Characterizing Defect Structures in Ordered Alloys Using Electron Microscopy and Computed Image Simulations
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CHARACTERIZING DEFECT STRUCTURES IN ORDERED ALLOYS USING ELECTRON MICROSCOPY AND COMPUTED IMAGE SIMULATIONS K.J. Hemker, B. Viguier, R. Schiublin and M.J. Mills1
Institut Genie Atomique, EPFL, CH-1015 Lausanne, Switzerland tSandia National Laboratories, Livermore, CA, 94551-0969 Abstract
The resolution offered by weak-beam transmission electron microscopy allows for the direct observation of partialdislocations in a large number of intermetallic alloys. In many instances, the comparisonof these observationswith computer simulated images leads to results that are much more quantitative and descriptive than are possible with ordinary analyses. Examples from several parallelstudies are presented: i) comparisons of experimental and simulated images thathave been used to characterizeextrinsic stackingfaults in TiAI are shown, ii) the need to correct the experimentally observed anriphaseboundarydissociations (dAPB) in Ni3Al in order to accountfor the image shifts that occur during microscopy are highlighted,and iii) dissociation distances of less than 2 nm, which were observed with (2g-5g) diffraction conditions, were verified and quantified by comparisons with computer simulated images. Antiphase boundary energies (,APB) and complex stackingfault energies (,tCSF) were calculated from the correctedobservations,and it wasfound that the APB energies did not depend on alloy content, but that the CSF energy of binaryNi3 AI is less than it isfor a boron containing alloy. 1. INTRODUCTION The dislocation mobility, and thus the unusual mechanical properties and inherent brittleness, of many ordered intermetallic alloys can be related to their dislocation core structure. The Burgers vectors of the dislocations that control deformation in ordered alloys are generally much larger than they are in ordinary metals and alloys, and the strain energy associated with these "super" dislocations leads to their dissociation into smaller partial dislocations that are connected by crystallographic faults. Dislocation motion is restricted when these dissociations results in a non-planar configuration, and there is general agreement in the literature that many of the interesting mechanical properties exhibited by intermetallic alloys can best be understood in terms of their dislocation mobility and thus their dislocation core geometry [ 1,2]. Transmission electron microscopy (TEM) is an especially powerful tool for characterizing dislocation structures in ordered alloys. The dissociations in ordered alloys are often too small to be seen with conventional electron microscopy, but the resolution offered by weak-beam microscopy allows for the direct observation of partial dislocations in a large number of these alloys. Weak-beam tilting experiments are commonly used to identify the partial dislocations and stacking faults that exist in intermetallic alloys, and weak-beam measurements of the separation distance between partial dislocations are considered to be the most reliable way of determining fault energies in these alloys. However, the close
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