A DISLOCATION MECHANISM FOR THE GROWTH OF TWINS IN DEFORMED HCP METALS

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A DISLOCATION MECHANISM FOR THE GROWTH OF TWINS IN DEFORMED HCP METALS

Ph. Komninou, E. Polychroniadis, J. Antonopoulos, Th. Karakostas, P.

Delavignette

To cite this version:

Ph. Komninou, E. Polychroniadis, J. Antonopoulos, Th. Karakostas, P. Delavignette. A DISLOCA-

TION MECHANISM FOR THE GROWTH OF TWINS IN DEFORMED HCP METALS. Journal

de Physique Colloques, 1990, 51 (C1), pp.C1-215-C1-219. �10.1051/jphyscol:1990133�. �jpa-00230291�

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PH. KOMNINOU, E. POLYCHRONIADIS, J.G. ANTONOPOULOS, TH. KARAKOSTAS and P. DELAVIGNETTE*

Department of Physics, Aristotle University of Thessaloniki, GR-540 06 Thessaloniki, Greece

Service de Physique des Surfaces, Universitk Libre d e Bruxelles, C.P.

234, Bd du Triomphe, B-1050 Bruxelles, Belgique

RBsumd

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Des macles mecaniques ont tltd examinees par MET dans les mdtaux hexagonaux comme Zn, Cd, Zr, Ti. En particulier, les joints contenant des dislocations intrinsbques ont Q6 examink. Les caracteristiques de ces dislocations ont dtd 6tudides a partir de leur contraste et par diffraction electronique. II est deduit que dans les cas du Zn et du Cd, les macles mdcaniques croissent par le mouvernent de dislocations dont les vecteurs de Burgers sont deduits des caracteristiques des macles mdcaniques. Dans le Zr et le Ti des familles de dislocations intrinsbques parallbles 21 la direction de coincidence sont observkes.

Abstract

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Deformation twins in hexagonal metals are examined by TEM. The materials studied are Zn, Cd, Zr, Ti. The observations are focussed o n the twin boundaries that contain intrinsic dislocation structures. The characteristics of these dislocations are analysed using contrast and electron diffraction techniques. It is deduced that in Zn and Cd deformation twinning proceeds by a dislocation mechanism in which the glide of the dislocation as well as its Burgers vector are connected with the crystallographic elements of the twinning mechanism. In Zr and Ti parallel arrays of intrinsic dislocations, with lines along coincident direction in both grains, are also observed.

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INTRODUCTION

Mechanical twinning is a common deformation mode in hexagonal close packed (hcp) metals. Plastic deformation by slip does not generally occur in these materials since possible Burgers vectors for the dislocations are limited: the mechanical twinning is therefore an important deformation mode at low temperature in such metals. According to the standard description of the deformation twinning the shear stress acts along a shear direction n which connects the positions of the atoms before and after the deformation. Since the glide characterisics of the principal twins are different from those observed in the single crystals in hcp metals, more complicated dislocation type mechanisms i.e zonal twinning dislocations have been proposed /l/.

In this work, mechanical twins in Zn, Cd, Zr, Ti are investigated by TEM. The observations are focussed on the precise characterization of the structure of the interfacial dislocation arrays using image contrast analysis and the determination of the relative crystallographic relationships between adjacent twins using electron diffraction methods /2/.

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THE CRYSTALLOGRAPHY OF THE TWINS

The crystallographic description of the mechanical twinning is given through the system of the planes and axes which are invariant by shear: i.e K,, 11

,,

^,^K^, ¹¹ ^/3/.

In order to have a better understanding of t i e physical mechanism, a description in terms of coincidence site lattices (CSL) using idealized c/a ratio may be also chosen /4/. For instance, for materials like Zn and Cd, an idealized c/a= (7/2)'12 gives a common CSL which can be used as a model in the experimental procedure.

