Mechanism of plastic deformation of Mn-added TiAl L 1 0 -type intermetallic compound
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I. INTRODUCTION
loading condition. Thus the actual image of loaded dislocations can be obtained.
The intermetallic compound TiAl is a possible structural material candidate for high-temperature applications. However, TiAl has not been used because of its room-temperature brittleness. l~* Recently, the addition of a small amount of Mn was found to have a beneficial effect for improving the room-temperature ductility.5 So far, several studies have been carried out to understand the mechanism of ductility improvement by Mn addition, but sufficient understanding has not been obtained except for several findings: the c/a ratio becomes closer to 1.00 from 1.02 with increasing Mn content, and Mn addition has a tendency to cause twin structures to form during plastic deformation at room temperature. In this study, with the purpose of better understanding the effect of Mn addition on improvement of room-temperature ductility, microcharacterization of twin Structures in Mn-added TiAl intermetallic compound has been carried out. The thermal stability of Mn-added TiAl in comparison to that of pure TiAl is studied by in situ high-voltage electron microscope (HVEM) observations in which each specimen is heated up to 1273 K inside the microscope and observed as the structure changes. To investigate the structures of twin boundaries of an Mn-added TiAl specimen, a tip of sharpened Mn-added TiAl specimen is observed and analyzed by atom-probe field-ion microscopy (APFIM). Finally, to better understand the twin mechanism of plastic deformation, in situ observation of plastic deformation is conducted using the HVEM, and micrographs are taken with the specimen under tensile 656
J. Mater. Res. 3 (4), Jul/Aug 1988
http://journals.cambridge.org
II. EXPERIMENTAL PROCEDURE A. Specimen preparation Specimens of TiAl and 2 mass % Mn-added TiAl were solidified by arc-melting of 99.8 mass % purity Ti and 99.99 mass % purity Al. The compositional ratio of Ti and Al was 50 : 50 in at. % for each specimen. In this study, specimens were used after heat treatment at 1273 K for 86.4 ks except the specimens used to study thermal stability of twin structures, in which specimens of as-cast condition were also used. Specimens with a cylindrical shape of 5 X 5 mm for a compression test were cut by a spark machine after the heat treatment (at 1273° K for 86.4 ks) and polished mechanically and chemically using emery papers and standard procedures. Specimens for transmission electron microcope (TEM) observations were electropolished until perforation began in a 10% perchloric acid and 90% acetic acid solution at room temperature. B. Compression test Compression tests were conducted at a strain rate of l X l 0 ~ 4 s ~ ' , using an Instron testing machine. C. In situ heating in an HVEM Electropolished specimens were heated in a highvoltage electron microscope (HVEM) with an accelerating voltage of 1000 kV. The same region was observed before and after heat treatment to determine the structural change for each specimen.
0003-6951 /88/040656-09S01.75
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