Synthesis, Properties, and Ternary Phase Stability of M6X Compounds in the Ti─Cu─O System

A section of the Ti─Cu─O ternary phase diagram was investigated at 925°C. The system contains two M6X‐type compounds, Ti4Cu2O and Ti3Cu3O, which are isostructural with the η‐carbides and are stabilized by O. The presence of two of these compounds and the size of their single‐phase fields contradict earlier predictions in the literature based on the valence and location of the elements in the periodic table. It is proposed that the atomic radii of the elements may also be a factor, especially for compounds containing Ti. The two observed M6X‐type compounds occur as products of reaction at the Ag─Cu─Ti/Al2O3 interface. The compounds were synthesized in bulk for characterization of their properties which influence the performance of the brazed joints. The compounds are stable in inert atmospheres up to their melting points, but are easily oxidized in ambients containing even trace amounts of O and form a coherent layer of reaction products. The values of elastic coefficients, thermal expansion, and resistivity of these compounds are intermediate between those of Al2O3 and Cu and would therefore provide a gradual transition across the interface.