Identification of Cleavage Planes in an Al 3 Ti-Base Alloy by Electron Channeling in the SEM
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IDENTIFICATION OF CLEAVAGE PLANES IN AN AI 3 Ti-BASE ALLOY BY ELECTRON CHANNELING IN THE SEM E. P. George, W. D. Porter and D. C. Joy Metals and Ceramics Division, Oak Ridge National Laboratory P. 0. Box 2008, Oak Ridge, TN 37831-6093 ABSTRACT Selected area electron channeling patterns were used to identify the cleavage planes in a polycrystalline Al3 Ti-base alloy having the L12 structure. In order to do this unambiguously in the scanning electron microscope (SEM), one needs to know that the cleavage facet from which any given channeling pattern is obtained is indeed normal to the electron beam. We accomplished this by utilizing a recently-developed technique in which an optical microscope with a short depth of focus is inserted in the SEM column and used to measure the elevations of several points on the cleavage facets. By appropriately tilting and rotating the sample, and using the optical microscope to measure elevations, it was possible to orient the facets normal to the beam. The cleavage planes in a cast and extruded alloy having an equiaxed grain structure were compared with those in a directionally-solidified (DS) alloy of the same composition. Of the eight cleavage facets examined in the DS material, six were of the {110} type and two were of the {111} type. Of the six facets examined in the cast and extruded material, two each were of the {110} and (1111 types, and one each were of the {100} and {013) types. Although it cannot be said that all possible cleavage planes have been identified in this alloy, the availability of several low-strength cleavage planes apparently exacerbates its brittleness. INTRODUCTION Among the attractive properties of AI3 Ti are a relatively high melting point (135000) [1], low density (3.4 g/cc) [2], and good oxidation resistance [3]. Although these are all desirable properties for a potential high-temperature structural material, the roomtemperature brittleness of AI3 Ti (thought to be a result of its low-symmetry, ordered tetragonal, D022 structure) will have to be first overcome before the alloy finds engineering application. In this regard, it is generally believed that if a low-symmetry structure can somehow be transformed into a higher-symmetry one that has a sufficient number of slip systems, it will result in improved ductility. Consistent with this point of view is the experimental observation that, in the pseudo-ternary system Fe 3 V-Co 3 V-Ni 3 V, it is possible to get the cubic L12 structure from the hexagonal structure of Co 3 V by partially replacing Co with Fe, and when this is done, ductility increases dramatically [4]. In the case of AI3 Ti, its D022 structure is closely related to the L12 structure (by a 1/2[1101 shift on every (001) plane), and it is known that partially replacing some of the aluminum atoms with Cu, Ni [5], or Fe [6] transforms it to the L12 structure. However, these modified AI3 Ti-base materials are also quite brittle [7-9], despite the availability of more than five independant slip systems in the L12 structure. Another intriguing obser
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