The Joining of Alumina to Hastelloy by a TiZrCuNi Filler Metal: Wettability and Interfacial Reactivity

A systematic microstructural characterization of alumina joined to Hastelloy C22((R)) by means of a commercial active TiZrCuNi alloy, named BTi-5, as a filler metal is reviewed and discussed. The contact angles of the liquid BTi-5 alloy measured at 900 degrees C for the two materials to be joined are 12 degrees and 47 degrees for alumina and Hastelloy C22((R)) after 5 min, respectively, thus demonstrating good wetting and adhesion at 900 degrees C with very little interfacial reactivity or interdiffusion. The thermomechanical stresses caused by the difference in the coefficient of thermal expansion (CTE) between the Hastelloy C22((R)) superalloy (approximate to 15.3 x 10(-6) K-1) and its alumina counterpart (approximate to 8 x 10(-6) K-1) were the key issues that had to be resolved to avoid failure in this joint. In this work, a circular configuration of the Hastelloy C22((R))/alumina joint was specifically designed to produce a feedthrough for sodium-based liquid metal batteries operating at high temperatures (up to 600 degrees C). In this configuration, adhesion between the metal and ceramic components was enhanced after cooling by compressive forces created on the joined area due to the difference in CTE between the two materials.