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 Nb₂₀Ti₄₀Ni₄₀, Nb₃₀Ti₃₅Co₃₅, Nb₁₃Zr₄₃Ni₄₄, and Nb₂₅Zr₃₅Co₄₀ have been investigated by means of transmission electron microscopy. The alloys Nb₂₀Ti₄₀Ni₄₀, Nb₃₀Ti₃₅Co₃₅, and Nb₂₅Zr₃₅Co₄₀ possess eutectic structures consisting of fine lamellar morphology with Nb-based bcc and B2 intermetallic phases. The Nb₁₃Zr₄₃Ni₄₄ 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 Nb₂₀Ti₄₀Ni₄₀ and the Nb₂₅Zr₃₅Co₄₀ alloys exhibit a cube-on-cube orientation relationship between the bcc and B2 phases. The eutectic structure in the Nb₁₃Zr₄₃Ni₄₄ 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 Nb₃₀Ti₃₅Co₃₅ alloy, as follows: (110)_{(Nb, Ti)}||(110)_TiCo, [\\bar221]_{(Nb, Ti)}||[001]_TiCo. The atomic arrangements at the eutectic interface are also discussed on the basis of high-resolution observations.