Microstructural Control of TiAl by HDDR using Milling in Hydrogen

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Titanium aluminides have attracted much attention from aerospace and automobile industries but they suffer from low ductility and toughness at the intermediate and low temperatures. Grain refinement is generally one of the most effective techniques to enhance ductility of metallic materials. In the present work, we attempt to prepare the nanocrystalline bulk TiAl by a combination of HDDR utilizing MG in a hydrogen atmosphere and hot-pressing. Disproportionation of TiAl into TiH_2 and the other phase powders was demonstarated by mechanical grinding in a hydrogen atmosphere (HD process) . Exothermic realloying occurred to form nanocrystalline TiAl powder around 700 K by heating these powders in an argon atmosphere (DR process) . Hot-pressing was conducted during or after the DR process in vacuum. A nearly full dense bulk TiAl (99 % relative density) with the grain size of about 30 nm was prepared by in situ hot-pressing at 873 K.This bulk TiAl could be prepared at the lower temperature which is lower than the usual consolidation temperature at least by 300 K,because the TiAl powder prepared by HDDR processing is very fine, fresh and reactive. The TiAl compacts prepared by the above method was brittle in the as-pressed state, but showed compressive ductility after annealing at 1373 K.