Cleavage fracture in an Al 3 Ti-based alloy having the Ll 2 structure
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B.F. Oliver Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996-2200 (Received 21 July 1988; accepted 15 September 1988) Selected area electron channeling patterns were used to identify the cleavage planes in a polycrystalline Al-23Ti-6Fe-5V alloy, which is an Al3Ti-based alloy having the Ll 2 structure. Alloys like this one are of scientific and technological interest because they fracture by brittle transgranular cleavage, despite the availability of more than five independent slip systems in the Ll 2 structure and their relatively low hardness when compared to ductile Ll 2 alloys like Ni3Al. Homogenized bars of the levitation zone melted and directionally solidified alloy were fractured at room temperature in three point bending. The fracture surfaces, which consisted almost entirely of brittle transgranular cleavage facets, were examined in a scanning electron microscope (SEM) equipped to take channeling patterns. An optical microscope with a short depth of focus was inserted in the SEM column just below the objective lens, and by focusing it on several points on the cleavage facets, it was possible to orient the facets normal to the optic axis of the SEM prior to taking the channeling patterns. Of the eight cleavage facets examined in this study, six were of the {110} type and two were of the {111} type. Although it cannot be said that these are the only two cleavage planes in this alloy, the availability of more than one plane with low cleavage strength contributes to the brittleness of this alloy. These results are examined in the light of theoretical treatments of cleavage fracture, and comparisons are made with earlier studies on other Ll 2 materials.
I. INTRODUCTION There is considerable interest in Al3Ti as a potential high temperature structural material because of its relatively high melting point (1350 °C),' low density (3.36 g/cc), 2 and good oxidation resistance. 3 Unfortunately, however, it has the low-symmetry, ordered tetragonal, DO22 crystal structure,2 making it brittle at ambient temperatures. One possible approach to ductilizing those ordered intermetallics that have low-symmetry crystal structures is to transform them to structures of higher symmetry, with the help of alloy additions. In this regard, it is known that in the pseudo-ternary system Fe 3 V-Co 3 VNi3V, as the electron concentration (defined as the average number of valence electrons per atom) decreases, the stacking character changes from purely hexagonal, through different ordered mixtures of hexagonal and cubic, to purely cubic.4 Partial substitution of Ni for Co increases the electron concentration, whereas partial substitution of Fe for Co decreases the electron concentration. In this way it is possible to get the cubic Ll 2 structure from the hexagonal structure of Co3V by partially replacing Co with Fe, and when this is done it is seen that ductility increases dramatically.4 In the case of Al3Ti, its DO22 structure is closely related to the Ll 2 structure {by a !/2[110] shif
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