angle bounda- ries were mostly vertical. Atomic structure of the ordered boundary did not agree with details of the geometrical model. Lattice .relaxa- tion was evident at the boundary. The regularity of atomic structure of CSL boundary decreased with increasing the missorientation from the perfect CSL orientation relationship. A low indices crystal plane was frequently parallel to the boundary plane in both fcc gold and bcc iron. The plane matching boundary was abundant at, or near, the rota- tion angle of high sigma CSL boundary in iron.

I. INTRODUCTION

Due to the absence of a suitable technique to observe the atomic structure of grain boundary, theoretica1,or rather geometrical model studies have been advanced(1)-(3). Reflecting the historical circumstances experimental grain boundary structure study by electron microscopy has also been based mostly upon coincidence-site-lattice (CSL) boundary model. The electron microscopic investigation,there- fore, tries to confirm the structural models first. New discoveries,- however, have been met especially during HREM observation in re- cent(4)-(6).

In the present paper certain structural features of the CSL type boundary not expected theoretically but observed experimentally is reported in fcc and bcc metals. The HREM observation of iron grain boundary showed certain structural features of bcc metal which is different in nature from that of fcc metals.

11. EXPERIMENTAL

Specimens observed were gold and iron prepared in the way described below.

a) Gold: A (110) surface of rock solt was polished and gold was evaporated on it at 670K and vacuum of 5x10-6torr. A policrystalline foil grew epitaxially on salt. Gold crystals in the foil were nearly paralell to (110). Most higher angle grain boundaries in the foil were pure tilt type with 110 rotation axis making the observation easy.

The thickness of the foil did not exceed 2 0 m .

b) Iron: Policrystalline thick foils, about 100 micro meter, were made by vacuum evaporation on (110) surface of a rock salt.The grain Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985403

diameter was 50nm on avarage so that the influence of the substrate orientation was small in the central layer of the thick foil. After annealing the thick foil at 870K for lhr, the specimen was thinned electrolitically from both sides. Some of the specimens were thinned by ion bombardment and observed by HREM.

c) HREMs employed were JEM-1000 high voltage high resolution electron microscope at Tohoku university and JEM-200CX HREM at JEOL.

Lens constants Cs and Cc were llmm and 25mm for JEM-1000 and 0.8mm and 1.2mm for JEM-200CX respectively. The beam divergence was less than 5x10-4 rad and the electrical stability was below 2x10-6.

111. RESULTS AND DISCUSSION

111-1 GRAIN BOUNDARIES IN A FCC METAL 111-1-1 SMALL ANGLE'BOUNDARY

Boundaries having appeared most frequently in statistics were, small angle boundaries which consisted of arrays of lattice dislocations.

Photo 1 shows a part of a small angle boundary with the rotation angle of 8 degree about <loo> axis. The Burgers vector is b=a/2<110>

which inclined by about 30 degrees with respect to the foil surface.

Photo l.A small angle boundary consisting of dislocations.

Diffused image like a tail of the comet on the left hand side of a mark indicates a trace of inclined dislocation line.

Letters b,u and R indicates the Photo 2. A small angle boundary trace of the Burgers vector still consisting of array of ,the dislocation line and the lattice dislocations with the grain boundary respectively. tilt angle beyond 15 degrees.

~etermination of the Burgers vector is easily done by conecting white spots representing lattice points on the photograph. The diffused lattice image seen left wardly from the dislocation mark like a tail of the comet appeares to be the trace of the dislocation line.

The dislocations and hence the boundary plane was not perpendicular but inclined with respect to the foil surface. The inclined boundary was common with small angle boundaries in contrast to high angle boundaries which seldom inclined to the foil surface.

Another feature of the small angle boundary is its range. As shown in Photo 2 boundaries consisted of arrays of. lattice disloca- tions even though the rotation angle exceeded the commonly considered critical angle of 15 deg.

111-1-2 LATTICE RELAXATION OF A HIGH ANGLE BOUNDARY

The 2=3 CSL boundary was the most popularly observed high angle boundary.

