Morphology, Deformation, and Defect Structures of TiCr 2 in Ti-Cr Alloys
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MORPHOLOGY, DEFORMATION, AND DEFECT STRUCTURES OF TiCr 2 IN Ti-Cr ALLOYS Katherine C. Chen, Samuel M. Allen, and James D. Livingston, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
ABSTRACT The morphologies and defect structures of TiCr2 in several Ti-Cr alloys have been examined by optical metallography, x-ray diffraction, and transmission electron microscopy (TEM), in order to explore the room-temperature deformability of the Laves phase TiCr 2. The morphology of the Laves phase was found to be dependent upon alloy composition and annealing temperature. Samples deformed by compression have also been studied using TEM. Comparisons of microstructures before and after deformation suggest an increase in twin, stacking fault, and dislocation density within the Laves phase, indicating some but not extensive room-temperature deformability. INTRODUCTION In prior studies, several alloys containing the TiCr2 Laves phase have shown promising mechanical properties and oxidation resistance at elevated temperatures [1,2]. Although Laves phases have the well-deserved reputation for low-temperature brittleness, recent studies have demonstrated room-temperature deformability of Laves phases in two-phase V-Hf-Nb [3] and Fe-Zr alloys [4]. HfV2 deforms by twinning and through bands of concentrated shear, while ZrFe 2 experiences a stress-induced phase transformation between the C36 and C15 crystal structures. The Laves phase will most likely have to be in a two-phase alloy to be used as hightemperature structural material [5]. The current study considers Ti-Cr alloys strengthened by precipitation hardening of TiCr 2 . The two-phase alloy consists of the hard and strong Laves phase TiCr2 reinforcing the more ductile P-Ti matrix. In addition to studying the microstructural dependencies of the mechanical behavior of the two-phase alloys, the question of whether the Laves phase deforms was also addressed. Compressed samples are studied to identify deformation mechanisms, and to offer insight into improving Laves phase ductility. Wright Percent Chromium EXPERIMENTAL PROCEDURES Alloy compositions of Ti-40 at pet Cr and Ti-30 at pet Cr were prepared by arc-casting. For heat treatments, samples were encapsulated in vacuum with tantalum getter and back-filled with argon. A range of times and temperatures were employed in the P3-Ti(Cr) + cc-TiCr 2 phase field, followed by air cooling. The Ti-Cr binary phase diagram is shown in
Figure 1 [6].
-
(O"* .lr.
,*** \ ri
-.
'. Ti
A.. ...........
nm
C,
Fig. 1 The titanium-chromium phase diagram [6].
Mat. Res. Soc. Symp. Proc. Vol. 288. @1993 Materials Research Society
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Cubes of 5 mm were spark-cut for compression tests. The compression was done at room-temperature on an Instron machine at a crosshead speed of 0.0025 cm/min. Loading was stopped before complete fracture of the test cubes. TEM samples were made by a combination of mechanically grinding, dimpling, jet polishing (using a Blackburn and Williams electrolyte [7] or
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