Interface Diffusion in Polysynthetically-Twinned Tiai
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Mat. Res. Soc. Symp. Proc. Vol. 481 ©1998 Materials Research Society
are parallel. At the same time, the introduction of field emission TEMs and the development of interatomic potentials that enable the effective treatment of bond covalency provide the experimental and theoretical tools with which to carry out an effective analysis of diffusion in this interesting alloy. In the present paper, the first results are presented of a study that is investigating the relationship between interface atomic structure and interface diffusion in PST TiAI using electron microscopy techniques, Auger electron spectroscopy and atomistic simulation. EXPERIMENT Polysynthetically-twinned (PST) TiAI crystals were grown by directional solidification in an optical float zone furnace at Kyoto University. The feed rod and seed of Ti-49.3%AI were first produced by arc melting of Ti and Al using a non-consumable W electrode in a gettered Ar atmosphere. The growth rate of the PST crystals was 5 mm/hr. Using the float zone method, PST crystals with diameters 1 cm ý by 10 cm long are produced. The PST crystals were mounted on a goniometer and back Laue x-ray diffraction was used to orient the crystals for processing. The orientation used in the present study was such that the lamellar interfaces and one set of directions lying on the interfaces were parallel to the x-ray beam. Oblong disks of approximately 250 lgm thickness were cut from the oriented crystal by spark cutting. From these disks, diffusion specimens of three mm diameter were cut using a cylindrical slurry cutter (see Figure Ia). The 3 mm disks were mounted in an automatic polishing apparatus and thinned using a dilute diamond paste solution in oil to a final thickness of less than 50 lI.m. The thinned disks were washed using acetone and ethanol. The disks were mounted in a sputter deposition system at Osaka University with a 1 mm collimating aperture centered on each disk. The central 1 mm area of each disk was sputter
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Lamellar Interfaces
Diffusion Direction Figure 1 (a) Diffusion specimen prior to thinning showing orientation of the lamellar interfaces with respect to the diffusion direction. (b) The final diffusion specimen after deposition of the diffusing species indicated by the shaded area. The region around this area has not been sputtered and acts as a surface diffusion block.
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cleaned using Ar ions and Ag was then sputter deposited on the disks to a thickness of approximately 1 ýtm (see Figure 1b). The disks were mounted on an in-situ heating holder within an Auger spectroscopy system at the Metals and Ceramics Division of Oak Ridge National Laboratory. The orientation was such that the side of the disks containing Ag, the source side, was mounted away from the electron beam and sputter gun of the Auger system. Some initial data on Ag diffusion was obtained. However, it was found to be necessary to characterize the sputter and annealing behavior of the TiAI. Therefore, after cleaning some specimens of Ag, sputter/anneal cycles were run at a variety of te
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