Among them, the 1111)A/B~=3 boundary was especially abundant in number. The structure of {111lA/B2=3 boundary was very close to that of geometrical model described in the text book. A recent computer analysis of the HREM picture,however, suggested a small lattice relaxation which was 0.7% of Illllplane distance even though the atomic arrangement is basically the same.

Photo 3; A {1121A/B 2=3 boundary. The 11111 planes are discontinuous across the boundary due to atomic relaxation at the boundary except at the bottom where the boundary is connected to {lll)A/B~=3boundary.

Photo 4. A Illl1A/Bz=3 boundary where the rotation angle exceeded that of the exact coincidence by 4 degrees.

The atomic arrangement of {1121A/BZ=3 boundary was undoubtedly different from that of geometrical CLS model as having been pointed out by Pond et al(8). A short segment of the boundary bound by (111)A/B boundaries forming a facet of finite length were exceptional;

the image of common (111) plane is continuous across the I112lA/B 2=3

boundary as shown in Photo 3 neare the facet corners. The tllllplane gap increased its amount in the middle. The largest gap amounted about a half of (111) plane distance or about a/6 < 111 > in the direction paralell to the boundary plane.

Such a lattice relaxation should arise on every ordered boundary even though details of the relaxation should differ to each other.

111-1-3 ANGULAR LIMIT OF CSL BOUNDARIES

Structural changes due to missorientation was investigated in representative CSL boundaries. The structural change of 2-3 CSL boundary corresponding to deviations from the perfect coincidence orientation relationship is achieved firstly by the introduction of arrays of structural dislocation. A n interesing feature was the difference between off-coincidence x=3 boundary and a just coincidence high indices CSL boundary;Photo 4 is a case of an off coincidence boundary with the miss orientation of 4 degrees with respect to Z=3 orientation relationship. Structural dislocations and corresponding boundary steps may be identified in the picture. In such a discrete two dimensional lattice image as this a Burgers vector of the dislocation could be determined unambiguously. The extra plane of the dislocation as well as the corresponding boundary step may be defined readily from the micrograph. The Burgers vector was determined to be b=a/3<111>. On this boundary the strain field of the dislocations was well defined about the dislocation core that atomic matching at the boundary appeares still similar to that of perfect E=3 boundary. At larger missorientation of 9 degrees ,however, the local strain due to the dislocations become so large that a recognition of the atomic arrangement of the boundary is prohibitive. It was found that neither the Burger's vector of the structural dislocation (b=a/3<111>) nor that of a lattice dislocation(b=a/2<110>) was justified in descri- bing the misorientation from the step hight and the length of each structure unit.

As the second example structural change of z=9 CSL boundary is shown in Photo 5,6,and 7. In Photo 5, the boundary with a missorienta- tion of 3 degrees, the image of strain field was not only limited but also arranged at equal distances to each other. The Burger's vector could be determined to be b=1/18<114> in the same way as mentioned above. Photo 6 shows a boundary missoriented by -4 degrees from perfect c=9 orientation. Only part of ordered region can be seen along the boundary which is slightly curved. With further misoriention of -6 degree(Phot0 7) no periodicity may be detected and the boundary is curving. The boundary is hardly called a CSL boundary any more.

Even the ttamorphous cement theoryqq presented by Ewing and Rosenhein(9) could describe the boundary.

The same kind of structural change was observed in boundaries neare E=3, E=ll, 2=17, x=19 andz=27 CSL relationship. A boundary in Photo 8 for example missoriented by 2 degree from 2=27 CSL boundary where even the only 2 degrees of miss orientation did not allow to present the periodic structure. A statistics of observed critical angles presenting the CSL periodicity was illustrated in Fig.1. The limit was approximated by a straight line represented by

8=15//f which was proposed by Brandon (10). But it should be noted that the periodicity decreased with increasing the amount of missorientation even though within the limit

.

Photo 5. A z =9 CSL related boundary with the missorientation of +3 degrees. A strain field is still limited,so that determination of the Burger$ vector is- easy.

Photo 6. A E = 9 CSL related boundary- with the missorientation -4 degrees. Only a limited portion appears to be periodically ordered.

