Microstructural Control of TiAl by HDDR using Milling in Hydrogen

钛铝化合物在航空航天和汽车工业中受到广泛关注，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塑性的最有效的技术之一。在本工作中，我们试图通过在氢气氛中利用MG的HDDR和热压相结合的方法制备纳米晶块体TiAl。在氢气氛下，用机械研磨法（HD法）研究了TiAl向TiH_2和其它相粉末的歧化反应。通过在氩气气氛中加热这些粉末（DR过程），在700 K左右发生放热再合金化以形成纳米晶TiAl粉末。在真空中在DR过程期间或之后进行热压。在873 K下原位热压制备了晶粒尺寸约为30 nm、相对密度约为99%的致密TiAl块体材料。由于HDDR工艺制备的TiAl粉末非常细小、新鲜且具有活性，因此可以在比通常的固结温度低至少300 K的较低温度下制备这种TiAl块体材料。用上述方法制备的TiAl压坯在压态下是脆性的，但在1373 K退火后显示出压缩塑性。

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.