Low-temperature bonding between F82H and Cr assisted by metastable Cr interlayers applied by pulsed laser deposition

Abstract

The joining of F82H and pure Cr metal has been demonstrated at temperatures as low as 798 K, assisted by the prior application of a thin metastable Cr interlayer to both bonding surfaces using pulsed laser deposition. Bonded F82H/Cr interfaces were characterised by SEM-EDS and TEM analysis, with respect to diffusion behaviour, chemical composition and microstructure. At higher bonding temperatures, an interface layer formed on the Cr side of the joint, which was rich in M23C6, though its structure and thickness was different depending on whether Cr interlayers were applied. As the bonding temperature was decreased stepwise, the interface layer thickness reduced to a minimum of ∼60 nm and appeared to adopt a film-like structure. Although the chemical composition of the thinner interface layers could not be determined, they were most likely M23C6 in a film-like form, which subsequently formed particles, and then columns, as the bonding temperature and interface layer thickness increased. Hardness around the interface increased as compared with the bulk F82H and Cr, though the peak hardness value decreased as the interface thickness decreased. The hardness of bulk F82H and Cr was similar before and after bonding at all temperatures, suggesting little impact on mechanical properties. Diffusion of Cr into F82H and Fe into Cr was not detected at any of the conditions tested, and the original tempered lath martensite microstructure of F82H appeared to have been retained.