Microstructural characterization of A γ-TiAl-Ni alloy produced by rapid solidification techniques

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An ingot of two-phase γ-TiAl containing a small amount of nickel [composition (Ti 52Al 48) 99.5Ni 0.5] was fabricated by the following series of processes: (1) high-purity buttons were arc melted; (2) the buttons were arc-melt spun into ribbons; (3) the ribbons were crushed into alloy powder using a hammer mill; (4) the powder was consolidated into an ingot by hot pressing, followed by hot isostatic pressing; and (5) the ingot was hot forged into a 1.5 cm thick plate. Microstructures of the material from different stages of the processing were examined. Although this series of fabrication steps was successful in producing ingots, the consolidated ingots showed numerous defects including unclosed pores and contamination by molybdenum and iron. These defects undoubtedly contributed to the brittleness of the ingots and the difficulty in machining tensile specimens for mechanical testing. It is believed that these defects can be eliminated by modification of the processing parameters, including increasing the HIP pressure and temperature to close pores, increasing forging temperature and reduction, and using processing hardware that will not contaminate the alloy. The lower level of oxygen contamination found by this fabrication route compared to other powder routes is encouraging and is probably a result of the larger powder particles (and hence lower amount of surface area exposed to the environment) produced by pulverization of the melt-spun ribbons.