Photo 7 . A E = 9 CSL related by the missorientation of 6 degrees. No regularity may be detected.

Photo 8, A boundary missoriented by 2 degrees from the 8=22 CSL system. The small missorientation prevented from reali- zing a periodically ordered structure in the CSL related boundary of high sigma value.

Fig.1. The angular limit of the missorientation of CSL boundaries.

Photo 9. E=19 CSL related boundary where the boundary trace is not straight but curved showing different atomic arrange- ment at each segment.

Fig.2

.

111-1-4 LOW INDICES CRYSTAL PLANE BOUNDARY

A Z=19 CSL boundary is shown in Photo 9, where the boundary trace is not straight but curved in spite of the smallness of the misorientation. The boundary plane takes none of the high density planes of the CSL system. A large difference in the atomic arrangement suggests that the information of the boundary plane should be the main factor of the physical properties.

In the following a notation is used which contains information of orientation relationship of both the grains and the boundary plane.

A rotation angle w is represented by w=81-82-, where 81 and 0 2 are the angles between the boundary trace and the low indises plane of one crystal and corresponding angle of the other crystal respectively.

A n inclination of the boundary plane from thehighest density CSL plane is represented by d8=3.14/2-(81+82)/2 ( F i g . 2 ) . By plotting 81 and 82 on the rectangular coordinate both rotation angle w and inclination of boundary plane d8 can be shown on the each side of the triangle (boundary triangle) as illustrated in Fig.3, where 28

boundaries except (111)A/B =3 boundary were plotted. One may recognize some distinctive tendency of the distribution of plotted black spots.

Tirty percent of the boundaries were parallel to the I1111 plane of one of the crystals. Such boundary was frequently recognized at the boundaries deviated by large angle from those of coincidence system (Photo 10 ) . The (1111 plane was some times selected as the boundary plane even in boundaries of exact coincidence orientation.relationship when the sigma value was rather large(Phot0 1 1 ) .

111-2 GRAIN BOUNDARIES IN BCC METALS 111-2-1 LOW INDICES CRYSTAL PLANE BOUNDARY

The nature of small angle boundaries were essentially the same as that of fcc gold,so that only large angle boundaries are described.

Fig .3. "A boundary triangle" by which both the rotation angle and the direction of the bounda- ry plane are plotted together.

Thick line shows the trace of I1111 plane.

Photo 10

.

Photo l a . A high density crystal plane boundary in exact but high sigma value coincidence orientation relationship. The boundary plane is partly parallele to the CSL plane.

A boundary plane was frequently appeared to be parallel to a low indices crystal plane also in iron. A boundary shown in Photo 12 is a high angle boundary with the rotation angle of about 62 degrees about

<loo> axis which corresponds to the orientation relationship of z=17a CSL system. The boundary plane was not at the symmetrical position with respect to the crystals, nor parallel to low indices CSL plane.

It is nearly parallel to (200)plane of upper side crystal. A feature of low indices crystal plane boundary in iron was the superimposition of energy saving criterion as shown in Photo 1 3 . At this boundary not only a (2001 plane of the left hand side crystal is parallel to the boundary but also a pair of (110) planes of neighboring crystals is parallel to each other.

111-2-2 PLANE M A T C H I N G BOUNDARY

The Plane matching boundary firstly described by Pumpherey(3) was in iron. A boundary in Photo 14 is with a rotation angle of 45degees, which may be classified as a random boundary according to the CSL theory, because the orientation relationship is intermediate between those of adjacent two 2 = 5 CSL boundaries(36.9 and 53.ldegrees). The angular deviation between those of the two2=5 boundries exceeded the Brandon's criterion. The atomic structure of this boundary,however,is different from the typical random boundary although the atomic arrangement may not be seen because it is inclined with respect to [I101 beam orientation. Two sets of I1101 planes are parallele to (200) planes across the boundary. Lattice plane of both crystals nearly coincide at every ten I1101 planes. Typical such boundary is shown in Photo 15

.

