HREM imaging of screw dislocation core structures in bcc metals
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HREM imaging of screw dislocation core structures in bcc metals B.G. Mendis, Y. Mishin *, C.S. Hartley † and K.J. Hemker Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218. * School of Computational Sciences, George Mason University, Fairfax, VA 22030-4444. † Air Force Office of Scientific Research, Arlington, VA 22203-1954. ABSTRACT Quantitative High Resolution Electron Microscopy (HREM) is used to characterize the in-plane displacements of atoms around a screw dislocation core in bcc molybdenum. The in-plane displacements have an important effect on the bulk mechanical properties of bcc metals and alloys. However, the largest displacements are predicted to be less than 10 pm, requiring that the atom positions in an HREM image be determined to sub-pixel accuracy. In order to calculate the displacements the positions of the atom columns in the undistorted crystal must be determined precisely from the information available in the HREM image. An algorithm for such a task is briefly discussed and the technique applied to several HREM images. It is seen that the atomic displacements are predominantly due to surface relaxation (i.e. Eshelby twist) of a thin TEM foil, thereby masking the finer displacements of the dislocation core. Nye tensor plots, which map the resultant Burgers vector at each point of a distorted crystal, are also used to characterize the core structure. Although the large displacements from the Eshelby twist were completely removed, no signal from the dislocation core region was observed. INTRODUCTION The core structure of a screw dislocation is thought to play a crucial role in the low temperature deformation of bcc metals and alloys. This is because the defect can potentially dissociate along three {110} type planes that contain the dislocation line vector. The non-planar character of the core results in an intrinsically high Peierls– Nabarro stress which makes the screw dislocation sessile and therefore deformation rate controlling. Early atomistic calculations using Johnson [1] and Finnis-Sinclair potentials [2] confirmed such a structure. The atomic displacements when projected onto the {111} plane perpendicular to the dislocation line, showed small (i.e. < 10 pm) non-zero components. In terms of energetics these ‘in-plane’ displacements arise due to the anisotropy of the γ surface [2]. Their significance is that they are able to interact with the non-glide components of the stress tensor [2] thereby making the critical resolved shear stress of the screw dislocation dependent on the orientation of the applied stress. Figures 1(a) and (b) show the direct and differential in-plane displacement maps for the screw dislocation in Mo calculated using the Finnis-Sinclair (FS) potential [2]. The former shows the displacement vector of individual atoms due to the introduction of the defect into an otherwise perfect crystal, while the latter shows the relative displacement
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vector between two neighboring atoms by plotting it between those two atoms. The d
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