This value of the c/a ratio is higher from 1.633, the ideal value for hcp metals. For Zr and Ti the c/a = (5/2)'12 is chosen which has a lower value than the ideal one. Based on these the corresponding CSLs hale the fol- lowin multiplicities: K,= (1072); c/a= (7/2)'12 --> E=13, c/a= (5/2)'12 --> E = l l b K,= (1122); c/a=

(5/219/' --> 1-7. It should be noticed 8/80 that, for K

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(1072

,

the twin for c/a= (712 '1' has a ~ s i t i v e mag- n i t u d e ~ f shear g while for c/a= (512)" a negative. kbr K,= '(1 122) with c/a= (5/2)'hthemagnirudeof shear

is also positive.

The twin relation can be described by 12 equivalent rotation operations /5,6/. Due to the idealization of the c/a ratio, all the axes of these rotations have rational indices and are normal to planes which have also rational indices.

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'work supported by the No ST2J 0289 European Community contract

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990133

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Cl-216 COLLOQUE DE PHYSIQUE

The 12 axes and the corres onding rotation angles, for the K{= (107~) c/%= (712) 'I2which approximates Zn and Cd and c/a= (5/2)1/Pfor Zr, together with the axes o the K,- (1122) ~ / a = ( 5 / 2 ) ' / ~ for Ti are given in Table 1. These axes and the poles of the normal planes can be placed on a stereographic projection which can

Table l

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The 12 equivalent rotation operations describing the:K1 = ( ~ 0 i 2 ) , q E ~ l ~ i i ] twin for c/a= (7/2~'/~, c/a =(5/2)lj2 and the 12 rotation operations of the =(1122),q1 =\l1231 twin for c/a= (5/2)'/

.

Rotation ,Rotation Rotation

(hkil) [uvtw] angle (hkil) [uvtw] angle (hkil) [uvtw] angle

1 85.59' 84.78' 64.62'

1210 1210 1510 1510

i i o o i i o o

2 94.41° 95.22' 1 15.38'

be used in the experimental procedure 171. In such a construction it is easily seen that the axes lay on two great circles having as an intersection the axis which is normal to the shear plane. One of the great circles defines the trace of K, and in the second circle, which is normal to K,, lay all the common reflections with small indices which are usually used in the dislocation contrast experiments.

3 -TWIN BOUNDARY DISLOCATIONS IN Zn AND Cd

Zn and Cd foils were deformed by cold rolling after a recrystallization treatment. Thin specimens have been prepared for TEM by a-double jet electropolishing technique. The TEM observations have shown that, in these materials, the K, = (1012) twin mode is connected with the imperfections present in the twin boundary as well as with the movements resulting inthe twin growth /7,8/. Two families of parallel dislocation arrays have been commonly observedin Zn, separated or together. The members of the first family, the "twinning" dislocations, havs b parallel to [l0111 and the members of the second family, the "tilt" dislocations, have b parallel to [l01 l](possibly with another component along [l2101 the rotation axis normal to the plane of shear). The twin boundary is obviously non planar.

The same family of the "twinning" dislocations is present in Cd with some differences in comparison to the preceding of Zn. In Zn the dislocation lines are almost normal to the foil plane while in Cd they are closely parallel to this, with the (1012) twin plane steeply inclined on the foil plane. The dislocations of an extrinsic origin are also inserted in the twin boundaries.

Typical examples of these "twinning" dislocations are illustrated in Fig.1.

The determination of the Burgers vector of the "twinning" dislocations, associated with its effect on the increase in thickness of the twinned part, is of a fundamental interest. This has shown that, twin growth may be oc- casionally quantified in small units, including only a few atomic layers and moving alarlg the twin boundary plane in a movement of the same type as the dislocation type.

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The spacing of the twinning dislocations in Zn was measured as 16nm and the deviation of the boundary in that area was about 7O off the direction of coherency. From an analysis of the accurate orientation of the habit plane, in the presence _of "twinning" dislocations, a possible magnitude of the Burgers vector of these dislocations is given: b = 1 /4[101 l ] /7/.

