Philosophical Magazine A

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Philosophical Magazine A

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Burgers vector determination of decorated dislocations in γ-TiAl by diffraction contrast and large-angle convergent-beam electron diffraction

J. M. K. Wiezorek a; A. R. Preston a; S. A. Court b; H. L. Fraser c; C. J. Humphreys a a

Department of Materials Science and Metallurgy, Cambridge University, Cambridge, England b Banbury Laboratory, Alcan International Ltd, Banbury, Oxfordshire, OX, England c Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio, USA Online Publication Date: 01 February 1994 To cite this Article: Wiezorek, J. M. K., Preston, A. R., Court, S. A., Fraser, H. L. and Humphreys, C. J. (1994) 'Burgers vector determination of decorated dislocations in γ-TiAl by diffraction contrast and large-angle convergent-beam electron diffraction', Philosophical Magazine A, 69:2, 285 - 299 To link to this article: DOI: 10.1080/01418619408244344 URL: http://dx.doi.org/10.1080/01418619408244344

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PHILOSOPHICAL MAGAZINE A, 1994, VOL. 69, No. 2, 285-299

Burgers vector determination of decorated dislocations in y-TiAl by diffraction contrast and large-angle convergent-beam electron diffraction By J. M. K. WIEZOREK~, A. R. PRESTON?, S . A. COURTS, H. L. FRASER~ and C. J. HUMPHREYS~ f Cambridge University, Department of Materials Science and Metallurgy,

Pembroke St, Cambridge CB2 3QZ, England Ltd, Banbury Laboratory, Southam Road, Banbury, Oxfordshire OX 16 7SP, England Q The Ohio State University, Department of Materials Science and Engineering, Columbus, Ohio 43210-1179, USA

1Alcan International

[Received 8 February 199311 and accepted 27 July 19931

AESTRACT Decorated and undecorated dislocations in y-TiAl have been studied by conventional transmission electron microscopy using diffraction contrast and by large-angle convergent-beam electron diffraction (LACBED). The results of the Burgers vector determination for undecorated and decorated dislocations using the different techniques have been compared. For the decorated dislocations the contrast from the decorating particles and the underlying dislocation have been separated and the misfit introduced by the precipitates has been determined. Computer simulations of the contrast from decorated dislocations in LACBED patterns have been performed. The rules governing the dislocation contrast in LACBED patterns have been discussed and extended to the case of decorated dislocations. A new fringe-counting rule to determine the Burgers vectors of dislocations decorated by misfitting precipitates has been proposed. Remarkably good agreement of the computer-simulated LACBED contrast with experimental observations has been achieved.

Q 1. INTRODUCTION Conventional methods of transmission electron microscopy (TEM) for defect studies employ diffraction contrast for the determination of the Burgers vectors of dislocations by exploiting the standard invisibility criteria (g b =0, g b x u =0) (Howie and Whelan 1961). However, since these criteria are fully valid only for elastically isotropic media, residual contrast from anisotropy effects can lead to ambiguous Burgers vector determinations in most real materials. For anisotropic materials, Head et al. (1973) suggested the use of computed electron micrographs for defect identification to determine the Burgers vectors b of dislocations, but this can be inconvenient and time consuming. In addition, if the dislocations are decorated by impurities, dislocation invisibility can be impossible to obtain because the strain fields of precipitates and dislocation interact (Montheillet, Handin and Frade 1973, Borggreen and Tholen 1976).The purpose of this paper is to study the problem of the

