Tantalum-vanadium diffusion bonding brittle intermetallic

Diffusion-bonded tantalum-to-vanadium joints shatter on tapping because Ta-V forms brittle intermetallic. Inserting a niobium foil between them — niobium being chemically close to tantalum (the "twin" ores, hence the names; Don Satterthwaite's doctoral thesis was Ta/Nb separation) but not forming Nb-V intermetallics — produces a good bond. No commercial application; pure teaching illustration of the brittle-intermetallic constraint on dissimilar diffusion bonding.

It's also used in many cases for joining of dissimilar materials, as long as there are no brittle intermetallics that form. One of my examples is tantalum to vanadium. Tantalum and vanadium are very close to each other on the periodic table but they form a brittle intermetallic, and you can diffusion bond them just fine — they dissolve away their oxide at higher temperatures — but then you form a brittle intermetallic, and you tap that joint and it shatters.