High-Resolution Electron Microscopy of Dislocation Cores in NiAl
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INTRODUCTION Among other factors the dislocation core structure has an important influence on the macroscopic behaviour of metals and intermetallic alloys [1,2]. In the intermetallic alloy NiAl the strong anisotropy of the flow stress is generally attributed to the fact that only dislocations with Burgers vector a can glide easily. a dislocations exhibit rather low mobility [3, 4]. High-resolution transmission electron microscopy has been applied to the study of dislocation cores in a number of material systems [5]. Since the high-voltage microscopes approach a point-to-point resolution of 0.1 nm [6], it has become possible to investigate dislocations in crystals with small lattice parameters. In the present study we produced dislocations in NiAl by room temperature compression in the so called "hard" direction and by preparing bi-crystals, respectively. The experimental images were compared with results obtained by molecular dynamics simulations using the embedded atom method (EAM). Since HRTEM images are taken from thin foils, the MD calculations were done with free boundary conditions along the dislocation line which give rise to surface relaxations that may affect the high-resolution image.
EXPERIMENT AND MD SIMULATIONS A stoichiometric -oriented NiAl single crystal (residual resistivity ratio 11, rod length 15mm, gauge length 6mm) was deformed in compression at room temperature with the strain rate 10- 4/s up to a final uniaxial strain of about 10-2. The dislocation microstructure was analyzed by conventional TEM. For HRTEM, thin foils perpendicular to were prepared. This allows the investigation of dislocations with (e.g. screw dislocations) and line directions. Slices of 3 mm diameter and 600 pm thickness were cut by spark erosion and subsequently thinned by electrochemical polishing using 10 parts ethanol and 1 part perchloric acid (233K,40V). The bi-crystals were prepared by solid state bonding under ultra high vacuum conditions [7]. A twist grain boundary was formed by twisting the crystals around the axis by 2.34'. This should produce a network of screw dislocations with separations of about 10 nm. In order to obtain a tilt grain KK9.8.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
boundary the two crystals were tilted around the axis by 2.8'. This results in an arrangement of edge dislocations separated by about 10 nm. The HRTEM investigations were performed at the side-entry configuration of the Stuttgart JEM 1250 ARM microscope (point-to-point resolution 0. 12nm). The thickness of the sample and the defocii of the objective lens were routinely estimated by comparing the experimental micrograph with the thickness and defocii map from NiA1. Along the direction the characteristic superlattice contrast of NiAl allows a definite comparison. For the EAM simulation the computer code of Schroll and Gumbsch [8] was used. The interatomic interactions were described by a potential developed by Ludwig and Gumbsch [9]. For HRTEM image simulations 10 nm by 10 nm areas were cut out from t
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