An analysis of such interfacial dislocations is presented by Pond /g/ in a review on line defects in interfaces where the core of the dislocations of this type is associated with a step on the boundary. In a recent paper Serra et al. /10/ investigated dislocations in hcp metals using computer simulation techniques. According to their study, a Burgers vector of the same type is possible but with a smaller length. Since such a discrepancy is significant the subject needs further investigation.

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DEFORMATION TWINS IN Zr AND Ti

Specimens of polycrystalline Zr and Ti, having the texture of the annealed structure, were deformed by cold rolling from 15% to 50%. These conditions are proper for the formation of deformation twins in these materials /l l/. Specimen for TEM were prepared by a double jet electropolishing technique.

In Zr, on the deformation twins no dislocation structure was observed when the deformation was of the order of 20%. The dislocations were appeared on higher rates of deformation

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In Ti, the introduction of twins by defor- mation was succeded in high deformation rate but the number of the deformation twins was low when the deformation was performed at room temperature. However, a large number was achieved by deformation at low temperature.

In the following we give an example of K, = (1072) deformation twin in Zr and a K, = (1 122) deformation twin in Ti with a preliminary analysis of the dislocation arrays present in the twin boundar~es.

In Zr, the deviation of the twin boundary was varying locally between 2' and 5O from K,. The boundary con- tains a closely spaced array of dislocations with an average distance of 5.5nm and direction parallel to the [72%] vector which is one of the invariant directions in both twin crystals, Fig. 2. This direction forms an angle of about 78Owith the [l2101 direction which was the direction of the dislocation line in Zn and Cd.

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Cl-218 COLLOQUE DE PHYSIQUE

a

b

Fig. 2

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Twin boundary in Zr. a) A parallel array of dislocations is visible using the 0002 reflection of the crystal A. b) The same twin boundary in edge-on position with the dislocations end-on. The corresponding diffraction pattern, inset, is the common zone axis along the direction of the dislocation line.

In Ti, the twin boundary deviates a few degrees from K,. The dislocations, with distances between 4nm to 8nm (depending upon the local deviation), were lying along the [2z23] direction forming an angle of 47.2O with the [1i00] axis which is the pole of the shear plane. This direction is also an invariant direction for both crystals and corresponds to one of the 12 rotation axes of the twin, Fig. 3.

Fig. 3 -Twin boundary in Ti. a) A parallel array of dislocations is visible. b) The same twin boundary in edge-on position with the dislocations end-on. The corresponding diffraction pattern, inset, is the common zone axis along the direction of the dislocation line.

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CONCLUSIONS

Dislocation structures in deformation twin boundaries were examined by TEM in Zn, Cd, Zr and Ti. In all the cases the dislocation lines lay along invariant directions for both twin crystals. In Zn and Cd the dislocation lines were parallel to the direction normal to the shear and their Burgers vector was parallel to the direction of shear

7,.

Th_e analysis confirms that they are responsible for an increase in thickness of the twinned part by glide on (1012). In Zr and Ti parallel arrays of intrinsic dislocations were also observed but there structure needs further analysis.

REFERENCES

/l/ Hirth, J.P. and Lothe, J., "Theory of Dislocations", MC Graw-Hill (1968) 746.

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/3/ Kelly, A., and Groves, G.W., "Crystallography and Crystal Defects", Longman (1973) 292.

/4/ Delavignette, P., J. Phys. Colloque C-6 suppl. n.12 a(1982) C6-1.

/5/ Bleris, G.L., Doni, E.G., Karakostas, Th., Antonopoulos, J.G. and Delavignette, P., Acta Cryst.

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/8/ Antonopoulos, J.G., Komninou, Ph., Karakostas, Th., and Delavignette, P., Scripta Met. 23 (1989) 417.

/g/ Pond, R.C., "Dislocations in Solids", Nabaro F.R.N., North-Holland 8 ch.38 (1989) 1.

/10/ Serra, A., Bacon, D.J. and Pond, R.C., Acta Met.

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