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

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, Nb30T65Co35, 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 bee 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), [(2) over bar 21]((Nb, Ti)) // [001](TiCo). The atomic arrangements at the eutectic interface are also discussed on the basis of high-resolution observations. [doi: 10.2320/matertrans.MA200803]