Observation of structural modulations and formation of H-carbides and zeta-silicides during alpha_2->gamma transformation in fully lamellar gamma-TiAl

摘要

The present paper illustrates the experimentally observed microstructures related to alpha_2->gamma + Ti_2AlC (H-carbide) and Ti_5Si_3 (zeta silicide) phase transformations in fully lamellar Ti-46Al-2Cr-3Nb-0.2W-0.2Si-0.1C alloy. These transformations were observed by in-situ heat treatments in the TEM at temperatures up to 800 deg C and by post heat treatment examination in a high-resolution transmission electron microscope. alpha_2 dissolution was observed to start both inside the lath and at the alpha_2/gamma interface. H-type carbides were observed to nucleate inside the alpha_2 lath, which explains the appearance of long arrays of well-aligned structures along prior alpha_2 laths in creep-deformed samples. zeta silicide precipitates nucleate at the alpha_2/gamma interface, this growth occurs epitaxially along the pre-existing <0001alpha_2 direction. Modulated structures were observed during the alpha_2->H, newly formed gamma and at the alpha_2/zeta interface. All these modulated structures consist of regular arrangement of stacking faults and dislocations.