Electron Microscopy Study of Eutectic Structure in Nb-Ti-X and Nb-Zr-X (X = Co, Ni) Hydrogen Permeation Alloys

Abstract

The morphology and crystallography of eutectic structures in several niobium-bearing hydrogen permeation alloys such as Nb 20 Ti 40 Ni 40 , Nb 30 Ti 35 Co 35 , Nb 13 Zr 43 Ni 44 , and Nb 25 Zr 35 Co 40 have been investigated by means of transmission electron microscopy. The alloys Nb 20 Ti 40 Ni 40 , Nb 30 Ti 35 Co 35 , and Nb 25 Zr 35 Co 40 possess eutectic structures consisting of fine lamellar morphology with Nb-based bcc and B2 intermetallic phases. The Nb 13 Zr 43 Ni 44 alloy possesses a eutectic structure consisting of rod-shaped bcc-(Nb, Zr) phase and B33-ZrNi phase which displacively transforms from the B2 high-temperature phase. The eutectic structures in the Nb 20 Ti 40 Ni 40 and the Nb 25 Zr 35 Co 40 alloys exhibit a cube-on-cube orientation relationship between the bcc and B2 phases. The eutectic structure in the Nb 13 Zr 43 Ni 44 alloy also exhibits a cube-on-cube orientation relationship by considering the lattice correspondence between the B2 and the B33 structures. On the other hand, a unique orientation relationship is found out in the Nb 30 Ti 35 Co 35 alloy, as follows: (110) (Nb, Ti) // (110) TiCo , [221] (Nb, Ti ) // [001] TiCo . The atomic arrangements at the eutectic interface are also discussed on the basis of high-resolution observations.