High-temperature oxidation of a rapidly solidified amorphous Ta-Ir alloy

The oxidation products formed at 500 and 700°C on an amorphous Ta-44.5 at% Ir alloy in an Ar-0.1% O₂ gas mixture were characterized using SEM, XRD, EPMA, TEM, STEM, AES, and XPS. Initially, a thin (3–4 nm) layer of Ta₂O₅ formed at the surface of the alloy. Continued growth of the Ta₂O₅, which occurred very rapidly, involved diffusion of oxygen anions from the Ta₂O₅/gas interface to the alloy/Ta₂O₅ interface, where tantalum was selectively oxidized. Because the oxide grew more quickly than iridium could diffuse back into the alloy, the iridium coalesced into platelets of crystalline iridium-rich alloy that were oriented approximately parallel to the oxide/alloy interface, and which became embedded in a matrix of Ta₂O₅. The unoxidized core remained in the glassy state. The oxidation process and/or the dissolution of oxygen into the unoxidized alloy caused the alloy to become embrittled.