The rotation angle of the boundary exceeded that ofZ=19 CSL boundary only by 2.7 degrees which is within the Brandon's angular limit ofZ=19 CSL boundary. Nevertheless the introduced dislocation was neither the structural nor the lattice dislocation. The boundary plane was not parallel to any of low indices CSL planes.

Photo 12

.

Photo 13

.

Photo 14

.

Photo 15

.

Following atomistic information is obtained by the present high resolution electron microscopy.

a) The atomic arrangement of the periodically ordered boundary analysed by present HREM may be accounted simply in terms of atomic relaxation at the boundary.

b) The periodicity of the atomic arrangement along the CSL boundary deteriorated with increasing the missorientation continuously beyond the so called Brandon's limit,which was represented roughly by e=15/ Jr

.

c) Low indices crystal plane boundary is abundant in both fcc and bcc metals.

d) Plane matching boundary prevailed in the orientational ranges that can be described only by high indices CSL system.

ACKNOWLEDGEMENT

The present authors thank professor M.Hirabayashi and S.Karashima,research assistants H.Ohota and E-Aoyagi of Tohoku University for their help in the high resolution observation.The authors also thank JEOL for use of high resolution electron microscope.

REFERENCES

(1) Brandon D.G., Acta.Meta11.14(1966)1479 (2) Bollman W., Phil.Mag.16(1967)363

(3) Pwnpherey P.H., Scripta Met.6(1972)107

(4) Ichinose H. and Ishida Y., Phil.Mag.A43(1981)1253

(5) Forwood C.T. and Clarebrough L.M., Acta Metall. 30 (1982) 1443

( 6 ) Forwood C.T. and Clarebrouqh L.M., Phil. Mag. A47 (1983) 135

(7) Pond R.C. and Vitek V., Proc.Roy.Soc.Lond.B35(1977)453 (8) Ewing J.A. and Rosenhein W., Phil.Trans.Roy.Soc.A195(1901)279 (9) Brandon D . G . , Ralph B., Ranganathan S . and Wald M.S.,

Acta.Met12(1964)813

DISCUSSION

M. Riihle: You studied the structure of GB in Fe. How do the ferromagnetic properties of Fe influence the quality (resolution) of the TEM &

H. Ichinose: Ferromagnetism is certainly a problem of the TEM observations but was not fatal with the present structure imaging because the specimen is very thin and the correction of resulted astigmatism is already included in the general photographing activity. Astigmatism correction is performed whenever the area of observation was changed.

D. Ast: Have you tried to match the "bondn in the micrograph to a known stable compound such as Fe3P?

H. Ichinose: No such compound structure was found.

J.M. Gibson: Your image s i m u l a t i o n s f o r a (211) tilt boundary show s e n s i t i v i t y o f images t o defocus changes o f 10 nm. Can you measure t h e defocus a c c u r a t e l y enough t o d i s t i n g u i s h between t h e s e simulations?

H. Ichinose: It depends on t h e e l e c t r o n microscope. I n t h e c a s e of a 200 kV microscope which is used i n my work, t h e defocus can b e changed by 35 1 s t e p s , s o t h a t it is easy t o d i s t i n g u i s h 100 1 o f defocus on t h e images. With an 1000 kV microscope it i s completely impossible. Lens c o n s t a n t s o f o u r 200 kV machine a r e Cs=0.8 nun, Cc=1.2 nun. The image change due t o defocus is very s e n s i t i v e .

M. Riihle: D i f f e r e n t a u t h o r s obtained d i f f e r e n t r e s u l t s on GB s t r u c t u r e s by computer simulation. Is it p o s s i b l e t o d i s t i n g u i s h between t h e d i f f e r e n t r e s u l t s and models with your experimental observations?

H . Ichinose: I n a paper by Y. I s h i d a , H. I c h i n o s e , M. Mori and M. Hashimoto (Trans. Jap. I n s t . Met. a (1983) 349) it was shown t h a t it is indeed p o s s i b l e t o d i s t i n g u i s h between t h e d i f f e r e n t proposed atomic s t r u c t u r e s o f incoherent twin boundaries and o f g r a i n boundaries.