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determination of Burgers vectors of decorated dislocations in elastically anisotropic materials. In particular the application of large-angle convergent-beam electron diffraction (LACBED) to this problem will be demonstrated. The traditional methods of convergent-electron beam diffraction (CBED) use a converged probe focused on to a specimen in the object plane resulting in the formation of a CBED pattern in the diffraction plane (the back focal plane of the objective lens). LACBED is a development of CBED which uses a converged but defocused probc (usually focused below the specimen) forming a ‘shadow image’ in the diffraction plane, that is the LACBED discs contain low-spatial-resolution image information superimposed upon high-angular-resolution diffraction information (Tanaka and Terauchi 1985).This allows the simultaneous observation of a defocused image of the defects in the specimen and their effects on the diffraction pattern. In recent years this method has been applied to study various crystalline defects and, of special concern to this work, for the determination of the total Burgers vectors b of dislocations (for example Cherns and Preston (1986,1989), Tanaka, Terauchi and Kaneyama (1 988), Niu, Wang and Lu (1991), Morniroli and Steeds (1991) and Chou, Preston and Steeds (1992)).Dislocations crossing the rocking-curve fringes of a reflection in a LACBED disc cause a characteristic bending and splitting of the fringes. For high-index reflections these effects can be explained by kinematical electron diffraction theory, leading to rules for the determination of the total Burgers vector b of the dislocation intersecting the reflection band (Cherns and Preston 1986, Preston 1989). We shall refer to these rules throughout this work as the Preston-Cherns rules, which can be summarized as follows: (1) Count the number n of minima (maxima) between the subsidiary fringe maxima (minima) for excess (deficiency) lines, and find )g* bl = IT. (2) The sign of b can be deduced from the sense of fringe bending, since the

deviation of the main intensity peak from the perfect crystal Bragg position depends on the sign of [g (dR/dz)]s and dR/dz reverses sign upon crossing the dislocation line.

To solve the problem of the Burgers vector determination of decorated dislocations we have studied dislocation structures in specimens of yTiA1 containing two different levels of oxygen impurities (250 and 1250 ppm respectively) using different techniques in the transmission electron microscope. We have investigated decorated dislocations in low-oxygen-content specimens and undecorated dislocations in high-oxygencontent specimens by bright-field (BF) and dark-field (DF) weak-beam (WB) techniques using conventional transmission electron microscopy (CTEM) and by LACBED. Computer simulations based on the kinematical theory of electron diffraction of the LACBED contrast from decorated dislocations are presented, which employ a model approximating the displacements from the rows of small decorating particles by a cylindrical displacement field along the dislocation line (Montheillet et al. 1973; Borggreen and Tholen 1977). These computer simulations are in good agreement with experiments. The dynamical theory of electron diffraction gives even better agreement with experiment, of course (Chou et al. 1992), but the kinematical theory gives more physical insight into fringe splittings and bendings and it is sufficient to explain o u r results. The LACBED contrast from the decorating particles and the dislocation will be discussed in terms of a theoretical treatment of the LACRED technique. Finally a new method to determine the total Burgers vectors of dislocations decorated by large precipitates will be proposed.


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5 2. EXPERIMENTAL PROCEDURE Alloy sections of Ti-52 at.% A1 were produced by arc casting on a water-cooled copper hearth under a high-purity argon atmosphere. The alloy was prepared from high-purity starting materials of 99.99% pure titanium and 99.998% pure aluminium, leading to two different initial oxygen impurity levels of about 250 and 1250ppm respectively. The alloy buttons were wrapped in tantalum foil and vacuum encapsuled into silica tubes, which were back filled with a partial pressure of argon, for a homogenization heat treatment of 100 h at 1100°C followed by air cooling. Finally cyclindrical compression coupons were cut from the alloy sections and deformed at a high temperature (T= 1173 K) in vacuum to total strains of about 3%. Thin specimen foils for examination in the transmission electron microscope were prepared by twin-jet electropolishing in a polishing solution of 7 v01.x sulphuric acid in methanol at an operating current of about 90 mA at about - 40°C. For the BF and D F WB studies we used a JEOL 2000FX operating at 160kV to avoid beam damage by mechanisms found active at 200 kV. For DF WB studies the foils were tilted into either the (g,3g) or the (g,4g) conditions obtaining deviations w g = sgtg 3 5, where s, and cg are the geometrical deviation parameter and the extinction distance for the reflection g respectively, and avoiding significant excitations of systematic and non-systematic reflections. The LACBED work was conducted with a JEOL 2010 operating at 200 kV maximizing penetration, since the lower beam flux (electrons per area) on the specimens reduced the likelihood of beam damage significantly and, unlike the diffraction contrast work such damage was not observed at 200 kV. The general principles needed to obtain good LACBED patterns with modern electron microscopes have been described by Tanaka and Terauchi (1985) and more recently by Vincent (1989). The important parameters to be observed for a satisfactory Burgers vector determination by LACBED are the following. For each dislocation three different values of g b for a non-coplanar set of reflections have to be obtained. For each of these reflection lines we should be able to count exactly the number of fringe displacements and check that they are sufficiently kinematic (generally the case for foil thickness t