Microstructural Control of TiAl by HDDR using Milling in Hydrogen

钛铝化物在航空航天和汽车工业中受到了广泛的关注，但其在中低温下的延展性和韧性较低。晶粒细化通常是提高金属材料延展性的最有效技术之一。在本工作中，我们尝试在氢气气氛中利用MG结合HDDR和热压制备纳米晶块状TiAl。在氢气气氛（HD）中进行机械研磨，证实了TiAl歧化成TiH_2和其他相粉末。在700 K左右的氩气气氛中加热，形成纳米晶TiAl粉末（DR工艺）。热压是在真空DR过程中或之后进行的。采用873 K原位热压法制备了晶粒尺寸约为30 nm的近全致密体TiAl（相对密度99%）。由于HDDR工艺制备的TiAl粉末非常细、新鲜且反应性好，因此可以在较低的温度下制备块状TiAl，比通常的固结温度至少低300 K。采用上述方法制备的TiAl合金在压制状态下呈脆性，但在1373 K退火后表现出抗压延展性。

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.