The effect of Si addition in NbTiZr coatings

Abstract

Nb based alloys are considered the most appropriate to replace Ni alloys for new generation aircrafts, since Ni alloys can no longer meet the extreme environmental requirements. Processing Nb based alloys as coatings might bring a wider use of alloys depending on the available understanding of their processability. This investigation takes an important step to process Niobium alloys containing high hardness low toughness silicide compounds as multilayer coatings, taking advantage of in-situ synthesis of the alloy during the deposition of powder mixtures. Procedures started by preparing elemental powder mixtures adding Si to a NbTiZr solid solution composition with good processability. In-situ synthesis of silicides occurs during the deposition of (Nb24Ti18Si + 1Zr) (at.%) powder mixtures by Plasma Transferred arc hardfacing (PTA). Si accounts for the hardness increase of the deposited coatings and their reduced mass gain. The addition of Si into the powder mixtures induced the formation of primary Nb5Si3 that competes with the primary Nb(Ti,Si,Zr)ss. Three solidification paths are identified to be competing as the PTA torch moves and the melt pool solidifies. The thermal cycles of multilayer coating processing favour the epitaxial growth of Nb5Si3 at the remelted interface between layer and induce a coarsening of the silicides in the microstructure. Phase distribution in coatings, with the more ductile and toughness phases surrounding the silicides, contributes for the confinement of cracks within the larger silicides formed after the deposition of multilayer coatings.